Cracking knuckles?
Text and picture by GOUTAM GHOSH
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Are you in the habit of cracking your knuckles? Then take care because you could be in trouble soon.
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Many in India snap their fingers, jerk their arms or their heads back for an "immensely satisfying" cracking sound. Happiness lights up their faces as the joints pop in a quick, sharp series like mild firecrackers. Some can crack all moveable joints — a distinction that brings no reward. You too may have learnt the "art" (if one may call it that).
If you crack your joints in a public place such as a restaurant or a meeting, people will stare at you with their eyebrows raised. If you are immune to subtle displays of disapproval, you may even wish to crack your joints, including your toes, and amplify the sound electronically.
Many parents — probably even yours — discourage the "disgusting habit". They worry that your joints may be damaged — a fear that is not baseless. One orthopaedic school insists that nothing happens if you crack your joints, while another says that over the years, the joints will be damaged.
How it happens
How is the sound generated? The simplified diagram shows that each joint is encased in a capsule - the joint capsule. Within this is the synovial membrane, which supplies a lubricant to aid the movement of bones. The lubricant is synovial fluid, a clear but viscous liquid that fills the space between the bones of a joint, known as joint space. The end of a bone has a cartilage, a living tissue that is strong, elastic and very durable (unless some disease damages it). It models itself continuously to adjust the thickness of the planes of highest stress where the bones knock on one another. Then there are muscles, and tendons (thick fibrous, inflexible tissues which act as pulleys; interestingly these tendons are flexible only in the legs of kangaroos) connecting the muscles to the bones.
Some experiments with microphones attached to joints showed that there were two sounds, not just one as believed. When joints are stretched beyond the natural limits, the synovial fluid film that is barely 10 microns (a micron is one-millionth of a metre) thick expands and the pressure within the fluid falls, and it sucks the ligaments in. The drop in pressure releases gases (oxygen, nitrogen and carbon dioxide) that are naturally trapped in the fluid. These gases form a bubble that bursts with a sharp crack. You cannot crack a knuckle again till the synovial fluid reabsorbs the gases.
Cracking joints
There is another sound when the ligaments are snapped back into place by the synovial fluid that pushes them out once the gas bubble bursts. This leads to another crack. The energy generated at the joint that you crack is about seven per cent of what is needed to damage it, but imagine the effect if joints are cracked repeatedly for years. Orthopaedicians say that bits of cartilage are torn away each time the joints are cracked. This leads to tendon swelling and joint pain similar to arthritis. Many complain that they lose the grip strength of their hands whose fingers they have been cracking habitually for years. They also complain that their finger joints ache. Many enthusiasts prefer to hide this fact but not for long.
Some knuckle-cracking enthusiasts claim that their joints pain when they do not crack them — a response similar to a withdrawal symptom.
Why risk joint aches later in life? If your joints crack when you move or bend, you need not worry. But if you plan to relish the sound and the fury of popping joints, think twice before you turn it into a habit. It may cause you grief in future.
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