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Simulating the shear flexibility behavior of transverse tibia fractures treated with intramedullary nails
Track 4. Implants for Trauma and Orthopedics - Joint ESB Track
4.5 AO T-plate. Load apply was at distal epiphysis. In 5 load cycles a bending moment of 7.5 Nm and a torsion moment of 8.3 Nm were applied. Deformations were determined at the distal epiphysis. The statistic analysis took place by means of one-way ANOVA. Results: The proximal humeral nail Sirus TM showed significantly higher values in bending and torsion stiffness. At a bending moment of 7.5 Nm the values were: Sirus: 14.2mm; PHN: 20.7mm; Philos: 28.1mm; T-plate: 29.3mm (p<0.0012). At 8.3Nm torsion moment the values were: Sirus: 8.4°; PHN: 12.3°; Philos: 16.4°; T-plate: 15.60 (p<0.02). The first clinical experience with the Sirus nail showed simple implantation technique, short operative and radiological exposure time as well as good functional results. Conclusion: In the here presented fracture model the intramedullary devices are biomechanically superior to the plate systems. By the possibility of minimal invasive technique those gain additional significance. 7171 Fr, 11:45-12:00 (P51) Biomechanical testing o f trauma implants have we forgotten to inculde the joints? P. Upadhyay 1, J. Allen 2, D. Walton 2, M. Blakemore 3, D. Griffin 1. 1University
of Warwick, Coventry, UK, 2The University of Birmingham, Birmingham, UK, 3 University Hospital Coventry and Warwickshire NHS Trust, Coventry, UK Introduction: A review of the literature was undertaken to analyse testing protocol and rig designs for tibial trauma implants. Fourteen papers were identified and analysed. It was found that none of the test rigs accounts for the presence of anatomical joints during testing. A test rig was design to replicate the freedom at the joint and compared against a system where the joints were constrained system. Methods: The Universal Joint Simulator was developed using a bi-axial linkage hinge system and a flexible end restraint which allows the user to programme the amount of movement desired at a particular joint. The above system was tested against a conventional test rig design which did not permit any movement between implant and test rig. The fracture configuration used for the testing was an unstable distal tibial fracture (AO Fracture Classification Type 43-A2.3) the fracture was fixed using T 2 intramedullary nail and the effect of one, two and three distal interlocking screws on the stability of the distal fragment was evaluated. Results: Using a conventional test rig, there was no statistical significance (P=0.758 unpaired Students T Test) in the load to failure between one vs. two distal interlocking screws, whereas testing with the presence of a 'joint' in the test rig using the 'universal joint simulator' showed a statistically significant difference (P =0.019 unpaired Students T Test) in load to failure between the use of one vs. two screws. The results obtained when the anatomical joints were accounted for are consistent with clinical studies showing that one distal locking screw fails more often than two distal interlocking screws. The above results show the importance of accounting for the presence of anatomical joints in biomechanical testing as the results obtained are clinically relevant. 7685 Fr, Experimental comparison o f the treatment o f unstable femur fractures using Uniflex nail, the Vector nail, Long Gamma nail G.O. Njus 1, C.R. Ruble 2, A.T. Kantaras 2, G.A. Vrabec 2, S.
12:00-12:15 (P51) subtrochanteric and the
Battula 1. 1. The University of Akron, Department of Biomedical Engineering, Akron, USA, 2Akron General Medical Center, Department of Orthopaedic Surgery, Akron, USA Objectives: To biomechanically compare the fatigue life, stiffness, fracture site motion, and failure site of three commonly used cephalomedullary nails in the treatment of unstable subtrochanteric femur fractures. Design: A biomechanical in vitro study in which identical synthetic femurs were randomly instrumented with the Long Gamma nail (G), the Uniflex nail (U), or the Vector nail (V) after an unstable subtrochanteric femur fracture model was created. Statistical Model: Sample size was determined to be six using power analysis and a single factor ANOVA model was used for analyzing the results. Main Outcome Measurements: Each construct was axially loaded, using an Instron material testing system, with a sinusoidal loading pattern between -200 N and -2000 N. Fatigue life was measured by the number of cycles to failure. Stiffness was calculated by load-displacement curves. Fracture site motion was measured with a spatial digitizer. Failure site of the nails was noted. Results: The unfractured, uninstrumented control specimens had a mean stiffness of 1764N/mm. All the Vector nails failed in the nail at the femoral fracture site, the Uniflex nails failed at the distal of the two proximal screw holes and the Long Gamma nails failed at the lag screw hole. The mean fatigue lives were 45,344 cycles, 52,891 cycles and 88,748 cycles; the mean stiffnesses were 294.3, 373.8 and 656.4N/mm; the mean axial displacements at the
4.6. Trauma Implants
$135
fracture site were 2.31,2.45, and 0.79 mm; the mean transverse displacements at the fracture site were 1.20, 1.22 and 0.28 mm respectively for V, U and G. Conclusions: The Long Gamma nails had a significantly longer fatigue life, significantly higher stiffness, and a significantly lower amount of fracture site motion than did the Uniflex and Vector nails. For these reasons, the Long Gamma nail may be better in the treatment of unstable subtrochanteric femur fractures. 5291 Fr, 14:00-14:15 (P52) Simulating the shear flexibility behavior of transverse tibia fractures treated with intramedullary nails T. Wehner, U. Simon, L. Claes. Institute ef Orthopaedic Research and
Biomechanics, University of UIm, Germany Introduction: The 3D stiffness of fracture fixations strongly influences the healing process due to the resulting interfragmentary movement (IFM). The gap between unreamed intramedullary nails and the endosteal surface can result in large shear IFMs, which might be critical for healing. The flexibility of unreamed tibial nails was measured in vitro for different load directions (axial compression, bending, torsion) [1,2], but not in the shear direction. The aim of this study was to determine the non-linear shear flexibility of the bone-implantcomplex (BIC) depending on nail diameter and fracture geometry using a Finite Element (FE) contact model. Method: 3D FE contact models were generated to represent transverse tibial fractures with different fracture gaps (3-20 mm) stabilized with intramedullary nails of different diameters (8-10 mm). The contact between the nail and the endosteal surface (diameter= 11 mm) was modeled with surface-to-surface contact elements. We calculated the force-deflection curves for shear forces of up to 1000N. Beside the shear case, the model was loaded axial, torsional and in bending, which allowed a comparison to in vitro results [1]. Results: The non-linear shear behavior consisted of two parts. Before the nail-endosteum contact (Fshea r < 4 0 N), the shear flexibility was mainly caused by deformations of the locking screws; we found relatively low stiffnesses (<20 N/mm). With an established contact the slope of the force~Jeflection curve increased significantly (e.g. 6500 N/mm, depending on fracture gap size and nail material). Predictions for the other load directions correlated well with experimental results [1] and confirmed our model assumptions. Discussion: The FE-contact model is a useful tool to determine the complex shear behavior of fractures treated with unreamed intramedullary nails. Combinations of thin nails and large fracture gaps were most flexible and might be critical for the healing outcome. Preliminary results of fracture healing simulations using a previous model [3] enhanced by the calculated nonlinear shear behavior of this study showed a delayed healing for those critical combinations. References [1] Schandelmaier et al. J Orth Tr 1996. [2] Schandelmaier et al. Injury 2000. [3] Simon et al. Proc. ESB 2004.
5390 Fr, 14:15-14:30 (P52) Finite element study of external fixation system for fracture healing W. Choromanski, G. Dobrzynski, A. Lesniewska. Warsaw University ef
Technology, Faculty of Transport, Warsaw, Poland This paper presents a new method of diagnosis and monitoring of union bone process. This method is based on measurement the tension of external fixator frame. The goal of treatment is to control the pain, promote healing, prevent complications, and restore bone to its original shape, structure and mechanical strength. This can be done by external fixation device which can be used in the early phase of treatment and when used gives an opportunity to achieve early full weight bearing after fracture stabilization. According to studies [1], rapid fracture healing is expected when the interfragmentary movement can be controlled using external fixation systems. One of them is Dynastab unilateral external fixator equipped with electronic measurement system which allows to evaluate the stress distribution during fracture healing. In other words, evolution of stress can help to estimate the current stiffness of healed fracture. This study is aimed to evaluate the mechanical performance of the external fixation system by means of finite element method. A finite element model of a long bone fracture stabilized by external fixation system was created using the CAD system. The three-dimensional model of proximal, distal bones and fracture zone was created from CT images based on the transverse and oblique fractures. As the fracture site is exposed to an axial loading, a variable load configuration was used. Also as mentioned above different regions were introduced in the model enabling the definition of different isotropic material properties Material properties for fracture zone based on the values given by [2]. The von Mises stress gives information about the stage of fracture healing and non linear characteristic of von Mises stress in the external fixator frame can be defined during fracture healing process.
4.5 AO T-plate. Load apply was at distal epiphysis. In 5 load cycles a bending moment of 7.5 Nm and a torsion moment of 8.3 Nm were applied. Deformations were determined at the distal epiphysis. The statistic analysis took place by means of one-way ANOVA. Results: The proximal humeral nail Sirus TM showed significantly higher values in bending and torsion stiffness. At a bending moment of 7.5 Nm the values were: Sirus: 14.2mm; PHN: 20.7mm; Philos: 28.1mm; T-plate: 29.3mm (p<0.0012). At 8.3Nm torsion moment the values were: Sirus: 8.4°; PHN: 12.3°; Philos: 16.4°; T-plate: 15.60 (p<0.02). The first clinical experience with the Sirus nail showed simple implantation technique, short operative and radiological exposure time as well as good functional results. Conclusion: In the here presented fracture model the intramedullary devices are biomechanically superior to the plate systems. By the possibility of minimal invasive technique those gain additional significance. 7171 Fr, 11:45-12:00 (P51) Biomechanical testing o f trauma implants have we forgotten to inculde the joints? P. Upadhyay 1, J. Allen 2, D. Walton 2, M. Blakemore 3, D. Griffin 1. 1University
of Warwick, Coventry, UK, 2The University of Birmingham, Birmingham, UK, 3 University Hospital Coventry and Warwickshire NHS Trust, Coventry, UK Introduction: A review of the literature was undertaken to analyse testing protocol and rig designs for tibial trauma implants. Fourteen papers were identified and analysed. It was found that none of the test rigs accounts for the presence of anatomical joints during testing. A test rig was design to replicate the freedom at the joint and compared against a system where the joints were constrained system. Methods: The Universal Joint Simulator was developed using a bi-axial linkage hinge system and a flexible end restraint which allows the user to programme the amount of movement desired at a particular joint. The above system was tested against a conventional test rig design which did not permit any movement between implant and test rig. The fracture configuration used for the testing was an unstable distal tibial fracture (AO Fracture Classification Type 43-A2.3) the fracture was fixed using T 2 intramedullary nail and the effect of one, two and three distal interlocking screws on the stability of the distal fragment was evaluated. Results: Using a conventional test rig, there was no statistical significance (P=0.758 unpaired Students T Test) in the load to failure between one vs. two distal interlocking screws, whereas testing with the presence of a 'joint' in the test rig using the 'universal joint simulator' showed a statistically significant difference (P =0.019 unpaired Students T Test) in load to failure between the use of one vs. two screws. The results obtained when the anatomical joints were accounted for are consistent with clinical studies showing that one distal locking screw fails more often than two distal interlocking screws. The above results show the importance of accounting for the presence of anatomical joints in biomechanical testing as the results obtained are clinically relevant. 7685 Fr, Experimental comparison o f the treatment o f unstable femur fractures using Uniflex nail, the Vector nail, Long Gamma nail G.O. Njus 1, C.R. Ruble 2, A.T. Kantaras 2, G.A. Vrabec 2, S.
12:00-12:15 (P51) subtrochanteric and the
Battula 1. 1. The University of Akron, Department of Biomedical Engineering, Akron, USA, 2Akron General Medical Center, Department of Orthopaedic Surgery, Akron, USA Objectives: To biomechanically compare the fatigue life, stiffness, fracture site motion, and failure site of three commonly used cephalomedullary nails in the treatment of unstable subtrochanteric femur fractures. Design: A biomechanical in vitro study in which identical synthetic femurs were randomly instrumented with the Long Gamma nail (G), the Uniflex nail (U), or the Vector nail (V) after an unstable subtrochanteric femur fracture model was created. Statistical Model: Sample size was determined to be six using power analysis and a single factor ANOVA model was used for analyzing the results. Main Outcome Measurements: Each construct was axially loaded, using an Instron material testing system, with a sinusoidal loading pattern between -200 N and -2000 N. Fatigue life was measured by the number of cycles to failure. Stiffness was calculated by load-displacement curves. Fracture site motion was measured with a spatial digitizer. Failure site of the nails was noted. Results: The unfractured, uninstrumented control specimens had a mean stiffness of 1764N/mm. All the Vector nails failed in the nail at the femoral fracture site, the Uniflex nails failed at the distal of the two proximal screw holes and the Long Gamma nails failed at the lag screw hole. The mean fatigue lives were 45,344 cycles, 52,891 cycles and 88,748 cycles; the mean stiffnesses were 294.3, 373.8 and 656.4N/mm; the mean axial displacements at the
4.6. Trauma Implants
$135
fracture site were 2.31,2.45, and 0.79 mm; the mean transverse displacements at the fracture site were 1.20, 1.22 and 0.28 mm respectively for V, U and G. Conclusions: The Long Gamma nails had a significantly longer fatigue life, significantly higher stiffness, and a significantly lower amount of fracture site motion than did the Uniflex and Vector nails. For these reasons, the Long Gamma nail may be better in the treatment of unstable subtrochanteric femur fractures. 5291 Fr, 14:00-14:15 (P52) Simulating the shear flexibility behavior of transverse tibia fractures treated with intramedullary nails T. Wehner, U. Simon, L. Claes. Institute ef Orthopaedic Research and
Biomechanics, University of UIm, Germany Introduction: The 3D stiffness of fracture fixations strongly influences the healing process due to the resulting interfragmentary movement (IFM). The gap between unreamed intramedullary nails and the endosteal surface can result in large shear IFMs, which might be critical for healing. The flexibility of unreamed tibial nails was measured in vitro for different load directions (axial compression, bending, torsion) [1,2], but not in the shear direction. The aim of this study was to determine the non-linear shear flexibility of the bone-implantcomplex (BIC) depending on nail diameter and fracture geometry using a Finite Element (FE) contact model. Method: 3D FE contact models were generated to represent transverse tibial fractures with different fracture gaps (3-20 mm) stabilized with intramedullary nails of different diameters (8-10 mm). The contact between the nail and the endosteal surface (diameter= 11 mm) was modeled with surface-to-surface contact elements. We calculated the force-deflection curves for shear forces of up to 1000N. Beside the shear case, the model was loaded axial, torsional and in bending, which allowed a comparison to in vitro results [1]. Results: The non-linear shear behavior consisted of two parts. Before the nail-endosteum contact (Fshea r < 4 0 N), the shear flexibility was mainly caused by deformations of the locking screws; we found relatively low stiffnesses (<20 N/mm). With an established contact the slope of the force~Jeflection curve increased significantly (e.g. 6500 N/mm, depending on fracture gap size and nail material). Predictions for the other load directions correlated well with experimental results [1] and confirmed our model assumptions. Discussion: The FE-contact model is a useful tool to determine the complex shear behavior of fractures treated with unreamed intramedullary nails. Combinations of thin nails and large fracture gaps were most flexible and might be critical for the healing outcome. Preliminary results of fracture healing simulations using a previous model [3] enhanced by the calculated nonlinear shear behavior of this study showed a delayed healing for those critical combinations. References [1] Schandelmaier et al. J Orth Tr 1996. [2] Schandelmaier et al. Injury 2000. [3] Simon et al. Proc. ESB 2004.
5390 Fr, 14:15-14:30 (P52) Finite element study of external fixation system for fracture healing W. Choromanski, G. Dobrzynski, A. Lesniewska. Warsaw University ef
Technology, Faculty of Transport, Warsaw, Poland This paper presents a new method of diagnosis and monitoring of union bone process. This method is based on measurement the tension of external fixator frame. The goal of treatment is to control the pain, promote healing, prevent complications, and restore bone to its original shape, structure and mechanical strength. This can be done by external fixation device which can be used in the early phase of treatment and when used gives an opportunity to achieve early full weight bearing after fracture stabilization. According to studies [1], rapid fracture healing is expected when the interfragmentary movement can be controlled using external fixation systems. One of them is Dynastab unilateral external fixator equipped with electronic measurement system which allows to evaluate the stress distribution during fracture healing. In other words, evolution of stress can help to estimate the current stiffness of healed fracture. This study is aimed to evaluate the mechanical performance of the external fixation system by means of finite element method. A finite element model of a long bone fracture stabilized by external fixation system was created using the CAD system. The three-dimensional model of proximal, distal bones and fracture zone was created from CT images based on the transverse and oblique fractures. As the fracture site is exposed to an axial loading, a variable load configuration was used. Also as mentioned above different regions were introduced in the model enabling the definition of different isotropic material properties Material properties for fracture zone based on the values given by [2]. The von Mises stress gives information about the stage of fracture healing and non linear characteristic of von Mises stress in the external fixator frame can be defined during fracture healing process.