Abstract The current benchmark for the assessment of fracture risk is the status of osteoporosis based on the measurement of bone mineral density (BMD) by dual-energy X-ray absorptiometry (DXA). However, DXA-based BMD has been shown to lack predictive ability for individual fracture risk. More than half of the hip fractures occur among people who are not classified as having osteoporosis. Osteoporosis (i.e. reduced bone mass) is only one risk factor for a fracture. In addition to bone mass, the mechanical strength of a bone is influenced by material and structural factors. However, we have limited information about the combined effects of BMD and bone structural properties in the evaluation of fracture risk, with regard to different types of hip fractures in particular. Therefore, this study investigated the radiograph-based structural factors of the upper femur for the assessment of bone mechanical competence and cervical and trochanteric hip fracture risk. The subjects of the clinical study comprised 74 postmenopausal women with non-pathologic cervical or trochanteric hip fracture and 40 age-matched controls. The impact of bone structure on the bone mechanical competence was studied using the experimental material of 140 cadaver femurs. The femora were mechanically tested in order to determine the failure load in a side impact configuration, simulating a sideways fall. In all study series, standard BMD measurements were performed, and the structural parameters of bone were determined from digitized plain radiographs. The present study showed that the large variation in the mechanical competence of bone is associated with the geometrical and architectural variation of bone. Moreover, the results strongly suggested that the etiopathology of different types of hip fractures significantly differs, and that fracture risk prediction should thus be performed separately for the cervical and trochanteric hip fractures. Furthermore, the study implied that the current clinical procedure can better be used for the assessment of the risk of trochanteric fracture, whereas cervical fracture is more strongly affected by the geometrical factors than by BMD. Finally, radiograph-based structural parameters of trabecular bone and bone geometry predicted in vitro failure loads of the proximal femur with a similar accuracy as DXA, when appropriate image analysis technology was used. Thus, the technology may be suitable for further development and application in clinical fracture risk assessment.

In this thesis a method was developed to reconstruct both the 3D shape and the BMD distribution of bone structures from Dual-energy X-ray Absorptiometry (DXA) images. The method incorporates a statistical model built from a large dataset of Quantitative Computed Tomography (QCT) scans together with a 3D-2D intensity based registration process. The method was evaluated for its ability to reconstruct the proximal femur from a single DXA image. The resulting parameters of the reconstructions were subsequently evaluated for their hip fracture discrimination ability. The reconstruction method was finally extended to the reconstruction of the lumbar vertebrae from anteroposterior and lateral DXA, thereby incorporating a multi-object and multi-view approach. These techniques can potentially improve the fracture risk estimation accuracy over current clinical practice. / En esta tesis se desarrolló un método para reconstruir tanto la forma 3D de estructuras óseas como la distribución de la DMO a partir de una sola imagen de DXA. El método incorpora un modelo estadístico construido a partir de una gran base de datos de QCT junto con una técnica de registro 3D-2D basada en intensidades. Se ha evaluado la capacidad del método para reconstruir la parte proximal del fémur a partir de una imagen DXA. Los parámetros resultantes de las reconstrucciones fueron evaluados posteriormente por su capacidad en discriminar una fractura de cadera. Por fin, se extendió el método a la reconstrucción de las vértebras lumbares a partir de DXA anteroposterior y lateral incorporando así un enfoque multi-objeto y multi-vista. Estos técnicas pueden potencialmente mejorar la precisión en la estimación del riesgo de fractura respecto a la estimación que ofrece la práctica clínica actual.

Les fractures de fragilité représentent un problème de santé publique pour les personnes âgées. L'évaluation de la résistance osseuse et du risque de fracture par la méthode de référence (absorption bi-photonique à rayons X, DXA) est limitée. Les micro-modèles en éléments finis (µFEM) ont montré de meilleures prédictions de la résistance osseuse, mais on ne peut confirmer qu’ils améliorent l’estimation du risque de fracture par rapport à la DXA. L'objectif de cette thèse était donc d'évaluer si la prédiction par simulation numérique pouvait être améliorée en prenant en compte des conditions réalistes de chargement. Tout d’abord, les conditions de chargement correspondant à une chute vers l’avant ont été reproduites sur 32 radius humain dans une expérimentation ex-vivo. Les résultats expérimentaux ont conduits à deux groupes : un fracturé et un non fracturé. Puis, la capacité de prédiction d’un modèle « ségment » (9 mm de radius distal) créé en utilisant un scanner à très haute résolution (High Resolution peripheral Quantitative Computed Tomography) a été évaluée. . Différentes configurations (axiale (configuration standard) et 5 non-axiales) ont été simulées. L’implémentation de chargement non-axial n’a pas amélioré la capacité de prédiction du modèle « segment ». Finalement, un modèle hétérogène du radius distal entier a été créé à partir d’un scanner clinique (Cone Beam Computed Tomography). Ce modèle a pris en compte les conditions d’une chute en termes d’orientation et de vitesse. Le modèle de radius distal entier a montré une meilleure prédiction de la charge à la rupture expérimentale que le modèle « segment ». Cette étude propose des données originales pour la validation de modèles numériques pour l’amélioration de la prédiction du risque de fracture / Fragility fractures represent a public health problem for elderly. The assessment of the bone strength and of the risk of fracture by the gold standard method (Dual X-ray Absorptiometry - DXA) is limited. Micro-finite element models (µFEM) have shown to better predict the bone strength, but it is not possible to confirm that they do better than the density measured by DXA to estimate the risk of fracture. Thus, the aim of this thesis was to evaluate whether including realistic loading conditions could improve the level of prediction of the FEM. First, we reproduced the loading conditions of a forward fall on 32 radii in an ex-vivo experiment. This experiment leaded to two groups: one fractured and one non - fractured. Then, we evaluated the prediction capability of a segment FEM (9 mm of the distal radius) created using the High Resolution peripheral Quantitative Computed Tomography. This segment FEM was tested under the axial loading (standard analysis), and under five additional non-axial configurations. It was found that the prediction capability of the segment FEM was not improved by the implementation of non-axial loadings. Finally, a heterogeneous FEM of the whole distal radius was created using data from a Cone Beam Computed Tomography. This model considers the fall loading configurations in orientation and speed of the ex-vivo experiment. The FEM of the whole distal radius has a better accuracy to predict the experimental failure load than the segment FEM. This study proposes original data for model validation dedicated to further improvements of fracture risk prediction

Finite element human body models have in recent years become widely used in the area of vehicle safety evaluations. They make it possible to predict injury risk in specific areas, down to the organ level in the human body. An existing human body model, SAFER HBM includes a rib cage representing an average male. However, humans have a large variability in rib geometry and material properties leading to uncertainties in non-linear phenomena such as rib fracture risk. Hence, it cannot be known if predictions based on an average male representation are applicable to other similar individuals. In simulation studies with the SAFER HBM, rib cortical bone thickness, rib cross-sectional width, and rib cortical bone material properties have been identified as the most influential for the magnitude of rib strains and thus, they have a large influence on the strain-based rib fracture risk. This means that the predicted injury outcome is sensitive to the particular rib properties of an individual, and in a real-world scenario, a distribution of injury outcomes is expected across a population. Knowledge of the injury risk distribution can aid vehicle designers in developing safer vehicles. This distribution can be found through repeated human body model simulations with various rib properties, but due to the lengthy simulation times, this is not feasible. This thesis aims to predict human body model rib strain histories, given variations in the three biomechanical parameters, rib cortical bone thickness, rib cross-section width and rib cortical bone material with the help of graph neural networks (GNNs) for both single and mixed impact scenarios. Several variations of GNNs were used and implemented with help of PyTorch and PyTorch Geometric. An extensive hyperparameter study was performed on a small part of one human body model rib, to find the optimal combinations of hyperparameters and GNNs. The data used in training and evaluation of the networks was generated in LS-DYNA with SAFER HBM v10 and post-processed in Meta post processor. To be able to generate many training examples, the HBM was subjected to a simplified impact scenario consisting of a pendulum impact to the chest. As final verification, the trained GNNs were applied to predict rib strains in a vehicle impact scenario. Evaluation of the GNNs' prediction accuracy on the whole rib cage for all impact scenarios was made by studying the root mean square error along with differences in predicted and actual peak strain, rib fracture risk, time the peak strain occurs and the euclidean distance between the locations within the rib of real and predicted peak strains. The results showed that it is possible to accurately predict strain histories. Further, a multilayer perceptron (MLP) model consistently achieved the lowest errors in all measurements for mixed impacts. However, the trained model produced slightly unexpected errors for test data extracted from vehicle simulations compared to simplified simulations. This is an indication that retraining the model on data from vehicle simulations may be necessary. In conclusion, this thesis has shown the possibility to predict strain histories from a SAFER HBM rib cage extracted from simplified simulations and simulations including the full vehicle model, the SAFER HBM and all safety systems, to investigate the effects of human variability in the rib cage.

No description available.

Long-duration spaceflight has been shown to have significant, lasting effects on the bone strength of astronauts and to contribute to age-related complications later in life. The microgravity environment of space causes a decrease in daily mechanical loading, which signals a state of disuse to bone cells. This affects the bone remodeling process, which is responsible for maintaining bone mass, causing an increase in damage and a decrease in density. This leads to bone fragility and decreases overall strength, posing a risk for fracture. However, there is little information pertaining to the timeline of bone loss and subsequent fracture risk. This study used finite element analysis to model the human femur, the bone most adversely affected by spaceflight, and to simulate the environments of Earth preflight, a six-month mission on the International Space Station, and one year on Earth postflight. Changes in the properties of cortical and trabecular bone in the femoral neck were measured from the simulations, and used to provide evidence for high fracture risk and to predict when it is most prominent. It was found that a risk for fracture is extremely evident in the femoral neck in both cortical and trabecular bone. Cortical bone in the inferior neck exhibited high magnitudes of damage, while the superior neck suffered the greatest increases in damage that proceeded to increase upon return to Earth. The density of trabecular bone decreased the most significantly and was not fully recovered in the following year. While it is still unclear exactly when these changes cause the greatest risk for fracture, it is possible that they will add to and advance the onset of medical complications such as osteoporosis. Additionally, the results of this study support the claim that the current countermeasure of inflight exercise is insufficient in sustaining bone mass and preserving skeletal health. The effects of long-duration spaceflight on bone health should continue to be investigated especially if future missions are to last as long as one to three years.

Abstract Hip fracture is a significant problem in health care incurring major costs to society. Therefore, it is necessary to study fracture mechanisms and develop improved methods to estimate individual fracture risk. In addition to conventional bone density measurements, computational finite element (FE) analysis has been recognized as a valuable method for studying biomechanical characteristics of a hip fracture. In this study, computed tomography (CT) based finite element methods were investigated and simulation models were developed to estimate experimental femoral fracture load and hip fracture type in a sideways fall loading configuration. Cadaver femur specimens (age 55–100 years) were scanned using a CT scanner and dual-energy X-ray absorptiometry (DXA), and the femurs were mechanically tested for failure in a sideways fall loading configuration. CT images were used for generating the FE model, and DXA was used as a reference method. FE analysis was done for simulation models of the proximal femur in a sideways fall loading configuration to estimate the experimentally measured fracture load and fracture type. Statistical analyses were computed to compare the experimental and the FE data. Cervical and trochanteric hip fractures displayed characteristic strain patterns when using a FE model mainly driven by bone geometry. This relatively simple FE model estimation provided reasonable agreement for the occurrence of experimental hip fracture type. Accurate assessment between experimental and finite element fracture load (r2 =  0.87) was achieved using subject-specific modeling, including individual material properties of trabecular bone for bilinear elastoplastic FE models. Nevertheless, the study also showed that proximal femoral fracture load can be estimated with reasonable accuracy (r2 =  0.73) by a relatively simple FE model including only cortical bone. The cortical bone FE model was more predictive for fracture load than DXA and slightly less accurate than the subject-specific FE model. The accuracy and short calculation time of the model suggest promise in terms of effective clinical use. / Tiivistelmä Lonkkamurtuma on huomattava ongelma terveydenhuollossa aiheuttaen merkittäviä kustannuksia yhteiskunnalle. Tämän vuoksi on tärkeää tutkia ja kehittää uusia yksilöllisen murtumariskin arviointimenetelmiä. Elementtimenetelmä on tehokas laskennallinen työkalu lonkkamurtuman biomekaanisten ominaisuuksien tutkimisessa. Tässä työssä tutkittiin ja kehitettiin tietokonetomografiaan perustuvia reisiluun simulaatiomalleja kokeellisten murtolujuuksien ja lonkkamurtumatyyppien arviointiin. Reisiluunäytteet (ikä 55–100 vuotta) kuvattiin tietokonetomografialaitteella ja kaksienergisellä röntgenabsorptiometrialla, jonka jälkeen reisiluut kuormitettiin kokeellisesti murtolujuuden ja murtumatyypin määrittämiseksi sivuttaiskaatumisasetelmassa. Tietokonetomografialeikekuvia käytettiin simulaatiomallien luomiseen, ja kaksienergistä röntgenabsorptiometriaa käytettiin vertailumenetelmänä. Reisiluun simulaatiomallit analysoitiin elementtimenetelmän avulla kokeellisten murtolujuuksien ja murtumatyyppien arvioimiseksi. Tilastoanalyysiä käytettiin verrattaessa kokeellista aineistoa ja simulaatioaineistoa. Reisiluun muotoon perustuva simulaatiomalli osoitti, että reisiluun kaulan ja sarvennoisen murtumilla on tyypilliset jännitysjakaumat. Tämän suhteellisen yksinkertaisen mallin murtumatyyppi oli lähes yhdenmukainen kokeellisen murtumatyypin kanssa. Reisiluun kokeellinen murtolujuus pystyttiin arvioimaan tarkasti (r2 =  0.87) käyttäen yksityiskohtaista simulaatiomallia, joka sisältää yksilölliset hohkaluun materiaaliominaisuudet. Toisaalta murtolujuus pystyttiin arvioimaan kohtuullisella tarkkuudella (r2 =  0.73) melko yksinkertaisellakin mallilla, joka käsittää ainoastaan kuoriluun. Kuoriluuhun perustuva malli oli tarkempi arvioimaan reisiluun kokeellista murtolujuutta kuin kaksienerginen röntgenabsorptiometria ja lähes yhtä tarkka kuin yksityiskohtaisempi simulaatiomalli. Mallin tarkkuus ja lyhyt laskenta-aika antavat lupauksia tehokkaaseen kliiniseen käyttöön.

Abstract Hip fracture is the most severe complication of osteoporosis. The occurrence of hip fracture is increasing worldwide as a result of the ageing of the population. The clinical assessment of osteoporosis and to some extent hip fracture risk is based on the measurement of bone mineral density (BMD) using dual X-ray absorptiometry (DXA). However, it has been demonstrated that most hip fractures occurring after a fall involve non-osteoporotic populations and that the geometry plays a critical role in the fracture risk assessment. A potential alternative for the assessment of hip fracture risk is finite element modelling, which is a computational method allowing simulation of mechanical loading. The aim of this study was to investigate different finite-element (FE) methods for predicting hip fracture type and eventually hip failure load in the simulation of a fall on the greater trochanter. An experimental fall on the greater trochanter was performed on over 100 cadaver femurs in order to evaluate the failure load and fracture type. In all studies, assessment of BMD, measurement of geometrical parameters and generation of finite element models were performed using DXA, digitized plain radiographs and computed tomography scans. The present study showed that geometrical parameters differ between specific hip fracture types. FE studies showed feasible accuracy in the prediction of hip fracture type, even by using homogeneous material properties. Finally, a new method to generate patient-specific volumetric finite element models automatically from a standard radiographic picture was developed. Preliminary results in the prediction of failure load and fracture type were promising when compared to experimental fractures. / Tiivistelmä Lonkkamurtuma on osteoporoosin vakavin seuraus. Lonkkamurtumatapaukset kasvavat maailmanlaajuisesti väestön ikääntymisen myötä. Osteoporoosin ja osin myös lonkkamurtumariskin kliininen arviointi perustuu luun mineraalitiheyden mittaamiseen kaksienergisellä röntgenabsorptiometrialla (Dual-energy X-ray absorptiometry, DXA). On kuitenkin osoitettu, että suurin osa kaatumisen seurauksena tapahtuvista lonkkamurtumatapauksista tapahtuu henkilöillä joilla ei ole todettua osteoporoosia, ja että myös luun muoto on tärkeä tekijä arvioitaessa lonkkamurtumariskiä. Laskennallinen mallintaminen elementtimenetelmällä mahdollistaa mekaanisen kuormituksen simuloinnin ja on potentiaalinen vaihtoehto lonkkamurtumariskin arviointiin. Tämän työn tarkoituksena on tutkia elementtimenetelmiä lonkkamurtumatyypin ja lopulta lonkan murtolujuuden ennustamiseksi simuloimalla kaatumista sivulle. Yli sataa reisiluuta kuormitettiin kokeellisesti murtolujuuden ja murtumatyypin määrittämiseksi. Luun mineraalitiheyden arviointi, muotoparametrien mittaus ja elementtimallit tehtiin käyttäen DXA:a, digitalisoituja röntgenkuvia ja tietokonetomografiakuvia. Tämä tutkimus osoittaa, että luun muotoparametrit vaihtelevat eri lonkkamurtumatyyppien välillä. Lonkkamurtumatyyppi voitiin ennustaa hyvällä tarkkuudella elementtimenetelmän avulla silloinkin, kun käytettiin homogeenisia materiaaliominaisuuksia. Lopuksi kehitettiin uusi menetelmä yksilöllisten kolmiulotteisten elementtimallien automaattiseen luontiin tavallisista röntgenkuvista. Alustavat tulokset lonkan murtolujuuden ja murtumatyypin ennustamisessa ovat lupaavia.

No description available.