“Don’t crack your knuckles, your bones will wear away!” Almost all of us grew up hearing this warning. Some say the sound comes from a loss of “water” in the joint, others that it is bones rubbing together, and many believe repeated cracking will lead to arthritis. But is that really so? Here is the real scientific explanation for the mysterious sound joints make.
What is inside a joint
Every movable joint in our body is enclosed in a special capsule containing synovial fluid, a clear, slippery liquid that helps the joint move easily and reduces friction between bones. Gases such as oxygen, nitrogen and carbon dioxide are dissolved in this fluid.
Where the ‘pop’ comes from
When we suddenly stretch a finger or other joint, the pressure inside the joint drops quickly. The fall in pressure causes the gas dissolved in the synovial fluid to form a small bubble in an instant, a process called tribonucleation. The popping or cracking sound is heard at the moment this bubble forms. It was once thought the sound came from the bubble bursting, but research shows it occurs as the bubble forms. In other words, the sound comes not from bones grinding but from a gas bubble forming inside the joint.
Why you cannot crack it again straight away
Many people notice that after cracking a knuckle, it will not crack again immediately. That is because the bubble takes some time to dissolve back into the synovial fluid. Usually the same joint can be cracked again after 15 to 30 minutes.
Do all joint sounds have the same cause?
No. Joint sounds are mainly of two kinds. The first is normal (physiological): no pain, no swelling, no problem moving, just an occasional pop or crack. This is usually harmless. The second is caused by disease (pathological). If the sound comes with pain, swelling, the joint locking, limited movement or a history of injury, it may be a sign of cartilage wear, ligament injury, a torn meniscus, tendon snapping or osteoarthritis.
Does cracking knuckles cause arthritis?
Current scientific research has not shown a direct link between knuckle cracking and osteoarthritis. But pulling joints very hard or habitually may in some people cause minor injury to ligaments or soft tissue, reduce grip strength or cause discomfort. So it is better not to make a habit of cracking joints for no reason.
When to see a doctor
You should see an orthopaedic specialist if you have any of these symptoms: pain with the sound, swollen joints, difficulty walking or working, joints locking repeatedly, new sounds after an injury, or a feeling of instability or weakness along with the sound.
In short, a popping joint does not mean disease. In most cases it is a normal physiological event caused by gas bubbles forming in the synovial fluid. But if the sound comes with pain, swelling or difficulty moving, it should never be ignored, as it may be an early sign of an orthopaedic condition. Remember: it is not the sound but the symptoms that come with it that are the most important reason to see a doctor.
The writer is an assistant professor (spine surgery) at the National Institute of Traumatology and Orthopaedic Rehabilitation (NITOR), Dhaka.
বাংলায় মূল প্রতিবেদন পড়ুন · Read the original Bengali report