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Monday, 25 July 2011

The Knee Joint

The knee joint is a hinge-type joint, which is capable of flexion and extension motions. Flexion and extension are not the only motions of the knee, it has been discovered that the knee performs slight rotational movement. It is this rotational component that accounts for the frequency of knee injuries. The distal femur articulates with the tibia and the patella. The distal femur is broadly adapted for articulation with the tibia. The articulation with the tibia is through its large prominent condyles that project posteriorly. A large intercondylar notch seen posteriorly separates the medial and lateral condyles of the femur. On the anterior surface the condyles are separated by a slight groove called the patellar surface over which the patella (kneecap) glides during flexion and extension of the knee joint. The patella is a sesamoid bone developed within the tendon of the quadriceps femoris tendon. The patella is triangular shaped having a broad base and an inferiorly pointed apex. The articular surfaces on the posterior aspect of the patella articulate with the medial and lateral condyles of the femur. The two primary functions of the patella are to strengthen the tendon of the quadriceps femoris muscle, and to protect the knee joint. The patella strengthens the quadriceps muscle by increasing its leverage as it extends (straightens) the knee joint. Essentially, the role of the patella is to aid in changing the direction of forces from the quadriceps muscle as they pass through the knee joint and are applied to the tibia.

Soft tissue injury can manifest as swelling about the knee, inability to bear weight, loss of function such as bending or straightening of the joint, or other clinical indicators. Radiographically, soft tissue surrounding the knee must be demonstrated when imaging for trauma. Fractures that involve the upper fourth of the tibia, may or may not involve the knee joint, and may be limited to ligaments supporting the joint. Fractures that enter the knee joint often render the joint defective and the once smooth joint surface made irregular. Additionally, fractures resulting in improper limb alignment may contribute to long-term morbidity like arthritis, instability, and functional loss of motion.

The proximal fibula also contributes to lateral stability of the knee joint by providing supportive attachment for the lateral collateral ligament of the knee. Ligaments within the knee joint also support the knee. These ligaments may be injured with trauma sparing bone.

The stabilizing ligaments of the knee include the medial collateral ligament (MCL) and lateral collateral ligament (LCL), and are located outside the knee joint proper; the anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL) are stabilizer ligaments located within the knee joint. The patellar ligament is located outside the knee joint; it also provides support for the knee by shielding it, and strengthening the actions of the quadriceps femoris muscle. Because the patella is integrated into the extensor knee apparatus it contains both passive and active elements. The patellar ligament is one of the passive elements of the knee. It originates at the apex of the patella and extends to the tibial tuberosity. The role of the patellar ligament is to limit proximal patellar ascent.

The knee contains two semi-lunar C-shaped menisci composed of fibrocartilage. The two menisci lie on the tibial plateaus along the lateral peripheries of the joint. The function of the meniscus is to provide shock absorption to the knee during the stress of weight-bearing and movement. The youthful healthy meniscus is only partially supplied with blood and is stronger than older cartilage. With age the meniscus deteriorates and can easily tear. A damaged torn meniscus can seed torn pieces into the joint (meniscal fragment) causing pain, swelling, and loss of function. The reason meniscal fragments are released is that the majority of the meniscus has no blood supply and does not properly heal when damaged. Instead, deteriorated portions of the meniscus tend to tear off and enter the joint space between the bones. A surgical procedure called arthroscopic surgery may be indicated to remove some types meniscal fragments or flaps.

Other soft tissue structures that are critical to proper knee function include the medial collateral ligament (MCL), lateral collateral ligament (LCL), anterior cruciate ligament (ACL), posterior cruciate ligament (PCL), articular cartilage, joint capsule and synovial fluid, and bursa. The collateral ligaments resist widening of the knee joint. The cruciate ligaments, which are within the knee joint proper, resist hyperflexion and hyper extension and also slight rotational movements of the knee. Articular cartilage is bathed by synovial fluid that lubricates the knee joint.

There are two collateral ligaments of the knee, the medial collateral ligament (MCL) and lateral collateral ligament (LCL). The MCL spans from the medial femoral condyle to the top of the lateral tibia (shin bone) and had multidirectional fibers on the inside of the knee joint too. The medial collateral ligament resists medial widening of the joint that would 'open-up' the knee. The LCL spans from the lateral condyle of the femur to the lateral portions of the fibula. Its main function is to resist lateral widening of the knee joint.

The anterior and posterior cruciate ligaments are located within the center of the knee joint. The PCL originates in a fan-shaped fashion from the anterolateral aspect of the medial femoral condyle near the intercondylar notch. It courses posteroinferomedially to insert on the backside of the tibial plateau. It functions to reduce internal rotational movements of the knee, and to prevent the tibia from sliding backwards on the femur. In other words, the PCL prevents hyperflexion of the knee joint. The ACL arises from the posterior part of the medial surface of the lateral condyle and courses anteroinferiorly and medially to the anterior plateau just posterior to a rather prominent synovial fold. It then inserts in a fossa in front of and lateral to the anterior intercondylar eminence of the tibia. It should be noted that the ACL lies within an intra-articular compartment of the knee joint, but is extrasynovial. The functions of the anterior cruciate ligament are to resist rotational motion of the knee and prevent the femur from translating backwards on the tibia. In other words, it limits hyperflexion of the knee joint. The most common mechanism of ACL injury is internal rotation of the femur when the knee is in full extension. A damaged ACL or PCL will result in instability of the knee when the foot is planted causing the knee to give way or to buckle.

Synovial joints are found where there is extensive movement of bone on bone such as the femur and tibia of the knee. Synovial joints are also classified as diarthroses, or freely movable joints. At the point of bone articulation there is a thin covering of hyaline cartilage covering the ends of bone called articular cartilage. This cartilage is maintained in apposition by ligaments of the knee and the surrounding joint capsule. Articular cartilage of the knee is found on the articular surfaces of the femur, tibia, and underside of the patella. Articular cartilage is void of blood vessels (avascular) and depends on diffusion of nutrients from synovial fluid that bathes it. The capsule of the knee joint is a sac that encloses the joint cavity. It is perhaps better thought of as a cavity rather a simple membrane. It completely surrounds the knee joint having compartments that surround the patella and the knee joint. The capsule is firmly attached to bone and is composed of a tough fibrous outer membrane and an inner synovial membrane. It is the inner layer of the synovial membrane that produces synovial fluid that bathes structures within the knee joint proper. Four bursa are found in the knee are found near tendons to provide smoothening of motions of their muscles. Like all bursa those of the knee are also subject to inflammation (bursitis) and to inflammatory reaction from trauma.