There are traditional and scientifically-based hypotheses regarding the mechanism of action of chiropractic and spinal manipulation. There is overlap between some of these theories, with research in several areas. However, the physiologic mechanism of spinal manipulation remains largely unknown.
Traditional theories: The vertebral subluxation hypothesis proposes that alterations in normal anatomical/physiologic relationships between contiguous articular structures result in disease, and that chiropractic/manipulative methods can reduce these positional abnormalities . "Vitalism" is the concept that the body has the innate ability to heal itself if relieved of spinal irritations or subluxations . Correction of subluxations has been suggested to restore the flow of life force throughout the body, resulting in a brief convalescence and a return to optimum health . There is limited scientific evidence in these areas .
The nerve compression hypothesis suggests that intervertebral subluxations can cause irritation or compression of spinal nerve roots and interfere with nerve transmission . The fixation hypothesis proposes that vertebral muscles become locked and lose range of motion, leading to the release of neurotoxic mediators and abnormal nerve conduction . The axoplasmic aberration hypothesis asserts that compression of spinal nerves or nerve roots may hinder axoplasmic transport and damage nerves.
It has been proposed that chiropractic may reduce nerve impingement at intervertebral foramina , alter the distribution of loads between joints , create gaps between joints and break up fibrous adhesions that interfere with normal function , improve range of motion , improve immune function ; and foster healing through the clinician-patient relationship .
Scientific research: Animal experiments report that vertebral displacement may alter the function of nerves arising from intervertebral muscles and influence heart rate and blood pressure . Human studies report possible changes in patterns of nerve conduction and reflexes during spinal manipulation, although the evidence is not definitive . Reduced sensitivity to painful stimuli has been reported in some studies of spinal manipulation , but not in others . Some studies report elevated plasma levels of substance P and endorphins following spinal manipulation, although other research reports no effects .
Problems in chiropractic research: Because spinal manipulation involves the hands-on application of a physical therapy, blinding in studies presents a challenge. Often, the effects of treatment are evaluated by those administering therapy. These individuals are not blinded to the type of treatment being administered (unlike assessors in pharmacologic studies in which active and placebo drugs are similar in appearance). This is a potential source of bias. Similarly, placebo control is difficult, and necessitates the use of "sham manipulation" . Existing studies are difficult to compare with each other, because methods of manipulation vary between trials, and definitions of medical conditions/diagnoses are inconsistent. Most research has used non-standardized, subjective outcome measures that cannot be pooled.