Clinical features and radiological findings of congenital proximal radioulnar synostosis with special reference to radial head dislocation type and ankylosed position.

Background: Congenital proximal radioulnar synostosis (PRUS) presents with diverse patterns, including variations in the range of synostosis and the type of radial head dislocation, as well as differences in the ankylosed position. However, detailed proportions and correlations among them remain unclear due to small sample sizes in previous reports. This study aimed to investigate the clinical features and radiological findings of congenital PRUS, with special reference to the type of radial head dislocation and ankylosed position.

Methods: We retrospectively examined 232 patients (366 limbs) with congenital PRUS using medical records from our institute between 2002 and 2018. We extracted patient profile data and analyzed correlations among the ankylosed position, type of radial head dislocation, and range of synostosis.

Results: Among the 366 limbs examined, 68% showed ankylosis in pronation, 19% in a neutral position, 4% in supination, and 9% exhibited incomplete fusion. Regarding radial head dislocation, 54% of the limbs had posterior dislocation, 26% had anterior dislocation, and 21% showed no dislocation. Analysis of the range of synostosis revealed that 10% of the limbs had severe synostosis (>20%), 56% had moderate synostosis (10-20%), 15% had mild synostosis (<10%), 10% had synchondrosis, and 9% had incomplete fusion. A significant correlation was found between the type of radial head dislocation and the ankylosed position (P<0.001). Residual analysis showed that specific combinations contributed notably to this correlation. These combinations included posterior dislocation of the radial head and pronated ankylosis, anterior dislocation of the radial head and neutral-positioned ankylosis, and the absence of radial head dislocation and incomplete fusion. Regarding the correlation between the type of radial head dislocation and the range of synostosis, a significant difference was observed among the three groups classified by the type of radial head dislocation (P<0.001). The posterior dislocation group exhibited a significantly wider range of fusion than the anterior dislocation (P<0.001) and non-dislocation groups (P<0.001). However, no significant difference was found between the anterior dislocation and non-dislocation groups (P=0.53).

Conclusions: The majority of limbs with posterior dislocation of the radial head showed ankylosis in pronation, whereas limbs with anterior dislocation of the radial head exhibited ankylosis in a neutral position. These findings may help us understand the pathophysiology of congenital PRUS.

Level of evidence: Level IV; Case Series; Descriptive Epidemiology Study.