The developmental spectrum of proximal radioulnar synostosis

Proximal radioulnar synostosis is a rare upper limb malformation. The elbow is first identifiable at 35 days (after conception), at which stage the cartilaginous anlagen of the humerus, radius and ulna are continuous. Subsequently, longitudinal segmentation produces separation of the distal radius and ulna. However, temporarily, the proximal ends are united and continue to share a common perichondrium. We investigated the hypothesis that posterior congenital dislocation of the radial head and proximal radioulnar fusion are different clinical manifestations of the same primary developmental abnormality. Records were searched for "proximal radioulnar fusion/posterior radial head dislocation" in patients followed at the local Children's Hospital and Rehabilitation Centre for Children. Relevant radiographic, demographic and clinical data were recorded. Ethics approval was obtained through the University Research Ethics Board. In total, 28 patients met the inclusion criteria. The majority of patients (16) had bilateral involvement; eight with posterior dislocation of the radial head only; five had posterior radial head dislocation with radioulnar fusion and two had radioulnar fusion without dislocation. One patient had bilateral proximal radioulnar fusion and posterior dislocation of the left radial head. Nine patients had only left-sided involvement, and three had only right-sided involvement.The degree of proximal fusion varied, with some patients showing 'complete' proximal fusion and others showing fusion that occurred slightly distal to the radial head: 'partially separated.' Associated disorders in our cohort included Poland syndrome (two patients), Cornelia de Lange syndrome, chromosome anomalies (including tetrasomy X) and Cenani Lenz syndactyly. The suggestion of a developmental relationship between posterior dislocation of the radial head and proximal radioulnar fusion is supported by the fact that both anomalies can occur in the same patient. Furthermore, both anomalies can be seen in different patients with the same genetic diagnosis, further supporting the notion that these defects are developmentally related. Posterior dislocation of the radial head and radioulnar fusion are considered to be related primary developmental anomalies of radioulnar differentiation/segmentation. We speculate that the eventual specific defect of this spectrum is influenced by very subtle differences in developmental timing. This is in contrast to patients with transverse forearm defects who can also display radial head dislocation but in an anterior or lateral direction. This direction of dislocation is seen when an abnormal force is exerted on a normally formed radial head later in development or postnatally in disorders such as multiple osteochondromatosis and various mesomelic dysplasias, or as a result of trauma.