Good afternoon and thank you for this opportunity. Myself and my co-authors have no relevant disclosures. While we have a relatively good understanding of glenoid bone loss as it relates to the outcomes in surgical management of anterior shoulder instability, not as much is known about the role of posterior glenoid bone loss in the management of posterior instability. A study last year in AJSM found that posterior glenoid bone loss greater than 13.5% was associated with a reoperation rate of 44% for posterior capsule labral repair. Additionally, in a biomechanical model, it was shown that 20% glenoid bone loss is the critical amount at which bony augmentation becomes necessary. So while we're starting to understand what meaningful glenoid bone loss is as it relates to outcomes, the natural history of glenoid bone loss in posterior instability remains largely unknown. Therefore, the purpose of this study was to define glenoid bone loss after posterior instability events in patients with and without prior history of instability. So this was a prospective cohort study in which all members of the United States Military Academy Class of 2010 were offered enrollment. All participants received bilateral shoulder MRIs and were followed through the year 2017. Patients who had a posterior instability event then got another MRI, which was compared to their pre-instability MRI. We ended up with 13 shoulders in 11 patients, 10 of which were first-time instability events and three of which were recurrent posterior instability events. Measurements of glenoid bone loss were performed by two orthopedic sports fellows and one senior orthopedic resident using the Best Fit Perfect Circle technique, utilizing the most on-fos sagittal image. We measured bone loss at the glenoid equator along the axis of greatest loss in total area of bone loss. We measured glenoid version using the Friedman technique, and we also looked at glenoid morphology both qualitatively and quantitatively by counting the amount of cuts on the axial sequence that had a dysplastic form of morphology. Our primary outcome was glenoid bone loss after posterior instability events, and secondary outcomes included change in glenoid morphology as well as the effect of baseline characteristics such as dysplasia, glenoid version, and age on glenoid bone loss. We found no difference in baseline demographics or exam findings other than a higher mean BITEN score in the recurrent instability group, which is not likely to be clinically meaningful. We had good to excellent intra- and inter-rater reliability for glenoid bone loss measurements. Review of baseline MRIs performed at the time of enrollment showed that patients did have a small amount of bone loss at baseline, and that patients with a history of prior posterior instability had a significantly greater amount of bone loss at baseline. When comparing pre- and post-instability MRIs, we found that overall each instability event was associated with a 4% to 5% increase in glenoid bone loss depending on the specific measurement we were looking at. Patients with recurrent instability had a greater change in bone loss as well as a greater absolute bone loss on final imaging than those with first-time instability. Most patients also did demonstrate a change in glenoid morphology, however, this did not result in a quantifiable statistically significant difference. On subgroup analysis, we found that age less than 21 years was associated with a two times greater rate of absolute bone loss than patients older than 21 at the time of their instability event. Additionally, glenoid retroversion greater than 10 degrees was also associated with greater bone loss than those with less retroversion, and that association with retroversion was a linear one in that for every degree of retroversion increase, there was a 0.76% increase in glenoid bone loss. Some of the strengths of this study is that we believe it's the first prospective evaluation of glenoid bone loss after posterior instability events. Use of the military health system provides us the optimum environment to capture posterior instability events being a closed system. The study does have some limitations. It's a small sample size. It may not be the most generalizable population given the unique nature of the military population. In the use of MRI for glenoid bone loss measurements, while it's been described, other studies have shown that 3DCT has been a little bit more reliable, more accurate. So that's another limitation. So to conclude, posterior instability events are associated with a 5% posterior glenoid bone loss. Recurrent posterior instability events are associated with more glenoid bone loss than first time instability. Glenoid retroversion greater than 10 degrees and age less than 21 degrees are associated with a greater amount of posterior glenoid bone loss after posterior instability events. Thank you.

posterior glenoid bone loss

shoulder instability

reoperation rate

glenoid bone loss

MRI scans