Biomechanical Study of Fused Lumbar Spine Comparing Ti and PEEK as Connecting Rod Considering Bone Degeneracy Using FEA

Nowadays, orthopedic fixation devices are used in treating spine diseases, but Osteoporosis is one of the most usual problems for lumbar fusion surgery with pedicle screws. Among the elderly people in the world, osteoporosis has become a common disease, which is a concern for spine surgeons, because a large number of the patients who need lumbar interbody fusion surgery are older people with osteoporosis. In this study, a finite element model was created from CT scan images of a healthy lumbar spine area of a 15-year-old teenager. This model includes three lumbar vertebrae: L2, L3 and L4. More precisely, the 3D model entailed three lumbar vertebrae, two intervertebral discs and the implant system comprises 4 pedicle screws with the precise geometry of threads and two types of titanium and polyetheretherketone (PEEK) connecting rod. The boundary conditions and loading were exerted on the model. Considering the applied loads and leaning moment on the model, stress dispensation was appraised on intervertebral discs and the whole model with implant systems entailed titanium or PEEK rods. To repute a decrease in bone quality, the bones are divided into four categories i.e., weak, normal, strong, and very strong. The derived finite element model was applied to multiple axial pressing loads (270 and 510 N). Also, a moment of 10nm was exerted for the behaviors of flexion, extension, transversal leaning, and torsion at the upside face of the L2, whereas the bottom face of the L4 settled motionless. ‘Abaqus CAE 6.14-1’ software was utilized to simulate the model with various fixed mechanisms. An inductive study among multiple fixation implant systems indicated that stress distribution amounts with the PEEK-based implant fixation system decreased over time with increasing osteoporosis. Likewise, lower equipollent pressure amounts were recorded with the PEEK-based implant fixation system, particularly in the whole model for cases with poor bone quality. In addition, both implant systems authorize to reduce the throughout loading stress in the entire spine model. Finally, it was concluded that the PEEK-based implant system significantly reduces the loading on the whole model and besides, appears as a preferable choice in reducing pressure and load multiplexing for cases with poor bone quality compared to the titanium implant system.