METHODS: The literature was accessed through MEDLINE. Key words used were "manipulation," "pain," and "chiropractic." This search was complemented by citation reviews of important research and chapters on the topic. Only studies that directly measured the effect of at least a single spinal manipulation on pain (eg, tenderness, biochemical assay, referred pain) were selected. The selected studies were reviewed descriptively; no systematic assessment of their quality was conducted.
RESULTS: The electronic search yielded 738 citations. Six hundred and forty-two were relevant to chiropractic. Of these, most were clinically descriptive articles about diagnostic and therapeutic procedures or case management. Most of the remaining articles were clinical trial reports or letters to the editor. Only 5 studies were selected according to the established criteria. Thus less than 1% of the indexed literature on chiropractic, manipulation, and pain involved studies that explored the mechanism of the putative effect of spinal manipulation on pain mechanisms. Six other studies were retrieved from citation reviews. These 11 studies were reviewed in order publication.
CONCLUSION: Few studies have investigated the effects of spinal manipulation on pain directly. If the theory of manipulation exerting its therapeutic effects posits that the sensory input created by the intervention results in some form of inhibition of pain, then the results of these studies are largely consistent with one another and with this theory. This review has highlighted the deficiencies in the extant studies and many remaining questions. Only more high-quality research will permit a full elucidation of the hypoalgesic effects of spinal manipulation.
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