Knee Ligament Anatomy

Anatomy

There are essentially four separate ligaments that stabilize the knee joint. On the sides of the joint lie the medial collateral ligament (MCL) and the lateral collateral ligament (LCL) which serve as stabilizers for the side-to-side stability of the joint. The MCL is a broader ligament that is actually made up of two ligament structures, the deep and superficial components, whereas the LCL is a distinct cord-like structure.

Front view of right knee in flexion

In the front part of the center of the joint is the anterior cruciate ligament (ACL). This ligament is a very important stabilizer of the femur on the tibia and serves to prevent the tibia from rotating and sliding forward during agility, jumping, and deceleration activities. Directly behind the ACL is its opposite, the posterior cruciate ligament (PCL). As you might predict, the PCL prevents the tibia from sliding to the rear.

The knee joint is a vulnerable joint that is easily injured. This is due in part to the fact that the joint is well exposed and in the middle of two long lever-arms, the femur and tibia. Unlike the hip joint which has a very stable ball-and-socket configuration, the bone anatomy of the knee imparts little support to the joint's stability. This makes the knee ligaments prone to injury with any contact to the knee, or often with just the force of a hard muscle contraction (e.g. performing a quick change of direction when sprinting).

 

Knee Ligament Injuries>

Ligament injuries are usually graded in terms of their severity:

Grade I sprain – some micro-tearing or slight stretching occurs, however the overall integrity of the ligament is preserved. The ligament hurts if stressed but is stable.

Grade II sprain – partial disruption of the ligament. Painful to stress, there is detectable laxity but the ligament has an eventual endpoint.

Grade III tear – complete ligament tear and laxity with no endpoint or stability to testing. As the nerves in the ligament are torn too, there is often minimal pain with stressing the joint.

The presentation and findings differ depending on which ligament is involved and to what degree it is injured. ACL tears are usually complete and are typically associated with immediate bleeding into the joint, hence fairly immediate swelling. This is a disabling injury and athletes are usually quite uncomfortable and unable to continue participating in their sport. If they try any agility activity the knee "goes out" with an instability event. Since this ligament rarely can heal, most athletes elect ACL reconstruction surgery so that they can return to high demand sports. Low demand individuals may elect to live with the disability (see Indications for ACL Reconstruction> on The ACL Page>)

MCL tears are common but fortunately are more often Grade I or II. They usually result from a clipping-type contact, a twist or any mechanism that forces the foot out and the knee in. Swelling occurs but it is typically just on the medial side of the joint rather than the whole joint. Many skiers have sprained this ligament and skied the rest of the day before noting increasing soreness the next morning. Treatment for even complete tears is usually nonsurgical since this ligament complex can heal itself provided that no other ligaments are also involved. Physical therapy and bracing have been shown to decrease the recovery time.

Isolated LCL injuries are infrequent with the mechanism of injury being the opposite of an MCL sprain. These too are often treated successfully without surgery.

Tears of the PCL can result from a hyperextension or hyperflexion injury. The classic PCL tear mechanism is a "dashboard" injury during a car accident in which a blow occurs to the front of the upper tibia. This forces the lower leg backwards at the knee, rupturing the ligament. The presentation of an isolated PCL tear is typically less swollen and sore than for an ACL tear and doesn't result in giving way instability episodes. Treatment recommendations for complete PCL tears are controversial, however most knee surgeons are not routinely performing reconstructions for isolated tears. On the other hand, if there is also complete tearing of either of the collateral ligaments (LCL or MCL) then PCL reconstruction combined with surgical treatment of the other ligament(s) is recommended.

High speed vehicular accidents, and occasionally sports injuries, can result in a complete dislocation of the knee joint (the tibia separates from the femur). This emergency can result in tears to all of the major four ligaments and has the potential to tear one of the main arteries behind the knee as well as damage important nerves to the lower leg. Surgery is needed for this major, disabling ligament tear combination.