The effects of pressure on arthritic knees in a rat model of CFA-induced arthritis.

BACKGROUND Pain is influenced by weather changes under certain circumstances, and inflammatory pain in animal models is ameliorated by pressure, but the underlying mechanism of atmospheric pressure has not been clearly elucidated. OBJECTIVE To examine the effect of pressure on pain in an arthritic animal model. STUDY DESIGN Controlled animal study. SETTING Laboratory animal study. METHODS Following an injection of complete Freund's adjuvant (CFA) into one side of a knee joint, 32 rats were assigned randomly to 2 groups and either placed under 1 or 2.5 atmospheres absolute (ATA) in a hyperbaric chamber for 5 hours. The pain levels were assessed daily for up to 2 weeks post-injection to determine the changes in weight bearing (WB) of the affected limbs. In addition, the levels of gelatinase, MMP-2, and MMP-9 expression in the synovial fluids of the knees were analyzed. RESULTS After arthritis induction, the rats in the 1 ATA group showed reduced WB of the affected limbs (< 10% of normal limbs). This reduction in WB peaked at 2 days after the injection and then decreased spontaneously. Nevertheless, the pain behavior lasted for more than 2 weeks. In the 2.5 ATA group, the WB was significantly better during the experiment.  The MMP-9/MMP-2 ratio increased at 7 and 14 days after the CFA injection in the 1 ATA group. However, repetitive exposure to 2.5 ATA significantly reduced this ratio in the 2.5 ATA group. LIMITATIONS Although a sufficient number of samples were used to support the hypothesis that high atmospheric pressure improves a painful condition in this study, an additional larger-scale study will be needed to confirm these findings. CONCLUSION Exposure to elevated pressures appears to relieve arthritic pain for extended periods by reducing the inflammatory process and should be considered as a possible alternative pain-reducing therapy.