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Pop! Crack! Crunch!

What makes that popping noise when your spine is adjusted? Does it matter if it pops, cracks, snaps, crunches or makes no sound at all? Here is everything you've ever wanted to know about spinal manipulation. (Well, not everything.)


“Crack your bones, crack your bones, crack your bones! … Make the noise! I live for the noise!”

— Eddie Izzard, Chiropractors Sketch from Dress to Kill. (1) Click the pic to watch the video.



What's a chiropractor's favorite cereal? You might think it's Rice Krispies® (2), but it's actually Cap'n Crunch® (3), or so the joke goes. The noise that's heard with a chiropractic adjustment, whether you call it a snap, crackle, pop, or crunch, is called cavitation. Audible release or articular release are other commonly used terms.


The terms manipulation and adjustment are virtually synonymous, but chiropractors prefer to use the latter one. An adjustment implies the movement of a single spinal joint or, at most, the joints between two vertebrae. We could say that an adjustment is a specific type of manipulation. We could say that, but it wouldn't be true. Chiropractic adjustments are not nearly as specific as chiropractors believe. They actually cause cavitation at several spinal segments. (4, 5)

Figure 1. A typical long-lever manipulation.

Is there a difference between a spinal adjustment and a manipulation?

Given that adjustments are not very specific, perhaps the differences have to do with more distinct contact points on the vertebrae and precise thrust directions. But even thrust direction, a factor that I was taught was very important when adjusting the spine and which I still practice to this day, has been shown to be irrelevant. Researchers using an in situ porcine model (pig spines) found that maximum force was obtained with thrusts at 90° to the target vertebra. Thrusts at 60° and 120° did not move the vertebra in the direction of thrust, and the angled thrusts resulted in reduced force to the vertebra. (6)


Many chiropractors, especially those toward the straight, more orthodox end of the spectrum, shun the term manipulation. It is, after all, a medical term. Manipulation is the term used by every healthcare discipline that performs this procedure, like osteopaths and physical therapists. (See Figure 1.) Chiropractors, it seems, use the term manipulation begrudgingly, Current Procedural Terminology, a set of codes produced by the AMA to indicate virtually every conceivable medical procedure, has assigned what are called CMT codes to chiropractic adjusting services. (CMT = chiropractic manipulative therapy.)


The etymology of the word manipulation is something like the meaning of the word chiropractic — done by hand. (See this post.) The Latin manus means “hand”, and plere is “to fill.” The French word manipule is a “handful”, which is a pharmacist’s measure. In one sense, manipulation is the “skillful handling of objects” (7), which can be extended to the “handling or managing of persons,” a skill that is at times used to the manipulator’s advantage. As it turns out, many chiropractors are practiced at both kinds of manipulation.


So what makes the popping sound?

The noise of a chiropractic adjustment has actually been the subject of some research. Cavitation can occur in almost any joint of the body. Cavitation in a knuckle is the same thing as cavitation in a spinal joint. If you grasp your index finger just below the knuckle with your other hand and pull gently, the two surfaces of the knuckle joint (the second metacarpophalangeal joint, to be technical) will begin to separate. Sometimes a quick tug is necessary. (See Figure 2.)

Figure 2. Joint cavitation.

Normally there is a negative pressure in the joint space that sucks the ends of the two bones together, but when the distractive force becomes great enough a vacuum suddenly forms within the joint. Just as suddenly, gas dissolved within the joint fluid bubbles into the vacuum (abhorred by nature, of course) and—POP! (Something similar occurs when you put your finger inside your cheek to make a popping noise.) Some researchers hypothesize that the noise results from the joint capsule, stretched and deformed by the distractive force, snapping back into place when gas is released.


Researchers testing chiropractic adjustment sounds.

Why is it therapeutic?

Scientific evidence for the benefit of vertebral adjustments for anything other than neck pain, back pain, and headache is lacking. But how do adjustments improve spine-related pain?


Several mechanisms have been proposed. They fall into two categories: mechanical and neurological. Here are the mechanical hypotheses:

  • Cavitation (POP!) increases joint range of motion.

  • Adhesions in the connective tissue of a joint are stretched or torn.

  • Fibrous tags trapped in a joint are released.

  • Nerve compression and irritation are relieved. (8)

Of course, the mechanical effects also produce neurological actions:

  • Cavitation and increased range of motion inhibit pain signals sent to the brain via the spinal cord. This inhibitory reflex has to do with proprioception (the sensation of joint position and movement). Nerves that transmit proprioceptive information do so faster than pain-transmitting nerves, so they beat the pain signals to the “gate” in the spinal cord.

  • Mechanoreceptors—nerves that are stimulated by joint movement—cause contracted muscles around the spine to relax.

  • A fibrous joint tag trapped in a spinal joint causes painful muscle spasms which are reflexively relaxed when the tag is released.

  • Cavitation and increased joint movement stimulate the autonomic nervous system which in turn affects the diameter of blood vessels in musculoskeletal tissue.

Does it have to pop to be therapeutic?

I'm curious to know what other chiropractors say to their patients when an adjustment fails to produce an audible release. This has occurred many times in my own practice; sometimes I tell the patient that the joint doesn't have to pop in order to move. In patient notes I indicate the absence of a pop with the initials nr for no audible release. Yet, I have never seen such annotations in the thousands of chiropractic records I've reviewed. Do chiropractors believe this sonic deficiency is insignificant, or do they avoid admitting to it in writing, fearing they will appear to be incompetent adjusters?


If we define an articular release as a change in a joint space, then we will be relieved to know that the release can be inaudible as well as audible. (9) The authors of this article stated, “It is uncertain whether there is any therapeutic value attached to the crack. …” However, “To ignore these noises would be foolish, as they might be used to help determine the effectiveness of treatment.”


There is also a strong placebo effect associated with the pop. The audible feedback suggests to the patient that the dysfunction has been corrected. And, as Eddie Izzard alludes to, the importance of the noise to the doctor’s self-esteem should not be minimized.


Some adjusting techniques such as DNFT (Directional Non-Force Technique®) don't produce audible releases. Other techniques rely on instruments which produce mechanical noises that may mask the sounds of cavitations if they are present. A randomized controlled trial comparing the Activator adjusting instrument to manual manipulation found greater benefit for the hands-on method. (10)


Is it always therapeutic?

Sometimes a joint will pop spontaneously. This can happen in almost any joint—knees, elbows, hips, shoulders, wrists, and ankles, as well as in the neck and back. An occasional audible release is normal, but when it occurs frequently, and especially when there are repeated episodes of pain and/or muscle spasm associated with minor exertions like rolling over in bed, then there may be a problem with hypermobility—too much movement in one or more joints. People who self-manipulate their backs and necks frequently can develop hypermobility. People with congenital conditions such as Marfan's and Ehlers-Danlos syndromes are also hypermobile. Hypermobility is not necessarily associated with frequent popping, although it seems likely that hypermobility could be caused by frequent and prolonged chiropractic treatments. Not all chiropractors recognize hypermobility or know how to treat it. (For more about hypermobility syndromes, see this post.)


Is it OK to crack your knuckles?

I've been asked this question many times over the years, so I'm guessing that a lot of people worry about it. Yes, it's OK. Many people believe that you can get arthritis by cracking your knuckles, but there's no harm in doing it as often as you like. (Disclaimer: please do not take this as professional advice.) One reason it's safe is that, unless you're adept at walking on your fingers, the knuckles are not weightbearing joints, unlike the joints of the spine.


A controlled but not blinded study was reported in Arthritis & Rheumatology in 1998. (11) The author, Donald Unger, MD, cracked the knuckles of his left hand at least twice a day. The right hand knuckles served as controls; he never cracked them. After 50 years (yes!) he compared the hands for the presence of arthritis. None was found in either hand. Dr. Unger also performed a literature search at that time and found one

study that came to the same conclusion. These researchers X-rayed 28 geriatric subjects who were able to recall whether or not they had performed knuckle cracking ("KC"). They reported that only one patient with a history of KC had arthritis while the X-rays of 14 other knuckle crackers showed no changes. (12)



Footnotes

  1. Eddie Izzard "Chiropractors" Sketch From Dress to Kill, https://www.youtube.com/watch?v=wc534eAmyrw, accessed September 23, 2022. back

  2. Rice Krisipies® is a registered trademark of The Kellogg Company. back

  3. Cap'n Crunch® cereal is manufactured by Quaker Oats Co., a subsidiary of PepsiCo. back

  4. Dunning J, Mourad F, Zingoni A, Iorio R, Perreault T, Zacharko N, et al. Cavitation Sounds During Cervicothoracic Spinal Manipulation. International Journal of Sports Physical Therapy. 2017 Aug;12(4). This study concluded: "Cavitation was significantly more likely to occur unilaterally, and on the side contralateral to the short-lever applicator contact, during cervicothoracic HVLA [high-velocity low-amplitude] thrust manipulation. Clinicians should expect multiple cavitation sounds when performing HVLA thrust manipulation to the [cervicothoracic junction]. Due to the presence of multi-peak energy bursts and sounds of multiple frequencies, the cavitation hypothesis (i.e. intra-articular gas bubble collapse) alone appears unable to explain all of the audible sounds during HVLA thrust manipulation, and the possibility remains that several phenomena may be occurring simultaneously." back

  5. Dunning J, Mourad F, Barbero M, Cescon C, Butts R. Bilateral and multiple cavitation sounds during upper cervical thrust manipulation. BMC Musculoskeletal Disorders. 2013;14(24). back

  6. Kawchuk GA, Perle SM. The relation between the application angle of spinal manipulative therapy (SMT) and resultant vertebral accelerations in an in situ porcine model. Manual Therapy. 2009 Oct;14(5):480–3. back

  7. Manipulation (n.) [Internet]. Online Etymology Dictionary. [cited 2020 Sep 25]. Available from: https://www.etymonline.com/word/manipulation. back

  8. I am not referring here to the traditional big-black-boot-stepping-on-the-little-green-garden-hose subluxation theory. Intervertebral foramina—the holes between two vertebrae where the nerves from the spinal cord exit—can be narrowed by arthritic bone spurs, a condition known as foraminal stenosis. back

  9. Protopapas MG, Cymet TC. Joint cracking and popping: understanding noises that accompany articular release. J Am Osteopath Assoc. 2002 May;102(5):283-7. back

  10. Schneider M, Haas M, Glick R, Stevans J, Landsittel D. Comparison of spinal manipulation methods and usual medical care for acute and subacute low back pain: a randomized clinical trial. Spine (Phila Pa 1976). 2015 Feb 15;40(4):209-17. back

  11. Unger DL. Does knuckle cracking lead to arthritis of the fingers? Arthritis Rheum. 1998 May;41(5):949-50. back

  12. Swezey RL, Swezey SE:,The consequences of habitual knucklecracking. WestJ Med: 122:377-379, May 1975. back



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About J. Michael Burke, D.C.

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During 45 years of diverse experience as a chiropractor, I have witnessed and documented many facets of the profession and the follies of many chiropractors, myself included. My involvement in various activities and endeavors has ranged from cracking necks and backs to developing a managed alternative healthcare network to investigating fraud.

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