Microsurgical anatomy of the lateral approach to extraforaminal lumbar disc herniations

During the "lateral" approach to extraforaminal lumbar disc herniations, the surgeon may be confronted with considerable variations in anatomy, making this approach extremely difficult in some patients. An anatomic study, therefore, was undertaken to examine the bony boundaries of the operative target, the medial intertransverse space. In 31 lumbar spine specimens taken from cadavers of people who had been between 30 and 93 years old at death, the relevant distances and proportions of the operative window were measured at the levels L1-L2 to L5-S1. Measurements revealed that the operative window in a systematic fashion becomes progressively smaller as the approach moves from L1-L2 toward L5-S1: 1) from L1 to L5, the medial boundary, the isthmus laminae, gradually extends farther laterally and eventually covers the waist of the respective vertebral body; 2) the lower boundary, the facet joint, gradually overlaps the disc space in an upward and lateral direction; 3) the upper boundary, the transverse process, gradually moves downward. Anatomic variations and abnormalities are found particularly often at the L5-S1 level. The anatomic findings led to important conclusions regarding the microsurgical approach to extraforaminal lumbar disc herniations; at levels L1-L2 to L3-L4, the midline approach with lateral retraction of the paraspinal muscles allows for efficient exposure of the lateral neural foramen and avoidance of trauma to the facet joint. Often at level L4-L5, and nearly always at level L5-S1, a tangential route through a paramedian transmuscular approach offers many advantages.