Supraspinatus tendon load during abduction is dependent on the size of the critical shoulder angle: A biomechanical analysis

Shoulders with supraspinatus (SSP) tears are associated with significantly larger critical shoulder angles (CSA) compared to disease‐free shoulders. We hypothesized that larger CSAs increase the ratio of joint shear to joint compression forces (defined as “instability ratio”), requiring substantially increased compensatory supraspinatus loads. A shoulder simulator with simulated deltoid, supraspinatus, infraspinatus/teres minor, and subscapularis musculotendinous units was constructed. The model was configured to represent either a normal CSA of 33° or a CSA characteristic of shoulders with rotator cuff tears (38°), and the components of the joint forces were measured. The instability ratio increased for the 38° CSA compared with the control CSA (33°) for a range of motion between 6° to 61° of thoracohumeral abduction with the largest differences in instability observed between 33° and 37° of elevation. In this range, SSP force had to be increased by 13–33% (15–23 N) to stabilize the arm in space. Our results support the concept that a high CSA can induce SSP overload particularly at low degrees of active abduction. © 2014 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 32:952–957, 2014.