We describe a variant muscular layer covering the superficial layer of the posterior sacrococcygeal membrane over the sacral hiatus, with longitudinally oriented muscular fibers, that in our opinion belongs to the multifidus muscle. We had the privilege to discover it in twelve patients while performing an open surgical approach of the sacral hiatus, which gives a prevalence of 11.11% among all cases treated with this intervention. Knowledge of this anatomical variant is important because it may render difficult any caudal approach to the sacral canal and might explain some failures of caudal epidural blocks. Significantly, our report is based on direct surgical visualization of an area until now described only indirectly, by magnetic resonance imaging, ultrasonography or cadaver sacral bone examination.
