Low profile polyaxial screw

A kit comprising:

• a spinal fixation element;
• a bone anchor assembly comprising
• bone anchor having a distal anchoring shaft and a proximal hollow hemispherical head defined by an outer convex surface and an inner concave surface, the head having a drive feature positioned between the outer convex surface and the inner concave surface,
• a rod-receiving member for receiving the spinal fixation element and the head of the bone anchor,
• a compression member positionable within the rod-receiving member and configured to engage the hollow head of the bone anchor, and
• a locking mechanism for selective locking the spinal fixation element relative to the bone anchor;
• and
• an instrument configured to engage the drive feature on the head.

A bone anchor assembly, comprising:

• a bone anchor having a distal shaft configured to engage bone and a hollow hemi-spherical proximal head defined by a convex outer surface and a concave inner surface, the convex outer surface and the concave inner surface being spaced apart to form a wall having a thickness, the thickness of the wall being approximately less than or equal to 33% of a radius of the convex outer surface;
• a receiving member for receiving a spinal fixation element and the head of the bone anchor;
• and
• a compression member positionable in the receiving member, the compression member having an upper portion including a groove to seat the spinal fixation element and a lower portion having a convex shape having a radius approximating a radius of the concave inner surface of the head of the bone anchor.