Paediatric musculoskeletal radiology

Developmental problems

 

Developmental dysplasia of the hips (DDH)

Developmental dysplasia of the hip (DDR) was formerly known as congenital dislocation of the hip (CDR) but since it sometimes may occur postnatally, the term developmental dysplasia had been now been accepted by most pediatric orthopedic surgeons. The condition is more common in girls than in boys and usually presents in the neonate. It can usually be diagnosed by clinical examination using the Ortolani and the Barlow manoeuvres. The radiographic findings in the neonate may be very subtle since the proximal fem oral head is usually not ossified until three to four months of age (Fig. 25 a).

The plain film of the pelvis in many cases, however, is suggestive; the acetabular angle is usually greater than normal often with a poor superior lip. The mean normal angle in neonates is 280. The 2 S.D. range is 180 - 370. After three months the + 2 S.D. upper limit is 300. The proximal femoral hip metaphysis may also be displaced laterally and superiorly (Fig. 25 c). When a Hilgenreiner line (Fig. 25 b) is drawn through the Y cartilage, displacement of the femoral metaphysis can be measured in relationship to this line. The useful view is the AP with the legs in neutral position. It is in this projection that dislocation tends to occur, while the frog leg view tends to relocate the hips in the acetabulum. A normal, plain radiograph, however, does not rule out DDR (Fig. 26 a). The optimal radiologic approach to evaluate possible DDR is ultrasonography with examination of the hip in the coronal plane. The static method as described by Graf examines the acetabular angle with ultrasound in this projection. The neonatal femoral head is usually clearly visualized on ultrasound examination as it contains many small signals within it. These are due to small vessels within the cartilaginous femoral head. A dynamic study can also be done to determine if the head is dislocatable.

a

Figure 26. Developmental dysplasia of the hip missed in infancy. a)  Radiograph at 1 day of age. There is a poor formation of the right acetabulum as compared to the left with a somewhat steeper acetabular angle and a less well developed lip. b) The child returned at 9 months of age and having received no treatment. There is now frank dislocation, lack of development of the right femoral head and a pseudoacetabulum, above the true acetabulum.

b

After several months another sign of DDH is a delay in ossification of the fem oral head (Fig. 26 b). This is more easily detected if the disease is unilateral. When the infant begins to stand a pseudoacetabulum may form (Fig. 26 b). Although MRI clearly shows the femoral heads relation to the acetabulum (Fig. 26 c), it is rarely used as the initial method of diagnosis of DDH since the clinical examination is usually sufficient. If there is a questionable clinical exam ultrasound and/or radiographs can be used.

There is some controversy on the value of ultrasound screening of children for DDR. Screening is probably not worthwhile in the general population because of the expense, the relatively low incidence of DDH and the fact that DDH can often be diagnosed clinically. It may be of value in the high risk infant such as in infants whose mothers had previous such babies, when there is breech presentation, or when there is other family history of DDR. In these populations there is a much higher incidence of DDR and, therefore, ultrasound screening may be worthwhile.

When diagnosed early in infancy DDR is usually easily treatable with some type of splinting and the success of reduction can be monitored with ultrasound. In children in whom DDR is first detected after six months or a year of age, reduction becomes more difficult. In these cases CT is of value to determine whether the femoral head has indeed been replaced following closed reduction. It is very difficult to determine whether the head had been reduced on radiographs of the child in plaster. In one comparison study we showed that correct evaluation by experienced pediatric radiologists and orthopedists of whether the femoral head was reduced was only 50%. CT on the other hand, clearly shows the position of the femoral head in relationship to the acetabulum. It also shows many of the reasons why the femoral head may be displaced such as the presence of a prominent pulvinar (fibrofatty tissue) or an indentation by the iliopsoas muscle upon the hip capsule. CT for postoperative evaluations should be done using very low mAs and, therefore, decreased radiation dose. We use 50 mAs. MRI outlines the soft tissues and cartilage much better than CT but its use is somewhat limited in the hip since many MRI gantries are small and most infants in a frog leg cast will not fit in them. Femoral anteversion and acetabular torsion can be determined with both CT and MRI.

Dislocation of the hip occurs secondarily in many contracture syndromes and in children with myelodysplasia or other neurologic problems (Fig. 27). These dislocations can occur later in childhood. In children with cerebral palsy dislocation usually occurs late and acetabular changes may be less apparent. They may have coxa valga which tends to promote dislocation of the hip (Fig. 27). The role of the radiologic examination in these cases is to see how well the femoral head is covered by the acetabulum. If there is still question of adequate coverage of the femoral head on radiographs of young children, MRI is useful since the actual cartilaginous portions of the acetabulum and femoral head are well visualized. In neonates with contracture syndromes an additional useful plain radiologic sign is the presence of pseudoacetabuli. Their presence in the neonate indicates a contracture syndrome because in otherwise normal children with DDR pseudoacetabuli occur only secondarily to weight bearing.

Figure 27. Child with cerebral palsy with lateral dislocation of the left hip. The left acetabulum is poorly formed; however, the femoral heads are not too dissimilar in size. They usually are much smaller on the affected side in DDH.

 

Foot deformities

Foot deformities may be due to congenital anomalies, may be secondary to position in utero, or they may be due to contractures or neuromuscular problems. The most common foot deformity that needs to be corrected is the club foot deformity (talipes equinovarus) (Fig. 28). However, there are many other foot deformities. In evaluation of foot deformities one should use weight bearing or simulated weight bearing views. The axes of the talus and calcaneus in both the AP and lateral projections must be determined. The angle between these bones as well as the relation of their axes to the axis of the first metatarsal and other metatarsals must be determined. In normal children in the AP projection the axis of the talus should pass through the first metatarsal and the axis of the calcaneus through the fourth. In varus deformities of the foot such as in the club foot, the axis of the talus passes lateral to the first metatarsal. The calcaneus may be lateral to all the metatarsals (Fig. 28). In club foot deformity the calcaneus and talus become parallel to each other, particularly in the lateral position. There is also plantar flexion of the foot (equinus deformity). In valgus deformity of the foot the axis of the talus lies medial to the first metatarsal. A common form of valgus deformity is associated with a vertical or oblique talus (Fig. 29). In this case the angle between the talus and the calcaneus is increased in the lateral projection with the talus pointing downwards. In severe cases there is dislocation between the talus and the tarsal navicular. In young infants, however, this cannot be visualized as the tarsal navicular is usually not ossified at birth. MRI shows some promise in defining the relationship between all

a

b

Figure 28.Clubfoot. Both views were obtained with weight bearing. a) In the AP view the axes of the talus and of the calcaneus are almost parallel and their axes project considerably lateral to their normal position. b) In the lateral view they are also somewhat parallel and the foot stays somewhat plantar flexed even with weight bearing. of these bones since those that are not ossified or partially ossified can be visualized.

Packing deformity

Occasionally children are born with a bowed tibia and fibula caused by a relatively tight uterus and pressure of one extremity on another (Fig. 30). This bowing usually straightens out postnatally. The condition must be separated from bowing due to congenital pseudoarthrosis or secondary to neurofibromatosis (Fig. 31). In neurofibromatosis the bowing is usually anterior while in packing deformity it is frequently from side to side. Bilateral bowing may also be due to a variety of congenital disorders and syndromes such as the campomelic syndrome.

	Figure 29. Vertical talus and rocker bottom foot. The talus is vertical in relation to the foot and is subluxed in relation to the navicular.
	Figure 30. Faulty fetal packing. This type of bowing of the leg results from pressure on the limb in utero and usually straightens with time. The bowing is usually from side to side rather than anterior or posterior.
	Figure 31. Tibial bowing in neurofibromatosis in 10-year old girl. There is a suggestion of an old pseudoarthrosis. In neurofibromatosis the bowing is usually in an anterior direction; however, it is not specific for neurofibromatosis.

Coxa vara and coxa valga

Coxa vara is when the angle between the femoral neck and the femoral shaft is decreased, while coxa valga is when it is increased. The normal angle after five years measures 122-149°.

Coxa vara may be congenital or acquired. The acquired form may occur in any situation where the bones are soft such as in rickets or osteogenesis imperfecta. It may occur secondarily to fractures, may be associated with a number of congenital malformation syndromes, or in association with a congenital short femur. Coxa valga is frequently secondary to lack of weight bearing and is seen in paralysis or various neuromuscular syndromes.

 

Andrew K. Poznanski