Aplasia


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Aplasia,
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failure of a structure to develop entirely or partially, leading to congenital abnormalities. Among the major tubular bones, the most frequently affected are the fibula, radius and femur. The limb defects can be divided into two types, transverse (in which all skeletal elements are absent distally along a designated axis and the entire width of the limb is involved) and longitudinal (in which portions of bone between the proximal and distal ends are involved and the involvement extends into the long axis of the bone).

Aplasia of the fibula is the most common form and varies in severity. Radiographs may show medial bowing of the tibia, equinovalgus foot deformity, absence of one or two of the lateral rays of the foot, tarsal aplasia or fusion, and delayed development or shortening of the ipsilateral femur (Fig.1). The deficiencies may be distal or proximal. When distal, an associated valgus deformity of the ankle may be seen, whereas proximal fibular hypoplasia can be accompanied by valgus knee deformity and instability at the proximal tibiofibular joint.

In the radius, bilateral abnormalities may be combined with hypoplasia or absence of the thumb or radial carpal bones. Systemic disorders such as VATER syndrome, cardiac abnormalities and TAR syndrome may be associated with an absent radius (Fig.2).

The ulna is involved less frequently and less severely than the radius but may reveal hypoplasia, partial aplasia or total aplasia. A radiographically invisible fibrocartilaginous band may cause tethering and produce ulnar deviation of the hand. Resection of this band may reduce the angular growth deformities.

Absence or shortening of the femur may lead to a spectrum of abnormalities termed proximal femoral focal deficiency (Fig.3). This disorder is of obscure cause and differs from total femoral agenesis. The various forms have been classified according to the presence and location of the femoral head and neck, and specific diagnosis may require both radiography and arthrography. MR imaging may also play a role.

Radiographs show a short femur that is displaced superiorly, posteriorly and laterally to the iliac crest, with delayed ossification of the femoral capital epiphysis. Other findings are subtrochanteric varus deformity or pseudarthrosis, a large unossified gap between the femoral capital epiphysis and dysplastic shaft, or ossification of only the distal femoral epiphysis. The disorder may be mistaken for developmental coxa vara, but the latter is characterized by abnormalities that are less severe, delayed in onset, progressive, and related to a true decrease in the neck-shaft angle as opposed to the subtrochanteric varus that appears in proximal femoral focal deficiency.

Rarely, a distal femoral focal deficiency may occur, marked by dysgenesis of a variable portion of the proximal end of the femur.

Congenital deficiency or absence of the tibia is usually associated with fibular hypoplasia and hemimelia. Four types of tibial deficiency are recognized:

type 1 is characterized by complete absence of the tibia and hypoplasia of the distal end of the femur or a rudimentary tibia that articulates with the distal end of a relatively normal femur;

type 2 reveals a well developed proximal end of the tibia and absence of the distal end of the tibia;

type 3 is manifested by a tibia that is merely an amorphous segment of bone; and

type 4 consists of a normal proximal end of the tibia with a shortened distal end and congenital diastasis of the ankle joint.

These abnormalities may be accompanied by flexion contracture of the knee, inversion and adduction of the foot, femoral hypoplasia, diastasis of the tibiofibular syndesmosis and anomalies of the hand.


DR/RB


The Encyclopaedia of Medical Imaging Volume III 1
Aplasia, Fig. 1
AP radiograph of the lower extremity demonstrates aplasia of the fibula.
Aplasia, Fig. 2
AP radiograph of the forearm demonstrates absence of the radius in a patient with the TAR syndrome.
Aplasia, Fig. 3
AP radiograph of the pelvis in a patient with proximal focal femoral deficiency syndrome demonstrates articulation of the tibia with the acetabulum at the deformed right hip joint. Also note the hypoplastic right iliac bone and dysplastic left hip joint.