Structure Design And Evaluation Of Orthopedic Implants Based On Topology Optimization

Orthopedic implants are widely used to repair the bone defects,promote regeneration of bone tissue and recover the functions of bone.However,the implantation occasionally failed in terms of prosthesis loosening and periprosthetic fracture,as results of the mechanical mismatch between the bone and implants.So far,extensive studies are devoted into tailoring the mechanical properties of the implant to match the host bone.Few studies investigated the impacts of orthopedic implants on the stress environment of host bones and subsequent bone’s adaptive remodeling.Furthermore,the attempts of coupling the mechano-driven bone regeneration into the designing and optimization of the implants are quite limited.A mathematical model of mechano-driven bone regeneration in the porous scaffolds was developed and utilized for evaluating the regenerative potential and mechanical properties of the bone/scaffold composite.In addition,another mathematical model which described the relationship between the bone cement distribution and bone mass changes in vertebrae was developed and used to optimize the bone cement injection protocol.The main research contents are as follows:1.Based on topology optimization(TO)algorithm,a three-dimensional micro-level mathematical model which reproduced the trabecular bone remodeling in the human third lumbar vertebra under three daily loading conditions(flexion,lateral bending and compression)was developed.The trabecular geometry and bone density distribution from the computational results were quantitatively consistent with CT analyses.The realistic simulated trabecular geometry determines the validity of the TO algorithm.2.A three-dimensional chiral mechanical metamaterial(also called as twist structure)with a twist was selected as the unit cell structure in the design of porous interbody fusion cages.Sequently,the trabecular bone remodeling model was used to explore the influence of mechanical stimuli on the adaptation or the regeneration of bone tissue.Firstly,the twist structure with tunable effective elastic modulus and twists per axial strain via a parametric method was designed and served as the unit cell of the Titanium fusion cage.Moreover,a mathematical model of mechano-driven bone regeneration in the porous scaffolds was established by means of TO and design space optimization algorithms.The numerical approach was used to evaluate and analyze the osseointegration effect of the fusion cage based on diagonal structure and twist structure with different parameters.Compared with the diagonal structure-based cage,the twist structure-based with similar effective elastic modulus better constructs a stress environment favorable for bone ingrowth by virtue of the twist property and results in excellent osseointegration.In addition,the twist structure-based fusion cage with a twist arm width of 0.6 mm better induces bone ingrowth and also significantly reduces the stress level at the bone/implant interface.3.For the design of bone cement injection protocol in the vertebroplasty,a time-dependent topology optimization algorithm of implants considering bone remodeling was improved and applied.Based on the algorithm,a mathematical model for bone cement distribution optimization and injection effect evaluation considering bone remodeling was developed.Additionally,the influence of different volume fractions of bone cement on the optimization results and the difference between the simulation results obtained by the novel algorithm and the traditional TO algorithm were discussed.Firstly,for the bone cement model considering bone remodeling,the incremental bone mineral density and the von mises stress of cancellous bone are negative correlation to the volume fraction of bone cement.Furthermore,for the model considering bone remodeling,the location of the injection space corresponding to different volume fractions of bone cement varies greatly,while in the conventional method,the volume fraction has less influence on the location of bone cement injection.Lastly,the bone cement injection protocol obtained from conventional TO is more compatible with the mechanical properties of cancellous bone in the early postoperative recovery phase,while the injection optimization protocol considering bone remodeling has a better long-term stability of the bone/cement composite.