La maladie rénale chronique (MRC) induit des anomalies du métabolisme phosphocalcique, avec des conséquences à la fois osseuses, vasculaires et biologiques. La prise en charge optimale de ces désordres représente un challenge quotidien pour le néphrologue pédiatre, à la fois sur le court terme (équilibre biologique) et sur le long terme (prévention des fractures, optimisation de la croissance et limitation de l’apparition des calcifications vasculaires). Peu d’outils sont actuellement disponibles pour évaluer ces atteintes, et de nouveaux outils prometteurs, à la fois biologiques (FGF23) et radiologiques (HR-pQCT) apparaissent. Néanmoins, les données pédiatriques sur ces outils restent rares. Cette thèse de doctorat a permis d’évaluer ces nouveaux moyens chez ces enfants MRC, notamment en évaluant l’HR-pQCT dans cette population, et en déterminant des valeurs de référence du FGF23 en fonction de l’âge, du sexe et de la fonction rénale. Nous avons pu aussi montrer que les concentrations circulantes de FGF23 ne sont pas dépendantes du sexe dans une population pédiatrique, mais qu’elles augmentent avec l’âge et l’indice de masse corporelle, mais aussi en cas d’antécédent de transplantation d’organe solide ou de traitement par corticostéroïdes. D’un point de vue plus fondamental, nous avons pu montrer que dans des monocytes issus de donneurs sains, une exposition au FGF23 induit une diminution de l’expression des 2 enzymes principales impliquées dans le métabolisme de la vitamine D (1α hydroxylase et 24 hydroxylase), en induisant également une diminution du peptide antimicrobien cathélicidine. Ces résultats permettent donc de décrire un nouveau rôle pour le FGF23 dans la régulation de l’immunité innée / Chronic kidney disease can induce mineral and bone disorders (CKD-MBD), with deleterious consequences for bone and vessels. The management of such abnormalities can be challenging, from the daily biological balance between calcium, phosphorus and PTH levels, to the long-term prevention of morbidities such as fractures, growth impairment and vascular calcifications. Some tools can help to accurately assess CKD-MBD, e.g., new bone imaging techniques (HR-pQCT) and FGF23, but they are rarely used in pediatric populations. In addition to evaluating HR-pQCT in CKD children and healthy controls, this PhD thesis allowed us to determine reference values for circulating FGF23 levels depending on age, gender and renal function; we also showed that FGF23 levels increased not only with age and BMI, but also in cases of solid organ transplant or corticosteroids therapy. We have also showed in vitro that FGF23 could inhibit the two key enzymes of vitamin D metabolism (1α hydroxylase et 24 hydroxylase) in monocytes issued from healthy donors, with in turn a decreased synthesis of the antimicrobial cathelicidin. These later results highlight a new role for FGF23 in innate immunity, and may bring new insights in the understanding of FGF23 deregulation during CKD

L’ostéoporose est une maladie du squelette caractérisée par une dégradation de la qualité osseuse conduisant à une majoration du risque fracturaire. Le seul examen permettant actuellement de prédire ce risque est l’ostéodensitométrie à double rayonnement X (DXA) par la mesure de la densité minérale osseuse (DMO). Cependant, la DMO seule ne rend compte que de 40 à 70% de la variation de la résistance mécanique osseuse. Les 3 buts ce travail étaient : 1/ d’évaluer les rôles respectifs de la microarchitecture corticale et trabéculaire dans le comportement mécanique vertébral, 2/ d’évaluer le rôle propre de l’hétérogénéité de la microarchitecture trabéculaire et 3/ de décrire le comportement mécanique vertébral post-fracturaire et d’en identifier les déterminants. Nous montrons que la mesure de l’épaisseur de la corticale antérieure et de son rayon de courbure ainsi que la détermination de la variation régionale de la microarchitecture trabéculaire améliorent significativement la prédiction du risque fracturaire. Il existe une variation marquée du comportement mécanique vertébral après une fracture de grade 1 de Genant. La microarchitecture osseuse, et non la masse osseuse, explique les propriétés mécaniques vertébrales plastiques et élastiques post-fracturaires. / Osteoporosis is characterized by altered bone quality and compromised bone strength leading to increased fracture risk. Measurement of areal bone mineral density (BMD) using dual-energy X-ray absorptiometry (DXA) is the most widely used index of bone strength. However, BMD alone can only account for 40 to 70% of the variation of bone strength. This study aimed to determine: 1/ the respective role of cortical and trabecular microarchitecture to vertebral mechanical behavior, 2/ the role of trabecular microarchitecture heterogeneity and 3/ the mechanical behavior of a vertebra after simulated mild fracture and its determinants. Our data imply that measurements of cortical thickness and curvature as well as determination of trabecular microarchitecture heterogeneity enhance prediction of vertebral fragility. We found marked variation in the post-fracture load-bearing capacity following simulated mild vertebral fractures. Bone microarchitecture, but not bone mass, was associated with post-fracture mechanical behavior of vertebrae.

Introduction: Puberty is a crucial period for rapid changes in bone mineral, size, geometry, and microarchitecture. The mechanostat theory postulates that increased mechanical loading will affect bone phenotype and strength during development and in later life. Individuals with cystic fibrosis (CF) have an increased risk of developing osteoporosis and fragility fractures in young adulthood, which may be caused by poor growth. The aim was to investigate whether sex and disease status modified the relationship between: 1) puberty and bone, and 2) muscle and bone. This would contribute to the understanding of how sex (males vs. females) and disease group (CF vs.controls) alters the relationship between bone and muscle in children and adolescents as they transition through puberty and who, on a population level, differ in the prevalence of osteoporosis and risk of fracture in later life. Methods and Analyses: This observational study used novel imaging and muscle assessment techniques to measure bone and muscle parameters in White Caucasian children and adolescents, aged 8 to 16 years, living in the UK, with children with CF (n=65) and controls (n=151). Anthropometry and pubertal status were assessed. Dual energy X-ray absorptiometry, peripheral quantitative computed tomography (pQCT), high-resolution pQCT, and jumping mechanography were used to measure bone and muscle outcomes. ANCOVA with Scheffé post hoc and multiple linear regression tests were performed. Data were adjusted according to the research aims and included covariates; sex, disease group, pubertal stage, age, quadratic age, height, weight, maximum force (Fmax), and maximum power (Pmax). Data are presented as beta-coefficient (%) and p-value, with the significance level set to p < 0.05. Results: In height adjusted analyses, among healthy participants, females had smaller bones and lower bone density compared to males. With pubertal maturation, females had lower apparent gains in the distal and proximal total area (Tt.Ar and CSA), distal cortical porosity (Ct.Po) and proximal bone strength (SSI) but higher apparent gains in distal and proximal cortical bone density(Ct.BMD, Ct.TMD, vBMD). Females had consistently lower distal total area (total CSA) and density (total vBMD), distal trabecular density(BV/TV) and number(Tb.N), and proximal cortical area(CSA) compared to males, across all stages of puberty. With increasing muscle force (Fmax), females had higher apparent gains in total body less head bone mineral (TBLH BMC) and bone area(BA), distal total and trabecular density (total and trab vBMD) compared to males. In contrast, with increasing muscle power (Pmax), females had higher apparent gains in distal total and cortical densities (D100, Ct.BMD and Ct.TMD), and distal trabecular thickness (Tb.Th), and proximal cortical density (cortical vBMD) but lower apparent gains in distal cortical porosity (Ct.Po) and trabecular number (Tb.N) compared to males. In height adjusted analyses, participants with CF had smaller bones and lower bone density compared to controls. With increasing pubertal maturation, participants with CF had lower apparent gains in total body less head bone mineral and bone area, and in distal trabecular density, cortical porosity, and trabecular thickness compared to controls. Participants with CF had consistently lower distal total and cortical area, distal total and trabecular densities and proximal bone strength compared to controls, across all stages of puberty. With increasing muscle force, participants with CF had lower apparent gains in total body less head bone mineral and bone area, distal total density, trabecular density, and trabecular number. In contrast, with increasing muscle power (Pmax), participants with CF had higher apparent gains in distal trabecular density (BV/TV) and trabecular number (Tb.N) compared to controls. Conclusion: These findings suggests that sex and disease status do modify the relationships between puberty and bone, and between muscle function and bone. Skeletal adaptation to muscle differs between sexes and in populations with chronic disease, which may explain sex and disease group differences in risks of osteoporosis and fracture. Bone adaptation to muscle in children with CF is altered, which may lead to narrow, under-mineralised bones, with lower bone strength in later life. Understanding better impairments in muscle functions may provide targets for intervention to improve skeletal health in later life.

There is a pressing need for better assessment of bone strength as current clinical tools do not directly measure bone mechanical properties, but offer only surrogate measures of bone strength. We conducted an ex-vivo study of emu bones to examine how two investigative devices, hr-pQCT and MRTA, compare to current clinical tools (DXA and QUS) in predicting true bone mechanical properties. We found that hr-pQCT parameters were able to assess bone strength as well as DXA and better than QUS, while MRTA was able to predict bone strength well in low-density but not high-density bones. Our results suggest that both hr-pQCT, which has the unique ability to specifically assess the various determinants of bone strength, and MRTA, which measures a bone mechanical property (stiffness), have great potential for use as clinical tools that can assess various components of bone strength not measured by current devices.

There is a pressing need for better assessment of bone strength as current clinical tools do not directly measure bone mechanical properties, but offer only surrogate measures of bone strength. We conducted an ex-vivo study of emu bones to examine how two investigative devices, hr-pQCT and MRTA, compare to current clinical tools (DXA and QUS) in predicting true bone mechanical properties. We found that hr-pQCT parameters were able to assess bone strength as well as DXA and better than QUS, while MRTA was able to predict bone strength well in low-density but not high-density bones. Our results suggest that both hr-pQCT, which has the unique ability to specifically assess the various determinants of bone strength, and MRTA, which measures a bone mechanical property (stiffness), have great potential for use as clinical tools that can assess various components of bone strength not measured by current devices.

Osteoporosis is an age-related skeletal disease characterised by an increased incidence of fragility fractures. In this thesis I develop a new technique capable of measuring the thickness of the previously unmeasured endocortical region, and providing an improved measure of the cortical bone mineral density (cBMD) from in-vivo clinical CT scans. These features are of interest as both have been linked to fracture risk. \\ The new technique is developed within the cortical bone mapping (CBM) framework so that it provides localised architectural measurements over a bone's surface. Its performance is assessed using simulated QCT data from three simulated phantoms with differing bone architecture, and two paired datasets of ex-vivo QCT and HR-pQCT scans across the proximal femur and the lumbar spine. The simulated data allows for inaccuracies in CBM measurements caused by beam hardening effects to be considered for the first time: I show that beam hardening leads to an underestimation in cortical thickness and an overestimation in trabecular BMD and that these inaccuracies can be reduced through adjustments to the CBM optimisation process. \\ A new technique of analysing HR-pQCT scans is also developed, for the validation of the new CBM method. It was used in place of other established HR-pQCT techniques for its ability to provide localised measurements of the endocortical region. A comparison with the well known full-width half-maximum (FWHM) method shows that it is less susceptible to errors caused by beam hardening. It also measures the mean cBMD, which has a greater clinical relevance than the peak cBMD measured by the FWHM method as it includes the impact of porosity. I demonstrate that the endocortical thickness can be measured to an accuracy of \(-0.15\pm0.71\thinspace\mathrm{mm}\), and that local cBMD measurements are possible down to \(300\thinspace\mathrm{mg/cm}^3\) from QCT scans over the proximal femur. I also validate CBM methods over the vertebrae for the first time and demonstrate that the cortical thickness and endocortical thickness can be measured with accuracies of \(0.10\pm0.30\thinspace\mathrm{mm}\) and \(-0.20\pm0.53\thinspace\mathrm{mm}\). \\ Two clinical trials involving Teriparatide are used to demonstrate that the new CBM method is able to detect significant regional changes in the dense cortical and endocortical bone over the proximal femur and lumbar spine, which can be attributed to changes in intracortical remodelling and endosteal apposition. The analysis of a cross-sectional fracture discrimination trial shows that fracture incidence is associated with significant decreases in endocortical thickness over specific regions.

La microarchitecture osseuse est un des déterminants de la qualité osseuse qui peut maintenant être évaluée in vivo au radius et au tibia distaux avec une résolution isotropique de 82μm par un nouveau scanner à haute résolution (XtremeCT, SCANCO Medical AG). Par ailleurs, l’utilisation d’analyse en éléments finis sur les volumes 3D obtenus permet d’évaluer les propriétés biomécaniques de l’os comme la résistance osseuse. Nous avons montré qu’il s’agissait d’une technique prometteuse pour évaluer la densité, la microarchitecture et les propriétés biomécaniques osseuses au niveau des sites périphériques, notamment parce que ces mesures étaient associées chez la femme avec des fractures ostéoporotiques de toutes sortes. Nous avons également montré que les mêmes mesures étaient tout aussi pertinentes chez l’homme, alors qu’il est moins sujet à l’ostéoporose. Les résultats étaient associés aux fractures ostéoporotiques de toutes sortes, notamment les fractures vertébrales. L’analyse en éléments finis permet donc la mesure in vivo de la résistance osseuse, ce qui pourrait fournir des informations sur la fragilité osseuse et le risque de fracture non accessible par les seules mesures de densité ou de microarchitecture osseuse. / Bone microarchitecture is one of the determinants of bone quality that can now be evaluated in vivo at the distal radius and tibia with an isotropic resolution of 82μm with a new high-resolution peripheral scanner (XtremeCT, SCANCO Medical AG). Moreover, the use of finite element analysis on the 3D bone volume acquired allows the assessment of bone biomechanical properties such as bone strength. Our studies show that this technique is promising to assess bone density, microarchitecture and strength at peripheral skeletal sites. Indeed those measures were associated with osteoporotic fractures of all kinds in women. We also demonstrated that those same measures were associated with osteoporotic fractures of all kinds, including vertebral fractures, in men, who are less prone to be affected by osteoporosis. Finite element analysis allows in vivo measurement of bone strength, which might provide additional information about bone fragility and fracture risk that are not assessed by measures of density or microarchitecture.

La microarchitecture osseuse est un des déterminants de la qualité osseuse qui peut maintenant être évaluée in vivo au radius et au tibia distaux avec une résolution isotropique de 82μm par un nouveau scanner à haute résolution (XtremeCT, SCANCO Medical AG). Par ailleurs, l'utilisation d'analyse en éléments finis sur les volumes 3D obtenus permet d'évaluer les propriétés biomécaniques de l'os comme la résistance osseuse. Nous avons montré qu'il s'agissait d'une technique prometteuse pour évaluer la densité, la microarchitecture et les propriétés biomécaniques osseuses au niveau des sites périphériques, notamment parce que ces mesures étaient associées chez la femme avec des fractures ostéoporotiques de toutes sortes. Nous avons également montré que les mêmes mesures étaient tout aussi pertinentes chez l'homme, alors qu'il est moins sujet à l'ostéoporose. Les résultats étaient associés aux fractures ostéoporotiques de toutes sortes, notamment les fractures vertébrales. L'analyse en éléments finis permet donc la mesure in vivo de la résistance osseuse, ce qui pourrait fournir des informations sur la fragilité osseuse et le risque de fracture non accessible par les seules mesures de densité ou de microarchitecture osseuse.

L'ostéoporose est une maladie du squelette caractérisée par une dégradation de la qualité osseuse conduisant à une majoration du risque fracturaire. Le seul examen permettant actuellement de prédire ce risque est l'ostéodensitométrie à double rayonnement X (DXA) par la mesure de la densité minérale osseuse (DMO). Cependant, la DMO seule ne rend compte que de 40 à 70% de la variation de la résistance mécanique osseuse. Les 3 buts ce travail étaient : 1/ d'évaluer les rôles respectifs de la microarchitecture corticale et trabéculaire dans le comportement mécanique vertébral, 2/ d'évaluer le rôle propre de l'hétérogénéité de la microarchitecture trabéculaire et 3/ de décrire le comportement mécanique vertébral post-fracturaire et d'en identifier les déterminants. Nous montrons que la mesure de l'épaisseur de la corticale antérieure et de son rayon de courbure ainsi que la détermination de la variation régionale de la microarchitecture trabéculaire améliorent significativement la prédiction du risque fracturaire. Il existe une variation marquée du comportement mécanique vertébral après une fracture de grade 1 de Genant. La microarchitecture osseuse, et non la masse osseuse, explique les propriétés mécaniques vertébrales plastiques et élastiques post-fracturaires.