The specific cause of PFFD is unknown. It is believed that an anomaly occurs during the 2nd -7th week of pregnancy, and it is considered nonhereditary. Viral and bacterial infections, irradiation, ischemia, hormones, chemicals and mechanical injuries have been included as possible causes.
PFFD involves a short femur in one or both legs, with varying levels of deficiency in the corresponding hip joint. There are several classifications for PFFD, the most common was developed by Dr. George T. Aitken in the early 1960's. Dr. Aitken developed a four class (Class A,B,C,D) classification system based on X-ray results.
- class A
- this is the least severe type where the femoral head is present and attached to the shaft by the femoral neck.
- the femur is shortened (as in all types), and a coxa vara deformity is present
- a cartilaginous neck is not seen on early radiognaphs but later ossifies.
- occasionally, the cartilaginous connection between the neck and the shaft forms a sub trochantenic pseudarthrosis.
- class B
- in this type, the acetabulum is “adequate” or moderately dysplastic and contains the femoral head.
- at maturity, no osseous connection is seen between the femoral head and the shaft.
- the femoral segment is short and usually has a bulbous bony tuft.
- class C
- the acetabulum is severely dysplastic in this form.
- the femoral head is absent or is very small and not attached to the femoral shaft.
- the shortened femoral segment has a tapered proximal end.
- class D
- this is the most severe form, with absence of the acetabulum and proximal femur.
- no proximal tuft is present.