Surface Electromyography Reveals Middle Deltoid as the Functionally Dominant Shoulder Muscle After Reverse Total Shoulder Arthroplasty.

BACKGROUND

Reverse total shoulder arthroplasty (RSA) increases deltoid muscle fiber recruitment and tension to compensate for deficient rotator cuff activity; however, it is unclear whether the anterior or middle deltoid becomes dominant and how the muscle activation profile changes postoperatively. Using minimally invasive electromyography, this study evaluated the activity of the deltoid and surrounding muscles during shoulder motion to assess muscle activation changes post-RSA.

METHODS

In this observational study, we assessed change in preoperative to postoperative shoulder muscle activation in 10 patients over six months. Muscle activation was measured using eight surface electrodes. Activation of the anterior, middle, and posterior deltoid, along with surrounding muscles, was recorded and quantified during shoulder abduction, flexion, and external and internal rotation. One-way analysis of variance was used to identify significant differences in activation and time or speed. The least significant difference post hoc test was used to determine specific differences in muscle activation at subsequent time points.

RESULTS

RSA shoulders at six months postoperatively showed a significant increase in activity of the middle deltoid predominantly. Middle deltoid activation increased during abduction (p<0.001), flexion (p=0.008), external (p<0.001), and internal (p<0.001) rotation.

CONCLUSIONS

Our study highlights the middle deltoid as the primary contributor to rotator cuff function in reverse shoulder arthroplasty (RSA), characterized by quantitative activation, significant involvement in all shoulder motions, and increased activation over time. These findings could inform future RSA designs to enhance deltoid wrapping, maximizing strength and efficiency.