Variations in Glenohumeral Bony Morphology May Predict Recurrent Instability after Arthroscopic Bankart Repair

OBJECTIVES: Variations in bony anatomy may be associated with failure of stabilization surgery. The aim of this study was to develop a method to measure bony morphology on magnetic resonance imaging (MRI) to identify risk factors for failure after Bankart repair. METHODS: This was a retrospective case-control study of 118 patients. Cases of postoperative dislocation were compared to matched controls. Demographic data was obtained by chart review and radiographic data from preoperative MRI. Volume was measured using a 3-D model. Radius of curvature of the humeral head and glenoid was measured on axial MRI images. Statistical analysis used student’s t-test for continuous variables and either Fisher’s exact or Chi-squared test for categorical variables; P value < 0.05 was significant. Interrater reliability between reviewers was calculated using interclass correlation coefficients (r). RESULTS: Forty-six patients who had a postoperative dislocation met inclusion criteria and were matched to 72 controls. There was no difference between groups for demographic (age, sex, percentage of contact athletes) or radiographic (glenoid bone loss, off-track Hill-Sachs lesions) parameters. The average number of preoperative dislocations was higher in the case group (3.2 vs. 2.0, p=0.003). The humeral head (68.8 ml vs 62.8 ml, p=0.05) volume was greater in the case group, though this did not reach statistical significance. Glenoid volume (13.5 ml vs 12.8 ml, p=0.31) was similar between groups. The radius of curvature of the glenoid was larger, or shallower, in the case group compared to the control group (23.6 mm vs 22.6 mm, p=0.05), though the difference did not reach statistical significance. A greater percentage of patients with a glenoid radius of curvature > 24.5 mm experienced a postoperative dislocation compared to those who had a smaller radius of curvature (62.0% vs 29.8%, p < 0.01). In fact, patients who had glenoid radius of curvature > 24.5 mm were 5 times as likely to experience a postoperative dislocation compared to those who did not (odds ratio 5.04, 95% CI 2.13 – 11.94, p < 0.01) There was no significant difference between the number of preoperative dislocations between patients with larger or smaller glenoid radius of curvature (2.6 vs 2.3, p = 0.55). There was a strong interrater reliability for measurement of humeral head volume, glenoid volume, radius of curvature of glenoid and radius of curvature of humeral head (r = 0.94, 0.88, 0.89, 0.95). CONCLUSION: The results of this study demonstrate that a larger radius of curvature, indicative of a shallower glenoid, is associated with failure following primary arthroscopic Bankart. These findings suggest that the bony concavity of the glenoid may play a role in stability.