Fatty change in rotator cuff muscles following reverse total shoulder arthroplasty: does the humeral component design matter?

Background: Fatty change progression is an important factor influencing treatment decisions for shoulder disorders. Appropriate evaluation and management of fatty change in the rotator cuff muscles and of its relationship to reverse total shoulder arthroplasty (rTSA) warrant careful consideration. This study aimed to examine the association between post-operative fatty change progression in individual rotator cuff muscles and humeral component design after rTSA. Methods: This retrospective study included 203 patients with large to massive rotator cuff tears or cuff tear arthropathy who underwent rTSA. Of these, 41 patients without glenoid-sided lateralization and with at least 2 years of follow-up imaging were included in the analysis. Patients were treated with either inlay or onlay humeral components. Fatty change progression was assessed using the Goutallier classification for each rotator cuff muscle and was categorized as mild or severe. Post-operative outcomes included active range of motion and patient-reported outcome measures, including the American Shoulder and Elbow Surgeons score and the Constant Murley score. Associations between implant design, fatty change in each rotator cuff muscle, and clinical outcomes were analyzed. The Minimal Clinically Important Difference (MCID) was explored for variables independently associated with outcomes in the primary multivariable models. Results: Sixteen patients underwent inlay and 25 underwent onlay humeral component implantation. Fatty change progression rates were similar between groups for the supraspinatus and teres minor but were lower with the onlay design for the infraspinatus (ISP) and subscapularis (SSC). The onlay design was independently associated with lower post-operative Goutallier grades of the ISP (odds ratio, 0.07; 95% confidence interval, 0.02-0.33) and SSC (odds ratio, 0.11; 95% confidence interval, 0.03-0.44). Post-operative fatty change correlated with pre-operative ones. Fatty change in individual muscles was not associated with post-operative range of motion. Greater post-operative SSC fatty change was independently associated with lower patient-reported outcome measure scores. All patients with mild SSC fatty change or those treated with an onlay component achieved the MCID. In contrast, MCID achievement rates in patients with severe SSC fatty change treated with an inlay component were 56% for the American Shoulder and Elbow Surgeons score and 44% for the Constant Murley score. Conclusion: Greater fatty change progression in the ISP and SSC was associated with the use of an inlay humeral component. An association between post-operative fatty change progression and clinical outcomes was observed in the SSC but not in the other rotator cuff muscles.