The rotator cuff muscles are antagonists after reverse total shoulder arthroplasty
Section snippets
Instrumented RTSA implant
In this in vitro biomechanical study, it was possible to measure joint loads and investigate the effects of systematic adjustments to implant geometry using a previously described custom modular implant system with a built-in load sensor (Fig. 1).14, 19 Four combinations of humeral and glenosphere lateralization were investigated (respectively: 0 and 0 mm, 0 and 10 mm, 10 and 0 mm, and 10 and 10 mm) where the configuration is considered to be neutral when both variables are at 0 mm,
Deltoid force
Results for total deltoid force indicated that rotator cuff loading (ie, clinical rotator cuff repair or intact anterior and posterior cuff) did not have a significant main effect (P = .3) when assessed across all implant configurations and abduction angles; however, in addition to the expected main effect of changing abduction angle (P = .001), humeral and glenosphere lateralization both significantly affected deltoid force (P < .021). Specifically, a 10-mm increase in humeral lateralization
Discussion
Although RTSA is primarily indicated as a treatment for rotator cuff tear arthropathy or massive cuff tears, in many cases, portions of the degenerated infraspinatus and subscapularis are amenable to repair despite having decreased contractile capabilities. Previous studies, focusing on the effects of cuff repair on RTSA stability and dislocation and IR and ER ROM,5, 6, 7, 10, 21, 23, 24 have disagreed on whether these tissues should be repaired when possible. This study is unique because it
Conclusion
Rotator cuff repair in the setting of RTSA produces an antagonistic effect that increases the deltoid muscle's work during elevation, and this effect is exacerbated by increased glenosphere lateralization. The combination of these rotator cuff forces and the increased demands on the deltoid muscle significantly increases joint loads. The long-term effects of these changes remain unknown, but their magnitude is likely to be clinically meaningful. Humeral lateralization, however, produces
Disclaimer
This study was supported by grant funding from the Natural Sciences and Engineering Research Council of Canada. The RTSA humeral cups were provided by DePuy, which had no input into this research project in any manner.
Dr. Athwal is a consultant for Wright Medical Technologies/Tornier, IMASCAP and Depuy. No company had any input with this research project in any manner. All other authors, their immediate families, and any research foundation with which they are affiliated did not receive any
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Institutional Review Board approval was not required for this in vitro biomechanical investigation.