Robert Souffrant

A convenient approach for finite-element-analyses of orthopaedic implants in bone contact: Modeling and experimental validation

With regard to the growing potential of finite-element-analysis (FEA) in the field of orthopaedic biomechanics, we present an approach helping in the development of appropriate models of the implant-bone compound. The algorithm is based on computed-tomography data of the bone and accordant computer-aided-design (CAD) data of the implant and aims at predicting the bone strains and interface mechanics of the included parts. The developed algorithm was validated exemplary using an acetabular cup in combination with a left and a right fresh-frozen human hemipelvis. The strains under maximum loads during the gait cycle as well as the micromotion in the bone-implant interface were measured and compared to results from equivalent finite-element-analyses. Thereby, we found strong correlation between the calculated and measured principal strains with correlation coefficients of r(2)=0.94 (left side) and r(2)=0.86 (right side). A validation of micromotion was not possible due to limited accuracy of the motion tracking system.

Finite Element Analysis of Osteosynthesis Screw Fixation in the Bone Stock: An Appropriate Method for Automatic Screw Modelling

The use of finite element analysis (FEA) has grown to a more and more important method in the field of biomedical engineering and biomechanics. Although increased computational performance allows new ways to generate more complex biomechanical models, in the area of orthopaedic surgery, solid modelling of screws and drill holes represent a limitation of their use for individual cases and an increase of computational costs. To cope with these requirements, different methods for numerical screw modelling have therefore been investigated to improve its application diversity. Exemplarily, fixation was performed for stabilization of a large segmental femoral bone defect by an osteosynthesis plate. Three different numerical modelling techniques for implant fixation were used in this study, i.e. without screw modelling, screws as solid elements as well as screws as structural elements. The latter one offers the possibility to implement automatically generated screws with variable geometry on arbitrary FE models. Structural screws were parametrically generated by a Python script for the automatic generation in the FE-software Abaqus/CAE on both a tetrahedral and a hexahedral meshed femur. Accuracy of the FE models was confirmed by experimental testing using a composite femur with a segmental defect and an identical osteosynthesis plate for primary stabilisation with titanium screws. Both deflection of the femoral head and the gap alteration were measured with an optical measuring system with an accuracy of approximately 3 µm. For both screw modelling techniques a sufficient correlation of approximately 95% between numerical and experimental analysis was found. Furthermore, using structural elements for screw modelling the computational time could be reduced by 85% using hexahedral elements instead of tetrahedral elements for femur meshing. The automatically generated screw modelling offers a realistic simulation of the osteosynthesis fixation with screws in the adjacent bone stock and can be used for further investigations.

Finite element analysis on the biomechanical stability of open porous titanium scaffolds for large segmental bone defects under physiological load conditions.

Repairing large segmental defects in long bones caused by fracture, tumour or infection is still a challenging problem in orthopaedic surgery. Artificial materials, i.e. titanium and its alloys performed well in clinical applications, are plenary available, and can be manufactured in a wide range of scaffold designs. Although the mechanical properties are determined, studies about the biomechanical behaviour under physiological loading conditions are rare. The goal of our numerical study was to determine the suitability of open-porous titanium scaffolds to act as bone scaffolds. Hence, the mechanical stability of fourteen different scaffold designs was characterized under both axial compression and biomechanical loading within a large segmental distal femoral defect of 30mm. This defect was stabilized with an osteosynthesis plate and physiological hip reaction forces as well as additional muscle forces were implemented to the femoral bone. Material properties of titanium scaffolds were evaluated from experimental testing. Scaffold porosity was varied between 64 and 80%. Furthermore, the amount of material was reduced up to 50%. Uniaxial compression testing revealed a structural modulus for the scaffolds between 3.5GPa and 19.1GPa depending on porosity and material consumption. The biomechanical testing showed defect gap alterations between 0.03mm and 0.22mm for the applied scaffolds and 0.09mm for the intact bone. Our results revealed that minimizing the amount of material of the inner core has a smaller influence than increasing the porosity when the scaffolds are loaded under biomechanical loading. Furthermore, an advanced scaffold design was found acting similar as the intact bone.

The influence of head diameter and wall thickness on deformations of metallic acetabular press-fit cups and UHMWPE liners: a finite element analysis.

BackgroundTo increase the range of motion of total hip endoprostheses, prosthetic heads need to be enlarged, which implies that the cup and/or liner thickness must decrease. This may have negative effects on the wear rate, because the acetabular cups and liners could deform during press-fit implantation and hip joint loading. We compared the metal cup and polyethylene liner deformations that occurred when different wall thicknesses were used in order to evaluate the resulting changes in the clearance of the articulating region.MethodsA parametric finite element model utilized three cup and liner wall thicknesses to analyze cup and liner deformations after press-fit implantation into the pelvic bone. The resultant hip joint force during heel strike was applied while the femur was fixed, accounting for physiological muscle forces. The deformation behavior of the liner under joint loading was therefore assessed as a function of the head diameter and the resulting clearance.ResultsPress-fit implantation showed diametral cup deformations of 0.096, 0.034, and 0.014xa0mm for cup wall thicknesses of 3, 5, and 7xa0mm, respectively. The largest deformations (average 0.084xa0±xa00.003xa0mm) of liners with thicknesses of 4, 6, and 8xa0mm occurred with the smallest cup wall thickness (3xa0mm). The smallest liner deformation (0.011xa0mm) was obtained with largest cup and liner wall thicknesses. Under joint loading, liner deformations in thin-walled acetabular cups (3xa0mm) reduced the initial clearance by about 50xa0%.ConclusionAcetabular press-fit cups with wall thicknesses of ≤5xa0mm should only be used in combination with polyethylene liners >6xa0mm thick in order to minimize the reduction in clearance.

HiL simulation in biomechanics: A new approach for testing total joint replacements

Instability of artificial joints is still one of the most prevalent reasons for revision surgery caused by various influencing factors. In order to investigate instability mechanisms such as dislocation under reproducible, physiologically realistic boundary conditions, a novel test approach is introduced by means of a hardware-in-the-loop (HiL) simulation involving a highly flexible mechatronic test system. In this work, the underlying concept and implementation of all required units is presented enabling comparable investigations of different total hip and knee replacements, respectively. The HiL joint simulator consists of two units: a physical setup composed of a six-axes industrial robot and a numerical multibody model running in real-time. Within the multibody model, the anatomical environment of the considered joint is represented such that the soft tissue response is accounted for during an instability event. Hence, the robot loads and moves the real implant components according to the information provided by the multibody model while transferring back the position and resisting moment recorded. Functionality of the simulator is proved by testing the underlying control principles, and verified by reproducing the dislocation process of a standard total hip replacement. HiL simulations provide a new biomechanical testing tool for analyzing different joint replacement systems with respect to their instability behavior under realistic movements and physiological load conditions.

Advanced material modelling in numerical simulation of primary acetabular press-fit cup stability

Primary stability of artificial acetabular cups, used for total hip arthroplasty, is required for the subsequent osteointegration and good long-term clinical results of the implant. Although closed-cell polymer foams represent an adequate bone substitute in experimental studies investigating primary stability, correct numerical modelling of this material depends on the parameter selection. Material parameters necessary for crushable foam plasticity behaviour were originated from numerical simulations matched with experimental tests of the polymethacrylimide raw material. Experimental primary stability tests of acetabular press-fit cups consisting of static shell assembly with consecutively pull-out and lever-out testing were subsequently simulated using finite element analysis. Identified and optimised parameters allowed the accurate numerical reproduction of the raw material tests. Correlation between experimental tests and the numerical simulation of primary implant stability depended on the value of interference fit. However, the validated material model provides the opportunity for subsequent parametric numerical studies.

Relationship Between Mechanical Properties and Bone Mineral Density of Human Femoral Bone Retrieved from Patients with Osteoarthritis

The objective of this study was to analyse retrieved human femoral bone samples using three different test methods, to elucidate the relationship between bone mineral density and mechanical properties. Human femoral heads were retrieved from 22 donors undergoing primary total hip replacement due to hip osteoarthritis and stored for a maximum of 24 hours postoperatively at + 6 °C to 8 °C. Analysis revealed an average structural modulus of 232±130 N/mm2 and ultimate compression strength of 6.1±3.3 N/mm2 with high standard deviations. Bone mineral densities of 385±133 mg/cm2 and 353±172 mg/cm3 were measured using thedual energy X-ray absorptiometry (DXA) and quantitative computed tomography (QCT), respectively. Ashing resulted in a bone mineral density of 323±97 mg/cm3. In particular, significant linear correlations were found between DXA and ashing with r = 0.89 (p < 0.01, n = 22) and between structural modulus and ashing with r = 0.76 (p < 0.01, n = 22). Thus, we demonstrated a significant relationship between mechanical properties and bone density. The correlations found can help to determine the mechanical load capacity of individual patients undergoing surgical treatments by means of noninvasive bone density measurements.

Evaluation of electric field distribution in electromagnetic stimulation of human femoral head.

Electromagnetic stimulation is a common therapy used to support bone healing in the case of avascular necrosis of the femoral head. In the present study, we investigated a bipolar induction screw system with an integrated coil. The aim was to analyse the influence of the screw parameters on the electric field distribution in the human femoral head. In addition, three kinds of design parameters (the shape of the screw tip, position of the screw in the femoral head, and size of the screw insulation) were varied. The electric field distribution in the bone was calculated using the finite element software Comsol Multiphysics. Moreover, a validation experiment was set up for an identical bone specimen with an implanted screw. The electric potential of points inside and on the surface of the bone were measured and compared to numerical data. The electric field distribution within the bone was clearly changed by the different implant parameters. Repositioning the screw by a maximum of 10 mm and changing the insulation length by a maximum of 4 mm resulted in electric field volume changes of 16% and 7%, respectively. By comparing the results of numerical simulation with the data of the validation experiment, on average, the electric potential difference of 19% and 24% occurred when the measuring points were at a depth of approximately 5 mm within the femoral bone and directly on the surface of the femoral bone, respectively. The results of the numerical simulations underline that the electro-stimulation treatment of bone in clinical applications can be influenced by the implant parameters.

Robot-Based HiL Test of Joint Endoprostheses

To simulate the dislocation behavior of total hip endoprostheses in their anatomical environment a novel Hardware-in-the-Loop (HiL) simulator is built up. It couples a real endoprosthesis with a numerical simulation of its environment by means of an industrial robot as actuator system. The simulation model describes the dynamics of the biomechanical motions including the tissue and muscle forces. The motion and joint constraint forces are calculated by the simulation model and applied to the endoprosthesis by the robot under hybrid position/force control. The actual position of the endoprosthesis in the constrained directions and torques in the unconstrained directions are measured and fed back into the simulation model closing the control loop. To demonstrate the functional principle of the HiL simulator the dynamic behavior of a test setup is numerically simulated.

Analytisches Berechnungsmodell zur Bestimmung des Einflusses konstruktiver und operativer Faktoren auf den Bewegungsumfang von Hüftendoprothesen / Analytical computational model for the determination of the influence of design and surgical factors on the range of motion of total hip replacements

Zusammenfassung Impingement und Luxationen zählen zu den häufigeren Versagensursachen von Hüftendoprothesen. Zur diesbezüglichen Optimierung der Implantatkomponenten werden Simulationsverfahren eingesetzt, die eine umfassende mathematische Beschreibung der Kinematik unter Berücksichtigung konstruktiver und operativer Parameter erfordern. Für die Untersuchung des Luxationsverhaltens sind räumliche Bewegungsbahnen bis zum Impingement mit den zugehörigen Belastungsszenarien zu generieren. In der vorliegenden Arbeit werden die Berechnungsgrundlagen zur Bestimmung des Bewegungsumfangs von Hüftendoprothesen unter Berücksichtigung mehrachsiger, überlagerter Bewegungen vorgestellt. Die wesentlichen Designparameter wie Kopf- und Halsdurchmesser, CCD-Winkel und Kopfüberdeckung werden dabei berücksichtigt. Darüber hinaus wird die Einbaulage der künstlichen Hüftpfanne über die Inklination und Anteversion sowie die Stielantetorsion erfasst. Mit diesem Ansatz für überlagerte Bewegungen ist es möglich, bei gegebenen Auslenkungen in z.B. Flexion/Extension die Restdrehwinkel für z.B. Abduktion oder Innenrotation zu ermitteln. So können für beliebige, mehrachsige Bewegungen die kritischen, luxationsbedingenden Gelenkstellungen bei einer Primär- oder Wechseloperation bestimmt werden, d.h., der Operateur kann bei vorgegebener Endoprothesenlage den maximalen Bewegungsumfang ermitteln. Die Berechnungen sind zudem nutzbar für weitere Geometrieoptimierungen der Implantate. Mit Hilfe dieser Berechnungsalgorithmen lassen sich ROM-Karten (grafische Darstellung der Range of Motion in Abhängigkeit der Implantatposition) erstellen, welche den Operateur bei der Implantatpositionierung unterstützen. Darüber hinaus werden die Ergebnisse in Prüfsystemen zur experimentellen Analyse von Impingement und Subluxation verwendet. Abstract Impingement and dislocations rank among the frequent failure causes of hip endoprotheses. The further optimization of endoprotheses requires a comprehensive mathematical description of the kinematics with consideration of surgical and design parameters. For the investigation of dislocation behavior, spatial movements up to impingement with associated load scenarios should be generated. We present fundamentals for the determination of the range of motion of total hip replacements with consideration of multidirectional, superimposed movements. Therefore, the remaining angle, e.g., of abduction/adduction or internal/external rotation depending on flexion/extension can be calculated. Thereby, the substantial design parameters such as head and neck diameter, CCD angle and head coverage are considered. Moreover, the position of the acetabular cup in terms of inclination and anteversion angle as well as neck anteversion is considered. Using this approach, especially designed for superimposed movements, residual range of motion for given movements, e.g., abduction or internal rotation for given angles of flexion/extension can be calculated. Thus, the critical dislocation-initiating joint positions for primary or revision total hip arthroplasty can be determined for arbitrary superimposed movements; subsequently, the operating surgeon can evaluate the maximum range of motion for a given implant position. Additionally, the calculations are of help for further geometrical optimization of implants. The calculation algorithms can be used to create ROM maps (graphical illustration of the range of motion depending on implant position) which support the operating surgeon in placement of the implant components. Moreover, our results are utilized for experimental test setups to analyze impingement and subluxation.