Comparison of the Design and Optimized Mechanical Performance of Variable Diameter Screws

Objective: To design a novel tapered screw for improving the modified cortical bone trajectory (MCBT) screw placement technique and to analyze the mechanical performance of optimized tapered lengths to provide a suitable tapered screw for the MCBT screw placement technique. Materials and Methods: The MCBT technique served as the basis of designing of a 40 mm long screw with a diameter of 5.5 mm at the proximal end and 4.0 mm at the distal end, and a taper length of 2 mm at both the proximal and distal joints. A four-factor, three-level orthogonal experiment was established with four parameters: taper position, thread depth, thread type, and pitch. The nine sets of screw parameters were modeled using SolidWorks software. Torsion, bending, and pullout forces were calculated for the designed screws using the finite element method. Analysis of variance (ANOVA) was performed on the results to determine the screw model based on the ANOVA results. Mechanical analysis of screw insertion was performed to determine the effect of taper length variation on insertion torque and to optimize the taper length of the existing taper screws. This optimization resulted in more stable screw insertion and reduced the risk of lumbar spine splitting. After the tapered screws were optimized, experiments were conducted using sheep lumbar vertebrae to compare the insertion torque and extraction force of the two tapered screw lengths. Results: The measured torque for the optimized tapered screw was 1.048 ± 0.16N·m, compared to 0.964 ± 0.189N·m for the pre-optimized tapered screw. The pullout force measurements were 1416.16 ± 212.47N for the optimized tapered screw and 1376.08 ± 236.62N for the pre-optimized tapered screw. Conclusion: The longer the tapered transition section of the tapered screw, the smoother the insertion process of the tapered screw. For the MCBT technique, when the posterior half of the screw remains the same, the diameter of the anterior half of the screw has minimal impact on pullout force and insertion torque results.