Osteosynthesis metal plate system for bone fixation using bicortical screws: numerical–experimental characterization

This study reports the numerical and experimental characterization of a standard immobilization system currently being used to treat simple oblique bone fractures of femoral diaphyses. The procedure focuses on the assessment of the mechanical behavior of a bone stabilized with a dynamic compression plate (DCP) in a neutralization function, associated to a lag screw, fastened with surgical screws. The non-linear behavior of cortical bone tissue was revealed through four-point bending tests, from which damage initiation and propagation occurred. Since screw loosening was visible during the loading process, damage parameters were measured experimentally in independent pull-out tests. A realistic numerical model of the DCP-femur setup was constructed, combining the evaluated damage parameters and contact pairs. A mixed-mode (I+II) trapezoidal damage law was employed to mimic the mechanical behavior of both the screw–bone interface and bone fractures. The numerical model replicated the global behavior observed experimentally, which was visible by the initial stiffness and the ability to preview the first loading peak, and bone crack satisfactorily.This research was funded by the Portuguese Foundation for Science and Technology (FCT), grant numbers SFRH/BD/143736/2019, UIDB/CVT/00772/2020, LA/P/0059/2020, UIDB/04033/2020, PTDC/EME-SIS/28225/2017, UID/EEA/04436/2019 and Laboratório Associado de Energia, Transportes e Aeronáutica (LAETA), grant number UID/EMS/50022/2020

An Investigation into the Plate Fixation for Periprosthetic Femoral Fractures

Periprosthetic femoral fractures are the third most reason for reoperation after the total hip arthroplasty with an incident rate of approximately 6%. The Vancouver type B periprosthetic femoral fractures account for over 70% of all cases, while the sub-type B1 fracture (when the total hip stem is stable) has remained a clinical challenge due to incidences of severe complications after the standard plate-screw fixation. To seek biomechanically sound fixations for the Vancouver type B1 fracture, this dissertation developed a combined modeling and testing framework to investigate the efficacy of fixation for a Vancouver type B1 fracture using different construct lengths and different plating systems. Specifically, the coupled musculoskeletal and finite element model of total hip stem and plate implanted femurs were developed to simulate the physiological bone strain and the plate stress under loads of common activities of daily living. The modeling results were shown to be able to effectively evaluate and compare the mechanics of different plating systems and construct lengths but were also able to shed light on the mechanisms of mechanical pathogenesis of PFFs. The models also showed good fidelity in predictions of bone strain and bone remodeling stimuli as compared with the previous clinical and biomechanical studies. The results of the coupled models were used as a basis for developing several new mechanical tests, which were shown to match the simulated physiological bone strain and plate stress in the coupled musculoskeletal and finite element models

Modelling and optimising the mechanical behaviour of fractures treated with locking plates

A large number of bone fractures are treated with stabilisation devices that utilise metal wires or screws, which traverse the bone and are connected to an external frame or internal plate. Clinically, fixation devices are required to be able to: sustain loads; minimise patient discomfort and possible implant loosening; and promote healing. In the recent years locking plates have become increasingly popular for osteoporotic or complex fractures, which can be difficult to manage. It, however, remains unclear as to how these devices need to be configured for optimum clinical performance. This thesis investigates the mechanics of locking plates, factors that influence their performance and provides guidance to optimise the placement of screws. Finite element simulation and analytical models were developed and validated using lab-based experimental models. The local behaviour around the screw-bone interface is considered and the implications of different modelling assumptions assessed. A novel method of simulating the effect of radial interference due to pilot-hole size is proposed. Different screw types are evaluated: osteoporotic bone is found to be particularly susceptible to the screw tightening preload used in compression screws; far-cortical locking screws are found to slightly reduce device stiffness but substantially increase strain levels around screw holes. Finite element simulations show that many of the local effects, such as preloads and contact modelling, can profoundly influence the prediction of strains around screws but do not generally influence the global load-displacement behaviour; the screw-plate connection and bone/plate material and geometric properties are found to have an influence on global stiffness predictions. The key determinants of load-displacement behaviour evaluated through models are the loading and restraint conditions, which explain the huge range of stiffness predictions in the literature (three orders of magnitude). An analytical model based on 7 bone-plate construct parameters is developed. Despite its simplicity, the model is found to be able to predict the axial stiffness for experimental tests conducted and for 16 other cases from five previous studies with an average error of 20%. The manner of load application, not considered in the literature, is shown to dramatically alter predictions of plate stress, strains within the bone and conclusions regarding screw placement. Even with the inclusion of muscles forces, the choice of restraint condition dominates the mechanical behaviour. Using the models, the influence of screw position is systematically evaluated in varying bone qualities under axial loading and torsion and guidance for optimising fixation is developed

Translational Models for Advancement of Regenerative Medicine and Tissue Engineering

At the root of each regenerative medicine or tissue engineering breakthrough is a simple goal, to improve quality of healing, thus improving a patient’s quality of life. Each tissue presents its own complexities and limitations to healing, whether it is the scarring nature of tendon healing or the mechanical complexity driving bone regeneration. Preclinical, translational models aim to reflect these complexities and limitations, allowing for effective development and refinement of tissue engineered therapeutics for human use. The following body of work explores several of these translational models, both utilizing them for tissue regenerative therapy development and evaluating the benefits and complications incurred with each model. This work begins with a discussion of the complexity of bone healing and how dysfunction in the mechanical, surgical, and systemic fracture environment can lead to delayed healing and nonunion. A comprehensive review of the advances in preventative and corrective therapeutics for bone nonunion is included next, with specific focuses on mechanical and tissue-engineered technology. Then, this work presents a tissue-engineered application of mesenchymal stem cells in acute tendon injury, highlighting experimentation in cell fate direction in vitro and intralesional mesenchymal stem cell implantation in vivo. Next, this work presents a series of experiments that evaluated and refined a commonly utilized preclinical model of delayed bone healing, the caprine segmental tibial defect stabilized using single locking plate fixation. First, the biomechanical stability of the model was evaluated in vivo using plantar-pressure analysis of gait. Then, the surgical technique was refined through a retrospective analysis of the effects of plate length and position on fixation stability in vitro and in vivo. Finally, the comorbidities of this preclinical model were explored via an analysis of the effect of long-term tibial locking plate fixation on cortical dimensions and density

Biomechanical analysis of femoral fracture fixation using the expert adolescent lateral femoral nail system

Femoral fracture in adolescents is a severe injury. Recent studies of intramedullary nail fixation with rigid titanium alloy helical nail viz. Expert adolescent lateral femoral nail (ALFN) have reported good results. However, there is no in vitro biomechanical data available on this nail in the literature. Experimental testing and finite element analysis (FEA) were used to establish the stiffness parameters of small composite femurs with simulated fractures stabilised using ALFN. In comparison to intact femur, construct stiffness ranged from maximum (114%) to minimum (20%) for healed fracture and segmental fracture, respectively. Simulation testing in SolidWorksTM was performed with validated FEA model to evaluate the effect of clinical and implant factors. Maximum predicted stress in the distal interlocking screw remained in an acceptable range (160.25 - 188.51 MPa) irrespective of the level of femoral shaft fracture with a relative decrease in stress values as the fracture callus healed over a 16 week period. The relative angle between the ALFN and proximal interlocking screw and implant material were two significant factors influencing stress at the interlocking screw and nail interface. In conclusion, a rigid helical titanium alloy femoral intramedullary nail can perform satisfactorily under physiological loading conditions experienced in the perioperative period

Computer-assisted pre-operative automatic segmentation and registration tool for malunited radius osteotomy: A proof-of-concept study

Corrective osteotomy is a standard treatment for distal radius fractures in malunited radius cases. In order to increase the efficiency of the osteotomy pre-operative plan, in this study, a proof-of-concept framework of automatic computer-assisted segmentation and registration tool was developed for the purpose of malunited radius osteotomy pre-operative planning. The program consisted of the functions of segmentation, virtual cutting, automatic alignment and registration. One computed tomography (CT) scanning dataset of a patient's bilateral forearm was employed as an illustration example in this study. Three templates of 3D models including the healthy radius, and the pre- and post-correction injured radius were output as STL geometries for pre-operative plan purposes

Computer Aided Tools for the Design and Planning of Personalized Shoulder Arthroplasty

La artroplastia de hombro es el tercer procedimiento de reemplazo articular más común, después de la artroplastia de rodilla y cadera, y actualmentees el de más rápido crecimiento en el campo ortopédico. Las principales opciones quirúrgicas incluyen la artroplastia total de hombro (TSA), en la quese restaura la anatomía articular normal, y, para pacientes con un manguito rotador completamente desgarrado, la artroplastia inversa de hombro (RSA), en la que la bola y la cavidad de la articulación glenohumeral se cambian. A pesar del progreso reciente y los avances en el diseño, las tasas de complicaciones reportadas para RSA son más altas que las de la artroplastia de hombro convencional. Un enfoque específico para el paciente, en el que los médicos adaptan el tratamiento quirúrgico a las características del mismo y al estado preoperatorio, por ejemplo mediante implantes personalizados y planificación previa, puede ayudar a reducir los problemas postoperatorios y mejorar el resultado funcional. El objetivo principal de esta tesis es desarrollar y evaluar métodos novedosos para RSA personalizado, utilizando tecnologías asistidas por ordenador de última generación para estandarizar y automatizar las fases de diseño y planificación.Los implantes personalizados son una solución adecuada para el tratamiento de pacientes con pérdida extensa de hueso glenoideo. Sin embargo, los ingenieros clínicos se enfrentan a muchas variables en el diseño de implantes (número y tipo de tornillos, superficie de contacto, etc.) y una gran variabilidad anatómica y patológica. Actualmente, no existen herramientas objetivas para guiarlos a la hora de elegir el diseño óptimo, es decir, con suficiente estabilidad inicial del implante, lo que hace que el proceso de diseño sea tedioso, lento y dependiente del usuario. En esta tesis, se desarrolló una simulación de Virtual Bench Test (VBT) utilizando un modelo de elementos finitos para evaluar automáticamente la estabilidad inicial de los implantes de hombro personalizados. A través de un experimento de validación, se demostró que los ingenieros clínicos pueden utilizar el resultado de Virtual Bench Test como referencia para respaldar sus decisiones y adaptaciones durante el proceso de diseño del implante.Al diseñar implantes de hombro, el conocimiento de la morfología y la calidad ósea de la escápula en toda la población es fundamental. En particular, se tienen en cuenta las regiones con la mejor reserva ósea (hueso cortical) para definir la posición y orientación de los orificios de los tornillos, mientras se busca una fijación óptima. Como alternativa a las mediciones manuales, cuya generalización está limitada por el análisis de pequeños subconjuntos de pacientes potenciales, Statistical Shape Models (SSMs) se han utilizado comúnmente para describir la variabilidad de la forma dentro de una población. Sin embargo, estos SSMs normalmente no contienen información sobre el grosor cortical.Por lo tanto, se desarrolló una metodología para combinar la forma del hueso escapular y la morfología de la cortical en un SSM. Primero, se presentó y evaluó un método para estimar el espesor cortical, a partir de un análisis de perfil de Hounsfield Unit (HU). Luego, utilizando 32 escápulas sanas segmentadas manualmente, se creó y evaluó un modelo de forma estadística que incluía información de la cortical. La herramienta desarrollada se puede utilizar para implantar virtualmente un nuevo diseño y probar su congruencia dentro de una población virtual generada, reduciendo así el número de iteraciones de diseño y experimentos con cadáveres.Las mediciones del alargamiento de los músculos deltoides y del manguito rotador durante la planificación quirúrgica pueden ayudar a los médicos aseleccionar un diseño y una posición de implante adecuados. Sin embargo, tal evaluación requiere la indicación de puntos anatómicos como referencia para los puntos de unión de los músculos, un proceso que requiere mucho tiempo y depende del usuario, ya que a menudo se realiza manualmente. Además, las imágenes médicas, que se utilizan normalmente para la artroplastia de hombro,contienen en su mayoría solo el húmero proximal, lo que hace imposible indicarlos puntos de unión de los músculos que se encuentran fuera del campo de visión de la exploración. Por lo tanto, se desarrolló y evaluó un método totalmente automatizado, basado en SSM, para medir la elongación del deltoides y del manguito rotador. Su aplicabilidad clínica se demostró mediante la evaluación del rendimiento de la estimación automatizada de la elongación muscular para un conjunto de articulaciones artríticas del hombro utilizadas para la planificación preoperatoria de RSA, lo que confirma que es una herramienta adecuada para los cirujanos a la hora de evaluar y refinar las decisiones clínicas.En esta investigación, se dio un paso importante en la dirección de un enfoque más personalizado de la artroplastia inversa de hombro, en el que el manejo quirúrgico, es decir, el diseño y la posición del implante, se adapta a las características específicas del paciente y al estado preoperatorio. Al aplicar tecnologías asistidas por computadora en la práctica clínica, el proceso de diseño y planificación se puede automatizar y estandarizar, reduciendo así los costos y los plazos de entrega. Además, gracias a los métodos novedosos presentados en esta tesis, esperamos en el futuro una adopción más amplia del enfoque personalizado, con importantes beneficios tanto para los cirujanos como para los pacientes.Shoulder arthroplasty is the third most common joint replacement procedure, after knee and hip arthroplasty, and currently the most rapidly growing one in the orthopaedic field. The main surgical options include total shoulder arthroplasty (TSA), in which the normal joint anatomy is restored, and, for patients with a completely torn rotator cuff, reverse shoulder arthroplasty (RSA), in which the ball and the socket of the glenohumeral joint are switched. Despite the recent progress and advancement in design, the reported rates of complication for RSA are higher than those of conventional shoulder arthroplasty. A patient-specific approach, in which clinicians adapt the surgical management to patient characteristics and preoperative condition, e.g. through custom implants and pre-planning, can help to reduce postoperative problems and improve the functional outcome. The main goal of this thesis is to develop and evaluate novel methods for personalized RSA, using state-of-the-art computer aided technologies to standardize and automate the design and planning phases. Custom implants are a suitable solution when treating patients with extensive glenoid bone loss. However, clinical engineers are confronted with an enormous implant design space (number and type of screws, contact surface, etc.) and large anatomical and pathological variability. Currently, no objective tools exist to guide them when choosing the optimal design, i.e. with sufficient initial implant stability, thus making the design process tedious, time-consuming, and user-dependent. In this thesis, a Virtual Bench Test (VBT) simulation was developed using a finite element model to automatically evaluate the initial stability of custom shoulder implants. Through a validation experiment, it was shown that the virtual test bench output can be used by clinical engineers as a reference to support their decisions and adaptations during the implant design process. When designing shoulder implants, knowledge about bone morphology and bone quality of the scapula throughout a certain population is fundamental. In particular, regions with the best bone stock (cortical bone) are taken into account to define the position and orientation of the screw holes, while aiming for an optimal fixation. As an alternative to manual measurements, whose generalization is limited by the analysis of small sub-sets of the potential patients, Statistical Shape Models (SSMs) have been commonly used to describe shape variability within a population. However, these SSMs typically do not contain information about cortical thickness. Therefore, a methodology to combine scapular bone shape and cortex morphology in an SSM was developed. First, a method to estimate cortical thickness, starting from a profile analysis of Hounsfield Unit (HU), was presented and evaluated. Then, using 32 manually segmented healthy scapulae, a statistical shape model including cortical information was created and assessed. The developed tool can be used to virtually implant a new design and test its congruency inside a generated virtual population, thus reducing the number of design iterations and cadaver labs. Measurements of deltoid and rotator cuff muscle elongation during surgical planning can help clinicians to select a suitable implant design and position. However, such an assessment requires the indication of anatomical landmarks as a reference for the muscle attachment points, a process that is time-consuming and user-dependent, since often performed manually. Additionally, the medical images, which are normally used for shoulder arthroplasty, mostly contain only the proximal humerus, making it impossible to indicate those muscle attachment points which lie outside of the field of view of the scan. Therefore, a fully-automated method, based on SSM, for measuring deltoid and rotator cuff elongation was developed and evaluated. Its clinical applicability was demonstrated by assessing the performance of the automated muscle elongation estimation for a set of arthritic shoulder joints used for preoperative planning of RSA, thus confirming it a suitable tool for surgeons when evaluating and refining clinical decisions. In this research, a major step was taken into the direction of a more personalized approach to Reverse Shoulder Arthroplasty, in which the surgical management, i.e. implant design and position, is adapted to the patient-specific characteristics and preoperative condition. By applying computer aided technologies in the clinical practice, design and planning process can be automated and standardized, thus reducing costs and lead times. Additionally, thanks to the novel methods presented in this thesis, we expect in the future a wider adoption of the personalized approach, with important benefits both for surgeons and patients.<br /

Development of novel mechanical diagnostic techniques for early prediction of bone fracture healing outcome

2021 Fall.Includes bibliographical references.To view the abstract, please see the full text of the document

Predictive modelling of the form and development of bone fracture healing

It is the contention of this study that the rate and formation of bone healing can be modelled mathematically and computationally, respectively, based on mechanical stimuli induced by the relative motion between bone fragments. To argue this case, the following hypotheses are tested; 1) the relationship between temporal rate of bone healing, measured in terms of callus stiffening per week, and the percentage of interfragmentary strain, can be described mathematically, and 2) the spatial distribution of callus tissue around a fracture can be modelled computationally based on compressive principal strains experienced by the immature healing tissues caused by interfragmentary motions. To test the first hypothesis, a comparative analysis of empirical relationships between rate of healing and level of mechanical stimulus found in the literature was conducted. Based on this, a mathematical phenomenological model was derived. To test the second hypothesis, the finite element method was employed to determine how bone-fixator position, fracture geometry, loading and the consequent strains experienced by the healing tissues, influence callus formation. An algorithm was proposed which iteratively removed lowly strained soft tissue from a large domain at the fracture site, thus producing a more efficient callus formation. The premise of this algorithm was based on the adage that ‘form follows function’, and a callus will inevitably strive to remodel itself to the point where greatest mechanical efficiency is achieved. The results of the comparative literature review and the proposed mathematical model revealed a positive correlation between the rate of callus stiffening and the initially applied interfragmentary strain. The results of computational models showed direct agreement with experimental findings and clinical observations which reinforces the hypothesis that compressive principal strains are the dominant driving force behind callus formation. Furthermore, it was shown that the proximity of a unilateral fixator to the fractured bone has a greater influence over asymmetric callus formation than the physical presence of the fixator itself. Finally, the implications of the proposed strategies show potential in pre-clinical testing of fixation devices and configurations, which was demonstrated using simulated comparisons with clinical case studies of healing bones under unilateral fixation

Failure Analysis of Biometals

Metallic biomaterials (biometals) are widely used for the manufacture of medical implants, ranging from load-bearing orthopaedic prostheses to dental and cardiovascular implants, because of their favourable combination of properties, including high strength, fracture toughness, biocompatibility, and wear and corrosion resistance. Owing to the significant consequences of implant material failure/degradation, in terms of both personal and financial burden, failure analysis of biometals has always been of paramount importance in order to understand the failure mechanisms and implement suitable solutions with the aim to improve the longevity of implants in the body. Failure Analysis of Biometals presents some of the latest developments and findings in this area. This includes a great range of common metallic biomaterials (Ti alloys, CoCrMo alloys, Mg alloys, and NiTi alloys) and their associated failure mechanisms (corrosion, fatigue, fracture, and fretting wear) that commonly occur in medical implants and surgical instruments