Knee pain and popping is a common problem and can be caused by a number of factors. Another term commonly used for popping in the knee is Crepitus, which essentially means a noisy joint, whether it be popping, cracking or snapping.
There are 3 main types of popping noises in the knee:
1) Pain-free Popping: Popping noises in the knee often occur without any pain, in which case they are nothing to worry about
2) Painful Popping Noise at time of Injury: Sometimes when the knee is injured eg twisting awkwardly, there is a sudden, loud "pop" at the same time indicating damage to part of the knee
3) Recurrent Painful Popping Noises not Caused by an Injury: Knee pain and popping can come on gradually with no obvious cause.
If knee popping occurs without any associated pain, it is usually due to either gas bubbles building up inside the joint and bursting, or ligaments/tendons snapping over the joint:
Changes in joint pressure can cause tiny bubbles of gas to slowly form in joints. When these gas bubbles burst quickly, they make a popping sound, in a similar way to when you pop bubble-wrap. The technical term for this is cavitation. There is no harm in this and the myth that it makes you prone to arthritis is unfounded.
Ligaments and tendons are soft tissues that are positioned around all the joints in our body. Sometimes, when you move a joint (eg your knee), a ligament or tendon may stretch slightly as it goes over a small bony lump around the joint and then snaps back into place making a popping sound. Again, there is no harm with this knee popping no pain and it doesn’t make you more prone to an injury.
If there is a loud "pop" at the time of injury it usually indicates a ligament injury. The 2 most common ligaments that produce knee pain and popping when they are injured are the ACL (Anterior Cruciate Ligament) and the MCL (Medial Collateral Ligament).
What is It? The ACL at the centre of the knee gets over-stretched and tears
How Does It Happen? Blow to the side of the knee, sudden twisting or deceleration, leg bending backwards too far
Symptoms? Approximately 50% of ACL tears are accompanied by a popping sound associated with immediate swelling and pain. The other classic sign of an ACL tear is the knee giving way.
Treatment? Visit the ACL Injuries section
What is It? The Medial Collateral Ligament on the inner side of the knee gets over-stretch and tears
How Does it Happen? Force through the outside of the knee eg tackle, or sudden twisting eg skiing
Symptoms? Inner knee pain, popping/tearing sensation, swelling, instability, difficulty bending the leg
Treatment? Visit the MCL Injury section
Sometimes knee pain and popping develops over time, without any obvious injury. The popping tends to happen frequently and is usually due to one of these conditions:
What is It? A tear in the meniscus – special cartilage that lines the joint.
What Causes the Noises? When the meniscus tears, small fragments of it can catch in the knee as it moves which results in the popping noise.
Frequency? Knee pain and popping tends to come and go, rather than happening all the time as the torn fragment moves around the joint.
Treatment? Visit the Meniscus Tear section
What is It? Wear and tear of the cartilage.
What Causes the Noises? As the cartilage thins, the joint surface becomes rough and friction occurs between the bones resulting in crepitus.
Who Does it Affect? It is most commonly seen in people over the age of 50 and develops gradually over time.
Frequency? Crepitus associated with arthritis tends to be persistent rather than coming and going.
Treatment? Visit the Arthritis section
What is It? Irritation and inflammation of the cartilage lining the back of the patella.
What Causes the Noises? Friction between the back of the kneecap and the underlying femur (thigh bone) can cause knee pain and popping.
Who Does it Affect? Healthy, often sporty adolescents and young adults. More common in women
Frequency? Tends to be a more constant problem. The crepitus can usually be felt as well as heard when you put your hand over the front of the knee and bend and straighten it.
Treatment? Visit the Chondromalacia Patella section
What is It? A problem in how the kneecap moves which causes an ill-defined ache around the knee.
What Causes the Noises? The patella rubs against the femur which is often associated with a grinding sensation when the knee moves.
Who Does it Affect? It is usually worse with prolonged activity eg running, coming downstairs or after prolonged inactivity eg office workers.
Frequency? Tends to come and go
Treatment? Visit the Runners Knee section
And remember, if your knee clicks but it doesn’t cause you any pain, don’t worry. It is entirely normal and nothing to worry about. You may find that strengthening your leg muscles actually eliminates the noise – see the strengthening exercises section for ideas on where to start.
If you would prefer to diagnose your pain using other specific symptoms, how the pain started or the specific location of the pain, go to the Diagnose Your Pain section for help working out was is causing your pain and to learn what you can do about it.
The only reliable way to diagnose your particular symptoms is to see a doctor but here I have shared with you the more common causes for knee pain and popping. Always see your doctor for a thorough examination to rule out any serious injury.