Why You Feel Your Knee Giving Out

Causes and Symptoms of Knee Instability

When you feel your knee is giving out, meaning that the knee suddenly feels unstable, it is usually due to a ligament injury. A ligament is a short, tough band of tissue that connects two bones. In the knee, there are four major ligaments and several minor ones.

If your knee suddenly gives out, it is often the result of a tear in one of three of these major ligaments. This article explores what it feels like to have your knee give out, which ligaments are commonly involved, and some of the treatments used to correct the injury.

causes of knee instability
Verywell / Brianna Gilmartin


Knee instability usually causes the sudden twisting or side-to-side motion of the knee, as if it is collapsing beneath you. This often occurs during sports and can also happen when doing something as simple as getting in and out of a car.

If the instability is due to a ligament injury, you may have:

  • A loud popping or snapping sound when the injury occurs
  • Pain, often sudden and severe
  • A feeling of looseness in the joint
  • Inability to put weight on the joint
  • Swelling within the first 24 hours

Knee Ligament Injuries

Ligament injuries tend to happen suddenly when the knee either buckles or is forced into an awkward position. They can range from a partial tear that may heal on its own to a complete rupture that requires surgery.

There are three knee ligaments prone to such injuries:

Anterior Cruciate Ligament (ACL)

The anterior cruciate ligament (ACL) runs diagonally in the middle of the knee. It supports the joint when making pivoting motions and prevents the shin bone from sliding out in front of the thigh bone.

People with an ACL tear often complain that their knee will suddenly buckle when changing directions. Most ACL tears require surgery.

Medial Collateral Ligament (MCL)

The medial collateral ligament (MCL) is on the inner side of the joint and prevents the knee from extending too far inward. It also helps keep the knee stable and allows it to rotate.

MCL tears most often occur when the knee is struck from the outside. Unlike ACL tears, most MCL injuries do not require surgery.

Posterior Cruciate Ligament (PCL)

The posterior cruciate ligament (PCL) crosses over the ACL and prevents the shin bone and thigh bone from sliding backward. It also prevents the excess rotation of the knee.

The PCL is most often injured by falling and landing directly on the front of the knee joint. PCL tears can often be treated without surgery but may require surgery if other ligaments, bones, and tendons are involved.


When your knee gives out, it is most often due to a tear of the anterior cruciate ligament (ACL), the medial collateral ligament (MCL), or the posterior cruciate ligament (PCL).

Non-Ligament Causes

It is also possible to experience knee instability with any injury that causes knee pain. Even if there is no major damage to the knee itself, the body may respond to pain with a protective reflex designed to safeguard the knee.

Knee strains and sprains are common reasons for this, but any condition that causes knee pain can trigger the response, including arthritis. If weight is placed on a painful knee, the brain will send signals to surrounding muscles to stop it. In doing so, the knee can suddenly collapse.

The best way to tell the difference is to have your knee looked at by a specialist known as an orthopedist. There are in-office procedures, like the Lachman's test, that can help pinpoint the cause. Imaging tests, like a CT scan or MRI, can help determine the nature and extent of the injury.


Knee instability is not always caused by a ligament injury. In some cases, a knee strain, sprain, or other painful condition can cause the knee to give out if weight is placed on it. It is a protective reflex designed to protect the knee.


There are surgical and non-surgical treatments for knee instability. These include physical therapy to strengthen the muscles around the joint and knee braces that help support and immobilize the knee. Pain may be treated with ice packs, pain medication, or steroid knee injections.

If the injury is severe, surgery may be needed to repair or reconstruct the damaged ligament. The most common type of ligament reconstruction is for ACL tears.


When your knee suddenly gives out, it is most often due to a ligament injury. The three ligaments usually involved are the anterior cruciate ligament (ACL), the medial collateral ligament (MCL), or the posterior cruciate ligament (PCL). An injury of the ACL usually requires surgery; the others may not.

Knee instability can also be caused by a knee strain or sprain or any other injury that causes knee pain. An orthopedist can diagnose the cause with in-office tests and imaging studies.

The treatment of knee instability depends on the severity of the injury. Some may heal on their own with rest and therapy, while others may require surgery.

A Word From Verywell

While knee instability doesn't always hurt, this doesn't mean you should just ignore it. The sooner you get it looked at, the better off you will feel. If you're tempted to "just live with it," an otherwise minor tear may worsen and eventually rupture, forcing you to undergo surgery.

If you notice that your knee wobbles or feels like it will collapse beneath you, it's best to see a doctor sooner rather than later.

Frequently Asked Questions

  • What can cause knee instability or giving out?

    The reason is usually due to a ligament injury, such as an ACL tear. Knee instability can also be caused by Plica syndrome, a condition in which the inner lining of the knee joint becomes inflamed. Arthritis and knee strains or sprains are other possible causes.

  • What does it mean to hyperextend your knee?

    Hyperextension of the knee is when the joint has extended further than is normal. This can cause stress on the ligaments inside the joint, leading to tears.

  • If I suffer a knee injury, what should I do?

    If you have a knee injury, immediately stop what you're doing to prevent further harm. Depending on the severity of the injury, you can call your healthcare provider or visit an emergency room to have your knee looked at as soon as possible.

  • How is a knee injury diagnosed?

    To diagnose a knee injury, the healthcare provider will examine your knee and ask about your symptoms and medical history. Imaging tests such as an X-ray or MRI are typically ordered. Some injuries require arthroscopy, a procedure in which a narrow, lighted scope is inserted into the joint space to examine the tissues.

5 Sources
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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  2. Andrews K, Lu A, Mckean L, Ebraheim N. Review: Medial collateral ligament injuries. J Orthop. 2017;14(4):550-554. doi:10.1016/j.jor.2017.07.017

  3. Encinas-Yllán CA, Rodríguez-Merchán EC. Isolated medial collateral ligament tears: an update on management. EFORT Open Rev. 2018;3(7):398-407. doi:10.1302/2058-5241.3.170035

  4. Vaquero-Picado A, Rodríguez-Merchán EC. Isolated posterior cruciate ligament tears: an update of management. EFORT Open Rev. 2017;2(4):89-96. doi:10.1302/2058-5241.2.160009

  5. Zlotnicki JP, Naendrup JH, Ferrar GA, Debski RE. Basic biomechanic principles of knee instability. Curr Rev Musculoskelet Med. 2016 Jun;9(2):114-22. doi:10.1007/s12178-016-9329-8

By Jonathan Cluett, MD
Jonathan Cluett, MD, is board-certified in orthopedic surgery. He served as assistant team physician to Chivas USA (Major League Soccer) and the United States men's and women's national soccer teams.