Simulation-based assessment for biomechanical behavior of scoliotic human thoracolumbar spine

Date

2018-12

Authors

Journal Title

Journal ISSN

Volume Title

Publisher

Abstract

Finite element analysis (FEA) is a valuable tool in investigating the biomechanics of human spine and the interaction between spine and medical device. Compared with experimental tests, FEA has lower cost and higher efficiency. FEA is also capable of capturing the biomechanical parameters internal to the bones and connective soft tissues in spine, which is difficult to measure by experimental approaches. In this dissertation, a novel FE modeling method of human thoracolumbar spine was first introduced and validated against experimental data. The validated spine FE models can be utilized to study the spinal biomechanics and the interaction between the spinal implant and spinal tissues in the following aspects: 1) Comparisons of pre- and post-operative scoliotic spines were made by investigating different biomechanical characteristics of ten patients. Simulation results showed that spine fusion surgery can improve the balance and stability of the spine and reduce the scoliotic deformation of the spine; 2) ten models of pedicle screws were generated to represent the commonly-utilized pedicle screw currently available in the market. Three FE modeling methods of screw/bone interaction were compared in pullout test and physiological spinal loading environment. Results showed that simplified screw model was able to accurately predict the screw/bone interaction force but not able to predict accurate von Mises stress value of the screw and bone compared to non-simplified (with thread) screw model; 3) Five healthy spines, and five scoliotic spines in both their pre-surgical and post-surgical conditions were tested under cyclic loads. The vibrational characteristics of healthy spine and scoliotic spine were compared. FEA results showed that untreated scoliotic spine is more sensitive to the vibration than spinal segments with normal anatomies, especially at the apical vertebrae. Further deformation is likely to be developed under long-term exposure to whole-body vibrations (WBV) environment. The spine fusion surgery is able to make the scoliotic spines less sensitive to the WBV at the fused level; 4) six different surgical methods of spine fusion surgery were simulated using FEA. The von Mises stress distribution in the posterior fusion fixation, spinal range of motion (ROM), and the screw-bone interaction force obtained from various fixation methods of short-segment spine SA under physiological spinal loading environment were investigated using a FE spine model. Fusing more spine levels, inserting more PS and adding interbody cage might help reduce the rod stress, screw force, and ROM of the fused spine. However, the fact that inserting more screws also increased the stress concentration points on the rods should also be considered; and 5) a FEA-based approach to predict the fatigue life of the spinal implants under physiological spinal loading environment was introduced. One conical PS and one cylindrical PS were inserted into spine FE model and tested under spinal loads. The implant stress was calculated by FEA in one loading cycle and the Smith-Watson-Topper (SWT) equation was utilized to predict the fatigue life of the implant under cyclic loads. The conical PS has longer predicted fatigue life than cylindrical PS did.

Description

Keywords

Human Spine, Biomechanics, Spinal Implants, Scoliosis, Spine Fusion Surgery, Finite Element Analysis, Whole-Body Vibration, Surgical Planning, Fatigue Life

Citation