Ulrich Simon

Publications of Ulrich Simon

• Optimization of intramedullary nailing by numerical simulation of fracture healing.

  Authors: Tim Wehner, Lutz Claes, Anita Ignatius, Ulrich Simon

  Journal of orthopaedic research : official publication of the Orthopaedic Research Society. 04/2012; 30(4):569-73.

  Due to the annular gap between intramedullary (IM) nails and the endosteal surface, high interfragmentary movement can occur under loading. This could prolong the healing time, particularly for thinDue to the annular gap between intramedullary (IM) nails and the endosteal surface, high interfragmentary movement can occur under loading. This could prolong the healing time, particularly for thin IM nails that are often used for unreamed IM nailing. The aims of our study were to determine the influence of the nail diameter on the healing time of human tibial shaft fractures and to investigate whether the healing time could be shortened by increasing the stiffness of the implant material. Therefore, a corroborated numerical model for simulating the fracture healing process in humans was used to simulate the healing process of human tibial fractures treated with IM nails. The calculated healing time (up to 71 weeks) was longest for transverse fractures treated with thin IM nails made of titanium. That the healing time was disproportionately long depended on the nail diameter, and could be greatly reduced by using a thicker nail or using steel instead of titanium. To avoid a prolonged healing time, the nail should be thick, and the annular gap should be as narrow as possible. Alternatively, using steel instead of titanium may also help to avoid a prolonged healing time. © 2011 Orthopaedic Research Society. © 2011 Orthopaedic Research Society Published by Wiley Periodicals, Inc. J Orthop Res 30:569-573, 2012.

• Optimization of Input Parameters for Fracture Healing Simulations

  Authors: Malte Steiner, Lutz Claes, Anita Ignatius, Frank Niemeyer, Ulrich Simon, Tim Wehner

  CBU 2011 proceedings: 17th International Symposium on Computational Biomechanics in Ulm, Jürgen Salk et al. (eds.); 07/2011

• Simulation von Knochendistraktionsvorgängen

  Authors: Frank Niemeyer, Tim Wehner, Lutz Claes, Anita Ignatius, Ulrich Simon

  Proceedings of the 7. Jahrestagung der Deutschen Gesellschaft für Biomechanik, Murnau; 05/2011

• Freies Tool zur Steifigkeitsberechnung knöcherner Strukturen aus klinischen Bilddaten – Entwicklung, Validierung und Erprobung von FEA4DICOM

  Authors: Ulrich Simon, Markus Helmer, F. Hauser, Dominik Lechler, Frank Niemeyer, Tim Wehner, Uwe Wolfram, Othmar Marti

  Proceedings of ACUM 2010, Aachen; 11/2010

• Improvement of the shear fixation stability of intramedullary nailing.

  Authors: Tim Wehner, Rainer Penzkofer, Peter Augat, Lutz Claes, Ulrich Simon

  Clinical biomechanics (Bristol, Avon). 10/2010; 26(2):147-51.

  The healing outcome of long bone fractures is strongly influenced by the mechanical environment. High interfragmentary movement at the fracture site is detrimental to the fracture healing process.The healing outcome of long bone fractures is strongly influenced by the mechanical environment. High interfragmentary movement at the fracture site is detrimental to the fracture healing process. Long bone fractures stabilized with thin intramedullary nails commonly used for unreamed intramedullary nailing might be very flexible in shear direction and therefore critical for the fracture healing outcome. The aims of this study were to simulate the shear interfragmentary movement during gait for a human tibia treated with intramedullary nailing and to investigate if this movement could be lowered by implant design modifications. The shear movement was calculated with a 3D finite element model based on computer tomograph images of a cadaver bone-implant complex of a transverse tibia fracture treated with a Stryker T2 Standard Tibial Nail. This model was validated through in vitro test results under pure shear, axial, bending and torsional loading. High shear movements of approximately 4mm were calculated during gait. These shear movements could be reduced by approximately 30% either by implant modifications or the use of a 1mm thicker nail. Combining the implant modifications with a 1mm thicker nail, the shear movements could be reduced by 54%. The increase of the fixation stiffness by using an implant material with a high Young's modulus in combination with an angle-stable nail-screw fixation helps to reduce the shear movement during gait and possibly to lower the risk of a prolonged healing time with unreamed intramedullary nailing.

• Internal forces and moments in the femur of the rat during gait.

  Authors: Tim Wehner, Uwe Wolfram, Thomas Henzler, Frank Niemeyer, Lutz Claes, Ulrich Simon

  Journal of biomechanics. 09/2010; 43(13):2473-9.

  The rat is of increasing importance for experimental studies on fracture healing. The healing outcome of long bone fractures is strongly influenced by mechanical factors, such as the interfragmentaryThe rat is of increasing importance for experimental studies on fracture healing. The healing outcome of long bone fractures is strongly influenced by mechanical factors, such as the interfragmentary movement. This movement depends on the stability of the fracture fixation and the musculoskeletal loads. However, little is known about these loads in rats. The musculoskeletal loads during gait were estimated using an inverse-dynamic musculoskeletal model of the right hindlimb of the rat. This model was based on a micro-CT scan of the lower extremities and an anatomical study using 15 rat cadavers. Kinematics were reconstructed from X-ray movies, taken simultaneously from two perpendicular directions during a gait cycle. The ground reaction forces were taken from the literature. The muscle forces were calculated using an optimization procedure. The internal forces and moments varied over the gait cycle and along the femoral axis. The greatest internal force (up to 7 times bodyweight) acted in the longitudinal direction. The greatest internal moment (up to 13.8 bodyweight times millimeter) acted in the sagittal plane of the femur. The validity of the model was corroborated by comparing the estimated strains caused by the calculated loads on the surface of the femoral mid-shaft with those from the literature. Knowledge of the internal loads in the femur of the rat allows adjustment of the biomechanical properties of fixation devices in fracture healing studies to the desired interfragmentary movement.

• Simulation of Distraction Osteogenesis

  Authors: Ulrich Simon, Tim Wehner, Lutz Claes, Anita Ignatius, Daniel Nolte, Frank Niemeyer

  Proceedings of the 17th Congress of the European Society of Biomechanics, Edinburgh; 07/2010

• Influence of the fixation stability on the healing time--a numerical study of a patient-specific fracture healing process.

  Authors: Tim Wehner, Lutz Claes, Frank Niemeyer, Daniel Nolte, Ulrich Simon

  Clinical biomechanics (Bristol, Avon). 07/2010; 25(6):606-12.

  The healing outcome of long bone fractures is strongly influenced by the interfragmentary movement of the bone fragments. This depends on the fixation stability, the optimum value of which is stillThe healing outcome of long bone fractures is strongly influenced by the interfragmentary movement of the bone fragments. This depends on the fixation stability, the optimum value of which is still not known. The aim of this study was to simulate a patient-specific human healing process using a numerical algorithm and to retrospectively analyse the influence of the fixation stability on the healing time. The healing simulation was processed as an initial value problem. This was iteratively solved based on two mechanical (invariants of the strain tensor, calculated through a finite element analysis) and five biological state variables (local tissue composition and blood perfusion) using a previously published fuzzy logic algorithm. For validation purposes, the calculated interfragmentary movement was compared to in vivo measurements of this patient. By changing clinically adjustable parameters of the fixation device, the influence of the fixation stability on the healing time was analysed. The time course showed good agreement of the interfragmentary movement compared with the in vivo measurements. The predicted healing time was strongly influenced by the fixation stability, i.e. by changing the parameters of the fixation device, it was possible to significantly reduce the healing time. The time to heal could be greatly reduced by modification of the fixator design, i.e. increasing the fixation stiffness. When using external fixation devices, this could be achieved by decreasing the free bending length of the pins, using a stiff fixation body and a stiff connection between the pins and the body.

• Numerische Simulation des Frakturheilungsfortschritts

  Authors: Ulrich Simon, Tim Wehner, Frank Niemeyer, Daniel Nolte, Markus Helmer, Lutz Claes

  06/2010;

• Simulation of the Callus Distraction Treatment

  Authors: Frank Niemeyer, Tim Wehner, Lutz Claes, Anita Ignatius, Ulrich Simon

  Proceedings of the 9th International Symposium on Computer Methods in Biomechanics & Biomedical Engineering, Valencia; 02/2010

• Simulation of Lateral Distraction Osteogenesis

  Authors: Ulrich Simon, Tim Wehner, Daniel Nolte, Lutz Claes, Frank Niemeyer

  Proceedings of the 9th International Symposium on Computer Methods in Biomechanics & Biomedical Engineering, Valencia; 02/2010

• A numerical model of the fracture healing process that describes revascularization and tissue development

  Authors: Ulrich Simon, Peter Augat, Lutz Claes

  Computer Methods in Biomechanics and Biomedical Engineering. 01/2010;

• Statistical osteoporosis models using composite finite elements: A parameter study.

  Authors: Uwe Wolfram, Lars Ole Schwen, Ulrich Simon, Martin Rumpf, Hans-Joachim Wilke

  Journal of biomechanics. 08/2009;

  Osteoporosis is a widely spread disease with severe consequences for patients and high costs for health care systems. The disease is characterised by a loss of bone mass which induces a loss ofOsteoporosis is a widely spread disease with severe consequences for patients and high costs for health care systems. The disease is characterised by a loss of bone mass which induces a loss of mechanical performance and structural integrity. It was found that transverse trabeculae are thinned and perforated while vertical trabeculae stay intact. For understanding these phenomena and the mechanisms leading to fractures of trabecular bone due to osteoporosis, numerous researchers employ micro-finite element models. To avoid disadvantages in setting up classical finite element models, composite finite elements (CFE) can be used. The aim of the study is to test the potential of CFE. For that, a parameter study on numerical lattice samples with statistically simulated, simplified osteoporosis is performed. These samples are subjected to compression and shear loading. Results show that the biggest drop of compressive stiffness is reached for transverse isotropic structures losing 32% of the trabeculae (minus 89.8% stiffness). The biggest drop in shear stiffness is found for an isotropic structure also losing 32% of the trabeculae (minus 67.3% stiffness). The study indicates that losing trabeculae leads to a worse drop of macroscopic stiffness than thinning of trabeculae. The results further demonstrate the advantages of CFEs for simulating micro-structured samples.

• Simulation der Frakturheilung

  Authors: Frank Niemeyer, Ulrich Simon, Daniel Nolte, Lutz Claes, Karsten Urban

  Stuttgart; 07/2009

• A novel model to study metaphyseal bone healing under defined biomechanical conditions.

  Authors: Lutz Claes, Andreas Veeser, Melanie Göckelmann, Ulrich Simon, Anita Ignatius

  Archives of orthopaedic and trauma surgery. 07/2009; 129(7):923-8.

  Experimental studies on metaphyseal fractures are rare and do not control the biomechanical conditions in the healing zone. This study aimed to develop an improved experimental model, whichExperimental studies on metaphyseal fractures are rare and do not control the biomechanical conditions in the healing zone. This study aimed to develop an improved experimental model, which characterizes and controls the biomechanical condition in the fracture gap of a metaphyseal fracture. A partial osteotomy model in the distal femur of the sheep was developed. The osteotomy was located in the region of the trochlea groove. The osteotomy gap was 3 mm wide. The retro-patellar force acting on the joint in vivo causes a bending of the trochlea resulting in a narrowing of the osteotomy gap. To limit and control this interfragmentary movement, stainless steel plates of various thicknesses were implanted into the osteotomy gap. Forces acting on the trochlea were analyzed and a load-deflection curve of the model was determined in vitro. A pilot study on two sheep was performed using the new model with two different interfragmentary movements of 0.3 or 1 mm. Eight weeks, post-operatively, the sheep were sacrificed and undecalcified histology was performed. The biomechanical analysis of the joint forces and the in vitro load-deflection behavior of the trochlea revealed that the forces acting on the trochlea were high enough to cause an interfragmentary movement of 1 mm in the osteotomy gap. This was confirmed by an X-ray of the sheep, which showed a closing of the proximal osteotomy gap under weight-bearing conditions. The histological section revealed no external callus formation. The sheep with the 0.3 mm interfragmentary movement showed almost complete bridging of the osteotomy gap with woven bone whereas the sheep with the 1 mm interfragmentary movement exhibited new bone formation only at the borderline of the osteotomy but larger areas with connective tissue or even fibrous cartilage in the center of the gap. This metaphyseal bone-healing model provides defined and adjustable biomechanical conditions. The histological images demonstrated intramembranous and endochondral bone healing in the osteotomy gap without callus formation. The model therefore seems appropriate to study metaphyseal bone healing under differing mechanical conditions.

• Die Berücksichtigung von Remodelling-Vorgängen ist essentiell für die Modellierung der Kallusentwicklung in späten Frakturheilungsphasen

  Authors: Frank Niemeyer, Ulrich Simon, Claes Lutz

  Proceedings of the 6. Jahrestagung der Deutschen Gesellschaft für Biomechanik, Münster; 05/2009

• Internal loads in the human tibia during gait.

  Authors: Tim Wehner, Lutz Claes, Ulrich Simon

  Clinical biomechanics (Bristol, Avon). 02/2009;

  BACKGROUND: Internal loads in long bones are of special interest, for testing and optimization of current implant designs for fracture treatment. Inverse dynamic musculoskeletal models wereBACKGROUND: Internal loads in long bones are of special interest, for testing and optimization of current implant designs for fracture treatment. Inverse dynamic musculoskeletal models were successfully used to determine the muscle forces in the lower extremities, however the internal loads expressed as forces and moments along the human tibia during gait have not been reported. METHODS: A musculoskeletal model of the lower extremities, developed and provided under public domain from the AnyBody research group (www.anybody.aau.dk), was modified to determine the three-dimensional internal loads along the tibial axis during gait. To corroborate this numerical model, the calculated resultant hip contact force as well as the axial force on the tibial plateau were compared to in vivo data from the literature. FINDINGS: The highest internal loads were the axial force with up to 4.7 bodyweight and the bending moment in the sagittal plane with up to 71.6 bodyweight times millimetre in the late stance phase. The extreme values of the internal loads along the tibial axis varied up to 1.5 bodyweight and 85.7 bodyweight times millimetre. In the distal part of the tibia, the axial force dominated the three-dimensional internal load case, whereas the internal moments became more significant with increasing distance from the ankle joint. INTERPRETATION: This study provides for the first time the three-dimensional internal loads, expressed as forces and moments along the human tibia during normal gait. The results of this study could be used to improve the mechanical behaviour of current implant designs for the treatment of tibial fractures to avoid implant failures under in vivo loading conditions.

• A novel model to study metaphyseal bone healing under defined biomechanical conditions

  Authors: Lutz Claes, Andreas Veeser, Melanie Göckelmann, Ulrich Simon, Anita Ignatius

  Archives of Orthopaedic and Trauma Surgery. 01/2009; 129:923--928.

  Abstract Introduction Experimental studies on metaphyseal fractures are rare and do not control the biomechanical conditions in the healing zone. This study aimed to develop an improved experimentalAbstract Introduction Experimental studies on metaphyseal fractures are rare and do not control the biomechanical conditions in the healing zone. This study aimed to develop an improved experimental model, which characterizes and controls the biomechanical condition in the fracture gap of a metaphyseal fracture. Materials and methods A partial osteotomy model in the distal femur of the sheep was developed. The osteotomy was located in the region of the trochlea groove. The osteotomy gap was 3 mm wide. The retro-patellar force acting on the joint in vivo causes a bending of the trochlea resulting in a narrowing of the osteotomy gap. To limit and control this interfragmentary movement, stainless steel plates of various thicknesses were implanted into the osteotomy gap. Forces acting on the trochlea were analyzed and a load-deflection curve of the model was determined in vitro. A pilot study on two sheep was performed using the new model with two different interfragmentary movements of 0.3 or 1 mm. Eight weeks, post-operatively, the sheep were sacrificed and undecalcified histology was performed. Results The biomechanical analysis of the joint forces and the in vitro load-deflection behavior of the trochlea revealed that the forces acting on the trochlea were high enough to cause an interfragmentary movement of 1 mm in the osteotomy gap. This was confirmed by an X-ray of the sheep, which showed a closing of the proximal osteotomy gap under weight-bearing conditions. The histological section revealed no external callus formation. The sheep with the 0.3 mm interfragmentary movement showed almost complete bridging of the osteotomy gap with woven bone whereas the sheep with the 1 mm interfragmentary movement exhibited new bone formation only at the borderline of the osteotomy but larger areas with connective tissue or even fibrous cartilage in the center of the gap. Conclusion This metaphyseal bone-healing model provides defined and adjustable biomechanical conditions. The histological images demonstrated intramembranous and endochondral bone healing in the osteotomy gap without callus formation. The model therefore seems appropriate to study metaphyseal bone healing under differing mechanical conditions.

• Mechanical characterization of external fixator stiffness for a rat femoral fracture model.

  Authors: Bettina Willie, Kyle Adkins, Xing Zheng, Ulrich Simon, Lutz Claes

  Journal of orthopaedic research : official publication of the Orthopaedic Research Society. 12/2008;

  Clinical and experimental studies have shown that several mechanical factors influence the fracture healing process. One such factor, interfragmentary movement, is affected by loading and theClinical and experimental studies have shown that several mechanical factors influence the fracture healing process. One such factor, interfragmentary movement, is affected by loading and the stiffness of the fixation device. This study evaluated the stiffness of different external fixation devices for a rat femoral fracture model, using in vitro and analytical methods. The contribution to the stiffness of the fixation construct was dominated by the flexibility of the pins in relation to their offset, diameter, and material properties. The axial stiffness increased with decreasing offset and increasing pin diameter. Titanium pins resulted in significantly lower axial stiffness compared to stainless steel pins of the same design. The fixator body material and fixator length had a less pronounced influence on fixation stiffness. Mechanically characterized external fixation devices will allow in vivo study of the fracture healing process utilizing pre-calculated fracture fixation stiffness. These characterized fixation devices will allow controlled manipulation of the axial and shear interfragmentary movement to achieve a flexible fixation resulting in callus formation compared to a more rigid fixation limiting callus formation in a rat femoral fracture model. (c) 2008 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res.

• Acquisition of full-field strain distributions on ovine fracture callus cross-sections with electronic speckle pattern interferometry.

  Authors: Michael Bottlang, Marcus Mohr, Ulrich Simon, Lutz Claes

  Journal of biomechanics. 02/2008; 41(3):701-5.

  This study evaluated the feasibility of assessing continuous strain distributions on fracture callus cross-sections with an electronic speckle pattern interferometry (ESPI) system. Mid-sagittalThis study evaluated the feasibility of assessing continuous strain distributions on fracture callus cross-sections with an electronic speckle pattern interferometry (ESPI) system. Mid-sagittal callus cross-sections were harvested from ovine tibiae. One low stiffness (LS) specimen and one high stiffness (HS) specimen were selected to evaluate the feasibility for strain acquisition over a range of callus properties. The HS specimen was 147 times stiffer in compression than the LS specimen. ESPI captured continuous strain distributions on both specimens. Peak strain was located adjacent to cortical boundaries in the osteotomy gap. In response to 5N compression, peak compressive strain of 5.8% in the LS specimen was over two orders of magnitude higher than peak compressive strain of 0.013% in the HS specimen. In conclusion, ESPI-based strain acquisition enables reproducible quantification of strain distributions on callus cross-sections. Such measurements may support validation of computational models and evaluation of experimental results in fracture healing research.