Sato EJ, Killian ML, Choi AJ, Lin E, Esparza MC, Galatz LM, Thomopoulos S, Ward SR.
Rotator cuff tears can cause irreversible changes (e.g., fibrosis) to the
structure and function of the injured muscle(s). Fibrosis leads to increased
muscle stiffness resulting in increased tension at the rotator cuff repair site.
This tension influences repairability and healing potential in the clinical
setting. However, the micro- and meso-scale structural and molecular sources of
these whole-muscle mechanical changes are poorly understood. Here, single muscle
fiber and fiber bundle passive mechanical testing was performed on rat
supraspinatus and infraspinatus muscles with experimentally induced massive
rotator cuff tears (Tenotomy) as well as massive tears with chemical denervation
(Tenotomy + BTX) at 8 and 16 weeks post-injury. Titin molecular weight, collagen
content, and myosin heavy chain profiles were measured and correlated with
mechanical variables. Single fiber stiffness was not different between controls
and experimental groups. However, fiber bundle stiffness was significantly
increased at 8 weeks in the Tenotomy + BTX group compared to Tenotomy or control
groups. Many of the changes were resolved by 16 weeks. Only fiber bundle passive
mechanics was weakly correlated with collagen content. These data suggest that
tendon injury with concomitant neuromuscular compromise results in extra-cellular
matrix production and increases in stiffness of the muscle, potentially
complicating subsequent attempts for surgical repair.
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