Risk Factors for instability and dislocation after reverse total shoulder arthroplasty: a systematic review.
BACKGROUND The number of reverse total shoulder arthroplasties has increased due to demographic changes, leading to more revision surgeries. Postoperative instability or dislocation is a frequent and challenging complication that impairs clinical outcomes. This study aimed to identify risk factors for instability following reverse total shoulder arthroplasty and to evaluate the underlying mechanisms described by Abdelfattah et al. METHODS: A systematic literature review was conducted using PubMed and Google Scholar with MeSH terms and keywords including "shoulder replacement," "shoulder arthroplasty," "joint instability," "risk factors," and "odds ratio." The search was completed on February 21, 2025. Instability mechanisms were classified according to the Frankle classification by Abdelfattah et al.. A meta-analysis of odds ratios was performed for risk factors reported in multiple studies. RESULTS Ten retrospective studies (six cohort studies, four case series) were included. Eleven significant risk factors were identified: male sex, younger age, opioid use disorder, depressive disorder, revision surgery, nonunion, rotator cuff disease, prior postoperative subluxation, absence of subscapularis repair, cemented humeral stem, and a expanded/ lateralized glenosphere >9mm. Additional less conclusive factors included high body-mass index, malunion, hypertension, electrolyte disorders, prior ipsilateral shoulder surgery, acute fractures, and glenosphere-related factors (size, positioning, neck-shaft angle). The most common mechanism of instability was loss of compression, followed by impingement. CONCLUSION Both established and novel risk factors for instability were identified, including younger age, opioid use disorder, depressive disorder, nonunion, and lack of bony lateralization. Loss of compression was the predominant mechanism. These findings may improve preoperative risk assessment and support individualized surgical planning. However, due to the retrospective nature of the available evidence and potential confounding by indication, particularly for implant-related factors, these findings should be considered hypothesis-generating rather than causal. Future studies should link risk factors to instability mechanisms to enable targeted treatments and develop a validated risk score. LEVEL OF EVIDENCE Level IV, Systematic Review.