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Background: Clinical use of platelet-rich plasma (PRP) and mesenchymal stem cells (MSCs) has gained momentum as treatment for muscle injuries. Exosomes, or small cell–derived vesicles, could be helpful if they could deliver the same or better physiological effect without cell transplantation into the muscle. Hypothesis: Local delivery of exosomes derived from PRP (PRP-exos) or MSCs (MSC-exos) to injured muscles hastens recovery of contractile function. Study Design: Controlled laboratory study. Methods: In a rat model, platelets were isolated from blood, and MSCs were isolated from bone marrow and expanded in culture; exosomes from both were isolated through ultracentrifugation. The tibialis anterior muscles were injured in vivo using maximal lengthening contractions. Muscles were injected with PRP-exos or MSC-exos (immediately after injury and 5 and 10 days after injury); controls received an equal volume of saline. Histological and biochemical analysis was performed on tissues for all groups. Results: Injury resulted in a significant loss of maximal isometric torque (66% ± 3%) that gradually recovered over 2 weeks. Both PRP-exos and MSC-exos accelerated recovery, with similar faster recovery of contractile function over the saline-treated group at 5, 10, and 15 days after injury (P <.001). A significant increase in centrally nucleated fibers was seen with both types of exosome groups by day 15 (P <.01). Genes involved in skeletal muscle regeneration were modulated by different exosomes. Muscles treated with PRP-exos had increased expression of Myogenin gene (P <.05), whereas muscles treated with MSC-exos had reduced expression of TGF-β (P <.05) at 10 days after muscle injury. Conclusion: Exosomes derived from PRP or MSCs can facilitate recovery after a muscle strain injury in a small-animal model likely because of factors that can modulate inflammation, fibrosis, and myogenesis. Clinical Relevance: Given their small size, low immunogenicity, and ease with which they can be obtained, exosomes could represent a novel therapy for many orthopaedic ailments. [ABSTRACT FROM AUTHOR]

Background: Massive rotator cuff tears (RCTs) begin as primary tendon injuries and cause a myriad of changes in the muscle, including atrophy, fatty infiltration (FI), and fibrosis. However, it is unclear which changes are most closely associated with muscle function. Purpose: To determine if FI of the supraspinatus muscle after acute RCT relates to short-term changes in muscle function. Study Design: Controlled laboratory study. Methods: Unilateral RCTs were induced in female rabbits via tenotomy of the supraspinatus and infraspinatus. Maximal isometric force and rate of fatigue were measured in the supraspinatus in vivo at 6 and 12 weeks after tenotomy. Computed tomography scanning was performed, followed by histologic analysis of myofiber size, FI, and fibrosis. Results: Tenotomy resulted in supraspinatus weakness, reduced myofiber size, FI, and fibrosis, but no differences were evident between 6 and 12 weeks after tenotomy except for increased collagen content at 12 weeks. FI was a predictor of supraspinatus weakness and was strongly correlated to force, even after accounting for muscle cross-sectional area. While muscle atrophy accounted for the loss in force in tenotomized muscles with minimal FI, it did not account for the greater loss in force in tenotomized muscles with the most FI. Collagen content was not strongly correlated with maximal isometric force, even when normalized to muscle size. Conclusion: After RCT, muscle atrophy results in the loss of contractile force from the supraspinatus, but exacerbated weakness is observed with increased FI. Therefore, the level of FI can help predict contractile function of torn rotator cuff muscles. Clinical Relevance: Markers to predict contractile function of RCTs will help determine the appropriate treatment to improve functional recovery after RCTs. [ABSTRACT FROM AUTHOR]