Looking at how your knee moves, it might seem like it simply hinges back and forth. However, the knee is one of most complex joints, and it must be able to withstand extreme force, from athletics and everyday life. (Read more about how knee injuries occur.)
The knee is the junction of three bones—the femur (thigh), tibia (shin), and patella (kneecap)—which are  held together by four primary ligaments. (Other structures critical for its function will be discussed in a future article.)
The knee primarily hinges to bend (flex) or straighten (extend) the leg. It also rotates to provide stability for walking or running. This movement is controlled by the popliteus, a small muscle in the back of the knee.
Despite all of its movement, the knee must stay strong and stable during running, changes of direction and landing from a jump. These may seem ordinary, but they place extreme force on the joint. The muscles surrounding the knee—particularly the quads and hamstrings—help to stabilize and absorb much of this force. Beyond that, the four ligaments of the knee are primarily responsible for keeping it safe and stable.

Medial Collateral Ligament (MCL)

• Location: Inside portion of knee.
• Purpose: Provides stability against lateral forces traveling toward the center of the body (valgus), especially when the leg is straight.
• Common Injuries: The MCL is often injured when suffering a blow on the outside of the leg when the foot is planted.
• Prognosis: The MCL has good blood flow and can heal without surgery after all but the most serious injuries.

Lateral Collateral Ligament (LCL)

• Location: Outside portion of knee.
• Purpose: Provides stability against lateral forces traveling away from the body (varus).
• Common Injuries: The LCL is not commonly injured because of its thickness and support from the I.T. band.
• Prognosis: The LCL has good blood flow and can heal without surgery after all but the most serious injuries.

Posterior Cruciate Ligament (PCL)

• Location: Center of knee crossing behind ACL.
• Purpose: Prevents the tibia from slipping backward
• Common Injuries: Hyperextension of the knee or contact when the knee is bent, like landing from a fall. Another common cause is a car accident due to the backward force placed on the shin.
• Prognosis: The PCL can heal without surgery depending on the amount of tearing. Some doctors think it is possible to play with torn PCL, and it may be advisable due to the complicated nature of the surgery.

Anterior Cruciate Ligament (ACL)

• Location: Center of knee crossing in front of PCL.
• Purpose: Prevents the tibia from slipping forward and rotating inward.
• Common Injuries: Excessive inward knocking of the knee when landing from a jump or changing direction. Also, contact to the outside of the knee when the joint is flexed.
• Prognosis: Surgery is typically needed to repair an ACL. It usually does not partially tear, but if it does, it does not heal well due to limited blood flow. Also, it is very difficult to return to play without an ACL, and it could cause further injury down the line.

For more information on the ACL please see the STACK ACL Guide.