Brodeur, R. (1995). The audible release associated with joint manipulation. Journal of Manipulative & Physiological Therapeutics, mars-avril 1995, 18(3), 155-64.

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. [References: 40]


1] Brodeur R: The audible release associated with joint manipulation. Journal of Manipulative & Physiological Therapeutics. 18(3):155-64, 1995

[...] Damage Due to Habitual Joint Cracking: There has been very little work done to determine the long-term effects of habitual joint cracking. The scant literature that is available on this topic indicates that habitual knuckle cracking may have a direct effect on the soft tissue of the hands and there is a case study that indicates it may also cause damage that leads to radiologic change (17). However, there is insufficient evidence to make any conclusive statement regarding the long-term effects of habitual knuckle cracking. [review of the Swezey and Swezey report: no significant difference.] [review of the Catellanos and Axelrod report.] The knuckle crackers had approximately 75% less grip strength and a higher incidence of hand swelling. Because the average duration of the knuckle cracking habit was 35 +/- 18 years, the habit seems to have little effect on joint cartilage. The increase in joint swelling and the decrease in hand function indicates that habitual knuckle cracking has a greater effect on the soft tissue than on the bone or cartilage. However, damage to the cartilage cannot be ruled out. Watson et al [review of Watson et al.] [ ... ] Joint Damage from Habitual Joint Cracking [ ... ] Watson et al estimated the energy released by the cracking sound to be 0.07 mJ/mm3 (15). Cartilage requires impact energies on the order of 1.0 mJ/mm3 to cause damage to the articular structure and chondrocyte death (26). The effect of a single joint crack is less than 10% of this value and the energy released would be released in terms of damage to cartilage. Watson et al. argue that the effects of habitual joint cracking may be additive: the energy released during caviatation may be, over a period of time, sufficient to damage the articular cartilage (5,17,23). Although Watson et al. proposed this thery as a cause of direct damage to the joint cartilage, there is little clinical evidence to support this mechanism. [ ... other possible damage mechanisms ...] The evidence of tissue swelling with habitual joint cracking indicates that the only area of injury would be at the proposed snap-back interface between the capsular ligament and the synovial fluid. Microtrauma may occur at the portion of the ligament involved in the snap-back and excessive joint cracking may ultimately lead to swelling of the ligament. This mechanism is completely speculative, but at least it has the support of clinical evidence (19). [ ... ]
[2] Castellanos J. Axelrod D. Effect of habitual knuckle cracking on hand function. Annals of the Rheumatic Diseases. 49(5):308-9, 1990
Discussion The development of arthritis of the hand as a result of habitual knuckle-cracking has been considered an old wives' tale. Swezey reviewed 28 nursing home patients who could recall whether or not they had cracked their knuckles. Among these patients, a relationship between knuckle cracking and arthritis could not be found[1]. Indeed, metacarpophalangeal osteophytes were found in patients who had not been habitual knuckle crackers. Yet a bioengineering study of cracking joints suggested the potential for significant joint damage.[2] When tension is applied to the joint, cavitation occurs within the synovial fluid. This creates an unstable condition as the pressure within the bubble is lower than that of the surrounding fluid. Because the joint separation occurs at a high rate the net flow of synovial fluid is toward the low pressure regions, with a collapse of the vapour phase of the cavity. There is a release of vibratory energy, which may be responsible for the cracking sound. It is this phenomenon which is responsible for the erosion of ship propellers and the blades of hydraulic turbines.[3] Given the potential damage caused by this cavitation phenomenon, one might expect habitual knuckle cracking to cause some decrement in hand function, if not accelerate the onset of osteoarthritis of the hand. Of the 300 patients studied, 74 admitted to habitual knuckle cracking for 35 (18) years. Their sex distribution was similar to that of those denying knuckle cracking. Those patients who were habitual knuckle crackers were more likely to have swelling of the hand and lower grip strength (table 2). Other factors which might influence hand function, such as carpal tunnel syndrome, contractures, surgery or trauma to the hand, and the presence of Heberden's or Beouchard's nodes, were equally present in both patient groups. Habitual knuckle crackers however, were more likely to be manual laborers with higher incomes (tables 1 and 2). Although the cause of habitual knuckle cracking was not considered in this study, patients admitting to it were more likely to bite their nails, smoke, and drink alcohol, as were members of their families (table 2). This study suggests that although habitual knuckle cracking does not relate to osteoarthritis of the hand, it may relate to decreased hand function. Therefore, habitual knuckle cracking should be discouraged.
[3] Simkin PA. Habitual knuckle cracking and hand function. Annals of the Rheumatic Diseases. 49(11):957, 1990
SIR: In a recent survey Castellanos and Axelrod evaluated 300 consecutive outpatients at Mount Carmel Mercy Hospital to determine whether habitual knuckle cracking is a risk factor for hand dysfunction. They found no relation with osteoarthritis, but noted that 'knuckle crackers were more likely to have hand swelling and lower grip strength' and concluded that 'habitual knuckle cracking results in functional hand impairment. I believe they have not established cause and effect in these interesting correlations. Not everyone can crack their knuckles. Some do so with ease, whereas others are quite incapable of performing this feat. No one has determined how the joints of these groups differ. It is quite possible, for instance, that metacarpophalangeal joint laxity may both facilitate knuckle cracking and impair hand function. As this hypothesis implies that hand swelling and diminished grip occur secondary to articular structure rather than abuse, it may be that nervous citizens of Detroit can continue to crack their knuckles without fear of injury. 'Will cracking my knuckles hurt my hands?' remains a common gambit when a rheumatologist is identified as such among new acquaintances striving to make conversation. I still believe that the answer to this question is no, but perhaps it is time that we really found out. 
[4] Watson P. Hamilton A. Mollan R. Habitual joint cracking and radiological damage. BMJ. 299(6715):1566, 1989
A 25 year old Malaysian man who habitually elicited cracking sounds from many of his joints was investigated during a study of joint cracking. He had no symptoms or obvious abnormalities of his joints, but a radiograph of his right hand showed ligamentous ossification on the ulnar side of his third metacarpophalangeal joint and chondrocalcinosis in the first and fourth metacarpophalangeal joints (figure). There was no evidence of osteoarthritis. Distraction of the articular surfaces during finger pulling lowers the pressure of the articular fluid. When the vapour pressure is reached the fluid evaporates, giving a cracking sound and forming an intra-articular bubble. Previous studies have reached conflicting conclusions about the radiological changes found in habitual finger cracking,[1,2], but we suggest that excessive joint cracking caused the changes seen in this subject, who had no signs of any other underlying disease.
[5] Swezey RL. Swezey SE. The consequences of habitual knuckle cracking. Western Journal of Medicine. 122(5):377-9, 1975
[ ... ] The patient population consisted of 28 persons, with an average age of 78.5 years, of whom 23 were women and 5 were men. [...] [ from Table 2: 11 women and 4 men were habitual knuckle-crackers ] 

Conclusion The data fail to support evidence that knuckle cracking leads to degenerative changes in the matacarpal phalangeal joints in old age. The chief morbid consequences of knuckle cracking would appear to be its annoying effect on the observer. 

[6] Unsworth A. Dowson D. Wright V. 'Cracking joints'. A bioengineering study of cavitation in the metacarpophalangeal joint. Annals of the Rheumatic Diseases. 30(4):348-58, 1971