Cracking joints

Cracking joints is the action of moving joints to produce a sharp cracking or popping sound. The most common form of this occurs during deliberate knuckle cracking. It is possible to crack many other joints, such as those between the back and neck vertebrae, hips, wrists, elbows, shoulders, toes, knees, jaws, and the Achilles tendon area.

Causes

To deliberately produce the clicking sounds, many people bend their fingers into unusual positions. These positions are usually ones that their own muscles are unable to achieve, and which are not commonly experienced in everyday use. For example, bending a finger backwards away from the palm (into extension), pulling them away from the hand (distraction), compressing a finger knuckle toward the palm (into flexion), or twisting a finger about (torsion).

The snapping of tendons or scar tissue over a prominence (as in snapping hip syndrome) can also generate a loud snapping or popping sound.^[1]

Source

The physical mechanism is as yet unproven, but suggested hypotheses include:

1. Cavitation within the joint—small cavities of partial vacuum form in the fluid and then rapidly collapse, producing a sharp sound. This explains the popping that can occur in any joint, such as during spinal manipulation. Synovial fluid cavitation is the most likely theory and substantial evidence exists in support of it. Cracking your knuckles releases gases from the joints.^[2]
2. Rapid stretching of ligaments.^[1]
3. Intra-articular (within-joint) adhesions being broken.^[1]

Of these hypotheses perhaps the most popular is cavitation. When a manipulation 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 gases that are dissolved in the synovial fluid (which are naturally found in all bodily fluids) leave the solution making a bubble or cavity, which rapidly collapses upon itself, resulting in a "clicking" sound. This process is known as cavitation. The contents of the resultant gas bubble are thought to be mainly carbon dioxide.^[3] The effects of this process will remain for a period of time known as the "refractory period", which can range from a few seconds to some hours while it is slowly reabsorbed back into the synovial fluid. There is some evidence that ligament laxity may be associated with an increased tendency to cavitate.^[4]

Repercussions

The long-term consequences of this practice have not been studied thoroughly, and the scientific evidence is inconclusive. The common parental advice "cracking your knuckles gives you arthritis" is not supported by any evidence, but habitual knuckle crackers are more likely to have hand swelling (due to the buildup of cartilage layers between the bones inside the joint) and lower grip strength attributed to stretched tendons.^[5] It is unknown whether these effects are caused by knuckle cracking or if knuckle cracking is a symptom, as it relieves excess pressure in joints.

References