BACKGROUND: The sacral ratio (SR) was proposed by Alberto Pena in 1995 as a reliable tool to evaluate sacral development in anorectal malformations (ARM). The SR is obtained by comparing sacrum size with fixed bony parameters of the pelvis. In previous studies, the average normal SR was calculated to be 0.74 for the anteroposterior view (APSR) and 0.77 for the lateral view (LSR). OBJECTIVES: (a) To evaluate the range of variability of the SR in a normal population with age as a possible variable factor. (b) To evaluate the reliability of the SR as a prognostic factor in patients with ARM. MATERIALS AND METHODS: We studied pelvic radiographs of 147 normal children in order to calculate a normal range of values for the SR. In 59 children with ARM, clinical parameters of faecal and urinary continence, spinal cord anomalies (as detected by MRI) and rectal innervation patterns (studied by enzymo-histochemistry) were correlated with SR values by Snedecor's 'F' test and Bravais-Pearson's 'r' correlation index. In 25 cases (15 children with ARM and 10 controls), multiple measurements of SR were obtained at different ages. RESULTS: Normal values for APSR ranged from 0.52 to 1.12, with an average of 0.74. Normal values for LSR ranged from 0.52 to 1.18, with an average of 0.75. Low SR values (below 0.52) correlated well with spinal cord anomalies and with unfavourable functional prognosis in children with ARM. APSR and LSR values increased with age in the normal population, while they decreased in patients with ARM. CONCLUSIONS: The SR has a wide range of normal values, and should be considered truly pathological when lower than 0.52, in either the AP or lateral views. Age has a significant influence on SR values.
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