Intervertebral disc removal is a common method for treating lumbar intervertebral disc protrusion. Symptom of most of patients can be improved obviously, and the short-term therapeutic effect is satisfactory, but the long-term effect is poor due to narrow intervertebral space and stress changes after intervertebral disc removal[3-4].
Spinal fusion and interbody cage technique are applied in clinic in order to reconstruct vertebral stability and correct abnormal load bearing mode. Though the spinal fusion and interbody cage system can recover the stability of anterior column and center pillar, they have been proved to increase the compensatory moment of adjacent segment, result in stress concentration, accelerate intervertebral disc degeneration, increase articuli intervertebrales stress, ultimately, lead to spinal stenosis, degenerative spondylolisthesis or pseudoarticulation formation, which affect the long-term therapeutic effect.
Zygapophysial joints have significant biomechanical values, which are the important structures in maintaining stability of lumbar motion segments, and play an critical role in stability, anti-torsion and weight bearing of spine. For a single motion segment, intervertebral disc and two zygapophysial joints construct a stable triangle at the horizontal plane, which form a three prismatic structure at the longitudinal direction. All of these formed a combo and can maintain the spinal stability. If one joint was abnormal, it would affect the others, and the sever abnormal would lead to corresponding clinical symptoms[5-6]. For example, intervertebral space turned to be narrow and secondary lumbar instability, spinal stenosis, or degenerative arthritis could be found in almost all the patients after resection of nucleus pulposus[7].
With the development of biomechanics and biomaterials, artificial lumbar disc replacement has been gradually used in clinic. The replacement of artificial lumbar disc can not only eliminate disease symptoms, but also maintain the physiological nature of lumbar motion segments. SB-Chaite III intervertebral disc endoprosthesis (Germany) are widely used in clinic[8-9]. Preliminary biomechanical studies demonstrated that the range of motion of 3-D lumbar segment was well after artificial lumbar disc replacement, and there were no significant differences between the replaced disc and normal segment. The compression stiffness of motion segment was greater than that after resection of nucleus pulposus, artificial lumbar disc may eliminate the damage of intervertebral disc on spinal biomechanics, and play its role as natural intervertebral disc[10-13]. However, factors such as the hard of stress measurement in clinic after artificial lumbar disc replacement, the difficulty in specimen acquisition, and high price of artificial lumbar disc, lead to difficulty in verify the biomechanical effects of artificial lumbar disc replacement on lumbar. Finite element analysis can solve all of these problems.
Finite element analysis can stimulate and alter the changes of material and structure, reflect local and internal interactions of subjects in experiment. The most prominent characteristics of which is that can simulate biomechanical behavior of living tissues, and are characterized by repeated uses, time saving, low cost, and wide application, thus, it is an ideal method for studying the biomechanics of artificial lumbar disc replacement. Finite element analysis has been widely used in simulating spinal biomechanics, and the obtained results were identical with solid model, which proved the effectiveness of finite element analysis[14-16]. However, studies concerning biomechanics of artificial lumbar disc are insufficient. Present research showed the range of motion of lumbar segment would be increased after artificial lumbar disc replacement[17].
Here, a 3-D finite element model of L4-5 lumbar motion segments artificial lumbar disc replacement was successfully constructed. The results of direction loads application showed: ①Under axial load, the stress of zygapophyseal joint was increased with load increasing, but, there were no significant differences between the zygapophyseal joint and normal segment (P > 0.01). ②The stress of zygapophyseal joint was decreased with load increasing under forward flexion moment, which changed to bear load when the forward torque was great. However, there were no significant differences between the zygapophyseal joint and normal segment (P > 0.01). ③The stress of zygapophyseal joint was increased with load increasing under posterior extension moment. However, there were no significant differences between the zygapophyseal joint and normal segment (P > 0.01). ④The ipsilateral zygapophyseal joint bear pressure under lateral bending moment, which was increased with load increasing; in contract, the contralateral zygapophyseal joint bear stress, which increased with loads decreasing, and changed to bear tension when the lateral bending torque was great. However, there were no significant differences between the zygapophyseal joint and normal segment (P > 0.01). The results demonstrated that there were no significant stress differences between the bilateral zygapophyseal joint and normal segment after artificial lumbar disc replacement under different loads. Artificial lumbar disc replacement can keep the stress of motion segment in normal level, which can meet the needs of spinal functional
