Locking compression members for use with bone anchor assemblies and methods
Various exemplary methods and devices are provided for fixing bone anchors to bone. In general, the methods and devices can allow for a bone anchor to be fixed to a bone at a desired angle to a surface of the bone. In an exemplary embodiment, a bone anchor assembly is provided that includes a bone anchor configured to engage bone, a receiver member for seating a head of the bone anchor, and a compression member for securing the bone anchor at fixed angle with respect to the receiver member when the compression member is seated within the receiver member. Corresponding engagement features of the compression member and the receiver member can be engaged in a secured configuration to inhibit or prevent removal of the compression cap from the receiver member and optionally to substantially prevent longitudinal and/or rotational movement of the compression member with respect to the receiver member.
1. A bone screw assembly, comprising:
a screw having a proximal head portion and a distal shank portion;
a receiver member having a polyaxial seat formed therein and configured to polyaxially seat the head portion of the screw;
a compression cap disposed within the receiver member and having a distal end configured to engage the head portion of the screw, the compression cap including opposed projections extending radially therefrom configured to mate with complementary recesses formed in the receiver member such that the compression cap is retained within the receiver member and exerts a frictional force on the head portion of the screw sufficient to prevent free movement of the screw relative to the receiver member while still allowing a user to polyaxially move the receiver member relative to the screw, wherein
at least one dimension of the recesses is slightly larger than a corresponding dimension of the projections to allow for some adjustment of the compression cap when the projections are seated in the recesses, and
the receiver member further includes ramped shoulders disposed proximal to the recesses, the ramped shoulders being angled such that an inner diameter of the receiver member decreases in a proximal to distal direction and such that the ramped shoulders engage and deform the projections as the compression cap is advanced distally into the receiver member;
wherein the compression cap further includes a lip disposed proximal to the projections and configured to engage the ramped shoulders of the receiver member when the projections are seated within the recesses.
2. The bone screw assembly of claim 1 , wherein the opposed projections and the complementary recesses are effective, when mated, to maintain the compression cap in a substantially fixed longitudinal position relative to the receiver member.
3. The bone screw assembly of claim 1 , wherein upon further distal advancement of the compression cap into the receiver member, the opposed projections snap into engagement with the complementary recesses such that the projections are no longer deformed.
4. The bone screw assembly of claim 1 , wherein an outside diameter of the compression cap is less than an inside diameter of the receiver member.
5. The bone screw assembly of claim 1 , wherein a distance between outer surfaces of the opposed projections is greater than the inside diameter of the receiver member.
6. The bone screw assembly of claim 1 , wherein a distal-facing surface of each projection is ramped to provide a lead-in surface geometry.
7. The bone screw assembly of claim 1 , wherein a proximal-facing surface of each projection and an inner superior surface of each complementary recess are planar such that, when mated, the compression cap is prevented from being decoupled from the receiver member.
8. The bone screw assembly of claim 1 , wherein the opposed projections and the complementary recesses are effective, when mated, to maintain the compression cap in a substantially fixed rotational position relative to the receiver member.
9. The bone screw assembly of claim 1 , wherein a width of each projection measured horizontally about the circumference of the compression cap is greater than a thickness of the opposed projection measured along a radial axis of the compression cap.
10. The bone screw assembly of claim 1 , wherein the receiver member further includes threads disposed proximal to the ramped shoulders and configured to engage a closure mechanism.
11. A bone screw assembly, comprising:
a screw having a proximal head portion and a distal shank portion;
a receiver member having a distal polyaxial seat configured to seat the head portion of the screw, opposed arms extending proximally from the polyaxial seat, recesses formed on interior surfaces of the opposed arms, and ramped surfaces formed proximally of the recesses on the interior surfaces of the opposed arms, the ramped surfaces being angled such that an inner diameter of the receiver member decreases in a proximal to distal direction; and
a compression member configured to be seated between the opposed arms of the receiver member and having a distal end configured to engage the head portion of the screw, opposed arms extending proximally from the distal end, wings formed on exterior surfaces of the opposed arms and configured to mate with the recesses in the receiver member, and flanges formed on proximal ends of the opposed arms and configured to mate with the ramped surfaces of the receiver member;
wherein, when the wings of the compression member are seated within the recesses of the receiver member, the compression member exerts a frictional force on the head portion of the screw sufficient to prevent free movement of the screw relative to the receiver member while still allowing a user to polyaxially move the receiver member relative to the screw.
12. The bone screw assembly of claim 11 , wherein once the wings are seated within the corresponding recesses, proximal-facing planar surfaces of the wings abut distal-facing planar surfaces of the recesses to prevent the compression member from being removed from the receiver member.
13. The bone screw assembly of claim 11 , wherein the wings are formed from a flexible material such that they are deformed as they pass over the ramped surfaces but return to an expanded configuration when seated in the recesses.