Effects of Tear-Associated Scapula Shape on the Stabilizing Potential of Glenohumeral Muscles

Glenohumeral muscles stabilize the humerus by providing compressive forces directed into the glenoid or by resisting opposing forces from larger muscles like the deltoid. Scapula shape is correlated to rotator cuff tear likelihood and previous research found that tear-associated shapes shift the supraspinatus’ line-of-action (LOA) anteriorly, potentially inhibiting its stabilizing potential. We predicted that the tear-associated scapula shape would cause an anterior shift in all glenohumeral muscles’ LOA. We expanded previous finite element models and computational scripts to compute the stability ratios of the teres major, teres minor, infraspinatus, supraspinatus and subscapularis on tear-associated and non-tear control scapulas. We identified insertion and attachment points for each muscle and created respective straight muscle meshes in Matlab. Then, we wrapped those meshes around the scapula and humerus surfaces and simulated an arm-raising motion in the open-source modelling package, Artisynth. In Matlab, we computed the muscle LOAs and their anterior-posterior (AP) and superior-inferior (SI) stability ratios, which describe how muscles tend to pull on the humerus relative to the glenoid. We found that the mean AP stability ratios of all five glenohumeral muscles were higher in the tear-associated scapula shape compared to the control shape, indicating an anterior shift in the muscles’ LOA. Aside from the teres minor, all other muscles exhibited slightly larger SI ratios for the tear-associated scapula shape, indicating a shift superiorly in their LOA. The tear-associated scapula shape may impact certain glenohumeral muscles’ ability to stabilize the humerus, placing higher demand on posterior fibres, subscapularis and teres major to maintain anterior stability. These findings indicate a potential injury mechanism caused by scapula shape and may help to inform rotator cuff tear prevention and rehabilitation. This method can also be applied to in vivo measurements of shoulder motions and bones to provide precise, patient-specific information on rotator cuff stabilizing potential.