In 2005, the chiropractic subluxation was defined by the World Health Organization as "a lesion or dysfunction in a joint or motion segment in which alignment, movement integrity and/or physiological function are altered, although contact between joint surfaces remains intact. It is essentially a functional entity, which may influence biomechanical and neural integrity." This differs from the medical definition of subluxation as a significant structural displacement, which can be seen with static imaging techniques such as X-rays. This exposes patients to harmful ionizing radiation for no evidentially supported reason. The 2008 book Trick or Treatment states "X-rays can reveal neither the subluxations nor the innate intelligence associated with chiropractic philosophy, because they do not exist." Attorney David Chapman-Smith, Secretary-General of the World Federation of Chiropractic, has stated that "Medical critics have asked how there can be a subluxation if it cannot be seen on X-ray. The answer is that the chiropractic subluxation is essentially a functional entity, not structural, and is therefore no more visible on static X-ray than a limp or headache or any other functional problem." The General Chiropractic Council, the statutory regulatory body for chiropractors in the United Kingdom, states that the chiropractic vertebral subluxation complex "is not supported by any clinical research evidence that would allow claims to be made that it is the cause of disease."
Among people seeking back pain relief alternatives, most choose chiropractic treatment. About 22 million Americans visit chiropractors annually. Of these, 7.7 million, or 35%, are seeking relief from back pain from various causes, including accidents, sports injuries, and muscle strains. Other complaints include pain in the neck, arms, and legs, and headaches.