With conventional designs of Schanz screws, with any additional load on the screw in a radial direction, the driving pressure is reinforced on one side while being relieved on the other because of the absence of pre-stress. This can lead to a loosening of the screw and subsequent bone resorption. (cf. S. M. Perren, R. Ganz, and A. Ruter, Med. Orthop. Technik, Heft 1/75, 95 Jahrg. (1975) pp.6-10). In addition, with conventional Schanz screws, two drilling procedures are necessary, one with a drill for the minor diameter d.sub.1 and the other with a drill for the major diameter d.sub.2. If one tries to simplify the procedure by forming a single, undersized hole, micro and macro fractures occur in the bone material.
In conventional Schanz screws the transition between the shank and thread segments, both of which have the same outside diameter, is formed as a channel that either punches the bone material along its leading edge or cuts it along its back edge. There is not merely a radial spreading of the bone material.
USPTO Backgrounds
USPTO Backgrounds