Spina bifida occulta in isthmic spondylolisthesis: a surgical trap

An 11-year-old girl presented with symptomatic grade IIB isthmic type spondylolisthesis, with an elongated pars, confirmed on magnetic resonance imaging (MRI). Posterolateral in situ fusion of L5/S1 was performed. At surgery, a significant bony defect in the posterior aspect of S1 was noted. Awareness of this possible co-existence is paramount if iatrogenic damage to neural elements is to be avoided during surgery.