(6.82Æ3.33 vs. 8.34Æ2.99, P<0.0001), and VAS (2.35Æ2.68 vs. 1.21Æ1.85, P<0.0001). The NPS group demonstrated significantly greater magnitudes of improvement compared to the PS group with SST (P¼0.026), but not ASES (P¼0.089) or VAS (P¼0.153). There were no significant differences detected in outcome scores or magnitudes of change in outcomes between patients undergoing RCR or any of the other procedures. Conclusion: While patients who have undergone prior ipsilateral shoulder surgery derive benefit from shoulder arthroplasty, these patients are significantly younger, and their magnitude of improvement and final scores are significantly lower than patients without prior surgery. Posterior Shoulder Instability with Glenoid Bone Loss: Morphology of Posterior Bone Defects SS-64 May 20, 2017, 10:00 AM MATTHEW PROVENCHER, M.D., PRESENTING AUTHOR BRENDIN BEAULIEU-JONES, B.A. JUSTIN ARNER, M.D. GEORGE SANCHEZ, B.S. ASHLEY TISOSKY, M.D. JAMES BRADLEY, M.D. Introduction: Although much less common than anterior shoulder instability with glenoid bone loss, posterior gle- noid 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 insta- bility. 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. Methods: A total of forty young, active individuals with recurrent posterior shoulder instability and a bony injury confirmed on either computed tomography (CT) (n¼18, 26.3Æ4.0 years) or magnetic resonance imaging (MRI) (n¼22, 20.0Æ4.9 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. Results: 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. Conclusion: Posterior glenoid bone loss morphology differs considerably from its anterior counterpart. There- fore, 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. Posterior Shoulder Instability in Athletes: An Analysis of the MOON Shoulder Stabili- zation Cohort SS-65 May 20, 2017, 10:05 AM MICAH NAIMARK, M.D., PRESENTING AUTHOR JESSICA BRYANT, M.D. ALAN ZHANG, M.D. BENJAMIN MA, M.D. BRIAN FEELEY, M.D. BRIAN WOLF, M.D. CAROLYN HETTRICH, M.D. Introduction: Posterior shoulder instability is an increasing recognized cause of disability and decreased performance in athletes. The purpose of this study is to compare the presentation of posterior and anterior shoulder instability among a large, multicenter patient cohort. Methods: The Multicenter Orthopedic Outcome Network (MOON) database of a prospective cohort of shoulder stabilizations was used to identify 541 patients with ante- rior or posterior shoulder instability. Instability was clas- sified by the surgeon as anterior or posterior instability. Patients with multidirectional instability were excluded from the study. All patients completed an intake ques- tionnaire, which included the Western Ontarian Shoulder Instability Index (WOSI) and Shoulder Activity Scale. Results: Among 541 shoulder stabilizations, there were 420(78%) anterior and 121(22%) posterior instability patients. The anterior and posterior shoulder instability groups did not statistically differ by age (24.0Æ8.7 and 23.1Æ7.8 years, respectively), BM I (25.3Æ4 and 26.3Æ4 kg/m2), or sex (86% and 88% male). Anterior and posterior cohorts also had similar shoulder activity level (13Æ4 and 13Æ4) and WOSI percentile scores (44Æ20% and 44Æ19%). Compared to anterior stabilizations, posterior stabilizations were performed more acutely after shorter duration of symptoms (p<0.02) and fewer dislo- cations (p<0.01). In the posterior cohort, 47% of patients reported no history of frank dislocation compared to 14% in the anterior cohort. Discrete sports injuries were re- ported in 73% of patients. Football was the most common injury mechanism in both instability subtypes (See Figure). Conclusion: Although historically considered uncommon, posterior instability accounted for almost one quarter of shoulder stabilization procedures. Posterior instability patients tended to present after a shorter dura- tion of symptoms and they were 3.3 times more likely to undergo stabilization without having a dislocation event compared to anterior instability patients. Understanding e28 ABSTRACTS