This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
As the functions of the lower extremity are support and mobility, it is evident that in order to obtain mobility without unduly weakening the limb the ligamentous connection of the bones must be exceptionally strong. The knee is placed half way down the extremity, hence it has the bulk of the body above to support; also, the bones on each side of the joint are the longest in the body, hence their lever action is exceptionally great, which likewise necessitates that the joint be firmly braced by ligaments.
The knee is primarily a hinge-joint; its main movements are extension and flexion. It can be extended to a straight line (180 degrees) and flexed until the thigh and upper portion of the leg come in contact, at about 45 degrees or even less. The movement is a combined gliding and rolling one. According to Morris ("Joints," p. 375), as extreme extension ends the leg rotates a little outward through a longitudinal axis, passing through the middle of the outer condyle of the femur, and as flexion begins it rotates inward. These rotatory movements are, however, slight, and may practically be ignored. When flexion has proceeded to 150 or 155 degrees, the joint becomes comparatively loose, and this increases as the joint is flexed, until a rotation of 36 degrees (Morris) is allowed. This is of decided practical importance because injuries and treatment are intimately associated with the presence of rotatory movements. No rotation is possible when the knee is fully extended, the bones being then immovable.
The knee-joint is between the femur, the tibia, and the patella; the fibula does not enter into it (Fig. 544). The patella is only a sesamoid bone developed in the quadriceps tendon, and is not essential. In some of the lower animals it has a synovial membrane separate from the knee-joint proper. The joint between the femur and tibia is built up of two separate lateral parts; the condyle and tuberosity of each side forming practically a separate joint and having a crucial ligament as one of its lateral ligaments. The object of thus combining two joints side by side to form one joint is to add to its strength and lateral stability. The condyles of the femur have their articular surfaces prolonged up on its anterior surface, not to aid in flexion and extension, but simply to facilitate the action of the patella. The outer condyle is the higher, to prevent external luxation of the patella.
Fig. 544. - Antero-external view of the bones of the knee.
The articular surfaces of the condyles are not perfect arcs of a circle. If they were the motion would be solely a gliding one and the lateral ligaments and crucial ligaments would be equally tense in all positions, which is not the case, for, particularly in flexion, they become slightly relaxed. The upper surface of the tibia is slightly hollow and its spine projects upward between the condyles, thus adding to the lateral stability of the joint.
The patella is divided by a longitudinal ridge into two articular facets, the outer for the external condyle being the larger; the ridge lies in the intercondylar space. The inner part of the patella is thicker than the outer because the inner condyle is lower. The patella is a sesamoid bone which is developed more toward the deep surface of the tendon of the quadriceps. The tendon goes over the surface of the patella and is practically continued longitudinally through the superficial portion of the bone. If the dried specimen is hammered the bone can be pulverized and removed, leaving the tendon of the quadriceps continuous with the tendo patellae. For this reason, when the patella fractures, the torn fibrous fringes are never on the articular surface but always on its superficial surface. The patella has its sharp apex below and fractures frequently tear it off, the small size of the fragment making repair difficult.