Arthroplasty implants
Arthroplasty implants replace an articulating surface of a bone joint such as bone joints of the foot, and is characterized by a three-component device comprising a bone screw, a button, and a cap. The button incorporates knurling on its distal end to fixate onto bone, a center hole, a spherical pocket to mate with the bone screw for stabilization, and a proximal retention structure to mate with the cap. The bone screw incorporates distal bone (cortical) threads or threading, a spherical head, and a drive structure used to insert the bone screw through the hole of the button and into the bone (e.g. a distal segment of the first metatarsal). The cap incorporates a distal mating structure that retains the cap onto the button, a proximal smooth spherical head that mates with the distal surface of the bone (e.g. a phalange bone), and a tang that prevents rotation after implantation.
1. An implant for replacing an articulating surface of a bone joint, the implant comprising:
a bone screw having a head with a top and a rounded underside, a shaft extending from the rounded underside of the head, bone threading on an outside surface of the shaft, and a drive structure in the top of the head configured to receive a drive tool for inserting the bone screw into bone of the bone joint;
a button comprising:
a proximal end and a distal end, the distal end configured to be received on bone of the bone joint and having an undersurface,
a first pocket extending from the distal end and having a bore configured to allow the shaft of the bone screw to pass through but retain the underside of the head of the bone screw, and
a retention structure comprising a ledge extending radially inwardly at the proximal end of the button to define an annular groove and an undercut on an undersurface of the ledge; and
a cap comprising:
a head having a distal surface and a curved proximal articulation surface, the curved proximal articulation surface configured to articulate with a bone joint, and
an annular collar extending from the distal surface and having a mating structure, mating structure includes a rib extending radially outwardly from the annular collar;
wherein rib of the mating structure engages the undercut of the retention structure by a snap engagement, whereby the cap is concentrically retained on the button;
wherein the button defines a first outside lateral dimension and the head of the cap defines a second outside lateral dimension larger than the first outside lateral dimension.
2. The implant of claim 1 , wherein:
the undercut and the rib cooperate to prevent the cap from separating from the button once the cap has been received on the button.
3. The implant of claim 2 , wherein:
the undercut of the button is formed by a radially inwardly extending flange; and
the rib of the cap is formed by a radially outwardly extending flange.
4. The implant of claim 3 , wherein:
the inwardly extending flange of the undercut comprises a first beveled surface;
the outwardly extending flange of the rib slides across the beveled surface prior to engagement between the rib and the undercut.
5. The implant of claim 4 , wherein the outwardly extending flange of the rib comprises a second beveled surface that slides across the first beveled surface of the inwardly extending flange during engagement between the rib and the undercut.
6. The implant of claim 1 , wherein the bone threading of the bone screw comprises cortical bone threading.
7. The implant of claim 1 , wherein the proximal surface of the cap is smooth.
8. The implant of claim 7 , wherein the proximal surface of the cap is also rounded.
9. The implant of claim 1 , wherein the first pocket of the button is spherical.
10. The implant of claim 1 , wherein the annular collar defines a second pocket which receives the head of the bone screw.
11. An implant for replacing an articulating surface of a joint of a small bone of the foot or hand, the implant comprising:
a bone screw having a head with a top and a spherical underside, a shaft extending from the spherical underside of the head, bone threading on an outside surface of the shaft, and a drive structure in the top of the head configured to receive a drive tool for inserting the bone screw into bone of the bone joint;
an annular button comprising:
a proximal end and a distal end, the distal end configured to be received on bone of the bone joint,
a bone screw pocket extending from the distal end and having a bore configured to allow the shaft of the bone screw to pass through but retain the underside of the spherical head of the bone screw, and
a retention structure comprising a ledge extending radially inwardly at the proximal end of the annular button to define an annular groove and an undercut on an undersurface of the ledge; and
a cap comprising:
a distal end and a proximal end, the proximal end having a curved proximal surface configured to articulate with an articulation surface of the bone joint, and
an annular collar extending from the distal end of the cap, the annular collar having a pocket and a mating structure, wherein the pocket receives at least a portion of the head of the bone screw and the mating structure includes a rib extending radially outwardly from the annular collar;
wherein the rib on the mating structure engages the undercut of the retention structure by a snap-fit engagement, whereby the cap is retained on the annular button.
12. The implant of claim 11 , wherein:
the ledge on the annular button comprises a first annular beveling on an upper edge of the ledge;
the rib on the cap comprises a second annular beveling on an underside edge of the rib.
13. The implant of claim 12 , wherein:
the first annular beveling and the second annular beveling cooperate during engagement between the ledge and the rib;
the undercut on the annular button and the rib concentrically retains the cap on the annular button and to prevent the cap from separating from the annular button once the cap has been received on the annular button.
14. The implant of claim 11 , wherein:
the cap comprises a tang; and
the tang is configured to prevent the cap from rotation of the annular button once the cap has been received on the button.
15. The implant of claim 11 , wherein the bone threading of the bone screw comprises cortical bone threading.
16. The implant of claim 11 , wherein the button defines a first lateral outside dimension and the head of the cap defines a second lateral outside dimension, wherein the second lateral outside dimension is larger than the first outside lateral dimension.