Topology optimization under 400N compression load for 40 and 20% mass fraction

1. Optimum Topology Design of an Interbody Fusion Implant for Lumbar Spine Fixation NSF Grant: DMI01-14975 – State of Indiana 21st Century Research and Technology Fund UNIVERSITY OF NOTRE DAME V I T A C E D O - D U L S P E S John E. Renaud1 & Andres Tovar2 Department of Aerospace and Mechanical Engineering University of Notre Dame, Notre Dame, Indiana, 46556 USA Phone: (574)631-8616 Fax: (574)631-8341 E-mail: Renaud.2@nd.edu 1 Professor 2 Graduate Research Assistant Motivation and Objectives A new surgical technique for interbody fusion is currently in development. The procedure makes use of an interbody fusion implant that is inserted between the vertebral bodies to be fused. The interbody fusion implant is used to provide support structure during fusion. The implant is packed with bone graft material to facilitate the fusion of the two vertebral bodies. Our goal in this work is to obtain the optimal topology (i.e., shape) of the interbody fusion implant. The implant must be capable of supporting the mechanical loads of the lumbar spine while solid fusion of the vertebral bodies occurs. The implant restrains the bone graft material and maintains proper intervertebral spacing during fusion. Materials and Methods The finite element analysis and topology optimization software GENESIS is used to drive the topology design. The implant is modeled by 8256 eight-node solid element CHEXA, with mechanical properties corresponding to the candidate implant material. The upper vertebra is modeled by one rigid element RBE2 that spans the compressive half of the implant. The lower surface of the implant is fully constrained in the model. The topology optimization process is performed for flexion/extension and lateral bending. The moment for each bending case is 7.5 Nm applied in the center of the upper vertebra. A distributed compression load of 400 N is applied as a third loading case. The optimization problem seeks to minimize the total strain energy of the implant subjected to a maximum mass fraction constraint for each load case. The resulting topologies of each load case are illustrated. The individual topologies are then superimposed subject to symmetry constraints in order to generate the optimum topology. The center structure supports shear loads in bending, while the outer structures bear the compressive loads. Vertebra Implant Superimposed topologies Candidate geometry Results and Conclusions The optimal topology (i.e., shape) of the interbody fusion implant is obtained for two different mass fraction constrained optimization trials. These implants will each restrain the bone graft material while maintaining proper intervertebral spacing during spinal fusion. The two implant topologies are both capable of supporting the mechanical loads of the lumbar spine while solid fusion of the vertebral bodies occurs. The topology with greater mass fraction provides greater structural support that yields smaller deflections under loading, while the lower mass fraction topology provides additional volume for the bone graft material experiences larger strains and deflections. These topologies must be converted to candidate geometries for the actual implant design as illustrated above. Students work on a cadaver Solid model of lumbar spine region Topology optimization under 400N compression load for 40 and 20% mass fraction Topology optimization under 7.5 Nm flexion/extension moment for 20 and 10% mass fraction Topology optimization under 7.5 Nm lateral bending moment for 20 and 10% mass fraction