J Simonovics, K Váradi
Department of Machine and Product Design,
Budapest University of Technology and Economics Budapest, Hungary P Bujtár, CME Avery
Department of Oral and Maxillofacial Surgery, University Hospitals of Leicester, Leicester, UK
GK Sándor
Regea Institute for Regenerative Medicine, University of Tampere, Tampere, Oral and Maxillofacial Surgery University of Oulu, Oulu, Finland
J Pan
Department of Mechanical Engineering, University of Leicester, Leicester, UK
Abstract:
The goal was to compare 4 different type reconstruction plate fixation technology in case of prophylactic internal fixation, focusing on stability. We examined unilocking and bicortical screw fixation methods with T-shaped and straight plates. These fixations were used for demonstration in other studies using human and animal models as well. In this study we performed finite element analysis (FEA) on a sheep tibia model with osteotomy defects to demonstrate certain clinical situations on the human radius. 4-point bending was done on the models giving superior result with the straight 3.5mm stainless steel unilocking plate.Keywords: FEA, biomechanics, tibia, fixation
1. Introduction
There is a limited amount of results on cadaveric biomechanical analysis due to ethical, technical and hygiene reasons. The finite element analysis (FEA) – as more recent technique – has a great potential to augment the understanding on, bypassing these limitations. This is the case for examinations on prophylactic internal fixation (PIF) on the of the radial osteocutaneous free flap donor site with the surgical plate fixation.
Retrospectively the frequency of fractures without reinforcement is reported to be between 15-25%.
The geometrical model of the tibia used constructed based on Computer Tomography (CT) data, and the 4 point bending examination in FEA prepared applying boundary conditions used in standard and corresponding previous studies.
The von Mises stresses stress criteria was applied for evaluating the resultant high stress around at the resection lines and at the closest screw holes near to the resections.
2. Methods
Initially 4 different bone implant plate were selected and modelled with the adherent screws. Three were uni-lock systems which means there are threads on the hole surface at the plates (titanium T-shaped radial locking compression plates (LCP) in 2.4 mm and 3.5 mm thickness, 3.5 mm stainless steel straight LCP plate) and the 4th plate
utilized bicortical non-locking screw fixation (3.5 mm stainless steel straight dynamic compression plate (DCP)) (SYNTHES, UK) (Table 1).
Table 1
T-plates Straight plates
Type of plate 3.5 mm T- plate1 2.4 mm T- plate 3.5 mm DCP plate 3.5 mm LCP plate
Thickness 3.5 mm 2.4 mm 3.5 mm 3.5 mm
18Cr-14Ni-2.5Mo 18Cr-14Ni-2.5
Plate materials TiCP TiCP Mo
Stainless Steel Stainless Steel Young Modulus
(GPa) 103 103 186 186
Poisson ratio 0.3 0.3 0.3 0.3
Screw – bone
engagement unicortical unicortical Bicortical Unicortical
Screw - plate
fixation locking locking non-locking locking
Screw materials Ti-6Al-7Nb Ti-6Al-7Nb 18Cr-14Ni-2.5 Mo Stainless Steel
18Cr-14Ni-2.5Mo Stainless
Steel Young Modulus
(GPa) 105 105 186 186
Poisson ratio 0.3 0.3 0.3 0.3
Screw diameter 2.4 & 3.5 mm 2.4 mm 3.5 mm 3.5 mm
Fig.1. Overview of T-shaped titanium plate over the defect and in the anterior position The ProEngineer Wildfire 5 CAD software environment were used for implant and defect modelling. The tibia model that we used based on computerized tomography scanner (Toshiba, Aquilion) data. The geometrical reconstruction was performed with ImageJ 1.42q software and software from DICOM data.
10-node quadratic tetrahedron elements were used to build the mesh within Abaqus (Simulia, Providence, RI, US). For the material assignment of the material properties the mesh of the assembled constructs were reimported in to Mimics.
For defining the mechanical elastic properties of the tibia model the radiographic density data were used after validation method preferred by our team [1]. The correlation between the HU unit (bone density) and the material density was defined based on validated data .
density ρ = 0.000732 x HU + 0.112715 [g/cm3] (1)
The linear elastic modulus ( E) is function of the bone density (ρ) by right of the literature
E = 10500 x ρ 2.29 [MPa]
(2)
The complete HU was decomposed into 100 different equal parts. These were allocated for the material definition for each separate isotropic, linear tetra elements. This preparation resulted in a non-homogenous model material property. The 0.3 value was set for Poisson ratio.
Orphan mesh was used to rebuild the FEA model in Abaqus. For the 4-point bending compression tests 4 main area were defined on every model to set the boundary condition and load parameters that based on earlier biomechanical studies. The settings can be seen on Fig. 3. The superior von Mises stress values with the corresponding principle stress values were collected.
Fig.3. Boundary conditions and loads on osteotomized tibia
400N preload values were applied on each individual screw in case of DCP plate simulation because of the non-locking technique.
The locking screws were modelled in a direct (non-limited) contact manner, and the non-locking screws were friction gripped using a recommended coefficient of 0.3. The goal was to find the strongest forms of reinforcement.
3. Results
The highest von Mises values were located around the screws closest to the defect and at angles of the osteotomy defects endings. This is consistent with previous biomechanical studies. [2, 9, 10] Data was collected and compared from these areas.
Fig. 4. Von Mises stress and region signings in the bone in case of 4-point bending with
3.5 mm straight DCP plate (400N preload)
region 3 - osteotomy 59 -5 -68
intact bone – control 35 37 1
region 4 - osteotomy 156 -15 -179
osteotomised bone –
control region 3 - osteotomy 178 -36 -220
The highest von Mises value at the intact bone was 35MPa, while the osteotomised bone were 178 MPa respectively. The highest stress values are the following: 3.5mm T-plate (65 MPa), 2.4 mm T-T-plate (60 MPa), straight 3.5mm DCP (120 MPa), and 3.5 mm straight LCP (48 MPa) (Table 2).
Based on the results from the table the higher stability offering fixation method was the 3.5mm straight stainless steel LCP unilocking plate in case of 4-point bending.
4. Conclusion
The 3.5mm straight stainless steel LCP unilocking plate construct is the superior reinforcement in this scenario. However this is not necessarily contributing to the fastest recovery but more likely providing higher level of security for the patient with smaller chance of loosening or acute fracture.
1 3
4 2
3
4
5. Acknowledgements
We thank Claire Robinson MSc, Advanced Practitioner, Forensic
Department of Radiology, University Hospitals of Leicester for imaging the sheep tibia.
The work reported in the paper has been developed in the framework of the project
„Talent care and cultivation in the scientific workshops of BME" project. This project is supported by the grant TÁMOP - 4.2.2.B-10/1--2010-0009
6. Conflict of Interest Statement
The consumables and a small research grant were supplied by Synthes (UK).
7. References
[1] Bujtar, P., et al., Finite element analysis of the human mandible at 3 different stages of life. Oral Surg Oral Med Oral Pathol Oral Radiol Endod, 2010. 110(3): p. 301-9.
[2] Keller, T.S., Predicting the compressive mechanical behavior of bone. Journal of biomechanics, 1994. 27(9): p. 1159-1168.
[3] Avery, C.M.E., et al., Biomechanical study of a unilocking T-plate system for
prophylactic internal fixation of the radial osteocutaneous donor site using the sheep tibia model. Oral Oncology, 2011. 47(4): p. 268-273.
[4] Avery, C.M.E., M. Danford, and P.A. Johnson, Prophylactic internal fixation of the radial osteocutaneous donor site. British Journal of Oral and Maxillofacial Surgery, 2007.
45(7): p. 576-578.