Paediatric musculoskeletal radiology

The normal pediatric skeleton

 

In evaluation of children's bones it is extremely useful to look carefully at the growth plate (physis), the metaphysis and the epiphysis (ossification centre). In many pediatric diseases useful diagnostic clues are seen in the se areas because they are areas of rapid growth. One must look carefully at the width and regularity of the growth plate, the shape and dens it y the ossification centres, the presence of lucent or dens e lines in the metaphyses, and the degree of modelling of the shaft. One must be aware of the normal variations in these structures. For example, some ossification centres such as the tarsal navicular often begin to ossify in many small fragments and may be sclerotic. This may involve only one foot and should not be confused with disease. Other common normal variants, which can often be mistaken for disease, include the prominent ischiopubic synchondrosis, which can be wide and irregular, sometimes even unilaterally (Fig. 2) and bowing of the legs in infants. The normal physiologic bowing should be differentiated from various diseases that cause bowing. It usually progresses to knock knee deformity which eventually straightens spontaneously. There are many other such possibilities. The reader is referred to textbooks such as Keats Normal Variants or Kohler's Borderland of Normal Pathology for further reading. As in any radiological diagnosis it is essential that the normal variation be known before pathological abnormalities can be detected. Because of the

Figure 3. Multiple growth lines in a 3-year-old boy. This boy has received several courses of chemotherapy for leukemia. The growth lines are evidence of arrest and restart of growth.

added factor of growth more of these normal variants are seen in children than adults.

There are a number of lines that are visible in the metaphysis. Small sclerotic lines in the metaphysis which are parallel to the growth plate may be seen normally. These have been called Park lines or growth lines and they simply represent an episode when growth has restarted after it was arrested. The growth lines are more common when a child undergoes major stresses such as may occur following serious disease. The lines are often multiple in children undergoing chemotherapy where many arrests and restarts of growth occur with each treatment (Fig. 3). Also, multiple growth lines associated with marked retardation of skeletal maturation can be seen in children with psychological deprivation (psychosocial dwarfs). The lines are a useful marker of growth of the end of a bone. For example, if a growth line does not form in a bone end on one side and forms on the other it indicates that growth has stopped on the side where no growth line is seen. A growth line further away from the physis on one side than the other indicates excess growth which could be due to hyperemia such as may occur in joint disease. Similarly, if after a fracture the growth line is not parallel to the growth plate this is an

	Figure 4. Ankle several years after trauma. The growth lines in the distal tibia are not parallel to the growth plate. The middle part of the growth line is very close growth plate, while both medially and laterally it is further away. This indicates a partial fusion in the middle portion.
a	Figure 5.Normal zone of provisional calcification and lead lines. a) Normal dense zone of provisional calcification in the metaphyses of the femur, tibia and to a lesser degree the fibula. This is simply a manifestation of active growth. b) Child with lead poisoning. The zones of provisional calcification are somewhat wider and somewhat denser than in the normal child. The fibular zone of  provisional calcification is much denser than in the normal.
b
	Figure 6. Lucent metaphyseal bands in a premature infant of fifteen days of age, with broncho-pulmonary dysplasia.

indication that growth is greater in one part of the growth plate and subsequent deformity will occur (Fig. 4).

A dense line at the metaphysis immediately adjacent to the growth plate is a normal finding in growing children. It is the zone of provisional ossification. This line can be dense and should not be mistaken for a lead line (Fig. 5 a). Dense metaphyseal lines caused by lead intoxication are usually more circumscribed and appear somewhat more dens e than the normal zone of provisional calcification (Fig. 5 b). It is often difficult to differentiate the lead lines from normal. The lead lines often appear in areas where the normal zones of provisional calcification are usually not visible such as in the proximal fibular metaphysis or in the iliac crest. However, their presence even in those areas does not prove that they are lead lines as sometimes in rapidly growing children they can be seen in these regions as well. The only way to be certain on radiographs that these lines are related to lead poisoning is if the lines move away from the metaphysis and there is a space between the distal metaphysis and the dense line.

Figure 7. Lucent bands at the time of birth in CVM. Similar bands can be seen in other TORCH infections.

Lucent bands (Fig. 6) at the ends of the bones are usually a sign of growth disturbance and poor ossification. If they are seen in the neonate at birth it usually means some growth disturbance occurred in utero such as may be caused by cytomegalovirus (CMV) (Fig. 7) or rubella. In rubella and to a lesser degree in CMV linear striations in the metaphysis parallel to the diaphysis may be seen. In syphilis lucent bands may also be seen or there may be more destructive changes (Fig. 8) often affecting the medial side of distal femoral and proximal tibial metaphyses. Postnatally metaphyseal lucent bands are very common in infants with severe distress and are particularly common in hyaline membrane disease and bronchopulmonary dysplasia (Fig. 6). They are of little diagnostic significance in this age group. In children over two years of age, however, lucent metaphyseal bands are usually the hallmark of leukemia (Fig. 9). The leukemic lines can be seen even in children who have a normal peripheral blood smear and when they are noted in multiple bones a bone marrow biopsy is indicated. They may be the presenting radiographic sign of leukemia in a child examined for joint pain. Occasionally rickets that has been treated can have a similar

	Figure 8.Syphilis in the neonate. The lucent bands have a more destructive appearance than in CMV, often the lucent defects in the femora and tibias are seen particularly on the medial side.
	Figure 9. Leukemic line. Lucent metaphyseal band in a child who complained of pain in the legs. When seen in children over two years of age these lucent bands if bilateral are usually indicative of leukemia.

Metaphyseal lucent bands. Lucent bands affecting only one bone or limb can be due to local disease or immobilization.

 

 

Andrew K. Poznanski