Bone screw retaining system

1. An implant comprising:

a joining member having a bone contact surface and an opposite upwardly facing surface and at least one opening with a groove formed around an inner circumference of the opening, said groove having an axially spaced upper and lower surface;

a bone-anchor capable of being accommodated in the opening between said groove and said bone contacting surface; and

an annular split-ring mounted in said groove, the annular split-ring sized to expand upon insertion of the anchor element into the opening and to come into direct contact with a head of the bone anchor to hold the bone anchor in the opening, said split-ring having an upper surface and a lower surface for respectively engaging said upper and lower surface of said groove, wherein the split-ring defines a distance between an inner and outer diameter of the annular split-ring, the outer diameter having a plurality of cutout portions so that the distance between the split-ring inner and outer diameters has alternating wider and narrower portions.

2. The implant according to claim 1 , wherein in the joining member comprises a plate.

3. The implant according to claim 2 , wherein the openings comprise a spherical seat.

4. The implant according to claim 3 , wherein the anchor comprises a complementary spherical part capable of coming into contact with the spherical seat.

5. The screw retaining system as set forth in claim 2 , wherein said at least one opening has a part spherical seat portion located between said groove and a bottom portion of said plate and open to said surface to receive a shank of said screw.

6. The screw retaining system as set forth in claim 5 , wherein said screw has a part spherical head portion extending from said shank towards said upwardly facing surface for engaging said spherical seat portion in said opening.

7. The screw retaining system as set forth in claim 5 , wherein said groove has a lower surface nearest the bottom surface of the plate and an upper inclined surface extending toward an upper surface of said plate and inwardly toward a central axis of said opening.

8. The screw retaining system as set forth in claim 7 , wherein the groove lower surface is generally flat and extends generally perpendicular to said central axis.

9. The screw retaining system as set forth in claim 7 , wherein said lower groove surface is located closer to said upper plate surface than an upwardly facing surface on said screw when said part spherical head is seated in said part spherical portion.

10. The implant according to claim 1 , wherein the anchor includes a drive portion.

11. The implant according to claim 1 wherein the split ring has at least three wider portions separated by at least three narrower portions.

12. The implant according to claim 11 wherein the split-ring has five wider and narrower portions.

13. The implant according to claim 1 , wherein the split-ring is specific to each opening.

14. An implant comprising:

a plate having a bone contact surface and an opposite upwardly facing surface and at least one opening with a groove formed around an inner circumference of the opening, said groove having an axially spaced upper and lower surface;

a bone-anchor capable of being accommodated in the opening between said groove and said bone contacting surface;

a split-ring mounted in said groove, the split-ring sized to expand upon insertion of the anchor element into the opening and to come into direct contact with a head of the bone anchor to hold the bone anchor in the opening, said split-ring having an upper surface and a lower surface for respectively engaging said upper and lower surface of said groove, wherein the split-ring has a variable width so as to optimize its flexibility;

wherein said at least one opening has a part spherical seat portion located between said groove and a bottom surface of said plate and open to said bottom surface to receive a shank of said screw; and

wherein said groove has a lower surface nearest the bottom surface of the plate and an upper inclined surface extending toward an upper surface of said plate and inwardly toward a central axis of said opening.

15. The screw retaining system as set forth in claim 14 , wherein the groove lower surface is generally flat and extends generally perpendicular to said central axis.

16. The screw retaining system as set forth in claim 15 , wherein said lower groove surface is located closer to said upper plate surface than an upwardly facing surface on said screw when said part spherical head is seated in said part spherical portion.

17. An implant comprising a joining member having a bone contact surface and an opposite upwardly facing surface and at least one opening with a groove formed around an inner circumference of the opening, said groove having an axially spaced upper and lower surface; a bone-anchor capable of being accommodated in the opening between said groove and said bone contacting surface; an annular split-ring mounted in said groove, the annular split-ring sized to expand upon insertion of the anchor element into the opening and to come into direct contact with a head of the bone-anchor to hold the bone-anchor in the opening, said annular split-ring having an upper surface and a lower surface for respectively engaging said upper and lower surfaces of said groove, wherein the annular split-ring defines a distance between an inner and outer diameter of the annular split-ring, the outer diameter having a plurality of cutout portions so that the distance between the annular split-ring inner and outer diameters has alternating wider and narrower portions; and wherein the annular split-ring inner diameter is circular and the outer diameter has portions spaced at two different distances from the inner diameter.