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Osteoarthritis (OA) of the glenohumeral joint constitutes the most frequent indication for nontraumatic shoulder joint replacement. Recently, a small critical shoulder angle (CSA) was found to be associated with a high prevalence of OA. This study aims to verify the hypothesis that a small CSA leads to higher glenohumeral joint reaction forces during activities of daily living than a normal CSA. A shoulder simulator with simulated deltoid (DLT), supraspinatus (SSP), infraspinatus/teres minor (ISP/TM), and subscapularis (SSC) musculotendinous units was constructed. The DLT wrapping on the humerus was simulated using a pulley that could be horizontally adjusted to simulate the 28° CSA found in OA or the 33° CSA found in disease-free shoulders. Over a range of motion between 6° and 82° of thoracohumeral abduction joint forces were measured using a six-axis load cell. An OA-associated CSA yielded higher net joint reaction forces than a normal CSA over the entire range of motion. The maximum difference of 26.4 N (8.5%) was found at 55° of thoracohumeral abduction. Our model thus suggests that a CSA typical for OA predisposes the glenohumeral joint to higher joint reaction forces and could plausibly play a role in joint overloading and development of OA. © 2015 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:1047-1052, 2016.

Shoulders with rotator cuff tears (RCT) tears are associated with significantly larger critical shoulder angles (CSA) (RCT CSA = 38.2°) than shoulders without RCT (CSA = 32.9°). We hypothesized that larger CSAs increase the ratio of glenohumeral joint shear to joint compression forces, requiring substantially increased compensatory supraspinatus loads to stabilize the arm in abduction. A previously established three dimensional (3D) finite element (FE) model was used. Two acromion shapes mimicked the mean CSA of 38.2° found in patients with RCT and that of a normal CSA (32.9°). In a first step, the moment arms for each muscle segment were obtained for 21 different thoracohumeral abduction angles to simulate a quasi-static abduction in the scapular plane. In a second step, the muscle forces were calculated by minimizing the range of muscle stresses able to compensate an external joint moment caused by the arm weight. If the joint became unstable, additional force was applied by the rotator cuff muscles to restore joint stability. The model showed a higher joint shear to joint compressive force for the RCT CSA (38.2°) for thoracohumeral abduction angles between 40° and 90° with a peak difference of 23% at 50° of abduction. To achieve stability in this case additional rotator cuff forces exceeding physiological values were required. Our results document that a higher CSA tends to destabilize the glenohumeral joint such that higher than normal supraspinatus forces are required to maintain modeled stability during active abduction. This lends strong support to the concept that a high CSA can induce supraspinatus (SSP) overload. © 2015 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:961-968, 2016.

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.