| Bone is an important part of human movement system, and its forming process isaffected by the mechanical and biological environments, which results in a complexhierarchical structure. With the aging of population, osteoporosis has become aserious problem in the elderly, while the female postmenopausal osteoporosis and therelated fracture have become the main threat of health, and it not only casts a gloomover the life, but also increases the medical cost of the country. In this study,age-related variations of macro-micro-nano multi-level femur bone qualities and theprocess of osteoporosis were investigated, which provided a theoretical basis for theclinical prevention of osteoporosis. In addition, pharmacologic, exercise and nutritiontreatments are the most popular regime in preventing osteoporosis, and non-invasiveand non-pharmacologic therapy is a focus of the current research on osteoporosistreatment. High-frequency, low-magnitude whole body vibration has been proven tobe a method with good application prospect for improving osteoporosis, and thecorrelative studies have made some progress. This study aimed to explore the effectsof high-frequency, low-magnitude mechanical vibration on osteoporosis caused bymechanical disuse at macro-micro-nano multi-levels when the long-durationmechanical stimulus was separated by several rest days. Combined with theexperimental study, in the present investigation the proximal femur fracture locationand type were predicted precisely with CT based nonlinear dynamic finite element(FE) method, which provided a theoretical basis for diagnosing and preventingosteoporosis in clinics. This study was composed of three sections:In section one, a macro-micro-nano multi-level study was conducted to explorethe age-related structural and mechanical properties of bone, as well as the effects ofaging on bone properties. A total of70male Wistar rats were used, ranging in the agesof1,3,5,7,9,11,14,15,16, and17months (n=7/age group). After micro-computedtomography (CT) scanning, longitudinal cortical bone specimens with a length of 5mm were cut along the femoral shaft axis from left femur shafts for mechanicaltesting, and the cross-sectional areas were measured. Scanning electron microscopy(SEM) was used for detecting the microarchitecture features of the fractured surfaces.The mineral phase of the left femoral shaft and head was analyzed by atomic forcemicroscopy (AFM). Longitudinal and transverse trabecular bone tissues, as well aslongitudinal cortical bone tissue, were used for nanoindentation test, and thequantitative chemical composition was evaluated by inductively coupled plasmaoptical emission spectroscopy (ICP-OES). The results showed that themacro-mechanical properties obtained in mechanical testing and microarchitectureparameters measured by micro-CT scanning were significantly correlated with theanimal age (r2=0.96, p<0.05), and varied in some statistical rules. Themicroarchitecture features of the fractured surfaces detected by SEM exhibitedage-related plate-fibrous-mixed fibrous-plate texture, resulting in changes inmacro-mechanical properties (r2>0.9, p<0.05). The correlations between mineralcontent and bone material properties (i.e., elastic properties of the bone tissue and sizeand roughness of bone mineral grains) were highly significant (r2>0.9, p<0.05).Macro-micro-nano multi-level femur morphology, mechanical property, and mineralcontent were significantly correlated with the animal age. The correlations betweenbone mineral content and bone material morphological and mechanical propertiespartly explained the increase in bone fragility with aging, which provided a theoreticalbasis for the investigation of age-related bone properties in clinics.In section two, the effects of low-magnitude, high-frequency whole bodyvibration with rest days on bone morphological and mechanical properties ofosteoporosis caused by mechanical disuse were investigated. Forty-ninethree-month-old male Wistar rats were randomly divided into seven groups, namely,vibrational loading for X day followed by X day rest (VLXR, X=1,3,5,7),vibrational loading every day (VLNR), tail suspension (SPD), and baseline control(BCL). The vibrational loading condition was35Hz,0.25g, and15min/day. Eightweeks later, femora were harvested to investigate macro-mechanical properties aftermicro-CT scanning was done, and fluorescence test were used to calculate the mineralapposition rate and bone formation rate. AFM analyses and nanoindentation test were used to analyze the nanostructure and mechanical properties of bone material,respectively. ICP-OES was used for quantitative chemical analyses. The Resultsshowed that bone mineral reorganization and spatial arrangement were modified byhigh-frequency, low-magnitude whole body vibration with different rest days, whichresulted in the recombination of mineral grains into different sizes and changes inmineral apposition rate and bone formation rate. The microarchitecture of bone wasaffected, which resulted in statistical differences in macro-mechanical properties.However, the mechanical properties of the bone material were not altered.High-frequency, low-magnitude whole body vibration with7d rest was moreefficacious in improving macro-biomechanical properties and microarchitecture thandaily loading, which provided a theoretical basis for improving bone quality using thismechanical intervention in clinics.In section three, how different hip fracture types and locations were affected byimpact directions was investigated. The geometry of a proximal femur was obtainedfrom the CT scan data of a67-year-old Chinese male. Mimics and ANSYS softwareswere applied to establish the cortical bone and cancellous bone models. Six differentloading cases, i.e., SW1(α=120°, β=0°), SW2(α=90°, β=0°), SW3(α=60°, β=0°),SW4(α=20°, β=0°), SW5(α=120°, β=15°), SW6(α=120°, β=45°) were defined bythe angle α with reference to the long axis of the femur in the frontal plane, and theangle β with reference to the femoral neck axis in the horizontal plane. They wereestablished to simulate sideways falls by explicit dynamic nonlinear FE analyses inANSYS-LS-DYNA software. The impact speed was3.17m/s. Stress and strainanalyses with time history of the fracture sites were taken to find the relationshipsbetween fracture types and impact directions. SW1–SW4caused femoral neckfractures, and the maximum principal stresses were4.5MPa,6MPa,5MPa and4.8MPa (compressive stress), respectively. SW5caused compound fractures includingneck fracture and trochanteric fracture, and the maximum principal stresses were6.8MPa (compressive stress) and6.5MPa (compressive stress), respectively. SW6caused trochanteric fracture, and the maximum principal stress was4.2MPa(compressive stress). The maximum principal strains of neck fracture (SW1:0.075,SW2:0.135, SW3:0.175, SW4:0.092)(compressive strain) and trochanteric fracture (SW6:0.045)(compressive strain) increased to the maximum values in10ms afterimpact, and were much higher than the ultimate compressive strain of cancellous bone.This study showed that falling posture was an important factor leading to differenttypes of fracture, and the results were consistent with the clinical findings. Dynamicnonlinear simulation was more effective in explaining the fracture mechanism anddetermining the load direction that caused hip fracture. The study also provided atheoretical basis for more targeted preventive measures for different types of hipfracture in clinics.In this thesis, age-related rats femur morphology, mechanical properties andmineral content were investigated in this study at the macro-micro-nano multi-levels.Osteoporosis model caused by long-term limb mechanical disuse was established, andthe effects of whole body vibration with rest days on multi-level femur morphology,mechanical property and mineral content were investigated. Finally, dynamicnonlinear FE method was used to clarify how hip fracture location and type wereaffected by the loading directions. This study provided a theoretical basis for theinvestigation of age-related bone properties, osteoporotic prevention and hip fractureprediction in clinics. |
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