Scapular morphology and posterior shoulder stability: biomechanical evidence from an advanced cadaveric shoulder simulator

Background: Static posterior shoulder subluxation (SPSL) is associated with both glenoid retroversion and altered acromial morphology. Although abnormal glenoid anatomy has been considered a crucial etiological factor, the biomechanical role of acromial anatomy remains incompletely understood. Hypothesis: Combined acromial and glenoid malalignment would produce greater posterior humeral head translation than either deformity alone, and targeted corrections could restore posterior stability. Study Design: Controlled laboratory study. Methods: Six fresh-frozen cadaveric shoulders underwent testing in a 6 degrees of freedom, 8-muscle actuated ex vivo cadaveric simulator. Seven conditions were tested: (1) intact, (2) posterior labral detachment, (3) isolated glenoid malalignment (–15° retroversion), (4) isolated acromial malalignment (high/flat), (5) combined malalignment, (6) acromial malalignment + glenoid correction + posterior acromial bone graft (PABG), (7) combined malalignment + PABG. Humeral head translation was measured during forward flexion at 30°, 50°, and 70° of elevation and normalized to glenoid width. Statistical analysis used repeated-measures analysis of variance with Bonferroni correction. Results: Posterior labral detachment showed minimal effect (1.3% ± 2.4% translation). On average, isolated glenoid malalignment increased posterior translation by 29%, whereas isolated acromial malalignment produced 31% posterior translation. Combined malalignment resulted in 54% posterior translation (P < .05 for all comparisons), demonstrating additive effects. Glenoid correction with PABG partially restored humeral head translation, but did not restore glenohumeral centering, with a residual 20% posterior translation compared with the intact shoulder. Adding a PABG to the combined malalignment led to a measurable reduction in posterior translation. However, although the graft decreased translation by approximately 13%, it did not restore native kinematics. Conclusions: Glenoid as well as acromial malalignment alone is associated with pathological posterior translation of the humeral head across the glenoid upon simulated active elevation. Combined acromial and glenoid malalignment produces significantly greater posterior translation than either deformity alone. Clinical Relevance: Complete anatomic correction of both deformities is necessary to restore normal posterior shoulder kinematics, supporting a comprehensive surgical approach for SPSL treatment.