Why You Feel Your Knee Giving Out

Causes and Symptoms of Knee Instability

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When you feel your knee is giving out, meaning that suddenly the knee 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.

A knee that feels unstable also can be due to another condition, such as arthritis or an injury to the patella (knee bone) itself.

This article explains what it feels like to have your knee give out and which ligaments are commonly involved. It also presents information about diagnosis and some of the treatments used to repair the knee.

causes of knee instability
Verywell / Brianna Gilmartin

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, your symptoms may include:

  • 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. When your knee gives out, it is most often due to a ligament tear, with the anterior cruciate ligament (ACL) tear a common injury.

People with an ACL tear often complain that their knee will suddenly buckle when changing directions. Most ACL tears require surgery, with some 400,000 procedures done in the United States each year.

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.

Arthritis at the knee is due to the loss of cartilage over time, and people often experience the feeling that their knee may give out or buckle under. Other symptoms include pain, joint stiffness, and swelling.

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.

Other conditions that can lead to a knee that feels unstable and painful include:

An injury to the patella (knee bone) itself also can cause a knee to feel unstable. These injuries can include a patella fracture or a kneecap dislocation.

What Can Cause Knee Instability or Giving Out?

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.

Diagnosis

To diagnose a knee injury, the healthcare provider will examine your knee and ask about your symptoms and medical history. They'll look for painful spots or any obvious deformity, problems with motion, and sounds of popping or cracking (crepitus) at the knee.

Your provider also may try some specialized exams, such as a Lachman test (bending the knee while you are lying down) or McMurray's test (rotating the knee to find tears).

Imaging tests, like a CT scan or MRI, can help determine the nature and extent of the injury. Some injuries require arthroscopy, a procedure in which a narrow, lighted scope is inserted into the joint space to examine the tissues.

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.

Preventing Knee Injuries

Diet and exercise can help you to protect your knees. A weight loss plan designed to limit the stress on your knee joints is often a first step that provides overall health benefits, too. Your healthcare provider can recommend workouts. Weight loss also may improve long-term outcomes after ACL surgery in people with obesity.

Prevention

There are several modalities that can help prevent knee injury. They include balance training, plyometric training, and strength and resistance training.

Balance Training

Improving your balance is a key part of injury prevention, and ACL injuries in particular. Some balance training exercises include simple exercises like balancing on one foot, walking heel to toe, and bodyweight exercises like walking lunges and Bulgarian split squats.

Plyometric Training

Also known as jump training, plyometric exercise includes using explosive movements to jump. Landing with soft knees and with your feet shoulder-width apart will ensure good form. Studies show that plyometric exercise can reduce ACL injuries in particular.

Strength and Resistance Training

Building adequate muscle is a key part of preventing injuries of all types, and knee injuries are no exception. Bodyweight exercises like squats and lunges are excellent for strengthening the muscles that support the knee.

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 physical therapy, while others may require surgery. While knee instability doesn't always hurt, be sure to see a healthcare provider for early diagnosis and treatment.

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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.