6thPan Hellenic Conference, Patra 10-12 October 2014 Mechanical behaviour of tensioned wires and half pins in Taylor Spatial Frame L.A. Spyrou1, P. Christodoulou2, A.T. Kermanidis2 1 Institute for Research & Technology-Thessaly, Centre for Research & Technology Hellas 2 Department of Mechanical Engineering, University of Thessaly

Taylor Spatial Frame (TSF) External fixator Two aluminum rings Six telescopic struts Pre-tensioned Wires Half Pins Uses Treatment of bone fractures and deformities Facilitating the osteogenesis in bones’ areas which suffer from fracture damage (tibia or femur) Alignment of angular deformities of bones through the external support process ΕΛΕΜΒΙΟ 6thPan Hellenic Conference Patra 2014

Why TSF? But… Fast & easy application Software based corrections Multi-level deformity correction Non parallel rings No concentric placement Strands in random angles (6DOF!) No concentric or perpendicular But… Limited information of biomechanical behaviour ΕΛΕΜΒΙΟ 6thPan Hellenic Conference Patra 2014

Thorough understanding of the actual biomechanical behavior of TSF Scope Thorough understanding of the actual biomechanical behavior of TSF Experiments FE Modeling Quantification of mechanical characteristics of the TSF’s key components: Pre-tensioned Wire Half Pin Present Study (1st step) Mounted on a TSF Ring ΕΛΕΜΒΙΟ 6thPan Hellenic Conference Patra 2014

Materials Tensile test Stress - strain curve Wire Metallurgical investigation of TSF’s Ring microstructure Measuring machine TESA Micro-Hite 3d Ring Elastic properties of materials used in FE calculations ΕΛΕΜΒΙΟ 6thPan Hellenic Conference Patra 2014

Mechanical Testing MTS Servo hydraulic system (5kN load capacity) TSF’s key components Ring + Pre-tensioned Wire Ring + Half pin Measuring devices MTS 50mm gage length clip-on extensometer Wire tensioner Control Constant crosshead velocity (1mm/min) Data Continuous data acquisition (Compressive load - displacement) MTS Servo hydraulic system (5kN load capacity) Mechanical response under axial loading ΕΛΕΜΒΙΟ 6thPan Hellenic Conference Patra 2014

Calculation of Wire’s tension loss due to slippage[1] FE Modeling (TSF Ring-Wire system) Geometry Ring dia.: 180mm (external) Wire dia.: 1.8mm Wire – Bone friction coeff.: frictionless Undeformed Plate – Bone friction coeff.: 0.2 Meshing Wire’s contact region with bone: solid structure with first order tetrahedral elements Wire’s free length: second order beam elements DOF ≈ 167,000 Solution 1st step: Wire pre-tension 2nd step: Axial loading Calculation of Wire’s tension loss due to slippage[1] Q: Loss of tension ΔL: Slip [1] A.R.Zamani, S.O.Oyadiji.(2007).”Analytical modelling of Kirschner wires in Ilizarov circular external fixators using a tensile model”. Proc. IMechE Vol.222.J.Eng. in Medicine

FE Modeling (TSF Ring-Half pin system) Geometry Ring dia.: 180mm (external) Half pin dia.: 1.8mm Half Pin – Bone friction coeff.: 0.3 [2] Undeformed Plate – Bone friction coeff.: 0.2 Modeling cases - Meshing Threaded hole in bone that matches the half pin thread ( ≈ 233,000 DOF) Cylindrical hole in bone with no threaded half pin ( ≈ 182,000 DOF) First order tetrahedral elements Solution Axial loading (a) (b) [2] A.R.MacLeod , P.Pankaj, A.HamishR.W.Simpson.(2012).” Does screw–bone interface modeling matter in finite element analyses? ”. Journal of Biomechanics 45 , 1712–1716

Results(1/2) Ring-Wire system ΕΛΕΜΒΙΟ 6thPan Hellenic Conference Patra 2014

Results(2/2) Ring-Half pin system ΕΛΕΜΒΙΟ 6thPan Hellenic Conference Patra 2014

Conclusions Detailed 3D FE models of TSF’s key components were developed and validated with experiments in order to predict accurately the axial stiffness behavior of: a pre- tensioned wire and a half pin mounted on a TSF ring importance of a rigid surface to apply a load on a half pin model because of bending Not significant difference in half pin stiffness whether the thread is included or not friction coefficient in both systems has no effect in mechanical response (F-d) Wire’s pre-tension loss due to slippage validated with experiments ΕΛΕΜΒΙΟ 6thPan Hellenic Conference Patra 2014

2nd Step: Full scale TSF’s biomechanical behaviour Future work... 2nd Step: Full scale TSF’s biomechanical behaviour ΕΛΕΜΒΙΟ 6thPan Hellenic Conference Patra 2014