Investigation on Bending Strength Between Different Designs of Pedicle Screw to Obtain an Optimum Design

Pedicle screw is made from titanium alloy which gives support in the treatment of specific condition like lumbar, acute and chronic instabilities or deformities of thoracic, sacral spine. Mostly, the screw comes with rods act as connector, nuts, sleeves and plates. As a system, it increases the rigidity and holds the bone in placed while the bone heals itself over time. Over the years, many researchers have studied the pullout strength and the bending strength of pedicle screw. They found out that factors like core design, thread design, geometry and insertion techniques influenced the strength in pullout and bending. Failure of a screw is unavoidable. Sometime breakage happened at the neck or at the radial hole of pedicle screw due to high local stress. Thus, this project focused on the finding the most optimize diameter and position of radial in cylindrical screw to increase its bending strength. The pedicle screw inserted 40 mm into cylindrical block as a replacement for real human bone. It was then validated using Ansys software after gone through several simulation until the error become less than 10%. As a validated model, it was modified into pedicle screw with 2 radial holes varied diameter of 1.0 mm, 1.5 mm and 2.0 mm at 3 different position of radial hole. It was given bending moment of 1.29 Nm for starters and increase 0.3 Nm until the pedicle screw reached the yield stress at 790 Mpa. The head fixed at all directions and bending moment applied at tip of pedicle screw. As a result, high stress mostly distributed around the radial holes. All of the proposed designs able to withstand bending moment of 1.29 Nm–2.79 Nm while maintaining its original shape. Unlike the original pedicle screw, it can only resist bending moment at best 2.03 Nm before it broke. Finally, the most optimized diameter and position of radial hole was 1.0 mm at parallel positioned with improvement of 10.75% and 21.51% better than 1.5 mm and 2.0 mm respectively despite any position of radial hole.