Coracoid Morphology and the Risk of Posterior Shoulder Instability: A Magnetic Resonance Imaging Study

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Abstract
Background: Posterior shoulder instability has been associated with abnormal acromial and glenoid morphology. However, the role of coracoid morphology in posterior instability remains unclear. Hypothesis: Patients undergoing surgery for posterior shoulder instability will have more inferiorly oriented and lateralized coracoids compared to patients with anterior instability or a noninstability comparison group. Study Design: Cohort study; Level of evidence, 3. Methods: Magnetic resonance imaging measurements were collected for 3 surgical cohorts: posterior instability (n = 41), anterior instability (n = 39), and a comparison group undergoing arthroscopy for snapping scapula (n = 38). Coracoid morphology metrics included: lateral coracoid offset, sagittal coracoid angle, coronal coracoid angle (CCA), transverse coracoid length, coracoacromial length (CAL), coracohumeral distance, and inferior coracoid coverage (ICC). Glenoid version and bone loss were also measured. One-way analysis of variance with Tukey post hoc testing or Kruskal-Wallis with Dunn testing was used as appropriate to compare groups. The P value was set a priori at <.05 to represent statistical significance. Analyses were performed using SPSS Version 30.0 and R Version 4.5.0. Results: The posterior instability group demonstrated a significantly higher CCA (69.4° ± 23.5°) than the anterior (52.8° ± 23.8°; P = .002) and comparison (56.9° ± 17.7°; P = .015) groups. The CAL and ICC were also significantly different in the posterior group (both P < .001). No significant differences in coracoid morphology were found between the anterior and comparison groups. Patients with posterior instability had significantly increased glenoid retroversion (–8.1° ± 3.7°) versus the anterior (–3.1° ± 5.3°) and comparison (–4.2° ± 4.3°) groups (both P < .001). Conclusion: Coracoid morphology differs significantly in patients undergoing posterior shoulder stabilization when compared to patients undergoing surgery for anterior instability or a comparison cohort. Increased CCA, increased CAL, and greater ICC are associated with patients undergoing posterior stabilization surgery. These findings suggest that a lower coracoid with increased inferior coverage of the humeral head may contribute to the pathogenesis of posterior instability.

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