A reference method for the evaluation of femoral head joint center location technique based on external markers

Accurate localization of joint centers is essential in movement analysis. However, joint centers cannot be directly palpated and alternative methods must be used. To assess the relative merits of these methods, a medical image based reference should be used. The EOS1 system, a new low dose bi-planar X-rays imaging technique may be considered. The aim of this study was to evaluate the accuracy of hip joint center (HJC) localization using the EOS1 system. Seventeen healthy young adults participated in the study. Femoral heads and pelvic external markers were localized using the EOS1 system and the HJCs were expressed in the movement analysis coordinate system. Results showed that external marker localization was reliable within 0.15 mm for trained assessors. Mean accuracy for HJC localization was 2.9 mm (SD: 1.3, max: 6.2). The EOS based method therefore appeared reliable and may be used for femoral head localization or as a reference to assess the accuracy of other methods for HJC localization.The authors are grateful to VICON (OMG-UK) for the loan of a motion capture system necessary for the overall study

Finite element modelling of shock-induced damages on ceramic hip prostheses

The aim of this work was to simulate the behaviour of hip prostheses under mechanical shocks. When hip joint is replaced by prosthesis, during the swing phase of the leg, a microseparation between the prosthetic head and the cup could occur. Two different sizes of femoral heads were studied: 28 and 32 mm diameter, made, respectively, in alumina and zirconia. The shock-induced stress was determined numerically using finite element analysis (FEA), Abaqus software. The influence of inclination, force, material, and microseparation was studied. In addition, an algorithm was developed from a probabilistic model, Todinov's approach, to predict lifetime of head and cup. Simulations showed maximum tensile stresses were reached on the cup's surfaces near to rim. The worst case was the cup-head mounted at 30^{\circ}. All simulations and tests showed bulk zirconia had a greater resistance to shocks than bulk alumina. The probability of failure could be bigger than 0.9 when a porosity greater than 0.7% vol. is present in the material. Simulating results showed good agreement with experimental results. The tests and simulations are promising for predicting the lifetime of ceramic prostheses

Three-Dimensional Reconstruction of Foot in the Weightbearing Position From Biplanar Radiographs: Evaluation of Accuracy and Reliability

The initial assessment and postoperative monitoring of patients with various abnormalities of the foot in clinical routine practice is primarily based on the analysis of radiographs taken in the weightbearing position. Conventional x-ray imaging, however, only provides a 2-dimensional projection of 3-dimensional (3D) bony structures, and the clinical parameters assessed from these images can be affected by projection biases. In the present work, we addressed this issue by proposing an accurate 3D reconstruction method of the foot in the weightbearing position from low-dose biplanar radiographs with clinical index measurement assessment for clinical routine practice. The accuracy of the proposed reconstruction method was evaluated for both shape and clinical indexes by comparing 3D reconstructions of 6 cadaveric adult feet from computed tomographic images and from biplanar radiographs. For the reproducibility study, 3D reconstructions from the biplanar radiographs of the foot of 6 able-bodied subjects were considered, with 2 observers repeating each measurement of anatomic landmarks 3 times. Baseline assessment of important 3D clinical parameters was performed on 17 subjects (34 feet; mean age 27.7, range 20 to 52 years). The average point to surface distance between the 3D stereoradiographic reconstruction and the computed tomographic scan-based reconstruction was 1 mm (range 0mm to 6mm). The selected radiographic landmarks were highly reproducible (95% confidence interval <2.0 mm). The greatest interindividual variability for the clinical parameters was observed for the twisting angle (mean 87°, range 73° to 100°). Such an approach opens the way for routine 3D quantitative analysis of the foot in the weightbearing position.The authors thank the ParisTech BiomecAM chair program on subject-specific musculoskeletal modeling, and in particular COVEA and Société Générale. The authors also thank Audrey Arts, Roxane Huet, and Thomas Joubert for their kind technical assistance

Etude de la dégradation par chocs de têtes et cupules de prothèses de hanche en biocéramique

Ceramics, with their unique properties of biocompatibility, ligthness and wear resistance, are often used for orthopaedics applications, including hip prostheses. Nevertheless, due to their brittleness, these bioceramics can exhibit microfractures caused by repeated mechanical shocks. Microseparation between the head and the cup, which has been showed to occur in vivo, leads to mechanical shocks that can enhance fracture risk of the different elements of the prosthesis. Using a unique shock machine, tests have shown that worn bands appear on the head and the cup, which probably caused the breaking of the cups, whether these were in alumina or zirconia. Moreover, potential rebounds of the head in the cup have been assessed, implying several secondary shocks. The lifetimes of the two bioceramics could not be compared clearly because of the rather important scattering of the results, but wear and rupture mechanisms have been identified for both of them. An original model has also been developed, combining finite element modeling and flaws growth simulations, in order to calculate failure probabilities and to compare theoretical lifetimes with experimental ones. The goal was to obtain a reliable model allowing us to study the influence of several parameters on the lifetime of the prostheses, like the cup angle for example. Even if further improvements are needed concerning wear simulation, the theoretical lifetimes calculated with this model were close enough to the experimental ones, which allowed us to determine that the cup angle seemed to have a rather important influence. Few researches have been made concerning this topic, therefore, more experimental tests have to be done to confirm the results obtained and the hypotheses introduced during this study.Les céramiques, présentant une combinaison unique de biocompatibilité, de légèreté, et de résistance à l'usure, sont d'excellents candidats pour les applications orthopédiques, et notamment les prothèses de hanche. Néanmoins, du fait de leur fragilité intrinsèque, ces biocéramiques sont particulièrement sensibles à la microfissuration suite à des chocs mécaniques répétés. Le phénomène de décoaptation, définissant la séparation de la tête et de la cupule, a été mis en évidence in vivo, et le choc mécanique en résultant peut donc fortement augmenter les risques de rupture des éléments consitutifs de la prothèse. Des essais réalisés à l'aide d'une machine de chocs originale ont permis de confirmer la formation de bandes d'usure au niveau de la tête et de la cupule, ce qui a vraisemblablement entraîné les ruptures des cupules, qu'elles soient en alumine ou en zircone. De plus, ces résultats ont conduit à émettre l'hypothèse de l'existence de rebonds de la tête dans la cupule, générant donc de multiples impacts secondaires. La dispersion des résultats ne permet pas de comparer avec certitude les durées de vie respectives des deux biocéramiques, néanmoins, des mécanismes d'usure et de rupture ont pu été proposés. En parallèle, un modèle a été développé, combinant simulations par éléments finis et simulation de croissance de défauts, afin de calculer des probabilités de rupture et de comparer les durées de vie théoriques obtenues avec les résultats expérimentaux. Le but est d'obtenir un modèle fiable afin d'étudier l'influence d'autres paramètres sur la durée de vie de la prothèse, comme par exemple l'orientation des cupules. Bien que d'autres aspects restent à développer, en particulier la prise en compte de l'érosion, ces simulations ont donné des durées de vie conformes à celles obtenues expérimentalement, ce qui a permis de mettre en évidence le rˆole prépondérant de l'orientation de la cupule. Cette étude originale, étant donné que peu de recherches avaient été menées à ce sujet auparavant, doit être complétée par un plus grand nombre d'essais, afin de confirmer les tendances observées et les hypothèses émises

Etude de la dégradation par chocs de têtes et cupules de prothèses de hanche en biocéramique

Les céramiques, présentant une combinaison unique de biocompatibilité, de légèreté, et de résistance à l'usure, sont d'excellents candidats pour les applications orthopédiques, et notamment les prothèses de hanche. Néanmoins, du fait de leur fragilité intrinsèque, ces biocéramiques sont particulièrement sensibles à la microfissuration suite à des chocs mécaniques répétés. Le phénomène de décoaptation, définissant la séparation de la tête et de la cupule, a été mis en évidence in vivo, et le choc mécanique en résultant peut donc fortement augmenter les risques de rupture des éléments constitutifs de la prothèse. Des essais réalisés à l'aide d'une machine de chocs originale ont permis de confirmer la formation de bandes d'usure au niveau de la tête et de la cupule, ce qui a vraisemblablement entraîné les ruptures des cupules, qu'elles soient en alumine ou en zircone. De plus, ces résultats ont conduit à émettre l'hypothèse de l'existence de rebonds de la tête dans la cupule, générant donc de multiples impacts secondaires. La dispersion des résultats ne permet pas de comparer avec certitude les durées de vie respectives des deux biocéramiques, néanmoins, des mécanismes d'usure et de rupture ont pu été proposés. En parallèle, un modèle a été développé, combinant simulations par éléments finis et simulation de croissance de défauts, afin de calculer des probabilités de rupture et de comparer les durées de vie théoriques obtenues avec les résultats expérimentaux. Le but est d'obtenir un modèle fiable afin d'étudier l'influence d'autres paramètres sur la durée de vie de la prothèse, comme par exemple l'orientation des cupules. Bien que d'autres aspects restent à développer, en particulier la prise en compte de l'érosion, ces simulations ont donné des durées de vie conformes à celles obtenues expérimentalement, ce qui a permis de mettre en évidence le rôle prépondérant de l'orientation de la cupule. Cette étude originale, étant donné que peu de recherches avaient été menées à ce sujet auparavant, doit être complétée par un plus grand nombre d'essais, afin de confirmer les tendances observées et les hypothèses émises.Ceramics, with their unique properties of biocompatibility, ligthness and wear resistance, are often used for orthopaedics applications, including hip prostheses. Nevertheless, due to their brittleness, these bioceramics can exhibit microfractures caused by repeated mechanical shocks. Microseparation between the head and the cup, which has been showed to occur in vivo, leads to mechanical shocks that can enhance fracture risk of the different elements of the prosthesis. Using a unique shock machine, tests have shown that worn bands appear on the head and the cup, which probably caused the breaking of the cups, whether these were in alumina or zirconia. Moreover, potential rebounds of the head in the cup have been assessed, implying several secondary shocks. The lifetimes of the two bioceramics could not be compared clearly because of the rather important scattering of the results, but wear and rupture mechanisms have been identified for both of them. An original model has also been developed, combining finite element modeling and flaws growth simulations, in order to calculate failure probabilities and to compare theoretical lifetimes with experimental ones. The goal was to obtain a reliable model allowing us to study the influence of several parameters on the lifetime of the prostheses, like the cup angle for example. Even if further improvements are needed concerning wear simulation, the theoretical lifetimes calculated with this model were close enough to the experimental ones, which allowed us to determine that the cup angle seemed to have a rather important influence. Few researches have been made concerning this topic, therefore, more experimental tests have to be done to confirm the results obtained and the hypotheses introduced during this study.ST ETIENNE-ENS des Mines (422182304) / SudocSudocFranceF

Finite Element Modelling of Shock-Induced Damages on Ceramic Hip Prostheses

International audienc

Flaws growth modelling in zirconia acetabular cups of hip prostheses under microseparation. Modélisation de la croissance de défauts dans des cupules de prothèses de hanche en zircone soumises au phénomène de décoaptation

National audienceCeramic hip prostheses are very reliable with respect to wear, however, there has been evidence of brittleness problems. Microseparation is likely to occur between the head and the cup during human gait and the mechanical shocks transmitted to these elements are believed to enhance the risk of potential failure of the prosthesis. A finite-element model has been developed and, based Oil its results. fracture probabilities for zirconia cups have been simulated over time. The results show good agreement with experimental work, nevertheless, additional work is needed to take into account the wear occuring at the surface of the elements. Les prothèses de hanche en biocéramiques sont résistantes à l'usure mais, du fait de leur fragilité intrinsèque, sont susceptibles de se rompre. Le phénomène de décoaptation, séparation de faible amplitude de la tête et de la cupule, a été mis en évidence lors de la marche, et le choc mécanique en résultant peut fortement augmenter les risques de rupture. Un modèle par éléments finis a été développé et les résultats ont été exploités afin de simuler l'évolution de la probabilité de rupture de cupules en zircone au cours du temps. Les résultats s'accordent bien avec l'expérience, même si des améliorations sont nécessaires afin de prendre en compte l'érosion des surfaces de contact