Synostosis (from Ancient Greek συν- (syn-) 'together' and ὀστέον (ostéon) 'bone'; plural: synostoses) is fusion of two or more bones.

Radioulnar synostosis is one of the more common failures of separation of parts of the upper limb.

Most cases are sporadic, congenital (due to a defect in longitudinal segmentation at the 7th week of development) and less often post-traumatic, bilateral in 60%, and more common in males.

Familial cases in association with autosomal dominant transmission appear to be concentrated in certain geographic regions, such as Sicily.

The condition frequently is not noted until late childhood, as function may be normal, especially in unilateral cases.