Cracking your knuckles

​​​​What causes bones to make popping sounds?

To understand what happens when we ‘crack’ our knuckles, or any other joint, first we need a little background about the nature of the joints of the body. The type of joints that we can most easily ‘pop’ or ‘crack’ are the diarthrodial joints. These are our most typical joints. They consist of two bones that contact each other at their cartilage surfaces; the cartilage surfaces are surrounded by a joint capsule. Inside the joint capsule is a lubricant, known as synovial fluid, which also serves as a source of nutrients for the cells that maintain the joint cartilage. In addition, the synovial fluid contains dissolved gases, including oxygen, nitrogen and carbon dioxide.

The easiest joints to pop are the ones in our fingers (the interphalangeal and the metacarpophalangeal joints). As the joint capsule stretches, its expansion is limited by a number of factors. When small forces are applied to the joint, one factor that limits the motion is the volume of the joint. That volume is set by the amount of synovial fluid contained in the joint. The synovial fluid cannot expand unless the pressure inside the capsule drops to a point at which the dissolved gases can escape the solution; when the gases come out of the solution, they increase the volume and hence the mobility of the joint.

The cracking or popping sound is thought to be caused by the gases rapidly coming out of the solution, allowing the capsule to stretch a little further. The stretching of the joint is soon thereafter limited by the length of the capsule. If we take an x-ray of the joint after cracking, we can see a gas bubble inside the joint. This gas increases the joint volume by 15 to 20 per cent; it consists mostly (about 80 per cent) of carbon dioxide. The joint cannot be cracked again until the gases have dissolved back into the synovial fluid, which explains why we cannot crack the same knuckle repeatedly.

Leave a Reply

Your email address will not be published. Required fields are marked *