on February 4th, 2022

It’s a common sight to see students or employees stretching their arms in front of them to perform that audible pop as they flex their wrists and fingers in the opposite direction.

Knuckle cracking is believed to be very common, and actually the body and mind’s response to pressure on your joints.

According to the two as you use your hands during the day it creates tightening in the muscles.

Photo by Bill Ringer on Unsplash

When you crack your knuckles, the joints stretch and it offers relief to the muscles.

While this can also be seen as the same compulsive behaviors like breathing deeply before attempting to do something you don’t really want to, there are positive feelings of stress relief attached to cracking your fingers.

When you crack your knuckles, your body is simply responding to a change in pressure while a joint is being manipulated.

To achieve a crack a part of the joint space is lengthened and it decreases intra-articular pressure which causes nitrogen gas that has dissolved in the synovial fluid to form microscopic bubbles, which collapse.

When the joint space reaches a maximum distraction, which could be as much as three times its resting space, the distance the fluids in your joints go into the areas with negative pressure.

Larger bubbles collapse into microscopic bubbles and that’s what creates the sound associated with knuckle cracking.

It has nothing to do with your bones or cartilage, but everything to do with fluids and nitrogen in your joint space.

How do fluids and gas create the sound of knuckle cracking?

Only in 2018 was new research that give mathematical formulas into why knuckle cracking comes with its specific sound. According to A Mathematical Model for the Sounds Produced by Knuckle Cracking there have been over 60 years of inconclusive experiments on what makes the sound.

Researchers developed a mathematical model to show all the events that led up to the noise that happens when you crack your knuckles.

The researchers looked at cavitation bubble collapse as a source of the sound and then shifted attention to the numerical simulations in resolving the origin of the sounds.

The aim was to establish future research into simulating the inception, terminal and long-term behavior of cavitation bubbles.

(a) The third metacarpophalangeal (MCP) joint 18.  

Researchers were able to use the Rayleigh-Plesset equation and their finding on how cavitation bubbles burst gave insight into the emanating sound of knuckle cracking – in that a very impressive engineering principle is needed to make the sound.

(a) Variation of the ambient pressure in the joint and the excess pressure of the bubble inside the joint from the solution of Eqs 1 and 5, respectively for an acceleration a of 72 m/s 2. The inset shows the eccentricity of the joint at 1 ms, the time up to which an excellent match was obtained with experiments. (b) Solution of the Rayleigh-Plesset equation (Eq. 5) for the ambient pressure obtained from Eq. 1 for an acceleration a of 72 m/s 2. (i) Evolution of the normalized bubble wall radius for the simulated parameters. (ii) Evolution of the normalized bubble wall velocity.
Sensitivity analysis of the model. (a) The model is sensitive to changes in the load but is relatively invariant to changes in acceleration and bubble radius.
(b) The model gives physiologically consistent results but is also sensitive to changes in the geometry as shown by varying the geometry through half a standard deviation (σ m = 1.08, σ p = 2.30) above or below the mean metacarpal and proximal phalanx radii. The model diverged for the case of R m − 0.5σ m , R p + 0.5σ p above a load of 6 N. 

Is knuckle cracking bad for you? Engineering principles like the cavitation effect might hold some clues

Studies show that knuckle cracking doesn’t really affect your health, but there is some correlation that it could aggravate cartilage damage.

Researchers used the mechanical processes of ship propellers and the bubble damage of water on propeller surfaces to show the potential harm.

Effect of cavitation number, and nuclei population on inception event rate, E, for test conditions investigated in this study. The event rate decreases with increasing cavitation number for the monodisperse nuclei population. The rate corresponding to the monodisperse case is eight times higher than that of the polydisperse for the same test section conditions.

The force required to crack your knuckle exceeds the energy threshold that could potentially lead to articular cartilage damage.

This is because of the cavitation effect of intra-articular bubble formation and collapse – a process that is also mechanically similar to cavitation of ship propellers, a process that has been shown to produce wear on propeller surfaces.

Based on this the paper Knuckle Cracking and Hand Osteoarthritis theorizes that habitual knuckle cracking could lead to the thinning of articular cartilage and in some cases to clinical osteoarthritis due to the breaking of bubbles and it’s constant reforming in the hand or joints that get cracked often.

It’s not all bad however, one study Knuckle cracking: secondary hyperparathyroidism and what your mother did not tell you found that in 13 patients that had undergone surgical parathyroidectomy (surgical removal of one or more parathyroid glands) patients, termed as habitual knuckle crackers, were extremely happy when they found out they were allowed to crack their knuckles again following their surgery.

Having surgery to improve their health and using knuckle cracking as a motivator is now under the spotlight.

The patients said they lost the ability to crack their knuckles before surgical parathyroidectomy.

The patients were from a group with secondary hyperparathyroidism with protean musculoskeletal manifestations. Some of the patients said they had lost the ability to crack their knuckles due to their condition that led to added calcium in their system.

Following surgical parathyroidectomy, many were able to resume cracking their knuckles in as little as a week after the surgery, and their experiences have been used to find out what the physiological and mechanical basis of cracking your knuckles is when it is affected by parathyroid-related mineral and bone disorders.

According to the researchers, four patients lost the ability to crack their knuckles completely, theorized to be from hyperparathyroidism that has created certain minerals within their bodies.

Picture: Wikipedia Commons

Two patients in the group were unable to crack their knuckles before the surgery and was fully restored and able to crack their knuckles again after parathyroidectomy.

The removal of the thyroid glands that released minerals into their systems seemed to have balanced, and the right chemicals were restored to resume knuckle cracking.

Six patients weren’t able to recollect if they cracked their knuckles, but out of the 13 patients, 67% reported they were unable to crack their knuckles pre-surgery.

What astounded the researchers was that knuckle cracking was extremely important to some of the patients.

These individuals were called ‘dismayed’ when they lost the ability to crack their knuckles and delighted to resume after a successful parathyroidectomy.

The ability to crack knuckles is now thought of as a motivator for health checks to improve adherence to bone and mineral disease regimens in patients. Since the researchers call the ability to crack knuckles a gratifying response to successful treatment of hyperparathyroidism.

At the end of the day, one research paper really sums up knuckle cracking, when the paper concludes that data has failed to support evidence that knuckle cracking can lead to any degenerative changes in the metacarpal phalangeal joints.

The only major morbid consequence of knuckle cracking appears to be its annoying effect on the observer.

Some facts about joint cracking

  • There’s a reason that cracked joints can’t immediately crack after you have performed a manipulation. It typically takes anywhere from 15 minutes to a few hours for the joint, after it has been cracked, to crack again. This is because the microscopic bubbles need to dissolve back into solution and the joint space needs to retract back into its resting position.
  • The same bubbles that collapse when you crack your knuckles are used on the ocean floor by Pistol Shrimps to create deadly noise and heat that stuns their prey. When the shrimp senses prey, it opens the top part of its big claw, allowing some water to enter a small chamber in the claw. When it is clamped down the pressure from a plunger line on the top of the claw forces water out of the chamber that creates bubbles that travel at 60 miles per hour. These bubbles stun and can even kill prey, and as the bubbles pop, it makes a cracking sound that gives the shrimp their name.

References:

Chandran Suja, Vineeth ‘Vinny & Barakat, A. (2018). A Mathematical Model for the Sounds Produced by Knuckle Cracking. Scientific Reports. 8. 10.1038/s41598-018-22664-4.

Kevin deWeber, Mariusz Olszewski and Rebecca Ortolano. (2011). Knuckle Cracking and Hand Osteoarthritis. The Journal of the American Board of Family Medicine March 2011, 24 (2) 169-174; DOI: https://doi.org/10.3122/jabfm.2011.02.100156

The Journal of the American Board of Family Medicine March 2011, 24 (2) 169-174; DOI: https://doi.org/10.3122/jabfm.2011.02.100156

Ross, Edward & Paugh-Miller, Jennifer & Nappo, Robert. (2013). Knuckle cracking: Secondary hyperparathyroidism and what your mother did not tell you. Clinical Kidney Journal. 6. 671-673. 10.1093/ckj/sft123.

Swezey RL, Swezey SE: The consequences of habitual knuckle cracking. West J Med 122:377-379, May 1975

Ocean Conservancy, 2020. The Real Power of the Pistol Shrimp [online] Available at: https://oceanconservancy.org/blog/2020/09/10/pistol-shrimp/ [Accessed 1 February]

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