Instability of the knee is usually due to an acute or chronic injury of the anterior cruciate ligament, posterior cruciate ligament, medial collateral ligament, and/or posterolateral complex of the knee. True instability results in the knee essentially subluxing or opening abnormally with specific activities. This can include problems with twisting, pivoting, or turning with an anterior cruciate ligament tear. Problems going down stairs or slopes with a posterior cruciate ligament tear, a varus thrusting of the knee with level gait with a posterolateral corner injury, or opening of the medial side of the knee with a sudden plant and pivot.
It's important to differentiate true instability of the knee from patients who have giving way of the knee. When a patient describes that they are walking along suddenly and the knee gives way, this is most commonly due to patellofemoral dysfunction rather than true knee instability.
The treatment of instability of the knee depends upon the overall grade (ie, amount) of instabilty and whether the injury is acute or chronic. In general, most grade 1 to 2 injuries (less than complete tears) can be treated initially without surgery, while most grade 3 (or complete tears) injuries of the knee ligaments (other than isolated grade 3 MCL injuries) usually need a surgical repair or reconstruction to return the knee to its normal stability with functional activities.