Posterior Shoulder Stability Depends On Acromial Anatomy: A Cadaveric Biomechanical Study.

Abstract

Aim: To evaluate the stabilizing effect of the acromion to posterior humeral head translation. Background(s): Posterior labral tears, glenoid retroversion and posterior glenoid bone loss are clinically and biomechanically proven factors that decrease resistance to humeral head translation in response to a posteriorly directed force. However, failure rates of conservative and surgical treatment are high. A high and flat acromion, implying less posterior humeral head coverage, has been shown to be associated with posterior instability. Method(s): Eight fresh-frozen cadaver shoulders were biomechanically tested in a shoulder simulator in the load-and-shift and the Jerk test position. Prior to testing, CT scans were performed on all shoulders to measure native glenoid width, glenoid retroversion, posterior acromial coverage (PAC), sagittal acromial tilt (SAT) and posterior acromial height (PAH). Each of the specimens underwent six testing conditions using 3D printed cutting and reduction guides: (1) Intact joint, native acromion (2) Intact joint, severe acromial pathology (SAT 69degree, PAC 47degree, PAH 26mm) (3) Scenario 2 + posterior acromial bone block (PABB) (4) Intact joint, medium acromial pathology (SAT 59degree, PAC 57degree, PAH 20mm) (5) Scenario 4 + PABB and (6) Intact joint, corrected acromial alignment (SAT 48degree, PAC 70degree, PAH 11mm). The degree of acromial malalignment and acromial reorientation was chosen based on a previous study that defined acromial anatomy in patients with posterior instability. The humeral head was translated posteriorly until either (1) a peak force of 150N or (2) a maximum posterior translation of 50% of the glenoid width were reached. Forces (N) and translations (mm) were recorded. Testing was repeated three times per condition and mean values were used for analysis. Result(s): Mean native glenoid width was 25.4mm (21-31mm), glenoid retroversion was 3.1degree (1.3-5.8degree), SAT was 58.6degree (41.1-72.6degree), PAC was 63.6degree (54.4-77.6degree), PAH was 18.9mm (8.5-24.5mm). Realigning the acromion or adding a PABB significantly increased stability compared with the medium and severe pathology in all tests (p<0.05). There were no significant differences between the native shoulder and the medium pathology. Conclusion(s): The acromion acts as a restraint to posterior humeral head translation. Correction of acromial malalignment or addition of a PABB can effectively restore glenohumeral stability.Copyright © 2024