Understand your pop in the knee symptoms, including 9 causes & common questions.
6 most common cause(s)
Symptoms of pop in the knee
The knee is a strong and complex joint, but it has a very limited range of motion. As a hinge joint, it can only flex in one direction, and that leaves its structures very vulnerable to being forced out of place if they are hit or twisted.
The "pop" is actually the sound of ligaments tearing or cartilage breaking. Small bits of torn cartilage floating loose inside the knee joint can also cause catching and popping.
Knee injuries are very troubling to athletes, but there are effective treatments and therapy available if an injury does occur.
Common characteristics of a pop in the knee
The sensation of a pop in the knee can be described by the following:
- An audible "pop": You may feel a sudden tear in or around the knee during exercise and hear a pop at the same time.
- Sudden, severe pain: Pain that prevents normal movement of the knee joint may occur.
- Weakness and instability in the knee: You will likely be unable to bear weight on the affected knee.
- Bruising and swelling
- A catching, locking sensation
Duration of symptoms
A sudden pop in the knee is considered an acute injury because it happens suddenly, with little to no warning.
Who is most often affected?
People who are most likely to experience a pop in knee include:
- Women: Female athletes have more knee injuries due to having greater strength in the front of the thigh and less in the back. This makes the knee more likely to be hyperextended (locked straight) when landing from a jump.
- Anyone over age 50: In an older person, cartilage and ligaments may become less flexible due to aging and long-term wear.
When is it most likely to occur?
This injury is likely to occur in the following situations:
- Playing fast-paced sports: Sports involving sudden starts, stops, and turns often result in knee injuries.
- Lifting heavy objects: This puts extra strain on the knee joint if proper lifting technique is not used.
Is a pop in the knee serious?
Depending on how much pain you are in or how limited your mobility is, you should have a good idea of the severity of your injury. However, it can be further evaluated by the following details.
- Not serious: An air bubble within the knee joint can form from time to time. Eventually, the bubble pops, which is painless, harmless, and is just like cracking your knuckles.
- Moderately serious: A knee ligament may be hyperextended (overstretched) but only partially torn through.
- Serious: A knee ligament and/or a meniscus (cartilage on the sides of the knee) is completely torn through.
Causes of pop in the knee
Many conditions can cause the symptom of a pop in the knee. We've listed several different causes here, in approximate order from most to least common.
Sports injuries
Various sports activities can lead to a pop in the knee and associated injury.
- Quick changes of speed and direction: This may include suddenly stopping or turning while running at a higher speed or landing on a hyperextended knee.
- Being struck in the knee: This may include getting a direct hit from another player or being hit by a piece of equipment.
- Falling or twisting with the foot planted: This may include wearing shoes with cleats that stick in the ground or getting the foot caught in the binding of a ski or snowboard, for example.
Aging
With normal aging and wear-and-tear, various structures around the body and in the knee specifically may not perform the same.
- Cartilage: Cartilage that normally helps cushion the ends of the bones in the knee joint becomes less rubbery and supportive.
- Ligaments: Ligaments in the knee, including the anterior cruciate ligament (ACL), posterior cruciate ligament (PCL), medial collateral ligament (MCL), and the lateral collateral ligament (LCL) may become less flexible.
Heredity
Hereditary causes may lead you to be predisposed to injuring your knee.
- Bone shape: Small malformations of the bones that form the joints may be present, leaving the joints prone to faster wear.
- Durability of cartilage: Cartilage is less durable in some people than in others, regardless of age, and therefore wears out faster.
We've listed some specific conditions that can cause a pop in the knee, along with how to identify each of them.
This list does not constitute medical advice and may not accurately represent what you have.
Repeated kneecap dislocation (patellar subluxation)
Recurrent patellar subluxation is the continued instability of the kneecap, which causes anterior knee pain and usually occurs laterally. It occurs unpredictably with varying durations.
Rarity: Uncommon
Top Symptoms: pain in one knee, dull, achy knee pain, pain in the front of the knee, pain in the inside of the knee, swollen knee
Symptoms that always occur with repeated kneecap dislocation (patellar subluxation): kneecap dislocation
Urgency: Primary care doctor
Severe kneecap dislocation
The kneecap connects the muscles in the front of the thigh to the shinbone (tibia). When the kneecap slips out of the groove, problems and pain often result.
Rarity: Rare
Top Symptoms: constant knee pain, pain in one knee, knee pain from an injury, knee injury, kneecap dislocation
Symptoms that always occur with severe kneecap dislocation: knee pain from an injury, kneecap dislocation, constant knee pain
Urgency: Hospital emergency room
Posterior cruciate ligament (pcl) injury
The PCL (posterior cruciate ligament) is located in the back of the knee and connects the big thigh bone (femur) to the big shin bone (tibia). It keeps the tibia from moving backwards. An injury to the PCL requires a huge force, making it much rarer than an ACL tear.
Your primary care provider or an urgent care provider can do an exam and imaging can determine the extent of the injury. Treatment would depend on the severity of the injury, ranging from just bracing to full surgery.
Rarity: Ultra rare
Top Symptoms: constant knee pain, severe knee pain, pain in one knee, knee pain from an injury, swollen knee
Symptoms that always occur with posterior cruciate ligament (pcl) injury: inability to bear weight immediately after injury, pain in one knee, knee pain from an injury, severe knee pain, constant knee pain
Urgency: Hospital emergency room
Patellofemoral pain syndrome
Patellofemoral pain syndrome (PFPS) is also called runner's knee, jumper's knee, anterior knee pain, chondromalacia patella, and patellofemoral joint syndrome.
Overuse through training for sports is a common cause, especially if there is a misalignment in the knee joint or a previous knee injury. This wears away the cartilage beneath the kneecap and causes pain on exercising.
It is most common in females and in young adults who are active in sports, but can affect anyone.
Symptoms include dull pain at the front of the knee and around the kneecap (patella) while running, squatting, or climbing stairs, or after prolonged sitting with knees bent.
Diagnosis is made through physical examination and through x-rays, CT scan, and/or MRI.
Treatment most often involves rest; over-the-counter pain relievers; low-impact exercise such as swimming or bicycling; physical therapy to strengthen and stabilize the knee; and orthotics (shoe inserts) to help correct a misaligned stride.
Surgery is needed only for severe cases, and is done through arthroscopy to remove any fragments of damaged cartilage.
Rarity: Common
Top Symptoms: knee pain, pain in one knee, knee pain that gets worse when going up stairs, dull, achy knee pain, knee pain that gets worse when squatting
Symptoms that always occur with patellofemoral pain syndrome: knee pain
Urgency: Primary care doctor
Osteochondritis dissecans
Osteochondritis dissecans, also called OCD, is a joint condition. It occurs when there is not enough blood flow within the end of a bone, under the protective cartilage. These bone layers begin to die and separate from the main bone, taking the cartilage with them.
The exact cause is not known. It may be due to overtraining a young person before the bone is entirely mature, which can interfere with blood supply.
Most susceptible are children and teenagers. The condition is found most often in only one joint, usually the ankle, knee, or elbow. However, any joint can be affected and there may be more than one.
Symptoms include swelling and pain in the joint during exercise, and sometimes "locking" of the joint. Osteochondritis dissecans may lead to osteoarthritis if not treated.
Diagnosis is made through patient history, physical examination, and imaging.
Treatment first involves rest because in children who are still growing, the bone and cartilage may heal spontaneously. If there are loose pieces of bone within a joint, surgery may be necessary.
Rarity: Rare
Top Symptoms: pain in one knee, knee stiffness, knee instability, knee pain that gets worse during a run, pop in the knee
Urgency: Primary care doctor
Meniscal injury
The menisci are the two pieces of cartilage serving as shock absorbers in the knee, between the lower end of the thighbone and the top of the shinbone. A torn meniscus is commonly referred to as "torn cartilage" in the knee.
Damage to a meniscus often happens along with another injury to the knee, especially when there is any forceful, twisting movement or a direct hit such as a tackle.
Older people may tear a meniscus through normal activity if the cartilage has become thin and worn due to aging.
Symptoms include pain, stiffness, and swelling. The knee will simply not work correctly and may catch, lock up, or give way.
Diagnosis is made through patient history, physical examination, simple motion tests, and imaging such as x-ray or MRI.
Depending on the exact form of the injury, the tear may be allowed to heal on its own with supportive care such as rest, ice, and nonsteroidal anti-inflammatory pain medication. In other cases, arthroscopic surgery followed by rehabilitation may be needed.
Rarity: Common
Top Symptoms: pain in one knee, knee stiffness, knee instability, pain in the inside of the knee, swollen knee
Urgency: Primary care doctor
Knee arthritis
Knee arthritis means that there is inflammation and abnormal wear of one or both of the two joints in the knee.
Arthritis in any joint is most often be caused by long-term wear and tear, called osteoarthritis; by an autoimmune condition that attacks the joints, called rheumatoid arthritis; or by an injury, called post-traumatic arthritis.
Symptoms include pain, which becomes worse with use of the joint; limited range of motion, meaning the shoulder joint cannot move as far as it once did; and pain when resting or trying to sleep.
Knee arthritis cannot be cured, but symptoms can be managed to improve quality of life and ease pain and discomfort.
Diagnosis is made through patient history, physical examination, and x-rays. To confirm, an injection of anesthetic may be placed into the joint. If the pain is eased, arthritis is almost certainly the cause.
Treatment involves physical therapy; nonsteroidal anti-inflammatory drugs to ease discomfort and inflammation; and corticosteroid injections into the knee to relieve pain. Surgery and knee joint replacement can be done in some cases.
Rarity: Common
Top Symptoms: pain in both knees, knee stiffness, knee instability, swollen knee, morning joint stiffness
Symptoms that always occur with knee arthritis: pain in both knees
Urgency: Primary care doctor
Dislocated kneecap
The kneecap connects the muscles in the front of the thigh to the shinbone (tibia). When the slips out of the groove, problems and pain often result.
You should visit your primary care physician within the next 24 hours. The first step is to return the kneecap to its proper place, a process called reduction.
Acl injury
The ACL (anterior cruciate ligament) is an important tendon that connects the femur (thigh bone) to the tibia (shin), stabilizing the knee. ACL injuries are commonly caused by sports-related injuries.
Your primary care provider or an urgent care provider can do an exam, and imaging can determine the extent of the injury. Treatment depends on the severity of the injury, ranging from rehabilitation to full surgery.
Rarity: Rare
Top Symptoms: knee pain, pain in one knee, knee instability, swollen knee, knee pain from an injury
Symptoms that always occur with acl injury: knee pain
Symptoms that never occur with acl injury: mild knee pain
Urgency: Hospital emergency room
Pop in the knee treatments and relief
As long as you are not in severe pain and you still have some use of your knee, treatment can begin at home while you reach out to a medical professional. It is likely you will have to have a consult and follow-ups for your injury in order to prevent further complications.
At-home treatments
While caring for the injured knee at home, you can try the following to hopefully provide relief or prevent injuries in the future.
- Rest, ice, compression, and elevation (RICE): When the injury first occurs, you can try resting the knee, icing the knee, compressing the knee with bandages, and keeping the knee elevated indicated by the mnemonic RICE. You can ice the area for 20 minutes at a time on and off as you see fit. You can also use over-the-counter pain relievers such as ibuprofen (Motrin/Advil) or other NSAIDs.
- Build strength: You should always stretch and warm up before exercising. Try to strengthen the legs as much as possible in preparation for sports, such as working in extra exercises or training during the off-season (if applicable to you), because strong muscles help protect and support the skeletal structure. You should also learn proper techniques for running, jumping, and landing.
- Use the correct sports equipment: You should wear proper, supportive footwear and not use cleats or spikes unless truly necessary. Make sure sports equipment is properly adjusted, such as skis or skates.
Medical treatments
You should follow-up with a physician sooner than later for the following.
- A moderately severe injury: This means one which follows a painful pop but still allows for some movement and walking.
- Referral for physical therapy: This will help with rehabilitation, including the possible temporary use of a knee brace and crutches.
- Discussion of surgery: This may include reconstructive surgery for torn ligaments, surgery for meniscus tears, and surgery for loose bits of cartilage within the knee.
When a pop in the knee is an emergency
Seek immediate treatment in the emergency room or call 911 if you have severe pain and you cannot move the injured knee at all.
FAQs about pop in the knee
Here are some frequently asked questions about pop in the knee.
Is a pop in the knee caused by a fracture?
No. A broken bone does not cause that sort of sound. A loud "pop" that is accompanied by immediate pain is caused by a tear in one of the four main ligaments that support the knee, or by a tear in the protective cartilage on either side of the knee.
Is a painful pop in the knee a sign of a permanent injury?
Not always. Some tears of the ligaments and/or cartilage are quite severe, requiring both surgery and a long period of rehabilitation, and may never heal entirely. However, less serious injuries, especially in someone under 40, can be treated with good supportive care and physical therapy and you can eventually return to previous activities.
Is a pop in the knee always caused by sports or heavy exercise?
It's true that a painful pop in the knee usually happens during sports, but in some cases, a piece of cartilage can break or a ligament can be sprained with mild exercises such as lifting boxes or climbing stairs. This usually happens to older people, especially those who are overweight or otherwise in poor physical condition.
Is a pop in the knee serious in a child?
If there is pain, then this is an injury that needs immediate treatment in anyone under 18. This is because the growth plates on the long bones are still growing in a young person. These plates are just above and just below the knee, and If they are damaged, the long bones may grow unevenly.
Is a pop in the knee the same as a sprained knee?
A sprain is the overstretching of a ligament the tough "cords" that hold the bones together. A sprain will be sore and uncomfortable but will heal with rest, while a pop followed by immediate, severe pain means that the ligament, or a piece of cartilage, has actually been torn or broken [5]. The pop, therefore, indicates a more serious injury.
Questions your doctor may ask about pop in the knee
- While you are standing, take a look at your knees. Do they appear to bend towards each other, rather than follow a straight line along your legs? (Knock-kneed)
- Did you recently injure your knee?
- Do you often feel your knees buckling?
- Did you feel your knee cap pop out of place?
Self-diagnose with our free Buoy Assistant if you answer yes on any of these questions.
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References
- Common knee injuries. American Academy of Orthopaedic Surgeons: OrthoInfo. Updated March 2014. OrthoInfo Link
- Types of knee ligaments. Stanford Health Care. Stanford Health Care Link
- Knee ligament repair. Johns Hopkins Medicine. Johns Hopkins Medicine Link
- Levy AS, Meier SW. Approach to cartilage injury in the anterior cruciate ligament-deficient knee. Orthopedic Clinics of North America. 2003;34(1):149-167. NCBI Link
- Types of Knee Sprains, Strains & Tears. NYU Langone Health. NYU Langone Link