| Bones have composite hierarchical structures to achieve diverse mechanical,biological and chemical functions,such as support and protection,transport,storage cells and mineral ion homeostasis.When the physiological load passing from macro to micro,the functional units of different scales show different properties,such as rigidity,strength,permeability,porosity,toughness,and flexibility,and they can keep close communication and coordination with each other to achieve unified macroscale forms and functions.The stress-strain distribution and fluid flow in bone play an important role in the effective realization of various functions of bone.Fluid flow in osteons can produce a series of effect,such as fluid shear stress,pore pressure gradient,solute transport and streaming potential,and some of these effects can be sensed by osteocytes as signals to trigger bone formation and bone resorption to adapt the continuous change of the mechanical environment.Bone microstructure and properties of materials can influence its macroscopic mechanical properties,and bone tissue constantly go through the processes of bone formation and bone resorption to adapt to the external environment.The complex microstructure of bone tissue is regulated by mechanical stimulation,and the bone will produce deformation which induced fluid flow in bone under physiological loading,and osteocytes are sensitive to fluid flow and its induced effects.It is significant to research the stress-strain field and the behaviors of fluid flow in different scales of bone under physiological load.To investigate the mechanotransduction and fluid behaviors in loaded bone,a multiscale method for the hierarchical structure of bone tissue was developed.Based on poroelastic theory,we established the theoretical and FE model of a segment bone tissue to provide basis for researching more complex bone model.The COMSOL Multiphysics software was used to establish different scales of bone models,and the properties of mechanical and fluid behaviors of different function unit in each scale were investigated.The main work and conclusion in this thesis as follows:(?)Macroscale model neglected the bone marrow cavity and trabeculae,only considers the tissue from the endosteum to periosteum,and the mechanical property and the flowing discipline of various functional units was observed.In order to provide the basis for researching the stress-strain field and fluid flow in various functional units by poroelastic FE method,the validity of poroelastic FE model was validated by comparing numerical result with simulation results.Analytic solutions of macroscale correlated very well with the FE results.Under cyclic physiological loads,the fluid pressure and flow velocity at macroscale level show periodic changes.At the macro level,we only considered the vascular porosity.When the value of the vascular poroscale and the lacunar-canalicular porescale were close,the pore pressure in macroscale will have a greater influence on the pore pressure of macro-meso scale.The previous studies almost considered at a small size level,however,most of the living bones are as a whole structure to bear external loads,such as walking,running and jumping.It is necessary to study the fluid flow behavior of macro whole bone structure in the process of dynamic loading for understanding the mechanism of fluid stimulation signals affecting the growth of osteocyte.It is also the physiological basis of bone therapy and bone reconstruction.(?)The macro-meso model considered the endosteum,osteon,interstitial bone and periosteum.The Haversian canal in osteon and the marrow cavity in bone were neglected.Under the periodic load,the fluid pressure(FP),flow velocity(FV),the von Mises stress(VMS)and the maximum principal strain(MPS)of bone tissue change periodically.This result is instrumental in the absorption and metabolism of the nutrients.The VMS in interstitial bone was significantly higher than that in the osteon.The large VMS in interstitial bone maked it easy to produce crack and fatigue damage,and the properties of high VMS and crack-prone of interstitial bone could provide a protection to the osteon,which helped to protect the osteocyte and maintain normal metabolic activity in the bone.This result was reasonable in the functions of maintain structural integrity and resistance to fatigue damage in the process of biology evolution.The maximum FV in interstitial bone and outer wall of osteon were both less than 2×10-8m/s,which means that the fluid stimulation generated by fluid flow in these locations were too small to cause the response of osteocyte's mechanoreceptors and was called"dead zone".This may be associated with the less osteocyte in interstitial bone and the outer wall of osteon.(?)The study of mesoscale model mainly divided into two aspects:osteon morphology and bone lamellae with different biological functional units.(1)Osteon morphology.The alterations of osteon morphology were caused by many factors,such as age,osteoporosis,and physical activity level.It is known that fluid flow in osteon play an important role in osteocyte mechanotransduction,but less is known about these alterations of osteon morphology affect the biomechanical properties and solute transport.The shape,cross-sectional curvature,cross-sectional area and wall thickness of osteon were observed under physiological load,and all these factors had great influence on the fluid flow in osteon.(2)Bone lamellae.Because of the difference of the mineral content and the arrangement direction of the collagen fiber,the material parameters on each osteon lamella were different.In this part,the osteon lamella,the interstitial bone and the cement line were refined,and the effect of the elastic modulus(E)and permeability(klcp)on its poroelasticity behaviors was investigated.Due to the different scales bewteen Macro-meso scale model and mesoscale model,so the submodel method was adopted.The results of the mesoscale structure in whole macro-meso scale model was compared with the mesoscale model which cut from whole macro-meso scale model to validate the mesoscale model.The E had significantly effect on the VMP and MPS,and had slight effect on FP and FV.The permeability had significantly effect on the FP and FV of osteon lamellae,but had little impact on the VMS and MPS.This part can be used to analyze the effect of bone scaffold,bone substitute or implants on the stress and fluid flow distributions of bone tissue to more accurately assess the potential beneficial and harmful effects,so as to accurately achieve better individual matching.It can also incorporate alternative material parameters obtained from different individuals.The suggested method is expected to provide dependable biological information for better understanding the bone mechanotransduction and signal transduction(?)At the microscale,(1)Firstly,the number and distribution of the bone canaliculi around the lacunae were calculated.Secondly,the permeability and porosity were estimated by using the calculated parameters and other microstructure data of bone tissue.Finally,the poroelastic finite element model of osteon was established according to the calculated parameters,and the influence of lacunae shape and direction on the fluid flow behavior in osteon under the axial displacement load was analyzed.The results showed that the lacunae shape and direction had a significant effect on the value and distribution of fluid pressure and velocity in osteon.For the range of parameters investigated,the influence of the lacunae shape on the maximum pressure and flow velocity in the same region of different osteon models can reach 86%and 18%,respectively,and the influence of the lacunae direction on that can reach 125%and 56%,respectively.In addition,the lacunae shape and direction had a great influence on the local pressure and velocity of a single osteon(up to 62%difference in fluid pressure between regions due to the influence of the lacunae shape,and up to 58%and 50%difference in fluid pressure and flow velocity due to the influence of the lacunae direction,respectively.(2)The bone lacunae and bone canalicules were separated according to the distinct structural parameters.Firstly,the sensitivity of pore pressure,fluid velocity and fluid shear stress to permeability of bone lacunae and bone canalicules was analyzed.Secondly,the pore pressure,fluid velocity and fluid shear stress along the radius direction of bone tissue and osteon were analyzed.Pore pressure and fliud shear stress in lacunae and canaliculus were highly sensitive to permeability.The pore pressure,fliud velocity and fliud shear stress at contact area of bone lacuna and bone canalicules fluctuates obviously,and the maximum occurred between the first lacuna and canaliculi near the Haversian canal.This is because the difference of pore scales of the lacuna and bone canalicules.The fuild velocity in the lacuna was significantly higher than that in the canaliculi.This study will help to provide a deeper understanding of the spatial characteristics of the bone lacuna-osteocyte-canalicules(including density and morphology)and their potential relationship with disease and aging. |
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