Abstract
OBJECTIVES: Posterior glenoid bone deficiency is an increasingly recognized entity in the setting of recurrent posterior shoulder instability; however, little is known about the subject. Due to the paucity of literature on posterior bone loss, historical comparisons to anterior bone loss may not be fully accurate. The purpose of this study was to systematically describe the morphology of posterior bone defects in the setting of recurrent posterior shoulder instability based on several quantitative parameters, including the mean location, orientation, and extent of bone loss on a clock face model, as well as the angle of the defect relative to the long axis of the glenoid. METHODS: 3-dimensional (3D) reconstructed computed tomography (CT) scans of serially collected patients with a history of recurrent posterior shoulder instability were evaluated by three separate reviewers. The posterior glenoid bone defect was characterized using the following measures: (a) the mean lesion location and orientation based on a clock face model with 6:00 o’clock denoted as inferior and 9:00 o’clock as directly posterior for all patients; (b) the total extent of the posterior bone defect based on the clock face; and (c) the average angle of the bone loss relative to the long axis of the glenoid. RESULTS: A total of 70 male patients and 1 female patient with mean age of 29.3 years (range = 24.4 to 35.1 years) were included in the analysis. The mean clock face location of the posterior glenoid defect originated at 6:44 (range = 4:16 to 8:12) and extended to a mean of 9:28 (range = 7:02 to 10:38). The mean extent of the posterior glenoid defect was 2:43 (range = 1:08 to 4:50), which corresponds to a mean total bone loss arc of 81.5° (range = 34.2° to 144.9°), nearly one quadrant of the glenoid. Posterior bone loss occurred in a posteroinferior direction at a mean angle of 30.7° (range = 8.0° to 80.0°) relative to the long axis of the glenoid. CONCLUSION: This study describes the location and orientation of posterior glenoid bone loss one can expect when treating this challenging patient population. Posterior bone defects in the setting of posterior shoulder instability most commonly occur in the posterior-inferior quadrant of the glenoid and extend on average from 6:44 to 9:28 (81.5° total degrees of arc) on a clock face model. Posterior bone loss occurs at a mean of 30° off the long axis of the glenoid in a posteroinferior direction, which is historically different from anterior bone loss, which occurs parallel to the long axis of the glenoid. This study serves to highlight the location and orientation of bone loss that one can expect in a patient with recurrent posterior shoulder instability, although additional work is needed to assess why this develops.