Why You Feel Your Knee Giving Out

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.

Symptoms

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.

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.

Treatment

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.

Summary

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.