Work ’til we break our back and you hear the crack of the bone…

Most of us will have heard a crack emanate from somewhere in our bodies at some point in our lives. The causes of these sounds are many; a stiff joint in the spine, a tendon or ligament rubbing over a bony prominence being relatively minor causes, right through to a fracture of a bone being a major cause! This article will focus on a technique that osteopaths (as well as chiropractors and some physiotherapists) carry out in order to restore normal function to a joint, be that in the spine or the upper or lower limbs of the body.

The technique, in full, is known as a high velocity and low amplitude thrust, HVLAT, or HVT for short. You may have heard your clinician refer to it as a click or crack of the joint. The full name explains exactly what is happening when this technique is applied – high velocity, meaning a quick technique, low amplitude, meaning a small force technique. When applied correctly to a restricted joint, often an audible sound can be heard, which is what the clinician may refer to as a click or crack.

So what is actually happening to cause this sound? Is bone being broken? This is a common misconception among patients, the answer to which is no! (Hopefully! Which is why you should only allow a qualified practitioner to HVT/manipulate your joints – I know plenty of cracked rib stories from lay people ‘cracking’ their friends!) The sound is caused by synovial fluid cavitation and/or the release of gas bubbles from the joint to which the technique has been applied to, and below is a diagram of the lumbar spine (lower back) facet joint to which the technique shown above is being applied to…

The suggested mechanism for the sound is thought to be cavitation within the joint. This is when small cavities of partial vacuum form in the synovial fluid, and then rapidly collapse, causing an audible sound, which varies from person to person. When the HVT is carried out, the force applied separates the articular surfaces of the synovial joint (which is fully encapsulated), which creates a reduction in the joint cavity pressure. In this low pressure environment, some of the gases dissolved in the synovial fluid, leave the solution, producing a bubble, or cavity. This cavity rapidly collapses upon itself, resulting in the clicking or cracking sound. The contents of the resultant gas bubbles are thought to be mainly carbon dioxide.

And to finish off with, a common question I get asked is “if I crack my knuckles, will I get arthritis?” The answer to this question is that there is no evidence to suggest this will cause arthritis. A medical doctor, Donald Unger, regularly cracked the knuckles of his left hand for over 60 years, whilst not cracking those of the right hand. No arthritis or other disease formed in either hand (in 2009 he was awarded the 2009 Ig Nobel Prize in Medicine).

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