Geometric analysis of selected press fit prosthetic systems for proximal humeral replacement.

The ability of a prosthetic system to replicate a wide range of normal anatomy appears dependent in part to its capacity to produce variable prosthetic geometries. Several modern designs have recently been developed in order to provide multiple prosthetic options. The purpose of this study was to compare the geometry of select press fit prosthetic systems in terms of their ability to match normal three-dimensional geometry of the proximal humerus. The anatomy of 60 humeral specimens from 30 cadavers known from CT data and computer-aided design (CAD) analysis was compared to a 1996 database of four conventional shoulder prosthetic systems (fixed inclination angle, one medial-lateral offset position per head) and one so-called anatomic system (variable inclination angles, multiple head offset positions). The prosthetic system with greatest geometric options allowed for a significantly better replication of the anatomy. Average displacement of the center of rotation was 2.1 mm compared to 9.7 mm for the other systems combined. Reduction in surface arc was 12 degrees compared to 32 degrees. The most problematic feature of conventional prosthetic systems in terms of replicating normal humeral anatomy is the gap created by the prosthetic collar and Morse taper.