Cracking Knuckles and Arthritis

Cracking knuckles does not cause arthritis. In 2009, Dr. Unger received a Nobel Prize in medicine for his work. For over fifty years he cracked his left hand at least twice a day and used his right hand as a control. At a young age of twelve, he decided to challenge the authoritative advice from his family that he would eventually acquire arthritis if he cracked his knuckles. Dr. Unger recorded his self experiment for over five decades and published his findings in the journal : Arthritis and Rheumatism. Many questions still remain regarding the event of cracking joints.  A cracking joint can happen spontaneously as synovial joints are forced into movement that is beyond its physiological position. This over stretched movement reaches the joint’s paraphysiological space. Para comes from the Latin root, around or beyond, as the joint is moved beyond its normal position. After a joint cracks, the spacing between the bones increase. This has been shown over and over again. But what is not known yet is the sound source of the cracking knuckles or any other synovial joint crack for that matter.

Current belief is that sound source is a process of a bubble formation and subsequent collapse called a cavitation. The research to define the sound source is not yet complete and Dr. Jerome Fryer believes that the sound source may be something else and would explain why Dr. Unger did not see the development of arthritis. All synovial joints have a ring-like structure called a synovial fold. This is a tissue (three types: adipose, fibrous, and fibro-adipose) that acts like an O ring to stabilize the joint.  This is very similar to the meniscus found in the knee or the labrum found in the shoulder joint. It is thought to increase the joint stability as well provide a bumper to minimize cartilage approximation. In the recent development of a synovial joint model, Dr. Jerome Fryer believes to have discovered the sound source of a joint crack.

To understand this mechanism of the sound of manipulation, one must appreciate the sound that is generated when a suction cup releases from a highly polished surface. With the suction cup analogous to the synovial fold and the highly polished surface as the hyaline cartilage, one can begin to understand what Dr. Jerome Fryer believes to be the sound source. Using a suction cup principle as an analogy for a joint crack, in order to induce a negative pressure, the suction cup must first achieve a negative pressure. This is performed by pressing the cup up against the flat surface and only if the suction cup is touching the flat surface around the edges of the cup, a negative pressure develops as the pressure under the cup reduces when the centre of the cup stalk is raised and atmospheric pressure “holds down” the elastic cup edges. If this seal is not broken, further negative pressure is induced as the cup centre is pulled away further from the flat surface. At some eventual point, the elastic end range of the cup material is reached and the seal is broken causing the suction cup to release and reform to its original shape. This is analogous to the synovial fold as it releases from the hyaline surface of a synovial joint.

The refractory period is the time that is required in order to re-crack the joint. Everyone knows that one cannot crack their joint repetitively and some time is required to wait (15-60 minutes approximately) to re-crack the same joint. Current belief is that this is the time to allow the bubble to dissolve. But Dr. Jerome Fryer believes that the refractory time is the time that is required to return the joint to the naturally occurring negative pressure. Each synovial joint, when healthy, is under a slightly negative pressure. J Levick writes about how the synovium works to produce this negative pressure. And just as the suction cup noise is dependent on creating a negative pressure, so does the synovial fold release as Dr. Jerome Fryer has proposed.

To see a suction cup release with slow motion video, click here. We know that cracking knuckles does not cause arthritis as Dr. Unger reported. But what we don’t know is how he cracked his knuckle. There are different ways to crack your knuckles. Some do it by bending the knuckle into flexion while others do it by pulling the fingers. This was not revealed in the publication. Dr. Fryer believes there is a right and a wrong way to crack knuckles. And if the current theory of the cracking noise is cavitation, why are we not seeing damage to the cartilage? Perhaps there is another sound source. Dr. Jerome Fryer believes as we unravel the different anatomical sound generating structures in the human frame, we will be better at prescribing and inducing the right mechanobiological strategies to optimize cartilage regeneration.