We’ve cracked it: why your knuckles pop

The Times

Oliver Moody

Science Correspondent

Gerry Gajadharsingh writes:

Joint “clicking” is one of the many different Osteopathic Manual Techniques that Osteopaths have been using for over 140 years and people have probably been clicking knuckles and other joints ever since we have been in existence. We have had many debates in our profession, over the cause of the “click” and the relevance of it when we use HVT (High Velocity Thrust) techniques. It is thought that cavitation, produced when we momentarily gap or slide a synovial joint, causes gasses dissolved in the synovial fluid mostly CO2 to form bubbles, the debate is that is it the formation or the dissolving of the bubbles that make the characteristic click. I suspect that it is the bubble popping that makes the noise. Cavitation is often seen when a boat’s propeller spins in water and bubbles are produced.

It probably makes no difference to patients what the mechanism is!

Osteopaths use HVT applied to synovial joints for a variety of reasons which are complicated and open to debate:

Mechanical effect to improve range and quality of movement of a stiff joint

To change the localised muscle tension via a neuromuscular loop

To influence reflex neurological connections (other somatic structures or visceral structures that receive a nerve supply via that particular joint (especially spinal facet joints)

Psychological reasons (the patient sometimes expects it, needs it and wants it)

Most of us agree that:

We are not manipulating intervertebral disc structures

We are not putting “bones back” into place

Patients fall into 3 groups:

Some love HVT

Some hate it

Most are happy to receive HVT if the Osteopath suggests that it may be helpful for their particular condition.

If HVT is clinically indicated, there are no contraindications and the patient consents to receive that particular treatment, it is generally a safe and useful technique to use.

I declare a particular interest in HVT, having taught it for over 30 years at both undergraduate and postgraduate level internationally. These days we encourage Osteopaths to aim to use MLT (minimal leverage thrust) techniques, which are gentler than traditional HVT techniques. Osteopathy has an enviable safety record in using manual treatment techniques, back up by our low professional indemnity insurance premiums.

Recurrently apply HVT to a joint can cause problems, usually due to developing hypermobility. Therefore, we do not usually recommend that patients constantly “crack” their knuckles or indeed any other joint.

Usually we will talk through the pros and cons of using HVT which each patient and if there are other technical manual treatment approaches that may be as effective.

While the sharpest minds in physics wrestle with the mechanics of the Big Bang, another group of scientists believe they have identified the source of the little popping noise that happens when people crack their knuckles.

The academic argument about this question has been going on since the aftermath of the Second World War. The latest study is unlikely to be the last word on the subject, but it does back up a theory that dominated the field for 40 years only to be knocked off its perch in 2015.

Abdul Barakat, an expert on cell dynamics at the École Polytechnique in Paris, and Vineeth Chandran Suja, a chemical engineer at Stanford University in California, found that the effect was probably down to the collapse of bubbles in the fluid inside the knuckle joint.

The gap between the metacarpal bone and the phalanx of each finger is full of synovia, a viscous fluid that resembles egg white.

When the knuckle is cracked, the pressure in the synovial fluid drops and the carbon dioxide and other gases dissolved in it form bubbles through a process called cavitation. As the pressure rises again, these tiny bubbles are supposed to pop with a small cracking sound.

The scientific wrangling over knuckle-cracking began in 1947, when JB Roston and R Wheeler Haines, a pair of doctors at St Thomas’s Hospital in London, argued that it was due to vibrations in the tissue around the joint.

In 1971, however, a group at the University of Leeds set out the bubble-popping hypothesis after building a machine that consisted of an arm splint and a finger ring attached to a string that was pulled upwards with gradually increasing force until the knuckle cracked.

The scientists took samples of the synovial fluid and made x-ray images of their subjects’ joints. Their models of the internal pressure suggested that bubbles were by far the likeliest explanation for the effects they saw.

This theory was challenged three years ago by yet another team of researchers, who claimed that it was not the collapse of the bubbles that made the noise, but their formation.

As if the picture were not already sufficiently confusing, Professor Barakat and Mr Suja, his former student, now argue that the Leeds faction were right all along.

They built a detailed computer simulation of a cracking knuckle and found the imploding bubbles were the closest fit to the results of real-world experiments.

They also calculated that the perfect knuckle-crack could be as loud as 83 decibels, roughly the volume of a passing lorry, and had a frequency of 129Hz, which is close to the bottom end of the acoustic register for the male voice.

Their paper, A Mathematical Model for the Sounds Produced by Knuckle Cracking, is published in Scientific Reports.