Study on anatomical parameters of modified cortical bone trajectories in the lower lumbar vertebrae

Modified Cortical Bone Trajectory (MCBT) technology offers advantages over Cortical Bone Trajectory (CBT) technology in anatomy, biomechanics, and therapeutic treatment. Currently, there is a lack of research that has comprehensively measured the MCBT screw trajectory parameter in the lower lumbar spine, which hinders its clinical use.

There are 60 lumbar spine CT images chosen, with an even split of 30 males and 30 females. The study measures the MCBT screw trajectory parameters, including entry angle α, screw trajectory lengths and diameters, and lateral and cephalic angle ranges, then compares measured parameters according to the sexes, and vertebrae. After measuring, the MCBT screws are inserted into the cadaveric specimen based on the measured data to evaluate the clinical feasibility of the measurement results by assessing the accuracy of screw placement and the amount of screw contact with cortical bone layers.

No significant variations are seen in the screw trajectories between the left and right sides of each vertebra. For males, the combined MCBT screw trajectory lengths in L4 and L5 vertebrae are 45.26 ± 1.26 mm and 44.61 ± 1.25 mm, with combined screw trajectory diameters of 9.46 ± 1.09 mm and 11.50 ± 1.19 mm. The ideal screw cephalic angulations in L4,5 vertebrae are 19.93°±2.54° and 20.05°±2.72°, while the ideal lateral angulations are 22.70°±4.25° and 23.53°±3.74°. The entry angles α in L4,5 vertebrae are 123.55°±4.13° and 121.07 ± 4.40°. In females, the combined MCBT screw lengths in L4 and L5 vertebrae are 43.02 ± 1.27 mm and 41.83 ± 1.94 mm, with the combined screw diameters of 7.86 ± 0.80 mm and 9.43 ± 0.98 mm. The ideal screw cephalic angulations in L4,5 vertebrae are 17.56°±2.37° and 17.83°±3.42°, while the ideal lateral angulations are 18.86°±2.89° and 18.98°±3.92°. The entry angles α in L4,5 vertebrae are 125.60°±3.13° and 125.15 ± 2.80°. Within different vertebrae in the one sex, the combined MCBT screw trajectory length and diameter in L5 vertebra are greater than in L4, with statistical differences. Within different sexes in the same vertebra, the MCBT screw combined trajectory length and diameter, cephalic and lateral angulation ranges, and entry angle α of males are greater than females, with statistical differences. Out of the 24 MCBT screws implanted in cadaveric specimens, 22 are Grade A, and 2 are Grade B, which are all clinically acceptable screws. Out of the 24 screws, 12 are in touch with 5 layers of cortical bone, 11 are in contact with 4 layers, and 1 is in contact with 3 layers. The implanted MCBT screws showed excellent precision and adequate contact with cortical bone layers according to the measurement data.

The study first measures the MCBT screw trajectory parameters in L4 and L5 vertebrae, and then validates the measured results through a cadaver specimen screw placement experiment, laying a strong groundwork for the clinical application of MCBT technology in conditions such as adjacent segment degeneration and degenerative spinal diseases.