Causes of Knee Popping or Snapping

Knee popping, snapping, cracking, or crunching noises are usually harmless and may simply be the consequence of aging joints. But they could also be an indication of a serious injury, such as a torn meniscus, or an early sign of knee arthritis. This is especially true if the noises are accompanied by pain or the loss of joint mobility.

Crepitus is the term used to describe the grating sounds or sensations produced by friction between bone and cartilage. It can affect different parts of the body but is common in the knee.

This article explains five common causes of knee popping and when it's time to seek treatment from your healthcare provider or a specialist known as an orthopedist.

Air Bubbles

Knee popping is often harmless and not only affects older people but younger ones as well. In such cases, the noise may arise when air seeps into the lubricating fluid within the joint space (called synovial fluid) and creates tiny bubbles.

When the knee is bent or stretched, the bubbles can burst, causing a popping or cracking sound. Even so, the condition doesn't cause pain and is considered harmless.

Meniscus Tear

Knee popping can happen when there's something mechanically wrong with the knee joint. It may feel as if something is "catching" in the knee whenever it is bent. Along with a popping or cracking sound, there may be knee pain, swelling, tenderness, redness, and locking.

These are all signs of a condition known as a meniscus tear. This is when the meniscus—a C-shaped piece of cartilage that acts as a cushion between the shinbone (tibia) and thighbone (femur)—is partially or fully torn due to a traumatic injury or the progressive degeneration of the knee joint.

While some meniscus tears heal on their own, those involving deeper tissue don't have the blood supply needed to enable healing. The severe tears may require a procedure called arthroscopic debridement to trim and repair the injury. In other cases, the surgical reattachment of the torn ends may be needed.

Runner's Knee

Knee crepitus may develop when excessive stress is placed on the femur and kneecap (patella). This may occur due to an overuse injury or when the bones of the knee joint are improperly aligned.

When this happens, the cartilage in the knee joint can start to soften and wear away, causing popping sounds and pain with certain movements. This includes climbing stairs, jogging on an incline, running on irregular terrain, or simply exercising the knees harder than usual.

These are all signs of patellofemoral stress syndrome (PFSS), also known as "runner's knee." PFSS is common among long-distance runners but can also affect someone who has had a bad fall or hit their knee badly (such as during an auto accident).

PFSS is most often treated with rest, ice application, anti-inflammatory drugs, physical therapy, and a structured exercise program.

Knee Osteoarthritis

Knee crepitus is often an early sign of knee osteoarthritis (OA), also known as "wear-and-tear arthritis." Knee OA is an aging-related condition in which knee cartilage is gradually worn down over time, causing pain, stiffness, and a loss of the range of motion of the knee joint.

Knee popping may occur without pain in the early stages and become progressively worse as the other OA symptoms develop.

With knee OA, the crepitus will tend to be chronic (persistent or recurrent) due to the progressive softening and breakdown of cartilage within the knee (known as chondromalacia patella).

Knee OA is treated with exercise, weight loss, heat or cold therapy, and over-the-counter anti-inflammatory drugs, In severe cases, prescription medications, physical therapy, cortisone knee injections, or surgery may be indicated.

Knee Tendonitis

Knee popping can also be caused by a problem with the tendons of the knee. There are times when swelling of tendons, known as tendonitis, can cause them to catch on the edges of a joint as the knee is bent. The most common type is called iliotibial band tendonitis.

The iliotibial (IT) band is a tendon that runs from your hip to just below your knee. When the band becomes swollen or irritated, it can get stuck on the end of the femur as the knee bends back and forth, causing an audible snap. 

Unlike mechanical popping where the problem is deep within the joint, this type of popping is felt just below the skin. You can often feel the swollen and displaced tendon as you move the knee.

Rest, ice, anti-inflammatory drugs, and physical therapy can help stretch the IT band and resolve the symptoms. If not, arthroscopic hip surgery may be recommended.

When to See a Healthcare Provider

Knee popping is not necessarily a cause for concern. However, if you feel pain when your knee pops, have a healthcare provider look at it as soon as possible. Doing so may prevent a more serious knee injury—including anterior cruciate ligament (ACL) injuries that affect between 100,000 and 200,000 people in the United States each year.

Even if the knee popping isn't painful, you may still want to have it checked out. In some cases, it may be an early sign of an overuse injury or osteoarthritis. Weight loss, a change of footwear, or knee-strengthening exercises may be all that is needed to resolve the condition or help ease symptoms.

The best treatments are targeted directly at the specific problem that is causing the abnormal popping or snapping inside the knee joint. 

Summary

Knee popping or snapping can be entirely harmless or a normal consequence of aging joints. It can also be a sign of a serious knee problem, especially if the sound is accompanied by pain or the loss of joint mobility.

Causes of knee crepitus with pain include a torn meniscus, patellofemoral stress syndrome (runner's knee), knee osteoarthritis, and knee tendonitis.