Exposure to radial extracorporeal shock waves modulates viability and gene expression of human skeletal muscle cells: a controlled in vitro study

Background: Recent clinical and animal studies have shown that extracorporeal shock wave therapy has a promoting influence on the healing process of musculoskeletal disorders. However, the underlying biological effects of extracorporeal shock wave therapy on human skeletal muscle cells have not yet been investigated. Methods: In this study, we investigated human skeletal muscle cells after exposure to radial extracorporeal shock waves in a standardized in vitro setup. Cells were isolated from muscle specimens taken from adult patients undergoing spine surgery. Primary muscle cells were exposed once or twice to radial extracorporeal shock waves in vitro with different energy flux densities. Cell viability and gene expression of the paired box protein 7 (Pax7), neural cell adhesion molecule (NCAM), and myogenic factor 5 (Myf5) and MyoD as muscle cell markers were compared to non-treated muscle cells that served as controls. Results: Isolated muscle cells were positive for the hallmark protein of satellite cells, Pax7, as well as for the muscle cell markers NCAM, MyoD, and Myf5. Exposure to radial extracorporeal shock waves at low energy flux densities enhanced cell viability, whereas higher energy flux densities had no further significant impact. Gene expression analyses of muscle specific genes (Pax7, NCAM, Myf5, and MyoD) demonstrated a significant increase after single exposure to the highest EFD (4 bar, 0.19 mJ/mm(2)) and after double exposure with the medium EFDs (2 and 3 bar; 0.09 and 0.14 mJ/mm(2), respectively). Double exposure of the highest EFD, however, results in a significant down-regulation when compared to single exposure with this EFD. Conclusions: This is the first study demonstrating that radial extracorporal shock wave therapy has the potential to modulate the biological function of human skeletal muscle cells. Based on our experimental findings, we hypothesize that radial extracorporal shock wave therapy could be a promising therapeutic modality to improve the healing process of sports-related structural muscle injuries.