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.