Mechanism of action of spinal manipulative therapy.

Spinal manipulative therapy (SMT) acts on the various components of the vertebral motion segment. SMT distracts the facet joints, with faster separation when a cracking sound is heard. Intradiscal pressure may decrease briefly. Forceful stretching of the paraspinal muscles occurs, which induces relaxation via mechanisms that remain to be fully elucidated. Finally, SMT probably has an inherent analgesic effect independent from effects on the spinal lesion. These changes induced by SMT are beneficial in the treatment of spinal pain but short-lived. To explain a long-term therapeutic effect, one must postulate a reflex mechanism, for instance the disruption of a pain-spasm-pain cycle or improvement of a specific manipulation-sensitive lesion, whose existence has not been established to date.