Dr. Martin's notes

Monday February 27, 2017

The knee joint is one of the most complex joints in the body. Because the knee supports the majority of the body weight, it is at risk of overuse and traumatic injuries in both contact and noncontact sports.

The knee is composed of three major bones and muscle groups. On top of the knee is the longest bone in the body, called the femur. The end of the femur flares at its distal end into a pair of rounded prominences called condyles. One is medial, the other lateral. The shape of the condyles allows the femur to roll and spin on the flattened top portion of the tibia, called the tibial plateau.

On the bottom of the knee is the tibia, which meets with the femur to form the tibiofemoral joint. The tibiofemoral joint is a weight-bearing, hinged joint held together with a joint capsule and several important ligaments. The motions at this joint are limited to flexion, extension, and a few degrees of rotation of the tibia on the femur.

Two types of cartilage are found within the knee joint. The ends of both the tibia and femur are coated with a protective layer of smooth articular cartilage. This articular cartilage provides a smooth surface for gliding of the joint. Interspersed between the tibia and femur are two crescent-shaped wedges of cartilage called menisci. The medial meniscus lies between the medial femoral condyle and the medial tibial plateau. The lateral meniscus lies between the lateral femoral condyle and the lateral tibial plateau. They aid in shock absorption, distribute forces, and improve stability of the femur as it rides on the tibia. The menisci are bathed by the synovial fluid of the knee.

Four major ligaments connect the tibia and femur. Two of the ligaments are on the outside of the joint capsule. These run roughly parallel to each other on the sides of the joint, going vertically. These ligaments are called the medial and lateral collateral ligaments. The medial collateral ligament (MCL) attaches to the femur above and the tibia below. The lateral collateral ligament (LCL) also attaches to the femur, but, unlike the MCL, it attaches to the head of the fibula instead of the tibia. These two ligaments provide medial and lateral stability of the knee joint.

Within the knee joint are two additional ligaments: the anterior cruciate ligament (ACL) and the posterior cruciate ligament (PCL). The ACL attaches to the anterior aspect of the tibial plateau, whereas the PCL attaches to the posterior tibial plateau. The cruciate ligaments each have a primary function. The ACL restricts anterior translation (movement) of the tibia on the femur; the PCL resists posterior translation of the tibia on the femur.

The patella, or kneecap, rides in the trochlear groove on the distal end of the femur. This is called the patellofemoral joint. The patella is a sesamoid, or plate-shaped, bone that is enveloped within the quadriceps tendon on the front of the knee, and is part of the extensor mechanism. Simply put, the presence of the patella allows knee flexion and extension to occur with a lesser amount of quadriceps force.The back side of the patella, which articulates with the femur, is called the retropatellar surface and is covered with a thick layer of articular cartilage.

The muscles that move the lower extremity are the strongest in the body. The large group of four muscles in the front of the thigh are collectively called the quadriceps muscles. These muscles are the vastus medialis, vastus intermedius, vastus lateralis, and the rectus femoris. They join together in the distal anterior thigh and attach to the patella through the quadriceps tendon. The tendon then encompasses the patella and extends distally across the front of the knee as the patellar tendon. The patellar tendon inserts onto the tibial tubercle on the proximal tibia. The quadriceps are very powerful extensors of the knee.

Two additional, long, strap-like muscles in the thigh are the sartorius and the gracillis. These muscles attach to the anteriomedial tibia near the attachment of the semitendinosus. They assist with flexion of the knee.
The hamstrings on the posterior thigh are divided into two groups. The medial hamstrings
include the semitendinosis and semimembranous, and the biceps femoris constitutes the lateral hamstrings. The hamstring muscles attach to the pelvis and femur proximally and insert onto the posterior tibia. Because they cross the hip joint, they are also extenders of the hip.