Aetiology

A precise cause is not recognized and there is no genetic formula, but it seems that some noxious physical, chemical or infective influence may modify normal development at a vulnerable stage at some time before the ninth week of uterine life when the hip joint and femur become fully differentiated. The sequence of normal development has been well described and illustrated by King (1969).

Femoral dysplasia has been described following thalidomide poisoning (Fixsen and Lloyd-Roberts, 1974) and as one of the syndromes associated with maternal diabetes (Kucera, Lenz and Maier, 1965). It is therefore interesting to note that femoral deficiency has followed insulin injections into the yolk sac of developing chick embryos on the sixth day (Duraiswami, 1952). Morgan and Somerville (1960) have proposed an alternative explanation for the more proximal deficiencies postulating that the orderly progression of vascular invasion is arrested late in foetal life so that ossification in the cartilaginous model is delayed and the capacity for longitudinal growth greatly impaired.

Clinical features at birth

These vary with the degree of malformation. Those destined to develop simple femoral shortening alone will show no abnormality, whereas those in whom the hip joint and the greater part of the femur are absent will seem to have no femur and lack stability between the lower leg and pelvis. Most, however, display marked femoral shortening with flexion, abduction and external rotation deformity at the hip which is however stable and mobile from the position of fixed deformity – Figure 1.1 – findings which may cause surprise when the radiograph is seen. These features are important for they indicate that continuity exists between femur and pelvis in spite of radiological translucency in the area normally occupied by the proximal femoral shaft (Figure 1.2).

Associated abnormalities when present may be local or general. Tibial shortening with fibular dysplasia is more likely to be seen in company with either severe femoral deficiency or in association with simple short femur when it takes precedence and dominates the clinical problem (Hootnick, et al., 1976).

It would be surprising if an insult to the embryo during the stage of differentiation was localized in its effect and we must therefore be alert to the possibility of developmental anomalies elsewhere, especially in the arms.

Difficulty in diagnosis is unlikely when the manifestation is florid but in minor degrees, i.e. simple short femur and coxa vara, this is possible. A healed birth fracture will not show evidence of distal femoral epiphyseal delay and the shortening, if any, will not progress. Idiopathic hemiatrophy will declare itself in later childhood and is not identifiable in infancy, but the shortening and loss of girth characteristic of spinal dysraphism may be confusing as may unexplained dystrophy of a limb without the stigmata of femoral dysplasia. Congenital coxa vara must be distinguished from the infantile variety (see below).

(a) Hip joint reconstruction impossible (Amstutz Type V)

The femur may very rarely be completely absent (Ring, 1961) but usually, although apparently absent at birth, later ossification will disclose persistence of the distal femoral epiphysis and the supracondylar area. The significant feature is absence of both components of the hip joint, i.e. femoral head and acetabulum. This can usually be recognized at an early stage for the acetabular concavity is non-existent or greatly reduced implying lack of the femoral head. The knee joint, however is normal. The prospect for establishing a stable and mobile hip is clearly non-existent (Figures 1.5 and 1.19).