Is cracking your knuckles bad for you?
Research hasn’t linked it to arthritis. The pop comes from a gas cavity forming in joint fluid, and one doctor cracked only one hand for 60 years to test it.
Quick Answer
The popping sound comes from the fluid that lubricates your joints. Pulling a knuckle apart lowers the pressure inside the joint, and a gas-filled cavity suddenly forms, something a 2015 study filmed in real time inside an MRI scanner. Scientists still debate whether the sound comes from that cavity forming or partly collapsing, but either way it’s gas in fluid, not bones grinding. What about arthritis? A 2011 study of 215 people found similar rates of hand osteoarthritis in knuckle crackers and non-crackers. And Dr. Donald Unger cracked the knuckles of only his left hand for about 60 years, leaving the right as a comparison; neither hand developed arthritis, which earned him a 2009 Ig Nobel Prize. Painless cracking appears harmless, but joint pain or swelling is a reason to see a doctor.
By the Numbers
Key Facts
- What pops
- A gas cavity forming in the fluid inside the joint
- Arthritis link
- Not found in a 2011 study of 215 people
- 60-year test
- Dr. Donald Unger cracked one hand only; neither developed arthritis
- Reset time
- A joint usually needs about 20 minutes before it will crack again
What makes the pop
- The joint is pulled apart
Bending or pulling a finger separates the surfaces inside the joint.
- Pressure drops
The space inside grows, lowering the pressure in the lubricating fluid.
- A cavity forms
Gas dissolved in the fluid suddenly forms a cavity, and you hear the crack.
- It needs time to reset
The gas slowly dissolves back into the fluid before the joint can pop again.
Watching a knuckle crack
In 2015, researchers filmed finger joints inside an MRI scanner as they were gently pulled until they cracked. The crack happened just as a cavity suddenly formed in the joint fluid, a process called tribonucleation.
The debate over the sound
A 2018 mathematical model argued the noise comes from the bubble partly collapsing rather than forming. The two explanations disagree on timing, but both agree it’s a gas bubble in fluid, not bone on bone.
The 60-year experiment
Dr. Donald Unger cracked the knuckles of his left hand for about 60 years and left his right hand alone. He found no arthritis in either hand and no difference between them, and won the 2009 Ig Nobel Prize in Medicine.
Myth vs. Fact
MythCracking your knuckles causes arthritis.
FactStudies haven’t found a link, including a 2011 study of 215 people and one doctor’s 60-year, one-handed experiment.
MythThe crack is bones rubbing together.
FactIt’s a gas cavity forming in the fluid that lubricates the joint, as MRI video showed in 2015.
The Bottom Line
Cracking your knuckles makes a gas cavity form in your joint fluid, and research hasn’t tied the habit to arthritis. Painless popping looks harmless; painful or swollen joints are a different story.
Sources & Further Reading
- PLOS ONE — Kawchuk et al., Real-Time Visualization of Joint Cavitation (2015)
- Scientific Reports — Chandran Suja & Barakat, A Mathematical Model for the Sounds Produced by Knuckle Cracking (2018)
- Journal of the American Board of Family Medicine — deWeber et al., Knuckle Cracking and Hand Osteoarthritis (2011)
- Live Science — Knuckle-Cracking Gets (Ig) Nobel Prize
- Cedars-Sinai — Ask a Doc: Is Knuckle Cracking Bad?