Restriction of joint motion by permanent paralysis early in development can result in the loss of the joint cavity by having a permanent fusion occur between the apposed surfaces of the articular cartilage. If the restriction of joint movement occurs later in development, the joint space may be present but the associated soft tissues of the joint are abnormal. An example is the nongenetic form of clubfoot (talipes varus) caused by the severe restriction of movement of the ankle joint before birth. The normal positioning of the fetus in the uterus allows a fair degree of movement of the upper limbs, but the lower limbs are folded together and pressed firmly against the body. The hip and knee joints are flexed and the feet are inverted in the pigeon-toed position. The ankle joint may become fixed in this inverted position because of the abnormal shortening of the muscles that invert the foot and the lengthening of their antagonists. Also, the ligaments on the medial side of the ankle joint may become abnormally shortened.

HIP JOINT

The upper limbs are far more functionally advanced at birth than are the lower limbs. The neonate can reflexly grasp objects firmly with the hands. In contrast, the underdeveloped lower limbs are reflexly maintained in the position they were held in before birth and in fact their straightening is strongly resisted. Relative to this, the very underdeveloped hip joint is prone to dislocation when the limbs are shortened. The hip socket, or acetabulum, is normally very small compared with the relatively large head of the femur (see Plate 1-7). When the lower limbs arc in the fetal position, the firm ligament of the head of the femur, by virtue of its attachments, strongly prevents the hip joint from becoming dislocated posterosuperiorly. However, if the ligament is abnormally long, it will not prevent a posterosuperior dislocation.

Normally, the ligament does not function to prevent hip dislocation in any limb position other than the fetal one. The thin, flimsy joint capsule is the chief resistance to dislocation when the limbs are not held in the fetal position. Once the infant tends to maintain the lower limbs in extension in the months after birth, the hip joint becomes secure and the ligament of the head of the femur serves no further useful function.

ERECT POSTURE

During the evolution of the human erect posture, the lumbar joints and especially the lumbosacral joint acquired the ability to undergo a pronounced extension that allows a marked lumbar curvature, or lordosis, of the vertebral column. Except for the fixed sacral curve, the vertebral column at birth has no curves. The thoracic part of the spine gradually develops a relatively fixed curve in the young child. A flexible cervical curve appears when the infant is able to raise the head, and a flexible lumbar curve appears at the end of the first year when the child starts to walk. The lumbar curve is necessary to attain the erect posture, because the pelvis remains essentially in the same position as that in a standing quadruped.

The fact that the pelvis did not shift from its quadruped position during evolution of the erect posture also necessitated placing the hip and knee joints into full extension. In addition, the arch of the foot evolved so that the bones were structurally arranged to bear the body weight with a minimum of muscular activity. Therefore, in the human, the passive ligaments of the foot bones and those of the fully extended hip and knee joints bear the brunt of the forces involved in standing erect.