A comparative finite element analysis of artificial intervertebral disc replacement and pedicle screw fixation of the lumbar spine

Titanium alloy-based Pedicle screw-rod fusion is a very common technique to provide higher fusion regularity than other methods. In recent times, Carbon-fibre-reinforced (CFR)-PEEK rod is used to better reduce the fusion rate. Alternatively, total disc replacement (TDR) is also very common for the non-fusion treatment method for degenerative disc disease (DDD). This study aims to investigate flexibility (ROM), stability, stress condition in implant, implant adjacent bone of the implanted lumbar spine during different physiological movements and loading environments. The finite element (FE) intact model of the lumbar spine (L2-L5) with two-level pedicle screw-rod fusion at L3-L4-L5 and two-level artificial disc replacement was developed. CFR-PEEK was taken for rod for semi-rigid fusion. UHMWPE was taken as core part of the artificial disc. The FE models were simulated under 6, 8 and 10 Nm moments in left right lateral bending, flexion and extension movements. The total ROM was reduced for two-level pedicle screw fixation and increased for the artificial disc replacement model during flexion extension compared to the intact spine. The total ROM was reduced by around 54% and 25% for two-level fixation and increased by 30% and 19.5% for artificial disc replacement spine, under flexion-extension and left-right lateral bending respectively. For screw fixation, the ROM increased by 15% and 18% reduced by 4.5% and 14% for disc replacement at the adjacent segments for flexion-extension and left-right lateral bending.

BIOMECHANICAL PERFORMANCE OF A MODIFIED DESIGN OF DYNAMIC CERVICAL IMPLANT COMPARED TO CONVENTIONAL BALL AND SOCKET DESIGN OF AN ARTIFICIAL INTERVERTEBRAL DISC IMPLANT: A FINITE ELEMENT STUDY

Most of the implants used for total disc replacement (TDR) surgery are designed as a ball and socket pair aimed at providing a three-dimensional unconstrained motion. However, one of the major concerns with ball and socket design is the wear of the implant which limits its life. In this study the biomechanical performance of two types of implant designs is compared — a conventional ball and socket type (Prodisc-C) and a modified design of dynamic cervical implant (DCI) using FE analysis. A 3-dimensional geometrical model of cervical spine (C1–T1) was developed using CT scan data of a middle-aged healthy male. Subsequently, using FE analysis, the ROM values were validated with the existing literature using a compressive load in combination with different physiological motions of the neck. Furthermore, FE analysis on the two implants, fitted at C5–C6 segment, showed a significant increase in the ROM of implanted segment using Prodisc and decrease in the ROM of inferior segment, but modified-DCI restored the motion of the implanted and adjacent segments. Analysis of average bone strains adjacent to the implant showed a possibility of stress shielding for Prodisc. However, higher stress distribution on the modified-DCI limited its clinical use.