Instability of the Knee: Why You Feel Your Knee Giving Out

Physical therapist stretching patient knee
Hero Images/Getty Images

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.

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.

Types of 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. Doctors 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.

Some of the more common ligament injuries include:

  • 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 are most commonly injured when the knee is struck from the outside, pushing the inner side of the knee open. Most MCL tears can 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.

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

    Non-Ligament Causes of Instability

    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 physician.

    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.

    Treatment of Instability

    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 doctor about the best treatment option for you and choose one that makes the most sense.

    Was this page helpful?
    Article Sources
    • DʼLima DD1, Colwell CW. "Intraoperative Measurements and Tools to Assess Stability" J Am Acad Orthop Surg. 2017 Feb;25 Suppl 1:S29-S32.