Implementing a Commercially Available Self-Locking Screw System in Additively Manufactured Medical Implants

Many medical implants require a fastening system to rigidly affix components. One such system for standard orthopedic implants is the PAX Polyaxial Locking System in which threaded screw heads are secured to the implant by cutting into holes of a tailored geometry. As most custom implants are fabricated by additive manufacturing (AM) techniques, investigating whether the PAX system can be printed along with the implant can facilitate its wider use and significantly decrease their cost. The geometry of PAX system was measured using an X-ray CT and recreated as a CAD for AM fabrication in stainless steel 316L and Ti6Al4V, two standard implant materials. The mechanical properties of the printed parts were altered using different heat treatments to receive various hardness of the material. This work characterizes the performance of a printed PAX system in terms of screw insertion and mechanical push-out force and compares it to the original system. It is found, that using AM 316L stainless steel may be utilized to produce a locking system in patient-specific additively manufactured implants.