Clicking or popping sounds in the knee, known medically as crepitus, are a common phenomenon. While often startling, the noise frequently results from normal mechanical changes within the joint and does not indicate damage. Understanding crepitus requires distinguishing between physiological sounds and symptoms of a structural issue.
The Most Common and Harmless Reasons for Knee Sounds
The most frequent cause of noise from a healthy knee joint is cavitation, which is completely benign. This is the same mechanism responsible for the sound made when cracking knuckles. The knee joint is filled with synovial fluid, which acts as a lubricant and contains dissolved gases, primarily nitrogen.
When the knee joint is quickly moved, the pressure inside the joint capsule rapidly changes, causing these dissolved gases to form tiny bubbles. The popping sound occurs when these bubbles abruptly collapse. This phenomenon is not associated with pain, swelling, or any long-term damage to the joint structure.
Another common source of noise is the movement of soft tissues, such as tendons or ligaments, snapping over a bony prominence. This happens when a tendon or ligament momentarily catches on a part of the femur or tibia as the joint bends. The sound is produced when the tissue quickly snaps back into its normal position.
A specific example involves the iliotibial (IT) band, which runs along the outside of the thigh. As the knee flexes and extends, the IT band can occasionally snap over the lateral femoral epicondyle, a bony bump on the side of the thigh bone. Like cavitation, this snapping is typically painless and is a mechanical action of soft tissue moving over bone.
Clicking Sounds Caused by Wear and Injury
When knee clicking is accompanied by pain or a grinding sensation, it often signals a structural issue. A widespread pathological cause is the roughening of the articular cartilage, often associated with osteoarthritis (OA). Articular cartilage is the smooth, protective layer covering the ends of the bones in the joint, allowing them to glide friction-free.
In OA, this cartilage wears down, becoming frayed or rough. When the uneven joint surfaces rub against each other, it generates a coarse, grating, or crunching sound known as crepitus. This sensation results from bones interacting with a damaged joint surface and is usually accompanied by discomfort or pain.
Mechanical clicking is frequently caused by a tear in the meniscus. The meniscus consists of two C-shaped pieces of fibrocartilage that sit between the thigh bone and the shin bone, acting as shock absorbers. A torn section of the meniscus can flip or shift, getting caught or jammed within the joint during bending or twisting movements.
This catching of the torn cartilage creates a distinct, often painful, pop or click. It may even cause the knee to lock, making it impossible to straighten the leg fully. Another cause is the presence of loose bodies, which are small fragments of bone or cartilage floating within the synovial fluid. These fragments can momentarily get wedged between the joint surfaces, leading to a sudden, painful click or sensation of the knee catching.
When a Knee Click Requires Medical Attention
While many knee noises are harmless, certain accompanying symptoms suggest the need for medical evaluation. Pain is the strongest indicator that the sound is a symptom of an underlying problem. Any clicking, popping, or grinding that is consistently associated with sharp or persistent pain should prompt a consultation.
Other warning signs include visible swelling or heat around the knee joint, indicating inflammation or fluid accumulation. Swelling following a sudden injury, especially one accompanied by a loud, immediate pop, can signal damage to structures like the menisci or ligaments. The sensation of instability, often described as the knee feeling like it is going to give out, is a serious symptom.
An inability to fully move the knee is a sign of a mechanical obstruction. If the joint feels locked or catches, preventing you from fully straightening or bending your leg, a torn meniscus or loose body may be physically blocking the movement. For acute injuries, initial management can involve RICE (Rest, Ice, Compression, and Elevation), but persistent symptoms or those that worsen over several weeks necessitate a diagnosis from a physician.