Compensatory Load Sharing by Residual Rotator Cuff Subregions Preserves Glenohumeral Mechanics in Partial and Massive Tears

BACKGROUND: Rotator cuff (RC) tears are common shoulder injuries that cause pain, dysfunction, and abnormal humeral head translation. Balanced force couples in the transverse and coronal planes help to maintain normal glenohumeral mechanics. Although clinical and biomechanical studies have suggested that compensatory activation of residual RC muscles preserves function, the contribution of individual RC subregions to glenohumeral contact mechanics and humeral head translations across progressively increasing tear sizes remains unclear. METHODS: Eight fresh-frozen male cadaveric shoulders (mean age, 56 years; 6 Caucasian; 2 Black) were dissected to isolate RC muscle subregions, and 4 progressive RC tear models were created: Tear I (supraspinatus [SSP] + superior region of the infraspinatus [ISP]), Tear II (SSP + complete ISP), Tear III (SSP + ISP + superior one-third of the subscapularis [SSC]), and Tear IV (SSP + ISP + superior one-third of the SSC + coracohumeral ligament). Each model underwent 3 loading conditions: loaded (as in the intact state), unloaded (i.e., unloading of the torn regions), and compensatory (i.e., increased loading of the remaining subregions). Humeral head translations and glenohumeral contact force, area, and pressure were measured at 10° of abduction with neutral rotation. RESULTS: Unloaded conditions significantly increased superior and posterior humeral head translations and reduced contact force and area in most models, particularly in Tears III and IV. Compensatory loading by residual RC subregions reduced superior translation by 34% to 44% and posterior translation by 60% to 68%, restoring the humeral head center to within 0.1 to 1.7 mm of its position in the intact condition. Contact forces and areas partially recovered under compensatory loading; however, contact pressure remained elevated in the largest tear model. CONCLUSIONS: Residual RC subregions can partially restore humeral head centering and glenohumeral contact mechanics in progressive RC tears. However, compensation is limited in advanced tear states, highlighting the potential need for surgical intervention to restore force-couple integrity in the transverse plane. CLINICAL RELEVANCE: These findings support targeted strengthening of the posterior cuff in patients with partial or early-stage massive RC tears to help maintain joint congruency, minimize abnormal glenohumeral contact mechanics and humeral head translation, and potentially delay the progression to cuff tear arthropathy. Surgical repair, particularly of the SSC, may be required in advanced tears to fully restore force coupling and load distribution.