Formation of a cavity within the synovial fluid, a slippery substance that lubricates the joints, has been found to be the cause.

For the first time ever, scientists have unassailable, experimental evidence on what causes the popping sound that is heard when knuckles are cracked. Formation of a cavity within the synovial fluid, a slippery substance that lubricates the joints, has been found to be the cause.

The finding puts to rest a 44-year-old perception that the sound is produced when gas-filled bubbles within the synovial fluid suddenly burst.

The results of a study by Gregory Kawchuk from University of Alberta, Canada and others are published today in the journal PLOS ONE. Dr. Kawchuk is the first author of the paper.

The finding confirms a 1947 study by J.B. Roston and R Wheeler Haines of St Thomas’s Hospital Medical School, London who described the origin of joint cracking. Unfortunately, a 1971 study countered the study by stating that the sound was produced when a cavity or bubble bursts.

The current study found that joint surfaces suddenly separate due to distraction force (or pulling force) and there is less amount of synovial fluid to fill the suddenly expanded joint volume. As a result, a cavity or bubble is formed. The creation of this bubble is what produced the popping sound.

The joints between ten fingers and the palm (metocarpophaangeal) of a volunteer were stretched and the events studied using a cine MRI. No bubble was found prior to the application of the force but only after the cracking sound was heard. This confirmed that the sound was produced not by the bursting of a bubble but by the formation of a bubble. The duration of cracking was found to be extremely short — less than 310 milliseconds.

The scientists found that only a single bubble was formed, which “varied in size, shape and location.” A bubble so formed did not disappear immediately after it was formed but persisted even after the sound was heard.

According to the scientists, the creation of a bubble is in line with tribonucleation — “a process where opposing surfaces resist separation until a critical point where they separate rapidly resulting in vapour cavities that do not collapse instantaneously.”

They found that every stage of the cracking sequence followed the sequence described by Roston and Haines.

The authors note that in the beginning, the distraction force results in only minimal joint separation due to viscous attraction between joint surfaces. But when the pulling force applied becomes more, the adhesive force is overcome and a rapid separation of joint surfaces results. When this happens, there is less amount of synovial fluid to fill the increased space. As a result, there is a drop in fluid pressure and the gas dissolved in the fluid is released. The gas so released produces the cavity or bubble.

Though Roston and Haines had correctly explained every stage prior to bubble creation that ultimately led to sound production, they were unable to be emphatic due to a technical “limitation” — they had used only serial radiographs. Hence, they could not see the cavity formation during sound production but could only see the lack of bubble prior to sound production and its presence after knuckle cracking.