The cruciate ligaments play the largest role in stability of the knee joint, crisscrossing from the front of one bone to the back of the other. The posterior cruciate ligament (PCL) prevents the tibia from sliding back on the femur. An acute blow to the front of the shin can injure the PCL. It can occur in motor vehicle crashes when the knee, in a seated position, strikes the dashboard. There is often swelling of the knee. Because of pain and swelling, the examination is difficult to perform on an acutely injured knee. A brace for support can be used as can crutches and anti-inflammatory medications. Once the swelling and pain have subsided, a thorough exam can be performed. Surgical reconstruction of an isolated PCL is rarely undertaken, but only for specific patients with symptomatic laxity.

The collateral ligaments run on the sides of the knee, either medially (close to the body, or, in this case, inside), or laterally (on the outside of the joint). The MCL is often injured when the outside of the knee sustains a blow. The force causes injury of the medial side (inside) of the knee and immediate pain is noted. Tenderness and bruising is noted along the inner aspect of the knee. MCL injuries do well without surgery. Rarely, it requires surgical repair when it is associated with other injuries of the knee. Supportive measures including weight-bearing modification, braces, and anti-inflammatory medications are the mainstay of treatment.

As a result of trauma to the joint, any of these may be injured or torn. The anterior cruciate ligament (ACL) often requires repair, depending on age, and is considered separately from the other ligaments because of its importance in knee stability. An injury to any ligament in the body is known as a "sprain," or, more commonly, a "tear" when it is most severe. Should the ligament separate from its attachment to the bone, it is known as an "avulsion."

A sprain to the ligament, most often brought on by a blow directly to the lower leg, can happen in one of three "grades"

• Grade 1, in which the ligament is stretched, but not torn.
  
• Grade 2, where the ligament is partially torn.
  
• Grade 3, in which the ligament is completely torn and instability, or looseness of the joint, occurs.

Depending upon the level of activity of the individual, an orthopedic surgeon may decide to operate or pursue conservative methods to manage the ligament tear. Depending upon the severity of the blow, a ligament injury may coincide with a fracture, meniscus injury, dislocation, or other knee injury.