| Background and purpose:Degenerative lumbar spine disease is common clinical phenomenon, at present more attention for it. Traditional treatment is discectomy and vertebral fusion, these two operations have given play to strong points in a certain period of time. But the discectomy disadvantage is intervertebral height loss, then it leads to lumbar verterbral instability and spinal stenosis. Although the vertebral fusion can resolve the loss of intervertebral height and the occurrence of instability, but the fusion can lead to lose the movement function of intervertebral joint, and accelerate adjacent segments degeneration. With the idea of spine functional reconstruction deepened and the long-term observation of above-mentioned postoperative complications, the prosthetic nucleus pulposus technology develops by leaps and bounds. It includes pre-fabricated and injectable prosthetic nucleus pulposus. The current research and clinical application is mainly pre-fabricated prosthetic nucleus pulposus. It is pre-made prosthetic nucleus pulposus in vitro, the core part is the hydrophilic gel. It can absorb water to expand after be implanted to intervertebral space, thereby restored vertebral height. However, in order to place the prosthesis, it need to remove more annulus fibrosus. Then it will be a greater damage of the annulus fibrosus. There is the risk of prosthesis extrusion. Secondly, as a result of pre-fabricated prosthesis has a fixed form, it can't fit for the space after discectomy. Thirdly, the stress from the adoption of water can not be controlled. It can prone to damage cartilage endplate. The injectable prosthetic nucleus pulposus is an elastomer by liquid monomer polymerized after injected into the space of post- discectomy. It avoids the pre-fabricated prosthesis shortcomings, but the liquid monomer can leak before the polymerization. Another there is a large number of heat in polymerization process, it can damage the tissue. In order to avoid the above shortcomings, we develop a new type of injectable artificial nucleus. By the use of the capsule, it effectively prevent the leakage of the liquid monomer. By the use of new material, the polymerization temperature can be controlled at 37℃to avoid thermal damage for the organization. The purpose of this project is to do biomechanical research of new injectable prosthetic nucleus pulposus in vitro, testing whether the new type of prosthetic nucleus pulposus accords with physiological nucleus. To establish the finite element model of the prosthetic nucleus pulposus and to calculate the theoretical biomechanical parameters by finite element method in all kinds of complicated conditions, it can supply aboratory and theoretical basis for the clinical use. Another it can supply the reference for the improving and perfecting of new prosthetic nucleus pulposus.Methods:1. Keeping the liquid vinyl silicone oil including the catalyzer and the liquid hydrogen silicone oil mix according to the ratio, and the mixture is injected into the capsules made of the poly carbonate urethane (PCU) by the injection device. Then put the capsules in the 37℃thermostat to make the polymerization. Finally we can get the prosthetic nucleus pulposus.2. Put the prosthetic nucleus pulposus(random selection, in 37℃saline for 24 hours) in the biomechanical testing machine(Bose Electroforce3500) and carry out compression modulus test, static stiffness test, dynamic stiffness test, the yielding limit test, grinding test,fatigue test,then record relevant data and do correlation analysis.3. Scanning the prosthesis by seven freedom degrees digital measurement and get the point -cloud model. Useing Imageware, ProE, Hyperworks software to do the data and graphics processing for set up finite element model of the prosthetic nucleus pulposus.4. Calculating the parameters such as static stiffness on the finite element model by the nonlinear finite element solver, Abaqus. Compared the results of experimental tests to validate the accuracy of the finite element model and the theoretical data.Results:1. When the prosthesis is slowly loaded vertically from 150N, 250N, 350N, 500N, 1000N, at 10N/s rates, the stress and stain of the prosthetic nucleus pulposus become a non-linear relationship and the elastic modulus is about 2.5MPa.2. The static stiffness increases gradually in the process of being loaded vertically at 10N/s rates to 1000N, and it will be non-linear.3. When the prosthesis is slowly loaded vertically to 400N at 10N/s rates, the dynamic stiffness'strain lags behind the stress of the phase angle of about 13 degrees in the 0.5 - 3Hz range; in the frequency of 3Hz, the dynamic stiffness is nearly 3.5 times the static stiffness.4. When the prosthesis is slowly loaded vertically from 500N to 2000N at 10N/s rates, there is no plastic deformation of filler; but in 1200N, one prosthesis appears yielding.5. When the prosthesis is loaded 100N, 400N in the grinding tests, the surface doesn't show obvious abrasion after 10000 times.6. Prosthesis'capsule is deformed or ruptured in the fatigue tests. There is the normal saline between the capsule and the filler. The individual filler core is ruptured.7. If human upper body payload is 500N, to test the finite element model of the prosthetic nucleus pulposus, the simulation result (displacement - load) is basically consistent with experimental result. That shows the finite element method is feasible to study the mechanical properties of the prosthetic nucleus pulposus.Conclusion:The capsule design is reasonable, flexible cores materials choice is appropriate. To do the biomechanical and materials performance testing of the new prosthetic nucleus pulposus in vitro, simulated human body environment, and the results show the prosthesis has good flexibility, and anti-deformation ability is high. The data and function are in line with or close to normal human lumbar nucleus pulposus. But there are some deficiencies in the production process of the prosthesis, it need to be improved and perfected.The construction process of the finite element model of the prosthetic nucleus pulposus is reasonable and accurate. The model can be applied to the finite element analysis in a variety of conditions which can effectively shorten the time during the experiment, reduce the experimental costs, can do the mechanical properties test in the complicated conditions. The tests'results are in accordance with the experimental results. The finite element model of the new prosthetic nucleus pulposus can be further applied to a wider range of biomechanical tests. The theoretical data will be very important to the improvement and perfection of the prosthesis. It is a very simple and practical method of biomechanical research and is the complement and extend of the experimental biomechanics.
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