What Causes Your Knee to Give Out?

Causes of Knee Buckling and Instability

Tennis player whose knee just went out

Westend61/Getty Images

What might cause your knee to go out, or the sensation that your knee is about to buckle? The sensation of knee instability or weak knees may occur suddenly or develop gradually over time. It may be the only symptom of a problem, or be accompanied by signs such as popping, locking, pain, bruising, or swelling. The possible causes range from damage to one of the knee ligaments, a meniscus tear, arthritis, patellar (kneecap) instability, or even nerve damage. If your knee feels unstable it's important to see your doctor, who can then perform an exam and order studies such as an MRI or X-rays. The best treatment will depend on the underlying cause of the problem and may range from rest to surgery. What should you know if your knee goes out completely, or if it simply feels like you can't trust your knees?

Anatomy of the Knee

In order to understand any knee symptoms you are experiencing, as well as the potential causes, it's helpful to briefly review the anatomy of the knee and how it functions. The knee is a hinge joint that is limited in its roles to flexion and extension of the knee, and connects the large thigh bone (the femur), to the shin bones (the tibia and fibula).

Ligaments connect bones to bones and play a large role in stabilizing the knee and keeping the bones in line. An injury to any of these ligaments can lead to instability. There are four major ligaments in the knee (as well as some minor ones). There are two collateral ligaments, an anterior cruciate ligament, and the posterior cruciate ligament.

  • The collateral ligaments are located on either side of the knee and limit sideways bending. The medial collateral ligament (MCL) is on the inner side of your knee and the lateral collateral ligament (LCL) is on the outer side of your knee.
  • The anterior cruciate ligament (ACL) connects the top of the tibia near the front (anterior) to the femur at the center of the knee and limits rotation and forward motion of the tibia.
  • The posterior cruciate ligament (PCL) connects to the top of the tibia near the rear (posterior) to the femur and limits backward motion of the tibia.

The cartilage pads known as menisci, are the "shock absorbers" of the knee and are located between the femoral and tibial components of the knee. Each meniscus cushions the knee joint and also plays a role in stabilizing the knee joint. Injury to one or both menisci increases instability.

Other structures within the knee are easiest to picture by looking at images of the knee.

Signs and Symptoms

As noted above, you may notice that your knee suddenly goes out on you completely, or instead, you may notice only a sensation that your knee may buckle that develops gradually over time.

With some conditions, the only symptom may be a feeling of having unstable knees, but with others, the feeling of a knee going out may be accompanied by signs and symptoms such as:

  • Popping
  • Clicking
  • Locking (a feeling that the knee joint is "stuck")
  • Sticking
  • Grinding
  • Pain
  • Bruising
  • Joint stiffness
  • Reduce range of motion


Having your knee go out can cause complications on its own (such as a fall or the need to have surgery for a torn ligament), but long-term instability that is less dramatic (so that it can be ignored and go untreated) can lead to complications as well. A 2016 study found that older adults with arthritis who had untreated knee instability were more likely to have falls, sustain fall-related injuries, and had significantly poorer balance confidence than similar adults who received treatment for their knee condition.


The knee joint is very complex, and there are many possible causes of the feeling that a knee has given out or is about to give out. These include:

Ligament Tears

Damage (either partial or complete tears) to one or more of the 4 ligaments in the knee is a common cause of knee instability. Some, such as complete ACL tears, often occur suddenly with a sensation of the knee going completely out, whereas with PCL tears, the knee doesn't usually "give out" per se, but feels like it could.

  • ACL tear: An ACL tear usually occurs with sudden movements, such as a change in direction during sports. It is more often a non-contact injury, in which a person may hear a pop while pivoting or landing after a fall. It may also occur with a motor vehicle accident. The giving out sensation is often followed rapidly by pain and swelling. ACL tears are much more common in female athletes than male athletes.
  • PCL tear: PCL tears often occur in one of two situations. In one, they may occur during a fall in which a person lands directly on the front of the knee. In the other, they occur when the shin is forcefully hit from in front. These tears have been coined "dashboard injuries," referring to hitting the front of the lower leg on the dashboard of a car in a motor vehicle accident. A PCL tear is often associated with injuries to other ligaments and/or menisci.
  • MCL tear: MCL tears usually occur when the outer side of the knee is hit (lateral forces), such as with clipping in football. If knee instability is present, it usually implies a significant MCL tear is present and often is associated with pain, bruising, swelling, and difficulty bending the knee.
  • LCL tear: An LCL tear usually occurs with excess pressure on the inside of the knee or excessive inward bending of the knee, such as with quick stops (for example, with skiing or soccer), or collisions (with hockey and football). As with MCL tears, a feeling that the knee will give out often implies a significant injury.

Meniscal Tears

A meniscus tear may occur with activities such as twisting, turning, pivoting, and symptoms such as stiffness and pain usually begin rapidly. It may be accompanied by a pop and locking sensation. They are more common in people over the age of 30, as the menisci or more resilient in younger people. With a torn meniscus, the sensation of a knee going out is often more related to pain than a true instability of the joint.

There are many degrees as well as patterns of tears, that are of different significance with regard to symptoms and treatment.

Patellar Instability, Dislocation, or Subluxation

Another cause of increased knee instability may involve the kneecap (patella) moving out of the groove where it sits on the femur. This type of instability is specifically known as patellar instability. Normally, the kneecap glides through in the patellofemoral groove, a track in the front of the femur (thigh bone) at the knee. The patella is attached to ligaments and tendons that serve to stabilize it. The patella lies within the quadriceps tendon, which anchors the quadriceps muscle to the upper tibia (shin bone). In addition, patellofemoral ligaments act as secondary ligament stabilizers from the sides of the patella. A direct blow to the kneecap, usually from an accident or sports injury, can dislocate it from the groove. Also, an unnatural twisting motion of the knee can have the same result. Patellar dislocation (partial or complete) may result, and increases instability.

Plica Syndrome

Plica syndrome is a condition that results from inflammation of the inner lining (synovial tissues) fo the knee joint. It may occur suddenly, with movements such as bending or may occur more gradually as part of an overuse syndrome. It can cause symptoms similar to a meniscal injury, including the sensation that a knee is about to go out.

Knee Osteoarthritis (Loose Bodies) and Inflammatory Arthritis

The aforementioned causes of knee instability were primarily related to a joint injury. But worn and uneven cartilage due to osteoarthritis of the knee is also associated with instability. This may be due to several problems that are seen in osteoarthritides, such as loose cartilage bodies inside the joint, degenerative meniscus, or ligament tears.

Loose bodies are small fragments of calcified cartilage that can become caught between the bones of the knee, in the joint space, causing sudden pain and possibly causing the knee to give way. Aside from causing the joint to give way, loose bodies in the knee can also cause locking or may interfere with straightening of the leg.

Even if the loose bodies have been detected on imaging studies (x-rays, MRI, or CT scan), the instability they may cause is somewhat unpredictable, meaning you don't know when you will feel your knee give out. It happens suddenly and without warning.

In addition to pain, knee arthritis often causes pain with movement, joint stiffness, reduced range of motion, and a sticking or grinding sensation when walking.

Nerve Damage

Neuropathy in the femoral nerve can disrupt normal sensations, making it feel like your knee could go out or is going out. There are a number of conditions associated with nerve damage, including diabetes, chronic alcohol use, and multiple sclerosis. In addition to changes in sensation or numbness, femoral nerve neuropathy may cause tingling or burning pain in the area around the knee.

Risk Factors

Risk factors for knee injuries that can lead to instability include:

  • Degenerative arthritis
  • Inflammatory arthritis (such as rheumatoid arthritis)
  • A prior knee replacement
  • History of minor injuries secondary to sports
  • Playing year-round sports
  • Ignoring knee instability and not receiving treatment
  • A history of surgery for a torn ligament


A careful history is a first step in evaluating a knee that goes out. This includes noting any mechanism of injury (what was happening when symptoms began), and any past history or risk factors for knee problems.

Physical Exam

An examination of the knee first looks for general findings, such as

  • Bruising
  • Tenderness in general, as well as along the joint line
  • Crepitus (a crunchy sound when pushing on the kneecap)
  • Any deformity
  • Range of motion

Special tests are then done to examine for meniscal tears (such as McMurray's test and Ege's test), and to evaluate the ligaments (such as side to side movements to test collateral ligament stability, and an anterior and posterior drawer test and Lachman test to evaluate the ACL and PCL).

Imaging Studies

The choice of imaging tests depends on the suspected problem. X-rays may be helpful (for example, finding calcium deposits in the MCL), but a knee MRI is often the test of choice for identifying soft tissue injuries such as ligament and cartilage injuries.


The treatment of knee instability and the sensation that a knee is giving out depends very much on the specific cause of the symptoms. The overall goal is to treat pain, restore strength, and restore the function of the joint.

Treatments may include RICE (rest, ice, and elevation), nonsteroidal anti-inflammatory medications for pain. Conditions such as a torn ACL or torn meniscus often require knee surgery, but other conditions may benefit more from physical therapy and knee rehab exercises. A knee brace or immobilizer may also be used in some cases.

When to See a Doctor

If you are experiencing the sensation of your knee giving up, you should see your doctor. As noted earlier, adults who do not seek treatment are more likely to experience fall-related injuries, and feeling less confident when it comes to balance can negatively impact your quality of life. You should seek immediate medical care if you are having severe pain, if your knee is bruised-looking, if you experience locking or a popping sensation in your knee, if you have significant swelling, or develop a fever.


An ounce of prevention is worth a pound of cure, and when it comes to knee instability, this adage rings truer than ever. Knee conditions that have not been addressed increase risk of the conditions that lead to instability, so it's important to see your doctor with any knee symptoms at all.

With athletes, greater attention is now being given to ACL tear prevention, especially in female athletes. The 15-minute ACL prevention program with plyometric exercises and balance drills was designed to reduce the risk of these tears in teens. Athletes should also consider avoiding playing year-round sports, and avoid playing when ill or not adequately rested. In some cases (and with some sports), wearing a knee brace may reduce the risk of a knee injury.

For those who have leg muscle weakness, a strengthening program may reduce risk, and avoiding overuse injuries is important for everyone. Finally, wearing a seatbelt is an easy way to reduce your risk of a dashboard injury (PCL) should you have a car accident.

A Word From Verywell

The sensation of having your knee give out can be challenging to diagnose as there are many potential causes. Fortunately, there are also many different treatment options that can restore function and reduce pain. It's not uncommon for people to ignore this symptom, or to dismiss it as a normal part of aging if it occurs with arthritis. Doing so, however, can increase your risk of other problems down the line. Make sure to visit your doctor to get an accurate diagnosis and treatment plan.

Was this page helpful?
Article Sources