Design,Fabrication And Performance Evaluation Of Bioinspired Intervertebral Disc

Lumbar back pain has become a global health problem,which increases national medical expenditure and brings heavy economic burden to patients and families.Low back pain involves many factors,and its causes have not been fully analyzed,but it is widely believed in clinical practice that biological intervertebral disc(IVD)degeneration is one of the key influencing factors.Artificial lumbar total disc replacement(L-TDR),as an alternative clinical treatment to fusion,can effectively restore the range of motion(ROM)of the patient’s damaged segment and effectively maintaining the intervertebral height of the vertebral segment.However,the existing LTDR devices with homogeneous stiffness in all directions are usually composed of mechanical joints and a single material.As a result,they do not match the threedimensional physiological motion of the patient’s spine after implantation and are prone to fatigue failure,causing many complications in clinical practice.Complications need to be urgently addressed.Based on the inspiration of the fiber interweaving structure and matrix hierarchical gradient characteristics of biological IVDs,this paper proposes a bioinspired design strategy with variable stiffness and fatigue resistance to reproduce the biomechanical functional characteristics of the variable stiffness and fatigue resistance of biological IVDs.The bioinspired variable stiffness IVD(BIVD-L)and bioinspired fatigue resistance IVD were respectively fabricated based on human lumbar spinal design using additive manufacturing technology.In addition,variable stiffness functional and fatigue resistance performance experimental tests were conducted.Therefore,the main contents and conclusions in this paper are as follows:(1)Human lumbar spine anatomical structure model reconstruction and structural design of the BIVD-L with variable stiffness.Based on CT medical images,the vertebrae and biological IVD models were reconstructed using reverse engineering technology;based on the fibrous interweaving structure and hierarchical gradient characteristics of biological IVD,the BIVD-L structure was designed;the proposed BIVD-L reproduces the fiber interweaving structure and hierarchical gradient characteristics of biological IVD;the structural parameters of BIVD-L are determined based on the size parameters of biological IVDs;(2)Material testing and establishment of finite element model.Biocompatibility tests and compression mechanical tests of the selected materials were carried out,combined with the reconstructed vertebral model of the human lumbar spine,the finite element models of the vertebrae,ligaments and BIVD-L were established;the relationship between load and displacement is determined by finite element analysis,comparison with biological IVDs showed that the simulation results have similar variable stiffness characteristics to biological IVDs;(3)BIVD-L physical sample fabrication and test platform construction and performance testing.In order to evaluate the mechanical properties of BIVD-L,a Stratasys J850 multi-material 3D printer was used to manufacture a physical sample of BIVD-L;at the same time,polyvinyl chloride(PVC)was used to prepare L4 and L5 vertebrae and an in vitro test model was constituted of BIVD-Lwith L4 and L5 vertebrae;to verify the variable stiffness characteristics of the explored BIVD-L,a 7-DOF KUKA robot system was used to simulate the physiological motion of the human lumbar spine,and a variable stiffness test device was developed and established;to further study the influence of structural parameters and material properties on stiffness,three types of physical samples with different fiber orientation angles,different lamellar harnesses and different disc heights were printed using a Stratasys J850 printer;the fatigue test results of TA Instruments showed that after 100000 cycles,the in vitro test model did not undergo any fracture;compared to the existing L-TDR prosthesis,the developed BIVD-L has characteristics similar to that of the biological IVD with fiber interweaving structure and multi-material coupling,achieving the function of variable stiffness;in addition,the variable stiffness mechanism of biological IVD is analyzed in this article,which is not available in current research work.(4)Structural design and finite element analysis of bioinspired fatigue resistant IVDs.Based on the structure-material-function of biological IVD,a design method with fatigue resistance is proposed to solve the fatigue failure difficult problem of LTDR prosthesis in clinical practice;two structures(parallel and crossed),as well as a control group without fiber structure were established based on the characteristics of fiber interweaving structure and multi-material coupling of biological IVDs;the mechanical properties of the three structures in single-material and composite conditions were evaluated by tensile testing;the optimal parameter combination of the structural model was determined by constructing the finite element model of the bioinspired fatigue resistant IVD;based on the finite element results of the optimal size parameter combination of the bioinspired fatigue-resistant IVD,its fatigue performance was further analyzed using Fe-safe,which provides good guidance for fatigue testing;(5)Physical sample preparation and fatigue test of bioinspired fatigue resistant IVD.Based on thermoplastic polyurethane(TPU),polylactic acid(PLA),polyvinyl alcohol(PVA)and polyetheretherketone(PEEK)materials with good biocompatibility,the method of the bioinspired fatigue resistant IVD was prepared using FDM printing technology combined with pouring technology was proposed;the fatigue performance test of the physical sample of the bioinspired fatigue resistant IVD was realized by using the TA Instruments dynamic test system;the results show that the physical sample of the proposed bioinspired fatigue resistant IVD in this paper did not fail after 10 million cycles.The overall height had not changed significantly,and the developed bioinspired fatigue resistant IVD can effectively maintain the intervertebral height of the patient’s damaged segment;compared to 3DF prosthesis and e Disc prosthesis,the fatigue life is extended by one million times and three million times respectively,achieving the 10 million times fatigue life standard of the Mo M type prosthesis.