The Research On The Ti-based Integrated Artificial Cervial Disc With Full Range Of Motion

In vivo, wearing fragments are more prone to be produced in ball-and-socket joint artificial cervical intervertebral discs, therefore resulting into inflammation; the over-mobility phenomenon is frequently occurred in non-limitation type artificial cervical intervertebral discs at the operative site, which leads to an increase of ligament stress and strain and accelerate the degeneration of adjacent soft tissue; as well as conventional dynamic cervical Implants(DCI) exhibits the high stress distribution, low range of motion(ROM), poor immediate fixedness. According to above mentioned disadvantages of existing artificial cervical intervertebral discs, two noval DCI-typeed intervertebral disc implants(modified DCI, MDCI) were designed based on conventional DCI and biomimetic optimization. The mechanical property and fatigue property of new MDCI were studied by static structural simulations, fatigue simulations and fatigue experiments in order to present the optimized design.By establishing a series of DCI within C5-C6 cervical spinal segments, the biomechanical finite element analysis for DCI were carried out to investigate the influence of materials, the width and thickness of DCI's curved section on their equivalent stress and ROM. The results show that titanium alloys with low elastic modulus and high strength are more suitable for MDCI; the width of MDCI's curved section has more obvious influence on the ROM of lateral bending and torsion, in comparison with the thickness of MDCI's curved section. By optimizing the width, thickness or cross-sectional shape of MDCI's curved section, MDCI meets the requirements for cervical segmental ROM in flexion, extension, lateral bending and axial torsion movements as far as possible, on the basis of meeting the required fatigue life. Two new-type initial design schemes of MDCI, corner-cutting and trapezoid cross-section of curved section, were proposed.The ROMs of the MDCI with corner-cutting design were increased greatly by increasing cutting angle. On meeting the fatigue life, the corner-cutting MDCI could achieve 100%, 100%, 99% and 100% of normal ROM in flexion, extension, lateral bending and axial torsion movements, respectively. The ROMs of MDCI with the trapezoid cross-section of curved section were increased greatly by removal of the redundant thickness in concave arc shape. On the basis of meeting the fatigue life, the MDCI with the trapezoid cross-section of curved section could achieve 77.6%, 100%, 100% and 77.9% of normal ROM in flexion, extension, lateral bending and axial torsion movements, respectively.To obtain a bimimetic fatigue-testing fixture for MDCI equivalenct to the human C5-C6 cervical spinal segments, the influence of the geometric parameters and materials of fixture on MDCI's equivalent stress and deformations were calculated and discussed. The results show that other factors being equal, the maximum equivalent stress and deformation decreases markedly with the increase of the elastic modulus of material, the thickness and width of the U-plate, respectively; whereas, other geometric parameters lack of the obvious influence on the MDCI's equivalent stress and maximum deformation. Therefore, the material and sizes of the U-plate were given the especial priority in optimization; the sizes of the cylindrical blocks are determined according to cervical vertebrae, while the distance between the center of the cylindrical blocks and the rear end of the U-plate should be evaluated based on that between intervertebral disc and ligaments.The fatigue simulations and experiments were carried out through the biomimetic fatigue-testing fixture, and fatigue fractures were analyzed as well. According to the fatigue simulation and experiment, the simulated and experimental fatigue lives of MDCI are 53.6 and 61.73 million times, respectively, of which the thickness of the curved section is 1.0mm, cutting angle is 25°, and distance between cutting angle and end face of bolt hole is 2.0mm(minimum width of the curved section is 8.3mm.). The the simulated and experimental fatigue lives of MDCI are 23.14 and 18.6 million times, respectively, of which the thickness of the curved section is 1.0mm, cutting angle is 25°, and distance between cutting angle and end face of bolt hole is 1.5mm(minimum width of the curved section is 7.8mm.). The fatigue lives of other corner-cutting MDCIs are more than 80 million times.By analysis of fatigue fracture, it was found that the cracks initiated on the outer surface of curved section, which was identical to the site of the maximum equivalent stress in the static structural simulation and minimum life in the fatigue simulation of MDCI. Therefore, the optimized structure of the corner-cutting MDCI is that with the thickness of the curved section at 1.0mm, cutting angle at 20°, and distance between cutting angle and end face of bolt hole at 1.5mm(minimum width of the curved section at 10 mm.), which could achieve 100%, 100%, 99% and 100% of normal ROM in flexion, extension, lateral bending and axial torsion movements, respectively.