Polyaxial bone anchors with increased angulation
A polyaxial bone anchor has a locking element shaped and configured to allow an anchoring member (e.g., a screw or hook) to polyaxially rotate at large angles about a central axis of the bone anchor before compression locking the anchoring member within an anchor head.
1. A polyaxial bone anchor for attaching a rod to a bone, comprising:
an anchor head comprising a longitudinal bore having a top opening and a bottom opening, a generally U-shaped channel transverse to the longitudinal bore for receiving the rod, and a central axis extending through the top opening and the bottom opening;
a bushing configured to be retained within the anchor head, the bushing defining a spherical exterior surface which is defined by a first sphere and a spherical inner surface which is defined by a second sphere, the first sphere having a first spherical centerpoint and the second sphere having a second spherical centerpoint, wherein one of the first and second spherical centerpoints is closer than the other of the first and second spherical centerpoints to the top opening;
an anchor member having a first portion configured to be supported by the bushing relative to the anchor head, and a second portion that extends from the first portion and through the bottom opening; and
a fastener removably mountable to the anchor head to lock the position of the rod in the U-shaped channel and the position of the anchor member,
wherein when the first portion of the anchor member is supported relative to the anchor head, the anchor member is configured to angulate with respect to the anchor head about the first and second spherical centerpoints from a neutral position whereby the anchor member is oriented substantially along with the central axis, to an angulated position whereby the second portion of the anchor member is angularly offset from the central axis, such that the anchor member abuts the anchor head when the anchor member is in the angulated position.
2. The polyaxial bone anchor of claim 1 , further comprises a sleeve retained within the anchor head, the sleeve configured to receive a force from the fastener, such that the sleeve is pushed against the bushing so as to 1) lock the bushing from angulating with respect to the anchor head, and 2) lock the anchor member from angulating with respect to the bushing and the anchor head.
3. The polyaxial bone anchor of claim 2 , wherein the sleeve has a generally U-shaped channel configured to receive the rod, the sleeve retained within the anchor head.
4. The polyaxial bone anchor of claim 3 , wherein the bushing has an upper end and a lower end opposed to the upper end, wherein the sleeve has a bottom surface that engages a portion of the upper end of the bushing, the bottom surface having a contour that matches the portion of the upper end of the bushing, such that the sleeve crush-locks the bushing around the first portion.
5. The polyaxial bone anchor of claim 4 , wherein the bottom surface of the sleeve is arcuate so as to mate with the upper end of the bushing.
6. The polyaxial bone anchor of claim 4 , wherein the bottom surface of the sleeve is tapered upward and inward with respect to the sleeve.
7. The polyaxial bone anchor of claim 1 , wherein the second portion of the anchor member abuts the anchor head when the anchor member is in the angulated position.
8. The polyaxial bone anchor of claim 1 , wherein the first portion defines an upper surface that is closer to the top opening than any other surface of the first portion, and the polyaxial bone anchor is devoid of structure that impedes movement of the upper surface during an entirety of the angulation of the anchor member between the neutral position and the angulated position.
9. The polyaxial bone anchor of claim 1 , wherein the anchor member is configured to non-elastically angulate about each of the first and second spherical centerpoints.
10. The polyaxial bone anchor of claim 1 , wherein the first and second spherical centerpoints are aligned along the central axis.
11. The polyaxial bone anchor of claim 10 , wherein the second spherical centerpoint is closer to the bottom opening than is the first spherical centerpoint.
12. The polyaxial bone anchor of claim 1 , wherein the angulation of the anchor member about the first and second spherical centerpoints forms a cone of about 100 degrees.
13. The polyaxial bone anchor of claim 1 , wherein the spherical exterior surface of the bushing is configured to angulate along the anchor head about the first spherical centerpoint, and wherein the anchor member is configured to angulate along the spherical inner surface of the bushing about the second spherical centerpoint.
14. The polyaxial bone anchor of claim 13 , wherein the spherical inner surface defines an internal opening that receives the first portion of the anchor member.
15. The polyaxial bone anchor of claim 1 , wherein the anchor member is elongate along an anchor member axis and the second portion is spaced from the first portion in a distal direction, wherein the first portion has a first portion cross-sectional dimension that is perpendicular to the anchor member axis, the first portion cross-sectional dimension being greater than or equal to a cross-sectional dimension of an entirety of the anchor member that is spaced from the first portion in the distal direction.
16. The polyaxial bone anchor of claim 1 , wherein the bushing has an upper end and a lower end opposed to the upper end, and the U-shaped channel is spaced from the upper end of the bushing.