Validation of Subject Specific Computed Tomography-based Finite Element Models of the Human Proximal Tibia using Full-field Experimental Displacement Measurements from Digital Volume Correlation

Quantitative computed tomography-based finite element (QCT-FE) modeling is a computational tool for predicting bone’s response to applied load, and is used by musculoskeletal researchers to better understand bone mechanics and their role in joint health. Decisions made at the modeling stage, such as the method for assigning material properties, can dictate model accuracy. Predictions of surface strains/stiffness from QCT-FE models of the proximal tibia have been validated against experiment, yet it is unclear whether these models accurately predict internal bone mechanics (displacement). Digital volume correlation (DVC) can measure internal bone displacements and has been used to validate FE models of bone; though, its use has been limited to small specimens. The objectives of this study were to 1) establish a methodology for high-resolution peripheral QCT (HR-pQCT) scan acquisition and image processing resulting in low DVC displacement measurement error in long human bones, and 2) apply different density-modulus relationships and material models from the literature to QCT-FE models of the proximal tibia and identify those approaches which best predicted experimentally measured internal bone displacements and related external reaction forces, with highest explained variance and least error. Using a modified protocol for HR-pQCT, DVC displacement errors for large scan volumes were less than 19μm (0.5 voxels). Specific trabecular and cortical models from the literature were identified which resulted in the most accurate QCT-FE predictions of internal displacements (RMSE%=3.9%, R2>0.98) and reaction forces (RMSE%=12.2%, R2=0.78). This study is the first study to quantify experimental displacements inside a long human bone using DVC. It is also the first study to assess the accuracy of QCT-FE predicted internal displacements in the tibia. Our results indicate that QCT-FE models of the tibia offer reasonably accurate predictions of internal bone displacements and reaction forces for use in studying bone mechanics and joint health

Multi-view information fusion using multi-view variational autoencoders to predict proximal femoral strength

The aim of this paper is to design a deep learning-based model to predict proximal femoral strength using multi-view information fusion. Method: We developed new models using multi-view variational autoencoder (MVAE) for feature representation learning and a product of expert (PoE) model for multi-view information fusion. We applied the proposed models to an in-house Louisiana Osteoporosis Study (LOS) cohort with 931 male subjects, including 345 African Americans and 586 Caucasians. With an analytical solution of the product of Gaussian distribution, we adopted variational inference to train the designed MVAE-PoE model to perform common latent feature extraction. We performed genome-wide association studies (GWAS) to select 256 genetic variants with the lowest p-values for each proximal femoral strength and integrated whole genome sequence (WGS) features and DXA-derived imaging features to predict proximal femoral strength. Results: The best prediction model for fall fracture load was acquired by integrating WGS features and DXA-derived imaging features. The designed models achieved the mean absolute percentage error of 18.04%, 6.84% and 7.95% for predicting proximal femoral fracture loads using linear models of fall loading, nonlinear models of fall loading, and nonlinear models of stance loading, respectively. Compared to existing multi-view information fusion methods, the proposed MVAE-PoE achieved the best performance. Conclusion: The proposed models are capable of predicting proximal femoral strength using WGS features and DXA-derived imaging features. Though this tool is not a substitute for FEA using QCT images, it would make improved assessment of hip fracture risk more widely available while avoiding the increased radiation dosage and clinical costs from QCT.Comment: 16 pages, 3 figure

Bone quality and mesenchymal stromal cell capacity in total hip replacement: Significance for stem osseointegration measured by radiostereometric analysis

Immediate implant stability is a key factor for success in cementless total hip arthroplasty (THA). Cementless techniques were originally designed for middle-aged patients with normal bone structure and healing capacity, but indications have expanded to also include elderly patients. Poor local bone quality, as a result of osteoporosis (OP), and age-related geometric changes of the proximal femur, may jeopardize initial implant stability and lead to increased migration of the implant components thereby compromising biological fixation and osseointegration. Mesenchymal stromal cells (MSCs) are essential in the process of osseointegration. Age-related dysfunction of MSCs is suggested to be a main contributory factor in altered bone repair with aging and therefore may influence osseointegration. The hypothesis of this prospective clinical study was that preoperative bone quality and MSC capacity dictate stability and osseointegration of femoral stems in cementless THA, especially in women after menopause. A total of 61 consecutive women (age <80 yrs) scheduled for cementless THA for primary hip osteoarthritis (OA) were screened for undiagnosed primary or secondary OP, vitamin D insufficiency and other metabolic bone diseases. Prior to THA, patients underwent aspiration of iliac crest bone marrow for analysis of MSC capacity using optimized isolation and culturing protocols. All patients received a cementless total hip implant with an anatomically designed hydroxyapatite (HA) coated femoral stem and ceramic-ceramic bearings. Per-operative biopsy of the intertrochanteric bone was taken for ex vivo analysis of the local cancellous bone quality using micro-CT imaging and biomechanical testing. After surgery, stem migration and osseointegration was monitored for two years using radiostereometric analysis. The majority of women with hip OA was osteopenic or osteoporotic. These conditions were associated with increased periprosthetic bone loss in the proximal femur and impaired initial stability and delayed osseointegration of the femoral stem. Altered intraosseous dimensions of the proximal femur, as well as aging, also had adverse effects on initial stem stability and were associated with delayed osseointegration. Local bone mineral density of the operated hip and the quality of intertrochanteric cancellous bone had less influence than expected on implant migration. The THA females showed differences in the osteogenic properties of their MSCs. Patients with MSCs of low in vitro osteogenic capacity displayed increased stem subsidence after the initial 3 months settling period and thereby delayed osseointegration. The results suggest that decreased skeletal health, such as low systemic BMD and decreased osteogenic properties of bone marrow MSCs, has major influence on early stability and osseointegration of cementless hip prostheses in female patients.Luun laadun ja mesenkymaalisten kantasolujen toiminnan vaikutus lonkan tekonivelen paranemiseen Tekonivelleikkaus on erinomainen toimenpide lonkan nivelrikon hoidossa. Jos leikkausmenetelmäksi valitaan biologisesti kiinnittyvä tekonivel, olennaisinta on saavuttaa komponenttien välitön stabiliteetti. Se mahdollistaa uudisluun kasvun implantin karhennetulle pinnalle. Ikääntymiseen liittyvä luuston haurastuminen ja reisiluun yläosan ydinontelon laajentuminen voivat heikentää tekonivelen komponenttien tukevuutta ja näin hidastaa niiden kiinnittymistä luuhun. Tällainen on mahdollista erityisesti naisilla vaihdevuosien jälkeen. Näiden potilaiden yksilölliset erot luun parantavien solujen (mesenkymaalisten kantasolujen) määrässä ja toiminnassa voivat osaltaan vaikuttaa heidän tekoniveltensä kiinnittymisnopeuteen. Tähän prospektiiviseen kliiniseen tutkimukseen osallistui 61 naispotilasta, joille tehtiin sementöimätön lonkan tekonivelleikkaus nivelrikon takia. Ennen leikkausta potilaille tehtiin seulontatutkimukset osteoporoosin ja muiden luuston aineenvaihduntasairauksien tunnistamiseksi. Leikkauksen yhteydessä potilailta otettiin luuydinnäyte suoliluusta, josta analysoitiin mesenkymaalisten kantasolujen jakautumis- ja erilaistumiskyky luunsoluiksi. Leikkauksen aikana otettiin näyte reisiluun yläosan hohkaluun hienorakenteen ja mekaanisten ominaisuuksien arvioimiseksi. Leikkauksen jälkeen tekonivelen reisikomponentin kolmiulotteista migraatiota ja kiinnittymistä seurattiin radiostereometrisellä analyysillä (RSA) 2 vuoden ajan. Valtaosalla potilaista oli alentunut luuntiheys (osteopenia tai osteoporoosi). Osteopeenisillä ja osteoporoottisilla potilailla todettiin kiihtynyttä luukatoa tekonivelen reisikomponentin ympärillä sekä komponentin lisääntynyttä migraatiota ja hidastunutta kiinnittymistä. Reisiluun yläosan ydinontelon laajentuminen ja potilaan korkea ikä lisäsivät reisikomponentin migraatiota, mutta reisiluun hohkaluun laatu ei vaikuttanut migraation määrään. Potilailla, joilla todettiin mesenkymaalisten kantasolujen alentunut kyky erilaistua luusoluiksi in vitro, todettiin reisikomponentin lisääntynyttä migraatiota ja hidastunutta kiinnittymistä. Tulokset osoittavat, että ikääntymiseen liittyvät luustomuutokset ja yksilölliset erot mesenkymaalisten kantasolujen määrässä ja toiminnassa voivat osaltaan vaikuttaa lonkan tekonivelten paranemiseen naisilla vaihdevuosien jälkeen.Siirretty Doriast

Improving Material Mapping in Glenohumeral Finite Element Models: A Multi-Level Evaluation

An improved understanding of glenohumeral bone mechanics can be elucidated using computational models derived from computed tomography data. Although computational tools, such as finite element analysis, provide a powerful quantitative technique to evaluate and answer a variety of biomechanical and clinical questions, glenohumeral finite element models (FEMs) have not kept pace with improvements in modeling techniques or model validation methods seen in other anatomic locations. The present work describes the use of multi-level computational modeling to compare, develop and validate FEMs of the glenohumeral joint. Common density-modulus relationships within the literature were evaluated using a multi-level comparative testing methodology to determine if relationships from alternate anatomic locations can accurately replicate the apparent-level properties of glenoid trabecular bone. Two different relationships were able to replicate the micro-level loading to within 1.4%, compared to microFEMs when accounting for homogeneous or heterogeneous tissue moduli. The multi-level comparative methodology was then used to develop a glenoid-specific trabecular density-modulus relationship. This allowed for controlled and consistent development of the relationship that was adapted for use in whole-bone scapular FEMs. The density-modulus relationship developed was able to simulate micro-level apparent loading to within 1.3%, using a QCT-density specific relationship. Micro-level FEM characteristics were then compared to determine the optimal parameters for microFEMs and the effect of down-sampled images as FEM input. This was accomplished by creating glenoid trabecular microFEMs from microCT images at 32 micron, 64 micron or down-sampled 64 micron, spatial resolution. It was found that microFEMs accounting for material heterogeneity at the highest spatial resolution were the most accurate. MicroFEMs generated from down-sampled images at 64 microns were found to differ from those generated from scanned 64 micron images, indicating that caution should be used with down-sampled images as input for microFEMs. The optimal QCT-FEM parameters and material mapping strategies (elemental or nodal) were then explored using the same multi-level computational methodology. Little difference was found when comparing elemental or nodal material mapping strategies for all element types; however, QCT-FEMs generated with hexahedral elements and mapped with elemental material mapping, most accurately replicated micro-level apparent loading. Comparisons by material mapping strategy are also presented for linear and quadratic tetrahedral elements. Experimental validation of whole-bone scapular models was then explored by loading cadaveric scapulae within a microCT and using digital volume correlation (DVC) and a 6-degree of freedom load cell to compare full-field displacements and reaction loads to whole-bone scapular QCT-FEMs generated with different material mapping strategies and density-modulus relationships from the literature. It was found that elemental and nodal material mapping strategies were able to accurately replicate experimental DVC displacement field results. There was only minimal variation between elemental or nodal material mapping, and although percentage errors in reaction forces varied from -46% to 965%, QCT-FEMs mapped with density-modulus relationships from the literature were able to replicate experimental reaction loads to within 3%. Finally, morphometric parameters and apparent modulus between non-pathologic normal and end-stage osteoarthritic humeral trabecular bone was compared. It was found that morphometric differences compared to normal bone only occurred in the most medial aspects of end-stage OA bone, within the subchondral region. Moving distally from the articular surface showed near identical morphometric parameters. The end-stage OA group also exhibited a more linear bone-volume-modulus relationship compared to non-pathologic normal bone. The largest differences were seen at bone volume fractions greater than 0.25. This indicates that if high bone volume OA bone is being modeled, then a linear bone-volume-fraction-modulus (or density-modulus) relationship may more accurately replicate bone loading; however, if the high bone-volume-fraction bone is removed (such as with humeral joint replacement surgery), a power-law relationship similar to normal non-pathologic bone may accurately replicate bone loading

Experimental and numerical investigations of bone drilling for the indication of bone quality during orthopaedic surgery

Bone drilling is an essential part of many orthopaedic surgical procedures, including those for internal fixation and for attaching prosthetics. Drilling into bone is a fundamental skill that can be both very simple, such as drilling through long bones, or very difficult, such as drilling through the vertebral pedicles where incorrectly drilled holes can result in nerve damage, vascular damage or fractured pedicles. Also large forces experienced during bone drilling may promote crack formation and can result in drill overrun, causing considerable damage to surrounding tissues. Therefore, it is important to understand the effect of bone material quality on the bone drilling forces to select favourable drilling conditions, and improve orthopaedic procedures. [Continues.

Patient specific finite volume modeling for intraosseous PMMA cement flow simulation in vertebral cancellous bone

Ph.DDOCTOR OF PHILOSOPH

Non-invasive prediction of bone mechanical properties of the mouse tibia in longitudinal preclinical studies

The mouse tibia is a common site to investigate bone remodelling and the effect of treatments preclinically. It can be monitored using in vivo micro-Computed Tomography (microCT) imaging in order to track longitudinal changes in its morphometric and densitometric properties. Additionally, microCT images can be converted into micro-Finite Element (microFE) models for the non-invasive estimation of mechanical properties. Therefore, the combination of in vivo imaging and microFE modelling can provide comprehensive analyses about bone changes over space and time. However, repeated ionizing radiation exposure can have a significant effect on the bone properties; also, microFE models need to be validated against experimental measurements before application. The aim of this PhD project was to provide the best practice for the definition and validation of the in vivo microCT scanning procedure for the mouse tibia in preclinical studies. First, different scanning protocols have been tested by quantifying the accuracy of the image-based measurements against high resolution scans. One of the procedures has been selected as the best compromise between measurement accuracy and nominal radiation dose. Afterwards, microFE predictions of local and structural mechanical properties obtained using the selected scanning protocol have been validated. The experimental data for the validation has been obtained using the Digital Volume Correlation (DVC) approach, the only method which can provide volumetric measurements of local displacements under loading. Good to excellent correlations between the measured and predicted displacements were found. Errors in predictions of structural properties were in the order of 10-15%. Lastly, the protocol has been tested in vivo. The right tibia of 24 mice has been scanned in vivo five times, while the left tibia has been used as non-irradiated control. Non-significant or minimal radiation effects were found on the morphometric, densitometric and mechanical properties of the tibia. In conclusion, a scanning procedure for longitudinal in vivo microCT imaging of the whole mouse tibia has been defined and validated. The protocol will be used in future studies for investigating the effect of bone interventions

In vivo morphometric and mechanical characterization of trabecular bone from high resolution magnetic resonance imaging

La osteoporosis es una enfermedad ósea que se manifiesta con una menor densidad ósea y el deterioro de la arquitectura del hueso esponjoso. Ambos factores aumentan la fragilidad ósea y el riesgo de sufrir fracturas óseas, especialmente en mujeres, donde existe una alta prevalencia. El diagnóstico actual de la osteoporosis se basa en la cuantificación de la densidad mineral ósea (DMO) mediante la técnica de absorciometría dual de rayos X (DXA). Sin embargo, la DMO no puede considerarse de manera aislada para la evaluación del riesgo de fractura o los efectos terapéuticos. Existen otros factores, tales como la disposición microestructural de las trabéculas y sus características que es necesario tener en cuenta para determinar la calidad del hueso y evaluar de manera más directa el riesgo de fractura. Los avances técnicos de las modalidades de imagen médica, como la tomografía computarizada multidetector (MDCT), la tomografía computarizada periférica cuantitativa (HR-pQCT) y la resonancia magnética (RM) han permitido la adquisición in vivo con resoluciones espaciales elevadas. La estructura del hueso trabecular puede observarse con un buen detalle empleando estas técnicas. En particular, el uso de los equipos de RM de 3 Teslas (T) ha permitido la adquisición con resoluciones espaciales muy altas. Además, el buen contraste entre hueso y médula que proporcionan las imágenes de RM, así como la utilización de radiaciones no ionizantes sitúan a la RM como una técnica muy adecuada para la caracterización in vivo de hueso trabecular en la enfermedad de la osteoporosis. En la presente tesis se proponen nuevos desarrollos metodológicos para la caracterización morfométrica y mecánica del hueso trabecular en tres dimensiones (3D) y se aplican a adquisiciones de RM de 3T con alta resolución espacial. El análisis morfométrico está compuesto por diferentes algoritmos diseñados para cuantificar la morfología, la complejidad, la topología y los parámetros de anisotropía del tejido trabecular. En cuanto a la caracterización mecánica, se desarrollaron nuevos métodos que permiten la simulación automatizada de la estructura del hueso trabecular en condiciones de compresión y el cálculo del módulo de elasticidad. La metodología desarrollada se ha aplicado a una población de sujetos sanos con el fin de obtener los valores de normalidad del hueso esponjoso. Los algoritmos se han aplicado también a una población de pacientes con osteoporosis con el fin de cuantificar las variaciones de los parámetros en la enfermedad y evaluar las diferencias con los resultados obtenidos en un grupo de sujetos sanos con edad similar.Los desarrollos metodológicos propuestos y las aplicaciones clínicas proporcionan resultados satisfactorios, presentando los parámetros una alta sensibilidad a variaciones de la estructura trabecular principalmente influenciadas por el sexo y el estado de enfermedad. Por otra parte, los métodos presentan elevada reproducibilidad y precisión en la cuantificación de los valores morfométricos y mecánicos. Estos resultados refuerzan el uso de los parámetros presentados como posibles biomarcadores de imagen en la enfermedad de la osteoporosis.Alberich Bayarri, Á. (2010). In vivo morphometric and mechanical characterization of trabecular bone from high resolution magnetic resonance imaging [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/8981Palanci

Mechanical and morphometric characterization of cancellous bone

[EN] Bone fracture is a social health problem of increasing magnitude because of its prevalence in aged population due to osteoporosis. Bone quality is often characterized by bone mineral density (BMD) measured at cancellous bone regions using dual-energy X-ray absorptiometry (DXA). However, BMD alone cannot predict several cases because not only density is important, but also microstructure plays an important role in cancellous bone strength. The mechanical properties can be used as indicators of bone integrity as a function of age, disease or treatment. Therefore, cancellous bone fracture characterization and its relationship to microstructure has not been completely solved in the literature and is relevant to improve fracture prediction. In this thesis, we aim at characterizing cancellous bone morphometry and mechanical behavior. Morphometry is estimated through the analysis of micro-computed tomography (micro-CT) images of vertebral cancellous bone specimens. With regards to the mechanical behavior, we calculate elastic, yield and failure properties at the apparent and tissue levels. To determine them, we followed different approaches: compression tests, finite element models and micro-CT phantoms. We have developed finite element models that reproduce the elastic and failure response of cancellous bone under compression conditions. We modeled failure as a combination of continuum damage mechanics and the element deletion technique. The numerical models permitted to estimate elastic and failure properties. Failure properties were consistent with results reported in the literature. Specifically, our results revealed that yield strain is relatively constant (0.7 %) over a range of apparent densities, while failure strain presents a wider range of variation. A single strain parameter (equivalent strain) was found as an accurate descriptor of cancellous bone compression failure. Image-based numerical models usually need for the action of a technician to segment the images. Therefore, we studied the sensitivity to variations of the segmentation threshold on the morphometry and the elastic properties of vertebral cancellous bone specimens of different bone volume fractions. The apparent modulus is highly sensitive to the segmentation threshold. We report variations between 45 and 120 % for a ± 15 % threshold variation. Other parameters, such as BS/BV, BS/TV, Tb.Sp, Tb.N, Conn.D and fractal dimension were influenced significantly. Digital image correlation (DIC) was applied to images taken during compression testing to analyze displacement fields at failure and characterize them. Some variables were explored to describe failure and a study is done about how DIC parameters influence the strain field obtained. Facet and step sizes have a relevant effect on the failure strain estimation, and an increment of both parameters reduces the strain estimation up to 40 %. Besides, several parameters combination led to correct failure pattern detection, so values reported in the literature should be referred to the parameters used. Furthermore, we explored if cancellous bone microstructure acts (non-speckle/texture approach) as a proper pattern to calculate displacements using DIC technique. As regards relationships between microstructure and mechanics, single and multiple parameter analysis were performed to assess the morphometric variables that control the explanation of mechanical properties variation. Bone volume fraction (BV/TV), bone surface to volume ratio (BS/BV), mean trabecular thickness (Tb.Th) and fractal dimension (D) presented the best linear correlations to the elastic properties, while both the yield and failure strains did not show correlation to any morphometric parameter. The regressions obtained permit to estimate those mechanical properties that describe the state of a specimen.[ES] Las fracturas óseas constituyen un problema social de salud con magnitud creciente por su prevalencia en la población de edad avanzada debido a la osteoporosis. La calidad del hueso suele caracterizarse mediante la estimación de la densidad mineral ósea (DMO) en regiones de hueso trabecular, utilizando absorciometría de rayos X de energía dual (DXA). No obstante, la DMO por si sola no es capaz de predecir numerosos casos de fractura porque no solo importa la pérdida de densidad, sino que la microestructura también tiene un papel principal en la resistencia del hueso. Las propiedades mecánicas del hueso pueden usarse como indicadores de su integridad en función de la edad, enfermedad o tratamiento. Por lo tanto, la caracterización de la fractura de hueso trabecular y su relación con la microestructura no se ha resuelto de forma completa en la literatura y es relevante para mejorar las predicciones de fractura. En esta tesis, nuestro principal objetivo es caracterizar la morfometría y el comportamiento mecánico del hueso trabecular. Estimamos la morfometría a través del análisis de imágenes obtenidas por micro tomografía computerizada (micro-CT) de muestras de hueso trabecular vertebral de cerdo. Respecto al comportamiento mecánico, calculamos propiedades elásticas, de plasticidad y fractura a escala aparente y de tejido. Para determinar esas propiedades, hemos seguido diferentes procedimientos: ensayos a compresión, modelos de elementos finitos y fantomas de calibración micro-CT. Los modelos de elementos finitos desarrollados reproducen la respuesta elástica y de fallo bajo condiciones de compresión en hueso trabecular, modelando el fallo como combinación de mecánica del daño contínuo y la técnica de eliminación de elementos. Los modelos numéricos desarrollados han permitido estimar propiedades elásticas y de fallo. En concreto, las deformaciones de inicio de fallo estimadas son relativamente constantes para las muestras analizadas (0.7 %), mientras que las deformaciones últimas de fallo presentan un rango de variación mayor. Por otro lado, encontramos que la deformación equivalente es el descriptor más preciso del fallo a compresión del hueso trabecular. Normalmente, los modelos numéricos basados en imágenes suelen necesitar la acción de un técnico para segmentar las imágenes. En este sentido, estudiamos la sensibilidad de la morfometría y la estimación de propiedades elásticas ante variaciones en el umbral de segmentación en muestras con distinta fracción en volumen. Hemos obtenido que la rigidez aparente es muy sensible a cambios en el umbral de segmentación, con variaciones entre 45 y 120 % para una variación de ± 15 % del umbral de segmentación. Otros parámetros, como BS/BV, BS/TV, Tb.Sp, Tb.N, Conn.D y la dimensión fractal se ven afectados significativamente. Por otro lado, hemos aplicado la técnica correlación digital por imagen (DIC) para caracterizar campos de desplazamientos en el fallo a compresión del hueso trabecular, a partir del análisis de imágenes tomadas durante el ensayo de las muestras. Además, estudiamos la influencia de algunos parámetros de la técnica DIC en el campo de deformaciones obtenido. También, hemos explorado la aplicación DIC sin el uso de moteado, utilizando como patrón de reconocimiento la propia microestructura trabecular. En relación al estudio de la influencia de la microestructura en la respuesta mecánica, hemos calculado correlaciones de uno y varios parámetros para analizar qué variables morfométricas explican la variación de las propiedades mecánicas. La fracción en volumen de hueso (BV/TV), la relación entre el área y el volumen de hueso (BS/BV), el espesor trabecular medio (Tb.Th) y la dimensión fractal (D) presentan las mejores correlaciones lineales respecto a las propiedades elásticas, mientras que las deformaciones de inicio de plasticidad y fractura no mostraron correlación con ningún parámetro morfométrico.[CA] Les fractures òssies constitueixen un problema social de salut amb magnitud creixent per la seua prevalença en la població d'edat avançada a causa de l'osteoporosi. La qualitat de l'os sol caracteritzar-se mitjançant l'estimació de la densitat mineral òssia (DMO) en regions d'os trabecular, utilitzant absorciometria de raigs X d'energia dual (DXA). No obstant això, la DMO per si sola no és capaç de predir nombrosos casos de fractura perquè no sols importa la pèrdua de densitat, sinó que la microestructura també té un paper principal en la resistència de l'os. Les propietats mecàniques de l'os poden usar-se com a indicadors de la seua integritat en funció de l'edat, malaltia o tractament. Per tant, la caracterització de la fractura d'os trabecular i la seua relació amb la microestructura no s'ha resolt de manera completa en la literatura i és rellevant per a millorar les prediccions de fractura. En aquesta tesi, el nostre principal objectiu és caracteritzar la morfometria i el comportament mecànic de l'os trabecular. Estimem la morfometria a través de l'anàlisi d'imatges obtingudes per micro tomografia automatitzada (micro-CT) de mostres d'os trabecular vertebral de porc. Respecte al comportament mecànic, calculem propietats elàstiques, de plasticitat i fractura a escala aparent i de teixit. Per a determinar aqueixes propietats, hem seguit diferents procediments: assajos a compressió, models d'elements finits i fantomas de calibratge micro-CT. Hem desenvolupat models d'elements finits que reprodueixen la resposta elàstica i de fallada sota condicions de compressió en os trabecular, modelant la fallada com a combinació de mecànica del dany continu i la tècnica d'eliminació d'elements. Els models numèrics desenvolupats han permés estimar propietats elàstiques i de fallada. Les nostres estimacions respecte a propietats de fallada són consistents amb valors reportats en la literatura. En concret, les deformacions d'inici de fallada estimades són relativament constants per a les mostres analitzades (0.7 %), mentre que les deformacions últimes de fallada presenten un rang de variació major. D'altra banda, trobem que la deformació equivalent és el descriptor més precís de la fallada a compressió de l'os trabecular. Els models numèrics basats en imatges solen necessitar l'acció d'un tècnic per a segmentar les imatges. En aquest sentit, estudiem la sensibilitat de la morfometria i l'estimació de propietats elàstiques davant variacions en el llindar de segmentació en mostres amb diferent fracció en volum. Hem obtingut que la rigidesa aparent és molt sensible a canvis en el llindar de segmentació, amb variacions entre 45 i 120 % per a una variació de ± 15 % del llindar de segmentació. Altres paràmetres, com BS/BV, BS/TV, Tb.Sp, Tb.N, Conn.D i la dimensió fractal es veuen afectats significativament. D'altra banda, hem aplicat la tècnica correlació digital per imatge (DIC) per a caracteritzar camps de desplaçaments en la fallada a compressió de l'os trabecular, a partir de l'anàlisi d'imatges preses durant l'assaig de les mostres. A més, estudiem la influència d'alguns paràmetres de la tècnica DIC en el camp de deformacions obtingut. També, hem explorat l'aplicació DIC sense l'ús de clapejat, utilitzant com a patró de reconeixement la pròpia microestructura trabecular. En relació a l'estudi de la influència de la microestructura en la resposta mecànica, hem calculat correlacions d'un i diversos paràmetres per a analitzar quines variables morfomètriques expliquen la variació de les propietats mecàniques. La fracció en volum d'os (BV/TV), la relació entre l'àrea i el volum d'os (BS/BV), la espessor trabecular mitjà (Tb.th) i la dimensió fractal (D) presenten les millors correlacions lineals respecte a les propietats elàstiques, mentre que les deformacions d'inici de plasticitat i fractura no van mostrar correlació amb cap paràmetre morfomètric.Belda González, R. (2020). Mechanical and morphometric characterization of cancellous bone [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/149376TESI

Design and testing of additively manufactured lattice structures for musculoskeletal applications

Additive manufacturing (AM) methods present a new frontier in engineering, allowing the fabrication of porous lattice structures with tailored mechanical properties. AM structures can be made using bio-inert metals, creating controlled stiffness biomaterials. As bone formation is strain dependent, these AM biomaterials can be used in implants to optimise the strain in surrounding trabecular bone for peak bone formation. However, the behaviour of AM lattices varies and is subject to manufacturing constraints. The aim of this PhD was to investigate the mechanical behaviour of AM lattices, and maximise the clinical benefits of AM for musculoskeletal applications. Lattice architecture was shown to affect the anisotropy of an AM lattice biomaterial, increasing the stiffness in directions not often tested in the literature. The mechanical and morphological properties of individual struts within powder bed fusion (PBF) lattices were also shown to vary depending on the orientation of the struts to the build direction. The ultimate tensile strength of titanium alloy (Ti6Al4V) struts more than doubled when built at a low angle versus perpendicular to the build platform, and other properties were substantially lower than for the bulk material. Geometric imperfections were found for struts built at low angles. As such, a low stiffness modified stochastic lattice was designed and tested which avoided the problems found with struts built at low angles. The resulting lattice had improved stiffness isotropy and could be used for musculoskeletal applications, tuned to match the mechanical properties in local trabecular bone and enhancing bone formation.Open Acces