Mechanical performance of standard and cannulated 4.0-mm cancellous bone screws

The mechanical performance of bone screws is determined by their pull-out s trength (holding power), compressive force, stripping torque, yield bending moment, ultimate bending moment, and fatigue strength, These parameters ar e related to the parameters of the screw design, including major thread dia meter, minor thread diameter, thread length, pitch, shaft diameter, cannula tion diameter, and material properties. The goal of the study was to theore tically predict the static performance of five 4.0-mm, 45-46-mm-long, cance llous, partially threaded standard and cannulated bone screws and compare t he predictions with experimental measurements. A secondary goal was to dete rmine if cannulation of the bone screw diminished its mechanical performanc e. The predicted values for pull-out force, compressive force, and strippin g torque were determined by the thread length, major thread diameter, and t hread shape factor. The screws with the largest major thread diameter and l ongest thread length had the greatest pull-out force, compressive strength, and stripping torque. However, when correcting for the thread length, a hi gher thread shape factor compensated for a smaller major diameter. The coef ficient of determination (r(2)) for the correlation between the predicted a nd measured pull-out force improved from 0.75 to 0.90 when the theoretical model included the thread shape factor. The yield and ultimate bending mome nts are a function of the section modulus and material properties of the sc rew. The Ace solid screw had the greatest section modulus and yield and ult imate bending moments. The experimental data support the theoretical models for predicting the mechanical performance of bone screws. The design of th e bone screws can be optimized on the basis of theoretical modeling. The st rong correlation between the predicted and measured parameters allows compa rison between bone screws without repeated experimental tests. Theoretical and experimental results show that cannulation of the bone screw did not in herently diminish its mechanical performance.