Cracking and Popping Joints

There are many myths about joint cracking. The two most common are that our knuckles will get bigger if we crack them, or we will get arthritis. Neither of these is likely, but there is truth to the idea that some forms of cracking are undesirable.

By Paul Grilley

There are two reasons why our joints crack and creak. One is that bones are rubbing together, and the other is that the bones of a joint are fixated. We will examine these one at a time.

Rubbing Bones
Most of the joint sounds we hear are due to bones rubbing. This is "friction popping." When we snap our fingers, we press our thumb and middle finger together hard enough to create friction. Then we try to overpower this friction with other muscles of the hand. This opposition of forces slightly bends the bones of the finger and thumb. When the two fingers finally slip past one another, the bones rebound violently and vibrate briefly, like tuning forks. This creates the snapping sound.

The snapping of our fingers is not at all painful or harmful, but sometimes we inadvertently create these popping sounds in other joints, such as our elbows. When our elbow briefly "catches" and then pops, it can be quite surprising and even slightly painful if the vibrating bones press a nerve. The popping sound has the same cause as finger snapping: the two bones of the elbow are temporarily in friction, and when they release, they vibrate violently and we hear a "pop."

A similar but more alarming instance of friction popping takes place in the knee. More specifically, it occurs in our patella, or kneecap. The patella sometimes rides up on the side of the groove it glides in and temporarily sticks there. It is being held on the lip of the groove by the pull of the thigh muscles. This is much like snapping our thumb and finger, but this moment is very brief because as the knee bends and moves, the patella loses its precarious balance of forces and "pops" violently back down into the groove where it belongs. There is nothing really harmful in this; the patella is not injuring the ligaments or cartilage. But it can be alarming for our knee to lock up for an instant and then release. At worst, there is a slight twinge to the tendon around the patella because it was stretched briefly.

The most common place to hear friction popping is in our neck. Most of us can roll our heads and hear these sounds, although they are not as loud here because the forces of friction are not as great. The bones involved are the facets of the cervical vertebrae-typically several of them, which is why the noise sounds "crunchy," like walking on pebbles.

Is It Bad for You?
If our elbow or knee inadvertently pops, there is nothing to worry about. There is just enough slack in our joints that these twinges are inevitable, and no harm is done. But there is little value in consciously trying to make these sounds happen. Just as it takes a certain effort to snap our fingers, many people can pop their hips over and over by doing sit-ups or leg lifts.

Other people can do similar things with their knees. This is not desirable. Even our thumb gets sore if we snap it enough. If a student insists on popping a joint repetitively, the joint may become inflamed and painful. This is because the body is trying to minimize the friction by swelling the fluid sacks that line our joints. These sacks are called bursae, and their inflamed condition is called bursitis. Bursitis most frequently occurs in the small joints of the shoulder and elbow.

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