Radioulnar synostosis

Radioulnar synostosis may be either congenital or post-traumatic. The precise cause of congenital synostosis is unknown.

Embryologically, the elbow forms from the three cartilaginous parts representing the humerus, radius and ulna. A programmed cavitation process leads to the formation of the elbow joint; if this process fails, enchondral ossification results in a bony synostosis. Because the forearm bones differentiate at a time when the fetal forearm is in pronation, almost all forearm synostoses are fixed in this position. Moreover, this process occurs at a time when all organ systems are forming, hence synostosis is seen in conjunction with, for example, Apert syndrome (acrocephalosyndactyly), Carpenter syndrome (acropolysyndactyly), arthrogryposis and Klinefelter syndrome.

Congenital radioulnar synostosis is bilateral in 60% of cases. Like tarsal coalition, although it is a prenatal condition and present at birth, it is often undetected until early childhood, when lack of forearm rotation, i.e. pronation or supination, is observed in day-to-day activities. In severe cases there is hyperpronation with a ‘back-handed grasp’. In these situations, early realignment is indicated to allow proper hand development. In minor cases, minor trauma is often blamed for restriction but the X-ray findings are typical. Occasionally, synostosis may be post-traumatic, secondary to abnormal healing of combined radius and ulna fractures.