Coronal plane geometry of the proximal humerus relevant to prosthetic arthroplasty.

Most prosthetic designs for the replacement of the proximal humerus have one head shaft angle and several head sizes allowing limited replication of the anatomy. The necessity for press fit designs to follow the reamed canal adds an additional constraint. To better understand these limitations, this study measures proximal humeral anatomy in relationship to the surgically reamed canal. Measurements were made from radiographs of 21 humeri. Radius of curvature averaged 25.3 mm, head height 18.5 mm, head-shaft angle 40.7 degrees, and offset 9.7 mm. Longer humeri had proportionally larger shafts and larger heads (r = 0.7, p < 0.0005). Proximal humeral anatomy is extremely variable. The relatively fixed geometry of most prosthetic systems limits the surgeon's ability to replicate the original anatomy. As a generalization for existing press fit, modular systems, the surgeon must either position a similarly sized prosthetic articular surface superomedial to that of the original anatomy or use one with less articular surface area.