What makes the popping sound when you crack your knuckles?

Answer

Research suggests it does not cause arthritis.

Context

Two explanations have competed for the sound. The traditional one says that pulling a joint apart lowers the pressure in the synovial fluid, a lubricating fluid that contains dissolved gases, and a gas bubble forms or collapses with a pop. A 2018 mathematical model of the knuckle supported the idea that a bubble collapsing produces the sound, according to Harvard Health's summary of the work.

In 2015 a Canadian team imaged a finger joint with real-time MRI while it was pulled until it cracked. A gas-filled cavity appeared at the moment the joint surfaces separated, coinciding with the sound, and the cavity was still visible afterward rather than seen to collapse. The joint gap widened from about 0.9 mm to about 1.9 mm. The authors argued that the cavity forms suddenly as the surfaces pull apart, a process called tribonucleation. The study used a single participant, so it is suggestive but not conclusive, and scientists still debate whether formation or collapse makes the noise.

After a crack, the same joint usually cannot be cracked again for roughly 15 to 30 minutes, which is generally thought to reflect the time the gas takes to redissolve into the fluid.

On health, a physician named Donald Unger cracked the knuckles of only his left hand for decades and found no arthritis difference between hands, a self-experiment that won an Ig Nobel Prize in 2009. Larger studies have generally found no link between habitual cracking and arthritis, though one 1990 study of about 300 people found lower grip strength and more hand swelling among habitual crackers. Rare injuries are reported in the medical literature, and these are findings, not guidance.