The inflammatory response of the supraspinatus muscle in rotator cuff tear conditions

Background Rotator cuff (RC) disorders involve a spectrum of shoulder conditions from early tendinopathy to full-thickness tears leading to impaired shoulder function and pain. The pathology of RC disorder is, nonetheless, still largely unknown. It is our hypothesis that supraspinatus (SS) tendon tear leads to sustained inflammatory changes of the SS muscle along with fatty infiltration and muscle degeneration, which are threshold markers for poor RC muscle function. The aim of this study was to determine the extent of this muscle inflammation in conjunction with lipid accumulation and fibrosis in RC tear conditions. Methods We used proteomics, histology, electrochemiluminescence immunoassay, and qPCR analyses to evaluate inflammatory and degenerative markers and fatty infiltration in biopsies from 22 patients undergoing surgery with repair of a full- thickness supraspinatus (SS) tendon tear. Results Bioinformatic analysis showed that proteins involved in innate immunity, extracellular matrix organization, and lipid metabolism were among the most upregulated whereas mitochondrial electronic transport chain along with muscle fiber function were among the most downregulated. Histological analysis confirmed changes in muscle fiber organization and the presence of inflammation and fatty infiltration. Inflammation appeared to be driven by a high number of infiltrating macrophages, accompanied by elevated matrix metalloprotease levels and changes in transforming growth factor-β and cytokine levels in the SS compared to the deltoid muscle. Conclusions We demonstrated massive SS muscle inflammation after tendon tear combined with fatty infiltration and degeneration. The regulation of tissue repair is thus extremely complex, and it may have opposite effects at different time points of healing. Inhibition or stimulation of muscle inflammation may be a potential target to enhance outcome of the repaired torn RC.