Posterior Shoulder Stability Can Be Restored by Posterior Acromial Bone Grafting (Scapinelli) in a Cadaveric Biomechanical Model With Normal Glenoid Anatomy

Background: A high and flat acromion seems to be a risk factor for posterior shoulder instability. Biomechanically, the surgical correction of acromial malalignment can restore glenohumeral joint stability. Purpose/hypothesis: The purpose was to assess (1) the stabilizing effect of a posterior acromial bone graft (PABG) in moderate and severe acromial malalignment (high and flat) and (2) contact patterns under posterior humeral head displacement. It was hypothesized that a PABG would significantly (1) increase resistance to posterior humeral head displacement, (2) restore stability, and (3) increase acromiohumeral contact pressure. Study design: Controlled laboratory study. Methods: A total of 8 fresh-frozen human cadaveric shoulders, with normal glenoid anatomy, were examined in a shoulder simulator in the load and shift and jerk test positions. Each specimen underwent 5 testing conditions using 3-dimensional printed cutting and reduction guides, with the joint left intact for each condition: (1) severe acromial malalignment, (2) severe acromial malalignment + PABG, (3) moderate acromial malalignment, (4) moderate acromial malalignment + PABG, and (5) corrected acromial alignment. The humeral head was translated posteriorly until reaching either a peak force of 150 N or a maximum posterior displacement of 50% of the glenoid width. Force, displacement, and acromiohumeral contact pressure were recorded. Conclusion: The study provides quantitative evidence showing that a PABG significantly enhanced resistance to displacement and compensated for deficient posterolateral acromial coverage by extending the natural mechanical buttress. Clinical relevance: Experimentally, a PABG provided comparable or superior stability to that after surgical acromial reorientation while representing a technically simpler and potentially less invasive approach.