A finite element study and mathematical modeling of lumbar pedicle screw along with various design parameters.

BACKGROUND: Lumbar pedicle screw is one of the most common and important elements in the field of lumbar surgery. It plays a great role in rectifying the spinal alignment and stabilization providing strength and stability to the affected area of spine. In spinal surgery, minimally invasive techniques and minor incisions are made which makes it less painful for the patients than the traditional methods. Moreover, the screws are not needed to be removed after the surgery which is yet another great advantage of the pedicle screw. METHOD: In this study, 3D Finite Element (FE) model of human L4 vertebrae is taken for analysis using image processing tool. Pedicle screw design with varying mechanical and geometrical properties has been carried out at different applied loads on it along with considering the effect of frictional forces between all contact surfaces. RESULT: Mathematical relationship among stress, strain, pitch of the screw and diameter have been developed for different thread profiles which will be beneficial for researchers for further development of pedicle screw implants. CONCLUSION: Results from the different analysis shows that bending stress on the screw for different loads at triangular pitch is higher than the trapezoidal. Hence, trapezoidal thread is efficacious than triangular thread. In case of vertebral bone, the magnitude of stress is less for trapezoidal screw than triangular and stress has a linear relationship with pitch length. In term of strain, triangular thread develops more strain than trapezoidal thread. A set of mathematical relation has been developed for different thread profile based on pitch length, stress and strain which gives the idea about von Mises stress and strain.

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.

Effect of single and multilevel artificial inter-vertebral disc replacement in lumbar spine: A finite element study.

Degenerative disc disease (DDD) in lumbar spine is one of the major musculoskeletal disorders that cause low back pain (LBP). The intervertebral disc structure and dynamics of the lumbar spine are significantly affected by lumbar DDD, leading to a reduced range of motion (ROM), muscle weakness and gradual degradation. Spinal fusion and inter-vertebral disc replacement prostheses are two major surgical methods used for treating lumbar DDD. The aim of this present study is to examine biomechanical impacts of single level (L3-L4 and L4-L5) and multi level (L3-L4-L5) inter-vertebral disc replacement in lumbar spine (L2-L5) and to compare the performance with intact spine. Finite element (FE) analysis has been used to compare the mobility and stress distribution of all the models for four physiological movements, namely flexion, extension, left and right lateral bending under 6, 8 and 10 Nm moments. Spinal fusion implants completely restrict the motion of the implanted segment and increase disc stress at the adjacent levels. In contrast to that, the results single level ADR models showed closer ROM and disc stress to natural model. At the spinal segments adjacent to the implantation, single level ADR shows lower chance of disc degeneration. However, significantly increased ROM was observed in case of double level ADR.