Comparative strength analysis of pedicle screw implants using biomechanical tests and finite element analysis

Spinal implants or instrumentation have been used for many years to re-align and stabilize unstable or deformed spines. The use of pedicle screws and spinal rods is one method of fusing the vertebrae in the spine. A pedicle screw is a particular bone screw designed for implementation into a vertebral pedicle. These screws are used to correct a deformity or treat trauma and to immobilize part of the spine to assist fusion by holding vertebrae together. Numerous variations in design have been developed for pedicle screws. These variations affect the load carload-carrying capacity of the pedicle screw system and thus the forces acting on the vertebral column. The Finite Element Method has grown into an important method in biomechanics and biomedical engineering to obtain approximate numerical solutions that predict the response of physical systems subjected to external influences. The objective of the present paper is to compare the load-carrying capacities of two types of pedicle screws commonly used for spinal fusion, namely monoaxial screws and polyaxial screws, using experimental testing, which includes Anterior-Posterior and Flexion-Extension tests according to the ASTM standards and to study the failure of the pedicle subassembly. Moreover, a Finite element model was developed as exact as the experimental environment by adjusting the boundary conditions, contact, and loading parameters. Once the testing and finite element results are studied, the model opens up ways to improve the strength and load-carrying capacity of the pedicle screw.