1. A digitizer adapter assembly comprising: a stem extending between opposite ends along a longitudinal axis; a target coupled to one end of the stem, the target having at least three markers disposed along a target surface; a spherical member fixedly secured to the opposite end of the stem relative to the target, the spherical member being made from a ferrous material; and a ring adapted to be fixedly secured to an outer surface of an object to be measured, the ring being magnetically charged to attract the spherical member and maintain the position of the target relative to the object to be measured.