The Mechanics of Knuckle Cracking
Scientific Evidence: Does Knuckle Cracking Cause Arthritis?
Potential Downsides of Knuckle Cracking
Possible Benefits of Knuckle Cracking
Knuckle Cracking: Harmless Habit or Cause for Concern?
To Crack Or Not To Crack: Final Verdict
References
How often have we been told not to crack our knuckles? The belief that cracking knuckles leads to arthritis is deeply embedded in popular culture. This notion has been perpetuated through generations, often cited as a cautionary tale against a seemingly harmless habit. The distinctive popping sound produced during knuckle cracking has also fueled concerns regarding potential joint damage.1
Despite the widespread nature of this belief, scientific research and scrutiny into this habit have revealed that the relationship between knuckle cracking and arthritis is more complex than it appears.
While the mere act of producing a sound from a joint can often evoke anxiety regarding joint health, understanding the underlying biomechanics and clinical evidence is crucial in separating fact from fiction.
In this article, we discuss the biological mechanism of knuckle cracking and explore scientific research on its mechanics and impact on the joints to determine whether this common habit poses a genuine risk to joint health.
Image Credit: Andrey_Popov/Shutterstock.com
The Mechanics of Knuckle Cracking
Knuckle cracking is a biomechanical phenomenon involving the synovial joints, particularly those in the fingers. These joints are surrounded by a capsule filled with synovial fluid, which lubricates and nourishes the cartilage. When a joint is manipulated beyond its resting position, the joint capsule stretches, leading to an increase in joint volume and a decrease in intra-articular pressure.2
This pressure drop allows gases, primarily nitrogen, to dissolve in the synovial fluid and form a bubble. This process, which is known as cavitation, produces the characteristic cracking or popping sound.3
Earlier theories suggested that the sound resulted from the collapse of the bubble; however, more recent imaging studies, including those involving magnetic resonance imaging (MRI), have suggested that the sound is generated at the moment the bubble forms rather than when it collapses.4
Knuckle cracking also causes temporary changes in joint pressure. One study determined that the pressure within the joint drops to approximately -3.5 atmospheres immediately before the cracking sound.
When the bubble forms, the pressure rapidly equalizes, resulting in the audible release. This sharp pressure fluctuation temporarily increases the joint space, which may explain the sensation of relief or increased mobility experienced by some individuals post-cracking.5
Additionally, after a joint is cracked, a refractory period follows, during which the joint cannot be broken again until gases are reabsorbed into the synovial fluid, which typically requires about 15 to 30 minutes.2
Although there has been a plethora of research on the biomechanical processes involved in knuckle cracking, there remains a longstanding concern regarding its potential impact on joint health.
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Scientific Evidence: Does Knuckle Cracking Cause Arthritis?
Anecdotal evidence often suggests that habitual knuckle cracking leads to arthritis, but scientific research paints a different picture. Several studies have examined the long-term impact of knuckle cracking on joint health. These studies consistently indicate that knuckle cracking is not associated with an increased risk of arthritis.
One of the earliest and most notable investigations is the self-experiment conducted by Dr. Donald Unger. Over the course of 50 years, Unger cracked the knuckles of his left hand every day while leaving his right hand uncracked.
After half a century, he found no difference in the presence of arthritis between his two hands.6,7 Although this study was anecdotal and had a sample size of one, it drew attention to the need for more rigorous research and earned Unger the satirical Ig Nobel prize in 2009.
A retrospective case-control study in 2011 involving 215 individuals aged 50 to 89, some with radiographically confirmed hand osteoarthritis and others without, further examined this link between knuckle cracking and arthritis.
Participants were surveyed regarding their knuckle-cracking history, including frequency and duration, and the study concluded that there was no significant correlation between knuckle-cracking and arthritis, regardless of the frequency or duration of the habit.8
Similarly, another study in 1990 conducted among 300 adults over the age of 45 compared the incidence of arthritis among habitual knuckle crackers and non-crackers.
Here too, the researchers found no increased prevalence of arthritis among knuckle crackers. Findings from another study among 28 elderly nursing home residents also revealed no association between knuckle cracking and arthritis, strengthening the consensus that this behavior is not a causative factor for joint degeneration.9
Here's what happens to your knuckles when you crack them
Potential Downsides of Knuckle Cracking
While arthritis might not be a serious concern, studies have identified some potential downsides of habitual knuckle cracking. Studies have reported that habitual knuckle crackers exhibited reduced grip strength compared to non-crackers. Moreover, the decrease in grip strength was statistically significant, raising questions about the potential impact on hand function over time.10
Temporary swelling is another commonly reported side effect. This mild inflammation is not indicative of joint damage but can cause discomfort and could be a result of repeated stretching of joint capsules and ligaments during the cracking process.10 However, research also shows that these effects are indeed temporary and do not appear to cause lasting damage.
In some rare cases, acute joint injuries related to knuckle cracking have been documented. These include ligament sprains, joint dislocations, and tendon damage, typically resulting from excessive force or aggressive joint manipulation.8
Such injuries underscore the importance of avoiding extreme or forceful joint cracking, particularly for individuals with pre-existing joint conditions or ligament laxity.
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Possible Benefits of Knuckle Cracking
Despite these concerns, some individuals have reported subjective relief following knuckle cracking. This relief is often attributed to the release of joint tension and increased joint mobility.
A prospective study involving ultrasound and physical examinations of 400 metacarpophalangeal joints found that knuckle cracking resulted in a small, immediate increase in joint range of motion without causing acute swelling or laxity.11
Additionally, the process of articular release, which underlies knuckle cracking, is a recognized component of osteopathic manipulative treatment. This technique aims to alleviate joint tension and restore a normal range of motion. Some studies have even suggested that knuckle cracking, when performed within normal limits, may offer temporary mechanical benefits.1
Knuckle Cracking: Harmless Habit or Cause for Concern?
While existing evidence largely dismisses the link between knuckle cracking and arthritis, concerns persist regarding other potential long-term effects. Excessive or forceful manipulation of joints can theoretically lead to ligamentous strain, joint instability, and soft tissue damage.10
Researchers have also hypothesized that the energy released during cavitation might damage articular cartilage over time.12
Furthermore, while this theoretical risk has not been substantiated by empirical research, individuals with hypermobility syndromes or pre-existing joint conditions should exercise caution, as habitual joint manipulation could exacerbate joint problems.1
Although there is limited evidence exploring the psychological aspects of habitual knuckle cracking, some individuals report feeling compelled to crack their knuckles to relieve tension. However, this phenomenon remains largely anecdotal and understudied in scientific literature.
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To Crack Or Not To Crack: Final Verdict
Overall, the hypothesis that knuckle cracking causes or increases the risk of arthritis is not supported by scientific evidence. Longitudinal studies and clinical evaluations consistently indicate that habitual knuckle cracking does not increase the risk of osteoarthritis.8,10
However, minor adverse effects, including reduced grip strength and transient swelling, have been reported in habitual knuckle crackers.10 While these issues are not indicative of degenerative joint disease, they highlight the importance of moderation.
Healthcare professionals generally view knuckle cracking as a benign habit for individuals without underlying joint conditions. Nevertheless, individuals experiencing pain, swelling, or functional limitations should seek medical advice. Furthermore, proper ergonomics, regular exercise, and a balanced diet remain the best approaches to preserving and improving joint health.
References
- Protapapas, M.G., & Cymet, T.C. (2002). Joint cracking and popping: Understanding noises that accompany articular release. The Journal of the American Osteopathic Association, 102(5), 283–287. https://doi.org/10.7556/jaoa.2002.102.5.283
- Rizvi, A., Loukas, M., Oskouian, R. J., & Tubbs, R. S. (2018). Let's get a hand on this: Review of the clinical anatomy of "knuckle cracking". Clinical anatomy, 31(6), 942–945. https://doi.org/10.1002/ca.23243
- Unsworth, A., Dowson, D., & Wright, V. (1971). 'Cracking joints'. A bioengineering study of cavitation in the metacarpophalangeal joint. Annals of the rheumatic diseases, 30(4), 348–358. https://doi.org/10.1136/ard.30.4.348
- Kawchuk, G. N., Fryer, J., Jaremko, J. L., Zeng, H., Rowe, L., & Thompson, R. (2015). Real-time visualization of joint cavitation. PLOS ONE, 10(4), e0119470. https://doi.org/10.1371/journal.pone.0119470
- Roston, J. B., & Haines, R. W. (1947). Cracking in the metacarpo-phalangeal joint. Journal of anatomy, 81(Pt 2), 165–173.
- Unger D. L. (1998). Does knuckle cracking lead to arthritis of the fingers?. Arthritis and rheumatism, 41(5), 949–950. https://doi.org/10.1002/1529-0131(199805)41:5<949::AID-ART36>3.0.CO;2-3
- Gaetano J. (2009). Cracking the cracked knuckle: a medical student's take. The Journal of rheumatology, 36(11), 2624. https://doi.org/10.3899/jrheum.090530
- deWeber, K., Olszewski, M., & Ortolano, R. (2011). Knuckle cracking and hand osteoarthritis. The Journal of the American Board of Family Medicine, 24(2), 169–174. https://doi.org/10.3122/jabfm.2011.02.100156
- Swezey, R.L., & Swezey, S.E. (1975). The consequences of habitual knuckle cracking. Western Journal of Medicine, 122(5), 377–379.
- Castellanos, J., & Axelrod, D. (1990). Effect of habitual knuckle cracking on hand function. Annals of the rheumatic diseases, 49(5), 308–309. https://doi.org/10.1136/ard.49.5.308
- Boutin, R. D., Netto, A. P., Nakamura, D., Bateni, C., Szabo, R. M., Cronan, M., Foster, B., Barfield, W. R., Seibert, J. A., & Chaudhari, A. J. (2017). "Knuckle Cracking": Can Blinded Observers Detect Changes with Physical Examination and Sonography?. Clinical Orthopaedics And Related Research, 475(4), 1265–1271. https://doi.org/10.1007/s11999-016-5215-3
- Watson, P., Kernohan, W. G., & Mollan, R. A. (1989). A study of the cracking sounds from the metacarpophalangeal joint. Proceedings of the Institution of Mechanical Engineers. Part H, Journal Of Engineering In Medicine, 203(2), 109–118. https://doi.org/10.1243/PIME_PROC_1989_203_019_01
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