Snapping Knee – Causes & Management

A large number of patients experience a snapping or popping sound in their knees. The sound is often described as an audible click, pop, or a snapping sensation while moving the knee. While mostly a normal finding, in some cases, it may signify an underlying pathology. The pathological snapping is usually associated with pain, swelling, and a history of injury.

The knee joint is the largest joint in the human body. The joint is formed by the lower end of the thigh bone and the upper part of the shinbone. The kneecap forms a joint with the thigh bone and acts as a lever for straightening the leg. The kneecap accounts for the smooth gliding of the quadriceps tendon during the straightening or bending of the leg.

 

MRI of the knee in the coronal section showing various structures.

The articular cartilage is a glistening white smooth tissue covering the end of the bones forming the joint. The cartilage also covers the inner surface of the kneecap. The cartilage is toughened yet flexible enough to allow smooth gliding of the joint surfaces.

The anterior cruciate ligament and the posterior cruciate ligament provide stability at the front and the back of the knee. The medial and lateral collateral ligaments provide stability on the sides of the knee joint.

There are small pads of tissue known as meniscus on the inner side and the outer side of the knee joint. The menisci cushion the impact on the ends of the bones and provide additional stability to the joint. The meniscus has a limited blood supply.

The tissue lining the inner surface of the joint is known as synovium. The synovium secretes a watery thin, clear, and sticky fluid known as synovial fluid. Just like machine oil, it acts as a lubricant allowing smooth gliding of the joint. Bursae are small fluid-filled sacs helping in smooth gliding of the structures around the knee joint.

Alignment between the femur and tibia is necessary for the proper distribution of joint forces. Muscles crossing the knee joint allow for bending and straightening the knee.

The abnormal sound may either be a normal finding or a sign of any underlying disease process. Normal knee sounds are generally not associated with pain and swelling. There is usually no history of any previous injury. The sounds are inconsistent and occur sporadically.

Commonly the causes of normal snapping/popping are :

• Cavitation of air bubbles or bursting of air bubbles in the synovial fluid.
• The tendons passing around the knee may stretch while passing over a bony bump and then snap back.
• After knee surgery.
• Catching of a fold of synovium in between the joint.

Pathological snapping/popping sound is commonly associated with pain and swelling. The sound is consistent and generally aggravating in nature. The sound may have a sudden onset in cases of acute meniscal or ligament injury. Old meniscal tears, ligament imbalance, and cartilage damages have a more chronic course.

• Meniscal tears may result in popping, clicking, or snapping sounds. There may be associated swelling. The patients often describe a catching sensation and a feeling of giving away.
• Loose cartilage in the knee joint often results in an abnormal knee sound. The patients commonly describe a feeling of having something caught within the knee. The sensation resulting in a clicking/popping sound as they bend/straighten the knee.
• Inflamed tendons may result in a snapping sound while bending the knee back and forth. The iliotibial band may get caught over the bony surface at the end of the thighbone and result in snapping.
• A grinding or grating sensation also known as crepitus is more often a result of arthritis.
• The softening of the inner surface of the kneecap cartilage may result in a popping/snapping sensation. Also known as patellofemoral pain syndrome or runners knee.

Whilst the normal sounds in the knee joint may just need reassurance, pathological sounds require further investigation. A sports specialist physician may order investigations such as an X-ray or MRI after a thorough physical examination. 

Meniscus tears rarely heal on their own due to limited blood supply, especially on the inside. Early diagnosis and management are important as chronic tears may lead to early arthritis.

Arthroscopic debridement/trimming or repair may be warranted for a meniscal tear. A small camera is inserted in the knee with minuscule tools for the surgery.

Arthroscopic surgery can also be used in the treatment of loose cartilage, ligament tears, articular cartilage damage, and chondromalacia patella.