Acromial morphology in painful posterior shoulder instability: a case-control radiographic study

Acromial morphology is a major contributor to posterior shoulder stability, notably in the sagittal plane. Painful posterior shoulder instability (P-PSI) is a rare form of posterior instability characterized by pain without a history of dislocation. The potential relationship between acromial morphology and P-PSI has yet to be studied.

We hypothesized that, as in other forms of PSI, patients with P-PSI would have more horizontally oriented and higher acromions.

A retrospective case-control study was conducted on 27 patients who underwent surgery for P-PSI between January 2008 to March 2023. Patients were compared in terms of radiographic parameters (posterior acromial height (PAH), posterior acromial tilt (PAT), anterior acromial coverage (AAC), posterior acromial coverage (PAC), and critical shoulder angle (CSA)) with 27 age- and sex-matched asymptomatic controls in bivariate and multivariable logistic regression analysis.

Patients with P-PSI had significantly larger AAC (mean ± standard deviation, -10.2 ± 13.8 vs -3.3 ± 5.2 °; p =  0.01), PAT (66.2 ± 8.2 vs 58.7 ± 9.0°; p =  0.002) and PAH (23.6 ± 7.9 vs 18.5 ± 7.9 mm; p =  0.02) that control individuals, and borderline significantly smaller PAC (58.9 ± 8.3 vs 63.4 ± 9.3°; p =  0.06). In multivariable logistic regression P-PSI was significantly associated with increased PAT (odds ratio, 1.15; 95% confidence interval, 1.03-1.20 per degree of angle; p = 0.006), but not with higher or lower AAC or PAH.

This group of patients with P-PSI had significantly higher AAC, PAT and PAH than age-and sex-matched controls, with multivariable analysis indicating that the strongest predictor of P-PSI was PAT. These findings are in keeping with previous results on other forms of posterior instability and with the occurrence of posterior subluxation in eccentric glenohumeral osteoarthritis.

III; Retrospective Case control study.