Joint cracking is the manipulation of

joint A joint or articulation (or articular surface) is the connection made between bones, ossicles, or other hard structures in the body which link an animal's skeletal system into a functional whole.Saladin, Ken. Anatomy & Physiology. 7th ed. McGraw- ...

s to produce a sound and related "popping" sensation. It is sometimes performed by

physical therapist Physical therapy (PT), also known as physiotherapy, is a healthcare profession, as well as the care provided by physical therapists who promote, maintain, or restore health through patient education, physical intervention, disease prevention ...

chiropractor Chiropractic () is a form of alternative medicine concerned with the diagnosis, treatment and prevention of physical disorder, mechanical disorders of the musculoskeletal system, especially of the vertebral column, spine. It is based on sever ...

s, and osteopaths pursuing a variety of outcomes. The cracking mechanism and the resulting sound is caused by dissolved gas (nitrogen gas)

cavitation Cavitation in fluid mechanics and engineering normally is the phenomenon in which the static pressure of a liquid reduces to below the liquid's vapor pressure, leading to the formation of small vapor-filled cavities in the liquid. When sub ...

bubbles suddenly collapsing inside the joints. This happens when the joint cavity is stretched beyond its normal size. The pressure inside the joint cavity drops and the dissolved gas suddenly comes out of solution and takes gaseous form which makes a distinct popping noise. To be able to crack the same knuckle again requires waiting about 20 minutes before the bubbles dissolve back into the synovial fluid and will be able to form again. It is possible for voluntary joint cracking by an individual to be considered as part of the

obsessive–compulsive disorder Obsessive–compulsive disorder (OCD) is a mental disorder in which an individual has intrusive thoughts (an ''obsession'') and feels the need to perform certain routines (''Compulsive behavior, compulsions'') repeatedly to relieve the dis ...

s spectrum.

Causes

For many decades, the physical mechanism that causes the cracking sound as a result of bending, twisting, or compressing joints was uncertain. Suggested causes included: *

Cavitation Cavitation in fluid mechanics and engineering normally is the phenomenon in which the static pressure of a liquid reduces to below the liquid's vapor pressure, leading to the formation of small vapor-filled cavities in the liquid. When sub ...

within the joint—small cavities of partial vacuum form in the

synovial fluid Synovial fluid, also called synovia, elp 1/sup> is a viscous, non-Newtonian fluid found in the cavities of synovial joints. With its egg white–like consistency, the principal role of synovial fluid is to reduce friction between the articul ...

and then rapidly collapse, producing a sharp sound. * Rapid stretching of ligaments. * Intra-articular (within-joint) adhesions being broken. *Formation of bubbles of joint air as the joint is expanded. There were several hypotheses to explain the cracking of joints. Synovial fluid cavitation has some evidence to support it. When a

spinal manipulation Spinal manipulation is an intervention performed on synovial joints of the spine, including the z-joints, the atlanto-occipital, atlanto-axial, lumbosacral, sacroiliac, costotransverse and costovertebral joints. It is typically applied w ...

is performed, the applied force separates the articular surfaces of a fully encapsulated synovial joint, which in turn creates a reduction in pressure within the joint cavity. In this low-pressure environment, some of the

gas Gas is a state of matter that has neither a fixed volume nor a fixed shape and is a compressible fluid. A ''pure gas'' is made up of individual atoms (e.g. a noble gas like neon) or molecules of either a single type of atom ( elements such as ...

es that are dissolved in the synovial fluid (which are naturally found in all bodily fluids) leave the solution, making a

bubble Bubble, Bubbles or The Bubble may refer to: Common uses * Bubble (physics), a globule of one substance in another, usually gas in a liquid ** Soap bubble * Economic bubble, a situation where asset prices are much higher than underlying fundame ...

, or cavity ( tribonucleation), which rapidly collapses upon itself, resulting in a "clicking" sound. The contents of the resultant gas bubble are thought to be mainly

carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...

oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...

and

nitrogen Nitrogen is a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...

. The effects of this process will remain for a period of time known as the " refractory period", during which the joint cannot be "re-cracked", which lasts about 20 minutes, while the gases are slowly reabsorbed into the

. There is some evidence that ligament laxity may be associated with an increased tendency to cavitate. In 2015, research showed that bubbles remained in the fluid after cracking, suggesting that the cracking sound was produced when the bubble within the joint was formed, not when it collapsed. In 2018, a team in France created a mathematical simulation of what happens in a joint just before it cracks. The team concluded that the sound is caused by bubbles' collapse, and bubbles observed in the fluid are the result of a partial collapse. Due to the theoretical basis and lack of physical experimentation, the scientific community is still not fully convinced of this conclusion. The snapping of tendons or scar tissue over a prominence (as in snapping hip syndrome) can also generate a loud snapping or popping sound.

Relation to arthritis

The common

old wives' tale An "old wives' tale" is a colloquial expression referring to spurious or superstitious claims. They can be said sometimes to be a type of urban legend, said to be passed down by older women to a younger generation. Such tales are considered super ...

that cracking one's knuckles causes

arthritis Arthritis is a general medical term used to describe a disorder that affects joints. Symptoms generally include joint pain and stiffness. Other symptoms may include redness, warmth, Joint effusion, swelling, and decreased range of motion of ...

is without scientific evidence. A study published in 2011 examined the hand radiographs of 215 people (aged 50 to 89). It compared the joints of those who regularly cracked their knuckles to those who did not. The study concluded that knuckle-cracking did not cause hand osteoarthritis, no matter how many years or how often a person cracked their knuckles. This early study has been criticized for not taking into consideration the possibility of confounding factors, such as whether the ability to crack one's knuckles is associated with impaired hand functioning rather than being a cause of it. The medical doctor Donald Unger cracked the knuckles of his left hand every day for more than sixty years, but he did not crack the knuckles of his right hand. No arthritis or other ailments formed in either hand, and for this, he was awarded 2009's satirical

Ig Nobel Prize The Ig Nobel Prize () is a satirical prize awarded annually since 1991 to promote public engagement with scientific research. Its aim is to "honor achievements that first make people laugh, and then make them think." The name of the award is a ...

in Medicine.

References

{{Reflist Articles containing video clips Habits Joints

Causes

Relation to arthritis

See also

References