MECHANICAL METRICS OF THE PROXIMAL FEMUR ARE PRECISE AND ASSOCIATED WITH HIP MUSCLE PROPERTIES: A MAGNETIC RESONANCE BASED FINITE ELEMENT STUDY

Proximal femoral (hip) fractures are a life-threatening injury which affects 30,000 Canadians annually. Improved muscle and bone strength assessment methods may reduce fracture occurrence rates in the future. Magnetic resonance (MR) imaging has potential to assess proximal femoral bone strength in vivo through usage of finite element (FE) modeling. Though, to precisely assess bone strength, knowledge of a technique’s measurement error is needed. Hip muscle properties (e.g., lean muscle and fat area) are intrinsically linked to proximal femoral bone strength; however, it is unclear which muscles and properties are most closely associated with bone strength. This thesis is focused on MR-based FE modeling (MR-FE) of the proximal femur and surrounding muscle properties (e.g., hip abductor fat area, hip extensor muscle area). The specific objectives of this research were 1) to characterize the short-term in vivo measurement precision of MR-FE outcomes (e.g., failure load) of the proximal femur for configurations simulating fall and stance loading, and 2) explore associations between upper thigh muscle and fat properties (e.g., hip abductor fat area, knee extensor muscle area) with MR-FE failure loads of the proximal femur. In vivo precision errors (assessed via root mean square coefficient of variation, CV%RMS from repeated measures) of MR-FE outcomes ranged from 3.3-11.8% for stress and strain outcomes, and 6.0-9.5% for failure loads. Hip adductor muscle area and total muscle area correlated with failure load of the fracture-prone neck and intertrochanteric region under both fall and stance loading (correlation coefficients ranged from 0.416-0.671). This is the first study to report the in vivo short-term precision errors of MR-FE outcomes at the proximal femur. Also, this is the first study to relate upper-thigh muscle and fat properties with MR-FE derived failure loads. Results indicate that MR-FE outcomes have comparable precision to computed tomography (CT) based FE outcomes and are related to hip muscle area

Investigation Of Biomechanical Risk Factors Of Medial Tibial Stress Syndrome Through Finite Element Analysis

Medial tibial stress syndrome (MTSS) is an overuse injury in the lower extremity associated with endurance running. MTSS is a palpitation of pain of at least 5 centimeters along the medial tibia with possible microfractures in the tibia. The various risk factors which may lead to the development of MTSS are body mass index, over pronation, heel striking, level of shod in the running shoe, type and angle of running surface, high volume training, age, gender, stride length, range of motion, and calf girth. Few investigations have been made to limit these risk factors through the utilization of finite element analysis (FEA). This study investigates the likelihood of MTSS developing and the possibility of microfractures in the tibia under varying conditions of pronation degree, body mass index, material property, and gait phase. FEA was used in order to measure the von Mises stress of 24 human tibia models. The simulations were run for three main phases of gait “impactâ€, “mid-stanceâ€, and “push-offâ€. The risk factors under investigation were intrinsic in nature, which are over pronation (OP) and body mass index (BMI). Forces were input for 2 male subjects running at 8 miles per hour on a flat surface. Simulations were run for isotropic and orthotropic tibia models with “normal pronation and normal BMIâ€, “over pronation and normal BMIâ€, “normal pronation and high BMIâ€, and “over pronation and high BMIâ€. FEA revealed that the combination of over pronation and high BMI consistently had the greatest von Mises stresses throughout each phase of gait for isotropic and orthotropic tibia models. Statistical results show that material properties had the greatest effect on the measured von Mises stress followed by pronation degree, gait phase, and BMI. A normality test with a confidence interval of 95% proved that the distribution of von Mises stress across was acceptable for all models with P=0.130. Factorial ANOVA was run for gait phase, BMI, pronation degree, and material property, which also confirmed the greatest effects on von Mises stress are material property, pronation degree, gait phase, and then BMI

Bio-implantable microdevices and structures for functional electrical stimulation applications

This dissertation describes the development of microstructures and devices for applications in functional electrical stimulation. A nerve cuff electrode design has been developed for applications in neural electrical stimulation and recording, which addresses limitations with existing cuff electrodes. The developed clip-on micro-cuff electrode design consists of a naturally closed cuff with inner diameter in the micro-scale or above. A novel pinch-hinge feature allows a user to easily open the cuff and place it on target nerve tissue for stimulation or recording purposes. Upon release of the pinch-hinge, the cuff assumes its normally closed nature. The device conducts and reads electrical signals in the amplitude and frequency range of typical neural signals. A typical clip-on cuff device with 800 µm inner diameter is opened to its maximum extent by a relatively low force of less than 0.8 N, offering an alternative to other designs requiring application of a force for cuff closure. For applications involving gastric muscle stimulation, a novel gastric pacing electrode is fabricated in biocompatible silicone elastomer. In response to physiological temperature of about 37 ˚C, polyethylene glycol embedded inside the device body melts due to which the structure changes from a more rigid state initially to a more flexible state. This is expected to reduce tissue penetration during and after electrode implantation. A comprehensive piece-wise discrete element equivalent circuit model has been developed to represent an electrode-neural tissue interface. This model addresses internal aspects of both the tissue and the electrode surface and is an improvement over previous models. The equivalent circuit is employed in conjunction with electronic circuit simulation software to study the electrical response of an axon to external stimulus. Simulation results broadly correlate with practical observations reported by others. Lastly, a new percutaneous access device functioning as an interface between implants and the external world is reported here. The device made of silicone elastomer incorporates stress concentration features and shows promise for improved robustness and reliability. The device also incorporates micro-scale porous structures to allow for tissue in-growth to facilitate anchoring of the device

Exercise and Proximal Femur Bone Strength to Reduce Fall-Induced Hip Fracture

Bone mass and structure, constituting its strength, adapt to prevalent mechanical environment. Physical activity and exercise provide natural ways to apply the mechanical loading to bone. Finding effective osteogenic exercise types to improve proximal femur bone strength is of great importance to reduce hip fracture incidence and consequent substantial socioeconomic burden. Importantly, almost all hip fractures are caused by falls. Therefore, the primary objective of the present doctoral research was to find such effective exercise types by exploring the effect of long-term specific exercise loading on proximal femur bone strength in the fall situation using a finite element (FE) method. The secondary objective was to analyze 3D morphological adaptation of proximal femur cortical bone to the specific exercise loading. The results from this secondary objective were anticipated to help understanding the findings pertinent to the primary objective. To achieve these objectives, proximal femur MRI data were obtained from 91 young adult female athletes (aged 24.7 ± 6.1 years, > 8 years competing career) and 20 nonathletic but physically active controls (aged 23.7 ± 3.8 years). The athletes were classified into five distinct exercise loading groups based on the typical loading patterns of their sports: high-impact (H-I: triple- and high-jumpers), odd-impact (O-I: soccer/football and squash players), high-magnitude (H-M: powerlifters), repetitive-impact (R-I: endurance runners), and repetitive non-impact (R-NI: swimmers). Based on their MRI data, proximal femur FE models were first created in a single fall configuration (direction) to compare 1) cortical stresses in eight anatomical octants of femoral neck cross-sections in the proximal, middle, and distal femoral neck regions and 2) fracture behavior (load, location, and mode) between each exercise loading and control groups. The athletic bones are adapted to the long- term specific exercise loading characterized by not only the loading magnitude, rate, and frequency but also direction. Given this, the study was extended to simulate the FE models in multiple fall directions to examine whether potentially identified higher proximal femur bone strength to reduce fall-induced hip fracture risk, attributed to the long-term specific exercise loading, depends on the direction of the fall onto the greater trochanter or hip. For the secondary objective, a new computational anatomy method called Ricci-flow conformal mapping (RCM) was implemented to obtain 3D distribution of the cortical thickness within the proximal femur and to perform its spatial between-group statistical comparisons. Key results from the present research demonstrated that young adult females with the exercise loading history of high ground impacts (H-I), ground impacts from unusual/odd directions (O-I), or a great number of repetitive ground impacts (R-I) had 10-22%, 12-16%, and 14-23% lower fall-induced cortical stress at the fracture-prone superolateral femoral neck and 11-17%, 10-11%, and 22-28% higher fracture loads (higher proximal femur bone strength) in the fall situations compared to the controls, respectively. These results indicate that the long-term H-I, O-I, and R-I exercise loadings may reduce the fall-induced hip fracture risk. Furthermore, the present results showed that the higher proximal femur bone strength to reduce hip fracture risk in athletes engaged in the high-impact or repetitive-impact sports are robust and independent of the direction of fall. In contrast, the higher strength attributed to the odd-impact exercise loading appears more modest and specific to the fall direction. The analysis of the minimum fall strength spanning the multiple fall directions also supported the higher proximal femur bone strength in the athletes engaged in these impact exercises. In concordance with the literature, the present results also confirmed in these young adult females that 1) the fall-induced hip fracture most likely initiates from the superolateral femoral neck’s cortical bone, particularly at its posterior aspect (superoposterior cortex) in the distal femoral neck region, and 2) the most dangerous fracture-causing fall direction is the one where the impact is imposed to the posterolateral aspect of the greater trochanter. It would be ideal if impact exercise loading could induce beneficial cortical bone adaptation in the fracture-prone posterior aspect of superolateral femoral neck cortex. However, such apparently beneficial cortical adaptation was not observed in any of the impact or nonimpact exercise loading types examined in the present research based on the supplementary RCM-based 3D morphological analyses of proximal femur cortical bone. This analysis importantly showed that the higher proximal femur bone strengths to reduce fall-induced hip fracture risk in athletes engaged in the high- or odd-impact exercise types are likely due to thicker cortical layers in other femoral neck regions including the inferior, posterior, and/or superior-to-superoanterior regions. Interestingly, the higher proximal femur strength in the athletes with the repetitive-impact exercise loading was not supported by such cortical adaptation. This suggests that other structural/geometrical adaptation contributes to their higher strength. This calls for further studies to elucidate the source of the higher proximal femur bone strength in this type of athletes. In contrast to the impact exercise loading histories, the exercise loading history of the high-magnitude (e.g., powerlifting) or repetitive, non-impact (e.g., swimming) was not associated with higher proximal femur bone strength to reduce fall-induced hip fracture risk. This most likely reflects the lack of any beneficial structural adaptations of cortical bone around the femoral neck in the athletes with these exercise loading histories. Considering the loading characteristics of the exercise types examined in the present doctoral research, the moderate-to-high loading magnitude alone appears insufficient but needs to be generated at the high loading rate and/or frequency to induce the beneficial adaptation in the proximal femur cortical bone. Therefore, in addition to aforementioned three impact exercise loading types, other exercise or sport types satisfying this condition may also be effective to increase or maintain the proximal femur bone strength to reduce fall- induced hip fracture risk. As a clinical prospect, the present findings highlight the importance of impact exercise in combating fall-induced hip fracture. Compared to the high-impact loading exercises (e.g., triple/long and high jumping exercise), the odd-impact [ball or invasion games (e.g., football/soccer, tennis)] and/or repetitive-impact loading exercises (e.g., endurance running, jogging, and perhaps vigorous walking) likely provide a safer and more feasible choice for the populations covering the sedentary adults to old people. This is due to the relatively more moderate ground impact involved in the odd- and repetitive-impact loading exercises than in the high-impact exercises. For young, physically active, and/or fit people, the above-mentioned or similar jumping exercises and any other exercise types consisting of the high ground impact (e.g., volleyball, basketball, gymnastics) can also be incorporated into their habitual exercise routines. Lastly, the present results were observed in the young adult females who had engaged in sport-specific training from their childhood/adolescence to early adulthood. Therefore, this calls for the prospective and/or retrospective observational studies to investigate whether the higher proximal femur bone strength to reduce fall-induced hip fracture risk obtained from the long-term specific impact exercise loading during these early phases of life can sustain into the later stages, especially after age of 65 years when the hip fracture is generally more common

Kombineeritud metalloosteosüntees pikkade toruluude murdude raviks väikeloomadel

The primary objective of present research was to compare the efficiency of a novel bone fixator for treatment of tibial fractures in sheep and to compare to bone plating and to describe the surgical technique used in experimental animals. Rod-through-plate fixator (RTP fixator) combines intramedullary and extramedullary osteosynthesis. It is composed of a pair of curved intramedullary rods, a connecting extramedullary plate, two cortical screws and two fixation screws. Connecting plate is relatively short and curved rods lengthen the intramedullary shoulder of the fixator up towards the metaphyses. Screws are connecting rods to the plates and the fixator to the bone. Experimental osteotomies were performed in the middle third of the left tibia in all animals. Animals were divided into two groups: in one group (four animals) rod-through-plate fixator was applied, and in the other group (three animals) bone plating was used. The duration of the experiments was 10 weeks during which fracture union was followed by radiography. The healing process was assessed by blood serum markers reflecting bone turnover and by histological and immunohistochemical investigations. More radiographic evidence of callus formation was seen in the RTP fixator group. No statistically significant differences in histological, immunohistochemical or serum biomarker values were noted between the experimental groups We conclude that RTP fixation is efficient for the treatment of experimental osteotomies of long bones in sheep. The fixator was used also in six clinical cases for the diaphyseal middle third fracture treatment in dogs.Doktoritöös uuriti uudse kombineeritud fiksaatoriga fikseeritud luumurdude paranemist loomadel. Fiksaatori tõhususe hindamiseks teostati eksperimentaalne uuring katseloomadel ja võrreldi luumurru paranemist traditsioonilise plaatfiksatsiooniga. Katseloomadena kasutati lambaid. Samal aja töötati välja fiksaatori paigaldamiseks vajalikud standardsed võtted. Fiksaatorit kasutati ka kliinilistel juhtudel toruluumurdude raviks koertel. Luumurru paranemist katseloomadel hinnati röntgenoloogiliselt, histoloogiliselt ja immunohistokeemiliselt. Samuti teostati vere biokeemilised uuringud. Röntgenoloogilisel hindamisel täheldati intensiivsemat kalluse moodustumist uudse fiksaatori kasutamisel võrrelduna plaatfiksatsiooniga. Histoloogiliste, immunihistokeemiliste ja biokeemiliste näitajate osas eksperimendi käigus mõlema rühma vahel olulisi erinevusi ei esinenud. Kokkuvõtteks võib öelda, et uudne kombineeritud fiksaator on osutunud efektiivseks toruluude murdude raviks väikeloomadel

Time-dependent circumferential deformation of cortical bone subjected to internal radial loading

Human cortical bone is a complex composite material that displays time-dependent deformation. The response of bone to loads in the transverse orientation is not well understood and has implications to the stability of a press-fit hip implant. Total hip arthroplasty is a surgical procedure to replace the hip joint. It involves inserting an implant into the shaft of the femur. One method of implant fixation is a press-fit of the implant into the bone canal. Press-fit fixation relies on the elastic response of the proximal femur to hold the implant in place until new bone growth occurs. The radial load may cause the bone to experience creep, deformation due to a constant load, which can influence initial implant stability. The objectives of this study were to (a) study the time-dependent hoop response of femoral bone to an intramedullary radial load, (b) study the creep response of specimens under constant load until failure, and (c) assess damage morphology and determine the damage mechanisms in cortical bone due to a radial load.;A test fixture was used to apply internal pressure to cylindrical bone specimens. Hoop strain was measured on the surface of the cylindrical specimens. Creep strain, creep strain rate and permanent strain exhibited similar linear behavior at low stress, until a particular stress level, or threshold, where nonlinear behavior began. This deformation, which occurred at low hoop stress levels, may change the press-fit between the implant and bone, result in the loss of initial implant fixation, and may cause creep fracture. A relationship between hoop stress and time to failure was obtained and a hoop creep function was determined to model the creep strain behavior as a function of hoop stress and time. Fractured specimens were prepared for microscopic evaluation of damage morphology. Tensile hoop and compressive radial stresses resulted in extensive radial matrix microcracking combined with osteonal delaminations and may serve as a damage mechanism in cortical bone. The results from this study will provide fundamental knowledge regarding the creep behavior of bone in a similar loading environment to in-vivo press-fit loading

A novel musculoskeletal joint modelling for orthopaedic applications

This thesis was submitted for the degree of Docter of Philosophy and awarded by Brunel University.The objective of the work carried out in this thesis was to develop analytical and computational tools to model and investigate musculoskeletal human joints. It was recognised that the FEA was used by many researchers in modelling human musculoskeletal motion, loading and stresses. However the continuum mechanics played only a minor role in determining the articular joint motion, and its value was questionable. This is firstly due to the computational cost and secondly due to its impracticality for this application. On the other hand, there isn’t any suitable software for precise articular joint motion analysis to deal with the local joint stresses or non standard joints. The main requirement in orthopaedics field is to develop a modeller software (and its associated theories) to model anatomic joint as it is, without any simplification with respect to joint surface morphology and material properties of surrounding tissues. So that the proposed modeller can be used for evaluating and diagnosing different joint abnormalities but furthermore form the basis for performing implant insertion and analysis of the artificial joints. The work which is presented in this thesis is a new frame work and has been developed for human anatomic joint analysis which describes the joint in terms of its surface geometry and surrounding musculoskeletal tissues. In achieving such a framework several contributions were made to the 6DOF linear and nonlinear joint modelling, the mathematical definition of joint stiffness, tissue path finding and wrapping and the contact with collision analysis. In 6DOF linear joint modelling, the contribution is the development of joint stiffness and damping matrices. This modelling approach is suitable for the linear range of tissue stiffness and damping properties. This is the first of its kind and it gives a firm analytical basis for investigating joints with surrounding tissue and the cartilage. The 6DOF nonlinear joint modelling is a new scheme which is described for modelling the motion of multi bodies joined by non-linear stiffness and contact elements. The proposed method requires no matrix assembly for the stiffness and damping elements or mass elements. The novelty in the nonlinear modelling, relates to the overall algorithmic approach and handling local non-linearity by procedural means. The mathematical definition of joint stiffness is also a new proposal which is based on the mathematical definition of stiffness between two bodies. Based on the joint stiffness matrix properties, number of joint stiffness invariants was obtained analytically such as the centre of stiffness, the principal translational stiffnesses, and the principal rotational stiffnesses. In corresponding to these principal stiffnesses, their principal axes have been also obtained. Altogether, a joint is assessed by six principal axes and six principal stiffnesses and its centre of stiffness. These formulations are new and show that a joint can be described in terms of inherent stiffness properties. It is expected that these will be better in characterising a joint in comparison to laxity based characterisation. The development of tissue path finding and wrapping algorithms are also introduced as new approaches. The musculoskeletal tissue wrapping involves calculating the shortest distance between two points on a meshed surface. A new heuristic algorithm was proposed. The heuristic is based on minimising the accumulative divergence from the straight line between two points on the surface and the direction of travel on the surface (i.e. bone). In contact and collision based development, the novel algorithm has been proposed that detects possible colliding points on the motion trajectory by redefining the distance as a two dimensional measure along the velocity approach vector and perpendicular to this vector. The perpendicular distance determines if there are potentially colliding points, and the distance along the velocity determines how close they are. The closest pair among the potentially colliding points gives the “time to collision”. The algorithm can eliminate the “fly pass” situation where very close points may not collide because of the direction of their relative velocity. All these developed algorithms and modelling theories, have been encompassed in the developed prototype software in order to simulate the anatomic joint articulations through modelling formulations developed. The software platform provides a capability for analysing joints as 6DOF joints based on anatomic joint surfaces. The software is highly interactive and driven by well structured database, designed to be highly flexible for the future developments. Particularly, two case studies are carried out in this thesis in order to generate results relating to all the proposed elements of the study. The results obtained from the case studies show good agreement with previously published results or model based results obtained from Lifemod software, whenever comparison was possible. In some cases the comparison was not possible because there were no equivalent results; the results were supported by other indicators. The modelling based results were also supported by experiments performed in the Brunel Orthopaedic Research and Learning Centre

Explore the Dynamic Characteristics of Dental Structures: Modelling, Remodelling, Implantology and Optimisation

The properties of a structure can be both narrowly and broadly described. The mechanical properties, as a narrow sense of property, are those that are quantitative and can be directly measured through experiments. They can be used as a metric to compare the benefits of one material versus another. Examples include Young’s modulus, tensile strength, natural frequency, viscosity, etc. Those with a broader definition, can be hardly measured directly. This thesis aims to study the dynamic properties of dental complex through experiments, clinical trials and computational simulations, thereby bridging some gaps between the numerical study and clinical application. The natural frequency and mode shapes, of human maxilla model with different levels of integrities and properties of the periodontal ligament (PDL), are obtained through the complex modal analysis. It is shown that the comprehensiveness of a computational model significantly affects the characterisation of dynamic behaviours, with decreasing natural frequencies and changed mode shapes as a result of the models with higher extents of integrity and preciseness. It is also found that the PDL plays a very important role in quantifying natural frequencies. Meanwhile, damping properties and the heterogeneity of materials also have an influence on the dynamic properties of dental structures. The understanding of dynamic properties enables to further investigate how it can influence the response when applying an external stimulus. In a parallel preliminary clinical trial, 13 patients requiring bilateral maxillary premolar extractions were recruited and applied with mechanical vibrations of approximately 20 g and 50 Hz, using a split mouth design. It is found that both the space closure and canine distalisation of the vibration group are significantly faster and higher than those of the control group (p<0.05). The pressure within the PDL is computationally calculated to be higher with the vibration group for maxillary teeth for both linguo-buccal and mesial-distal directions. A further increased PDL response can be observed if increasing the frequency until reaching a local natural frequency. The vibration of 50 Hz or higher is thus approved to be a potential stimulus accelerating orthodontic treatment. The pivotal role of soft tissue the PDL is further studied by quantitatively establishing pressure thresholds regulating orthodontic tooth movement (OTM). The centre of resistance and moment to force ratio are also examined via simulation. Distally-directed tipping and translational forces, ranging from 7.5 g to 300 g, are exerted onto maxillary teeth. The hydrostatic stress is quantified from nonlinear finite element analysis (FEA) and compared with normal capillary and systolic blood pressure for driving the tissue remodelling. Localised and volume-averaged hydrostatic stress are introduced to describe OTM. By comparing with clinical results in past literature, the volume average of hydrostatic stress in PDL was proved to describe the process of OTM more indicatively. Global measurement of hydrostatic pressure in the PDL better characterised OTM, implying that OTM occurs only when the majority of PDL volume is critically stressed. The FEA results provide new insights into relevant orthodontic biomechanics and help establish optimal orthodontic force for a specific patient. Implant-supported fixed partial denture (FPD) with cantilever extension can transfer excessive load to the bone surrounding implants and stress/strain concentration which potentially leads to bone resorption. The immediate biomechanical response and long-term bone remodelling outcomes are examined. It is indicated that during the chewing cycles, the regions near implant necks and apexes experience high von Mises stress (VMS) and equivalent strain (EQS) than the middle regions in all configurations, with or without the cantilever. The patient-specific dynamic loading data and CT based mandibular model allow us to model the biomechanical responses more realistically. The results provide the data for clinical assessment of implant configuration to improve longevity and reliability of the implant-supported FPD restoration. On the other hand, the results show that the three-implant supported and distally cantilevered FPDs see noticeable and continuous bone apposition, mainly adjacent to the cervical and apical regions. The bridged and mesially cantilevered FPDs show bone resorption or no visible bone formation in some areas. Caution should be taken when selecting the FPD with cantilever due to the risk of overloading bone resorption. The position of FPD pontics plays a critical role in mechanobiological functionality and bone remodelling. As an important loading condition of dental biomechanics, the accurate assignment of masticatory loads has long been demanded. Methods involving different principles have been applied to acquire or assess the muscular co-activation during normal or unhealthy stomatognathic functioning. Their accuracy and capability of direct quantification, especially when using alone, are however questioned. We establish a clinically validated Sequential Kriging Optimisation (SKO) model, coupled with the FEM and in vivo occlusal records, to further the understanding of muscular functionality following a fibula free flap (FFF) surgery. The results, within the limitations of the current study, indicates the statistical advantage of agreeing occlusal measurements and hence the reliability of using the SKO model over the traditionally adopted optimality criteria. It is therefore speculated that mastication is not optimally controlled to a definite degree. It is also found that the maximum muscular capacity slightly decreases whereas the actual muscle forces fluctuate over the 28-month period