چكيده به لاتين
Abstract:
Pedicle screws are medical implants that are implanted in spinal vertebrae posteriorly and generally are connected to a rod to form a structure that correct spinal deformity or improve its stability. In many spinal surgeries these screws are used for vertebrae stabilization and fixation. The biomechanical behavior of pedicle screws has been investigated in several studies. One of the most important parameters for pedicle screws is fatigue behavior under dynamic loads after surgery. Since many repetitive dynamic loads are applied on spine during walking, these loads also are applied on the pedicle screw located inside the vertebrae of the spine. In this study the goal is to investigate the effect of these loads on the fatigue behavior of screws.
In the current study, the effect of dynamic loads on the fatigue behavior and the impact of this fixation on the other segments were investigated by creating a three-dimensional model of the lumbar spine, with MIMICS 17 software, and fixation and placement of pedicle screws in the L3 and L4 vertebrae of lumbar spine, using the finite element method (FEM), with ABAQUS 2016. In this study vertebra was modeled with three cancellous, cortical and posterior bone as elastic material. Discs with two parts, annulus and nucleus, were modeled as viscoelastic material with prony series, ligaments as elastic material and implants as elastoplastic material. The effect of aging was considered for cancellous bone and disc. Fluctuating loads were applied on the model in four states: compression, flexion-extension, lateral bending and axial rotation. Three levels of compression load with maximum values of 250, 400 and 750 Nm and maximum to minimum load ratio of ten were applied on vertebrae as fluctuating. Flexion-extension loading was applied with 250 and 400 N of constant compression and moment of ± 7.5 N.m. Lateral bending loading was applied with 250 and 400 N of constant compression and moment of ± 7.8 N.m. And axial rotation loading was applied with 250 and 400 N of constant compression and torque of ± 5.5 Nm on the model. After loads applying, the fatigue life obtained via formulations for each loading. The effect of fusion on the other adjacent segments were investigated by evaluating intervertebral disc pressure variable.
Results showed that the compression loading with 75 – 750 N value, flexion-extension loading and lateral bending loading with 400 N constant pressure, lead to implant failure in less than five million cycles. In the other loadings, implants life was more than five million cycles. It was also shown that the aging have a negative impact on fatigue life, and implant life in higher grades is less than lower grades. The results of fusion effect on adjacent vertebrae showed that, in general fusion in compressive loading increases the pressure of the intervertebral disc in segments adjacent to implanted segment, and in flexion-extension loading, fusion increases the intervertebral disc pressure of segments above implanted segment and reduce the intervertebral disc pressure of segments below implanted segment.
As conclusion, fatigue life is different in different dynamic loading that body experiences and lateral bending loads has the greatest impact on fatigue life. Aging has negative impact on the fatigue life and fusion surgery in compression loading increase the intervertebral disc pressure of adjacent segments near the implanted segment.
Keywords: Pedicle Screw, Spinal Fusion Surgery, Fatigue, Finite Element Method.
