Extracorporeal shock wave therapy (ESWT) evolved from extracorporeal shock wave lithotripsy as a urinary tract lithotripsy procedure in the early 1990s [1]. ESWT effectively treats musculoskeletal pain, tendinopathy, and fasciitis by suppressing inflammation [[2], [3], [4]]. ESWT can be categorized into two groups: radial pressure waves (RPW) and focused shock waves (FSW). While FSW reaches its maximum energy at a focal point located deep in the body tissue, RPW has a more superficial effect. Nevertheless, RPW does not possess the typical physical features of authentic shock waves, such as a quick increase in time in the nanosecond range, a substantial peak pressure, and nonlinearity [5]; however, RPW and FSW seem to act similarly [6].

Both RPW and FSW have been widely used in the treatment of many musculoskeletal disorders, such as proximal plantar fasciitis, lateral and medial epicondylitis, calcifying tendonitis of the shoulder, and Achilles tendinopathy, and good clinical outcomes have been reported [4,6,7]. Several studies have been conducted on the inflammation and pain improvement mechanism of FSW. The application of FSW leads to the production of proteins, nitric oxide, and specific growth factors causing increased neo-angiogenesis, tenocyte and fibroblast proliferation, and collagen synthesis, further enhancing tissue catabolism, healing, and remodeling [8,9]. However, there have been few studies on the pain-relief mechanisms of RPW. Although Fu et al. suggested that RPW inhibits pain signal transmission by stimulating algesioreceptors and inducing the release of pain-inhibiting substances from the body, the pain relief mechanism of RPW is not fully clarified [10]. Using immunohistochemistry, Ohtori et al. reported that FSW induced degeneration and reinnervation of sensory nerve fibers in the skin of rats, which is one of the pain relief mechanisms of FSW [11]. Besides, complete Freund's adjuvant (CFA) has been used for the inflammatory pain model to assess pain sensitivity in several studies [[12], [13], [14]]. Chakrabarty reported that cutaneous hyperinnervation occurs after CFA injection and appears to be closely associated with inflammation and hypersensitivity [12]. Therefore, we hypothesized that RPW improves inflammatory pain, which is induced by the degeneration and reinnervation of sensory nerve fibers. This study aimed to elucidate the efficacy of RPW in a rat model of adjuvant arthritis.