Poster 182: Acromial Morphology Influences Instability Risk and Direction: An MRI Study

OBJECTIVES: To determine if acromial morphology is associated with posterior or anterior shoulder instability as measured on MRI. METHODS: MRI measurements of posterior acromial coverage (PAC), posterior acromial height (PAH), posterior acromial tilt (PAT), and anterior acromial coverage (AAC) were completed for three separate matched groups who underwent surgical intervention: posterior instability, anterior instability, and a comparison group of patients with who underwent arthroscopic surgery for snapping scapula. Inclusion criteria were patients with recurrent instability younger than 40 years of age without multidirectional instability, glenoid bone loss greater than 13.5% or glenoid retroversion greater than 10%. RESULTS: Thirty-seven patients were included in each group. PAC was significantly less in the posterior instability group when compared to anterior and the comparison groups (68.3° vs. 88.7° vs. 81.7°, p<.001). PAH was significantly greater in the posterior group than compared to the anterior instability patients (11.0 mm vs -0.1 mm, p<.001) as well as the comparison patients (0.7 mm, p<.001). There was no difference between the posterior and anterior groups in terms of PAT or AAC (p=0.45, p=0.05). PAT was significantly smaller in the posterior instability group than the comparison group (55.2 degrees vs 62.2 degrees, p=0.026). The anterior and comparison groups were not significantly different in PAH or PAT (p=8.74, p=0.067) but were significantly different with AAC (p=0.26). CONCLUSIONS: The posterior acromion is significantly higher and flatter in patients with posterior shoulder instability who require arthroscopic capsulolabral repair when measured on pre-operative MRI. This information may help clinicians to both diagnose and predict the need for operative intervention for patients with posterior labral tears.