What’s the Crack?

What’s the crack? Recently, I have seen a lot of misinformation and misunderstanding about the “popping” or “cracking” sound that can be heard during spinal manipulation or when you crack your knuckles. The cause of the popping sound is also a question I get a lot from my students and patients. To help combat both the misinformation and to help educate the public on this topic, I thought an evidence-based blog and video for our YouTube channel was in order.

What the crack is has got to be one of the most misunderstood phenomena in chiropractic/physical therapies. I have seen and heard several well-known chiropractors on social media discussing the phenomenon when they are asked about it on podcasts. What was staggering and incredibly frustrating is that these chiropractors who are “famous” on social media, gave the wrong explanation of the phenomenon. This adds to the problem of misunderstanding among students, the public, and other ill-informed clinicians. This doesn’t paint us in a good light if we as professionals who utilise spinal manipulation every day don’t understand the basic mechanism and outcome of our treatment method.

As an educator teaching students the mechanics of manipulation, and what happens to the joint and synovial fluid during spinal manipulation, any misinformation they receive from other healthcare professionals is detrimental to their education. These videos are easily accessible and are viewed as experts speaking knowledgeably about the science of their jobs. Unfortunately, these “famous” chiropractors are ill-informed and don’t understand it themselves. It is not their fault entirely; they were probably taught this incorrectly during their education. However, the scientific studies done in this area are as readily available and easily accessible as their unfortunate videos, so continued ignorance of this topic doesn’t look good either.

What’s the Crack?

Many joints of the body are called synovial joints (Fig 1). Synovial joints are the more mobile joints of the body, e.g., your fingers, wrists, elbows, and shoulders. The list of synovial joints also includes the 24 mobile joints of the spine and 1/3 of the sacroiliac joints. Their joint capsule provides a relatively loose connection, allowing the bones to move through a large range of motion without separation. The strength and stability of these joints are created by strong ligaments and muscles that attach to either side of the joint. These muscles pull across the joint, acting as primary movers of them. Synovial joints are filled with synovial fluid that lubricates the joint by reducing friction and nourishes the cartilage at the ends of the two bones. Synovial fluid is a thick (viscous) fluid that fills the joint capsule and the space between the bones, the intra-articular space, separating the bone surfaces (the articular surfaces) from one another (Fig 1).

Fig 1.

During normal movement of these joints, the articular surfaces glide across one-another with the synovial fluid being spread equally across the joint. However, as a result of injury, damage, or other causes of inflammation, these synovial joints can become dysfunctional, leading to changes in the pressure within the joint capsule. The articular surfaces do not have such an even distribution of the synovial fluid across them, which leads to joint stiffness and loss of motion. When a chiropractor or other healthcare professional applies a manipulation, there is a sudden separation of the articular surfaces, which causes a rapid change in fluid pressure within the synovial fluid¹. This rapid change in fluid pressure is called a cavitation in physics and results in the formation of a bubble of air to form within the joint, between the two articular surfaces (Fig 2). Where does the bubble come from? I hear you ask. The bubble comes from the dissolved molecules within the synovial fluid (e.g., carbon dioxide [CO₂] or oxygen [O₂]). Once the pressure returns to normal, these air molecules dissolve back into the synovial fluid. They do not make you burp or fart as one of my patients asked when I explained this concept to her during an appointment session recently… You know who you are!!! 😀

Fig 2.

So, What’s the Crack?

The first published scientific article published on this topic was back in 1947 when Roston et al used serial x-rays of the volunteers cracking their knuckles by pulling their fingers to make them crack⁸. The authors described a phenomenon of a “clear space” forming within the joint between the two bones they described as a “vapour cavity”. This finding has been confirmed by several studies over the years, most recently by Kawchuk et al (2015) who subsequently published the video of the MRI scan of someone cracking their knuckle on YouTube in this video². If you do nothing else or take nothing else from this blog post, I encourage you to watch the video of the joint cavitation, it is the holy grail when answering this question.

Misinformation on Social Media:

This is the raison d’etre of this blog post really isn’t it? I commend you for sticking with me this far! In a nutshell, the explanation I have heard from these social media sources is that they believe the popping sound is created by the popping of the cavitation bubbles. Unfortunately, this is the exact opposite of the truth… it is the formation of the bubble that creates the sound of joint cracking. Scientists have known this since the 1940s, and yet it is still being taught and explained arse-about-face. Some of you are probably thinking: “is that all your problem is Mark?” “So what if they get it the wrong way around?” This information is important to get right because of the misinformation from the scientific and medical literature we will discuss next.

Misinformation in the Scientific Literature

We have all heard it as children: “you shouldn’t crack your knuckles; it will give you arthritis!” But exactly where did this statement come from? Is there any truth behind it?

Several authors hypothesised that cavitation damages the joints or cartilage surfaces of the bones ^3–5 which led the medical and scientific communities to discourage people from cracking their joints for fear of it causing arthritis and joint damage. It doesn’t by the way, just in case anyone was unsure if arthritis is the result of joint cracking^2,6,7. Watson et al (1989) found that the energy produced by the cavitation formation does not exceed the threshold forces that cause joint/cartilage damage⁶, and deWeber et al (2011) found that frequent and habitual joint cracking does not cause or contribute towards arthritis or cartilage degeneration⁷.

I am very sorry for those of you who are parents and want your children to stop cracking their knuckles, but the old saying “it gives you arthritis” is a myth. A myth that needs to die a horrible and painful death because it has been used to discourage people from seeking the care of chiropractors and other manual therapists for decades! I have had this nonsense used by other medical professionals to argue against patients seeking care from therapists who utilise manipulation. It is a true shame to see this information is either still taught incorrectly or misunderstood by colleagues in my profession. I hope this blog goes some way to address the information gap regarding joint health and why spinal manipulation is a safe therapeutic intervention.

So, there you have it. The cracking, clicking, or popping noises made during the manipulation of a joint is the formation of an air bubble during the rapid change in fluid pressure, not the popping of them!

By Mark Spriggs DC, MChiro, MSc, FRCC

References:

1. Evans DW, Lucas N. What is “manipulation”? A reappraisal. Man Ther. 2010;15(3):286-291. doi:10.1016/j.math.2009.12.009
2. Kawchuk GN, Fryer J, Jaremko JL, Zeng H, Rowe L, Thompson R. Real-Time Visualization of Joint Cavitation. Zhang Q, ed. PLoS One. 2015;10(4):e0119470. doi:10.1371/journal.pone.0119470
3. Chen YL, Israelachvili J. New mechanism of cavitation damage. Science (80- ). 1991;252(5009):1157-1160. doi:10.1126/SCIENCE.252.5009.1157
4. You Lung Chen, Kuhl T, Israelachvili J. Mechanism of cavitation damage in thin liquid films: Collapse damage vs. inception damage. Wear. 1992;153(1):31-51. doi:10.1016/0043-1648(92)90259-B
5. Zeng H, Zhao B, Israelachvili JN, Tirrell M. Liquid- to solid-like failure mechanism of thin polymer films at micro- and nanoscales. Macromolecules. 2010;43(1):538-542. doi:10.1021/MA901845Z
6. Watson P, Kernohan WG, Möllan RAB. A Study of the Cracking Sounds from the Metacarpophalangeal Joint. Proc Inst Mech Eng Part H J Eng Med. 1989;203(2):109-118. doi:10.1243/PIME_PROC_1989_203_019_01
7. deWeber K, Olszewski M, Ortolano R. Knuckle Cracking and Hand Osteoarthritis. J Am Board Fam Med. 2011;24(2):169-174. doi:10.3122/jabfm.2011.02.100156
8. Roston JB, Haines RW. Cracking in the Metacarpo-phalangel Joint. J Anat. 1947;81(2):165-173