Title of Invention

ORTHOPAEDIC FIXATION SYSTEM

Abstract

A fixation system includes a plate comprising a top surface, a bottom surface, a central longitudinal axis, at least one fixation hole extending between the top and bottom surfaces and comprising an undercut therein, and at least one passage intersecting one of the undercuts. At least one resilient clip is disposed in a portion of the undercut, with the clip having a pair of generally parallel sides and an end tab. At least one fastener is provided and comprises a head and a threaded shaft, with the head comprising a perimetral groove extending around at least a portion thereof and an instrument receiving portion. The clip is configured and dimensioned to seat in the undercut with the end tab extending within the passage, and the fastener is configured and dimensioned to be received in the fixation hole and securable therein when the clip abuts the perimetral groove of the head.

Full Text

BONE PLATE WITH CAPTIVE CLIPS FIELD OF THE INVENTION The invention is related to a fixation system. More particularly, the invention is related to a plate with a clip for resisting post-operative fastener back-out. BACKGROUND OF THE INVENTION Orthopaedic fixation devices such as plates are frequently coupled to bone with fasteners inserted through plate holes. It is known that securing such fasteners to the bone plate, for example through the use of expansion-head screws, can decrease the incidence of loosening of the fixation assembly post-operatively. It is also known that a bushing may be disposed in each plate hole to receive the fastener to permit polyaxial movement so that the fastener may be angulated at a surgeon-selected angle. However, typically as the fastener is inserted into bone through the bushing and plate hole, a threaded head of the fastener engages a threaded internal surface of the bushing to expand the bushing against the wall of the plate hole, thereby locking the screw at a given angular orientation with respect to the plate, Despite these fixation systems, there exists a need for bone plates that allow post-operative angulation and/or movement. In particular, with respect to fixation of the spine, there exists a need for anterior cervical compression plates and associated fasteners that allow the vertebral bodies to compress over grafts post-operatively. In order for compression to occur, there exists a need for a plate/fastener construct that allows for translational and/or rotational settling that both occur post-operatively between the plate and fasteners that have been rigidly placed into vertebral bodies. More particularly, there exists a need for a fixation system that facilitates translational settling by permitting a fastener to slide within a plate hole. There further exists a need for a fixation system that facilitates rotational settling by permitting a fastener head to toggle or pivot within the plate hole. In addition, there exists a need for a fixation system that permits fastener motion associated with translational and/or rotational settling while also resisting back-out of the fastener from the plate. SUMMARY OF THE INVENTION The invention relates to a fixation system that includes a plate comprising a top surface, a bottom surface, a central longitudinal axis, at least one fixation hole extending between the top and bottom surfaces and comprising an undercut therein, and at least one passage intersecting one of the undercuts. At least one resilient clip is disposed in at least a portion of the undercut, with the at least one clip having a pair of generally parallel sides and an end tab. At least one fastener is provided and comprises a head and a threaded shaft, with the head comprising a perimetral groove extending around at least a portion thereof and an instrument receiving portion. The at least one clip is configured and dimensioned to seat in the undercut with the end tab extending within the passage, and the at least one fastener is configured and dimensioned to be received in the at least one fixation hole and securable therein when the at least one clip abuts the perimetral groove of the head. The fixation system may further comprise a slot extending through the centra] longitudinal axis. The slot may be aligned along the central longitudinal axis or may be disposed transverse to the central longitudinal axis. The slot may include a dog-bone shape or the slot may include a pair of overlapping circular shapes. Also, the at least one passage may extend transverse to at least one of the fixation holes. The instrument receiving portion of the head of the at least one fastener may at least partially intersect the undercut. A portion of the undercut may be disposed closer to the top surface than the bottom surface, and may extend completely around the at least one fixation hole, Each of the fixation holes may further include a lower portion disposed between the bottom surface and the undercut, with the lower portion tapering toward a central axis of the fixation hole. The taper of the lower portion may be semispherical. The lower portion also may have a first maximum inner dimension and the undercut may have a second maximum inner dimension, wherein the first maximum inner dimension is less than the second maximum inner dimension. The head of the fastener may further include a tapered portion disposed between the threaded shaft and the perimetral groove, with the tapered portion tapering toward a central axis of the fastener. At least one fixation hole may be circular. Also, the plate may include at least two pairs of fixation holes. One pair of fixation holes may be generally circular and the other pair of fixation holes may be generally oblong. In addition, the perimetral groove of the fastener may be interrupted by at least one corner, at least (wo corners, or at least four corners. Each comer may be configured and dimensioned as a cam to abut an inner wall of the clip. The perimetral groove of the fastener may include an upper surface and a lower surface disposed at between about 10° and about 70° with respect to each other. In some embodiments, the perimetral groove includes an upper surface and a lower surface disposed at between about 30° and about 50° with respect to each other, and in some embodiments, the perimetral groove includes an upper surface and a lower surface aligned at about 40° with respect to each other. The instrument receiving portion of the head of the at least one fastener may intersect the perimetal groove in at least one location, at least two locations, or at least four locations. The instrument receiving portion of the head of the at least one fastener may have two substantially perpendicular slots. Also, the instrument receiving portion may include an internal thread that may extend within the shaft. The clip may be generally wishbone-shaped. In some embodiments, the clip includes a generally circular portion or a generally arcuate portion. The clip may also include a discontinuity. In some embodiments the clip may be configured and dimensioned to slide in the undercut, while in other embodiments the clip may be configured and dimensioned to be fixed and stationary in the plate. The undercut of the plate may be sized to retain the clip at least partially therein while permitting expansion thereof. The at least one fixation hole may include at least two pairs of fixation holes, with at least one of the pairs of fixation holes being configured and dimensioned to permit toggling of fasteners disposed therein. The head of the fastener may be configured and dimensioned to permit toggling in the at least one fixation hole, or the head may be configured and dimensioned for coupling to the plate at a fixed angle. The fastener may be permitted to toggle. The head of the fastener may further include at least one scallop disposed proximate a top peripheral portion of the head. In some embodiments, four scallops are provided. The at least one scallop may include an arcuate portion. At least one surface of the head may include roughening for interacting with the clip. In some embodiments, the roughening is formed by steps on a surface. The at least one resilient clip may be configured and dimensioned to permit toggling of the at least one fastener through a greater angular range in a cephalad-caudal direction of the plats than in other directions of the plate. In particular, the at least one resilient clip may be configured and dimensioned to permit toggling through a greater angular range in a cephalad-caudal direction of the plate than in a medial-lateral direction of the plate. Toggling of the at least one fastener may be permitted between about 0° and about 32° along a plane extending parallel to the central longitudinal axis of the plate, while toggling of the at least one fastener may be permitted between about 0° and about 20° along a plane extending perpendicular to the central longitudinal axis of the plate. The invention also relates to a fixation system including a plate having a top surface, a bottom surface, a central longitudinal axis, and a slot extending through the central longitudinal axis. The plate may also have at least two pairs of fixation holes, with each of the fixation holes extending between the top and bottom surfaces and including an undercut therein. In addition, at least one passage may extend transverse to one of the fixation holes and intersect one of the undercuts. The fixation system may also include a resilient clip disposed in at least one of the undercuts, with the clip having a pair of generally parallel sides and an end tab. At least one fastener having a head and a threaded shaft may be provided, with the head including a perimetral groove extending around at least a portion thereof and an instrument receiving portion that at least partially intersects the groove. Each clip may be configured and dimensioned to seat in an undercut with the end tab extending within the passage, and each fastener may be configured and dimensioned to be received in one of the fixation holes and securable therein when the clip abuts the perimetral groove of the head. BRIEF DESCRIPTION OF THE DRAWINGS Preferred features of the present invention are disclosed in the accompanying drawings, wherein: HO. 1A shows a top view of a plate for use with a first embodiment of a fixation system; FIG. IB shows a partial cross-sectional view taken along line IB-IB of the plate of FIG. 1A; FIG. 1C shows a partial cross-sectional view taken along line IC-IC of the plate of FIG. IB; FIG, ID shows a partial cross-sectional view taken along line ID-ID of the plate of FIG. 1C; FIG. IE shows another partial cross-sectional view taken along line ID-ID of the plate of FIG. 1C; FIG. 2A shows a captive clip for use with the plate of FIG. 1A; FIG. 2B shows a cross-sectional view taken along line IIB-IIB of the captive clip of FIG. 2A; FIG. 3 shows a partial cross-sectional perspective view of the plate of HG. 1A with fasteners installed therewith; FIG. 4A shows a perspective view of an embodiment of a fastener; FIG. 4B shows a side view of the fastener of FIG7 4A; FIG. 4C shows a partial cross-sectional side view taken along line FVC-IVC of the fastener of FIG. 4B; FIG. 4D shows a partial side view of the head of the fastener of FIG. 4A; FIG. 4E shows a top view of the fastener of HG. 4A; FIG. 4F shows a bottom view of the fastener of FIG. -4A; FIG. 4G shows a partial cross-sectional side view of the shaft and threads of the fastener of FIG. 4A; FIG. 5A shows a top view of a plate for use with a different embodiment of a fixation system; FIG. 5B shows a partial cross-sectional view taken along line VB-VB of the plate of FIG. 5A; FIG. 5C shows a partial cross-sectional view taken along line VC-VC of the plate of FIG. SB; FIG. 5D shows a partial cross-sectional view taken along line VD-VD of the plate of FIG. 5C; FIG. 5E shows another partial cross-sectional view taken along line VE-VE of the plate of FIG. 5A; FIG. 6A shows a circular captive clip for use with the plate of FIG. 5A; FIG. 6B shows a cross-sectional view taken along line VIB-VIB of the circular captive clip of FIG. 6A; FIG. 7A shows an oblong captive clip for use with the plate of FIG, 5A; FIG. 7B shows a cross-sectional view taken along line VIIB-VITB of the circular captive clip of FIG. 7A; FIG. 8A shows a perspective view of an embodiment of a self-drilling fastener; . . FIG. SB shows a side view of the fastener of FIG. 8A; FIG. 8C shows a partial cross-sectional side view taken along line VIIIC-VIHC of the fastener of FIG, 8B; FIG. 8D shows a partial side view of the head of the fastener of FIG. 8A; FIG. 8E shows a top view of the fastener of FIG. 8A; FIG. SF shows a bottom view of the fastener of FIG. 8A; FIG. 8G shows a partial cross-sectional side view of the shaft and threads of the fastener of FIG. 8 A; FIG. 9A shows a top view of a plate for use with a third embodiment of a fixation system; FIG. 9B shows a partial cross-sectional view taken along line IXB-IXB of the plate of FIG. 9A; FIG. 9C shows a top view of end 221a of the plate of FIG. 9A; FIG. 9D shows a top view of end 221b of the plate of FIG. 9A; FIG. 9E shows a partial cross-sectional view taken along line IXE-IXE of the plate of FIG. 9B; FIG. 9F shows a partial cross-sectional view taken along line IXF-BXF of the plate of FIG. 9E; FIG. 9G shows another partial cross-sectional view taken along line IXF-IXF of the plate of FIG. 9E; FIG. 10A shows a captive clip for use with the plate of FIG. 9A; FIG. 10B shows a cross-sectional view taken along line XB-XB of the captive clip of FIG. 10A; FIG. 11 shows a perspective view of another plate with fasteners installed therewith along with the captive clip of FIG. 10A; FIG. 12 shows an alternate captive clip for use with the plate of FIG. 9A or 11; PIG. 13 shows a partial cross-sectional side view of the captive clip of FIG. 12 installed in the place of FIG. 11; FIG. 14 shows another alternate captive clip; FIG. 15 shows yet another alternate captive clip; FIG. 16A shows a bottom perspective view of a plate and cam element used in a fourth embodiment of a Fixation system; FIG. 16B shows a partial cross-sectional perspective view of the plate and cam element of FIG. 16 A with the cam element in a locked position with respect to a captive clip; FIG. 16C shows a bottom view of the plate and cam element of FIG. 16A with the cam element in an unlocked position with respect to a captive clip; FIG. 16D shows a bottom view of the plate and cam element of FIG, 16A with the cam element in a locked position with respect to a captive clip; FIG. 17A shows a top view of a plate for use with a fifth embodiment of a fixation system; FIG. 17B shows a partial cross-sectional view taken along line XVIIB-XVIIB of the plate of FIG. 17A; FIG. 17C shows a partial cross-sectional view taken along line XVIIC-XVIIC of the plate of FIG. 17B; FIG. 17D shows a partial cross-sectional view taken along line XVIID-XVIID of the plate of FIG. 17C; FIG. 17E shows another partial cross-sectional view taken along line XVIID-XVIID of the plate of FIG. 17C; FIG. 18A shows a captive clip for use with the plate of FIG. 17A; FIG. 18B shows a cross-sectional view taken along line XVIIIB-XVIIIB of the captive clip of FIG. 18 A; FIG. 19A shows a perspective view of another embodiment of a fastener; FIG. 19B shows a side view of the fastener of FIG. 19A; FIG. 19C shows a partial cross-sectional side view taken along line XIXC-XIXC of the fastener of FIG. 19B; FIG. 19D shows a partial side view of the head of the fastener of FIG. 19A; FIG. 19E shows a top view of the fastener of FIG. 19A; FIG. 19F shows a bottom view of the fastener of PIG. 19A; FIG. I9G shows a partial cross-sectional side view of the shaft and threads of the fastener of FIG. 19A; FIG. 20A shows a schematic of the captive clip of FIG. I18A in an unexpanded state with a cross-section of a screw head inserted through the clip; . , FIG. 20B shows a schematic of the captive clip of FIG, 18A in an expanded state with a cross-section of a screw head inserted through the clip; FIG, 21A shows a top view of a plate for use with a sixth embodiment of a fixation system; FIG. 21B shows a partial cross-sectional view taken along line XX1B- XXIB of the plate of FIG. 21 A; .; FIG, 21C shows a partial cross-sectional view taken along line XXIC-XXIC of the plate of FIG. 21B; FIG, 2ID shows a partial cross-sectional view taken along line XXID-XXID of the plate of FIG. 21C; FIG. 21E shows another partial cross-sectional view taken along line XXID-XXTD of the plate of FIG. 21C; FIG. 22 shows a partial cross-sectional perspective view of the plate'of FIG. 21A with a fastener and captive clip installed therein; FIG. 23A shows a top view of a one level plate for use with a seventh embodiment; FIG, 23B shows a partial cross-sectional side view taken along line XXIIB-XXIIIB of the pJate FIG. 23A; FIG. 24A shows a top view of a two level plate for use with a seventh embodiment; FIG. 24B shows a partial cross-sectional side view taken along line XXIVB-XXIVB of the plate FIG. 24A; FIG. 25A shows a top view of a three level plate for use with a seventh embodiment; FIG. 25B shows a partial cross-sectional side view taken along line XXVB-XXVB of the plate FIG. 25A; FIG. 26A shows a top view of a four level plate for use with a seventh embodiment; F]G. 26B shows a partial cross-sectional side view taken along line XXVIB-XXVIB of the plate FIG. 26A; FIG. 27A shows a cross-sectional view taken along line XXVIIA-XXVIIA of the plate of FIG. 23B; FIG. 27B shows a partial bottom view of a fixation hole and passage of the plate of FIG. 23A; FIG, 27C shows a partial side view of a portion of the slot of the plate as indicated at detail N in FIG. 23B; FIG. 27D shows a cross-sectional side view taken along line XXVIID-XXVIID of the plate of FIG, 23B; FIG. 27E shows another cross-sectional side view of one of the fixation holes taken along line XXVDD-XXVIID of the plate of FIG. 23B; FIG. 28A shows a captive clip for use with the plates of FIGS. 23A, 24A, 25A,,and 26A; FIG. 28B shows a cross-sectional view taken along line XXVIIIB-XXVIIIB of the captive clip of FIG. 28A; FIG. 29 shows a partial cross-sectional perspective view of the plate of FIGS. 23A, 24A, 25A, and 26A with a fastener and the captive clip of FIG. 28A installed therein; FIG. 30A shows another embodiment of a captive clip; FIG. 3OB shows a cross-sectional view taken along line XXXB-XXXB of the captive clip of FIG. 30A; FIG. 30C shows another view of the captive clip of FIG. 30A along with a cross-section of a fastener head taken along the minor diameter of a perimetral groove in the fastener head; FIG. 31A shows a perspective view of a four level plate for use with an eighth embodiment; FIG. 3 IB shows a partial perspective view of the plate of FIG. 31A with captive clips and a fastener installed therein; FIG. 31C shows a partial cross-sectional perspective view of the plate of FIG. 31A with a captive clip and a fastener installed therein; FIG. 3 1D shows another partial cross-sectional perspective view of the plate of FIG. 31A with captive clips and fasteners installed therein; FIG. 3 1E shows a top view of another embodiment of a plate; FIG. 3 1F shows a partial cross-sectional side view of the plate of FIG. 3 1E; FIG. 31G shows a cross-sectional view taken along line XXXIG-XXXIG of the plate of FIG. 3 1E; FIG. 31H shows a cross-sectional view taken along line XXXIH-XXXIH of the plate of FIG, 3 1E; FIG. 311 shows a cross-sectional view taken along line XXXII-XXXII of the plate of FIG. 3 IE; FIG. 31J shows a cross-sectional view of a keyhole-shaped fixation hole of the plate of FIG. 3 IE taken in the circular portion of the fixation hole along line XXXIG-XXXIG; FIG. 3IK shows a. partial cross-sectional side view'taken along the midline 1084 of the plate of FIG. 3 IE proximate a circular fixation hole; FIG. 31L shows a partial bottom view of a circular fixation hole of the plate of FIG. 3 IE; FIG. 31M shows a partial cross-sectional side view of a keyhole-shaped fixation hole of the plate of FIG. 3 IE taken in the straight portion of the fixation hole along line XXXTH-XXXIH; FIG. 31N shows a top view of another multi-level fixation plate; FIG. 310 shows a top view of another embodiment of a captive clip; FIG. 3 1P shows a cross-sectional side view taken along line XXXP-XXXP of the plate of FIG. 3 1E; FIG. 32A shows a side view of an embodiment of a variable angle, self-tapping fastener; FIG, 32B shows a partial cross-sectional side view taken along line XXXIIB-XXXIIB of the fastener of FIG. 32A; FIG. 32C shows a partial side view of the head of the fastener of FIG, 32A; FIG. 32D shows a top view of the fastener of FIG. 32A; FIG. 32E shows a partial cross-sectional side view of the shaft and threads of the fastener of FIG. 32A configured and dimensioned for use with cancellous bone; FIG. 32F shows another partial cross-sectional side view of the shaft and threads of the fastener of FIG. 32A configured and dimensioned for use with cortical bone; HO. 33A shows a side view of an embodiment of a variable angle, self-drilling fastener; FIG. 33B shows a partial cross-sectiona] side view taken along line XXXIIIB-XXXfIIIB of the fastener of FIG. 33 A; FIG. 33C shows a partial side view of the head of the fastener of FIG. 33A; FIG. 33D shows a top view of the fastener of FIG. 33A; FIG. 34A shows a side view of an embodiment of a fixed angle, self-tapping fastener; FIG. 34B shows a partial cross-sectional side vievwtaken along line XXXIVB-XXXIVB of the fastener of FIG. 34A; FIG. 34C shows a partial side view of a fixed-angle head of the fastener of HG. 34A; FIG. 34D shows a top view of the fastener of FIG. 34A; FIG. 34E shows a partial cross-sectional side view taken along line XXXrVF-XXXTVF of the fastener of FIG. 34A; FIG. 35A shows a side view of an embodiment of a fixed angle, self-drilling fastener; FIG. 35B shows a partial cross-sectional side view taken along line XXXVB-XXXVB of the fastener of FIG. 35A; FIG. 35C shows a partial side view of the head of the fastener of FIG. 35A; FIG, 35D shows a top view of the fastener of HG. 35A; FIG, 35E shows a partial cross-sectional side view taken along line XXXVF-XXXVFof the fastener of FIG. 35A; FIG. 36A shows a first perspective view of an embodiment of a plate having a fastener with scallops installed therewith; FIG. 36B shows another perspective view of the embodiment of FIG, 36A; FIG. 36C shows another perspective view of the embodiment of FIG. 36A; FIG. 36D shows another perspective view of the embodiment of FIG. 36A; FIG. 37A shows a side view of an embodiment of a fastener; FIG, 37B shows a partial cross-sectional side viesv taken along line XXXVIEB-XXXVIIB of the fastener of FIG. 37A; FIG. 37C shows a partial side view of the head of the fastener of FIG. 37A; FIG. 37D shows a partial cross-sectional side view taken along line XXXVIID-XXXVIID of the fastener of FIG, 37E; FIG. 37E shows a top view of the fastener of FIG. 37A; FIG. 37F shows a partial top view of the fastener of FIG. 37 A FIG. 37G shows a partial side view of area S of FIG. 37C; FIG. 37H shows an alternate a top view of the fastener of FIG. 37A; FIG. 371 shows an alternate partial top view of the fastener of FIG. 37A; FIG. 38A shows a perspective view of an embodihlent of a plate having gussets; FIG. 38B shows a top view of the plate of FIG. 38A; FIG. 38C shows a cross-sectional side view taken along line XXXVIIIC-XXXVIIIC of the plate of FIG. 38 A; FIG. 38D shows a cross-sectional side view taken along line XXXVIIID-XXXVIIID of the plate of FIG. 38A, with a fastener disposed in a hole in the plate; FIG. 38E shows a perspective view of an embodiment of a fastener with slits; FIG. 38F shows a cross-sectional side view of the fastener of FIG. 38E with a fastener head in an expanded state; and, FIG. 38G shows a cross-sectional side view of the fastener of FIG. 38E with a fastener head in a contracted state as being inserted or removed from the plate. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Turning to FIGS. 1A-4G, a first embodiment of a fixation system is shown. The fixation system includes a plate 20 with two pairs of fixation holes 22, 24. Fixation holes 22 are oblong in shape so as to have a different length than width (i.e., the hole is non-circular), while fixation holes 24 are circular in shape. The fixation holes are preferably oblong in the direction along the longitudinal axis 28a of the bone plate 20. Although plate 20 is provided with two pairs of fixation holes 22, 24, more than two pairs may instead be provided, for example so that plate 20 may span a greater length and thus be fastened to multiple locations along the spine. Single holes alternatively maybe provided as opposed to pairs. A slot 26 is aligned along central longitudinal axis 28a for receiving a drill/screw guide and for graft visualization. Preferably, slot 26 does not receive any fasteners. In alternate embodiments, more than one slot may be provided, and the slot or slots may be disposed transverse to central longitudinal axis 28a, Preferably, slot 26 includes straight portions 26a and semicircular portions 26b, Each of fixation holes 22, 24 extends between top and bottom surfaces 28, . 30 and includes an undercut 32. In one embodiment, undercut 32 is disposed closer to top surface 28 than bottom surface 30 while in another embodiment, undercut 32 is disposed closer to bottom surface 30 than top surface 28. Undercut 32 also may be disposed intermediate top and bottom surfaces 28, 30, or transverse to the~surfaces. Preferably, an undercut 32 extends completely around each of fixation holes 22, 24. A lower portion 34 of each fixation hole 22, 24 may be disposed between bottom surface 28 and undercut 32 and may taper toward a central axis 36 of the respective fixation hole as shown for example in FIG. 1D. Preferably, lower portion 34 has a first maximum inner dimension LI and the undercut has a second maximum inner dimension L2, wherein the first maximum inner dimension LI is less than the second maximum inner dimension L2. Turning to FIGS. 2A and 2B, a generally circular captive clip 50 is shown with a slit 52 that permits elastic expansion/compression of clip 50 as will be explained shortly, as well as an outer edge 54 and an inner edge 56. Clip 50 includes a generally rectangular cross-section, and in one embodiment, is provided with a radius Rl adjoining outer edge 54 of about 0.1 mm and a second radius R2 adjoining inner edge 56 of about 0.2mm. Referring to FIG. 3, plate 20 is shown with a fastener 60 in each of fixation holes 22, 24. A captive clip 50 is provided for each fastener 60 around head 62." Preferably, a clip 50 is pre-installed in each fixation hole 22, 24, and snap-fits around a head 62 of a fastener 60 such that post-operative back-out of a fastener 60 from a hole 22, 24 is resisted, Preferably, a captive clip 50 disposed in an undercut 32 of an elongated, oblong hole 22 is permitted to travel (slide) along a longitudinal axis 22a of the hole, as shown in FIGS. 1A and IE. Thus, a fastener 60 on a captive clip 50 and disposed in an oblong hole 22 can slide across the length of hole 22. However, a captive clip 50 disposed in an undercut 32 of a circular hole 24 is not permitted to travel (slide) along a longitudinal axis 24a of hole 24, a shown in FIGS. 1A and 1D. As will be described in detail herein, various embodiments of fasteners are contemplated. In particular, two main types of fasteners for "variable angle" and "fixed angle" applications are provided. "Variable angle" refers to fasteners and/or plates for which: (1) the trajectory of insertion of the fastener into bone (through a fixation hole in the plate) may be selected by the surgeon (although only a limited range of motion may be permitted); and/or (2) the trajectory of the fastener with respect to the plate is allowed to change following insertion into bone, for example to toggle to accommodate any translational and/or rotational settling that occur post-operati vely between the plate and the fastener that has been rigidly placed into a vertebral body (although only a limited range of motion may be permi tted). " " "Fixed angle" refers to fasteners and/or plates for which: (1) the trajectory of insertion of the fastener into bone (through a fixation hole in the plate) is pre-selected and thus fixed; and/or (2) the trajectory of the fastener with respect to the plate is not allowed to change following insertion into bone. Each of the two main types of fasteners may be provided with features for use preferably with either cancellous or cortical bone. Moreover, each of the two main types of fasteners may be provided preferably with either self-tapping or self-drilling features. Finally, the diameters of the fasteners may be varied as well as the lengths. In selecting a fastener for a given application, therefore, a surgeon must decide which type of fastener and which combination of features are appropriate. A the minimum, the surgeon must select among eight categories of fasteners: (1) variable-angle, cancellous, self-tapping, (2) variable-angle, cancellous, self-drilling, (3) variable-angle, cortical, self-tapping, (4) variable-angle, cortical, self-drilling, (5) fixed-angle, cancellous, self-tapping, (6) fixed-angle, cancellous, self-drilling, (7) fixed-angle, cortical, self-tapping, and (8) fixed-angle, cortical, self-drilling. As shown in FIGS, 4A-4G, an exemplar fastener 60 includes a head 62 with a self-tapping, threaded shaft 64 with a flute 65 provided proximate the bottom of the threading. In an alternate embodiment, as described above, a self-drilling, threaded shaft 64 instead may be provided. Head 62 includes a perimetral groove 66 extending around at least a portion thereof, and an instrument receiving portion 68 that at least partially intersects groove 66 at one or more openings 70. Instrument receiving portion 68 preferably includes an internal, unthreaded, cylindrical, annular wall 69, although in an alternate embodiment wall 69 may be threaded. In the exemplar embodiment of FIG. 4E, . instrument receiving portion 68 is cross-shaped and thus intersects groove 66 at four openings 70. A pair of slots 70a, 70b (similar to the look of a Phillips-head screw) form instrument receiving portion 68. However, in alternate embodiments, only one opening 70, two openings 70, or any number of openings 70 may be provided. Fastener 60 preferably is used with a plate 20 such that fastener 62 may toggle in the fixation holes. In addition, fastener 60 preferably is used for fixation to cancellous bone. However, threading appropriate for cortical bone instead may be provided. In one preferred exemplary embodiment, perimetral groove.66 includes an upper surface 66a and a lower surface 66b disposed at an angle 5 of between about 10° and about 70° with respect to each other. More preferably, uppelr Surface 66a and lower surface 66b are disposed at an angle 5 of between about 30° and about 50° with respect to each other, and most preferably, they are an angle 6 of about 40° with respect to each other. The angulation of surfaces 66a, 66b with respect to each other permits toggling of fastener 60 when coupled to a captive clip 50. Head 62 preferably is partially spherical and includes a bottom section 68 extending to the top end 70 of shaft 64, with bottom section 68 tapering inward toward longitudinal axis 72 from perimetral groove 66 to top end 70. Advantageously, such tapering permits angulation of fastener 60 when disposed in a fixation hole 22, 24. In use, a fastener 60 is received in a captive clip 50, The shaft 64 is initially screwed into bone until the partial-spherical head 62 of fastener 60 reaches captive clip 50. Upon further insertion of fastener 60 into captive clip 50, the partial-spherical head 62, particularly bottom section 68, bears against the inside edge 56 of captive clip 50 and expands captive clip 50. Once fastener 60 is inserted far enough, captive clip 50 contracts so that it "snaps" into perimetral groove 66 in head 62, thereby preventing fastener 60 from backing out of plate 20, as previously described. In addition, captive clip 50 subsequently may be clastically expanded to permit removal of fastener 60. When a screwdriver tip is inserted in the slots 70a, 70b of instrument receiving portion 68, the screwdriver tip protrudes into the perimetral groove 66 through openings 70. Inserting the screwdriver tip thus elastically expands captive clip 50 to allow fastener 60 to be unscrewed from bone without interference from captive clip 50. A second embodiment of a fixation system is shown in FIGS. 5A-8G. The fixation system includes a plate 120 with two pairs of fixation holes 122, 124. Fixation holes 122 are oblong in shape so as to have a different length than width (i.e., the hole is non-circular), while fixation hoJes 124 are circular in shape. Although plate 120 is provided with two pairs of fixation holes 122,124, more than two pairs may instead be provided, for example so that plate 120 may span a greater length and thus be fastened to multiple locations along the spine. Plate 120 is not provided with a slot for receiving a drill/screw guide or for graft visualization. However, in alternate embodiments, one or more slots may be provided, and the slot or slots may be disposed transverse to central longitudinal axis 128a. Each of fixation holes 122, 124 extends between top and bottom surfaces 128,130 and includes an undercut 132. In one embodiment, undercut 132 is disposed closer to top surface 128 than bottom surface 130 while in another embodiment, undercut 132 is disposed closer to bottom surface 130 than top surface 128. Undercut 132 also may be disposed intermediate top and bottom surfaces 128,130, or transverse to the surfaces. Preferably, an undercut 132 extends completely around each of fixation holes 122,124. A lower portion 134 of each fixation hole 122,124 may be disposed between bottom surface 128 and undercut 132. Lower portion 134 optionally may taper toward a central axis 136 of the respective fixation hole as shown for example in FIG. 5D. Preferably, lower portion 134 has a first maximum inner dimension L3 and the undercut has a second maximum inner dimension L4, wherein the first maximum inner dimension L3 is less than the second maximum inner dimension L4. Plate 120 may be provided with two types of captive clips. First, a circular captive clip 150a is shown in FIGS. 6A and 6B with a slit 152a that permits elastic expansion/compression of clip 150a as previously explained, as well as an outer edge 154a and an inner edge I56a. Clip 150a includes a generally rectangular cross-section, and in one embodiment, is provided with a radius R3 on outer edge 154a of about 0.1 mm and a second radius R4 on inner edge 156a of about 0.2 mrn. In a relaxed state, ends 158a, I59a forming slit 152a of clip I50a preferably are angled at between about 5° and about 15° with respect to each other, and more preferably at about 10°, Clip 150a is sized to be received in fixation hole 124. Second, an oblong captive clip 150b is shown in FIGS. 7 A and 7B with a slit 152b that permits elastic expansion/compression of clip 150b as previously explained, as well as an outer edge 154b and an inner edge 156b. Clip 150b includes a generally rectangular cross-section, and in one embodiment, is provided with a radius R5 at outer edge 154b of about 0.1 mm and a second radius R6 at inner edge 156b of about 0,2 mm. Clip 150b is sized to be received in a fixation hole 122. In a relaxed state, ends 158b, 159b forming slit 152b of clip 150b preferably are angled at between about 5° and about 15° with respect to each other, and more preferably at about 10°. In an exemplary preferred embodiment, a captive clip 150a or 150b is provided for each fastener 160 around head 162, based on whether the fastener 160 is disposed in an oblong fixation hole 122 or a circular fixation hole 124. Preferably, a clip 150 is pre-installed in each fixation hole 122,124, and snap-fits around at least part of a head 162 of a fastener 160 during installation of the fastener such'lhat post-operative back-out of a fastener 160 from a hole 122, 124 is resisted. Preferably, when a captive clip 150b is disposed in undercut 132 of elongated, oblong hole 122, captive clip iSOb serves as a rail upon which a fastener 160 is permitted to travel (slide) along a longitudinal axis 122b of the hole. Thus, a fastener 160 on a captive clip 150b and disposed in an oblong hole 122 can slide across the length of the hole. However, neither captive clip 150a disposed in an undercut 132 of a circular hole 124, nor a fastener captured by the captive clip 150a, is permitted to travel (slide) along a longitudinal axis 124aof the hole. Turning to FIGS. 8A-8G, an exemplar fastener 160 according to the present invention includes a head 162 with a self-drilling, threaded shaft 164. In an alternate embodiment, a self-tapping, threaded shaft 164 instead may be provided. Head 152 includes a perimetral groove 166 extending around at least a portion thereof, and an instrument receiving portion 168. Although instrument receiving portion 168 is not shown to at least partially intersect groove 166 at one or more openings, provision for such may be made as described previously. In the exemplar embodiment of FIG. 8E, instrument receiving portion 168 is hexagonal-shaped. However, in alternate embodiments, other shapes may be provided. Fastener 160 preferably is used with a plate 120 such that fastener 160 may toggle in the fixation holes. In addition, fastener 160 preferably is used for fixation to cancellous bone. In one preferred exemplary embodiment, perimetral groove 166 includes an upper surface 166a and a lower surface 166b that are disposed at between about 5° and about 50° with respect to each other. Preferably, upper surface 166a is disposed at an angle ?q of between about 5° and about 15° with respect to a line 173 disposed approximately intermediate upper inside edge 173 and lower inside edge 173b and perpendicular to longitudinal axis 172. Most preferably, angle  1is about 10°. In addition, lower surface 166b preferably is disposed at an angle  2of between about 15° and about 35° with respect to line 173. Most preferably, angle  2 is about 25°. Thus, most preferably, angles \\,  2 total about 35°. The angulation of surfaces I66a, 166b with respect to each other permits toggling of fastener 160 when coupled to a captive clip 15Oa, 150b. Head 162 preferably is partially spherical and includes a bottom section 168 extending to the top 170 of shaft 164, with bottom section 168 tapering inward toward longitudinal axis 172 from perimetral groove 166 to top 170, Advantageously, such tapering permits angulation of fastener 160 when disposed in a fixation hole 122, 124. In use, a fastener 160 is received in a captive clip 150a or 150b. The shaft 164 is initially screwed into bone until the partial-spherical head 162 of fastener 160 reaches captive clip 150a or 150b. Upon further insertion of fastener 160 into captive clip 150, the partial-spherical head 162, particularly bottom section 168, bears against the inside edge 156a, 156b of captive clip 150a, 150b, respectively, and expands the clip. Once fastener 160 is inserted far enough, captive clip 150a or 150b contracts so that it "snaps" into perimetral groove 166 in head 162, thereby preventing fastener 160 from backing out of plate 120, as previously described. A third embodiment of a fixation system is shown in FIGS. 9A-12. The fixation system includes a plate 220 with two pairs of fixation holes 222, 224. Fixation holes 222 are oblong in shape, while fixation holes 224 are circular in shape. Although plate 220 is provided with two pairs of fixation holes 222, 224, more than two pairs may instead be provided, for example so that plate 220 may span a greater length and thus be fastened to multiple locations along the spine. A slot 226 is aligned along central longitudinal axis 228a for receiving a drill/screw guide and for graft visualization. Preferably, slot 226 does not receive any fasteners. In alternate embodiments, more than one slot may be provided, and the slot or slots may be disposed transverse to central longitudinal axis 228a. Preferably,, slot 226 includes straight portions 226a and semicircular portions 226b. Each of fixation holes 222, 224 extends between top and bottom, surfaces 228, 230, As shown in particular in FIGS. 9C and 9D, each pair of the fixation holes is partially intersected by a common slot 232. . . . Turning to FIGS. 10A and 10B, an unslitted captive clip 250 is shown. Preferably, captive clip 250 is elastically flexible to permit expansion and.contractiqn, although such flexibility is not necessary. Clip 250 has an outer edge 254 and an inner edge 256. Clip 250 also has a generally rectangular cross-section, and in one . embodiment, has flat upper and lower surfaces 25Sa, 258b, as well as rounded sides 259a, 259b. . : In another embodiment of a plate 220b, shown in FIG, 11, two pairs of circular fixation holes 224 are provided. As explained with respect to plate 220, each pair of fixation holes 224 is partially intersected by a common slot 232. A captive clip 250 is inserted into each slot 232, and is sized such that it protrudes into each fixation hole 224. Preferably, a clip 250 is pre-instalJed in each slot 232. Once a fastener, such'as one of the fasteners previously described with a perimetral groove in its head, is inserted far enough in the fixation hole, captive clip 250 bears against the perimetral groove, thereby locking the fastener in place and preventing the fastener from backing out of plate 220b. In order to permit removal of a fastener, a captive clip 250 may be removed from either the cephalad or caudal side of plate 220b where the fastener is located, thereby unlocking that fastener from plate 220b. Plate 220b is similar to plate 220 as shown in FIGS. 9A-9G, with a clip 250 assembled therewith. An alternate embodiment of a clip 270 is shown in FIGS. 12-13. Clip 270 is generally shaped like a paper clip, and thus may confer spring-like behavior. As with the embodiment of plate 220b, clip 270 may be inserted in, and/or removed from, a slot 232 from either the cephalad or caudal side of plate 220b. Preferably, a first end 272 of clip 270 is arcuate, while a second end 274 is straight. The corners 276a, 276b of end 274 may be rounded, beveled, or provide a sharp transition from side edges 278a, 278b such as a right-angle transition between side edges 278a, 27Sb and end 274. Second end 274 is straight for stability, and preferably is disposed toward the outer edge of the plate. In some embodiments, one or more edge surfaces may be straight to allow for a flush resting surface with a greater surface area for abutting a surface of a plate in which clip 270 is installed. Two free ends 279a, 279b may be provided. Additional alternate embodiments of captive clips are shown in FIGS. 14 and 15. A clip 280, shown in FIG. 14, includes a slit 282 that permits elastic expansion/compression of clip 280 as previously explained. Clip 280 has an outer edge 284 and an inner edge 286. Clip 290, as shown in FIG. 15, includes a slit 292 that permits elastic expansion/compression, an outer edge 294, an inner edge 296, and further includes opposing recesses 298 the potential use of which will be explained below. Captive clip 290 further includes a pair of generally parallel sides 299a, 299b. Turning to FIGS. 16A-16D, a fourth embodiment of a fixation system is shown. The fixation system includes a plate 320 with two pairs of circular fixation holes 222 (only one pair is shown in FIG. 16A). A slot 326 is aligned along central longitudinal axis 328a for receiving a drill/screw guide and for graft visualization. Preferably, slot 326 does not receive any fasteners. Each of fixation holes 322 extends between top and bottom surfaces 328, 330. Each pair of fixation holes 322 is partially intersected by a common slot 332. As shown in each of FIGS. 16A-16D, a captive clip 290 is inserted in slot 332 from a side, preferably cephalad or caudal, of plate 320. In an alternate embodiment, slot 332 is disposed transverse to central longitudinal axis 328a and such that it opens on a side of plate 320, preferably the cephalad and/or caudal sides. In addition, slot 332 alternatively may open on a side of plate 320 that extends between the cephalad and caudal sides of plate 320. A cam element 334 is inserted from the bottom 330 of plate 320, and is received in recesses 298 of captive clip 290 to govern expansion and contraction thereof. Cam element 334 is turned using a locking screw 336 fastened thereto. In its unlocked position, shown in FIGS. 16A and 16C, captive clip 290 can freely expand and contract, and thus a fastener is not captured as securely by the clip. In its locked position, shown in FIGS. 16B and 16D, cam element 334 engages recesses 298, and thus compression of captive clip 290 is prevented by cam element 334 thereby permitting captive clip 290 to securely capture one or more fasteners inserted in fixation holes 322. In an exemplary preferred embodiment, cam element 334 has four rounded corners to facilitate engagement with recesses 298. Turning to FIGS. I7A-20B, a fifth embodiment of a fixation system is shown. The fixation system includes a plate 420 with two pairs of fixation holes 422, 424. Fixation holes 422,424 are circular in shape. Although plate 420 is provided with two pairs of fixation holes 422, 424, more than two pairs may instead be provided, for example so that plate 420 may span a greater length and thus be fastened to multiple locations along the spine. A "dogbone" shaped slot 426 is aligned along central longitudinal axis 427 for receiving a drill/screw guide and for graft visualization. Preferably, slot 426 does not receive any fasteners. Each of fixation holes 422,424 extends between top and bottom surfaces 428, 430 and includes an undercut 432. In one embodiment, undercut 432 is disposed' transverse to top and bottom surfaces 428,430, respectively, along line 432a, but other orientations are also possible as previously described with respect to other embodiments. Preferably, an undercut 432 extends completely around each of fixation holes 422,424. At least one passage 429 extends transverse to each of the fixation holes 422,424, . preferably from bottom surface 428, and intersects an undercut 432 associated with the hole 422 or 424. Passages 429 are used for alignment of captive clips, as will be described shortly. . Turning to FIGS. 18A and 18B, a generally U-shaped captive clip 450 is shown with a slit 452 that permits elastic expansion/compression of clip 450, as well as t an outer edge 454, an inner edge 456, and generally parallel sides 459a, 459b. Clip 450. includes a generally rectangular cross-section, and in one embodiment, is provided with a radius R7 at edges 454 and 456 of between about 0.1 mm and about 0.2 mm. An end tab 458 extends from side 459a, preferably transverse to the plane of the page. In some embodiments, end tab 458 extends generally parallel to the plane of the page. When a captive clip 450 is installed in a fixation hole 422, 424, end tab 458 extends into a passage 429 so that captive clip 450 may be aligned and properly placed in undercut 432 in the fixation hole. In an exemplary preferred embodiment, captive clips 450 are pre-installed in fixation holes 422,424 in plate 420 prior to installation of fasteners therein. A captive clip 450 is provided for each fastener 460 around head 462. Preferably, clip 450 snap-Fits around head 462 of a fastener 460 such that post-operative back-out of a fastener 460 from a hole 422, 424 is resisted. A fastener 460 retained on a captive clip 450 disposed in an undercut 432 of a circular hole 422, 424 may be permitted to toggle along a central axis of the hole. As shown in FIGS. 19A-19G, an exemplar fastener 460 includes a head 462 with a self-tapping, threaded shaft 464. In an alternate embodiment, a self-drilling, threaded shaft 464 may be provided. Head 462 includes a perimetral groove 466 thai is interrupted by four corners 470 separating back wall 471. In the exemplar embodiment of FIG. 19E, an instalment receiving portion 468 is hexagonal-shaped. However, in alternate embodiments, other shapes may be provided. Fastener 460 preferably is used for fixation to cancellous bone, although threading appropriate for fixation to conical bone instead may be provided. In one preferred exemplary embodiment, perimelral groove 466 includes an upper portion 466a and a lower portion 466b that are disposed generally parallel to each other. Head 462 preferably is partially spherical and includes a bottom section 475 extending to the top end 473 of shaft 464, with bottom section 475 tapering inward toward longitudinal axis 472 from perimetral groove 466 to top end 473. Advantageously, such tapering permits angulation of fastener 460 when disposed in a fixation hole 422,424. In use, fastener 460 is received in captive clip 450. The shaft 464 is initially screwed into bone until the partial-spherical head 462 of fastener 460 reaches captive clip 450. Upon further insertion of fastener 460 into captive clip 450, the partial-spherical head 462, particularly bottom section 475, bears against the inside edge 456 of captive clip 450 and expands captive clip 450. Once fastener 460 is inserted far enough, captive clip 450 contracts so that it "snaps" into perimetral groove 466 in head 462, thereby preventing fastener 460 from backing out of plate 420, as previously described, As evident from FIGS. 20A and 20B, in some orientations, the four corners 470 of fastener 460 may be retained on captive clip 450 while the clip is in an unexpanded stele. In other orientations, two of the corners act as cams on the inner wall 456 of (he clip to elastically expand it such that the fastener is no longer locked by captive clip 450. Thus, it is possible to remove a fastener by turning the fastener to engage the cams with the clip to expand the clip. A fastener 460, retained on captive clip 450, can toggle on captive clip 450 in a fixation hole 422, 424, because of the geometry of the perimetral groove 466 in head 462 of fastener 460 and the geometry of the fixation hole 422, 424. During toggling, captive clip 450 remains essentially fixed in place, while the axis of fastener 460 is allowed to angulate with respect to the central axis of the fixation hole 422, 424, In embodiments in which surfaces 466a, 466b of groove 466 are generally parallel to each other, toggling may be substantially limited. A sixth embodiment of a fixation system.is shown in FIGS. 21A-22. The fixation system includes a plate 520 with two pairs of fixation holes 522, 524. Fixation holes 522, 524 are circular in shape. Although plate 520 is provided with two pairs of fixation holes 522, 524, more than two pairs may instead be provided, for example so that plate 520 may span a greater length and thus be fastened to multiple locations along the spine. Although no slot is provided along central longitudinal axis 528a for receivings drill/screw guide and for graft visualization, one may be provided as described previously with respect to other embodiments, Each of fixation holes 522, 524 extends between top and bottom surfaces 528, 530, respectively, and includes an undercut 532. In one embodiment, undercut 532 is disposed transverse to top and bottom surfaces 528, 530, respectively, but other orientations are also possible as previously described with respect to other embodiments. Preferably, an undercut 532 extends completely around each of fixation holes 522, 524. The geometry of one pair of fixation holes 522, preferably the caudal pair of holes, is sized such that movement of a fastener 560 therein is very limited. The geometry of the remaining pair of fixation holes 524 allows a fastener 560 to toggle between about 5° and about 30°, and more preferably between about 15° and about 20°, as shown in FIG. 22. In particular, a fastener 560, shown in FIG. 22, is retained on a captive clip 550. The fastener 560 may toggle on captive clip 550 in fixation hole 524 because of the geometry of perimetral groove 566 in the head 562 of the fastener and the geometry of fixation hole 524. In particular, lower portion 534a widens proximate bottom surface 530 of plate 520. The captive clip 550 preferably remains fixed in place, while the axis of fastener 560 is allowed to angulate with respect to the axis of fixation hole 524. With reference to FIGS. 23A-29, a seventh embodiment of a fixation system is shown. Plates 620,720, 820, 920 represent "one level," "two level," "three level," and "four level" constructions, respectively. Each "level" is provided by one or more fixation holes, and preferably pairs of adjacent fixation holes, bridging two vertebrae. Thus, a first level plate bridges two vertebrae and includes at least one fixation hole, preferably two adjacent fixation holes for each vertebrae, while a second level plate bridges three vertebrae and includes at least one fixation hole, preferably two adjacent fixation holes for each vertebrae. For example, plate 620 has two pairs of fixation holes 622,624, and thus only represents one level; in contrast, plate 720 has three pairs of fixation holes, and thus the middle pair of fixation holes is adjacent two other pairs thus forming a two level construction. As can be seen from FIGS. 23A-29, the construction of plates 620, 720, 820, 920 is very similar, and thus although plate 620 will be described in detail herein, the description of plate 620 applies also to plates 720, S20, 920. Referring to FIG. 23A and 27A-27E, the fixation system includes a plate 620 with two pairs of fixation holes 622, 624. Fixation holes 622, 624 are circular in shape. A "figure eight" shaped slot 626 is aligned along central longitudinal axis 628a for receiving a drill/screw guide and for graft visualization. Preferably, slot 626 does not receive any fasteners, and is beveled along upper inner edge 626a at an angle a between about 30° and about 60°, and more preferable about 45°. It should be noted that as the length of plates 620, 720, 820, 920 increases along the central longitudinal axis, slot 626 becomes elongated and generally "dog-bone" shaped as shown for example in FIGS. 25A and 26A. Each of fixation holes 622,624 extends between top and bottom surfaces 628, 630 and includes an undercut 632. In one embodiment, undercut 632 is disposed transverse to top and bottom surfaces 628, 630, respectively, but other orientations are also possible as previously described with respect to other embodiments. Preferably, an undercut 632 extends completely around each of fixation holes 622, 624. At least one passage 629 extends transverse to each of the fixation holes 622, 624, preferably from bottom surface 628, and intersects undercut 632 associated with the hole 622 or 624. In some embodiments, passage 629 may extend generally parallel lo each of the fixation holes 622, 624, preferably from bottom surface 628, and intersects undercut 632 associated with the hole 622 or 624. Passages 629 are used for alignment of captive clips, as will be described shortly. In the preferred exemplary embodiment, holes 629 are disposed along lines generally parallel to central longitudinal axis 628a, for example along line 628b. As shown in FIGS. 28A and 28B, a generally "wishbone-shaped" captive clip 650 includes a slit 652 that permits clastic expansion/compression of clip 650, as well as an outer edge 654, an inner edge 656, and generally parallel sides 659a, 659b. Clip 650 also includes a generally rectangular cross-section, as shown in FIG. 28B. In a preferred exemplary embodiment, an end tab 658 extends between, and preferably halfway between, sides 659a, 659b and transverse to the plane of the page. In some embodiments, end tab 658 extends generally parallel to the plane of the page. When a captive clip 650 is installed in a fixation hole 622, 624, end tab 658 extends into passage 629 so that captive clip 650 may be aligned and properly placed in the fixation hole. Preferably, captive clips 650 are pre-installed in fixation holes 622, 624 in plate 620 prior to installation of fasteners therein. Referring to FIG. 29, a captive clip 650 is provided for fastener 660 around head 662. Preferably, clip 650 snap-fits around head 662 of fastener 660 such that post-operative back-out of fastener 660 from hole 622, 624 is resisted. Preferably, captive clip 650 is disposed in an undercut 632 of a fixation hole 622, 624 and is permitted to toggle along a central axis of the hole, as shown for example in FIG, 29 as indicated for example by arrows A. Another preferred embodiment of a "wishbone-shaped" captive clip 680 is shown in FIGS. 30A to 30C. Similar to the captive clip 650 shown in FIGS. 28A and 28B, captive clip 680 includes a slit 682 that permits elastic expansion/compression of . clip 680, as well as an outer edge 684, an inner edge 686, and generally .parallel sides 689a, 689b. Clip 680 also includes a generally rectangular cross-section, as shown in FIG. SOB. In a preferred exemplary embodiment, an end tab 688 extends between, and preferably halfway between, sides 689a, 689b, along clip central longitudinal axis 688a and transverse to the plane of the page. In some embodiments, end tab 688 extends generally parallel to the plane of the page. When captive clip 680 is installed in fixation hole 622, 624, end tab 688 extends into a passage 629 so that captive clip 680 may be aligned and properly placed in the fixation hole. Preferably, captive clips 680 are pr&-installed in fixation holes 622, 624 in plate 620 prior to installation of fasteners therein. Preferably, the captive clips 680 are sized and configured so that once installed in the bone plate they are fixed in position. In the embodiment of the clip shown in FIG. 30A, the comers of the clip are wedged into the undercut so that movement of the clip .in the undercut is prevented. In addition, once the fastener is inserted in the captive clip, the captive clip is even more securely wedged into position to prevent movement in the undercut. A preferred exemplary embodiment of captive clip 680 includes the following dimensions: L5 of about 5.5 mm, L6 of about 4.4 mm, L7 of about 2.5 mm, L8 of about 7-2 mm, L9 of about 3 mm, L10 of about 3.8 mm, and HI of about 2.85 mm. Captive clip 680 also has a thickness L12 of between about 0.3 mm and about 0.4 mm, more preferably aboul 0.37 mm. Preferred radii of curvature include: R8 of about 0.25 mm, R9of about 2.8 mm, R10 of about 1 mm, Rll of about 0.3 mm, R12 of about 0.4 mm, and R13 of about 6.8 mm. A preferred angle P between outer edges of straight portions 6S9a, 689b and adjacent outer edges of straight portions 690a, 690b, respectively, is about 150°. Preferably, captive clip 680 is symmetric about central longitudinal axis 688a. Advantageously, as shown for example in FIG, 30C, captive clip 680 is sized such that a fastener has a greater freedom to toggle in some directions as compared to other directions. For example, axis 688a is oriented parallel to the cephalad-caudal direction of a plate, while central axis 692 may be oriented perpendicular to axis 688 and parallel to the medial-lateral direction of the plate. A cross-section of an exemplary fastener head 694 is also shown, with the cross-section taken through the center of a perirnetral groove in head 694 as described in the embodiments of fasteners disclosed herein. An outer edge 696 of the perirnetral groove in head 694 is relatively close to edge 686 of captive clip 680 proximate central axis 692, while outer edge 696 is spaced a greater amount from edge 686 of captive clip 680 proximate axis 688a. Thus, toggling of the fastener is more limited in the direction of axis 692 because fastener head 694 may only be angulated until it contacts edge 686 which serves as a stop. In one preferred exemplary embodiment, a fastener inserted in captive clip 680 is permitted to toggle through between about 0° and about 32° along the plane extending perpendicular to the page through line 688a, while only being permitted to toggle through between about 0° and about 20° along the plane extending perpendicular to the page through line 692. Preferably, a fastener is permitted a greater freedom to toggle in the cephalad-caudal direction of a plate. The preferred exemplary embodiment of captive clip 680, as well as each of the other captive clips described herein, preferably is formed of Elgiloy® (Cobalt-Chromium-Nickel alloy), ASTM F-1058 Grade I, burr free and electropolished. Preferably, each of the captive clips of the present invention may have high strength, ductility and good mechanical properties including an ultimate tensile strength between . about 250,000 psi and about 350,000 psi (about 1,700 MPa and 2,000 MPa, respectively), a hardness (HRC) of between about 45 and about 60, an elastic modulus of up to about 30,000,000 psi (about 270 GPa), excellent fatigue life, and corrosion resistance. Alternatively, each of the captive clips described herein may be formed of another elastic material such as Nitinol superelastic alloy memory material per ASTM-2063. Referring to FIGS. 31A to 3ID, an eighth embodiment of a fixation system is shown. A plate 1020 is provided with one or more pair(s) of fixation holes 1022. that are generally "keyhole-shaped" and oblong, and one or more pair(s) of fixation holes . 1024 that are generally circular. Each of the circular and keyhole-shaped holes includes an undercut 1026 that receives an "omega-shaped" captive clip 1028. Captive clip 1028 includes a pair of generally parallel sides 1030a, 1030b and two end tabs 1032a, 1032b protruding from each side 1030a, 1030b, respectively. The geometry of fixation holes 1022,1024 and their associated undercuts 1026 is sized such that movement of a captive clip 1028 once seated in an, undercut 1026 is prevented, except that the undercuts 1026 accommodate elastic expansion of captive clip 1028 as previously explained. One tab 1032a, 1032b of captive clip 1028 is received in a hole 1034 diat extends from undercut 1026 to a side of plate 1020, while the other tab 1032a, 1032b is received in a hole 1036 that extends from undercut 1026 toward the midlinc 1036 of plate 1020. Tabs 1032a, 1032b are used to align the "omega-shaped" captive clip 1028 in plate 1020. Preferably, captive clip 1028 is symmetrical about clip central axis 1029. Captive clip 1028 also preferably is installed in plate 1020 such that central axis 1029 is parallel midline 1036 of plate 1020 with intermediate portion 1031 of clip 1028 oriented so as to not interfere with movement of a fastener 1040 along the length of fixation hole 1022. The "keyhole" shaped fixation holes 1022 include a rounded portion 1038 in which a head 1040a of a fastener 1040 is received, and a straight portion 1042 with an integrally formed ridge 1044 that may extend about the length of straight portion 1042 and under which the head 1040a is permitted to move. Captive clips 1028 are disposed in each rounded portion. In use, the shaft 1040b of fastener 1040 is initially screwed into bone through the rounded portion 1038 of fixation hole 1022 until the partial-spherical head 1040a of fastener 1040 reaches captive clip 1028. Upon further insertion of fastener 1040 into captive clip 1028, the partial-spherical head 1040a bears against the inside edge 1046 of captive clip 102S and expands the captive clip, Once fastener 1040 is inserted far enough, captive clip 1028 contracts so that it "snaps" into the perimetral groove 1050 in the head 1040a, thereby preventing fastener 1040 from backing out of plate 1020, as previously described. Fastener 1040 is then allowed to travel along the length of fixation hole 1022, for example under integrally formed ridge 1044 of the plate which provides additional resistance to back-out of fastener 1040. However, in order to allow shaft 1040b to be removed from the bone without interference from ridge 1044 on the straight portion 1042 of the "keyhole-shaped" fixation holes 1022, fastener 1040 must be moved so that head 1040a is disposed proximate the "omega-shaped" captive clip 1028. As described previously, captive clip 1028 may then be elastically expanded to permit removal of fastener 1040. With respect to the pair of circular holes 1024, again, the screw may be removed by expansion of the captive clip 1028. Thus, a fastener 1040 disposed in a "keyhole-shaped" fixation hole 1022 is permitted to travel (slide) along the longitudinal axis of the hole 1022. The fastener 1040 can slide along the parallel sides 1030a, I030b of the "omega-shaped" captive clip 1028 or under the integrally formed ridge 1044. A fastener disposed in a circular hole may be permitted to toggle but remains relatively stationary also as previously described. It should be noted that plate 1020 shown in FIG. 31A is a four level construct, and includes five pairs of holes. Plate 1020 thus includes three additional pairs of "keyhole-shaped" fixation holes 1060, 1062,1064. In alternate embodiments of plate 1020, more than one pair of circular fixation holes may be provided, or as few as one "keyhole-shaped" fixation hole may be provided. While no slots are provided along midline 1036 of plate 1020, slots may be included as described with respect to other plate embodiments. Another embodiment of a fixation system similar to that shown in FIGS. 31A to 31D is shown in FIGS. 31E to 31P. In the "one-level" embodiment shown, a plate 1066 is provided with a pair of fixation holes 1068 that are generally "keyhole-shaped" and oblong, and a pair of fixation holes 1070 that are generally circular. An embodiment of a plate 1066b with four pairs of fixation holes 1068 and one pair of fixation holes 1070 also is shown in FIG. 3IN. Thus, as described herein with respect to other embodiments, each plate may be provided with one or more pairs of oblong fixation holes and one or more pairs of circular fixation holes. Each of the circular and keyhole-shaped holes includes an undercut 1072 that receives an "omega-shaped" captive clip such as captive clip 1074, shown in FIGS. 310 to P. Captive clip 1074 includes a pair of generally parallel sides 1076a, 1076b and two end tabs 1078a, 1078b protruding from each side 1076a, 1076b, respectively. Also, captive clip 1074 includes extensions 1093a, 1093b for additional stability. As can be seen from FIG. 310, the periphery of captive clip 1074 lies entirely within circle 1094 except portions of end tabs 1078a, 1078b, and thus captive clip 1074 is interchangeably useable with keyhole-shaped and circular holes 1068,1070, respectively. Advantageously, captive clip 1074 may be used both with plates in which fasteners are not permitted to travel along the length of any of the holes, as well as: with plates in which fasteners are allowed to move across the fixation holes. . In addition, as shown for example in FIGS. 31E and 31L, at least one passage 1085 optionally may be provided, and are used for alignment of captive clips'as previously described. Thus, for example, wishbone-shaped captive clip 650 optionally may be used in at least one fixation hole 1070 in plate 1066, while "omega-shaped" captive clip 1074 optionally at the same time or instead may be used in at least one fixation hole 1068,1070 in plate 1066. Plate 1066 therefore accommodates use of more than one design of captive clip. "" . A preferred exemplary embodiment of captive clip 1074 includes the following dimensions: Ml of about 7.5 mm, M2 of about 5,96 mm, M3 of about 4.4 mm, M4 of about 5.6 mm, M5 of about 0.5 mm, M6 of about 1.4 mm, and M7 of about 5.2 mm. Captive clip 1074 also has a thickness M8 of between about 0.3 mm and about 0.4 mm, more preferably about 0.35 mm. Preferred radii of curvature include: Rla of about 1 mm, R2a of about 0.1 mm, R3a of about 0.15 mm, R4a of about 0.5mm, R5a of about 2.7 mm, and R6a of about 3.3 mm. The geometry of fixation holes 1068,1070 and their associated undercuts 1072 is sized such that movement of a captive clip 1074 once seated in an undercut 1072 is prevented, except that the undercuts 1072 accommodate elastic expansion of captive clip 1074 as previously explained. One tab 1078a, 1078b of captive clip 1074 is received in a hole 1080 that extends from undercut 1072 to a side of plate 1066, while the other tab 1078a, 1078b is received in a hole 1082 that extends from undercut 1072 toward the midline 1084 of plate 1066. As shown for example in FIGS. 31G and 31H, holes 1082 from adjacent undercuts of adjacent fixation holes preferably communicate with each other and may be circular in cross-section. Tabs 1078a, 1078b are used to align the "omega-shaped" captive clip 1074 in plate 1066. Preferably, captive clip 1074 is symmetrical about clip central axis 1086. Captive clip 1074 also preferably is installed in plate 1066 such that central axis 1086 is parallel to midline 1084 of plate 1066 with intermediate portion 1087 of clip 1074 oriented so as to not interfere with movement of a fastener along the length of fixation hole 1068. The "keyhole" shaped fixation holes 1068 include a rounded portion 1088 in which a head of a fastener is received such as head 1040a of a fastener 1040, Fixation holes 1068 also include a straight portion 1089 with an integrally formed ridge 1090 that may extend about the length of straight portion 1089 and under which the head 1040a is permitted to move. Captive clips 1074 are disposed in each rounded portion. Plate 1066 further may be provided with a slot 1091 for visualization and instrument-receiving holes 1092 for receiving a suitably configured drill guide. Slots 1091 may be any suitable shape including other shapes disclosed herein such as dog-bone shapes. In addition, for multi-level plates, the length and/or spacing of the slots may vary as shown in FIG. 3 IN. The general operation of captive clip 1074 and plate 1066 is as described above with respect to FIGS. 31A to 31D. Turning to FIGS. 32A to 32F, an exemplar preferred self-tapping fastener 1160 is shown preferably for use in variable angle applications. Fastener 1160 includes a head 1162 with a self-tapping, threaded shaft 1164 ending in a blunt tip 1164a. Head 1162 includes a perimetral groove 1166 extending around at least a portion thereof, and an instrument receiving portion 1168 that preferably at least partially intersects groove 1166 at one or more openings 1170. In the exemplar embodiment of FIG. 32D, instrument receiving portion 1168 is cross-shaped and thus intersects groove 1166 at four openings 1170. As shown in the figures, exemplar preferred dimensioning for self-tapping fasteners is listed in Tables I-HJ below. Because fasteners for use with cancellous bone differ from fasteners for use with cortical bone particularly with respect to the threads on shaft 1164, FIGS. 32E and 32F are provided for cancellous bone and cortical bone, respectively, along with appropriate dimensions in the tables below. (Table Removed) Table I Table II (Table Removed) Table III (Table Removed) Dimensions particularly important for self-tapping fasteners include L15, L16, L19, L20, R14, R15, and y7. Threading dimensions particularly important for fasteners used with cancellous bone include L33a, L34a, Rl9a, R20a, R21a, ysa, and -yea. Threading dimensions particularly important for fasteners used with cortical bone include L33b, L34b, R19b, R20b, R21b, y5b, and y6b. Turning next to FIGS. 33A to 33D, an exemplar preferred self-drilling fastener 1260 is shown preferably for use in variable angle applications. Fastener 1260 includes a head 1262 with a self-drilling, threaded shaft 1264 ending in a sharp tip 1264a. Head 1262 includes a perimetral groove 1266 extending around at least a portion thereof, and an instrument receiving portion 1268 that at least partially intersects groove 1266 at one or more openings 1270. In the exemplar embodiment of FIG. 33D, instrument receiving portion 1268 is cross-shaped and thus intersects groove 1266 at four openings 1270. As shown in the figures, exemplar preferred dimensioning for self-drilling fasteners is listed in Tables IV-YI below. As discussed previously, fasteners for use with cancellous bone differ from fasteners for use with cortical bone particularly with respect to the threads on shaft 1264. Thus, the threads and dimensions shown in FIGS. 32E and provided for cancellous bone and cortical bone, respectively, also may be used with fastener 1260, along with appropriate dimensions in the tables below. Table IV (Table Removed) Table V (Table Removed) Table VI (Table Removed) Dimensions particularly important for self-drilling fasteners include L37, L40, With respect to the threaded portion of shaft 1264, (he threads preferably are provided in accordar.ee with the dimensions described previously with respect to fastener 1160 as shown in FIGS. 32E and 32F. Turning to FIGS. 34A to 34E, another exemplar preferred self-tapping fastener 1360 is shown preferably for use in fixed-angle applications. Fastener 1360 includes a head 1362 with a self-tapping, threaded shaft 1364. Head 1362 includes a perimetra! groove 1366 extending around at least a portion thereof, and an instrument receiving portion 1368 that preferably at least partially intersects groove 1366 at one or more openings 1370, In the exemplar embodiment of FIG. 34E, instrument receiving portion 1368 is cross-shaped and thus intersects groove 1366 at four openings 1370. Preferably, fastener 1360 is provided for cancellous bone. As shown in FIGS. 34A to 34E, in one exemplary preferred embodiment, fastener 1360 is provided with dimensioning as listed in Table VII below: Table VII (Table Removed) Fastener 1360 is disposed about central longitudinal axis 1372 which is oriented perpendicular to a plane that includes uppermost surface 1362a. Head 1362 preferably is provided with a first surface 1362b oriented at an angle 04 with respect to uppermost surface 1362a, as well as a second surface 1362c oriented at an angle O5 with respect to a plane 1374 disposed midway across groove 1366 and perpendicular to axis 1372. Groove 1366 is bounded at its upper and lower ends by second and third surfaces 1362c and 1362d. respectively. Preferably, third surface 1362d is oriented at an angle 05 with respect to a plane 1374 disposed midway across groove 1366 and perpendicular to axis 1372. A transition surface 1362s also may be disposed at an angle o7 with respect to axis 1372. Also, a generally flat surface 1362e connecting surfaces 1362b and 1362c preferably is oriented at an angle 0-3 with respect to axis 1372. Dimensioning for a preferred exemplary embodiment of FIGS. 34A to 34E is listed in Table VIII below: Table VIII (Table Removed) With respect to the threaded portion of shaft 1364, the threads preferably are provided in accordance with the dimensions described previously with respect to fastener 1160 as shown in FIGS. 32E and 32F. Dimensions particularly important for "fixed-angle" fasteners are shown in FIG. 34C. As shown in FIGS, 35A to 35E, another exemplar preferred self-drilling fastener 1460 is shown preferably for use in fixed angle applications. Fastener 1460 includes a head 1462 with a self-drilling, threaded shaft 1464. Head 1462 includes a perimetral groove 1466 extending around at least a portion thereof, and an instrument receiving portion 1468 that preferably at least partially intersects groove 1466 at one or more openings 1470. In the exemplar embodiment of FIG. 35E, instrument receiving portion 1468 is cross-shaped and thus intersects groove 1466 at four openings 1470. Preferably, fastener 1460 is provided for cancellous bone. As shown in FIGS. 35A to 35E, in one exemplary preferred embodiment, a fixed angle, self-drilling fastener 1460 is provided with dimensioning as listed in Table IX below: Table IX (Table Removed) Fastener 1460 is disposed about central longitudinal axis 1472 which is oriented perpendicular to a plane that includes uppermost surface 1462a. Head 1462 preferably is provided with a first surface 1462b oriented at an angle O10b with respect to uppermost surface I462a, as well as a second surface 1462c oriented at an angle aioc with respect to a plane 1474 disposed midway across groove 1466 and perpendicular to axis 1472. Groove 1466 is bounded at its upper and lower ends by second and third surfaces 1462c and 1462d, respectively. Preferably, third surface 1462d is oriented at an angle Oiod with respect to a plane 1474 disposed midway across groove 1466 and perpendicular to axis 1472, Also, a generally flat transition surface 1462e connecting surfaces I462b and 1462c preferably is oriented at an angle O10a with respect to axis 1472. Dimensioning for a preferred exemplary embodiment of FIGS. 35A to 35E is listed in Table X below: Table X (Table Removed) With respect to the threaded portion of shaft 1464, the threads preferably are provided in accordance with the dimensions described previously with respect to fastener 1160 as shown in FIGS, 32E and 32F. Some surfaces of head 1462 of fastener 1460 may be provided with features such as roughening in the form of grooves, rounded bumps, ridges, steps, serrations, etc., to provide tactile and/or audio feedback when a captive clip interacts therewith. For example, in one preferred exemplary embodiment, surfaces 1462c, 1462d, and 1462f may be provided with such roughening. When comparing the fixed angle fastener 1360 of FIGS. 34A to 34E with the variable angle fastener 1160 of FIGS. 32A to 32F, preferably the following conditions are met: maximum shaft diameter L76 > maximum shaft diameter L23; maximum head diameter L78b > maximum head diameter L25; and groove vertical Width L72 Dimensions particularly important for "fixed angle" fasteners are shown in FIG.35C. In yet another exemplary embodiment of an exemplary fastener 1560, shown in FIGS, 36A to 36D, head 1562 is provided with scallops 1563 so that a captive clip 1580 in a fixation plate 1582 may be visualized by a surgeon, thereby allowing the surgeon to confirm that head 1562 has been captured by captive clip 1580 to resist backout from fastener hole 1584. In one embodiment, scallops 1563 are arcuate in shape. However, other shapes may be provided as long as visualization of captive clip 1580 is permitted when fastener 1560 is coupled thereto. Preferably, four equally radially.spaced scallops are provided proximate a top peripheral portion of head 1562. Further details concerning exemplary fastener 1560 are shown in FIGS.. 37A to 37F. Fastener 1560 includes a head 1562 with a self-tapping, threaded shaft 1564. Head 1562 includes a perimetral groove 1566 extending around af least a portion thereof, and an instrument receiving portion 1568 that preferably at least partially intersects groove 15(56 at one or more openings 1570. In the exemplar embodiment of FIG. 37E, instrument receiving portion 1568 is cross-shaped and thus intersects groove 1566 at four openings 1570. Fastener 1560 is disposed about central longitudinal axis 1572 which is oriented perpendicular to a plane that includes uppermost surface 1562a. Preferably, fastener 1560 is provided for cancellous bone. In an exemplary embodiment, the dimensioning listed in Table IX and Table X for fastener 1460, as well as the threads and dimensioning therefor as disclosed for example with respect to FIG. 35D, also are applicable to fastener 1560. As shown in FIG. 37C, however, fastener 1560 is further provided with a generally cylindrical portion 1562g disposed between surface 1562d with rounded transition 1562e and surface 1562f. Cylindrical portion 1562g may be provided with a diameter L96 of about 4.9 mm. In addition, the distance between uppermost surface 1562a and the plane defined at the intersection of rounded transition 1562e and smface 1562g may be provided with a length L97 of about 1.8 mm. Finally, surface 1562e may transition to surface 1562g at an angle GO of between about 0° and about 45°, and more preferably about 20°. Further detail of area S in FIG. 37C is shown in FIG. 37G. Radiuses R37 and R38 of about 0.1 mm may be provided. Thus, in operation, when a fastener abuts a clip while being installed in a plate, the clip crosses from the tapered surface 1562f to the flat cylindrical surface 1562g, and then catches on surface 1562e. Such a configuration permits tactile feedback while the fastener is interacting with the clip. As shown in FIGS. 37E and 37F, instrument receiving portion 1568 preferably has a maximum width L98 of about 5 mm and a width L99 of each section 1568a of about 1 mm. Preferably, the midway points of adjacent scallops 1563 are disposed about 90° from each other. Moreover, the midway point of each scallop 1563 is disposed at an angle O14of about 45° from a line extending through the midway points of opposing outermost portions 1570. Tangents from rounded corners 1563a, 1563b of each scallop 1563 preferably are disposed at about 50° with respect to each other, with corners 1563a, I563b being provided with a radius R35 of about 0.1 mm. Each scallop preferably lias a radius R36 of about 0.75 mm and extends inward toward central longitudinal axis 1472 so that the radial distance L99 between the innermost portion of scallop 1563 and the central longitudinal axis 1572 may be about 2.1 mm. Turning to FIGS. 37H to 371, a variation of the fastener head shown in FIG. 37E is shown. In particular, scallops 1563B are each provided with a central portion 1580 having a radius R39 that is bordered by adjacent portions 1582 having a radius R40. Also, each of the portions of cross-shaped instrument receiving portion 1568B may extend to the periphery of the screw head and thus open along the upper portion thereof as shown at openings 1584. In an exemplary preferred embodiment, each of the fasteners and fixation plates disclosed herein may be formed of a titanium alloy such as titanium-aluminum-niobium, preferably anodized. One preferred material for use with each of the plates and screws described herein is Ti-6Al-7Nb, with a density of about 4.52 gm/cc, a modulus of elasticity of about 105 GPa, an ultimate tensile strength of about 900 MPa, and a yield strength of about 800 MPa. Surfaces of the fasteners preferably also are burr free, with all sharp edges broken to a maximum of 0.1 mm. Each of the embodiments of plates, fasteners, and clips disclosed herein may be provided in sets or kits. For example, as shown in FIG. 23 A, each of the exemplary "one-level" plates according to the present invention may be provided with maximum lengths X1, lengths X2 between the centers of cephalad and caudal holes, and lengths X3between the center of the plate along longitudinal axis 628a and the center of a fastener hole, as follows: Table XI (Table Removed) Thus, kits of plates may be provided including two or more plates, such as plates selected from exemplary plates 1-7 as listed in Table XI. For example, plates may be provided with lengths X| that differ by about 2 mm each, lengths X2 that differ by about 2 mm each, and lengths X3 that differ by about 1 mm each. Similarly, as shown for example in FIG. 24A, each of the exemplary "two-level" plates according to the present invention may be provided with maximum lengths X1, lengths X2 between the centers of cephalad and caudal holes, and lengths Xs between the centers of adjacent holes, as follows: Table XH (Table Removed) Thus, kits of plates may be provided including two or more plates, such as plates selected from exemplary plates 8-14 as listed in Table XII. For example, plates may be provided with lengths X4 that differ by about 4 mm each, lengths X5 that differ by about 4 mm each, and lengths Xe that differ by about 2 mm each. In addition, as shown for example in FIG. 25 A, each of the exemplary "three-level" piates according to the present invention may be provided with maximum lengths X7, lengths X8 between the centers of cephalad and caudal holes, and lengths X9 between the centers of adjacent holes, as follows: Table XIII (Table Removed) Thus, kits of plates may be provided including two or more plates, such as plates selected from exemplary plates 15-23 as listed in Table XIH. For example, plates may be provided with lengths X^ that differ by about 3 mm each, lengths Xs that differ by about 3 mm each, and lengths X6 that differ by about 1 mm each. Also, as shown in FIG. 26A, each of the exemplary "four-level" plates according to the present invention may be provided with maximum lengths X10, lengths X11 between the centers of cephalad and caudal holes, and lengths X12 between the centers of holes of a two-level construct, as follows: Table XIV (Table Removed) Thus, kits of plates may be provided including two or more plates, such as plates selected from exemplary plates 24-34 as listed in Table XIV. For example, plates may be provided with lengths X1 that differ by about 4 mm each, lengths X2 that differ by about 4 mm each, and lengths X3 that differ by about 2 mm each. In addition, kits with one or more plates selected from the group of "one-level", "two-level", "three-level", and "four-level" plates may be provided. Thus, a kit . may optionally include one or more plates selected from exemplary plates 1-7, one or more plates selected from exemplary plates 8-14, one or more plates selected from exemplary plates 15-23, and one or more plates selected from exemplary plates 24-34, as listed in Tables XI to XIV. In another exemplary embodiment of the present invention, an exemplary plate 1600 is provided for use without captive clips. As shown in FIGS. 38A to 3SG, plate 1600 includes fastener holes 1602 provided therein with opposed gussets 1604, 1606. Gussets 1604, 1606 serve a similar function as the clips disclosed with respect to other fixation systems described above, in that when a fastener is disposed in a hole 1602, the gussets are accommodated in groove 1610 in the head of the fastener. Thus, unlike the clips disclosed herein which may be plastically deformed to accommodate passage of the fastener and lock the fastener in place, gussets 1604, 1606 are immobile and thus do not deform. As shown in FIG. 38E, fastener 1608 may be provided with slits 1612 extending from an upper portion of head 1614 toward shaft 1616. Slits 1612 permit the head of fastener 1608 to expand and contract in a spring-like fashion, thus accommodating the decreased width of hole 1602 at gussets 1604,1606 when fastener 1608 is installed or removed from hole 1602. In particular, fastener 1608 may be captured in hole 1602 as permitted when the head of fastener 1608 expands such that gussets 1604,1606 are received in groove 1610. Optionally, head 1614 may be internally threaded. Advantageously, the need for a clip is eliminated in plate designs using gussets, allowing a one-piece plate construct requiring less manufacturing parts and easing assembly. Shown in FIG. 38G, the fastener 1608 is in a dynamic position; the fastener head 1614 is shown in the contracted state that occurs during insertion or removal of fastener 1608 from plate 1600. Advantageously, gussets 1604, 1606 provide the same function as described previously with respect to captive clip 680 shown in FIG. 30C. While various descriptions of the present invention are described above, it should be understood that the various features can be used singly or in any combination thereof. Therefore, this invention is not to be limited to only the specifically preferred embodiments depicted herein. Further, it should be understood that variations and modifications within die-spirit and scope of the invention may occur to those skilled in the art to which the invention pertains. For example, each of the plates described herein preferably are provided with curvature to conform to the spinal anatomy. In alternate embodiments, however, the plates instead may be provided without pre-Iordosis. In addition, each of the plates described herein instead may be provided in embodiments that only include two fixation holes. Accordingly, all expedient modifications readily attainable by one versed in the art from the disclosure set forth herein that are within the scope and spirit of the present invention are to be included as further embodiments of the present invention. The scope of the present invention is accordingly defined as set forth in the appended claims. WE CLAIM: 1. An orthopaedic fixation system comprising: A) a plate (420) comprising a top surface (428), a bottom surface (430), a central longitudinal axis (427), at least one fixation hole (422, 424) extending between the top and bottom surfaces (428, 430) and comprising an undercut (432) therein, and at least one passage (429) intersecting the undercut (432); B) at least one resilient clip (450) disposed in at least a portion of the undercut (432), the at least one clip (450) having a pair of generally parallel sides (459a, 459b) and an end tab (458); C) at least one fastener (460) comprising a head (462) and a threaded shaft (464), the head (462) comprising a perimetral groove (466) extending around at least a portion thereof and an instrument receiving portion (468); wherein D) the at least one clip (450) is configured and dimensioned to seat in the undercut (432) with the end tab (458) extending within the passage (429); and E) the at least one fastener (460) is configured and dimensioned to be received in the at least one fixation hole (422, 424) and securable therein when the at least one clip (450) snaps into said perimetral groove (466) of the head (462), characterized in that F) the perimetral groove (466) of the at least one fastener (460) is bounded at its upper and lower ends with an upper and a lower surface, wherein said upper and lower surface are oppositely arranged with respect to a plane disposed midway across said groove (466) and perpendicular to a longitudinal axis (472) of said fastener (460) and wherein said upper and lower surface are disposed at between 10° and 70° with respect to each other. 2. The fixation system as claimed in claim 1, wherein said plate (420) is provided with a slot (426) extending through its central longitudinal axis (427). 3. The fixation system as claimed in claim 2, wherein said slot (426) is aligned along the central longitudinal axis (427). 4. The fixation system as claimed in claim 2, wherein said slot (426) is disposed transverse to the central longitudinal axis (427). 5. The fixation system as claimed in claim 2, wherein said slot (426) comprises a dog- bone shape. 6. The fixation system as claimed in claim 2, wherein said slot (426) comprises a pair of overlapping circular shapes. 7. The fixation system as claimed in claim 1, wherein said at least one passage (429) extends transverse to at least one of the fixation holes (422, 424). 8. The fixation system as claimed in claim 1, wherein said instrument receiving portion (468) of the head (462) of the at least one fastener (460) at least partially intersects the undercut (432). 9. The fixation system as claimed in claim 1, wherein a portion of the undercut (432) of the plate (420) is disposed closer to the top surface (428) than the bottom surface (430). 10. The fixation system as claimed in claim 1, wherein the undercut (432) of the plate (420) extends completely around the at least one fixation hole (422, 424). 11. The fixation system as claimed in claim 1, wherein each of the fixation holes (422, 424) comprises a lower portion disposed between the bottom surface (430) and the undercut (432), said lower portion tapering toward a central axis of the fixation hole (422, 424). 12. The fixation system as claimed in claim 11, wherein said lower portion has a first maximum inner dimension and the undercut (432) has a second maximum inner dimension, wherein the first maximum inner dimension is less than the second maximum inner dimension. 13. The fixation system as claimed in claim 11, wherein the head (462) of the fastener (460) comprises a tapered portion disposed between the threaded shaft (464) and the perimetral groove (466), the tapered portion tapering toward a central axis of the fastener (460). 14. The fixation system as claimed in claim 1, wherein at least one fixation hole (422, 424) is circular. 15. The fixation system as claimed in claim 1, wherein the plate (520) includes at least two pairs of fixation holes (522, 524). 16. The fixation system as claimed in claim 1, wherein one pair of fixation holes (522, 524) is generally circular and the other pair of fixation holes (522, 524) is generally oblong. 17. The fixation system as claimed in claim 1, wherein the perimetral groove (466) of said at least one fastener (460, 560) is interrupted by at least one corner (470). 18. The fixation system as claimed in claim 1, wherein the perimetral groove (466) of the at least one fastener (460, 560) is interrupted by at least two corners (470). 19. The fixation system as claimed in claim 1, wherein the perimetral groove (466) of the at least one fastener (460, 560) is interrupted by at least four corners (470). 20. The fixation system as claimed in claim 19, wherein each corner (470) is configured and dimensioned as a cam to abut an inner wall (456) of the clip (450). 21. The fixation system as claimed in claim 1, wherein the instrument receiving portion (468) of the head (462) of the at least one fastener (460, 560) intersects the perimetral groove (466) in at least one location. 22. The fixation system as claimed in claim 1, wherein the instrument receiving portion (468) of the head (462) of the at least one fastener (460, 560) intersects the perimetral groove (466) in at least two locations. 23. The fixation system as claimed in claim 1, wherein the instrument receiving portion (468) of the head (462) of the at least one fastener (460, 560) intersects the perimetral groove (466) in at least four locations. 24. The fixation system as claimed in claim 1, wherein the instrument receiving portion (468) of the head (462) of the at least one fastener ((460, 560) comprises two substantially perpendicular slots. 25. The fixation system as claimed in claim 1, wherein the instrument receiving portion (468) of the head (462) of the at least one fastener (460, 560) is provided with an internal thread. 26. The fixation system as claimed in claim 25, wherein the internal thread extends within the shaft (464). 27. The fixation system as claimed in claim 1, wherein the clip (450) is generally wishbone-shaped. 28. The fixation system as claimed in claim 1, wherein the clip (450) comprises a generally circular portion. 29. The fixation system as claimed in claim 1, wherein the clip (450) comprises a generally arcuate portion. 30. The fixation system as claimed in claim 1, wherein the undercut (432) is sized to retain the clip (450) at least partially therein while permitting expansion thereof. 31. The fixation system as claimed in claim 1, wherein the at least one fixation hole (422, 424) comprises at least two pairs of fixation holes, and wherein at least one of the pairs of fixation holes is configured and dimensioned to permit toggling of fasteners (460) disposed therein. 32. The fixation system as claimed in claim 1, wherein the head (462) of the fastener (460) is configured and dimensioned to permit toggling in the at least one fixation hole (422, 424). 33. The fixation system as claimed in claim 1, wherein the head (462) of the fastener (460) is configured and dimensioned for coupling to the plate (420) at a fixed angle. 34. The fixation system as claimed in claim 1, wherein the clip (290) comprises a slit (292). 35. The fixation system as claimed in claim 1, wherein the perimetral groove (466) of the at least one fastener (460) comprises an upper surface and a lower surface disposed at between about 30 and about 50 with respect to each other. 36. The fixation system as claimed in claim 1, wherein the perimetral groove (466) of the at least one fastener (460) comprises an upper surface and a lower surface aligned at about 40 with respect to each other. 37. The fixation system as claimed in claim 1, wherein the clip (450) is configured and dimensioned to slide in the undercut (432). 38. The fixation system as claimed in claim 1, wherein the clip (450) is configured and dimensioned to be fixed and stationary in the plate (420). 39. The fixation system as claimed in claim 38, wherein the fastener (460) is permitted to toggle. 40. The fixation system as claimed in claim 1, wherein the head (462) of the fastener (460) is further provided with at least one scallop (1563) disposed proximate a top peripheral portion of the head (462). 41. The fixation system as claimed in claim 40, wherein the at least one scallop (1563) comprises four scallops. 42. The fixation system as claimed in claim 40, wherein the at least one scallop (1563) comprises an arcuate portion. 43. The fixation system as claimed in claim 1, wherein at least one surface of the head (462) comprises roughening for interacting with the clip (450). 44. The fixation system as claimed in claim 43, wherein the roughening comprises steps. 45. The fixation system as claimed in claim 11, wherein the taper of the lower portion is semispherical. 46. The fixation system as claimed in claim 1, wherein the at least one resilient clip (450) is configured and dimensioned to permit toggling through a greater angular range in a cephaladcaudal direction of the plate (420) than in a medial-lateral direction of the plate (420). 47. The fixation system as claimed in claim 1, wherein the at least one resilient clip (450) is configured and dimensioned to permit toggling of the at least one fastener (460) through a greater angular range in a cephalad-caudal direction of the plate than in other directions of the plate. 48. The fixation system as claimed in claim 47, wherein toggling of the at least one fastener (460) is permitted between about 0° and about 32° along a plane extending parallel to the central longitudinal axis of the plate (420). 49. The fixation system as claimed in claim 47, wherein toggling of the at least one fastener (460) is permitted between about 0° and about 20° along a plane extending perpendicular to the central longitudinal axis of the plate (420).

Documents:

680-delnp-2006-Abstract-(03-05-2011).pdf

680-delnp-2006-abstract.pdf

680-delnp-2006-assignment.pdf

680-delnp-2006-Claims-(03-05-2011).pdf

680-delnp-2006-claims.pdf

680-delnp-2006-Correspondence Others-(03-05-2011).pdf

680-DELNP-2006-Correspondence-Others-(16-07-2010).pdf

680-DELNP-2006-Correspondence-Others-(23-03-2010).pdf

680-DELNP-2006-Correspondence-Others-(30-03-2010).pdf

680-delnp-2006-correspondence-others-1.pdf

680-delnp-2006-correspondence-others.pdf

680-delnp-2006-description (complete).pdf

680-delnp-2006-drawings.pdf

680-delnp-2006-Form-1-(03-05-2011).pdf

680-delnp-2006-form-1.pdf

680-delnp-2006-form-13-(04-04-2008).pdf

680-delnp-2006-form-18.pdf

680-delnp-2006-Form-2-(03-05-2011).pdf

680-delnp-2006-form-2.pdf

680-delnp-2006-Form-3-(03-05-2011).pdf

680-delnp-2006-form-3.pdf

680-delnp-2006-form-5.pdf

680-delnp-2006-form-6-(14-11-2006).pdf

680-delnp-2006-form-6.pdf

680-DELNP-2006-GPA-(23-03-2010).pdf

680-DELNP-2006-GPA-(30-03-2010).pdf

680-delnp-2006-gpa.pdf

680-delnp-2006-pct-304.pdf

680-delnp-2006-Petition-137-(03-05-2011).pdf

680-DELNP-2010-Correspondence-Others-(23-04-2010).pdf