BackgroundUnstable shoulders with a large glenoid defect and small bone fragment are at higher risk for postoperative recurrence after arthroscopic Bankart repair. The purpose of the present study was to clarify the changes in the prevalence of such shoulders during conservative treatment for traumatic anterior instability.MethodsWe retrospectively investigated 114 shoulders that underwent conservative treatment and computed tomography (CT) examination at least twice after an instability event in the period from July 2004 to December 2021. We investigated the changes in glenoid rim morphology, glenoid defect size, and bone fragment size from the first to the final CT.ResultsAt first CT, 51 shoulders showed no glenoid bone defect, 12 showed glenoid erosion, and 51 showed a glenoid bone fragment [33 small bone fragment (<7.5%) and 18 large bone fragment (≥7.5%); mean size: 4.9 ± 4.2% (0-17.9%)]. Among patients with glenoid defect (fragment and erosion), the mean glenoid defect was 5.4 ± 6.6% (0-26.6%); 49 were considered a small glenoid defect (<13.5%) and 14 were a large glenoid defect (≥13.5%). While all 14 shoulders with large glenoid defect had a bone fragment, small fragment was solely seen in 4 shoulders. At final CT, 23 of the 51 shoulders persisted without glenoid defect. The number of shoulders presenting glenoid erosion increased from 12 to 24, and the number of shoulders with bone fragment increased from 51 to 67 [36 small bone fragment and 31 large bone fragment; mean size: 5.1 ± 4.9% (0-21.1%)]. The prevalence of shoulders with no or a small bone fragment did not increase from first CT (71.4%) to final CT (65.9%;P= .488), and the bone fragment size did not decrease (P= .753). The number of shoulders with glenoid defect increased from 63 to 91 and the mean glenoid defect significantly increased to 9.9 ± 6.6% (0-28.4%) (P< .001). The number of shoulders with large glenoid defect increased from 14 to 42 (P< .001). Of these 42 shoulders, 19 had no or a small bone fragment. Accordingly, among a total of 114 shoulders, the increase from first to final CT in the prevalence of a large glenoid defect accompanied by no or a small bone fragment was significant [4 shoulders (3.5%) vs. 19 shoulders (16.7%);P= .002].ConclusionsThe prevalence of shoulders with a large glenoid defect and small bone fragment increases significantly after several instability events.