The Anterior Cruciate Ligament and Posterior Cruciate Ligaments (ACL and PCL, crucial ligaments or ligamenta cruciata genu) are found in the middle of the knee joint. They are nearer the back than the front of the knee and sit within the intercondylar notch. They are called cruciate (like a cross) because they pass across each other in opposite directions and, when viewed from the side, look the letter ‘X’. The front ligament is called the anterior cruciate ligament (ACL) and the back one is called the Posterior Cruciate Ligament (PCL). Their names are derived from where they attach to the tibia.
The ACL is critical for rotational stability of the knee. People with a torn ACL often complain that their knee gives out under them on uneven ground, when stepping down off the curb or when twisting their body with their foot planted on the ground. They are unable to play side stepping sports without the knee giving way but are usually able to run in a straight line. Most patients who tear their ACL tear choose to have an ACL reconstruction.
The Anterior Cruciate Ligament
(ACL, ligamentum cruciatum anterius, external crucial ligament) is contained within the capsule of the knee but outside the synovial layer. It is made up of 3 bundles which are tight in different positions of knee bending. The main function of the ACL is to cause the tibia to twist outwards on the femur as the knee becomes fully extended (straight). The ‘screwing home’ mechanism starts when the knee is bent 30 degrees and continues until the knee is fully straight. Side stepping with the knee bent or a blow to the outside of the knee when it is bent puts large loads on the ACL. The ACL can then tear as it resists this action. The ACL is attached to a depression in front of the intercondyer eminence of the tibia and passes upward, backward and towards the outside part of the leg. It attaches to the inside and back part of the lateral (outside) condyle of the femur.
The Posterior Cruciate Ligament
(PCL, ligamentum cruciatum posterius, internal crucial ligament) is stronger but shorter than the ACL. It is attached at the back of intercondylar fossa of the tibia and passes upward, forward and towards the inside of the knee to be fixed into the outer and front part of the medial condyle of the femur. A direct blow to a bent knee with pain at the back of the knee but little swelling usually indicates a PCL tear. The knee is less swollen and painful than an ACL injury and the patient may have been able to continue playing their sport after the injury. It can result in difficulty stopping when running, pain running downhill and patellofemoral (kneecap) pain. A hyperextension injury (when the knee is forced too far straight) can also injure the PCL alone or both the ACL and PCL.
The Ligamentum Patellae is also known as the patella tendon. It is the central portion of the tendon of the thigh muscle (Quadriceps femoris) which encases the kneecap (patella) and runs down to the sin bone (to the tuberosity of the tibia). It is a strong and flat and about 8 cm long. It’s back surface is separated from the synovial membrane of the joint by a large infrapatellar pad of fat and from the tibia by a bursa.
The Medial Collateral Ligament
(MCL, Tibial Collateral Ligament, internal lateral ligament) is a broad, flat, thickened band of tissue which is located (closer to the back than to the front of the knee joint) near where the 2 knees touch each other. It is typically injured with a direct valgus (the foot is forced directly sideways stretching the inside of the knee) blow to the knee or an external twist to the shin (like 2 soccer players kicking the ball at the same time). At the top end it is attached to the femur just below the adductor tubercle and on the bottom to the medial condyle and surface of the tibia. The front part of the ligament is a flattened band which angles forward for about 10 cm. It inserts into the medial surface of the body of the tibia about 2.5 cm. below the level of the condyle. The per anserinus crosses it containing the the Sartorius, Gracilis and Semitendinosus tendons. There is a bursa between the ligament and the pes. The deep surface of the MCL covers the inferior medial genicular vessels and nerve and the anterior portion of the tendon of the Semimembranosus. It is also attached to the medial meniscus and is often associated with a medial meniscal tear if the deep fibres of the ligament are torn. The fibers of the posterior (back) part of the ligament are short and incline backward as they descend. These insert into the tibia above the groove for the Semimembranosus.
The lateral meniscus should be checked carefully in anyone with a MCL tear because it will have been squashed when the MCL was stretched.
The Lateral Collateral Ligament
(LCL, Fibular Collateral Ligament, external lateral or long external lateral ligament) is a strong, rounded, fibrous cord. It attaches at the top end to the back part of the lateral condyle of the femur (immediately above the groove for the tendon of the Popliteus). At the bottom end it attaches to the lateral side of the head of the fibula (in front of the styloid process). Most of its outer surface is covered by the tendon of the Biceps femoris The Popliteus tendon and the inferior lateral genicular vessels and nerve run under the LCL but the ligament does not attach to the lateral meniscus.
Injury to a ligament in the knee will usually require surgical reconstruction of that ligament. This is particularly true of the ACL if you wish to return to pivoting or side stepping sports. It may also be needed if your knee gives way with your day to day activities.
Isolated PCL injuries do not require surgery for most people but when combined with an ACL injury a PCL reconstruction is usually performed at the same time as the ACL reconstruction.
Most MCL injuries can be treated with a brace and physiotherapy programme but most LCL injuries (other than a mild sprain) are treated with a ligament reconstruction.
Patella tendon and quadriceps tendon injuries always require surgery.