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.
