Joint cracking is something that we've all experienced at some point or another, whether it is the random pop from your spine as you twist, the crunch of someone's knees or the macho-man clicking his knuckles.
Before answering what actually happens when a joint clicks, it is important to know there are different causes of cracks, pops, creaks, grinds and clicks; not all of which emanate from our joints.
The technical word for joint cracking, grinding and creaking is Crepitus, but again this can mean many things.
The grinding that some people experience from their knees on walking up stairs can be caused by roughness of cartilage behind the knee-cap.
Sometimes people experience what is often described as crunching from their neck as the turn to look over a shoulder. While this could be caused by roughness of cartilage, because the joints of the neck don’t bear much load this is less likely..
What is more likely, and more common, is for muscles of the neck to flick over bits of bone as the spine rotates. If you look at the irregular shape of the vertebrae and the string-like muscles that surround the spine, it is easy to see how this flicking can take place.
But neither of the causes of joint clicking above describe the main source of joint popping – the cavitation.
A cavitation is the technical word for what happens when somebody clicks their knuckles, or the pop that is sometimes associated with a chiropractic adjustment.
So what is actually going on when you have a cavitation? Bear with us, it gets a little bit sciency...
What causes a joint to pop?
As mentioned above, a cavitation is quite different to the other causes of clicking and grinding people experience.
Actions in video not to be recommended!
A cavitation is a word borrowed from physics. It means the formation of a bubble within a liquid because the pressure that liquid is under decreases.
When you take the top off a coke bottle, bubbles suddenly appear. This isn’t because you have pumped any gas into the liquid.
Bubbles appear because dissolved gasses, which are prevented from escaping due to the pressure that the liquid is stored under, are released when the pressure inside the bottle decreases.
A similar effect is seen when a boat's propeller slices through the water:
The action of the propeller creates regions of higher and lower pressure. In the regions of lower pressure cavitations occur, leading to the production of bubbles, seemingly from nothing.
This may seem a long-winded way round to describe what happens when you pop a joint, but the process is strikingly similar.
Some Joint Anatomy
Most joints in your body, including those in your spine and fingers are what is called synovial joints.
A synovial joint is one that is encased in a membrane. This membrane contains fluid that helps lubricate and nourish the joint.
Think back to the physical act of clicking one's knuckles.
The knuckle is stretched past its normal limit, and at the point this limit is passed a pop occurs.
The membrane surrounding the joint is stretched along with the joint.
Because the stretch increases the volume of this membrane, we now have the same amount of fluid inside the membrane as before, but this fluid now occupies a bigger space.
This means the fluid is under less pressure.
The result is that some of the gasses which are ordinarily dissolved within this synovial fluid come out of solution. They are released.
This release causes the formation of a bubble, just like taking the top off a coke bottle.
But it isn't the formation of that bubble that creates a noise; it is the collapse of that bubble.
Very quickly after stretching a joint the elastic membrane returns to its normal shape and pressure within the joint is returned to normal, causing the collapse of the bubble.
The same process can occur in nearly any joint in your body and is often associated with a chiropractic adjustment.
But does that cavitation have any therapeutic effect? And if not, why does clicking your neck feel so good?
What does clicking a joint do?
Specifically regarding a chiropractic adjustment, or spinal manipulation, there has been great debate around whether it matters if a joint clicks or not.
Often patients report that there is a feeling of relief associated with a cavitation.
However, great numbers of scientific studies have been done on this topic, measuring outcomes amongst people who experience a cavitation versus those that do not.
The result of most of these studies is that any relationship between improvement in symptoms and cavitation is very tenuous or not present at all.
That is to stay that there seems to be little to no difference in the outcome of manipulation whether or not a pop occurs.
The picture is clouded slightly by psychological effects of hearing a pop, which can enhance the placebo affect associated with treatment.
But it is generally thought that the most important aspect of manipulation or a chiropractic adjustment is the movement of a stiff joint.
There are some really interesting physiological effects of rapidly moving a joint, as with an adjustment, but this is a whole other topic.
As to whether clicking your knuckles give you arthritis? Probably not.
And should you be clicking your own spine? Definitely not…