A 2008 critical review found that with the possible exception of back pain, chiropractic SM has not been shown to be effective for any medical condition, and suggested that many guidelines recommend chiropractic care for low back pain because no therapy has been shown to make a real difference,[11] but a 2008 supportive review found serious flaws in the critical approach and found that SM and mobilization are at least as effective for chronic low back pain as other efficacious and commonly used treatments.[96] Most research has focused on spinal manipulation (SM) in general,[97] rather than solely on chiropractic SM.[15] A 2002 review of randomized clinical trials of SM[35] was criticized for not distinguishing between studies of SM in general, and studies on chiropractic SM in particular;[98] however the review's authors stated that they did not consider this difference to be a significant point as research on SM is equally useful regardless of which practitioner provides it.[97]

There is a wide range of ways to measure treatment outcomes.[99] Chiropractic care, like all medical treatment, benefits from the placebo response.[100] It is hard to construct a trustworthy placebo for clinical trials of spinal manipulative therapy (SMT), as experts often disagree about whether a proposed placebo actually has no effect.[101] The efficacy of maintenance care in chiropractic is unknown.[16]

• The sound is caused by a bubble of nitrogen gas forming in the joint. This occurs through a pressure drop which is created when the joint is forced to the extreme of its range of motion. After the "pop", the joint capsule is temporarily enlarged, which also increases the neural firing in the joint's proprioception receptors. These signals transmit over the local nerve root. This increased neural activity inhibits pain signals from smaller nerve fibres in the same dermatome - the area of skin supplied by the same nerve root.

The only known effect of repeated joint-popping is in the joints of the spine, and results in a reduction in the financial content of the patient's purse: chiropractic spinal treatments rely on eliciting the sound - and offering brief relief - but later when symptoms recur (presumably as the gas is reabsorbed) the cyclic need for treatment resumes.

Don L Jewett, Professor Emeritus of Orthopaedic Surgery, University of California, San Francisco, US

We agree that many of the randomized trials we described were on spinal manipulation rather than specifically on chiropractic manipulation itself, but we believe that this is not a significant point. Chiropractors use all forms of manipulation. In the United States, more than 90% of all spinal manipulation services are provided by chiropractors, and research on spinal manipulation, like that on any other treatment method, is equally of value regardless of the practitioner providing it.