Four major ligaments provide support to the knee joint, and any one of these or combination of these ligaments can become strained or torn due to acute injury.  Strains and tears are graded according to a grading system of grade 1 – grade 3 tears depending on the amount of ligament fibres that have been torn.  In a grade one tear relatively few fibres are torn and pain in the area of the ligament is the primary symptom.  In a grade three tear there is complete rupture of the ligament and laxity at the joint with or without pain is the common presentation.

The four major knee ligaments that may become injured are the anterior crutiate ligament (ACL), the posterior crutiate ligament (PCL), the medial collateral ligament (MCL), and the lateral collateral ligament (LCL).