Introduction: Rotator cuff tears are a common cause of upper-extremity disability. For chronic, full-thickness, rotator cuff tears, repair surgery can be technically challenging due to large gap distances and increased stiffness of the muscle-tendon unit. In contrast, acute rotator cuff tears are associated with lower repair tensions, less fibro-fatty infiltration of the muscle, and better functional outcomes. Animal models can be used to understand rotator cuff muscle function and these results can be translated into clinical recommendations using computational modeling.;Methods: In vivo muscle function, electromyography, and passive muscle-tendon unit properties were studied before and after supraspinatus tenotomy in a rodent rotator cuff injury model (acute vs chronic). Chemical denervation and Fung's QLV model were used to assess neural contributions to in vivo stress-relaxation biomechanical properties. Then, a series of simulation experiments were conducted using a validated computational human musculoskeletal shoulder model to assess both passive and active tension of rotator cuff repairs based on surgical positioning.;Results: Muscle function was impaired at the tensions required to repair a chronically torn rotator cuff (45% reduction from maximal twitch amplitude, p<0.05). Dysfunction in the chronic tear setting was detectable via EMG (p<0.05). Although the chronically injured muscle-tendon unit becomes more stiff; pharmacological modulation of the nervous system with BoNT-A improves compliance by approximately 20% (p<0.05). At adducted postures, computational data from simulated surgical repair of chronically torn rotator cuff indicated that passive repair tension markedly exceeds the pullout strength of fixation techniques typically used in these surgeries and there was approximately a 50% reduction in moment generating capacity after repair of chronically torn rotator cuff.;Discussion and Conclusion: Rotator cuff surgical outcomes may be improved by earlier intervention, which results in lower surgical repair tensions and fewer electromyographic neuromuscular changes. Our data suggest a direct experimental connection between high repair tensions---chronic tear setting---and impaired contractile force. The pharmacological modulation of increased muscle-tendon unit stiffness using BoNT-A has the potential to facilitate the surgical manipulation of the muscle-tendon unit and protect the repaired tendon. Simulation analysis stresses the importance of proper arm positioning during intra-operative repair, post-operative healing, and rehabilitation.
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