Pectoralis major insertional ratio in proximal humerus fractures: a method to reconstruct humeral head height in arthroplasty.

A key factor for a successful outcome after hemiarthroplasty for a 4-part proximal humerus fracture is accurately restoring humeral length. Our hypothesis was that the pectoralis major insertion is not at a constant distance on the humerus, as has been previously suggested, but varies depending on the length of the humerus, and our goal was to determine if a consistent ratio exists for the insertion as it relates to total humeral length. Thirty-eight cadaver arms were dissected to expose the pectoralis major insertion. Using a digital caliper, measurements were made from the top of the humeral head to the superior aspect of the pectoralis major insertion (HP), and from the pectoralis insertion to the lateral epicondyle (PL). The predictive ability of PL for HP was examined via regression, and the average prediction error was computed. The final predictive regression model had the following formula: pHP=0.2323xPL, where pHP is predicted HP. This equation had an average prediction error of 4.11 mm. The PL can be measured intraoperatively during hemiarthroplasty for proximal humerus fractures. The proportionality relationship can then be used to predict HP with an average prediction error <5 mm. This relationship may facilitate accurate intraoperative reconstruction of prosthetic head height and enhance existing techniques for assessment of implant positioning.