You don't say? Cracking the code

Do you ever wonder what causes the distinctive "popping" sound of a "cracked" knuckle?

First off, you don't really crack a knuckle. The sound is caused by popping bubbles in the synovial fluid that lubricates tendons and joints.

When you pull the finger bones apart, either by stretching the fingers or bending them backward or forward, you stretch the capsule containing the synovial fluid. This action decreases the pressure in the capsule and extends the gas bubbles until eventually they burst. Once the gas in the synovial fluid builds up again, the knuckle is ready to take another crack.

Knuckle cracking appears to be harmless. Researchers have found no reliable association between knuckle cracking and a higher risk of arthritis (the most common worry). Other studies have found conflicting evidence: one suggested chronic knuckle cracking could cause swelling in the hands and lessen grip strength, while another found the habit may increase range of motion in the fingers.

However, if your joints make noises other than the typical crack or pop, or if you feel pain, see your doctor. The symptoms could signify other issues. For instance:

• Pain with a grinding sound during joint movement could indicate osteoarthritis or another form of joint disease.
• If the joint seems unstable (if it locks or gives way) or produces a "clunking" sound, this could suggest torn cartilage, a dislocation, or another internal joint injury.

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