Modelling tri-cortical pedicle screw fixation in thoracic vertebrae under osteoporotic condition: A finite element analysis based on computed tomography

The technique of tri-cortical pedicle screw (TCPS) has been used to improve the anchoring strength in the sacral vertebrae. However, no studies have reported their application in the thoracic vertebrae. Our research is aimed to assess the stability and strength of the TCPS in thoracic vertebrae under osteoporotic condition by three dimensional (3D) finite element method on the basis of medical image reconstruction using computed tomography (CT), and verifying its effectiveness in clinical application. The 3D finite element models were constructed using Mimcs to transfer two dimensional CT images into 3D models by marching cubes algorithm of six-partition. Six physiological activities were simulated in 3D finite element models. Compared with the strength and stability of the uni-cortical pedicle screw (UCPS) and bi-cortical pedicle screw (BCPS), the effectiveness of TCPS was assessed. The stress distribution and maximum stress were measured to evaluate the strength. The maximum displacement and the range of motion were analysed to assessed the stability. Four geometrically accurate and nonlinear T7–T9 finite element models were constructed successfully by 3D finite element method based on the CT images. Three kinds of internal fixation methods in the osteoporotic thoracic vertebral body can improved the maximum stress, decrease the maximum displacement and range of motion in six physiological activities. The range of motion and maximum displacement of TCPS decreased more significantly than that of UCPS and BCPS. The maximum von Mises stress of TCPS was minimum and UCPS was maximum under the condition of extension, right lateral bending, left rotation and right rotation. Effectively, TCPS can provide better stability and strength than that of UCPS and BCPS techniques in the osteoporotic thoracic vertebrae. In practice, the technique of TCPS can be applied in the osteoporotic thoracic vertebral body to enhance the griping strength of the screws and reduce the risk of pedicle screw loosening. However, further cadaver experiments and more biomechanical analysis are necessary to confirmed our findings.