Plasticity of Muscle Architecture After Supraspinatus Tears

Abstract

Study Design Controlled Laboratory Study Objectives To measure the architectural properties of rat supraspinatus muscle after a complete detachment of its distal tendon. Methods Supraspinatus muscles were released from the left humerus of 29 rats (Sprague-Dawley, mass 400 - 450 g) and animals were returned to cage activity for 2 weeks (n = 12), 4 weeks (n = 9), or 9 weeks (n=8) before euthanasia. Measurements of muscle mass, pennation angle, fiber bundle length (sarcomere number), and sarcomere length permitted calculation of normalized fiber length, serial sarcomere number, and physiological cross-sectional area (PCSA). Results Coronal oblique sections of the supraspinatus confirmed surgical transection of the supraspinatus muscle at 2 weeks, with reattachment by 4 weeks. Muscle mass and length were significantly lower in released muscles at 2 weeks, 4 weeks, and 9 weeks. Sarcomere lengths in released muscles were significantly shorter at 2 weeks but not different by 4 weeks. Sarcomere number was significantly reduced at 2 and 4 weeks, but returned to control values by 9 weeks. The opposing effects of smaller mass and shorter fibers produced significantly smaller PCSA at 2 weeks, but PCSA returned to control levels by 4 weeks. Conclusions Release of the supraspinatus muscle produced early radial and longitudinal atrophy of the muscle. The functional implications of these adaptations would be most profound at early time points (particularly relevant for rehabilitation), when the muscle remains smaller in cross-sectional area and, due to reduced sarcomere number, would be forced to operate over a wider range of the length-tension curve and at higher velocities, all adaptations resulting in compromised force generating capacity. These data are relevant to physical therapy because they provide tissue-level insights into impaired muscle and shoulder function following rotator cuff injury.

The Clinical and Sonographic Effects of Kinesiotaping and Exercise in Comparison With Manual Therapy and Exercise for Patients With Subacromial Impingement Syndrome: A Preliminary Trial

“…Also, architectural parameters such as physiological cross-sectional area and normalized muscle fiber length (serial sarcomere number) can predict muscle maximal force, excursion, and velocity, which is particularly important for physical therapists. 72 Thus, as an evidence-based outcome measure, the sonographic assessment of cross-sectional supraspinatus tendon thickness was used because significant functional improvement may be related to structural changes of the tendon. 18,63,73 The finding of this study pointed out that not only the manual therapy intervention but also the kinesiotape intervention with exercise had no effect on tendon structure within 6 weeks.…”

Contributions of Neural Tone to In Vivo Passive Muscle–Tendon Unit Biomechanical Properties in a Rat Rotator Cuff Animal Model

“…17,18,39 Studies in rat, 14,15 sheep, 6,13 dog, 39 and human rotator cuff 7 have demonstrated that the muscle-tendon unit becomes retracted and stiffer after injury, which is attributed to changes in the muscle's structure. 6,[13][14][15]39 Specifically, as the muscle atrophies following tendon injury, it undergoes sarcopenia, 45 decreased muscle volume, 2,29,39 fibrofatty infiltration, 2,29,30,39 decreased capillary density, 6,13,20,39 and muscle fiber-type switching. 2 In some instances, full excursion of the injured muscle-tendon unit back to its anatomical footprint on the humeral head is not possible during surgical repair of torn rotator cuffs.…”

Mechanisms of fatty degeneration in massive rotator cuff tears

“…The unloading of tensile forces results in radial and longitudinal atrophy of the muscle. 47 Ultrastructural changes include myofibril disorganization with decreased sarcomere numbers and lengths. 3,29 A reduction in muscle mass and volume occurs after myofibril dissolution and degeneration.…”