Causes of Knee Popping

Knee popping, snapping, cracking, or crunching noises are usually harmless and may be the consequence of aging joints. However, the sensation can also indicate 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 for the grating sounds or sensations produced by friction between bone and cartilage. It can affect different body parts but is common in the knee.

Air Bubbles

Knee popping is often harmless and affects both older and younger people. In such cases, the noise may arise when air seeps into the lubricating fluid within the joint space (called synovial fluid), creating tiny bubbles.

When the knee is bent or stretched, the bubbles can burst, causing a popping or cracking sound. Air bubbles do not cause pain and are 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, knee pain, swelling, tenderness, redness, and locking may occur.

These are all signs of a condition known as a meniscus tear. This occurs 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 lack the blood supply needed to enable healing. Severe tears may require arthroscopic debridement to trim and repair the injury. In other cases, 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 can happen due to an overuse injury or when the knee joint bones are improperly aligned.

Overuse injuries and misalignment can cause the cartilage in the knee joint to soften and wear away, causing popping sounds and knee pain with specific movements. Specific movements include climbing stairs, jogging on an incline, running on irregular terrain, or simply exercising the knees harder than usual.

Overuse injury and misalignments can lead to patellofemoral stress syndrome (PFSS) or patellar femoral syndrome, also known as "runner's knee." PFSS is common among long-distance runners but can also affect someone who has fallen 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.

Patellar Tendon Tear

The patellar tendon is involved in straightening the front of the leg. Patellar tears are a common injury and may be complete or partial. A popping noise is often heard when the tendon tears, with other symptoms following the tear, such as pain and difficulty walking.

Partial tear: In a partial tear, the tendon is stretched and potentially frayed but not torn through. Conservative treatments, such as immobilization and physical therapy, are often effective, especially in smaller tears.

Complete tear: In a complete tear, the tendon is completely torn through and separated from the kneecap. In a complete tear, you can't physically straighten your knee. Surgical intervention is typically required to re-attach the tendon to the kneecap. Following surgery, therapies include immobilization and physical therapy.

Knee Osteoarthritis

Knee crepitus is often an early sign of knee osteoarthritis (OA). Knee OA is an aging-related condition in which knee cartilage gradually wears down over time, causing pain, stiffness, and a loss of the knee joint's range of motion.

Knee popping may occur without pain in the early stages and worsen 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 knee's tendons. Sometimes, tendon swelling, 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. However, surgical intervention may be necessary if conservative treatments are ineffective.

Osteonecrosis

Osteonecrosis occurs when the blood supply to a bone in the body is disrupted. When it occurs in the knee, it is known as osteonecrosis of the knee.

It is a painful condition typically occurring in the femur or tibia (thigh or shin bones, respectively). The knee joint becomes damaged without a healthy blood supply, and arthritis can occur.

Treatment for less severe cases includes pain management and limiting the use of the knee. However, surgical interventions are often required, especially as the condition progresses.

Ligament Injuries

Four main ligaments in your knee hold it in place. They include the:

• Medial lateral ligament (MCL)
• Lateral collateral ligament (LCL)
• Anterior cruciate ligament (ACL)
• Posterior cruciate ligament (PCL)

The ligaments on the side of your knee are called the collateral ligaments, and the ones inside your knee joint are the cruciate ligaments. ACL tears are one of the most common ligament injuries, affecting between 100,000 and 200,000 people in the United States each year.

Collateral ligaments: The collateral ligaments include the MCL and LCL. MCL injuries are common in sports. Outside hits to the knee can injure the MCL. In contrast, hits to the knee's inside aspect injure the LCL and are less common.

Cruciate ligaments: The cruciate ligaments include the ACL and PCL. ACL injuries are common and occur often in sports —especially in soccer, football, and basketball, where players change direction rapidly. PCL tears are less common and occur in specific instances where impact to the front of the knee happens while the knee is bent. Car crashes are a common cause of PCL tears.

You may hear a popping sound when any of these ligaments are initially injured. Other symptoms may include pain and difficulty moving the knee.

Treatment for ligament tears often includes a combination of immobilization, pain management, and physical therapy. Surgical interventions may be necessary depending on the severity of the tear.

How to Prevent Knee Injuries

If you are an athlete, exercise regularly, or play sports regularly, consider adding an exercise-based knee injury prevention program to your training and practices. Several programs exist, and your coach or trainer can advise you on the most appropriate for your sport or activity.

Properly warming up before a workout, cooling down, and stretching can also help prevent knee injury. Warming up helps to get blood flowing, increases the temperature and flexibility of your muscles, and can help prevent a fall or injury from stopping an exercise abruptly.

If you stop too quickly after an intense workout without a cool down, you may feel lightheaded or dizzy and could fall.

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.

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

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

Summary

Knee popping or snapping can be harmless or a normal consequence of aging joints. It can also signify a severe 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.