Cracking sound in knee

Though most doctors use the term crepitus to refer to joint noises, there are specific terms to refer to the kind of noise and the possible reasons for the noise, too. These terms are based on the duration, frequency and loudness of the sound and can be listed as:

• Cracking or popping: Occurs due to popping of gas bubbles in the joint.
• Pop: A pop is a sudden sharp sound that occurs during an injury. Popping sound from the knee may indicate an anterior cruciate ligament (ACL) injury or a meniscal tear. (Each of our knees has two rubbery pieces of cartilage to prevent any friction between the thigh bone and the shin bone. This cushiony layer is known as the meniscus. A tear in this layer can produce a pop accompanied by pain when you try to turn or twist the knee.)
• Clunking: It is a single loud sound that occurs when you flex your knee. The clunking sound occurs when you flex your knee against some form of resistance. In patients with total knee arthroplasty, clunking happens when the patella (knee cap) repositions itself in the socket of the femur (thigh bone).
• Clicking: A clicking sound is again a single sound that you hear when you flex your knee. Though it occurs due to a meniscus tear.
• Grinding or grating sound: It is used to describe a continuous scratching sound from the knee joint. This kind of sound is very common in those with patellofemoral pain syndrome and osteoarthritis of the knee.

Depending on the cause, noises from the knee can be divided into:

• Physiological: The word physiological means anything pertaining to normal body function. So, physiological sounds from the knee are obviously nothing to worry about. Most commonly these occur when the structures of your joints rub against each other. Though nobody really knows the exact cause of physiological knee popping, some possible physiological causes of cracking sounds from the knee are:

  • Bursting of gas bubbles in the synovial fluid (the fluid surrounding and lubricating the knee joint).
  • Tribonucleation or formation of cavities or open spaces in the knee joint. According to this theory, when two solid surfaces are stuck close to each other in a liquid (synovial fluid in this case) which contains gas, they tend to resist being separated and when they do part, they do so loudly with the formation of a gas bubble and a popping sound.
  • Stretching of ligaments or supporting structures of the joint as in snapping knee syndrome. The movement creates friction and when it happens suddenly or abnormally, it causes a popping sound.
  • Presence of infrapatellar plica: A plica is a fold of synovial membrane that normally forms as a part of embryonic development of the knee. This membrane usually gets reabsorbed before birth. In some people, this fold is only partially reabsorbed and can create popping sounds, especially when you have another knee condition such as a repetitive strain injury.
  • Some patients just start to notice knee popping more often after a minor injury or knee surgery. 
• Pathological: Pathological, as the name suggests, includes knee noises that are caused due to an underlying condition. The following causes are included under pathological knee crepitus:
  • Pathological plica: Though a lot of people get snapping sounds from their knees due to the presence of plica, inflammation of plica may cause knee popping with knee pain. The plica may become inflamed due to repeated bending of the knee (this is common in some sports), knee twisting or any wear and tear in this connective tissue.
  • Osteoarthritis of the knee: Degeneration or wear and tear of the knee joint with age is one of the possible causes of knee cracking. If you have osteoarthritis, you would also have other symptoms like joint pain and swelling.
  • Cartilage tear: Knee crepitus may occur when any cartilage around the knee joint gets torn or damaged. The knee joint has two pieces of cartilage, called meniscus, that act as shock absorbers. Meniscus tears usually happen as a sports injury and are associated with a popping sound. Also, wear and tear with old age increases the chances of a meniscus tear. If you get one, you’ll also have joint stiffness, and pain and swelling in your knee joint, which will show up within two to three days of the injury. Additionally, you may lose some range of motion in your knee joint.
  • Pathological snapping knee syndrome: Pathological snapping knee occurs due to lipomas, synovial nodules (non-cancerous growths in the thin layer of tissue that covers the knee joint), cysts in the knee joint or the conditions of the supporting structures of the knee joint. 
  • Unstable knee cap: Normally, your knee bone fits in a groove between your thigh and shinbone and slides as per your movement. In the case of patellofemoral instability, the knee cap just slides out of the grove, either partially or completely. The latter usually happens due to sports injuries and is much more painful than the former. Patellofemoral pain syndrome or runner’s knee is one condition caused due to unstable knee cap. 
  • Knee sounds after surgery: Knee popping may also show up in some people after surgery. For example, when chondrocalcinosis occurs after a meniscectomy (removal of meniscus). Chondrocalcinosis refers to a condition in which calcium crystals start to accumulate around or inside a joint. It is often associated with pain. Also, a lot of people note knee crepitus after a total knee arthroplasty. This occurs due to the placement of the new knee cap. Arthrofibrosis or formation of scar tissue after surgery may also lead to popping noises from the knee.

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Physiological knee popping does not need any treatment and treatment for pathological knee popping depends on the underlying cause of the sounds. 

In most cases, it can be treated with lifestyle management, strengthening exercises, and other non-surgical methods to manage the symptoms. 

Hot and cold compression is suggested for a knee injury. Plica and cysts can be removed with surgery and unstable knee cap can also be corrected with surgical procedures.

For persistent post-surgical crepitus, doctors may go for an arthroscopic debridement: a small surgery to remove the foreign matter or the loose tissue to relieve pain.

If you already experience frequent knee popping and don’t have an underlying health condition, you can do the following to protect your knees and prevent pathological knee crepitus:

• Practice exercises that stretch the calf muscle, iliotibial band and hips. Inner thigh squats (Sumo squats) and strengthening exercises for the legs can also protect the knees. However, experts suggest that older people should not squat so low that it puts undue pressure on their knee.
• Always warm-up before exercises before and stretch after a work out session.
• Wear the right kind of shoes to prevent injury.
• Lose some weight if you are obese or overweight.

If you have a knee problem, try to avoid running or jumping for a while or at least until your doctor tells you it's safe to resume.