Radiographic method to assess the prevalence of sacral spina bifida occulta.

Spina bifida occulta of the sacrum is the most common type of spinal deformity. Many authors have published data on the frequency of spina bifida occulta, with varying results. Some possible reasons for this variability could include the differing methods used to gather data and differing ways of classifying the condition. This study attempts to develop an X-ray method to study sacral spina bifida occulta in a standardized fashion, using an angulated antero-posterior technique. This technique is then used to estimate the frequency of sacral spina bifida occulta in an Australian sample. The sacra of 53 cadavers were X-rayed and the level of closure of the sacral spinal canal recorded. The X-ray technique was validated by open dissection of six of the cadavers studied and was shown to be accurate to half a sacral segment. No sacra with a completely open sacral canal were found, two sacra (4%) were open from S2 down to S5 and ten sacra (19%) were open from S3 down to S5. The most common condition (43%) recorded was where S4 and S5 only were open. Eighteen cadavers (34%) showed only S5 open, and interestingly, no sacra were recorded as having the dorsal sacral arch completely closed. A study of a larger sample will follow using the validated X-ray technique.