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.

Results: At 30° of flexion, the force needed to displace the humeral head 50% increased by 659% when a PABG was added to a moderately malaligned acromion and by 1249% when a PABG was added to a severely malaligned acromion. At 60° of flexion, it increased by 293% and 348%, respectively. This stabilizing effect increased progressively with increasing displacement (P < .05 for all comparisons after ≥5% of displacement). Compared with acromial correction, a PABG allowed comparable posterior displacement but required different amounts of force, depending on the scenario. At 30° of flexion after 30% of displacement, a PABG provided significantly greater stability (P < .05 for all comparisons). Mean contact pressure was significantly reduced on the rotator cuff and significantly increased on the acromial undersurface in moderate and severe acromial malalignment, whereas a PABG restored acromiohumeral contact pressure comparable with corrective osteotomy, particularly at 30° of flexion.

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.