Neer Award 2006: Biomechanical assessment of inferior tuberosity placement during hemiarthroplasty for four-part proximal humeral fractures.

Tuberosity malpositioning commonly occurs and is associated with a decline in clinical function after prosthetic shoulder reconstruction for proximal humeral fractures. This study assesses the biomechanical effects of inferior tuberosity position on glenohumeral joint forces and humeral head position at multiple positions. Eight fresh-frozen cadaveric shoulders were tested. Hemiarthroplasty was performed with preservation of anatomic tuberosity height and with 10 mm and 20 mm of inferior tuberosity displacement. The rotator cuff, deltoid, pectoralis major, and latissimus dorsi muscles were statically loaded. Contact forces and humeral head position were recorded within a functional range of motion. Glenohumeral joint forces shifted significantly superiorly (P < .05) at 30 degrees of abduction after both 10 mm and 20 mm of tuberosity displacement. At 60 degrees of glenohumeral abduction, glenohumeral joint forces remained significantly altered after tuberosity displacement of 10 mm and 20 mm compared with the intact height (P < .005). This study demonstrates that, during hemiarthroplasty performed for proximal humeral fractures, malpositioning the tuberosities inferiorly results in significant superior glenohumeral joint force displacement. These findings suggest that the mechanical advantage of the shoulder abductor muscles is compromised with inferior tuberosity malpositioning and may help to explain inferior functional results seen in these patients.