1. 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

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

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

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

11. 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

12. 2nd Step: Full scale TSF’s biomechanical behaviour
Future work...
2nd Step: Full scale TSF’s biomechanical behaviour
ΕΛΕΜΒΙΟ 6thPan Hellenic Conference Patra 2014