Subtalar joint implant
A subtalar joint implant ( 20 ) is disclosed for replacing a natural subtalar joint existing between the talus ( 102 ) and calcaneum ( 104 ) bones of a patient. In an embodiment the implant comprises talar ( 22 ) and calcaneal ( 24 ) portions that are configured to slide over each other in the implanted state. The implant is shaped so that the sliding is constrained to trajectories that involve pivoting of the talar portion relative to the calcaneal portion about a pivot point ( 36 ) or a line ( 38 ) of pivot points.
1. A subtalar joint implant for replacing a natural subtalar joint existing between the talus and calcaneum bones of a patient, the implant comprising:
a talar portion configured to be rigidly attached to the talus; and
a calcaneal portion configured to be rigidly attached to the calcaneum, wherein, where upper and lower directions are defined relative to the orientation of the implant when implanted in a weight bearing or standing patient:
a first and a second region of a lower surface of the talar portion are configured to be in contact with respective first and second regions of an upper surface of the calcaneal portion during use;
the first regions are shaped so as to allow sliding of the first region of the lower surface of the talar portion over the first region of the upper surface of the calcaneal portion; and
the second regions are shaped so as to define a pivot point or line of pivot points and constrain said sliding of the first region of the lower surface of the talar portion over the first region of the upper surface of the calcaneal portion to trajectories that involve pivoting of the talar portion relative to the calcaneal portion about the pivot point or pivot points.
2. The implant of claim 1 , wherein the sliding of the first regions relative to each other is along a curved trajectory.
3. The implant of claim 1 , wherein the sliding of the first regions relative to each other constitutes rotation of the talar portion relative to the calcaneal portion about an axis that is fixed during a portion or all of an allowed range of sliding.
4. The implant of claim 3 , wherein, when viewed from above, said axis passes through said pivot point, passes through said line of pivot points, passes through a region adjacent to the pivot point that overlaps with one or both of the second regions, or passes through a region adjacent to said line of pivot points that overlaps with one or both of the second regions.
5. The implant of claim 1 , wherein one of the first regions of the lower and upper surfaces comprises a portion of an external surface of a frustum.
6. The implant of claim 5 , wherein the other of the first regions of the lower and upper surfaces comprises a portion of an internal surface of a frustum.
7. The implant of claim 1 , wherein one of the second regions comprises a ball-like protrusion and the other of the second regions comprises a cup-like depression or a channel with a rounded base.
8. The implant of claim 1 , wherein the talar portion comprises an upper component configured to be rigidly attached to the talus and a lower component that presents the lower surface of the talar portion, the lower component being formed from a material having a lower coefficient of friction than the upper component.
9. The implant of claim 8 , wherein the upper component is formed from metal and the lower component is formed from plastic or ceramic.
10. The implant of claim 8 , wherein the upper surface of the calcaneal portion is formed from a metal.
11. The implant of claim 1 , wherein the calcaneal portion comprises a lower component configured to be rigidly attached to the calcaneum and an upper component that presents the upper surface of the calcaneal portion, the upper component being formed from a material having a lower coefficient of friction than the lower component.
12. The implant of claim 11 , wherein the lower component is formed from metal and the upper component is formed from plastic or ceramic.
13. The implant of claim 11 , wherein the lower surface of the talar portion is formed from a metal.
14. The implant of claim 1 , wherein the talar portion or the calcaneal portion comprises one or more abutment members configured to restrict the sliding of the first regions relative to each other to within a predetermined range.
15. The implant according to claim 14 , wherein the sliding of the first regions relative to each other constitutes rotation of the talar portion relative to the calcaneal portion and the one or more abutment members are configured to restrict the rotation to a range of 40 degrees or less.
16. The implant of claim 14 , wherein the talar portion comprises at least one of the abutment members and the calcaneal portion is configured to be brought into contact with one of the abutment members of the talar portion at one or both extreme ends of the predetermined range of sliding of the first regions relative to each other.
17. The implant of claim 14 , wherein the calcaneal portion comprises at least one of the abutment members and the talar portion is configured to be brought into contact with one of the abutment members of the calcaneal portion at one or both extreme ends of the predetermined range of sliding of the first regions relative to each other.
18. The implant of claim 1 , wherein the talar and calcaneal portions are configured to be implanted in such a way that, when viewed from above, the first regions overlap with a region where the posterior facet of the subtalar joint was located prior to implantation of the implant.
19. The implant of claim 1 , wherein the talar and calcaneal portions are configured to be implanted in such a way that, when viewed from above, the second regions overlap with a region where the middle facet of the subtalar joint was located prior to implantation of the implant.
20. A method of implanting a subtalar joint implant for replacing a natural subtalar joint existing between the talus and calcaneum bones of a patient,
wherein the implant comprises:
a talar portion configured to be rigidly attached to the talus; and
a calcaneal portion configured to be rigidly attached to the calcaneum, wherein, where upper and lower directions are defined relative to the orientation of the implant when implanted in a weight bearing or standing patient:
a first and a second region of a lower surface of the talar portion are configured to be in contact with respective first and second regions of an upper surface of the calcaneal portion during use;
the first regions of the lower and upper surfaces are shaped so as to allow sliding of the first region of the lower surface over the first region of the upper surface; and
the second regions of the lower and upper surfaces are shaped so as to define a pivot point or line of pivot points and constrain said sliding of the first region of the lower surface over the first region of the upper surface to trajectories that involve pivoting of the talar portion relative to the calcaneal portion about the pivot point or pivot points,
wherein the method comprises:
preparing a space between the talus and calcaneum that is suitable for accommodating the implant;
implanting the implant in the space in such a way that, when viewed from above, the first regions overlap with a region where the posterior facet of the subtalar joint was located prior to implantation of the implant and the second regions overlap with a region where the middle facet of the subtalar joint was located prior to implantation of the implant.