Anatomic study of the lumbar lamina for safe and effective placement of lumbar translaminar facet screws

OBJECTIVE: To evaluate the anatomic characteristics of the lumbar lamina and suggest a safe and effective strategy for setting lumbar translaminar facet screws. METHODS: The width and length of the lumbar lamina, screw path, lateral angle of the lamina, and maximum and minimum caudal angles of the lamina from L3 to L5 were measured with Mimics software using 32 patients’ computed tomographic data. RESULTS: The optimal screw entry point was located at the median of the spinous process base or slightly lower. The ideal screw trajectory was from the entry point to the base of the transverse process and across the center of the facet joint. A length of 35 to 45 mm was suitable for L3 to L4 in most cases, and a length of 45 to 50 mm was safe for L5 in most cases. The screw should be inserted at an angle of 49.4° to 59.29° laterally and 43.68° to 57.58° caudally at L3 to L5. For the ideal caudal angle, error of <3° was considered safe. CONCLUSION: The optimal entry point, ideal screw trajectory, ideal screw-setting angles, and safest range of the angle and length of the lumbar lamina were identified in this anatomical study.