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Knee Instability Symptoms, Causes & Common Questions

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Last updated April 9, 2024

Knee instability quiz

Take a quiz to find out what's causing your knee instability.

The feeling of a loose knee cap is most commonly caused by an injury that has sprained or tore ligaments. A meniscal or anterior cruciate ligament (ACL) injury can also cause knee instability and pain. Read below for more information about causes and how to seek medical treatment for an unstable knee.

Knee instability quiz

Take a quiz to find out what's causing your knee instability.

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Knee instability symptoms

Ligaments are the tough fibrous bands that hold the bones together. If the ligaments within the knee become damaged, the knee joint will be painful, wobbly, and unstable. It may feel as though it could give way at any moment while you are walking, exercising, or even just standing.

If the muscles of the knee are damaged, they cannot contract properly, and you will experience weakness and pain. Swelling may have the same effect.

Knee injuries are usually sports injuries, though they can happen during any physical activity that leads to twisting of the knee or to a traumatic injury of the joint.

Common characteristics of knee instability

The following can likely describe your knee instability.

  • Your knee feels out of place: The sensation that the knee is popping out of place, or is about to collapse under you.
  • Looseness: The sensation that the structures within the knee are loose.
  • Knee locking: Feeling that the knee joint is locking or catching.
  • A pop in the knee: Feeling, or even hearing, a popping or clicking from the knee.
  • Finding it impossible to straighten the knee
  • Pain: This may be mild, moderate, or quite severe.
  • Swelling

Who is most often affected by knee instability symptoms

The following individuals are more likely to experience knee instability.

  • Young athletes: Fit, active athletes in their teens and twenties are the ones who most often suffer sports injuries.
  • Dancers
  • Anyone doing hard physical labor
  • Anyone who is overweight or obese
  • Women: Women may be more prone to knee problems, due to naturally having a wider pelvis. Therefore, women have a predisposition to misalignment between the top of the kneecap and the bottom of the thighbone.
  • Older people: May experience gradual wearing away of the cartilage of the knee
  • If you have a prior injury: This can predispose you to another one.

When are knee instability symptoms most likely to occur?

The following situations can result in either acute or chronic knee instability.

  • Acute injury: This can occur when playing sports, when falling down, or during an automobile accident.
  • Chronic pain: This can occur with mild exercise or while climbing up and down stairs.

Are knee instability symptoms serious?

The severity of your knee instability is ultimately dependent on the cause and the impact it has on your activity levels or quality of life.

  • Not serious: A mild strain or sprain is a common sports injury and can be healed with rest and physical therapy.
  • Moderately serious: Damage to the cartilage can lead to arthritis. The pain and instability of an acute injury may become chronic if not treated right away.
  • Serious: A more severe injury, if neglected, can suddenly worsen if you overdo it. For example, a partial tear may become a complete tear.

Causes of knee instability

Most common knee instability causes

The most common causes of knee instability include the following.

  • Ligament sprain: Sprain of the knee ligaments, which means these ligaments have had their fibers damaged due to wear and tear, sudden stress, or direct trauma.
  • Ligament tear: Tear of the ligaments, meaning the fibers have been partially or completely torn through. This usually happens when you run and then make a sudden change of direction the knee stays pointing one way while the rest of you twists the other way.

Less common knee instability causes

Causes of knee instability that are less common include the following.

  • Muscle damage: Inability of the damaged muscle to physically contract properly
  • Fluid buildup from the injury: Thus interfering with muscle action

Rare and unusual knee instability causes

Causes of knee instability that occur even less frequently include the following.

  • Partial dislocation: If you do something strenuous enough to damage the ligaments, it is likely that you will have damage to the cartilage around the knee bone as well. The patella, or kneecap, can become partially or completely dislocated. A partial dislocation is called a subluxation. Dislocation can occur if the ligaments are damaged and are not holding the kneecap firmly in place as usual.
  • Cartilage tears: The cartilage at the ends of the upper and lower leg bones, which meet under the kneecap, can become torn. This often happens along with a ligament sprain.
  • Cartilage wear: Cartilage can gradually wear away over time, causing pain and difficulty moving since there is no longer enough cushioning material between the bones.
  • Birth deformity: You may have been born with knee structure that makes the joint vulnerable to slipping out of place.

This list does not constitute medical advice and may not accurately represent what you have.

Knee (mcl) sprain

The medical collateral ligament (MCL) links the thigh bone and the shin bone on the inner side of the knee joint. An MCL sprain is any damage done to this ligament (usually through twisting/force during sports).

Rarity: Uncommon

Top Symptoms: pain in one knee, knee pain from an injury, knee injury, pain in the inside of the knee, sports injury

Symptoms that always occur with knee (mcl) sprain: pain in one knee, knee pain from an injury

Symptoms that never occur with knee (mcl) sprain: mild knee pain

Urgency: Primary care doctor

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 exercises 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 upstairs, 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

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 sprain (LCL)

The LCL is the ligament on the outside of the knee, keeping it from bending away from the body. It is most commonly injured while playing sports (ouch!) when a force is placed on the knee from the inner half of the knee.

Depending on the severity of your knee pain, you should go see your doctor or go to an urgent care clinic as soon as possible. Your doctor may prescribe you crutches, a brace, pain medication, or physical therapy. You may be asked to refrain from physical activity.

Rarity: Rare

Top Symptoms: pain in one knee, knee pain from an injury, knee injury, pain in the outside of the knee, sports injury

Symptoms that always occur with knee sprain (lcl): pain in one knee, knee pain from an injury

Symptoms that never occur with knee sprain (lcl): mild knee pain

Urgency: Hospital emergency room

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 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 the 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

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

Knee instability treatments and relief

At-home treatments

Knee instability remedies that you can try at home include:

  • Rest: Do not try to continue playing or exercising if you have sudden pain in and around the knee.
  • Over-the-counter NSAIDs: You can use ibuprofen (Advil, Motrin) to treat minor pain.

When to see a doctor

You should see your physician if your knee instability is interfering with your life. He or she can help you with the following.

  • Physical therapy: This can provide you with a program of proper stretching and exercise to help strengthen the ligaments, muscles, and tendons of the knee.
  • Possible fitting of braces and supports
  • Surgery to repair damaged cartilage and ligaments

When it is an emergency

You should seek immediate treatment in the emergency room or call 911 if you have severe and sudden pain following an injury, especially if you are unable to move the knee or bear weight on it.

Questions your doctor may ask about knee instability

  • What is your body mass?
  • Do you often feel your knees buckling?
  • Did you feel a pop in your knee?
  • Do you feel like your knee is locking from time to time?

Self-diagnose with our free Buoy Assistant if you answer yes on any of these questions.

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The stories shared below are not written by Buoy employees. Buoy does not endorse any of the information in these stories. Whenever you have questions or concerns about a medical condition, you should always contact your doctor or a healthcare provider.
Dr. Rothschild has been a faculty member at Brigham and Women’s Hospital where he is an Associate Professor of Medicine at Harvard Medical School. He currently practices as a hospitalist at Newton Wellesley Hospital. In 1978, Dr. Rothschild received his MD at the Medical College of Wisconsin and trained in internal medicine followed by a fellowship in critical care medicine. He also received an MP...
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