Pre-operative deltoid muscle volume predicts post-operative elevation after reverse total shoulder arthroplasty in patients with pre-operative contracture.

BACKGROUND

Although reverse total shoulder arthroplasty (rTSA) can restore shoulder function in patients with rotator cuff deficiency, it relies heavily on the deltoid muscle. Low muscle volume, particularly in rheumatoid arthritis (RA), may lead to suboptimal outcomes. The purpose of this study was to investigate the relationship between pre-operative deltoid muscle volume and post-operative range of shoulder elevation, focusing on the influence of pre-operative joint stiffness.

METHODS

We retrospectively reviewed 45 consecutive patients (49 shoulders) with RA or cuff tear arthropathy (CTA) who underwent rTSA. The mean follow-up period was 30 months. Active and passive range of motion were evaluated pre-operatively and post-operatively. Pre-operative deltoid cross-sectional area was measured on magnetic resonance imaging to calculate the "deltoid index" (area [mm2]/height2 [m2]). Correlations between the deltoid index and post-operative elevation were analyzed, stratified by pre-operative contracture status.

RESULTS

Pre-operative shoulder contracture was significantly more frequent in the RA group (59%) than in the CTA group (23%). The deltoid index was significantly smaller in patients with RA compared to those with CTA (738 vs. 946 mm2/m2, P = .0038). The deltoid index showed a strong positive correlation with post-operative active shoulder elevation only in patients with pre-operative contracture (r = 0.61, P = .0020). No significant correlation was observed in patients without contracture.

CONCLUSION

Pre-operative deltoid muscle volume (deltoid index) was an independent predictor of post-operative shoulder elevation after rTSA, particularly in patients with pre-operative contracture. rTSA may compensate for moderate deltoid atrophy in mobile shoulders, whereas the combination of severe deltoid atrophy and contracture may limit post-operative elevation.