Objective A novel variable-diameter cortical threaded screw used in a modified cortical bone trajectory(MCBT)was designed to verify its mechanical properties using the MCBT technique.Methods According to MCBT technology,the screw pitch was fixed at 2 mm,the total length was 45 mm,the diameter of the thick rod was 5.5 mm,the diameter of the thin rod was 4.0-4.5 mm,and the length of variable-diameter position connecting the thick rod and the thin rod was 2 mm.The parameters were set based on three aspects:variable-diameter position,thread depth,and thread type.Three-factor and three-level L9 tests were conducted and screw models were established.The torsion and the bending and pull-out force of the designed screws were calculated based on the finite element method,the results were analyzed using range analysis,and then the screw models were determined.The three-dimensional(3D)model of L4 vertebral body in osteoporosis specimens was established and screws were placed according to the MCBT technique.The pull-out force of the novel variable-diameter cortical threaded screw was compared with that of a conventional non-variable-diameter cortical threaded screw.Results Range analysis showed that screw No.6(variable-diameter position:24 mm from the screw head,thread depth:0.7 mm,45° symmetrical thread)was the optimal screw.The anti-pull-out force of the No.6 variable-diameter cortical threaded screw was 13.1%higher than that of the 4.5 mm conventional non-variable-diameter cortical threaded screw,and no statistical difference in anti-pull-out force was found between the No.6 variable-diameter cortical threaded screw and the 5.5 mm conventional non-variable-diameter cortical threaded screw.Conclusions The variable-diameter position has the smallest influence on pull-out force of the screw,the thread type has the largest influence on pull-out force,and the thread depth has the largest influence on torsion and bending.Compared with that of the conventional non-variable-diameter cortical threaded screw,the variable-diameter cortical threaded screw had a smaller front end,which prevented splitting at the entrance point of the screw.The screw has a large diameter at rear end,thereby showing improved pull-out performance.The results provide a new theoretical basis for the clinical application of MCBT technology.