Joint and bone fixation
Eccentrically shaped bone fixation implants interact with bone fragments or joints to compress the bone fragments or joint bones together. The eccentrically shaped bone fixation implants can have longer and shorter portions along an axis and a plurality of teeth with opposing beveled surfaces in different configurations to vary both the direction and amplitude of the compressive forces applied to the bone fragments or joint bones. Instrumentation for implanting and orienting the eccentrically shaped bone fixation implants are also disclosed herein.
1. A bone fixation implant assembly, comprising:
a fastener having a partially spherical fastener head having a head diameter and a shaft extending along a longitudinal axis; and
an eccentrically shaped monolithic cap comprising:
a first lobe;
a second lobe adjoining the first lobe;
a through aperture extending through the eccentrically shaped monolithic cap and configured to receive the fastener, the through aperture is offset from a geometric center of the cap, the through aperture comprising an aperture diameter and an aperture geometric center, the aperture exposed to a spherically shaped capsule comprising a lip having a lip diameter, wherein the aperture diameter is larger than the head diameter, the head diameter is larger than the lip diameter, and the fastener head is polyaxially retained within the spherically shaped capsule by the lip; and
a pair of slots recessed into the cap on opposite sides of the aperture for interaction with a guide tool to hold the cap at a specific orientation; wherein:
a first axis intersects an outer edge of the first lobe, an outer edge of the second lobe, and the aperture geometric center, the first axis defining an axis of maximum length of the cap, a second axis intersects the first axis at the aperture geometric center and extends perpendicular to the first axis, and the aperture defines an aperture axis perpendicular to the first and second axes;
the first lobe of the eccentrically shaped cap is longer, when measured from the aperture geometric center to the outer edge of the first lobe along the first axis, than the second lobe of the eccentrically shaped cap, when measured from the aperture geometric center to the outer edge of the second lobe along the first axis;
the first lobe of the eccentrically shaped cap comprises at least one width, wherein the at least one width is larger, when measured in the direction of the second axis perpendicular to the first axis, than a largest width of the second lobe, when measured in the direction of the second axis perpendicular to the first axis;
the cap comprises a bone engaging side, each of the first and second lobes comprises a planar bone engaging surface on the bone engaging side and an upper surface opposite the planar bone engaging surface, wherein the planar bone engaging surface of the first lobe abuts and is coplanar with the planar bone engaging surface of the second lobe, providing a uniplanar surface on the bone engaging side of the cap, the uniplanar surface extending between the outer edges of the first and second lobes;
the spherically shaped capsule is contained entirely between the uniplanar bone engaging surface of the cap and the upper surface of the cap; and
a plurality of teeth projecting from the first and second lobes on the bone engaging side continuous with the outer edges of the first and second lobes, said plurality of teeth configured to pierce into bone, the plurality of teeth comprising beveled surfaces which oppose one another and at least partially diverge away from each other moving away from the bone engaging side of the cap toward free ends of the teeth, and an additional tooth projecting from the planar bone engaging surface of the first lobe between one of the plurality of teeth and the aperture.
2. The assembly of claim 1 , wherein the pair of slots extend into the cap intermediate the bone engaging side of the cap, and the upper surface, wherein the slot is shaped to receive a boss of the guide tool, in response to insertion of the boss into the slot along a direction non-parallel to the aperture to couple the cap to the guide tool, to facilitate positioning of the cap through the use of the guide tool.
3. The assembly of claim 1 , wherein the upper surface of the first lobe of the eccentrically shaped cap tapers toward the planar bone engaging surface.
4. The assembly of claim 1 , wherein the aperture is intermediate the first lobe and the second lobe.
5. The assembly of claim 1 , wherein the additional tooth is located in an interior of the first lobe, and the plurality of teeth surround the additional tooth.
6. The assembly of claim 1 , wherein the beveled surfaces of the plurality of teeth are flat.
7. The assembly of claim 1 , wherein the fastener head is received within the capsule, and wherein the fastener is rotatable within the aperture and concentrically pivotable along its longitudinal axis.
8. The assembly of claim 3 , wherein the upper surface tapers toward the planar bone engaging surface along the first lobe moving away from the second lobe toward the outer edge of the first lobe.