Crepitus, the crackling, popping, or grinding sensation in a joint or body tissue, usually indicates nothing serious. About 36% of pain-free adults have detectable knee crepitus alone, making it one of the most common harmless body sounds. But in certain contexts, crepitus can signal cartilage wear, trapped air under the skin, or fluid in the lungs, so the details matter: where it happens, whether it hurts, and what other symptoms come with it.
Why Joints Make Noise
Joint crepitus has three main causes, and only one of them points to a real problem.
The first and most common is gas release. Your joints are lubricated by synovial fluid, which contains dissolved oxygen, nitrogen, and carbon dioxide. When you stretch or move a joint, the capsule around it expands, and gas rapidly escapes from the fluid to form bubbles. That’s the pop you hear when you crack your knuckles. You typically can’t crack the same joint again right away because the gas needs time to redissolve into the fluid.
The second cause is tendons and ligaments shifting. When a joint moves, tendons can slip slightly out of their groove and snap back into place. Ligaments can also tighten during movement, producing a cracking sound. This is especially common in the knee and ankle and is generally painless.
The third cause is roughened cartilage surfaces. When the smooth cartilage lining a joint wears down, the exposed surfaces grind against each other and produce a gritty, crunching sensation. This type of crepitus is the one most likely to indicate a developing problem.
Crepitus and Osteoarthritis
Crepitus is one of the key physical signs doctors use when evaluating for osteoarthritis. The European League Against Rheumatism includes crepitus alongside restricted movement and bony enlargement as examination findings that, combined with symptoms like persistent pain and morning stiffness, can support a confident diagnosis of knee osteoarthritis without imaging.
The numbers are striking. In people with even mild structural changes visible on X-ray (early-to-moderate stages), the presence of crepitus means there’s an 80% chance of an osteoarthritis diagnosis. Crepitus in the knee has also been specifically linked to cartilage damage behind the kneecap, confirmed on MRI in women with knee osteoarthritis. A systematic review and meta-analysis in the British Journal of Sports Medicine found that 41% of the general population has some degree of knee crepitus, which means the sound alone isn’t diagnostic. It becomes meaningful when paired with other symptoms.
Interestingly, people with knee osteoarthritis who have crepitus report slightly lower quality of life and higher pain levels (by about 3 to 11%) compared to those without it. But when researchers tested actual physical function, like strength and mobility, there was no measurable difference between the two groups. The grinding sensation seems to affect how people feel about their knees more than how their knees actually perform.
Painless vs. Painful Crepitus
The single most important distinction is whether crepitus comes with pain. Painless popping and cracking, even if it happens often, is almost always benign. Your joints are designed to move, and movement produces sound.
Painful crepitus is different. When a grinding or clicking sensation is accompanied by sharp pain, swelling, or a feeling that the joint catches or locks, it can point to specific injuries. A torn meniscus in the knee, for example, often produces a clicking or clunking sound during movement, along with pain and a locking sensation where the knee briefly gets stuck. That mechanical catching is distinct from the diffuse grinding of cartilage wear. If you notice swelling that comes and goes around a joint, or if the crepitus started after a specific injury, those are signs worth investigating.
Crepitus Under the Skin
Crepitus doesn’t only happen in joints. When air gets trapped in the tissue just beneath the skin, pressing on the area produces a distinctive crackling sensation called subcutaneous emphysema. The skin may also look smoothly puffed up over the affected area.
This type of crepitus has a completely different set of causes and is almost always a reason to seek immediate care. Air can leak into the tissue from a collapsed lung (often alongside a rib fracture), a tear in the airway or esophagus, facial bone fractures, or diving injuries. It can also occur after certain medical procedures that involve tubes placed into the body. Rarely, certain infections, including gas gangrene, produce gas that accumulates under the skin. If you notice crackling under the skin rather than inside a joint, especially after trauma or a medical procedure, that needs urgent evaluation.
Crepitus in the Lungs
Doctors also use the word “crepitus” (or “crepitation”) to describe a specific type of lung sound. Fine crackles heard through a stethoscope are short, high-pitched, interrupted sounds, sometimes described as the noise of pulling apart strips of Velcro. They differ from coarser crackles, which are lower-pitched and longer.
Fine crackles typically indicate mucus or fluid in the small airways of the lungs. The most common underlying causes are swelling or inflammation in the airways and conditions like pneumonia. You can’t hear these sounds yourself in most cases. They’re detected during a physical exam and help guide your doctor toward further testing or a diagnosis.
What Crepitus Means for You
If your joints pop or crack without pain, you’re in the same category as roughly a third of the adult population. No treatment is needed, and there’s no evidence that painless joint cracking causes long-term damage.
If the noise comes with pain, swelling, stiffness that’s worst in the morning, or a sensation of the joint catching or giving way, those added symptoms shift crepitus from a curiosity to a clinical clue. In that context, it can be an early indicator of cartilage wear or a soft tissue injury like a meniscus tear. The combination of crepitus, persistent pain, and risk factors like age or higher body weight is often enough for a doctor to diagnose osteoarthritis based on a physical exam alone.
Crackling under the skin or unusual lung sounds occupy a different category entirely. Both suggest air or fluid is somewhere it shouldn’t be, and both call for prompt medical attention.