Anatomic study on lumbar cortical bone trajectory of adults

Objective To determine anatomic parameters related to the surgery of lumbar cortical bone trajectory of normal adults and the feasibility of screw application in cortical bone and its specification and dimension. Methods Lumbar 3D im-age data of 80 adults between 18 and 40 years old (40 for each gender) were randomly selected as subjects of our research. With the help of the imaging software for 3D interactive viewing, the ideal starting point was supposed to be the junction of the center of the superior articular process and 1 mm below the inferior border of the transverse process. The diameter, length, lateral angle to the vertebral horizontal plane, cephalad angle to the vertebral sagittal plane of the trajectory and the horizontal distance from the insertion point to the lateral edge of the vertebral plate were measured. Differences of anatomic parameters for each gender, side and segment were analyzed. Results Differences of anatomic parameters on both sides of each segment had no statistical significance. At the ideal trajectory, the mean screw length was 37.56±2.41 mm at L1, 38.72±2.36 mm at L2, 39.51±2.51 mm at L3, 39.78±2.87 mm at L4 and 38.83±2.74 mm at L5. The mean screw diameters from L1 to L5 were 6.04±1.23 mm, 6.17±1.24 mm, 7.15±1.22 mm, 8.02±1.41 mm and 8.68±1.42 mm respectively. However, differences of ideal entry angle of L1 to L5 had no statistical significance. The mean lateral angle from L1 to L5 were 8.46°±2.11°, 9.37°±2.84°, 9.62°±2.16°, 9.53°±1.98°, 9.04°±1.97°, while the mean cephalad angle to the vertebral sagittal plane from L1 to L5 were 26.49°±4.97°, 25.94°±4.56°, 26.42°±4.42°, 26.29°±3.48°, 26.89°±3.69°. The mean distance from the insertion point to the lateral edge of the vertebral plate gradually increased from L1 to L5, which were 1.19±0.75 mm, 1.54±1.08 mm, 2.01±1.45 mm, 3.49±1.52 mm, 4.47±1.32 mm respectively. The screw diameters of each segment for men were greater than those for women. Conclusion The common length of screw for lumbar cortical bone trajectory of normal adults might be from 35 mm to 40 mm. The safe upper limits of the screw diameter were 5.5 mm at L1, 5.5-6.0 mm at L2, 6.5-7.0 mm at L3, 7.5 mm at L4 and 8 mm at L5. The average lateral angle of all 5 lumbar segments was 9.20°±2.11° and the average cephalad angle was about 26.41°±4.22°. Key words: Lumbar vertebrae; Bone screws; Tomography, X-ray computed