CONGENITAL RADIOULNAR SYNOSTOSIS
Congenital radioulnar synostosis is an uncommon condition in which the proximal ends of the radius and ulna are joined, fixing the forearm in pronation. The deformity is due to a failure of the developing cartilaginous precursors of the forearm to separate during fetal development. Radioulnar synostosis is bilateral in 60% of patients and is frequently associated with other musculoskeletal abnormalities. Chromosomal abnormalities have been reported in some patients with bilateral involvement. Two types of synostosis are seen. In the first, called the headless type, the medullary canals of the radius and ulna are joined and the proximal radius is absent or malformed and fused to the ulna over a distance of several centimeters. The radius is anteriorly bowed and its diaphysis is larger and longer than that of the ulna. In the second type, the fused segment is shorter and the radius is formed normally but the radial head is dislocated anteriorly or posteriorly and fused to the diaphysis of the proximal ulna. The second type is often unilateral and sometimes associated with deformities such as syndactyly or supernumerary thumbs.
Radioulnar synostosis is present at birth but is usually not noticed until functional problems arise, most often in patients with bilateral involvement. Commonly, the only clinical finding is lack of rotation between the radius and the ulna, which fixes the forearm in a position of midpronation or hyperpronation. Range of motion in the elbow and wrist joints is usually normal or excessive, although some patients cannot completely extend the elbow. The degree of functional disability varies with the amount of fixed pronation and whether the condition is unilateral or bilateral. Unilateral deformity with less fixed pronation may be able to compensate with shoulder motion. However, in patients with bilateral involvement in which both hands are hyperpronated, many daily activities become problem- atic, such as turning a doorknob, buttoning clothing, drinking from a cup or eating, and personal hygiene.
Because the disability is so varied, treatment should be specific to the patient. Mild, unilateral abnormalities are typically left untreated. Synostosis resection to regain forearm rotation in more severe cases has not been successful, with new bone often rebridging the resected gap. In patients with hyperpronation, particularly if bilateral, rotational osteotomy, either through the distal end of the fused area or through the radius and ulna distal to the fusion, can be performed to put the forearm in a more functional position. Typically, the dominant forearm is positioned in 0 to 20 degrees of pronation while the nondominant forearm is positioned in 20 to 30 degrees of supination. Compart- ment syndrome can occur as a complication of this procedure.