Early-stage inflammation changes in supraspinatus muscle after rotator cuff tear.

Background: Rotator cuff (RC) tendon tear leads to impaired shoulder function and pain. The supraspinatus (SS) tendon is most often affected, but the biological response of the SS muscle to SS tendon tear is largely unknown. This study aimed to investigate time-dependent muscle inflammation, degeneration, fatty infiltration, and regeneration in experimental SS tear conditions.

Methods: Forty-five C57BL/6 mice were subjected to SS tendon tear and allowed to recover for 1, 3, 5, 7, 14, or 28 days. The extent of muscle damage was examined using histologic, flow cytometric, proteomic, and chemiluminescence analyses.

Results: We found that muscle inflammation peaked around day 5 with increased monocyte infiltration and increased cytokine levels in the ipsilateral compared to the contralateral SS muscle. Bioinformatics analysis of proteomics on mice that survived 5 days after RC tendon tear revealed upregulated proteins involved in "neutrophil activation involved in immune response" and "extracellular matrix organization," whereas "skeletal muscle tissue development and contraction" and "respiratory electron transport chain" were among the most downregulated. Histologic analysis of collagen showed increased collagen accumulation and fatty infiltration of the ipsilateral SS over time. Finally, we observed time- and lesion-dependent changes in satellite cell and fibro-adipogenic progenitor populations.

Conclusion: Altogether, we demonstrate that the SS muscle shows severe signs of acute inflammation, early degeneration, and fatty infiltration, as well as reduced regenerative potential following SS tendon tear.