Fourth International Conference on Experimental Mechanics, 2009
In connection with technological advances in the manufacturing of medical ceramics, a newly devel... more In connection with technological advances in the manufacturing of medical ceramics, a newly developed ceramic femoral component was introduced in total knee arthroplasty (TKA). The motivation to consider ceramics in TKA is based on the allergological and tribological benefits as proven in total hip arthroplasty. Owing to the brittleness and reduced fracture toughness of ceramic materials, the biomechanical performance has
Dislocation of the artificial joint is a serious and relatively frequent complication of total hi... more Dislocation of the artificial joint is a serious and relatively frequent complication of total hip replacement. Despite increasing clinical experiences there are still numerous open questions about influence parameters on the stability against dislocation like implant design, implant position, neck length, offset, actual loads as well as anatomic conditions like hip muscles, ligaments and capsule structures. Because in vivo measure-ments of the process of dislocation are not possible, a novel mechatronic hardware-in-the-loop (HiL) joint simulator is developed. The system consists of a simulation computer running a biomechanical multibody model of the anatomic environment of the hip joint and a six-axis robot which moves and loads a real hip endoprosthesis. The biomechanical multibody model delivers motions and constraint forces in complemen-tary spatial directions that are applied by the robot on the endoprosthesis by means of hybrid position and force control. Actual loading and po...
Closed cell polymer foams are often used as bone material substitutes in biomechanical experiment... more Closed cell polymer foams are often used as bone material substitutes in biomechanical experiments, due to better reproducibility and availability compared to human bone specimens. However, the numerical simulation of such materials is difficult, when high deformations are necessary, e.g. in simulations of press-fit implants. The present study analysed the usability of the crushable foam plasticity model for the numerical
A major problem of total knee endoprostheses is the instability of the artificial joint. Contribu... more A major problem of total knee endoprostheses is the instability of the artificial joint. Contributing factors are, for example, insufficient ligament structure, inappropriate implant design or implant malposition. Despite increasing clinical experiences the reoperation rate caused by knee instability has not reduced significantly during the last years. There are still numerous open questions about influence parameters on postoperative instability of
Two revisions of broken β-titanium total hip stems had to be performed in our hospital after 2 an... more Two revisions of broken β-titanium total hip stems had to be performed in our hospital after 2 and 4 years in situ. Since both fractures were located at the level of a laser engraving, a failure analysis was conducted. Both retrieved hip stems were disinfected and collected in our retrieval database after patient's signed agreement. Each fragment was macroscopically photographed. Fracture surfaces were analyzed using scanning electron microscopy (SEM). Quantification of element content was conducted using energy dispersive X-ray (EDX) analysis. Both stems show fatigue fracture, as displayed by the lines of rest on the fracture surface. The origin of fracture was identified directly at the laser engraving of the company logo at both stems by means of SEM. The EDX analysis showed an oxygen level beneath the laser engraving about twice as high as in the substrate, causing material embrittlement. Laser engravings need to be reduced to a minimum of necessary information, and should be placed at locations with minimum mechanical load. Biomechanical analyses are recommended to identify less loaded areas in implant components to avoid such implant failures.
An adequate primary stability and a subsequent stable osseous fixation (secondary stability) of a... more An adequate primary stability and a subsequent stable osseous fixation (secondary stability) of artificial hip cups are required for long-term implant survival. The aim of this study was to analyse the design of cementless press-fit cups as an influencing factor of primary stability. Different hemispherical and conical cup designs were analysed. The fixation stability of the cups was detected experimentally using a spongiosa and a cortical model based on artificial bone as well as a numerical simulation using a spongiosa model by pull-out and lever-out tests. In addition, the stress on the osseous cup cavity was determined in the finite-element analysis. All tested cup designs revealed higher fixation stability in the cortical bone model compared to the spongiosa model. The experimental tests did not show an increase of fixation stability with the conical cup profile in comparison to hemispherical cup profiles. Therefore, cementless press-fit cups with conical cup profile do not pro...
The aim of the present wear simulator study was to assess the effect of steep acetabular cup posi... more The aim of the present wear simulator study was to assess the effect of steep acetabular cup positions on the wear propagation of highly cross-linked-PE (HX-PE) liners. Furthermore, a finite element analysis (FEA) was performed in order to calculate the stress within the HX-PE material in case of steep cup positions under physiological loadings. The higher stress in the HX-PE at a steep acetabular cup position did not result in increased wear in the present wear simulator study. The gravimetrical wear rates at normal (45°) and steep cup inclinations (75°) showed wear amounts of 3.15±0.27mg and 2.18±0.31mg per million cycles (p=0.028), respectively. However, FEA revealed clear increase in stress at the HX-PE liners with respect to steep cup positions.
Finite element analyses (FEA) as well as multibody system dynamics (MSD) are the main tools used ... more Finite element analyses (FEA) as well as multibody system dynamics (MSD) are the main tools used for numerical simulation in the field of musculoskeletal research. While FEA is utilized for field problems, such as calculation of stress and strain distribution, MSD is applied for solving kinematic analyses, such as calculation of muscle and joint forces. Depending on the focus of investigation, modelling of biological tissue may vary from simple homogeneous behavior to modelling biochemical processes on the microscale and nanoscale. An important milestone in biomechanical research was the analysis of stress shielding, which led to further research on bone remodelling. Various models of implant-bone fixation used for the prediction of micromotion have been published. New possibilities for biomechanical analyses are achieved by consideration of complex muscle forces which are generated by MSD simulation and imported into FEA models as limiting conditions. A numerical model always requi...
... 9. Kaehler M, Woernle C, Bader R (2009) Hardware-in-the-loop-simulaton of constraint elements... more ... 9. Kaehler M, Woernle C, Bader R (2009) Hardware-in-the-loop-simulaton of constraint elements in mechanical systems, Proc.of the 5th International Workshop on Computational Kinematics, Duisburg, Germany, 2009, pp 159-166. 10. ... Author: Dipl.-Ing. Sven Herrmann, M.Sc. ...
Proceedings of SPIE - The International Society for Optical Engineering, 2010
The finite element method is used in various approaches to solve biomechanical problems. We prese... more The finite element method is used in various approaches to solve biomechanical problems. We present a concept helping in the development of appropriate models of the implant-bone compound based on different software packages. The reconstruction of bone morphology is based on computed tomography (CT) data of the designated bone. After the bone is three-dimensionally reconstructed in the CAD-environment, virtual implantation
Die Häufigkeit von Kniegelenksarthrosen nimmt durch erhöhte Prävalenzraten im Alter und stetig st... more Die Häufigkeit von Kniegelenksarthrosen nimmt durch erhöhte Prävalenzraten im Alter und stetig steigende Lebenserwartung der Bevölkerung zu. Nach Ausschöpfung aller konservativen und operativen Maßnahmen bleibt der endoprothetische Kniegelenksersatz häufig die einzige Möglichkeit, Schmerzfreiheit und Gelenkfunktion wieder herzustellen sowie vorhandene Deformitäten zu korrigieren. Die Wiederaufnahme der individuellen Alltagsaktivitäten sowie die Selbstversorgung stehen dabei für den Patienten im Vordergrund. Nach Schätzungen von
The major reason for revision surgery of load bearing implants is aseptic loosening, especially w... more The major reason for revision surgery of load bearing implants is aseptic loosening, especially with regard to total hip arthroplasty. Osseous anchorage of loosened implants needs to be measured exactly in order to ensure the indication for revision surgery and additionally to enable early detection of the loosening process. However, current diagnostic methods show unsatisfying results concerning sensitivity and specificity. Therefore, a new sensor system is being developed based on an acousto-mechanical approach, which can be used non-invasively and radiation-free to measure the stability of endoprosthesis fixation on the bone stock. This paper intends to show initial results of functional models, which are used to prove the measuring principle.
Dislocation remains a serious complication of total hip replacement. An insufficient range of mot... more Dislocation remains a serious complication of total hip replacement. An insufficient range of motion can lead to impingement of the prosthetic neck on the acetabular cup. Together with the initiation of subluxation and dislocation, recurrent impingement can cause material failure in the liner. The objective of this study was to generate a validated finite element (FE) model capable of predicting the dislocation stability of different femoral head sizes with regard to impingement in different implant positions as well as the corresponding stress distribution in the liner. In order to cover posterior and anterior dislocation, two total hip dislocation associated manoeuvres were simulated using a three-dimensional nonlinear finite element model. The dislocation stability of two head sizes was determined numerically and experimentally. After validation, the FE model was used to analyse the dislocation stability of four different head sizes in variable implant positions. Range of motion (ROM) until impingement, the resisting moment that was developed and ROM until dislocation were evaluated. Additionally, stress distribution within the polyethylene liner during impingement and subluxation was determined. For both dislocation modes, a cup position of 45 degrees lateral abduction and 15 degrees up to 30 degrees anteversion resulted in appropriate ROM and dislocation stability. In general, larger head diameters revealed an increase in ROM and higher resisting moments. Stress analysis showed decreased contact pressures at the egress site of the liners with the larger inner diameters during subluxation. The analysis shows that an optimal implant position and a larger head diameter can reduce the risk of dislocation induced by impingement. The finite element model that was developed enables simplification of design variations compared to experimental studies since prototyping and assembling are replaced by prompt numerical simulation.
A certain failure mode using a newly developed cemented ceramic femoral component in total knee r... more A certain failure mode using a newly developed cemented ceramic femoral component in total knee replacement was observed in clinical application, i.e. fracture of the femoral component during intraoperative impaction. This may be caused by unintentional deflection of the saw blades during cutting with consecutive higher resection angle of the distal femur than desired, leading to bending of the femoral component during implantation. A finite-element-analysis was carried out to simulate implantation of the femoral component and to evaluate the influence of distal femur preparation on implant stress. We developed and validated a numerical model of the ceramic femoral component including a contact formulation which allowed calculating the principal stresses of the implant during implantation onto the resected femur. The analysis considered different anterior and posterior resection angles with a total of 17 variations. By increasing the femoral resection angle in the finite-element-model it could be shown that a deviation of three degrees from the intended resection angle can cause critical stress amounts during implantation. When implanting the ceramic component in total knee arthroplasty, the femoral resection angles should be prepared very precisely, in particular anterior saw blade deflection has to be avoided. The implant manufacturer increased implant safety through an additional resection template. Moreover, the impaction of the ceramic femoral component during cementing was not further recommended by using a hammer.
Fourth International Conference on Experimental Mechanics, 2009
In connection with technological advances in the manufacturing of medical ceramics, a newly devel... more In connection with technological advances in the manufacturing of medical ceramics, a newly developed ceramic femoral component was introduced in total knee arthroplasty (TKA). The motivation to consider ceramics in TKA is based on the allergological and tribological benefits as proven in total hip arthroplasty. Owing to the brittleness and reduced fracture toughness of ceramic materials, the biomechanical performance has
Dislocation of the artificial joint is a serious and relatively frequent complication of total hi... more Dislocation of the artificial joint is a serious and relatively frequent complication of total hip replacement. Despite increasing clinical experiences there are still numerous open questions about influence parameters on the stability against dislocation like implant design, implant position, neck length, offset, actual loads as well as anatomic conditions like hip muscles, ligaments and capsule structures. Because in vivo measure-ments of the process of dislocation are not possible, a novel mechatronic hardware-in-the-loop (HiL) joint simulator is developed. The system consists of a simulation computer running a biomechanical multibody model of the anatomic environment of the hip joint and a six-axis robot which moves and loads a real hip endoprosthesis. The biomechanical multibody model delivers motions and constraint forces in complemen-tary spatial directions that are applied by the robot on the endoprosthesis by means of hybrid position and force control. Actual loading and po...
Closed cell polymer foams are often used as bone material substitutes in biomechanical experiment... more Closed cell polymer foams are often used as bone material substitutes in biomechanical experiments, due to better reproducibility and availability compared to human bone specimens. However, the numerical simulation of such materials is difficult, when high deformations are necessary, e.g. in simulations of press-fit implants. The present study analysed the usability of the crushable foam plasticity model for the numerical
A major problem of total knee endoprostheses is the instability of the artificial joint. Contribu... more A major problem of total knee endoprostheses is the instability of the artificial joint. Contributing factors are, for example, insufficient ligament structure, inappropriate implant design or implant malposition. Despite increasing clinical experiences the reoperation rate caused by knee instability has not reduced significantly during the last years. There are still numerous open questions about influence parameters on postoperative instability of
Two revisions of broken β-titanium total hip stems had to be performed in our hospital after 2 an... more Two revisions of broken β-titanium total hip stems had to be performed in our hospital after 2 and 4 years in situ. Since both fractures were located at the level of a laser engraving, a failure analysis was conducted. Both retrieved hip stems were disinfected and collected in our retrieval database after patient's signed agreement. Each fragment was macroscopically photographed. Fracture surfaces were analyzed using scanning electron microscopy (SEM). Quantification of element content was conducted using energy dispersive X-ray (EDX) analysis. Both stems show fatigue fracture, as displayed by the lines of rest on the fracture surface. The origin of fracture was identified directly at the laser engraving of the company logo at both stems by means of SEM. The EDX analysis showed an oxygen level beneath the laser engraving about twice as high as in the substrate, causing material embrittlement. Laser engravings need to be reduced to a minimum of necessary information, and should be placed at locations with minimum mechanical load. Biomechanical analyses are recommended to identify less loaded areas in implant components to avoid such implant failures.
An adequate primary stability and a subsequent stable osseous fixation (secondary stability) of a... more An adequate primary stability and a subsequent stable osseous fixation (secondary stability) of artificial hip cups are required for long-term implant survival. The aim of this study was to analyse the design of cementless press-fit cups as an influencing factor of primary stability. Different hemispherical and conical cup designs were analysed. The fixation stability of the cups was detected experimentally using a spongiosa and a cortical model based on artificial bone as well as a numerical simulation using a spongiosa model by pull-out and lever-out tests. In addition, the stress on the osseous cup cavity was determined in the finite-element analysis. All tested cup designs revealed higher fixation stability in the cortical bone model compared to the spongiosa model. The experimental tests did not show an increase of fixation stability with the conical cup profile in comparison to hemispherical cup profiles. Therefore, cementless press-fit cups with conical cup profile do not pro...
The aim of the present wear simulator study was to assess the effect of steep acetabular cup posi... more The aim of the present wear simulator study was to assess the effect of steep acetabular cup positions on the wear propagation of highly cross-linked-PE (HX-PE) liners. Furthermore, a finite element analysis (FEA) was performed in order to calculate the stress within the HX-PE material in case of steep cup positions under physiological loadings. The higher stress in the HX-PE at a steep acetabular cup position did not result in increased wear in the present wear simulator study. The gravimetrical wear rates at normal (45°) and steep cup inclinations (75°) showed wear amounts of 3.15±0.27mg and 2.18±0.31mg per million cycles (p=0.028), respectively. However, FEA revealed clear increase in stress at the HX-PE liners with respect to steep cup positions.
Finite element analyses (FEA) as well as multibody system dynamics (MSD) are the main tools used ... more Finite element analyses (FEA) as well as multibody system dynamics (MSD) are the main tools used for numerical simulation in the field of musculoskeletal research. While FEA is utilized for field problems, such as calculation of stress and strain distribution, MSD is applied for solving kinematic analyses, such as calculation of muscle and joint forces. Depending on the focus of investigation, modelling of biological tissue may vary from simple homogeneous behavior to modelling biochemical processes on the microscale and nanoscale. An important milestone in biomechanical research was the analysis of stress shielding, which led to further research on bone remodelling. Various models of implant-bone fixation used for the prediction of micromotion have been published. New possibilities for biomechanical analyses are achieved by consideration of complex muscle forces which are generated by MSD simulation and imported into FEA models as limiting conditions. A numerical model always requi...
... 9. Kaehler M, Woernle C, Bader R (2009) Hardware-in-the-loop-simulaton of constraint elements... more ... 9. Kaehler M, Woernle C, Bader R (2009) Hardware-in-the-loop-simulaton of constraint elements in mechanical systems, Proc.of the 5th International Workshop on Computational Kinematics, Duisburg, Germany, 2009, pp 159-166. 10. ... Author: Dipl.-Ing. Sven Herrmann, M.Sc. ...
Proceedings of SPIE - The International Society for Optical Engineering, 2010
The finite element method is used in various approaches to solve biomechanical problems. We prese... more The finite element method is used in various approaches to solve biomechanical problems. We present a concept helping in the development of appropriate models of the implant-bone compound based on different software packages. The reconstruction of bone morphology is based on computed tomography (CT) data of the designated bone. After the bone is three-dimensionally reconstructed in the CAD-environment, virtual implantation
Die Häufigkeit von Kniegelenksarthrosen nimmt durch erhöhte Prävalenzraten im Alter und stetig st... more Die Häufigkeit von Kniegelenksarthrosen nimmt durch erhöhte Prävalenzraten im Alter und stetig steigende Lebenserwartung der Bevölkerung zu. Nach Ausschöpfung aller konservativen und operativen Maßnahmen bleibt der endoprothetische Kniegelenksersatz häufig die einzige Möglichkeit, Schmerzfreiheit und Gelenkfunktion wieder herzustellen sowie vorhandene Deformitäten zu korrigieren. Die Wiederaufnahme der individuellen Alltagsaktivitäten sowie die Selbstversorgung stehen dabei für den Patienten im Vordergrund. Nach Schätzungen von
The major reason for revision surgery of load bearing implants is aseptic loosening, especially w... more The major reason for revision surgery of load bearing implants is aseptic loosening, especially with regard to total hip arthroplasty. Osseous anchorage of loosened implants needs to be measured exactly in order to ensure the indication for revision surgery and additionally to enable early detection of the loosening process. However, current diagnostic methods show unsatisfying results concerning sensitivity and specificity. Therefore, a new sensor system is being developed based on an acousto-mechanical approach, which can be used non-invasively and radiation-free to measure the stability of endoprosthesis fixation on the bone stock. This paper intends to show initial results of functional models, which are used to prove the measuring principle.
Dislocation remains a serious complication of total hip replacement. An insufficient range of mot... more Dislocation remains a serious complication of total hip replacement. An insufficient range of motion can lead to impingement of the prosthetic neck on the acetabular cup. Together with the initiation of subluxation and dislocation, recurrent impingement can cause material failure in the liner. The objective of this study was to generate a validated finite element (FE) model capable of predicting the dislocation stability of different femoral head sizes with regard to impingement in different implant positions as well as the corresponding stress distribution in the liner. In order to cover posterior and anterior dislocation, two total hip dislocation associated manoeuvres were simulated using a three-dimensional nonlinear finite element model. The dislocation stability of two head sizes was determined numerically and experimentally. After validation, the FE model was used to analyse the dislocation stability of four different head sizes in variable implant positions. Range of motion (ROM) until impingement, the resisting moment that was developed and ROM until dislocation were evaluated. Additionally, stress distribution within the polyethylene liner during impingement and subluxation was determined. For both dislocation modes, a cup position of 45 degrees lateral abduction and 15 degrees up to 30 degrees anteversion resulted in appropriate ROM and dislocation stability. In general, larger head diameters revealed an increase in ROM and higher resisting moments. Stress analysis showed decreased contact pressures at the egress site of the liners with the larger inner diameters during subluxation. The analysis shows that an optimal implant position and a larger head diameter can reduce the risk of dislocation induced by impingement. The finite element model that was developed enables simplification of design variations compared to experimental studies since prototyping and assembling are replaced by prompt numerical simulation.
A certain failure mode using a newly developed cemented ceramic femoral component in total knee r... more A certain failure mode using a newly developed cemented ceramic femoral component in total knee replacement was observed in clinical application, i.e. fracture of the femoral component during intraoperative impaction. This may be caused by unintentional deflection of the saw blades during cutting with consecutive higher resection angle of the distal femur than desired, leading to bending of the femoral component during implantation. A finite-element-analysis was carried out to simulate implantation of the femoral component and to evaluate the influence of distal femur preparation on implant stress. We developed and validated a numerical model of the ceramic femoral component including a contact formulation which allowed calculating the principal stresses of the implant during implantation onto the resected femur. The analysis considered different anterior and posterior resection angles with a total of 17 variations. By increasing the femoral resection angle in the finite-element-model it could be shown that a deviation of three degrees from the intended resection angle can cause critical stress amounts during implantation. When implanting the ceramic component in total knee arthroplasty, the femoral resection angles should be prepared very precisely, in particular anterior saw blade deflection has to be avoided. The implant manufacturer increased implant safety through an additional resection template. Moreover, the impaction of the ceramic femoral component during cementing was not further recommended by using a hammer.
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