The rotator cuff muscles are antagonists after reverse total shoulder arthroplasty

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Introduction

There is disagreement regarding whether, when possible, the rotator cuff should be repaired in conjunction with reverse total shoulder arthroplasty (RTSA). Therefore, we investigated the effects of rotator cuff repair in RTSA models with varying magnitudes of humeral and glenosphere lateralization.

Methods

Six fresh frozen cadaveric shoulders were tested on a validated in vitro muscle-driven motion simulator. Each specimen was implanted with a custom adjustable, load-sensing RTSA after creation of a simulated rotator cuff tear. The effects of 4 RTSA configurations (0 and 10 mm of humeral lateralization and glenosphere lateralization) on deltoid force and joint load during abduction with and without rotator cuff repair were assessed.

Results

Deltoid force was significantly affected by increasing humeral lateralization (−2.5% ± 1.7% body weight [BW], P = .016) and glenosphere lateralization (+7.7% ± 5.6% BW, P = .016). Rotator cuff repair interacted with humeral and glenosphere lateralization (P = .005), such that with no humeral lateralization, glenosphere lateralization increased deltoid force without cuff repair (8.1% ± 5.1% BW, P = .012). This effect was increased with cuff repair (12.8% ± 7.8% BW, P = .010), but the addition of humeral lateralization mitigated this effect. Rotator cuff repair increased joint load (+11.9% ± 5.1% BW, P = .002), as did glenosphere lateralization (+13.3% ± 3.7% BW, P < .001). These interacted, such that increasing glenosphere lateralization markedly increased the negative effects of cuff repair (9.4% ± 3.2% BW [P = .001] vs. 14.4% ± 7.4% BW [P = .005]).

Conclusion

Rotator cuff repair, especially in conjunction with glenosphere lateralization, produces an antagonistic effect that increases deltoid and joint loading. The long-term effects of this remain unknown; however, combining these factors may prove undesirable. Humeral lateralization improves joint compression through deltoid wrapping and increases the deltoid's mechanical advantage, and therefore, could be used in place of rotator cuff repair, thus avoiding its complications.

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.

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