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.

Design and development of a multiscale model for the osteoporotic fracture prevention: a preclinical tool

Se espera que la osteoporosis sea partícipe de más de 9 millones de nuevas fracturas en todo el mundo en un futuro no muy lejano, ya que es una de las enfermedades con mayor índice de impacto entre la población de los países desarrollados. Se define como una enfermedad sistémica caracterizada por la pérdida de masa ósea y una alteración de su microestructura interna con la consiguiente susceptibilidad a la fractura. Actualmente, la estimación del riesgo de fractura se lleva a cabo mediante tomografía axial computerizada (TAC), Rayos X o densitometrías. Sin embargo, las simulaciones por elementos finitos para un paciente determinado, pueden contener una gran cantidad de información que permitirían unas predicciones más precisas. Una metodología multiescala ayudaría al desarrollo y caracterización de modelos de fractura más robustos que permitirían conocer de una manera más detallada el comportamiento del hueso. Además, dichos modelos podrían incorporar parámetros relacionados con la edad, el grado de osteoporosis o el tratamiento mediante fármacos. De hecho, debido a que el hueso trabecular interviene, en gran medida, en las fracturas de cadera osteoporóticas, un tratamiento preventivo alternativo para reducir el riesgo de fractura osteoporótica consistiría en la inyección de cemento óseo (PMMA) en el fémur osteoporótico.Por lo tanto, el principal objetivo de esta tesis doctoral es el desarrollo de un modelo multiescala para la prevención de la fractura ósea osteoporótica. Este modelo nos permitirá conocer más acerca de los mecanismos de fallo asociados a la osteoporosis desde el nivel tisular hasta el nivel macroscópico a fin de evaluar la factibilidad de la femoroplastia. Para alcanzar este objetivo, en primer lugar, se ha llevado a cabo una caracterización in vitro e in silico de estructuras artificiales de hueso artificial, denominadas open-cell (Sawbones, Malmö, Sweden), con propiedades próximas al hueso sano y osteoporótico, de manera que permita elucidar mecanismos de fractura asociados a la osteoporosis desde el nivel tisular. De esta manera, se han empleado métodos experimentales y computacionales basados en el procesado de imagen con el fin de estimar el módulo elástico y las porosidades de las diferentes estructuras open-cell. Las resultados computacionales y experimentales fueron comparados con los datos aportados por el fabricante. Se apreciaron importantes diferencias no sólo en términos del módulo de Young sino también en las porosidades. Posteriormente, se desarrolló un modelo discreto de partículas basado en la Teoría del Movimiento Aleatorio para simular la infiltración de cemento a través de las estructuras open-cell, previamente caracterizadas. Los parámetros del modelo incluyeron no sólo la viscosidad del cemento (alta o baja) sino la dirección de inyección (vertical o diagonal). De nuevo, se llevó a cabo una caracterización in vitro e in silico de las estructuras cementadas, validando el modelo computacional mediante ensayos experimentales. Dichos resultados mostraron que el modelo discreto de partículas era suficientemente robusto para su aplicación en la escala macroscópica. También, se inyectó cemento in vivo en fémures de conejo a fin de evaluar la factibilidad de la femoroplastia. Finalmente, se utilizaron fémures sanos y osteoporóticos para la predicción computacional del grado de mejora de las propiedades mecánicas cuando se inyectaba cemento de alta o baja viscosidad. El cemento de baja viscosidad mejoraba notablemente las cargas de fractura con respecto a los fémures no cementados. Los resultados finales mostraron que el cemento óseo mejora definitivamente las propiedades del hueso osteoporótico y la metodología propuesta puede llegar a utilizarse como una herramienta preclínica para un diagnóstico más preciso.<br /

Numerical investigation of femoral augmentation

Femoral augmentation is a minimally invasive procedure involving injection of bone cement into osteoporotic femora in order to enhance their load capacity. However, this treatment poses significant risks such as bone thermal necrosis or embolism when large amounts of bone cement are injected in the femur. This thesis presents methods developed to find the ideal bone cement volume and distribution needed to restore the load capacity of osteoporotic femora depending on their level of osteoporosis. Material properties of augmented tissue were modelled using a proposed scheme that combines Voigt-Reuss-Hill average and bone cement porosity. These ideal bone cement distributions were used as a reference to propose several feasible and generalised augmentation strategies, which comprised placing bone cement in up to three spheres or in up to two pre-drilled channels. Bone cement location was found to be more significant in the augmentation result than bone cement volume or augmentation strategy. Fracture analysis of augmented femora was also conducted, demonstrating that approximately 7ml of bone cement can result in an increase of 74% in yield load, 62% in fracture load, and 117% in energy to fracture. After finding the optimum bone cement volume and distribution, the bone cement injection and polymerisation process was studied in a 2D femur model, and results suggest that risk of thermal necrosis was limited to the regions in the bone-PMMA interface while stress levels required to develop debonding between the materials were not reached. However, results were obtained from a 2D model and the bone-PMMA interface was not modelled in detail. Some other limitations involved in the present study are the use of a single femur, with virtually introduced osteoporosis that only represents senile osteoporosis and a single set of boundary conditions. Additionally, despite results were compared against experiments in the literature, an experimental validation may be necessary to ensure the validity of the model. Despite the limitations of the present study and lack of direct experimental validation, the methods presented in this thesis can be applied to any femur to evaluate the requirements of femoral augmentation and the risks that it may entail. When applied to the studied femur, we conclude that femoral augmentation can increase significantly the femur yield and fracture load and only present risk of thermal necrosis in the bone-PMMA interface

DEVELOPMENT OF A SUBJECT SPECIFIC FINITE ELEMENT MODEL USED TO PREDICT THE EFFECTS OF A SINGLE LEG EXTENSION EXERCISE

The study presented attempts to prove the concept that mechanical changes in the structure of a bone can be predicted for a specific exercise by a subject specific model created from CT data, MRI data, EMG data, and a physiologic FE model. Previous work generated a subject specific FE model of a femur via CT and MRI data as well as created a set of subject specific biomechanical muscle forces that are required to perform a single leg extension exercise. The FE model and muscle forces were implemented into a single leg extension FE code (ABAQUS) along with a specialized bone remodeling UMAT. The UMAT updated the mechanical properties of the femur via a damage-repair bone remodeling algorithm. The single leg extension FE code was verified by applying walking loads to the femur and allowing the system to equilibrate. The results were used to apply the appropriate walking loads to the final FE simulation for the single leg extension exercise. The final FE simulation included applying the single leg extension loads over a one year period and plotting the change in porosity at various regions of the femoral neck. Although only two regions were found to generate valid results, the data seemed counterintuitive to Wolff’s Law which states that bone adaptation is promoted when the material is stressed. The model was successful in creating a subject specific model that is capable of predicting changes in the mechanical properties of bone. However, in order to generate valid FE model results, further understanding of the bone remodeling process and application via a FE model is required

Characterisation of trabecular bone behaviour under impact

Increasing ageing population around the world implies deteriorated bone quality due to diseases such as osteoporosis. Osteoporosis results in reduced bone density and strength and increases the risk of bone fractures due to impact loads such as those caused by a fall. While numerous studies have been undertaken to evaluate the mechanical properties of bone using experimental or computational approaches most of these have focused on static or cyclic loads. Little is known about the behaviour of bone under dynamic loading, such as that caused by a fall. This study focuses on characterising the impact behaviour of spongy bone which is also known as trabecular or cancellous bone using drop tower tests. A closed-form solution of drop test on an elastic material is developed in detail to serve as a benchmark. Sequential compressive stress wave equations are derived then coded in programming language to obtain desired number of equations, leading to the resulting force history at the end struck. The number of required intervals to obtain the whole pulse depends on the mass ratio; the developed code can be readily expanded to include a wide range of these ratios. A parametric analysis is undertaken to evaluate the influence of parameters such as elastic modulus and drop height. This evaluation defines the key outputs of the force response, namely peak load, pulse width, interval frequency, number of stress intervals and loading rate; hence it establishes the output variable-parameter relationships, e.g. peak load-elastic modulus relation. The force response from the closed-form solution is used to verify the finite element (FE) models. The Poisson's ratio is found to have insignificant effect on the resulting peak load and pulse width. Stiffer response is indicated by an increase in peak load or a decrease in pulse width for the same impact. Fast Fourier transform (FFT) is applied to the pulses obtained from the closed-form solution and FE analysis; the returned frequency values reveal that the discrepancy observed in FE results can be attributed to its solver rather than the FFT technique. The time-independent elastic FE model is extended to include the time-dependent viscoelastic behaviour; the latter provides a stiffer response. The output variableparameter relations from FE analysis are verified by similar expressions derived from the closed-form solution. Drop tests are conducted in the lab on trabecular bone samples with varying bone volume fraction (bone volume to total volume ratio or BV/TV) to investigate the resulting force response. These responses are categorised on the basis of BV/TV of the tested samples. Samples with large BV/TV are shown to behave in an apparently elastic manner. Low BV/TV samples respond inelastically which the study attempts to simulate using plasticity. Behaviour of samples with growth plates shows inelastic behaviour even though they have large BV/TV. Regression analysis is carried out on the outputs of pulses obtained from higher BV/TV samples to find the output variable-parameter relations, e.g. inverse relation of pulse width-BV/TV. Inverse modelling using FE analysis is also performed to estimate a representable elastic modulus by matching the peak load and pulse width of the resulting force response. The elastic moduli are found to be in the range of 650 to 1400 MPa for samples with BV/TV ranging between 35 to 53%. Elastic modulus is also evaluated using the initial load-response curve for both lower and higher BV/TV samples to establish linear and power-law loading rate versus BV/TV relationships. The inelastic behaviour of low BV/TV samples consistently shows initial peak, followed by a drop to a finite non-zero value in the post-elastic regime which is maintained for a considerable prolonged time before the load returns to zero. This pattern has not been previously demonstrated. The duration of plasticity is found to be larger for lower BV/TV samples, while shorter drop time is observed exhibiting brittle breakage after the peak load is attained. The study also considers simulation of the post-yield inelastic response using strain-softening plasticity. It is shown that strain-softening is capable of approximately replicating the post-yield impact response pattern by assuming appropriate value of tangent modulus. Four dynamic elastic modulus expressions as functions of BV/TV are derived by correlating the relationships regressed experimentally with their correspondences from the closed-form solution. The performance of these linear and power law equations are compared to several relations from the literature and the response from tests conducted at an apparent strain rate of 0.01 /sec on similar samples. It is found that the dynamic elastic moduli found in this study are higher than their quasi-static and monotonic loading counterparts; a conclusion showing the effect of strain rate magnitude on the stiffness of the trabecular bone. The study evaluates the strain rate experienced by bone due to impact loading considered in this study to be a maximum of 44.3 /sec at the initial stage with secant value of 30 /sec at the peak strain. Assuming the high BV/TV samples behave fully elastic, the apparent strain due to impact loads can be as high as 3% in compression

NONINVASIVE EVALUATIONS OF SLENDER GRAPHITE RODS AND HUMAN THORACOLUMBAR SPINE

Mechanical properties, internal condition and fracture risk of structural components can be assessed by noninvasive techniques being preferred mainly because of their efficiency and speed. This study presents noninvasive evaluations of slender graphite rods and human thoracolumbar spine. An experimental approach for graphite rods and numerical approaches for both graphite rods and human thoracolumbar spine were developed. Internal cracks may occur in the graphite rods during the manufacturing process. In an effort to develop a nondestructive testing approach to evaluation of the graphite rods, transient elastic impact was used. Wave theory was used for solid rods. Subsequently, numerical models were developed to determine the response of rods containing cracks. Experiments on graphite rods with and without cracks were conducted and the internal condition was determined from the recorded signals. The rods were then cut lengthwise to reveal the internal condition and verify the predicted results. The knowledge gained from simulations allowed for the presence of cracks to be detected. For fracture risk assessment of vertebra, finite element (FE) models with simplified geometry, material properties or loading conditions were developed in the past. To investigate the role of these parameters, two FE models were created from CT images: an isolated L1 vertebra and a T12-L2 spinal segment with ligaments, discs and facets. Each model was examined with both homogeneous and spatially varying bone tissue properties. Stresses and strains were compared for uniform compression and flexion. Inclusion of heterogeneous bone properties and physiological loading in FE models was critical to assess vertebral fracture risk. The fracture risk of an osteoporotic thoracolumbar junction was assessed using the FE model of L2-T12 spinal segment. Osteoporosis was simulated in four stages, which included disc stiffening and stiffness losses in cancellous core and cortical cortex. Overall stiffness of the segment, and stresses and strains in two sections of L1 were computed for uniform compression and flexion at each stage. This study clearly delineated that osteoporotic bone was at high risk for fracture through not only increased bone stresses and strains with loading, but also changes in the volume and location of bone experiencing these high strains

Development of a 50th Percentile Female Femur Model

This study illustrates the development of a generic femur model representative of a 50th percentile female in terms of geometry, material data, and injury risk curve. A female femur model consisting of 14,520 hexahedral elements was developed, calibrated, and validated. The outer shape and cortical thickness of the femur shaft were adjusted to meet a regression model reported in literature for an average 50 year old female. For the proximal femur, five computed tomography scans were morphed to the target geometry and the mean thickness of the cortical bone was calculated. Material properties for the cortical bone were calculated from experimental data for both tension and compression loading. To validate the proximal femur mode and calibrate an injury risk curve, 15 dynamic drop-tower tests were reproduced. For the validation of the femur shaft, 16 bending tests were simulated. The characteristics of the experimental curves were generally well captured for experiments with normal bone density. Maximum principal strains and 99th percentile strains of the cortical bone at the time of fracture were used to develop risk curves for fractures of the proximal femur and the femur shaft, which were identified as the most relevant femoral injuries in an accident analysis. The model as well as the post-processing scripts are openly available and can be applied or further enhanced by other researchers

Participant-Specific Modelling of the Proximal Femur during Lateral Falls: A Mechanistic Evaluation of Risk Factors

Falls among older adults are a common occurrence with the potential to result in substantial injury. Hip fractures are among the most frequent and devastating fall induced injuries, resulting in increased morbidity and mortality, as well as significant socioeconomic costs. From a mechanistic perspective, the risk of a hip fracture during a fall is dictated by the ratio between the impact loading and the ability of the femur to withstand such loads. Investigations of clinical fracture risk factors have generally focused on the latter, neglecting the influence of these factors on impact dynamics. Experimental fall simulations provide a means to investigate factors modulating impact dynamics; however, these studies are limited to the skin surface with limited ability to draw conclusions on femoral loading and fracture risk. Investigations into the mechanical basis of clinical risk factors (sensitive to both loading and femur morphology) could provide insights to inform the development of protective devices and increase the accuracy of screening tools. Therefore, the purpose of this thesis was to evaluate the influence of previously identified hip fracture risk factors on impact characteristics during lateral falls and how the application of these loading conditions influence femoral neck stresses and fracture risk. Specifically, the influence of fall simulation paradigm (FSP: a surrogate for fall type), sex, and trochanteric soft tissue thickness (TSTT) were evaluated through coupling of experimental impact dynamics with participant-specific proximal femur models. Healthy young males and females, encompassing a wide range of body compositions underwent a series of fall simulation paradigms. These paradigms varied in fall trajectory and impact configuration, ranging from highly controlled vertical drops (pelvis release) to releases more representative of falls observed in older adults (kneeling and squat releases). Peak impact force magnitude and localization over the proximal femur, as well as orientation and point of application with respect to the femur were extracted (Chapters 3 and 4). A subset of the participants subsequently underwent dual energy X-ray absorptiometry (DXA) imaging, enabling participant-specific modelling and tissue level analysis - driven by experimental loading conditions (reginal force magnitude, orientation, and point of application; Chapter 5). FSP significantly influenced skin surface loading conditions, as well as femoral neck stresses and fracture risk. Compared to kneeling and squat, pelvis release elicited lower peak force magnitude; however, this force was applied closer to and was more concentrated over the greater trochanter. Despite the differences in force distribution, kneeling and squat release still elicited greater force directed over the proximal femur compared to pelvis release. Beyond force magnitude and distribution, these FSP varied significantly in impact vector orientation with respect to the femur. Kneeling release was associated with the most perpendicular loading vector, while squat release elicited the most distally directed vector in the frontal plane. In the anterior-posterior plane, pelvis release was directed posteriorly, while kneeling and squat release were directed anteriorly. Observed difference in skin surface loading conditions across FSP interacted with underlying femoral geometry to influence stress generation and fracture risk. Compressive stress at the superior-lateral femoral neck was greatest in kneeling release, while tensile stress at the inferior-medial femoral neck was greatest in squat release (driven by proportion of force resulting in axial compression vs. bending stress). While no differences in femoral neck fracture risk were observed between kneeling and squat release, kneeling release may elicit a greater risk of local compressive failure in the superior femoral neck. At the skin surface, sex and TSTT significantly influenced impact dynamics; however, underlying differences in femur morphology influenced the translation of these loading conditions to femoral neck stresses and fracture risk. Compared to females, males exhibited greater impact force magnitude, which was applied closer to and was more concentrated over the greater trochanter of the proximal femur. This increased loading in males was mitigated by differences in femur morphology (greater resistance to bending and shear stress generation, as well as strength), resulting in no differences in femoral neck stresses or fracture risk. The increased risk of hip fracture in females may be explained by age related changes in femur morphology, as well as sex-differences in the circumstances of falls. High-TSTT individuals exhibited greater impact force magnitude; however, these loads were applied further from and less focally over the greater trochanter compared to low-TSTT individuals. Combined, no differences were observed in the amount of force directed over the proximal femur across TSTT. Despite similar loading conditions, low-TSTT individuals elicited greater femoral neck stresses and fracture risk compared to their high-TSTT counterparts, driven by differences in underlying femur morphology (reduced resistance to bending and shear stress generation). The protective influence of TSTT to redistribute impact force peripherally away from the greater trochanter appears to play an important role in fracture risk. When global impact force was utilized instead of local force during modelling, no differences in femoral stresses or fracture risk were observed across TSTT. In summary, this thesis combined two previously exclusive approaches (experimental fall simulations and tissue level modelling) to gain novel insights into the influence of FSP, sex, and TSTT on femoral neck stresses and fracture risk. Through a participant-specific multi-level approach, this analysis was sensitive to both impact dynamics and underlying femoral geometry. FSP influenced fracture risk, as well as the location and magnitude of peak femoral stresses. Inclusion of muscle activation in future versions of the current approach may inform ‘safe-falling’ strategies, designed to reduce fracture risk. The current results support epidemiological findings suggesting TSTT is a protective factor against hip fracture; however, sex differences in fracture risk are likely driven by age related changes in femur morphology not included in this analysis. Based on the apparent importance to fracture risk, future work should aim to quantify the translation of skin surface pressure distributions to impact energy delivered to the proximal femur