Posterior Shoulder Instability with Glenoid Bone Loss: Morphology of Posterior Bone Defects

Abstract

Although much less common than anterior shoulder instability with glenoid bone loss, posterior glenoid bone defects leading to shoulder instability are severely limiting and significantly affect a patient's quality of life. Moreover, given the rarity of incidence of this pathology, posterior instability with bone loss has been much less investigated in comparison to anterior instability. Given this paucity in literature, we sought to better define the geometrical pattern of posterior bone defects. The purpose of this study was to geometrically describe posterior bone defects based on slope and version as well as extent of bone loss along equal intervals spanning the long axis of the glenoid fossa. A total of forty young, active individuals with recurrent posterior shoulder instability and a bony injury confirmed on either computed tomography (CT) (n=18, 26.3±4.0 years) or magnetic resonance imaging (MRI) (n=22, 20.0±4.9 years) were identified. The posterior glenoid bone defect was characterized using the following measures: (a) percent bone loss; (b) glenoid vault version; (c) the slope of the posterior defect relative to the glenoid surface; (d) the superior-inferior height of the defect; and (e) the anterior-posterior width of the defect at five intervals along the glenoid fossa. Posterior glenoid bone loss morphology was characterized by MRI and CT as more sloped relative to the glenoid than the more perpendicular pattern of bone loss seen in anterior instability. Although CT featured a more severe pattern of bone loss, including greater glenoid version and a greater slope of the posterior defect, this pattern of glenoid bone loss remained consist regardless of the imaging modality. Posterior glenoid bone loss morphology differs considerably from its anterior counterpart. Therefore, this affirms the notion that shoulder instability with bone loss should not be grouped all together, but treated on a case-by-case basis given the direction of instability.