Posterior shoulder dislocation shows a higher, flatter acromion, and increased glenoid retroversion compared with anterior dislocation: A matched computed tomography/magnetic resonance imaging analysis

INTRODUCTION: Posterior shoulder dislocation (PSD) is less common than anterior dislocation (ASD) but is increasingly recognized. Understanding the osseous characteristics of PSD is essential for guiding management. However, direct comparative radiographic analyses between PSD and ASD remain limited. The purpose of this study was to perform a matched-pair radiographic comparison of acromial, glenoidal, and (reverse) Hill-Sachs lesion morphology between patients with first-time traumatic PSD and ASD, and to identify osseous features that may predispose to posterior shoulder instability. METHODS: In this retrospective single-center study, 24 patients (24 shoulders) with cross-sectional imaging (computed tomography [CT] or magnetic resonance imaging [MRI]) following first-time traumatic PSD between 2011 and 2020 were included and matched 1:1 by sex, age, and laterality to 24 patients with first-time traumatic anterior shoulder dislocation. Two independent raters performed standardized radiological measurements on CT/MRI, assessing the following parameters: posterior acromial height (PAH), posterior acromial coverage (PAC), anterior acromial coverage (AAC), and total acromial coverage (TAC), acromial tilt (AT), glenoid bone loss (GBL, glenoid offset, glenoid retroversion, glenoid depth, scapular neck angle, defect size, width and depth of Hill-Sachs lesion (HSL)/reverse Hill-Sachs lesion (rHSL). RESULTS: PAH was greater in PSD than ASD (22.5 mm vs. 15.9 mm, p < 0.001). The PAC was lower in PSD (58.7° vs. 68.0°, p < 0.001), while AAC was higher (4.9° vs. -3.0°, p = 0.0015). No statistically significant difference was found for TAC. The AT was greater in PSD (65.5° vs. 55.2°, p < 0.001). The GBL was smaller in PSD than ASD (8.1% vs. 12.2%, p = 0.0073). Glenoid offset and neck angle showed no statistically significant differences. Retroversion was higher in PSD (8.0° vs. 3.3°, p < 0.001), and glenoid depth was greater (2.0 mm vs. 1.2 mm, p = 0.0053). The rHSL width was smaller in PSD than HSL width in ASD (13.0 mm vs. 17.0 mm, p = 0.037), while lesion depth and surface area did not differ. CONCLUSION: PSD is associated with distinct alterations in bony shoulder anatomy including a higher, flatter acromion resulting in reduced posterior containment of the humeral head, increased glenoid retroversion, less GBL, and deeper glenoid concavity compared with ASD. LEVEL OF EVIDENCE: Retrospective matched-pair cohort study, 3.