OBJECTIVE: The objective of this paper is to review the literature on the audible release associated with manipulation.

DATA SOURCES: Bibliographic information in pertinent articles and papers located in the MEDLINE database containing the keywords joint, joints, cartilage, crack, cracking, cavitation, crepitus and noise.

STUDY SELECTION: All articles relevant to the objectives were selected.

DATA EXTRACTION: All available data was used.

DATA SYNTHESIS: The audible release is caused by a cavitation process whereby a sudden decrease in intracapsular pressure causes dissolved gasses in the synovial fluid to be released into the joint cavity. Once a joint undergoes cavitation, the force-displacement curve changes and the range of motion of the joint increases. The gasses released from the synovial fluid make up about 15% of the joint volume and consist of approximately 80% carbon dioxide. Habitual joint cracking does not correlate with arthritic changes, but does correlate with loss of grip strength and soft-tissue swelling. During the "crack" associated with a joint manipulation, there is a sudden joint distraction that occurs in less time than that required to complete the stretch reflexes of periarticular muscles. Theories on the cavitation mechanism were reviewed and new information on the cavitation process is introduced. In this paper, it is proposed that the cavitation process is generated by an elastic recoil of the synovial capsule as it "snaps back" from the capsule/synovial fluid interface.

CONCLUSIONS: Because the sudden joint distraction during a manipulation occurs in a shorter time period than that required to complete the stretch reflexes of the periarticular muscles, there is likely to be a high impulse acting on the ligaments and muscles associated with the joint. This is an important conclusion, because others have proposed that reflex actions from high threshold periarticular receptors are associated with the many beneficial results of manipulation. This suggests that the cavitation process provides a simple means for initiating the reflex actions and that without the cavitation process, it would be difficult to generate the forces in the appropriate tissue without causing muscular damage.

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