Locking device for fixation mechanism of medical implant

1. A receiving member for a bone fixation assembly, the receiving member having opposite first and second sides spaced apart by a thickness, the receiving member comprising a primary socket and a secondary socket on the receiving member first side, the primary socket being adapted to receive a fixation mechanism, the secondary socket being adapted to receive a locking device, the locking device being adapted to engage the fixation mechanism and prevent the fixation member from becoming disengaged from the receiving member primary socket, the primary socket having a bore extending from the primary socket to the receiving member second side thereby allowing a fixation mechanism to be directed into the primary socket and through the bore to a bone anchor site, the secondary socket having first and second bores with the secondary socket first bore being larger in diameter than the secondary socket second bore and forming a shoulder therebetween, the secondary socket first bore intersecting a portion of the primary socket and forming an arcuate cut-out, the secondary socket second bore extending from the secondary socket shoulder into the receiving member thereby enabling a locking device to be directed into the secondary socket and through the secondary socket second bore, the secondary socket having a chord member extending into the secondary socket first bore, the chord member forming a radial undercut between the chord member and the secondary socket shoulder, and the secondary socket having a detent extending from the radial undercut into the chord member.

2. The receiving member of claim 1 , wherein the primary socket has a spherical contour.

3. The receiving member of claim 1 , wherein the receiving member is an anterior lumbar plate.

4. The receiving member of claim 1 , wherein the chord member is generally opposite the arcuate cut-out.

5. The receiving member of claim 1 , wherein the secondary socket second bore extends from the shoulder to the receiving member second side.

6. A bone fixation assembly comprising:

a receiving member having opposite first and second sides spaced apart by a thickness, the receiving member comprising a primary socket and a secondary socket on the receiving member first side, the primary socket having a bore extending from the primary socket to the receiving member second side, the secondary socket having first and second bores with the secondary socket first bore being larger in diameter than the secondary socket second bore and forming a shoulder therebetween, the secondary socket first bore intersecting a portion of the primary socket and forming an arcuate cut-out in the first socket, the secondary socket second bore extending from the secondary socket shoulder into the receiving member, the secondary socket having a chord member extending into the secondary socket first bore, the chord member forming a radial undercut between the chord member and the secondary socket shoulder, and the secondary socket having a detent extending from the radial undercut into the chord member;

a fixation mechanism adapted to extend through the receiving member primary socket and primary socket bore to a bone anchor site; and

a locking device having a stem that is configured to be received in the receiving member secondary socket second bore, the locking device having a cap extending from the stem, the cap having an engagement surface and an arm extending from the cap with a pawl extending axially from the arm, the locking device being rotatable in the secondary socket between an unlocked position in which the cap engagement surface is angularly displaced from the arcuate cut-out, and a locked position in which the cap engagement surface occupies the arcute cut-out and engages the fixation mechanism, and the pawl engages the detent with the arm being positioned under the chord member.

7. The bone fixation assembly of claim 6 , wherein the chord member deflects the arm as the locking device is rotated to the locked position.

8. The bone fixation assembly of claim 6 , wherein the locking device cap engagement surface has a contour that matches a head of the fixation mechanism.

9. The bone fixation assembly of claim 6 , wherein the detent is generally centered on the chord portion.

10. The bone fixation assembly of claim 6 , where the cap has a protuberance that engages the chord member when the locking device is in the locked position.

11. The bone fixation assembly of claim 6 , where in the cap has chord portions defining a minor diameter of the cap.

12. The bone fixation assembly of claim 11 , wherein the chord portions provide clearance for the cap in the secondary socket first bore adjacent to the arcuate cut-out when in the locking device is in the unlocked position.

13. The bone fixation assembly of claim 6 , wherein the cap has a clearance area to provide clearance for the cap in the secondary socket first bore adjacent to the chord member when the locking device is rotated to the locked position.

14. The bone fixation assembly of claim 6 , wherein the secondary socket second bore extends from the shoulder to the receiving member second side.

15. The bone fixation assembly of claim 14 , wherein the stem extends through the secondary socket second bore.

16. The bone fixation assembly of claim 15 , wherein the stem has a distal end with a diameter larger than the secondary socket second bore.

17. The bone fixation assembly of claim 6 , wherein the receiving member primary socket has a spherical contour.

18. The bone fixation assembly of claim 6 , wherein the receiving member is an anterior lumbar plate.

19. The bone fixation assembly of claim 6 , wherein the secondary socket chord member is generally opposite the arcuate cut-out.

20. The bone fixation assembly of claim 6 , wherein the arm extending from the cap is generally diametrically opposite the cap engagement surface.

21. The bone fixation assembly of claim 10 , where the protuberance has a flat positioned under the chord member when the locking device is in the locked position.