Contact forces in the subacromial space: effects of scapular orientation.

The purpose of this study was to examine the effects of scapular orientation on clearance in the subacromial space. Eight glenohumeral joints from fresh-frozen human cadavers were secured to an Instron mechanical testing machine via a custom-made translation table. Forces were applied to simulate rotator cuff and deltoid contraction. Superior translation of the glenohumeral joint was simulated, and the distance before the development of significant subacromial contact force was measured. Specimens were tested at varying orientations of scapular posterior tilting, upward rotation, and external rotation. Results demonstrated no significant effect of posterior tilting and external rotation. Subacromial clearance was found to decrease with an increase in upward rotation, which is contrary to what was expected. These results suggest that changes in upward rotation observed in patients with impingement syndrome may serve to open the subacromial space. Future work needs to focus on confirming these results and determining contact location.