Variation in external rotation moment arms among subregions of supraspinatus, infraspinatus, and teres minor muscles

Abstract

A rotator cuff tear causes morphologic changes in rotator cuff muscles and tendons and reduced shoulder strength. The mechanisms by which these changes affect joint strength are not understood. This study's purpose was to empirically determine rotation moment arms for subregions of supraspinatus, infraspinatus, and for teres minor, and to test the hypothesis that subregions of the cuff tendons increase their effective moment arms through connections to other subregions. Tendon excursions were measured for full ranges of rotation on 10 independent glenohumeral specimens with the humerus abducted in the scapular plane at 10 and 608. Supraspinatus and infraspinatus tendons were divided into equal width subregions. Two conditions were tested: tendon divided to the musculotendinous junction, and tendon divided to the insertion on the humerus. Moment arms were determined from tendon excursion via the principle of virtual work. Moment arms for the infraspinatus (p < 0.001) and supraspinatus (p < 0.001) were significantly greater when the tendon was only divided to the musculotendinous junction versus division to the humeral head. Moment arms across subregions of infraspinatus (p < 0.001) and supraspinatus (p < 0.001) were significantly different. A difference in teres minor moment arm was not found for the two cuff tendon conditions. Moment arm differences between muscle subregions and for tendon division conditions have clinical implications. Interaction between cuff regions could explain why some subjects retain strength after a small cuff tear. This finding helps explain why a partial cuff repair may be beneficial when a complete repair is not possible. Data presented here can help differentiate between cuff tear cases that would benefit from cuff repair and cases for which cuff repair might not be as favorable. ß

Shoulder muscle function depends on elbow joint position: An illustration of dynamic coupling in the upper limb

“…For example, the peak abduction moment arm of the middle deltoid in the model was 29.0 mm, which is close to the peak value of 30 mm measured by Poppen and Walker (1978) and identical to that measured by Kuechle et al (1997). Also, the internal rotation moment arm of the inferior infraspinatus at 601 of abduction was 17.5 mm, compared to 17 mm reported by Langenderfer et al (2006). As segment anthropometry and muscle lines-of-action were based on detailed measurements taken from eight cadaver specimens, we believe the model used in the present study provides a realistic representation of the capacities of the individual muscles of the shoulder to accelerate the glenohumeral joint.…”

Effects of rotator cuff tears on muscle moment arms: A computational study

“…Boundary conditions were applied to replicate an experimental measurement of external rotation muscle moment arms for infraspinatus and teres minor (Langenderfer et al, 2006a) (Fig. 1).…”

“…Muscle origin and insertion sites were identified relative to scapular landmarks (Langenderfer et al, 2006a). The muscle insertions were defined by dividing the respective tendon into equal-width sub-regions and attaching to the center of each sub-region.…”

“…Moment arm reductions were calculated by subtracting divided tendon moment arms from intact normal moment arms. Calculated moment arms and reductions were compared to empirically determined results (Langenderfer et al, 2006a).…”

“…These alterations decrease the ability of muscles to generate force. Recently, division of the rotator cuff tendons in a manner similar to cuff tears has been shown to cause alterations in muscletendon excursions and reductions in muscle moment arms (Langenderfer et al, 2006a). Additionally, the alterations in muscle excursions, combined with changes in muscle fascicle length due to contracture and increased tendon lengths, may result in reduced rotator cuff muscle forces by altering the muscle length-tension operating range.…”