Glenoid Bone Loss After First-Time Posterior Instability Events: A Prospective Cohort Study

OBJECTIVES: To prospectively determine the amount of bone loss associated with posterior instability and to determine predisposing factors based on pre-instability imaging. METHODS: A total of 714 athletes were evaluated prospectively for 4+ years. At baseline, every patient received a subjective history of shoulder instability as well as bilateral noncontrast shoulder magnetic resonance imaging (MRI) regardless of any reported history of shoulder instability. The cohort was prospectively followed during the study period, and those who were diagnosed with posterior glenohumeral instability were identified. Postinjury MRIs with contrast were obtained and compared with the screening MRI. Glenoid version, perfect-circle-based bone loss was measured for each patient’s pre- and post-injury MRI using previously described methods. RESULTS: Of the 714 athletes (1428 shoulders) who were prospectively followed during the 4-year period, 5 athletes (7 shoulders) sustained a first-time posterior instability event (1 dislocation, 5 subluxations), and one athlete (1 shoulder) sustained a recurrent posterior instability event. At baseline, 6 out of 8 shoulders were noted to have some level of dysplasia and/or bone loss. Glenoid bone loss increased significantly after instability event in terms of percentage linear bone loss (6.96 ± 4.66 versus 13.54% ± 4.66, p=0.014) and total area of bone loss (3.51 ± 2.78 versus 8.10 ± 3.25, p= 0.009). Baseline glenoid retroversion >10° was associated with a significant greater percentage of bone loss (2.58 ± 1.94 versus 5.79 ± 1.28, p=0.028). CONCLUSIONS: Patients with posterior instability tend to have baseline dysplasia and/or glenoid bone loss. Glenoid bone loss of 8.1% was observed after index posterior instability event, with baseline glenoid retroversion >10° being associated with more bone loss.