What are the Causes of Knee Instability?

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  • Written By: Alex Paul
  • Edited By: R. Halprin
  • Last Modified Date: 07 December 2019
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In most cases, knee instability is caused by damage to one of the joint’s ligaments. These ligaments hold the knee in place, but can often be injured during athletic activity. The ligaments are particularly susceptible to an injury during twisting movements. If the knee regularly gives way, this may not be a sign of true knee instability, although the symptoms can feel similar. Instead, this can be caused by problems including patellofemoral dysfunction, weak quadriceps muscles, and a reduced range of motion.

The most common cause of knee instability is damage to one of the ligaments. Knee ligaments help to secure the knee in place; if one is damaged, it causes the forces in the joint to become unbalanced. This can lead to a feeling of instability. Injuries to knee ligaments often occur during athletic activities, especially those which require twisting movements or changes of direction at high speed. A severe sprain to a knee ligament is a painful injury, although minor sprains are more common.


Three ligaments that are commonly injured are the anterior cruciate, posterior cruciate, and medial collateral ligaments — more commonly known as the ACL, PCL, and MCL respectively. The amount of knee instability depends on the severity of the injury. A full rupture to a ligament can result in significant instability initially, but the joint is often then protected by large amounts of swelling. Ligament sprains are graded from one to three, with a Grade One sprain being the least severe, and Grade Three sprains usually requiring surgery.

Even if a patient is suffering from a knee that gives way, this may not be a sign of true knee instability. When a knee is unstable, it will open up or sublux during activity, which means that the kneecap doesn’t track properly in its groove. If a knee gives way, this may be a sign of another condition, and is not always the result of instability. It is important for a physician to diagnose the problem accurately so the correct treatment can be prescribed.

A number of problems can cause the knee to become instable during weight bearing activities. Patellofemoral dysfunction, a catch-all term to describe conditions that affect how the kneecap moves, is a common cause. Weak quadriceps muscles, or an inability to achieve full range of motion in the joint, can also have the same result. Treatment for these conditions depends on the underlying problem and whether it is an acute or chronic issue.


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