Congenital radioulnar synostosis is a rare condition in which there is an abnormal connection (synostosis) of the radius and ulna (bones in the forearm) at birth. The condition is present in both arms (bilateral) in approximately 60% of cases. Signs and symptoms depend on the severity of the abnormality and whether it is bilateral; people with the condition often have limited rotational movement of the forearm. Pain is usually not present until the teenage years. There are 2 types of radioulnar synostosis: type 1 and type 2. In type 1, the fusion involves 2-6 cm of the area between the radius and ulna bones which is closer to the elbow and the knobby end of the radius that meets the elbow is absent (radial head). In type 2, the fusion is farther from the elbow and there is dislocation of the radial head. Both types result in a limitation of inward roll (pronation) and outward roll (supination) of the forearm, and in type 2 there is also a restriction of extension at the elbow. Congenital radioulnar synostosis is due to abnormal fetal development of the forearm bones, but the underlying cause is not always known. It is sometimes a feature of certain chromosome abnormalities or genetic syndromes. Some cases appear to be inherited in an autosomal dominant manner. Treatment may be conservative or involve surgery depending on the severity of the abnormality and the range of motion.

Congenital radioulnar synostosis is caused by abnormal development of the forearm bones during the fetal period, although the underlying cause of the developmental abnormality is not always known. The condition may be isolated (occur without other abnormalities) or it may be associated with various other skeletal, heart (cardiac), neurologic, or gastrointestinal abnormalities. When other abnormalities are present, the condition may be due to an underlying genetic cause, including a variety of syndromes or chromosome abnormalities. Some chromosome abnormalities that may include radioulnar synostosis are Klinefelter syndrome and XXXY syndrome. Examples of genetic syndromes that may include radioulnar synostosis are Apert syndrome, Crouzon syndrome, Carpenter syndrome, arthrogryposis, Treacher Collins syndrome, Williams syndrome, amegakaryocytic thrombocytopenia, and Holt-Oram syndrome.

Congenital radioulnar synostosis may also occur with a variety of other abnormalities and may be associated with a chromosome abnormality or with a genetic syndrome. In these cases, the inheritance pattern may depend upon that of the underlying genetic abnormality.

Congenital radioulnar synostosis may also occur sporadically as an isolated abnormality, in which case the cause may be unknown.

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