Effects of Vibration Therapy on Immobilization-Induced Hypersensitivity in Rats.

BACKGROUND Cast immobilization induces mechanical hypersensitivity, which disturbs rehabilitation. Although vibration therapy can reduce various types of pain, whether vibration reduces immobilization-induced hypersensitivity remains unclear. OBJECTIVE The purpose of this study was to investigate the preventive and therapeutic effects of vibration therapy on immobilization-induced hypersensitivity. DESIGN The experimental design of the study involved conducting behavioral, histological, and immunohistochemical studies in model rats. METHODS Thirty-five Wistar rats (8 weeks old, all male) were used. The right ankle joints of 30 rats were immobilized by plaster cast for 8 weeks, and 5 rats were used as controls. The immobilized rats were divided randomly into the following 3 groups: (1) immobilization-only group (Im, n=10); (2) vibration therapy group 1, for which vibration therapy was initiated immediately after the onset of immobilization (Im+Vib1, n=10); and (3) vibration therapy group 2, for which vibration therapy was initiated 4 weeks after the onset of immobilization (Im+Vib2, n=10). Vibration was applied to the hind paw. The mechanical hypersensitivity and epidermal thickness of the hind paw skin were measured. To investigate central sensitization, calcitonin gene-related peptide (CGRP) expression in the spinal cord and dorsal root ganglion (DRG) was analyzed. RESULTS Immobilization-induced hypersensitivity was inhibited in the Im+Vib1 group but not in the Im+Vib2 group. Central sensitization, which was indicated by increases in CGRP expression in the spinal cord and the size of the area of CGRP-positive neurons in the DRG, was inhibited in only the Im+Vib1 group. Epidermal thickness was not affected by vibration stimulation. LIMITATIONS A limitation of this study is that the results were limited to an animal model and cannot be generalized to humans. CONCLUSIONS The data suggest that initiation of vibration therapy in the early phase of immobilization may inhibit the development of immobilization-induced hypersensitivity.