Compensatory activation of the deltoid and remaining rotator cuff muscles in patients with rotator cuff tears using...

Rotator cuff (RC) tears are a prevalent condition leading to dysfunction and shoulder pain. Spatial changes in the behavior of the deltoid and remaining RC muscles during arm elevation in patients with RC tears have not been investigated. The purpose of this study was twofold: (1) to investigate factors related to the changes in the deltoid muscle behavior during scapular plane abduction (scaption) in patients with RC tears, and (2) to examine the functional relationship between the deltoid and remaining RC muscles, particularly remaining infraspinatus (ISP) and teres minor (TMin) muscles, in these patients using ultrasound elastography.

Twenty-four patients (mean age, 66 ± 6 years) with RC tears (mean antero-posterior diameter, 17.4 ± 8.3 mm) were evaluated. Antero-posterior and medio-lateral diameters of the posterosuperior RC and Subscapularis (SSC) tears were measured intra-operatively. Muscle elasticity was evaluated at passive and active states, as a proxy for muscle activation, at 30°, 60°, and 90° during scaption for the deltoid, ISP, and TMin muscles using real-time tissue elastography. Based on the deltoid muscle activity ratio at 30° and 90° scaption, patients were divided into normal and abnormal activation groups.

Activity of the deltoid muscle in the abnormal activation group exhibited consistently high activity across various elevation angles, significantly higher compared to that of the normal activation group at 30° scaption (p = 0.037). Although there was no statistically significant between-group difference in TMin muscle activity at 30° scaption, TMin muscle activity in the normal activation group was highest at 30° and significantly higher than at 90° scaption (p = 0.032). Abnormal activation group had a higher incidence of SSC tears combined with posterosuperior RC tears (p = 0.009).

SSC tears combined with posterosuperior RC tears may be associated with compensatory activation of the deltoid muscle in the early phase of arm elevation, while relatively greater TMin muscle activity in the initial range of elevation may reflect a compensatory contribution of the remaining posterior RC muscles.