Paper 04: Acromial Morphology Differences in Primary versus Revision Posterior Shoulder Instability Patients

Objectives: Predisposing factors for failure of shoulder posterior stabilization surgery are not well defined. Characterization of revision posterior instability patients allows for insight into potentially identifiable risk factors for failure. Methods: A series of patients who underwent stabilization surgery for posterior shoulder instability as well as revision posterior instability cases performed between 2005-2019 were identified. Patient demographics were collected and glenoid morphology was quantified by measuring posterior bone loss, acromial tilt, and posterior acromial height on shoulder MRIs. Continuous variables were compared with Student’s t-test. Results: 42 active duty servicemembers who underwent successful primary posterior stabilization (primaries, P) and 41 who underwent revision posterior stabilization (revisions, R) and were identified. Mean ages of the primaries and revisions at time of primary surgery were 28.7 (±6.0; 18-39) and 23.0 (±5.2; 18-41) years respectively (p < 0.005). Males made up 92.9% (P) and 92.7% (R) of each group. Mean posterior glenoid bone loss at index MRI was 4.5% (±7.2; P) and 6.3% (±7.8; R) (p = 0.316). Mean index acromial tilt (from horizontal) was 40.9 degrees (±10.2; P) and 28.8 degrees (±10.6; R) (p < 0.005). Index posterior acromial height was 14.2 mm (±7.4; P) and 19.8 mm (±7.6; R) (p = 0.001). In the revision group, the progression of bone loss from index MRI to pre-revision MRI was 6.3% (±7.8) to 6.6% (±6.2) (p = 0.456). Index bone loss greater than 15% was present in 5/41 revision patients; when excluding these, mean index bone loss was 3.4% (±3.3), significantly less than that at pre-revision MRI (p = 0.038). Additional surgery occurred in 9.8% (4/41) of the revision group, and 2/4 separated from the military. In the revision group, 17.1% (7/41) separated, while an additional 7.3% (3/41) remained on active duty with upper extremity activity limitations. Conclusions: Index acromial tilt and posterior acromial height were respectively lower and higher in a population of revision posterior stabilization than in a cohort of patients who underwent successful posterior stabilization. A “high and flat” acromial morphology may predispose patients to failure of standard posterior stabilization procedures, potentially justifying further study into supplemental techniques to decrease failure risk.