Supraspinatus and deltoid muscle fiber composition in rotator cuff tear conditions

Abstract

Background Rotator cuff (RC) tears are associated with RC muscle atrophy and changes in composition that are crucial to the prognosis of RC repair. The aim of this study was to characterize muscle fiber composition in the supraspinatus (SS) muscle under tear conditions. Methods Muscle biopsies were obtained from 21 patients undergoing surgery for an RC tendon tear. Biopsies were obtained from the musculotendinous junction of the SS muscle, and control biopsies were harvested from the deltoid muscle (DT). Biopsies were immunohistochemically processed for detection of type 1 (slow type) and type 2 (fast type) fibers and analyzed using unbiased, stereological principles. We counted the total numbers of type 1 and 2 muscle fibers/mm 2 , and fiber diameter was used to estimate muscle fiber atrophy and hypertrophy. Results We found significantly more type 2 cells/mm 2 in the SS compared with the DT ( P < .01). In addition, we found a significantly higher fraction of type 1 fibers than type 2 fibers in the DT ( P < .01), whereas both fiber types were equally present in the SS. The diameters of SS cells were generally smaller than those of DT cells. Atrophy of especially SS type 2 fibers was also demonstrated. Fiber atrophy was more pronounced in men than women. Conclusion The changes in the composition of SS muscle cell types suggest a shift from type 1 to type 2 muscle fibers and atrophy of both type 1 and 2 fibers. This composition indicates loss of endurance and rapid fatigue of the SS muscle under RC tear conditions.

Electromyographic Evaluation of the Shoulder Muscle after a Fatiguing Isokinetic Protocol in Recreational Overhead Athletes

“…Several studies assessed changes in muscle activity and recruitment of the shoulder girdle after internal and external isokinetic protocols in athletes and healthy active subjects [ 1 , 2 , 4 ], as well as in individuals with impingement syndrome [ 36 , 37 ]. It should be noted that most of the analyzed muscles, i.e., UT, IS and DP, are slow-twitch muscles (type I) [ 38 , 39 , 40 ] However, AD is a fast-twitch muscle (type II) [ 40 ]. According to our results, we found that all investigated muscles showed signs of muscular fatigue after isokinetic fatigue protocol.…”

A Cadaveric Study of the Rotator Cable: Interrogating the Suspension Bridge Model

“…Furthermore, the rotator cuff atrophies in response to tendon tears [22]. More in-depth studies of the supraspinatus have shown that in addition to atrophy of both type 1 and 2 muscle bers, there is also a shift towards type 2 bers, leading to a loss of muscle endurance and increased fatigability in the presence of rotator cuff tears [19]. Increased rotator cuff fatigability alters shoulder kinematics, possibly leading to increased stress on neighboring structures [20].…”