Anatomic parameters of the isthmus from L1 to L5 were measured in 30 dried lumbar spines. All measured parameters were fairly constant from L1 to L5. The mean values of the core length, thickness of the superior and inferior borders of the isthmus, superoinferior diameter and posterior and medial inclinations of the lumbar isthmus at L5 were 39.9 +/- 2.3 mm, 2.0 +/- 0.9 mm, 8.9 +/- 1.0 mm, 13.2 +/- 2.5 mm, 35.9 degrees +/- 5.7 degrees and 31.8 degrees +/- 6.3 degrees, respectively. This study shows that a 40 mm long, 4 to 5 mm diameter screw should be inserted in the lumbar vertebral isthmus at an angle of 30 degrees laterally and anteriorly.