Bottom-loading bone anchor assemblies and methods

1. A bone anchor assembly, comprising:

a shank having a distal threaded portion and a cylindrical proximal head portion;

a single piece non-expandable ball having a spherical exterior surface and a central lumen sized to receive the head portion of the shank;

a clip configured to be engaged between the head portion and the ball such that the clip is effective to lock the ball in engagement with the shank; and

a receiver member having an aperture formed in a distal end thereof sized such that the head portion of the shank can pass through the aperture and such that the ball cannot pass through the aperture,

wherein the head portion of the shank includes a tapered blank that is reversibly mated to the proximal end of the head portion with a mechanical engagement mechanism,

wherein the tapered blank protrudes proximally from the non-expandable ball, and

wherein the head portion has a first annular groove formed therein and the central lumen includes a second annular groove formed therein, and wherein the clip is configured to extend into the first annular groove and the second annular groove when the clip is engaged between the head portion and the ball.

2. The bone anchor assembly of claim 1 , wherein a major diameter of the distal threaded portion of the shank is greater than a diameter of the aperture formed in the receiver member.

3. The bone anchor assembly of claim 1 , wherein the central lumen of the ball is cylindrical.

4. The bone anchor assembly of claim 1 , wherein a height of the first groove and a height of the second groove are substantially the same as a height of the clip such that, when mated, the ball and the shank are locked in a fixed axial position relative to one another.

5. The bone anchor assembly of claim 1 , wherein the clip comprises a circlip.

6. The bone anchor assembly of claim 1 , wherein the clip comprises a C-shaped band having a radial cut formed therein, a first side of the radial cut having a tab configured to interlock with a complementary recess formed in a second side of the radial cut.

7. The bone anchor assembly of claim 1 , wherein the clip is a continuous ring formed from an expandable material.

8. The bone anchor assembly of claim 1 , wherein at least one of the first and second grooves has a depth that is equal to or greater than a width of the clip.

9. The bone anchor assembly of claim 1 , wherein the tapered blank of the head portion tapers towards its proximal end to provide a lead-in surface for expanding the clip as the clip is slid distally over the head portion and into the first groove during assembly.

10. The bone anchor assembly of claim 1 , wherein when the ball is mated to the shank, the tapered blank and the exterior surface of the ball each define sections of a common sphere.

11. The bone anchor assembly of claim 1 , wherein the head portion includes a driving interface formed therein.

12. The bone anchor assembly of claim 11 , wherein the first annular groove intersects with the driving interface and wherein the clip is configured to bear against instruments inserted into the driving interface to retain the shank on such instruments.

13. The bone anchor assembly of claim 1 , wherein the non-expandable ball is a truncated sphere.

14. The bone anchor assembly of claim 1 , wherein the spherical exterior surface of the non-expandable ball extends a full 360° around a central axis extending through the central lumen.

15. The bone anchor assembly of claim 1 , wherein the non-expandable ball has a cross section in the shape of a circle.