This cracking or popping sensation, known as crepitus, is usually due to air bubbles being caught in body tissues. It can happen in the chest or the knee. In the knee, it can cause a sound when the knee is extended.

Knee crepitus can happen at any age, but it is more common as people get older. It can affect one or both knees. The sound may be audible to other people, or it may not.

Crepitus is often harmless, but if it happens after a trauma or if there is pain and swelling, it may need medical attention.

Structure of the knee

To understand how crepitus affects the knee, let's look at the structure of the knee joint.

The knee joint is formed of three bones: the femur, tibia, and patella.

The knee is the largest joint in the body.

The three bones in the knee joint are the thighbone (femur), the shinbone (tibia), and the kneecap (patella). The kneecap rests in a groove of the thighbone, called the trochlea. When a person bends or straightens their knee, the patella moves back and forth inside this groove.

Soft tissue provides padding and protection.

Two wedge-shaped pieces of cartilage between the thighbone and the shinbone are called the meniscus. These enable the bones to glide smoothly against each other.

The cartilage is tough and rubbery, and it helps to cushion the joint and keep it stable.

There is also a thin layer of tissue called the synovial membrane that covers the joints and produces a small amount of synovial fluid, which helps to lubricate the cartilage.

The underside of the kneecap is lined with cartilage. This cartilage "rubs" against the end of the femur in the trochlear area and with abnormal wear can cause grinding (crepitus).

Causes of crepitus

There are various causes of crepitus.

In most cases, the popping sound comes from air seeping into the soft tissue, finding its way into the area around the joint and causing tiny bubbles in the synovial fluid.

When a person bends or stretches their knees, the bubbles can burst with a popping or cracking sound. While it may sound alarming, this is harmless.

However, crepitus can also happen as cartilage rubs on the joint surface or other soft tissues around the knee when the joint moves, and when the cartilage becomes thin and wears away.

In this case, medical attention may be necessary.

If there is pain as the knee snaps or catches, it can be because scar tissue, a meniscus tear, or a tendon is moving over a protruding bone within the knee joint.

Pain or swelling can be a sign of a more serious problem, such as patellofemoral pain syndrome, torn cartilage or other soft tissue, or osteoarthritis (OA).

These issues may need medical attention. Let's look at them now in more detail.

Patellofemoral pain syndrome

When the pressure between the kneecap and the femur is greater than usual, the cartilage in the joint can start to soften and wear away, losing its smoothness and leading to a condition called patellofemoral pain syndrome (PFS), or "runner's knee."

Runner's knee is a common source of knee pain in young people and athletes.

PFS can be caused by trauma or overuse, or if a part of the person's knee is badly aligned. It is a common source of knee pain in young people and athletes.

Rigorous exercise such as jogging, squatting, and climbing stairs can put strain on the area between the femur and the kneecap joint.

A sudden increase in physical activity, such as exercising more frequently, or running further or on rougher terrain than usual, can also cause it.

Another risk factor for crepitus related to PFS is trauma to the knee, for example, due to a fall, or hitting the knee on the dashboard of the car in a road traffic accident.

The individual may experience crepitus when climbing stairs or after sitting for a long time with the knees bent, as well as pain, swelling, puffiness, and stiffness.

The first line of treatment for this condition includes rest, ice, compression, and elevation, or "RICE." Anti-inflammatory medication and physical therapy exercises can also relieve it.

If not, splinting, surgery, or both may be needed - for example, to realign part of the knee.

To prevent this problem, anyone who is exercising or participating in sports should make sure they always use appropriate techniques, footwear, and equipment, and be sure to warm up before starting.

Torn cartilage

Crepitus can also be a sign of a torn meniscus. A torn meniscus can happen during sports activities, such as when the knee is twisted. It can also happen as people get older and the meniscus wears thin.

Symptoms include swelling, stiffness, and difficulty extending the knee.

The American Academy of Orthopaedic Surgeons (AAOS) explain that when the meniscus tears, the individual may experience a "popping" sensation. Normally, the knee can still be used, but stiffness and swelling may appear over the next 2-3 days.

As with patellofemoral pain syndrome, the first line of treatment is RICE and anti-inflammatory medication. Sometimes surgical repair is needed.

Osteoarthritis

Crepitus accompanied by pain can be an early sign of osteoarthritis (OA) of the knee. OA is normally a result of wear and tear, and it tends to develop and worsen with age.

In OA, the cartilage that covers the ends of bones in the joints gradually wears away. Bones rub on this increasingly rough surface, resulting in pain and mobility issues. It is more likely among people with obesity or past injury.

A study published in the journal Osteoarthritis and Cartilage found that women aged 45 to 60 years who had both crepitus and patellofemoral pain had a 72 percent chance of developing OA, although they did not yet have a diagnosis of OA.

The Centers for Disease Control and Prevention (CDC) estimate that 49.7 percent of adults aged 65 years and above were living with doctor-diagnosed arthritis between 2010-2012.

Early intervention can make it possible to tap into nonsurgical options to slow progression of OA, maximize mobility, and improve the person's strength. These can include lifestyle modifications such as weight loss and exercise, medication, and physical therapy.

As OA progresses, treatment through medication or even knee replacement surgery may be necessary.

Crepitus following surgery

Research published in Clinics in Orthopedic Surgery shows that up to 18 percent of patients who have a total knee arthroplasty (TKA), or knee replacement, will experience crepitus, sometimes due to the design and fit of the new knee.

This type of crepitus has been described as "typically benign and self-limited," usually righting itself without intervention.

However, patients who experience ongoing problems may be referred for debridement, a minor surgical procedure to remove debris from around the joint.

Protecting the knees

To help protect the knees, the Mayo Clinic suggest exercises such as walking, cycling, or swimming to strengthen the quadriceps, which are muscles in the front of the thigh.

Warming up before exercise is one way to protect the knees and reduce the risk of crepitus.

Stronger quadriceps can decrease the load on the patellofemoral joint, reducing the risk of cartilage wearing away.

Other exercises can be carried out, some with weights, but patients who are concerned about their knees should consult a physician before embarking on a new exercise regime.

Other ways to protect the knees including wearing suitable shoes, warming up before exercise and stretching afterward, and maintaining a healthy body weight to reduce stress on the knees.