Why You Feel Your Knee Giving Out

Instability of the Knee

Table of Contents
View All
Table of Contents

The symptom of a knee giving out is most often due to a ligament injury. The knee is held together by ligaments, structures that connect two bones. There are four major and many secondary ones.

The sensation of instability—the feeling of your knee giving out—is often due to an injury to at least one of them, which leads to the bones not being held tightly enough in position.

causes of knee instability
Verywell / Brianna Gilmartin

Knee Instability Symptoms

Symptoms of knee instability are usually noticed with twisting or side-to-side movements. This may occur in sports activities or may occur with simple tasks, such as twisting your knee to get in and out of a car.

Knee Ligament Injuries

Ligament injuries generally occur as the result of a sudden injury where the knee buckles or is forced awkwardly into the wrong position. When a ligament is injured, it may be either partially or completely torn. Healthcare providers often use the description of "grading" a ligament injury, although the truth is that ligaments can be injured in an endless number of ways.

Injuries range from microscopic tearing within the ligament to complete tears of the ligament that may not heal without surgery. As you might expect, more minor tears generally heal with some simple steps, where more major injuries often require more invasive treatments.

Knee ligament injuries can cause the knee to feel unstable and the sensation that the knee joint will give out.

Anterior Cruciate Ligament (ACL) Tears

The ACL is central within the knee and critical to supporting the joint with cutting and pivoting maneuvers. The ACL is most often injured with sudden shifts in direction in non-contact injury situations.

People with an ACL tear often complain that their knee buckles or wants to give out when they suddenly shift direction. Most ACL tears require surgery.

Medial Collateral Ligament (MCL) Tears

The MCL is on the inner side of the knee joint and prevents the knee from opening up too much on the inside. MCL tears most often occur when the knee is struck from the outside, pushing the inner side of the knee open. Most MCL tears heal with nonsurgical treatment.

Posterior Cruciate Ligament (PCL) Tears

The PCL crosses over the ACL and prevents forward shifting of the shin bone. The PCL is typically injured by falling and landing directly on the front of the knee joint.

PCL tears can often be treated with nonsurgical treatment when sustained as an isolated injury, but are more commonly treated surgically when combined with other injuries.

Non-Ligament Causes

It is also possible for people to experience instability symptoms with injuries that cause knee pain. Often the body protects itself from pain with involuntary movements. This may feel as though the knee wants to give out, causing a sensation of instability, but it is not due to a ligament injury as described above.

The best way to tell the difference between instability caused by a ligament injury, versus a sensation of instability, is by having your knee examined by a skilled healthcare provider.

There are tests used to determine the function of each ligament. For example, the Lachman's test is used to test the anterior cruciate ligament.


There are both surgical and non-surgical treatments for knee instability. These can include physical therapy to strengthen the muscles around the knee joint and knee braces to better support the knee joint.

Surgical treatments generally involve repairing or reconstructing the damaged ligament to restore the normal structure of the knee joint. The most common type of ligament reconstruction is for ACL tears.

A Word From Verywell

Knee instability usually feels uncomfortable. The sooner you address it, the better you'll feel. If you're tempted to "just deal with it" because you're worried about invasive treatments, remember that nothing will be forced on you. Have a discussion with your healthcare provider about the best treatment option for you and choose one that makes the most sense.

Frequently Asked Questions

  • What can cause knee instability or giving out?

    The reason for a knee giving out is usually due to a ligament injury, such as a torn ACL. Knee instability can also be caused by Plica syndrome, a condition in which the inner lining of the knee joint becomes inflamed, or arthritis. If you experience knee instability or feel your leg giving out, see a healthcare provider. A knee injury has the best chance of recovery if treated as soon as possible.

  • What does it mean to hyperextend your knee?

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

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

    If you have suffered a knee injury, immediately stop doing anything that could cause it further harm. Depending on the severity of the injury, call your healthcare provider or visit the ER to have your knee examined as soon as possible.

  • How is a knee injury diagnosed?

    To diagnose a knee injury, a healthcare provider will likely start by examining the knee and asking about your symptoms and medical history. The next step may be imaging tests such as an X-ray, MRI, or arthroscopy. These tests can help identify damaged bones or tissues within the knee.

Was this page helpful?
Article 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.
  1. Spindler KP, Wright RW. Anterior cruciate ligament tear. N Engl J Med. 2008;359(20):2135-42. doi:10.1056/NEJMcp0804745

  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. Kiapour AM, Murray MM. Basic science of anterior cruciate ligament injury and repair. Bone Joint Res. 2014;3(2):20-31. doi:10.1302/2046-3758.32.2000241

  4. Encinas-ullán 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

  5. 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

Additional Reading
  • DʼLima DD1, Colwell CW. "Intraoperative Measurements and Tools to Assess Stability" J Am Acad Orthop Surg. 2017 Feb;25 Suppl 1:S29-S32.