Evaluation of the muscle volumes of the transverse rotator cuff force couple in nonpathologic shoulders

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The balance between the subscapularis muscle and the infraspinatus/teres minor muscles, often referred to as the rotator cuff transverse force couple (TFC), has been proposed to be a critical component for normal shoulder function. The relationship between the muscle volume and the power means that TFC can be evaluated with the measurement of the muscle volume of the subscapularis muscle and the infraspinatus/teres minor. The aim of this study is to evaluate an innovative computed tomography (CT)-based technique to measure the muscle volume and to evaluate if there is a significant difference between muscle volumes of both the subscapularis muscle and the infraspinatus/teres minor in nonpathologic shoulders. CT images of 27 shoulders (21 patients) with a full scapula and a proximal humeral head were evaluated. Two volume masks (subscapularis and infraspinatus/teres minor) were calculated on the basis of the assigned muscle contours on the transverse slices. The intraobserver and interobserver correlation coefficient was calculated. The intraobserver and interobserver correlation coefficient was excellent. The correlation between the anterior and posterior part of the TFC was strong. There was no significant difference between the volume masks. Muscle volume of the TCF can be quantified using CT images. In nonpathologic shoulders, there is no significant difference between the muscle volume of the anterior (subscapularis) and posterior part (teres minor/infraspinatus) of the TFC.