Uni-planer bone fixation assembly
View Patent ↗A bone fixation assembly includes a plurality of bone fixation elements that each include a bone anchor configured to be implanted into underlying bone, such as a vertebra. Each bone anchor is received in an anchor seat, and the anchor seats are joined by a fixation rod so as to operatively couple and fix the position and orientation of the vertebrae relative to each other. The bone anchor is free to rotate relative to the anchor seat, and is also free to pivot in a desired direction relative to the anchor seat.
1. A bone fixation subassembly configured to receive a fixation rod and a locking cap, the bone fixation subassembly comprising:
an anchor seat including an anchor seat body extending along a central axis and defining an upper end and a lower end, wherein the upper end comprises a pair of opposing fixation rod-receiving gaps therebetween that are spaced along a longitudinal axis, and a bore disposed between the rod-receiving gaps;
a collet including a collet body configured to be disposed in the anchor seat, the collet body defining an upper end and an expandable lower end, the expandable lower end comprising a plurality of fingers separated by slots; and
a bone anchor that extends along an axis of rotation and is configured to attach to the collet such that the bone anchor is permitted to rotate about the axis of rotation relative to the collet, the bone anchor is further permitted to pivot relative to the collet along a desired plane, and the bone anchor is prevented from pivoting in another plane that includes the central axis and is angularly offset with respect to the desired plane.
2. The bone fixation subassembly of claim 1 , wherein the bone anchor can pivot only along the desired plane about an axis that is perpendicular to the longitudinal axis.
3. The bone fixation subassembly of claim 1 , further comprising a guide configured to engage an abutment surface of the bone anchor so as to direct pivotal movement of the bone anchor in the desired plane.
4. The bone fixation subassembly of claim 3 , wherein the guide is configured to abut opposing abutment surfaces of the bone anchor that are separated along an axis that is perpendicular to the longitudinal axis.
5. The bone fixation subassembly of claim 3 , wherein the lower end of the anchor seat body defines a pair of inner guide surfaces that define the guide.
6. The bone fixation subassembly of claim 5 , wherein the inner guide surfaces extend along the desired plane.
7. The bone fixation subassembly of claim 5 , wherein the bone anchor defines a head, a shaft, and a neck that extends between the head and the shaft, the neck having a neck diameter that is less than a diameter of the head, and the opposing guide surfaces define a distance therebetween that is substantially equal to the neck diameter, such that the guide surfaces are configured to abut opposing abutment surfaces of the neck.
8. The bone fixation subassembly of claim 7 , wherein the distance is less than the diameter of the head.
9. The bone fixation subassembly of claim 5 , wherein the lower end defines at least one lower end surface that extends in a direction substantially perpendicular to the guide surfaces, the at least one lower end surface being upwardly displaced with respect to the guide surfaces so as to provide a stop that limits pivoting of the bone anchor along the desired plane.
10. The bone fixation subassembly of claim 5 , wherein the lower end defines a pair of spaced lower end surfaces that extend in a direction substantially perpendicular to the guide surfaces, the lower end surfaces being upwardly displaced with respect to the guide surfaces so as to provide stops that limit pivoting of the bone anchor along the desired plane on either side of the central axis.
11. The bone fixation subassembly of claim 1 , wherein the plurality of fingers of the collet include a first plurality of fingers disposed on either side of the desired plane and a second plurality of fingers extending between adjacent ones of the first plurality of fingers, the second plurality of fingers having lower ends that terminate above lower ends of the first plurality of fingers.
12. The bone fixation subassembly of claim 1 , wherein the collet body defines at least one locking lip extending radially out from the collet body, and the anchor seat body defines at least one corresponding recess configured to receive the at least one locking lip so as to prevent the collet from backing out through the upper end of the anchor seat.
13. The bone fixation subassembly of claim 1 , wherein the upper end of the collet body includes a pair of radially opposing, upwardly facing seat portions and a pair of flanges extending from the upper end of the collet body at locations radially between the seat portions, wherein the seat portions are configured to support at least a portion of the fixation rod between the flanges.
14. The bone fixation subassembly of claim 1 , wherein the collet body defines an axial bore extending through the upper and expandable lower ends of the collet body.