Background: Pelvic osteotomies are frequently used as part of the surgical management of bladder exstrophy. The outcomes are often measured on the basis of the residual symphyseal diastasis. The aims of this study were to evaluate and validate a more reliable radiographic measure of ischiopubic rotation, to utilize this measure in analyzing pelves from patients with exstrophy and controls, and to propose a model for rediastasis in a pelvis with exstrophy.

Methods: Pelvic radiographs of 164 normal children two months to eighteen years of age were used to determine the changes in interpubic and interischial distances and in the interischial/interpubic (IS/IP) ratio with age. Twenty-one pelvic CT (computed tomography) studies of normal children, two to sixteen years of age, were also used to study the change in the ischiopubic divergence angle. The same parameters were measured on radiographs or CT or magnetic resonance imaging studies of seventy-three patients with classic bladder exstrophy who were followed for two to nineteen years after exstrophy closure with or without pelvic osteotomies.

Results: In normal children, the interpubic distance and the ischiopubic divergence angle had a narrow range and were constant with age, whereas the interischial distance and the IS/IP ratio increased progressively and were strongly correlated with age. In the patients with exstrophy, the interpubic distance was positively correlated with the interischial distance, whereas the IS/IP ratio was lower than that in normal controls and was not correlated with age.

Conclusions: The IS/IP ratio is a useful measure of ischiopubic rotation and can be used to characterize pelvic growth, including the phenomenon of rediastasis in patients with exstrophy. Pelvic rediastasis is a progressive increase in interpubic distance resulting from growth without loss of rotational correction, as shown by the constancy of the IS/IP ratio with age in these patients. A better rotational position at the time of osteotomy may lead to a better pelvic shape at maturity.