Alexander J. Vervaecke

Background: Traumatic posterior shoulder dislocation is rare, and the role of posterior acromial morphology in acute traumatic posterior dislocation is unknown. We compared posterior acromial morphology in first-time traumatic posterior versus first-time traumatic anterior dislocations. Methods: In this retrospective comparative cohort study, 34 consecutive patients with a first-time acute traumatic posterior shoulder dislocation were compared with 34 consecutive patients with a first-time acute traumatic anterior shoulder dislocation. The posterior dislocation cohort included 26 males and 8 females, with a mean age of 43.4 ± 16.5 years, and the anterior dislocation cohort included 29 males and 5 females, with a mean age of 41.3 ± 15.4 years. Posterior acromial tilt (PAT), posterior acromial coverage (PAC), posterior acromial height (PAH), and the critical shoulder angle (CSA) were measured on standardized radiographs. Comparisons between the groups were performed, and logistic regression analyses were used to assess associations between acromial morphology parameters and posterior dislocation. Interobserver and intraobserver reliability were assessed using intraclass correlation coefficients. Results: Compared with the anterior dislocation group, patients with posterior dislocation demonstrated a significantly higher PAT (65.8° ± 6.7° versus 58.9° ± 8.4°, p < 0.001), lower PAC (55.6° ± 6.6° versus 59.5° ± 8.4°, p = 0.038), greater PAH (27.0 ± 4.2 versus 22.0 ± 6.7 mm, p < 0.001), and smaller CSA (32.9° ± 4.0° versus 35.5° ± 4.1°, p = 0.009). Logistic regression showed that higher PAH and PAT were most strongly associated with posterior dislocation. Measurement reliability was good to excellent for all parameters. Conclusions: Acute traumatic posterior shoulder dislocation is associated with distinct posterior acromial morphology similar to that described in recurrent posterior instability. Level of Evidence: Diagnostic Level IV . See Instructions for Authors for a complete description of levels of evidence.