 Finite Element modeling to predict the implant holding strength of osteoporotic bone Bhishan Kayastha ( ) Supervisor: Karen Reynolds

Project Objective  To validate Finite element model and use it to predict the optimal implant holding strength (pullout strength) of osteoporotic bone.

Background  Osteoporosis is a disease that makes the bone fragile and easily susceptible to fracture.  Surgeons face difficulties in fixing screw securely in such fragile bone.  Surgeons do not know what level of insertion torque should they apply to fix the screw securely (optimal holding strength) into osteoporotic bone.  Lack of this knowledge leads them to either fixing the screw too tight and fracture the bone internally during screw insertion or fixing too loose to let it come off.  That is why we need to investigate the effect of insertion torque on holding strength of osteoporotic bone.

Past Works  Mechanical tests and finite element models for bone holding power of tibial locking screws (Hou et al.)  Total strain energy and total reaction force of screws used as an estimator of holding power of implant.  Co-relation of experimental and numerical results for the holding power of dental, traumatic and spinal screws (Lee et al.)  Total strain energy and reaction force are used for estimation of holding power of implant.

Continued….  Predicting Cancellous bone failure during screw insertion (K. J Reynolds et al.)  Maximum torque used as holding power of implant  Investigation of fixation screw pullout strength of human spine (Zhang et al.)  Maximum von-mises stress used as deciding factor

What’s in this project?  Use of insertion torque as an indicator for the estimation of optimal implant holding power of the osteoporotic bone.

Why Finite element modeling (FEM) ?  FEM is a numerical analysis method used to simulate the experiments.  Getting bone samples is an expensive option and not always an easy one.  Experiments can be performed any no. of times.  Experiment parameters can be changed any no. of times.

Project Outline  Develop Finite element (FE) model of bone-screw construct.  Apply some torque to screw head to rotate the FE screw model.  Pull out the FE screw model after every certain degree of rotation to measure the pull-out strength (holding power) for that degree of rotation.  Validate the FE model by comparing the FE predicted pull-out strength against experimental data.

FE Model of Bone screw construct  FE software ‘Ansys workbench 15’ is used to create FE model of Bone-screw construct.  Screw model is made by using CAD software and then imported into Ansys.  Then 3D cylindrical Solid is created around the screw and then thread imprint is made inside the solid by a process called boolean subtraction.  The screw is rotated just after the head contact with the bone to find the tightening torque.  The contact interface between bone and screw is considered frictionless and sliding.  Boundary condition is simulated exactly like in experiment. The cylindrical bone is fixed at its top face in order to prevent it from rotating along with the screw.

Fig 1. Screw Thread imprint in bone

Fig 2. Finite Element model of Screw-bone construct

Material properties  The FE model created is a linear elastic model as it is computationally cheap and is enough to simulate the screw insertion.  Material properties of Osteoporotic bone:  Young’s modulus: Gpa  Poisson’s ratio: 0.3  Material Properties of Screw:  Young’s modulus: 200 Gpa(Steel)  Poisson’s ratio: 0.3

Meshing  Element size of 0.5 mm has been chosen.  Tetrahedral element with 10 nodes.  Total no. of elements: 69,987  Total no. of nodes: 110,354

Fig 3. Meshing of Bone

Rotating the screw  The screw is rotated in clockwise direction with step wise (5 deg. at a time) increase of angle of rotation.  Then the torque for each stepwise rotation is estimated by Ansys.  Failure of bone is defined when the level of tightening torque reaches the peak.

Torque Vs. Rotation

Pull-out test  Pull-out test is performed to find the relationship of tightening torque and pull-out strength.  This test is performed by pulling out the head of the screw from the bone.  The pull-out test is performed after every stepwise rotation of screw into the bone.  The pull-out force is calculated by Ansys.  Maximum pull-out strength is defined by pull-out force relating to maximum torque.

Validation of FE model  Set of pull-out force calculated by Ansys will be compared against the experimental data.

References 1. Hou, S. M., Hsu, C. C., Wang, J. L., Chao, C. K. & Lin, J., Mechanical tests and finite element models for bone holding power of tibial locking screws. Clin Biomech (Bristol, Avon), : p Lee, C. C., Lin, S. C., Wu, S. W., Li, Y. C. & Fu, P. Y., Correlation of the experimental and numerical results for the holding power of dental, traumatic, and spinal screws. Med Eng Phys, : p Reynolds, K. J., Cleek, T. M., Mohtar, A. A. & Hearn, T. C., Predicting cancellous bone failure during screw insertion. J Biomech, : p Zhang, Q. H., Tan, S. H. & Chou, S. M., Investigation of fixation screw pull-out strength on human spine. J Biomech, : p

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