Dynamic fixation implant and method of use
Implants and related methods for achieving dynamic fixation are disclosed. The implant comprises a head portion at a proximal end of the implant comprising external threads and a first axial through hole, and an anchor portion extending from the head portion at a distal end of the implant comprising external threads and a second axial through hole in communication with the first axial through hole. The implant further comprises a flexible constraint member extending within the first and second axial through holes comprising a first end portion coupled to the head portion and a second end portion coupled to the anchor portion. At least one of the head portion and the anchor portions forms a breakaway portion with aligned internal and external circumferential grooves configured to concentrate stress thereat such that the breakaway portion fractures via forces acting on the implant to separate the head and anchor portions.
1. An implant, comprising:
a head portion at a proximal end of the implant, comprising external threads and a first axial through hole;
an anchor portion extending from the head portion at a distal end of the implant comprising external threads and a second axial through hole in communication with the first axial through hole; and
a flexible constraint member extending within the first and second axial through holes comprising a first end portion coupled to the head portion and a second end portion coupled to the anchor portion,
wherein at least one of the head portion and the anchor portions forms a breakaway portion configured to concentrate stress thereat such that the implant fractures at the breakaway portion via forces acting on the implant to separate the head and anchor portions, and
wherein the breakaway portion comprises an external circumferential groove and an internal circumferential groove axially aligned with the external groove, the internal circumferential groove formed between a radially-extending internal end surface portion of the head portion and a radially-extending internal end surface portion of the anchor portion, and
wherein the head portion and the anchor portion are separate and distinct components that are welded together at a weld zone that is axially adjacent to the external and internal circumferential grooves such that the weld zone is axially spaced from the radially-extending internal end surface portion of the head portion and the radially-extending internal end surface portion of the anchor portion.
2. The implant according to claim 1 , wherein the internal end surface portion of the head portion comprises a radially-extending internal arcuate end surface portion of the head portion, and the internal end surface portion of the anchor portion comprises a radially-extending internal arcuate end surface portion of the anchor portion.
3. The implant according to claim 1 , wherein the internal end surface portion of the head portion comprises a radially-extending internal beveled end surface portion of the head portion, and the internal end surface portion of the anchor portion comprises a radially-extending internal beveled end surface portion of the anchor portion.
4. The implant according to claim 1 , wherein a proximal end portion of the anchor portion comprises a coupling cavity and a distal end portion of the head portion comprises a coupling projection corresponding to the coupling cavity, the coupling projection being received within the coupling cavity.
5. The implant according to claim 4 , wherein a bottom portion of the coupling cavity defines the internal end surface portion of the anchor portion, and a tip portion of the coupling projection defines the internal end surface portion of the head portion.
6. The implant according to claim 1 , wherein the flexible constraint member comprises an elastic suture loop.
7. The implant according to claim 1 , further comprising an anchor post member positioned within the second axial through hole and coupled with the flexible constraint member, wherein the anchor post member comprises an external groove, and wherein a proximal coupling portion of the anchor member is deformed into the external groove.
8. The implant according to claim 1 , wherein the head portion comprises:
a shaft portion with a first end and a second end;
a head extending from the first end of the shaft portion; and
a first breakaway coupling portion extending from the second end of the shaft portion.
9. The implant according to claim 8 , wherein a portion of the shaft portion of the head portion comprises external threads.
10. The implant according to claim 8 , wherein the head comprises a non-circular drive opening at an axial free end thereof, the non-circular drive opening forming a portion of the first axial through hole.
11. The implant according to claim 8 , wherein the first breakaway coupling portion comprises a coupling projection with an inner surface that defines the internal end surface portion of the head portion, the inner surface of the coupling projection forming a portion of the first axial through hole.
12. The implant according to claim 11 , wherein the first breakaway coupling portion further comprises a stop surface extending radially from an outer surface of the coupling projection and positioned axially between the internal end surface portion thereof and the head.
13. The implant according to claim 12 , wherein the anchor portion comprises a second breakaway coupling at a first end thereof comprising a coupling cavity with an inner bottom surface that defines the internal end surface portion of the anchor portion, the inner surface of the coupling projection forming a portion of the first axial through hole.
14. The implant according to claim 13 , wherein the coupling projection is mated within the coupling cavity.
15. The implant according to claim 14 , wherein the coupling projection and the coupling cavity comprise the weld zone such that they are welded together.
16. The implant according to claim 13 , wherein an end surface of the second breakaway coupling abuts the stop surface of the first breakaway coupling.
17. The implant according to claim 12 , wherein the anchor portion further comprises:
a shaft portion with a first end and a second end; and
a crimp portion extending from the second end, and
wherein the second breakaway coupling extends from the first end of the shaft portion.
18. The implant according to claim 17 , wherein a portion of the shaft portion of the anchor portion comprises external threads.
19. The implant according to claim 17 , wherein a proximal portion of the shaft portion of the anchor portion comprises a plurality of outer circumferentially arranged planar surfaces forming a proximal external drive feature.
20. The implant according to claim 1 , wherein the first axial through hole of the head portion comprises a first enlarged portion positioned proximate to a proximal end of the head portion and a second narrow portion positioned proximate to the breakaway portion, and further comprising a head post member positioned within the first enlarged portion of the first axial through hole and coupled to the first end portion of the flexible constraint member.
21. The implant according to claim 20 , further comprising at least one resilient member positioned within the first enlarged portion of the first axial through hole axially between the second narrow portion thereof and the head post member.
22. The implant according to claim 21 , wherein the second narrow portion, the at least one resilient member and the head post member are configured such that the at least one resilient member and the head post member are prevented from axially translating through the second narrow portion.
23. The implant according to claim 21 , wherein the at least one resilient member comprises at least one tube formed of thermoplastic urethane, polycarbonate urethane or a combination thereof.
24. The implant according to claim 1 , wherein the flexible constraint member comprises a suture loop.
25. The implant according to claim 1 , comprising a cannulated opening extending through an entire axial length of the implant.
26. The implant according to claim 1 , wherein a radial thickness of the breakaway portion extending between the internal and external circumferential grooves is a thinnest wall portion of the implant.
27. The implant according to claim 1 , wherein the breakaway portion comprises a medial portion of the implant that is void of external threads.
28. The implant according to claim 1 , wherein the internal circumferential groove comprises a space between the radially-extending internal end surface portion of the head portion and the radially-extending internal end surface portion of the anchor portion.