Changes in Rotator Cuff Muscle Volume, Fat Content, and Passive Mechanics After Chronic Detachment in a Canine Model

Abstract

The chronically detached muscle is not merely a smaller version of the original muscle but, rather, a different muscle. The detached muscle becomes stiffer, and the passive loads required to repair it can become excessive. A significant reduction in muscle volume occurs within days to weeks following tendon detachment (p < 0.0001). The nonuniformity of changes in muscle fat suggests that fat content should be used cautiously as an indicator of muscle quality.

Mechanisms of fatty degeneration in massive rotator cuff tears

“…2). 20,25,26,42,45 Whereas some studies have suggested that chronic unloading of the muscle-tendon unit leads to fatty degeneration, other groups have postulated that the massive tear induces a neurologic injury, with the ensuing fatty degeneration being a byproduct of the denervation of the affected muscle. Investigators have accounted for anatomic differences among the different species through rigorous anatomic and biomechanical testing, such as the use of the subscapularis in the rabbit model.…”

“…3,29 A reduction in muscle mass and volume occurs after myofibril dissolution and degeneration. 45 Over time, muscle retraction leads to disuse atrophy with a progressive pattern of fibrosis and increased fatty content. 38 Morphologic studies report that fat accumulates in intrafascicular, extrafascicular, and intratendinous locations within the muscle.…”

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.…”

Chemical Denervation with Botulinum Neurotoxin A Improves the Surgical Manipulation of the Muscle–Tendon Unit: An Experimental Study in an Animal Model

“…2,12,13 After tendon rupture, the muscle undergoes many structural changes as it atrophies, including fibro-fatty infiltration of the muscle belly, 14 -18 decreased capillary density, 19 and reduced muscle volume. 15,16,20 These structural changes result in a stiff and retracted muscle-tendon unit that is difficult to manipulate surgically. 15,[21][22][23] After these changes, the repair tension required to reattach the ruptured muscle-tendon unit to its anatomical insertion progressively increases from the time of injury.…”

“…15,16,20 These structural changes result in a stiff and retracted muscle-tendon unit that is difficult to manipulate surgically. 15,[21][22][23] After these changes, the repair tension required to reattach the ruptured muscle-tendon unit to its anatomical insertion progressively increases from the time of injury. This increased repair tension results in impaired healing biomechanics.…”