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//NEONATAL LOWER LIMB

NEONATAL LOWER LIMB

In the neonate the pelvis is cone-shaped. The transverse diameter of the true pelvis is 2.2 cm, its anteroposterior diameter 2.8 cm, and the length between the inlet and outlet is 2 cm. The sacrum is proportionately larger than in the adult and the sacral promontory is higher. When walking begins, the sacrum descends between the ilia and the promontory develops. The ilia, ischia and pubic bones are variably ossified at birth: they meet at the acetabulum, which in the neonate is cartilaginous, relatively large and shallow.

The lower limbs are underdeveloped in the neonate when compared to the upper limbs. They are retained in a flexed position and the leg is proportionately shorter than the thigh. Although the legs appear to be bowed, the tibia and fibula are straight: the illusion of bow legs is caused by the shape of the soft tissues and the slightly more advanced development of the lateral head of gastrocnemius compared to its medial head. The femoral neck is much shorter and forms an acute angle with the shaft. The latter is quite straight, because the adult curvature is acquired with walking. The head of the femur is larger than the acetabular fossa and nearly one-third remains external, which means that the ligamentum teres is relatively very long. Dislocation of the hip joint is relatively easy; the femoral head can be removed from the acetabular fossa laterally, but not posteriorly. The calcaneus and the talus have an ossification centre at birth, and a centre is present in the cuboid in 50% of neonates.

The muscles of the lower limb are much less developed than those in the upper limb. The fetal position often assumed by postnatal babies keeps the thighs in continuous abduction, stretching the adductors. The muscles that will be used for walking are weak; the lack of gluteal development in particular gives the typically diminutive buttocks of the neonate.

In neonates, the feet are usually inverted and they have a greater degree of dorsiflexion, caused by the relatively greater area of the trochlea of the talus. Plantar flexion is limited, in part reflecting the shortness of the extensor muscles of the foot. At birth the footprint outlines the whole plantar surface, reflecting the deposition of subcutaneous fat beneath the longitudinal and transverse arches, and so most babies appear flat-footed.

LOWER LIMB ANOMALIES

The categories of limb defects described by Swanson (1976) are given in Chapter 52. Although devised for the upper limb, they apply in the same manner to the lower limb.

Developmental dysplasia of the hip is a term that covers an alteration in hip development before and after birth. The term is now preferred to ‘congenital dislocation of the hip’. In the most extreme cases, the femoral head is completely dislocated out of the acetabulum. This condition occurs in approximately 1 in 100 live births; the ratio of females to males is 6 : 1. The aetiology is considered to be multifactorial and is associated with first pregnancies, suggesting that both maternal and uterine musculature restrict fetal movement and put postural strain on the fetal hips. Developmental dysplasia of the hip is seen more frequently in breech delivery, especially if the child’s knees are extended. The left hip is more frequently affected than the right, which may be because in breech presentation the fetus lies with the right shoulder anterior and the left thigh closest to the maternal sacrum. The physiological effects that cause the maternal ligaments to become temporarily lax prior to delivery are also considered to affect the fetus and to contribute to laxity of the hip capsule.

Congenital talipes equinovarus, or club foot, derives its name from a combination of talus and pes, together with terms describing an elevated heel resembling that of a horse (equino), which is also turned inwards (varus). It is a common neonatal anomaly and occurs in approximately 1 per 1000 live births, with males affected twice as often as females. There is some degree of inheritance, so it is not entirely an effect of intrauterine positioning or of oligohydramnios (abnormally little amniotic fluid). Both bones and soft tissues are affected and it is difficult to tell which are primary effects and which are secondary. A number of theories have been proposed to explain the underlying pathogenetic mechanism(s), including abnormal tendon and ligament attachments and delayed muscle maturation. The most widely held theory is that the talus undergoes defective development and that all the other deformities arise as a consequence of this initial defect. The talus is decreased in size by up to 25%. It has a foreshortened neck and decreased body/neck angle, and the subtalar facets are medially rotated. The navicular is small and medially deviated relative to the talus. The calcaneus is also small and shows varus displacement and equinus tilt: the anterior facets correspond to those of the talus (Barlow & Clarke 1994). The entire affected foot and calf are smaller than their normal counterparts. The foot is inverted and supinated and the forefoot is adducted. The heel is small, rotated inwards and elevated. The calcaneus is inverted beneath the talus. Treatment varies between splintage and repeated complex surgery, which reflects the highly variable severity of the condition and individual response to therapy.

‘Flat feet’ are common in childhood. The majority are ‘flexible’ and simply related to posture, whereas ‘rigid’ flat feet are caused by structural abnormalities. The description of ‘rocker bottom’ foot is associated with many congenital syndromes. The foot is flat and rigid and the plantar surface appears curved with the apex of the curve at the midtarsal joint. The condition shows an equinus position of the hindfoot. The talus may be vertical and palpable on the plantar surface. A majority of infants with this condition have neurological abnormalities.

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