Patent ID: 8617225

Claim:
A bone anchor comprising: a shaft that defines a distal end and a proximal end, the shaft extending along a longitudinal direction from the distal end to the proximal end, the shaft includes external threads that extend radially outward from the shaft at a location between the distal end and the proximal end such that the external threads define a first outer diameter; and a head connected to the proximal end of the shaft, the head including a body that defines an outer surface and a second outer diameter measured between opposed locations of the outer surface in a direction substantially perpendicular to the longitudinal direction, the head further including a spline drive mechanism that has a plurality of equidistantly spaced, longitudinally elongate recesses that extend into the body along a direction substantially perpendicular to the longitudinal direction such that each recess defines a pair of spaced side walls and a base extending between the pair of spaced side walls, such that the base and the side walls define a region configured to engage a mating driver instrument, at least one of the plurality of recesses defining a height measured from the base of the respective recess to a location of the body that is spaced from the base along a radial direction that is perpendicular to the longitudinal direction, and the height of the at least one of the plurality of recesses is different at a first location of the respective recess than a second location of the respective recess that is spaced from the first location along the longitudinal direction; wherein the first outer diameter is greater than the second outer diameter.