Posterior Acromial Bone Block (PABB) As a Modification of The Scapinelli Procedure to Correct Posterior Shoulder Instability

Posterior shoulder instability is increasingly recognized as a morphology-driven disorder in which abnormal glenoid and acromial anatomy reduces resistance to posterior translation of the humeral head. When the underlying scapular deformity is not corrected, isolated soft-tissue or glenoid-based procedures show limited and inconsistent success. Within the SCOPE (Scapular Corrections for Posterior Escape) concept, a short, high posterior acromion with reduced posterior coverage can be addressed by restoring posterior acromial height and sagittal coverage. This technical note describes a modification of the Scapinelli posterior acromial bone block (PABB) that uses patient-specific three-dimensional planning to correct an isolated posterior acromial deficiency. In carefully selected patients with increased posterior acromial height and decreased posterior sagittal coverage but normal lateral acromial extension and normal glenoid orientation, patient-specific restoration of posterior acromial morphology re-establishes the normal posterior osseous buttress. A computed tomography scan of the scapula is segmented and compared with a statistical mean shape model derived from a control cohort of 40 stable shoulders. Scapular parameters, including glenoid version, posterior acromial height, posterior sagittal coverage, sagittal tilt, lateral acromial extension, and the critical shoulder angle, are measured against these normative values. Because these parameters are continuous variables, deviations exceeding one standard deviation from the normal mean are used as pragmatic decision thresholds, always interpreted together with the overall morphology and the clinical context, rather than as absolute cut-offs. An explicit stepwise algorithm guides the choice between an isolated PABB, a corrective acromial osteotomy, a glenoid osteotomy, or their combination. The surgical technique, including graft harvest, patient-specific positioning and fixation, deltoid refixation, and a structured rehabilitation protocol, is described and supplemented by an operative video. To date, the technique has been performed in 15 patients, of whom 10 have a minimum follow-up of 12 months; clinical outcomes remain to be established. This patient-specific procedure addresses the underlying morphologic abnormality while preserving native glenoid anatomy. Its clinical superiority has not yet been demonstrated, and further studies are required to refine the indications and to evaluate mid- to long-term outcomes.