Why do my knees crack?

Experiencing cracking within your knees can be disconcerting and lead to many different negative thoughts; “My knee is wearing away”, “Something is not right”, “It’s bone grinding on bone”, “It must be arthritis”, “I am worried about long term damage”.    

Often these negative beliefs lead to fear, altered loading and an avoidance of activity which in turn can cause negative consequences (Robertson et al, 2017).    

What we need to understand is that this cracking or crepitus is normal. There has been no definitive link between noise and pathology. In fact, a key study examined 247 symptomatic knees and compared them to 250 normal, pain free knees. The results showed that 99% of subjects who were pain free still had crepitus (McCoy et al, 1987). This indicates that crepitus can be a normal presentation within a healthy knee joint.    

If crepitus within the knee joint is normal, what is actually taking place?    

A number of different explanations have been given for this noise or sensation. The most common reason given is the release of gas bubbles within the synovial fluid of the joint. A change in joint pressure can cause tiny gas bubbles to be formed within the joint which are then released when the joint is moved causing a ‘popping’ sound. This is known as cavitation.  

Other explanations include:

• The snapping of tendons or ligaments over a bony protuberance, the structure is stretched when moved and then snaps back into place causing a clicking sound.
• The cartilage that covers the bone can become uneven as we age. Sound can come from these rougher surfaces gliding across each other.
• The movement of fluid particularly through the patellofemoral joint can cause similar sounds and sensations.
• Those with hypermobility often report regular clicking of their joints.

No matter the cause, often a noisy knee is no reason for concern.

Crepitus and arthritis, should I be worried?

Having highlighted some of the causes of crepitus which are of no concern to an individual, should you be worried if you have crepitus and arthritis? Crepitus can be a common feature of knee osteoarthritis however it is unlikely to be your sole symptom. A recent study investigated how likely it was for those with crepitus and osteoarthritis to go on and have a knee replacement in the future observing 4566 people. The results showed that the presence of knee crepitus at baseline does not predict the need for a total knee replacement 36 months later. It was found that those with crepitus had a slight reduction in knee extensor strength however this did not impact general physical function over time (Pazzinatto et al, 2019).  

When should I get it investigated?

If you have crepitus alongside knee pain or if you have injured your knee and are now experiencing crepitus that is new to you, it would be worth being assessed by one of our expert physiotherapists to rule out pathology.  

References

• McCoy, G., McCrea, J., Beverland, D., Kernohan, W., & Mollan, R. (1987). Vibration arthrography as a diagnostic aid in diseases of the knee. A preliminary report. The Journal of Bone and Joint Surgery. British volume, 69-B(2), 288-293
• Pazzinatto, M. F., De Oliveira Silva, D., Azevedo, F. M., & Pappas, E. (2019). Knee crepitus is not associated with the occurrence of total knee replacement in knee osteoarthritis – a longitudinal study with data from the osteoarthritis initiative. Brazilian Journal of Physical Therapy, 23(4), 329-336.
• Robertson, C. J., Hurley, M., & Jones, F. (2017). People’s beliefs about the meaning of crepitus in patellofemoral pain and the impact of these beliefs on their behaviour: A qualitative study. Musculoskeletal Science and Practice, 28, 59-64.

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