MICROSURGICAL ANATOMY OF THE LATERAL APPROACH TO EXTRAFORAMINAL LUMBAR DISC HERNIATIONS

OBJECTIVE: During the ''lateral'' approach to extraforaminal lumbar di sc herniations, the surgeon may be confronted with considerable variat ions in anatomy, making this approach extremely difficult in some pati ents. An anatomic study, therefore, was undertaken to examine the bony boundaries of the operative target, the medial intertransverse space. METHODS: In 31 lumbar spine specimens taken from cadavers of people w ho had been between 30 and 93 years old at death, the relevant distanc es and proportions of the operative window were measured at the levels L1-L2 to L5-S1. RESULTS: Measurements revealed that the operative win dow in a systematic fashion becomes progressively smaller as the appro ach moves from L1-L2 toward L5-S1: 1) from L1 to L5, the medial bounda ry, the isthmus laminae, gradually extends farther laterally and event ually covers the waist of the respective vertebral body; 2) the lower boundary, the facet joint, gradually overlaps the disc: space in an up ward and lateral direction; 3) the upper boundary, the transverse proc ess, gradually moves downward. Anatomic variations and abnormalities a re found particularly often at the L5-S1 level. CONCLUSION: The anatom ic findings led to important conclusions regarding the microsurgical a pproach 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 an d avoidance of trauma to the facet joint. Often at level L4-L5, and ne arly always at level L5-S1, a tangential route through a paramedian tr ansmuscular approach offers many advantages.