MXPA05013630A - Polyaxial bone screw - Google Patents

Abstract

The present invention generally provides a polyaxial fixation device having a shank with a spherical head formed on a proximal end thereof, and a receiver member having an axial passage formed therein that is adapted to polyaxially seat the spherical head of the shank. The polyaxial bone screw further includes an engagement member that is adapted to provide sufficient friction between the spherical head and the receiver member to enable the shank to be maintained in a desired angular orientation before locking the spherical head within the receiver member.

Description

SCREW OSEO PO IAXIAL TECHNICAL FIELD OF THE INVENTION The present invention relates to polyaxial bone screws and, in particular, to a polyaxial bone screw assembly wherein the bone thyme can be maintained in a desired angular orientation before fixing the bone screw with respect to the rod receiving member.

BACKGROUND OF THE INVENTION Spinal fixation devices are used in orthopedic surgery to align and / or fix a desired relationship between aent vertebral bodies. Said devices generally include a spinal fixation element, such as a relatively rigid attachment rod, which engages the aent vertebrae joining the element with different anchoring devices, such as hooks, bolts, wires or screws. The fixation rods may have a predetermined contour that has been designed in accordance with the properties of the target implantation site and, once installed, the instrument holds the vertebrae in a desired spatial relationship, when either the desired healing has occurred. or spinal fusion or for a longer period of time.

Spinal fixation devices can be anchored in specific portions of the vertebra. Since each vertebra varies in shape and size, a whole series of anchoring devices have been developed to facilitate coupling of a particular portion of the bone. For example, pedicle screw assemblies have a shape and size configured to mate with the pedicle bone. Generally said screws include a bone screw with a threaded body that is adapted to be screwed into a vertebra, as well as a rod receiving element, generally in the form of a U-shaped groove formed in the head. The rod receiving body and assembly can be provided as a monoaxial screw, whereby the rod receiving member is fixed with respect to the body, or as a polyaxial screw, by means of which the rod receiving element. It has a free angular movement with respect to the body. In use, the body portion of each screw is screwed into a vertebra and, once it has been properly positioned, a fixing rod is seated in the rod receiving element of each screw. The rod is then fixed in place by tightening a clamping screw, pin or similar type clamping mechanism on the rod receiving element. Although current spinal fixation systems have been shown to be effective, it may be difficult to mount rods on the rod receiving member of different securing devices. In particular, it may be difficult to align and seat a rod in the rod receiver of a polyaxial implant, since the rod receiver has freedom of polyaxial movement with respect to the body. More particularly, the freedom of polyaxial movement of the rod receiver can allow the receiver to "flap", thus requiring the surgeon or an assistant to hold the receiver in the desired position during the introduction of the rod. Accordingly, there is still a need for a polyaxial bone screw assembly where the rod receiving element can be maintained in a desired angular orientation before fixing the body with respect to the receiving member.

BRIEF DESCRIPTION OF THE INVENTION The present invention generally provides a polyaxial spinal fixation device (eg, bone screws, hooks, etc.) having a body with a spherical head formed at a proximal end thereof, as well as a receiving member having an axial passage formed in that place that adapts to seat the spherical head of the body in a polyaxial way. The polyaxial fixation device also includes a coupling member that is adapted to provide sufficient friction between the spherical head and the receiving member and allow the body to be maintained in a desired angular orientation before attaching the spherical head within the receiving member. The coupling member can have a whole series of configurations and, in one embodiment, the coupling member may be a ring member, such as a snap ring, which is positioned to engage with a portion of the spherical head and provide friction engagement between the head and the receiving member. The ring member may be disposed within a notch formed around an outer surface of the spherical head of the body and / or may be disposed within a notch formed around an inner surface of the receiving member. The notchabout the inner surface of the receiving member preferably has a depth that is equal to or greater than the thickness of the ring member, to allow the ring member to be fully disposed within the notch. Alternatively or in addition, the ring member may be adapted to expand or contract and be disposed completely within the notch. In another embodiment, the coupling member may be a compression cap that is disposed within the receiving member and having a concave distal surface adapted to seat at least a portion of the spherical head of the body. Preferably, the compression cap is capable of coinciding with the receiving member, so that the compression cap is effective to maintain the spherical head of the body in a spherical recess formed in the receiving member. The compression cap may have a variety of configurations and, in one embodiment, may include opposed leaf spring members that adapt to contract inwardly, tilting the cap distally, to frictionally engage the spherical head of the cap. body. In another embodiment, at least a portion of the compression cap has a diameter that is expandable to engage frictionally with the spherical head. By way of non-limiting example, the compression cap may include a plurality of distally extending tab-like members formed around a distal edge of the compression cap to engage frictionally with the spherical head. In yet another embodiment, the compression cap can include at least one longitudinally oriented slot formed therein to allow the compression cap to contract to frictionally engage the spherical head. In other aspects, a polyaxial fixation assembly having a body with a spherical head formed at a proximal end thereof, as well as a receiving member having a first proximal opening adapted to receive a spinal fixation rod and a second opening is provided. distal that has a size of diameter that allows the passage of the body through that place, at the same time that the spherical head is maintained in that place. The receiver member also includes a spherical seat adjacent to the second distal opening for polyaxial seating of the spherical head of the body. The polyaxial fastening assembly also includes means for frictionally coupling the spherical head and maintaining the body in a desired angular orientation, so that greater force is required to a frictional coupling force to change the angular orientation of the threaded body with respect to the receiving member.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be understood more fully from the following detailed description taken in conjunction with the accompanying drawings, wherein: The figural is a perspective view of a polyaxial bone screw of the prior art; Figure 2A is a partially transverse enlarged view of a polyaxial bone screw assembly having a ring member disposed therein in accordance with an embodiment of the present invention; Figure 2B illustrates various embodiments of a ring member that can be used with the polyaxial bone screw assembly shown in Figure 2A; Figure 2C is an illustration of another embodiment of a bone screw having a ring member disposed about that location; Figure 3A is a perspective view of a polyaxial bone screw assembly, in the disassembled state, having a compression cap with a collar for coupling with the head of a bone screw in accordance with another embodiment of the present invention; Figure 3B is a partially transverse enlarged view of a portion d of the polyaxial bone screw assembly shown in Figure 3A; Figure 3C illustrates the polyaxial bone screw assembly of Figure 3B with a rod and a locking mechanism disposed therein; Figure 4A is a perspective view of another embodiment of a polyaxial bone screw assembly, in the disassembled state, having a compression cap with a leaf spring for coupling with the head of a bone screw in accordance with the present invention; Figure 4B is an enlarged cross-sectional view of a portion of the polyaxial bone screw assembly shown in Figure 4A; Figure 5A is a perspective view of yet another embodiment of a polyaxial bone screw assembly, in the disassembled state, having a compression cap with a groove formed therein to allow the compression cap to engage with the head of the head. a bone screw according to the present invention; And Figure 5B is a partially transverse enlarged view of a portion of the polyaxial bone screw shown in Figure 5A.

DETAILED DESCRIPTION OF THE INVENTION Figure 1 illustrates a polyaxial bone screw assembly 10 of the prior art which includes a bone screw 12, a receiver member 18 and a compression cap 24. As shown, the bone screw 12 generally includes a threaded body 14 having a spherical head 16 formed on a proximal end 14a of that place. An Alien type inlet or other female inlet 15 is formed in the head 16 to apply torque along the axis of the body 14 and insert the body 14 in the bone. The receiving member 18 is generally U-shaped and includes opposite side legs or walls 20a, 20b that are substantially parallel to each other and that define a rod receiving portion 22 for seating a spinal fixation rod. A distal end 18b of the receiving member 18 includes an axial opening (not shown) formed therein and having a diameter size that allows passage of the body 14 through that location, while maintaining the spherical head 16 in that place. The receiving member 18 also includes a spherical seat (not shown) adjacent to the distal opening for polyaxial seating of the spherical head 16 of the bone screw 12. The compression cap 24, which is adapted to be positioned within the receiving member 18, has generally a cylindrical shape and includes a proximal stem receiving surface 26 and a distal concave surface (not shown) that is adapted to fit around and seat a portion of the spherical head 16 of the bone screw 12. In use, the body threaded 14 is disposed through the distal opening in the receiving member 18 and the spherical head 16 of the bone screw 12 is placed inside the spherical seat in the receiving member 18. The compression cap 24 is then inserted into the receiving member 18, so that the concave distal surface of the compression cap 24 is disposed around and seats a portion of the spherical head 16 of the bone screw 12. To maintain In the compression cap 24 within the receiving member 18, the receiving member 18 includes perforations of opposite sides (only a side perforation 28a is shown) having a deformable material (not shown) extending lengthwise on an inner surface of the receiving member 18. The lateral perforations 28a allow the material to deform inwardly to extend toward and engage opposite holds (only a retainer 30a is shown) formed in the compression cap 24. A tool can be used to deform the material and form the detents 30a once the compression cap 24 is disposed within the receiver 18. As a result, the compression cap 24 is held within the receiving member 18, thereby preventing removal of the bone screw 12 from the receiving member. 18. The compression cap 24 is also effective for fixing bone screw 12 in a desired angular orientation with respect to the rec member. eptor 18 once a rod is disposed and fixed within the receiving member 18. A person skilled in the art will note that a variety of techniques may be used to maintain the compression cap 24 within the receiving member 18 and is not intended to present invention is limited to use with compression caps 24 having detents for receiving the deformable material disposed within the receiving member. By way of non-limiting example, the compression cap 24 can be maintained within the receiver 18 using a cross pin. Once the bone screw 12 is implanted within the bone, and prior to the insertion of a rod into the receiving member 18, the receiving member 18 of the prior art assembly is free to rotate and / or angularly locate with respect to to the bone screw 12. Although advantageously this allows the alignment of the receiving member 18 with a rod adapted to be disposed there, said free axial movement of the receiving member 18 may represent challenges during surgery, since it is required that the surgeon keep the receiving member 18 in the desired position during the introduction of the rod. Accordingly, the present invention provides mechanisms for creating friction between the spherical head 16 and the receiving member 18 to allow the receiving member 18 to be temporarily held in a desired angular orientation before fixing the receiving member 18 with respect to the receiving device. polyaxial fixation. This is particularly advantageous because it allows a surgeon to position and maintain the receiving member 18 in a desired orientation prior to the introduction of the rod, thereby preventing the receiving member 18 from moving with respect to the bone screw 12 during the introduction of a rod. . Although several different techniques can be used to create the frictional forces necessary to allow the angular orientation between the receiving member 18 and the bone screw 12 to be maintained, FIGS. 2A through 5B illustrate several examples of embodiments for frictionally coupling the spherical head. of a bone screw with respect to a rod receiving member. For purposes of convenience, the reference numbers used in the embodiments shown in Figures 2A to 5B correspond to the reference numbers used in Figure 1, except that a different prefix is added to the reference numbers for each modality. The person skilled in the art will notice that a whole series of other techniques can be used to create the frictional forces necessary to maintain the angular orientation of the body with respect to the receiving member. In addition, the techniques used to create friction between the spherical head and the receiver member can be adapted for use with virtually any polyaxial spinal fixation device, in addition to the illustrated bone screw assembly, and it is not intended that the invention be limited to the assembly of specific polyaxial bone screw shown. Figure 2A illustrates one embodiment of a polyaxial bone screw assembly 210 using a ring member, e.g. a pressure ring 234, for frictionally coupling the spherical head 216 of the bone screw 212. The pressure ring 234 may have a variety of configurations, shapes and sizes, but must be adapted to expand and fit around at least a portion of the spherical head 216. As shown, the pressure ring 234 is in the form of a loop with an opening 235 formed therein that allows the diameter of the pressure ring 234 to expand to fit about a portion. of the spherical head 216 of the bone screw 212. Although the pressure ring 234 shown is C-shaped, the pressure ring 234 may have a number of other configurations. By way of non-limiting example, Figure 2B illustrates a series of distinct pressure rings 234a, 234b, 234c, 234d, 234e that can be used with the polyaxial bone screw assembly 210 shown in Figure 2A. For example, each of the pressure rings 234a and 234c has an irregular shape that allows the pressure rings 234a, 234c to expand to fit around the spherical head 216 of the bone screw 212. On the other hand, the ring pressure 234e includes several cutting portions 235e that allow the pressure ring 234e to expand. In other embodiments, the pressure ring 234 may have a plurality of different transverse shapes, such as, for example, a circular cross-sectional shape as shown in pressure ring 234d or a C-shaped cross section, as shown in FIG. the pressure ring 234b. Although the pressure ring 234 may have a variety of configurations, the pressure ring 234 must adapt to fit within a corresponding notch 236 formed around an inner surface of the receiving member 218. The notch 236 maintains the pressure ring 234 in a particular location with respect to the spherical head 216 of the bone screw, so that the pressure ring 234 expands around the head 216. More particularly, the notch 236 must be formed in a proximal portion of the spherical seat 219 formed at the distal end. 218b of the receiving member 218. The notch 236 is not only effective to maintain the position of the pressure ring 234 around the spherical head 216, but is also effective to fully seat the pressure ring 234 when the head 216 is fixed Within the receiver 218. As mentioned above, when a rod sits within the receiving member 218, the compression cap 224 is forced distally to fix the bone screw 216 with respect to the receiver 218. The notch 236 receives the ring of pressure 234 to prevent pressure ring 234 from interfering with the fixing function of compression cap 224. Accordingly, notch 236 preferably has a depth d which is at least equal to and greater preferably, greater than the thickness tr of the pressure ring 234. Alternatively or further, the pressure ring 234 may be adapted to expand or contract to be fully disposed within the notch 2 36. By way of non-limiting example, the pressure ring 234 can be formed from a compressible material. or deformable that allows the pressure ring 234 to be forced completely into the notch 236. Still with respect to Figure 2A, the bone screw assembly 210 can be assembled by first placing the pressure ring 234 within the notch 236. in the receiving member 218. The threaded body 214 of the screw 212 can then be inserted through the axial opening 237 formed in the distal end 218b of the receiving member 218. As a result, the spherical head 216 will rest on the upper part of the ring pressure 234. The compression cap 224 can then be placed in the receiver 218 and can be used to push the head 216 into the recess 219, thereby causing the pressure ring 234 to expand around the head 216 for engaging the head 216. This can be achieved by using a tool to push the compression cap 224 in a distal direction. To prevent the compression cap 224 from suddenly projecting out of the receiver 218, another tool may be inserted into each of the opposed perforations 228a, 228b to deform the deformable material 232a, 232, which extends along the inner surface of the cap. receiving member 218, and forming the corresponding detents 230a, 230b formed in the compression cap 224. As a result, the compression cap 224 is prevented from moving in a proximal direction, thereby preventing the spherical head 216 from move proximally and disengage with the pressure ring 234. The person skilled in the art will note that a variety of other techniques and clamping members are known to be used in the maintenance of the spherical head 216 (and any shank) within the receiver 218. Once the device 210 is assembled, the frictional forces created by the pressure ring 234 acting on the spherical head 216 of the screw The screw 212 will allow the screw 212 to be fixed in a desired angular orientation with respect to the receiver member 218, as shown in Figure 2A. Friction forces can simply be overcome by gripping and moving the bone screw 212 with respect to the receiver member 218 to change the angular orientation. In other words, a force greater than the frictional engagement force is required to change the angular orientation of the bone screw 212 with respect to the receiving member 218.

In another embodiment, shown in Figure 2C, the pressure ring 234 may be disposed within a notch 236 'formed around the spherical head 216 * of the bone screw 2121, rather than in a notch 236 formed within the receiving member 218. In this embodiment, the notch 236 'around the head 216' of the bone screw 212 'preferably extends at an angle a, with respect to a longitudinal axis L of the screw 212', around the proximal half of the screw. the spherical head 216 ', to allow the head 216' to fit within the spherical recess 219 in the receiving member 218. The angle a of the notch 236 'also allows the pressing ring 234 to exert pressure against the concave inner surface of the the compression cap 224, thereby creating the frictional forces necessary to allow the angular orientation of the bone screw 212 'to be maintained with respect to the receiver member 218. Figures 3A to 3C illustrate another embodiment of a torso assembly. polyaxial bone ring 310 that includes a coupling feature that is effective to maintain the angular orientation of a bone screw 312 with respect to a receiver member 318. In this embodiment, rather than being provided with a separate coupling member, such as the pressure ring 234 shown in Figures 2A to 2C, the compression cap 324 is modified to include an expandable portion that is adapted to fit around and engage frictionally with the ball head 316 of the bone screw 312. Although the expandable portion may have virtually any In one exemplary embodiment, the distal end 324b of the compression cap 324 includes a collar 333 formed around that location having several spaced-apart tongue members 334 that are separated by slots 335 that allow the tongue-like members. 334 expand. The collar 333 may include any number of tongue-like members 334 that may be spaced apart at varying distances. Once the cap is held in place within the receiving member, the tabs 334 will exert pressure on the spherical head 316 of the screw 312. This can be achieved by deforming the tabs 334 in the cap inward prior to assembly, so that they enter contact with the spherical head 316 of screw 312 once inserted. Alternatively, the concave underside of cap 324 can be adjusted such that the radius is less than a radius r of the ball head 316 of screw 312. This interference will also cause tabs 334 to exert pressure on screw head 316 312. In use, as the compression cap 324 moves distally relative to the head 316, the collar 333 is forced to expand around the spherical head 316 of the bone screw 312 to engage with the head and creating the necessary friction to maintain the angular orientation of the screw 312 with respect to the receiver 318, as shown in Figure 3B. As previously indicated, the compression cap 324 can be maintained in this position by deforming the material 332a, 332b in the receiving member 318 to form corresponding catches 330a, 330b in the compression cap 324.

Still with respect to Figure 3B, in a further embodiment, the receiving member 318 may include an annular groove 336 formed therein to receive the expandable tongues 334 of the collar 333. The groove 336, which is similar to the groove 236 shown in FIG. Figure 2A, prevents collar 333 from interfering with the function of fixing the compression cap 324. In other words, when a rod 50 sits within the stem receiving recess 326 formed in the compression cap 324 and is applied a locking mechanism 60 for securing the rod 50 within the receiving member 318, as shown in Figure 3C, the compression cap 324 fixes the position of the spherical head 316 within the receiving member 318. The notch 336 receives from this the collar 333 to prevent the collar 333 from interfering with the clamping forces created between the compression cap 324 and the ball head 316. Figures 4A to 4B illustrate yet another embodiment of a polyaxial bone screw assembly 410 in which a cap The leaf spring compression 424 is used for coupling the ball head 416 of the bone screw 412 to create the frictional forces necessary to maintain the angular orientation of the bone screw 412 with respect to a receiver member 418. As shown in FIG. figure 4A, the compression cap 424 includes a first pair of grooves 431 a-i, 431 a2 formed on opposite sides of the first detent 430a, as well as a second pair of grooves 431 b- ?, 431 b2 formed on opposite sides of the second retainer 430b. Each pair of slots 431 a-i, 431 a, 431 b ?, 431 b extends from a proximal end 424 a of the compression cap 424 to the distal end 424 b, ending just proximal to the distal end 424 b. As a result, the grooves 431 a-i, 431 a2, 431b !, 431 b2 form lateral wall portions 434a, 434b therebetween that are flexible, thereby forming a leaf spring. In use, as shown in Figure 4B, when the deformable material 432a, 432b in the receiving member 418 is deformed to form corresponding detents 430a, 430b in the compression cap 424, the sidewall portions 424a, 424b are they bend inward, then contracting around them and, preferably, creating a downward pressure on the spherical head 416 of the bone screw 412. As a result, friction is created between the compression cap 424 and the spherical head 416 to maintain the angular orientation of the screw 412 with respect to the receiving member 418. The person skilled in the art will note that a number of other techniques can be used to create a spring-like compression cap 424 that is effective for coupling the spherical head 416 of the screw 412. Figures 5A to 5B illustrate yet another embodiment of a polyaxial bone screw assembly 510. In this embodiment, the compression cap 524 includes a ring axial ura 534 formed therein to allow the compression cap 524 to contract for engagement of the spherical head 516 of the bone screw 512. Although the slot 534 can be formed anywhere in the compression cap 524, preferably the groove 534 is formed in the portion of the side wall that extends between opposing detents 530a, 530b and, most preferably, slot 534 is equidistant from each detent 530a, 530b to allow compression cap 524 to be embossed uniformly in a distal direction. Figure 5B illustrates the compression cap 524 in a contracted state around the head 516 of the bone screw. The deformable material 532a, 532b in the receiving member 518 is deformed to form the detents 530a, 530b in the compression cap 524 to contract the compression cap 524 about the spherical head 516. As a result, the friction forces created by the compression cap 524 which contracts radially around the spherical head 516 are effective to allow the bone screw 512 to remain in the desired angular orientation with respect to the head of the receiver 518. The person skilled in the art will warn that a variety of other techniques can be used to apply friction to the spherical head of a polyaxial bone screw and allow the bone screw to remain in a desired angular orientation before fixing the bone screw within the recipient member. By way of non-limiting example, the spherical head of the polyaxial screw may include a cover or surface treatment therein to impede the movement of the head of the screw with respect to the receiving member. Alternatively or in addition, the spherical head, the compression cap and / or the receiver member may include one or more projections formed therein to engage frictionally with the spherical head and allow the orientation of the head to be maintained in a configuration desired. For example, the projections may be formed from a plastic material that is effective to interfere with the free rotary movement of the screw within the receiver. The person skilled in the art will notice that this design can be applied to other polyaxial fixation devices, including other screws, cross-type connectors, hooks, bolts, etc., and is not intended to be limited to use with a polyaxial bone screw. The person skilled in the art will also notice additional features and advantages of the invention based on the modalities described above. Accordingly, the invention should not be limited by what has been shown and described in a particular way, except for that indicated by the appended claims. All publications and references cited herein are expressly incorporated herein by reference in their entirety.

Claims (31)

NOVELTY OF THE INVENTION CLAIMS

1. - A polyaxial fixation device comprising: a body having a spherical head formed at a proximal end thereof; a receiving member having an axial passage formed in that location and adapted to seat the spherical head of the body in a polyaxial manner; and a coupling member adapted to provide sufficient friction between the spherical head and the receiving member, to allow the body to remain in a desired angular orientation before attaching the spherical head within the receiving member.

2. The device according to claim 1, further characterized in that the coupling member comprises a ring member positioned to engage frictionally with a portion of the spherical head.

3. The device according to claim 2, further characterized in that the ring member is disposed within a notch formed around one of an internal surface of the receiving member and an outer surface of the spherical head of the body.

4. The device according to claim 3, further characterized in that the notch has a depth that is equal to or greater than a thickness of the ring member to allow the ring member to be fully disposed within the notch.

5. The device according to claim 3, further characterized in that the ring member is adapted to expand or contract to be fully disposed within the notch.

6. The device according to claim 2, further characterized in that the ring member has an irregular shape.

7. The device according to claim 2, further characterized in that the ring member is substantially C-shaped.

8. The device according to claim 2, further characterized in that the ring member includes an opening formed in a wall thereof to allow the ring member to expand.

9. The device according to claim 1, further characterized in that the coupling member comprises a collar having a plurality of spaced apart expandable members disposed about that location and adapted to mate with the spherical head of the body.

10. The device according to claim 9, further characterized in that the collar is arranged at least partially within a notch formed around an internal surface of the receiving member.

11 - The device according to claim 1, further characterized in that the coupling member comprises a compression cap disposed within the receiving member and having a concave distal surface adapted to seat at least a portion of the spherical head of the body, the compression cap coinciding with the receiving member, so that the compression cap is effective to maintain the spherical head of the body in a spherical recess formed in the receiving member.

12. The device according to claim 11, further characterized in that the compression cap includes spring members of opposite leaves and in that the receiving member includes opposite deformable portions which, when deformed, are effective for contracting the leaf spring members. and causing the compression cap to engage frictionally with the spherical head of the body.

13. The device according to claim 11, further characterized in that at least a portion of the compression cap has a diameter that is expandable or deformable to engage frictionally with the spherical head.

14. The device according to claim 13, further characterized in that the compression cap includes a plurality of distally extending tongue-like members formed around a distal edge of the compression cap and adapted to fit in a secure manner. frictional with the spherical head.

15. - The device according to claim 13, further characterized in that the compression cap includes at least one longitudinally oriented slot formed therein to allow the compression cap to contract to frictionally engage the spherical head.

16. The device according to claim 14, further characterized in that the concave distal surface of the compression cap has an internal radius that is less than a radius of the spherical head to allow the compression cap to frictionally engage with the spherical head when the compression cap is held within the receiving member.

17. A polyaxial fastening assembly comprising: a body having a spherical head formed at a proximal end thereof; a receiving member having a first proximal opening adapted to receive a spinal fixation rod and a second distal opening having a diameter size that allows the passage of the body through that location, while maintaining the spherical head in that location, the receiving member further including a spherical seat adjacent to the second distal opening for polyaxial seating of the spherical head of the body; and means for frictionally coupling the spherical head and maintaining the body in a desired angular orientation, so that a force greater than the coupling friction force is required to change the angular orientation of the body with respect to the receiving member.

18. - The polyaxial fastening assembly according to claim 17, further characterized in that the means for coupling the spherical head comprises a ring member located to engage frictionally with a portion of the spherical head.

19. The polyaxial fixation assembly according to claim 18, further characterized in that the ring member is disposed within a notch formed around one of an outer surface of the spherical head of the body and an inner surface of the receiving member .

20. The polyaxial fixation assembly according to claim 19, further characterized in that the notch has a depth that is equal to or greater than a thickness of the pressure ring, to allow the pressure ring to expand or contract completely towards the inside of the notch.

21. The polyaxial fastening assembly according to claim 19, further characterized in that the ring member is adapted to expand or contract to be fully disposed within the notch.

22. The polyaxial fixation assembly according to claim 18, further characterized in that the ring member has an irregular shape.

23. The polyaxial fastening assembly according to claim 17, further characterized in that the means for coupling the spherical head comprises a collar having a plurality of expandable members disposed about that location and adapted for coupling the spherical head of the body.

24. The polyaxial fixation assembly according to claim 23, further characterized in that the collar is disposed at least partially within a notch formed around an internal surface of the receiving member.

25. The polyaxial fastening assembly according to claim 17, further characterized in that the means for coupling the spherical head comprises a compression cap disposed within the receiving member and having a concave distal surface adapted to settle at least a portion of the spherical head of the body, the compression cap coinciding with the receiving member so that the compression cap is effective to maintain the spherical head of the body in the spherical seat in the receiving member.

26. The polyaxial fastening assembly according to claim 25, further characterized in that the compression cap includes spring members of opposite blades and in that the receiving member includes opposite deformable portions which, when deformed, are effective for contracting the limb member. leaf spring towards each other to make the compression cap frictionally engage the spherical head of the body.

27. - The polyaxial fastening assembly according to claim 25, further characterized in that at least a portion of the compression cap has a diameter that is expandable or deformable for coupling frictionally with the spherical head.

28. The polyaxial fixation assembly according to claim 27, further characterized in that the compression cap incs a collar portion with a plurality of distally extending tab-like members formed around a distal edge of the cap. compression cap and adapted for frictional coupling with the spherical head.

29. The polyaxial fastening assembly according to claim 27, further characterized in that the compression cap incs at least one longitudinally oriented groove formed therein to allow the compression cap to contract frictionally to engage with the spherical head.

30. The polyaxial fixation assembly according to claim 28, further characterized in that the concave distal surface of the compression cap defines an internal radius that is less than a radius of the spherical head, to allow the compression cap to be Frictionally engage with the spherical head when the compression cap is held within the receiving member. 31.- The polyaxial fastening assembly according to claim 17, further characterized in that the means for coupling the spherical head comprises a surface cover disposed over at least a portion of the spherical head of the body and effective to create friction between the spherical head and the receiving member.