In this uncommon condition there is congenital fusion of the proximal ends of the radius and ulna, fixing the forearm in varying degrees of pronation. In about 60 percent of cases, involvement is bilateral.
Male and female incidence is approximately equal.
There are three types of radioulnar synostosis.
First type is true congenital radioulnar synostosis where ulna and the upper end of the radius are closely fused together. The radial head may be fused to the ulna, or it may be completely absent.
The radial shaft is bowed to a greater degree than normal and is longer and thicker than the ulna. A
The second type is that in which the radial head is malformed and posteriorly dislocated.The proximal end of the radius is fused with the upper shaft of the ulna.
In the third type, the rarest, the ulna and radius may be attached, at a point just distal to their upper ends, by a short, thick, interosseous ligament that prevents any pronation or supination, just as if the bones were fused together. This is not a true synostosis.
Congenital radioulnar synostosis is hereditary in some cases, appearing to be a dominant trait . It is caused by a developmental arrest of longitudinal segmentation.
There is no motion of the radius or the ulna, and the forearm is usually fixed in a position of mid-or hyperpronation.
The lack of supination of the forearm is compensated for somewhat by rotation at the glenohumeral joint, though it is impossible for the palm to be fully supinated. The elbow joint and the wrist are able to move freely, though extension of the elbow may be somewhat limited.
The degree of functional disability varies according to the position in which the forearm is fixed. It may be minimal if the condition is unilateral.
The child may have difficulty with such activities as turning a doorknob, buttoning shirts, and handling eating utensils.
The involved forearm is thinner than normal and somewhat twisted in appearance.
Each patients should be individually evaluated to determine the treatment required. Surgical separation of the synostosis is not advised, as results have been poor.
In cases of extreme pronation, an osteotomy in the proximal thirds of the radius and ulna may be performed to place the forearm in functional position.