When Your Knee Gives Out

Possible Causes and Treatments

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Feeling weak in the knees or having your knee give out may occur suddenly or develop slowly over time. It may be the only symptom you have, or you may also experience others such as popping, pain, bruising, or swelling of the knee.

Possible causes of knee weakness include tears, knee injuries, and underlying inflammatory conditions, which occur when the immune system views healthy body tissue as an invader and attacks it.

Woman sitting on stairs outside, holding her knee.

Martin-dm / Getty Images

This article explains the possible causes of knee weakness. It also covers symptoms of weak knees, how knee conditions that can cause it are diagnosed, and available treatment options.

How Does the Knee Work?

The knee is a hinge joint that connects the large thigh bone to the lower leg bones. Connective tissues called ligaments connect bones to bones and help stabilize the joint.

There are several ligaments in the knee, but the four major ones are:

  • Collateral ligaments (MCL and LCL): These 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. The lateral collateral ligament (LCL) is on the outer side of your knee.
  • Anterior cruciate ligament (ACL): This runs through the middle of the knee and connects the shinbone and the thighbone. It limits rotation and the forward motion of the shinbone.
  • Posterior cruciate ligament (PCL): This connects the top and rear of the shinbone to the thighbone and limits the backward motion of the shinbone.

The cartilage pads, known as menisci, are the shock absorbers of the knee and help stabilize it. There are two per knee—one on the inside and one on the outside.

Recap

The knee is a hinge joint made up of four major ligaments. These ligaments stabilize the knee, while the cartilage pads in each knee act as shock absorbers.

What Are the Signs and Symptoms of Knee Weakness?

With some conditions, you may only experience the symptom of having unstable knees.

However, with others, you may also have:

Complications of Knee Buckling

Complications of weak knees may include:

  • Recurrent falls
  • Lack of confidence with balance
  • Feeling scared or nervous about future falls
  • Injuries because of falls

You should see your healthcare provider if you are experiencing the sensation of your knee giving out.

Seek immediate medical care if your knee gives out and you experience severe pain, significant swelling, or a fever.

What Causes My Knee to Suddenly Give Out?

There are multiple reasons why your knee may feel weak or go out, each of which are detailed below:

  • Tears, including to a ligament or meniscus
  • Other injuries
  • Medical conditions

Ligament Tears

Partial or complete tears to one or more of the ligaments in the knee can cause knee instability. Types of ligament tears include:

  • ACL tear: An ACL tear usually occurs with sudden movement, such as a change in direction during sports. Individuals may hear a "pop" while pivoting or landing after a fall. It may also occur with a car accident. The giving out sensation is often followed by pain and swelling.
  • PCL tear: PCL tears may occur during a fall where an individual lands directly on the front of the knee. These may also occur if the shin is forcefully hit from the front, like during a car accident. This type of tear tends to occur along with other injuries, rather than on its own.
  • MCL tear: MCL tears usually occur when the outer side of the knee gets hit. If knee instability is present, it usually implies that a significant MCL tear has occurred. Pain, swelling, and difficulty with movement are common symptoms.
  • LCL tear: An LCL tear usually causes pain on the outer part of the knee. Like a PCL, this type of tear typically occurs along with other injuries. Feeling like your knee will give out often implies a significant injury.

Meniscal Tear

A meniscus tear may occur with activities such as twisting, turning, and pivoting. A meniscal tear is more common in people ages 30 and older, as the menisci start to weaken around this time.

Symptoms may include:

  • Stiffness
  • Swelling
  • Pain
  • Popping or locking feeling
  • Difficulty with movement

Patellar Dislocation or Subluxation

The kneecap, or patella, normally glides through a groove. A direct blow to the kneecap, usually from an accident or sports injury, can dislocate it from this groove. This is called patellar instability.

Two types of patellar instability include:

Plica Syndrome

Plica syndrome is a condition that is caused by inflammation of the inner lining of the knee joint. It may occur suddenly with movements, such as bending, or more gradually.

Symptoms include:

  • Knee pain
  • Clicking
  • Popping feeling, especially when squatting
  • Difficulty with movement

Arthritis

Arthritis, an inflammatory condition, can lead to knee instability. There are many types of arthritis that can impact the knee. These include osteoarthritis, rheumatoid arthritis, and inflammatory arthritis.

In general, symptoms of knee arthritis may include:

  • Pain
  • Stiffness
  • Swelling
  • Difficulty with movement, including walking

Nerve Damage

The femoral nerve is located in the pelvis and goes down the thigh and the shinbone. Femoral nerve damage can make it feel like your knee could give out or is giving out.

It can also cause a tingling or burning pain around the knee.

Femoral nerve damage can happen if:

  • An individual is injured
  • The nerve experiences a lot of pressure
  • An individual has a condition associated with nerve damage, such as diabetes

Recap

Several issues can cause your knees to give out. A ligament or meniscus tear are obvious ones, but knee instability can also be due to arthritis, nerve damage, or another cause.

Who Is at Risk for Injuries Related to Knee Instability?

Risk factors for injuries related to knee weakness include:

  • Engaging in sports, especially football, lacrosse, basketball, or skiing
  • Ignoring knee pain: Pain is your body's sign something is wrong. If you let it persist unchecked, you could be allowing its cause to worsen.
  • History of torn ligament surgery
  • Arthritis
  • A prior knee replacement

How Is Knee Instability Diagnosed?

To diagnose knee instability your doctor will first take a detailed medical history. You may also get a physical exam, as well as imaging tests done.

Physical Exam

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

  • Bruising
  • Tenderness
  • Crepitus, which is a crunchy sound when pushing on or moving the kneecap
  • Any deformity
  • Range of motion abnormalities

Special tests may also be done during this example. For example, to examine for meniscal tears and to evaluate the ligaments, a healthcare provider may perform the:

  • McMurray's test, which involves rotating the knee to find tears
  • Ege's test, which involves squatting while the doctor feels for a click
  • Lachman test, when a doctor bends the knee while you are lying down

Imaging Studies

The type of imaging test done depends on the suspected problem.

For example, X-rays may be helpful for evaluating a dislocation, but magnetic resonance imaging (MRI) may be used to identify ligament and cartilage injuries.

How Do You Treat Weak Knees?

Treatment for knee instability will vary depending on the specific cause. Treatments may include rest, ice, compression, and elevation, better known as the RICE method, as well as nonsteroidal anti-inflammatory medications for pain.

Conditions such as a torn ACL or arthritis may require knee surgery, but other conditions may benefit more from physical therapy and knee rehab exercises.

A knee brace may also be used in some cases.

Recap

Treatment for weak knees may include the RICE method, knee surgery, physical therapy, and/or using a knee brace.

How Do You Prevent Knee Instability?

See your healthcare provider if you are experiencing any knee-related symptoms so a knee condition, if present, can be treated as early as possible. In some cases, this may help you avoid knee instability.

Other preventative measures include:

  • Doing exercises like toe raises and squats to strengthen the ACL, which is especially important for athletes
  • Wearing a knee brace
  • Avoiding overuse injuries, which are when knee trauma repeatedly occurs
  • Wearing a seat belt to reduce your risk of a dashboard-related knee injury

Summary

Knees may buckle due to tears, dislocation, certain conditions, as well as nerve damage. Your knees giving out may be the only symptom, or you may also experience symptoms such as popping, pain, stiffness, and reduced movement.

To diagnose knee instability your doctor will take a detailed medical history, give you a physical exam, and may also have you get an imaging test. Depending on your diagnosis, treatment may include the RICE method, medication, and possibly surgery.

To help prevent knee instability, be sure to see your doctor as soon as possible if you start to have knee related symptoms. You can also try knee strengthening exercises and avoid overusing your knee.

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Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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