Anatomy of the Knee

Bones and Articulations

The knee joint is the largest joint in the body. The knee is a synovial hinge type joint. The entire weight-bearing load is transferred through the knee joints. When describing the knee, four bones and their articulations should be discussed: the femur, the tibia, the fibula, and the patella. Each articulation surface is covered with hyaline cartilage. The primary articulation of the knee is between the condyles of the femur and tibia. This articulation is separated by the medial and lateral menisci, which serve to deepen the articular surfaces and aid in lubrication and cushioning of the joint (Jenkins, 1991, pp. 233-239). Although not a part of the knee joint, the articulation of the tibia and fibula is significant due to its importance in weight bearing. The patellofemoral joint is a synovial gliding type joint. The patella is a sesamoid bone contained in the tendon of the quadriceps muscle. The articulation consists of the underside of the patella and the patellar grove of the femur. The gliding of the patella in the femoral grove allows for increased efficiency of the quadriceps muscle.

Musculature

Many muscles acting on the thigh have their insertions around the knee. Although not participating in gross knee movements these muscles are significant in the dynamic stabilization of the knee joint. Only the muscles specifically participating in knee flexion, extension, internal, and external rotation are discussed here.

The anterior muscles of the knee act primarily as knee extensors. The quadriceps femoris muscle is the principle muscle involved in knee extension. This muscle can be divided into four distinct parts: the rectus femoris, vastus medialis, vastus lateralis, and the vastus intermedius. All four parts of this muscle come together to insert on the proximal edge of the patella, which then transfers their action, by way of the patellar tendon, to the tibia.

The principle muscles involved in knee flexion are the hamstring muscle group. This group is comprised of the biceps femoris, semitendinosus, and the semimembranosus muscles. Their insertion occurs on the proximal tibia and head of the fibula. The biceps femoris muscle has an additional action of externally rotating the tibia. While the semitendinosus and semimembranosus muscles also have an additional role of internally rotating the tibia. Other muscles participating in knee flexion and internal rotation are the sartorius, and gracilis muscles. The popliteus muscle also serves to internally rotate the knee in a non-weight bearing position. Additional muscles involved in isolated knee flexion include the gastrocnemius and plantaris muscles.

Range of Motion

The principle motions of the knee joint are flexion and extension; however, it does allow for some degree of rotation (Hoppenfeld, 1976, pp. 186-188). The arc of motion of the knee defined by Hoppenfeld (1976) is typically about 0° extension to 135° of flexion. The amount of internal and external rotation about the knee is approximately 5° to 10° in each direction (Hoppenfeld, 1976, pp. 186-188). It is in extension that the rotational component of the knee joint is necessary. The knee is unable to reach full extension without a small amount of external rotation of the tibia on the femur. This need for external rotation is due to the fact that the medial femoral condyle is approximately 1/2-inch longer than the lateral femoral condyle (Hoppenfeld, 1976, pp. 186-188). The external rotation of the tibia allows the knee to achieve full extension. This mechanism is known as the "screw home" mechanism and it allows the knee to be held in full extension without undue fatigue of the surrounding musculature (Hoppenfeld, 1976, pp. 186-188).

Static Stabilizers

Static stabilization of the knee is provided by the ligamentous structures and to a lesser extent the joint capsule surrounding the knee articulations. The principle stabilizing ligaments of the knee are discussed here.

The anterior portion of the knee joint is stabilized partly by the medial and lateral patellar retinacula, which are extensions of the quadriceps femoris muscle (Tortora, 1992, pp. 202-205). The patellar tendon gives added support to the anterior portion of the knee.

The oblique popliteal ligament and the arcuate popliteal ligament stabilize the posterior aspect of the knee (Tortora, 1992, pp. 202-205). The oblique popliteal runs from the intercondylar fossa of the femur to the head of the tibia. While the arcuate popliteal rises from the lateral condyle of the femur to attach to the styloid process of the head of the tibia.

The tibial (medial) collateral and the fibular (lateral) collateral ligaments serve to stabilize the medial and lateral aspects of the knee joint respectively. These ligaments also serve to restrain rotation of the knee (Jenkins, 1991, pp. 233-239). The tibial collateral ligament is a broad, flat ligament that runs from the medial condyle of the femur to the medial condyle of the tibia. A deep portion of this ligament blends posteriorly with the joint capsule of the knee, which is also attached to the medial meniscus. The fibular collateral ligament is more rounded and cordlike and extends from the lateral epicondyle of the femur to the lateral aspect of the head of the fibula. These ligaments are especially important stabilizers of rotational and lateral movement when the knee is in the extended position (Jenkins, 1991, pp. 233-239).

Two important intra-articular ligaments that provide static support to the knee are the anterior (ACL) and posterior (PCL) cruciate ligaments. Although the ligaments are intra-articular they are not contained within the joint capsule of the knee (Hall-Craggs, 1990, pp. 400-406). The ACL extends from the anterior area between the condyles of the tibia in a posterior and lateral direction to a posterior area on the medial surface of the lateral condyle of the femur (Hall-Craggs, 1990, pp. 400-406). The ACL functions to prevent anterior displacement of the tibia on the femur. The PCL runs from a posterior depression between the condyles of the tibia in an anterior and medial direction to the lateral side of the medial femoral condyle (Hall-Craggs, 1990, pp. 400-406). The PCL functions to prevent posterior translation of the tibia on the femur. Additionally both the ACL and PCL serve to reduce rotation of the femur on the tibia. The ligaments are tense in all positions, but increase their tension in the extremes of flexion and extension (Jenkins, 1991, pp. 233-239).

Arteries, Nerves, and Bursa

Blood is supplied to the knee via the popliteal artery. The popliteal artery originates from the external iliac artery, which gives rise to the femoral artery in the proximal thigh. The femoral artery passes posterior to the knee and becomes the popliteal artery.

The knee joint and surrounding musculature is innervated by a number of nerves of the lower limb. Originating from the lumbosacral plexus the femoral and obturator nerves innervate the front and anteromedial sides of the thigh. The sciatic nerve, which rises from the sacral plexus supplies the posterior thigh and divides above the knee to form the common peroneal and tibial nerves.

Various bursae are located about the knee joint for purposes of decreasing friction over tendons and bones. The suprapatellar bursa is located between the deep surface of the quadriceps muscle and the distal part of the femur. This bursa is in communication with the joint capsule of the knee. The prepatellar bursa is located between the superficial surface of the patella and the skin. An infrapatellar bursa is located between the patellar ligament and the skin. The deep infrapatellar bursa is situated between the proximal tibia and the patellar ligament. Other bursae decrease friction at the attachment sites of the gastrocnemius, gracilis, sartorius, semitendinosus, and semimembranosus muscles (Jenkins, 1991, pp. 233-239).