Stiffening of the femoral head due to inter-trabecular fluid and intraosseous pressure

The role of bone marrow on the mechanical properties of trabecular bone: a systematic review

Background

Accurate evaluation of the mechanical properties of trabecular bone is important, in which the internal bone marrow plays an important role. The aim of this systematic review is to investigate the roles of bone marrow on the mechanical properties of trabecular bone to better support clinical work and laboratory research.

Methods

A systematic review of the literature published up to June 2022 regarding the role of bone marrow on the mechanical properties of trabecular bone was performed, using PubMed and Web of Science databases. The journal language was limited to English. A total of 431 articles were selected from PubMed (n = 186), Web of Science (n = 244) databases, and other sources (n = 1).

Results

After checking, 38 articles were finally included in this study. Among them, 27 articles discussed the subject regarding the hydraulic stiffening of trabecular bone due to the presence of bone marrow. Nine of them investigated the effects of bone marrow on compression tests with different settings, i.e., in vitro experiments under unconfined and confined conditions, and computer model simulations. Relatively few controlled studies reported the influence of bone marrow on the shear properties of trabecular bone.

Conclusion

Bone marrow plays a non-neglectable role in the mechanical properties of trabecular bone, its contribution varies depending on the different loading types and test settings. To obtain the mechanical properties of trabecular bone comprehensively and accurately, the solid matrix (trabeculae) and fluid-like component (bone marrow) should be considered in parallel rather than tested separately.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12938-022-01051-1.

Effects of mechanobiological signaling in bone marrow on skeletal health

Bone marrow is a cellular tissue that forms within the pore space and hollow diaphysis of bones. As a tissue, its primary function is to support the hematopoietic progenitor cells that maintain the populations of both erythroid and myeloid lineage cells in the bone marrow, making it an essential element of normal mammalian physiology. However, bone's primary function is load bearing, and deformations induced by external forces are transmitted to the encapsulated marrow. Understanding the effects of these mechanical inputs on marrow function and adaptation requires knowledge of the material behavior of the marrow at multiple scales, the loads that are applied, and the mechanobiology of the cells. This paper reviews the current state of knowledge of each of these factors. Characterization of the marrow mechanical environment and its role in skeletal health and other marrow functions remains incomplete, but research on the topic is increasing, driven by interest in skeletal adaptation and the mechanobiology of cancer metastasis.

Physiological cyclic hydrostatic pressure induces osteogenic lineage commitment of human bone marrow stem cells: a systematic study

Background

Physical loading is necessary to maintain bone tissue integrity. Loading-induced fluid shear is recognised as one of the most potent bone micromechanical cues and has been shown to direct stem cell osteogenesis. However, the effect of pressure transients, which drive fluid flow, on human bone marrow stem cell (hBMSC) osteogenesis is undetermined. Therefore, the objective of the study is to employ a systematic analysis of cyclic hydrostatic pressure (CHP) parameters predicted to occur in vivo on early hBMSC osteogenic responses and late-stage osteogenic lineage commitment.

Methods

hBMSC were exposed to CHP of 10 kPa, 100 kPa and 300 kPa magnitudes at frequencies of 0.5 Hz, 1 Hz and 2 Hz for 1 h, 2 h and 4 h of stimulation, and the effect on early osteogenic gene expression of COX2, RUNX2 and OPN was determined. Moreover, to decipher whether CHP can induce stem cell lineage commitment, hBMSCs were stimulated for 4 days for 2 h/day using 10 kPa, 100 kPa and 300 kPa pressures at 2 Hz frequency and cultured statically for an additional 1–2 weeks. Pressure-induced osteogenesis was quantified based on ATP release, collagen synthesis and mineral deposition.

Results

CHP elicited a positive, but variable, early osteogenic response in hBMSCs in a magnitude- and frequency-dependent manner, that is gene specific. COX2 expression elicited magnitude-dependent effects which were not present for RUNX2 or OPN mRNA expression. However, the most robust pro-osteogenic response was found at the highest magnitude (300 kPa) and frequency regimes (2 Hz). Interestingly, long-term mechanical stimulation utilising 2 Hz frequency elicited a magnitude-dependent release of ATP; however, all magnitudes promoted similar levels of collagen synthesis and significant mineral deposition, demonstrating that lineage commitment is magnitude independent. This therefore demonstrates that physiological levels of pressures, as low as 10 kPa, within the bone can drive hBMSC osteogenic lineage commitment.

Conclusion

Overall, these findings demonstrate an important role for cyclic hydrostatic pressure in hBMSCs and bone mechanobiology, which should be considered when studying pressure-driven fluid shear effects in hBMSCs mechanobiology. Moreover, these findings may have clinical implication in terms of bioreactor-based bone tissue engineering strategies.

Electronic supplementary material

The online version of this article (10.1186/s13287-018-1025-8) contains supplementary material, which is available to authorized users.

Contribution of fluid in bone extravascular matrix to strain-rate dependent stiffening of bone tissue - A poroelastic study

Osteoporotic fractures represent an increasing cost to society, and its diagnosis methods based on bone density still lack accuracy in identifying risk of fracture. This is why a better understanding of mechanical behavior of bone tissue is of importance, especially when it comes to relating experimental observations to realistic physiological fall loading conditions. This study aims at exploring the stiffening effect of pore fluid in bone extravascular matrix subject to high strain rate loading that is more realistic to simulate a physiological fall. A computational approach is used, where bone tissue microstructure extracted from micro-CT images is modeled using finite elements. The solid phase of bone tissue is modeled as a poroelastic material, a porous matrix filled with fluid. When the extravascular matrix experiences certain volumetric deformation, the fluid in pores presents load carrying capacity, which consequently varies the apparent stiffness of bone tissue. It is shown that effects of fluid stiffening in bone can be significant, depending on the chosen material properties, the amount of volumetric strain in tissue and the loading rate with respect to hydraulic conductivity and drainage conditions. It is also shown that such stiffening effect is influenced by bone microstructure, and is more significant in cortical bone than in trabecular bone.

A simulation study on marrow fat effect on biomechanics of vertebra bone

Trabecular bone and bone marrow are main components of cancellous bone. Most mechanical studies for bone mainly focus on hard tissues, while if bone marrow contributes to bone biomechanics is not clear yet. This study was proposed to investigate marrow fat effect on trabecular bone biomechanics by simulation. Finite element (FE) bone models were established based on quantitative CT images at L3 lumbar spine, from which trabecular structures with and without marrow fat were investigated respectively. Auni-static compressive test was applied on the proposed models until to the appearance of fracture. Simulation results showed that trabecular models filled with marrow fat had about 3%-9% less maximum stress in volume than models with only trabeculae. However, its average stress in volume was about 9%-56% larger than those with only trabeculae. The strain energy density of the bone model with marrow fat showed a more uniformed distribution. As a conclusion, marrow fat has contributions to the bone mechanics. It can balance the stress distribution of the bone tissue, which may reduce bone deformation under a compressive loading. The mixture of trabecular structure and marrow fat would be against higher compress load before the failure point.

A simulation study of marrow fat effect on bone biomechanics

Bone marrow was assumed to be negligible on the aspect of bone mechanical behavior, where bone mass and bone mineral density were most studied. As a result, if the bone marrow, especially the marrow fat, plays a role in the bone mechanical properties is unknown yet. Marrow fat content was found increased in osteoporotic bone. However, the relationship between such change of bone marrow and bone strength is not clear yet. This study was proposed to investigate the effect of marrow fat on the bone biomechanical performance by computer simulations. A finite element model was established based on trabecular structure extracted from quantitative CT at L3 vertebrae. Simulations were conducted on the models with and without marrow fat under the same condition, respectively. The results showed that the cancellous bone with marrow fat had a 7.56%~18.81% higher maximum stress in trabeculae. Further, trabeculae with higher Young's modulus tend to sustain a higher maximum compressive stress when considering the marrow fat. As a conclusion, the marrow fat has effect on bone biomechanics, which cannot be ignored. Such effect in osteoporosis should be further investigated in deep.

The in situ mechanics of trabecular bone marrow: the potential for mechanobiological response

Bone adapts to habitual loading through mechanobiological signaling. Osteocytes are the primary mechanical sensors in bone, upregulating osteogenic factors and downregulating osteoinhibitors, and recruiting osteoclasts to resorb bone in response to microdamage accumulation. However, most of the cell populations of the bone marrow niche,which are intimately involved with bone remodeling as the source of bone osteoblast and osteoclast progenitors, are also mechanosensitive. We hypothesized that the deformation of trabecular bone would impart mechanical stress within the entrapped bone marrow consistent with mechanostimulation of the constituent cells. Detailed fluid-structure interaction models of porcine femoral trabecular bone and bone marrow were created using tetrahedral finite element meshes. The marrow was allowed to flow freely within the bone pores, while the bone was compressed to 2000 or 3000 microstrain at the apparent level.Marrow properties were parametrically varied from a constant 400 mPas to a power law rule exceeding 85 Pas. Deformation generated almost no shear stress or pressure in the marrow for the low viscosity fluid, but exceeded 5 Pa when the higher viscosity models were used. The shear stress was higher when the strain rate increased and in higher volume fraction bone. The results demonstrate that cells within the trabecular bone marrow could be mechanically stimulated by bone deformation, depending on deformation rate, bone porosity, and bone marrow properties. Since the marrow contains many mechanosensitive cells, changes in the stimulatory levels may explain the alterations in bone marrow morphology with aging and disease, which may in turn affect the trabecular bone mechanobiology and adaptation.

Pressure and shear stress in trabecular bone marrow during whole bone loading

Skeletal adaptation to mechanical loading is controlled by mechanobiological signaling. Osteocytes are highly responsive to applied strains, and are the key mechanosensory cells in bone. However, many cells residing in the marrow also respond to mechanical cues such as hydrostatic pressure and shear stress, and hence could play a role in skeletal adaptation. Trabecular bone encapsulates marrow, forming a poroelastic solid. According to the mechanical theory, deformation of the pores induces motion in the fluid-like marrow, resulting in pressure and velocity gradients. The latter results in shear stress acting between the components of the marrow. To characterize the mechanical environment of trabecular bone marrow in situ, pore pressure within the trabecular compartment of whole porcine femurs was measured with miniature pressure transducers during stress-relaxation and cyclic loading. Pressure gradients ranging from 0.013 to 0.46 kPa/mm were measured during loading. This range was consistent with calculated pressure gradients from continuum scale poroelastic models with the same permeability. Micro-scale computational fluid dynamics models created from computed tomography images were used to calculate the micromechanical stress in the marrow using the measured pressure differentials as boundary conditions. The volume averaged shear stress in the marrow ranged from 1.67 to 24.55 Pa during cyclic loading, which exceeds the mechanostimulatory threshold for mesenchymal lineage cells. Thus, the loading of bone through activities of daily living may be an essential component of bone marrow health and mechanobiology. Additional studies of cell-level interactions during loading in healthy and disease conditions will provide further incite into marrow mechanobiology.

Subchondral cysts create increased intra-osseous stress in early knee OA: A finite element analysis using simulated lesions

AIM OF STUDY

To investigate the role of intra-osseous lesions in advancing the pathogenesis of Osteoarthritis (OA) of the knee, using Finite Element Modeling (FEM) in conjunction with high-resolution imaging techniques.

METHODS

Twenty early stage OA patients (≤ Grade 2 radiographic score) were scanned with a prototype, cone-beam CT system. Scans encompassed the mid-shaft of the femur to the diaphysis of the proximal tibia. Individual bones were segmented to create 3D geometric models that were transferred to FE software for loading experiments. Patient-specific, inhomogeneous material properties were derived from the CT images and mapped directly to the FE models. Duplicate models were also created, with a 3D sphere (range 3-12 mm) introduced into a weight-bearing region of the joint, mimicking the size, location, and composition of a subchondral bone cyst (SBC). A spherical shell extending 1mm radially around the SBC served as the sample volume for measurements of von Mises equivalent stress. Both models were vertically loaded with 750 N, or approximately 1 body weight during a single-leg stance.

RESULTS

All FE models exhibited a physiologically realistic weight-bearing distribution of stress, which initiated at the joint surface and extended to the cortical bone. Models that contained the SBC experienced a nearly two-fold increase in stress (0.934 ± 0.073 and 1.69 ± 0.159 MPa, for the non-SBC and SBC models, respectively) within the bone adjacent to the SBC. In addition, there was a positive correlation found between the diameter of the SBC and the resultant intra-osseous stress under load (p = 0.004).

CONCLUSIONS

Our results provide insights into the mechanism by which SBC may accelerate OA, leading to greater pain and disability. Based on these findings, we feel that patient-derived FE models of the OA knee - utilizing in vivo imaging data - present a tremendous potential for monitoring joint mechanics under physiological loads.

The mechanical environment of bone marrow: a review

Bone marrow is a viscous tissue that resides in the confines of bones and houses the vitally important pluripotent stem cells. Due to its confinement by bones, the marrow has a unique mechanical environment which has been shown to be affected from external factors, such as physiological activity and disuse. The mechanical environment of bone marrow can be defined by determining hydrostatic pressure, fluid flow induced shear stress, and viscosity. The hydrostatic pressure values of bone marrow reported in the literature vary in the range of 10.7-120 mmHg for mammals, which is generally accepted to be around one fourth of the systemic blood pressure. Viscosity values of bone marrow have been reported to be between 37.5 and 400 cP for mammals, which is dependent on the marrow composition and temperature. Marrow's mechanical and compositional properties have been implicated to be changing during common bone diseases, aging or disuse. In vitro experiments have demonstrated that the resident mesenchymal stem and progenitor cells in adult marrow are responsive to hydrostatic pressure, fluid shear or to local compositional factors such as medium viscosity. Therefore, the changes in the mechanical and compositional microenvironment of marrow may affect the fate of resident stem cells in vivo as well, which in turn may alter the homeostasis of bone. The aim of this review is to highlight the marrow tissue within the context of its mechanical environment during normal physiology and underline perturbations during disease.

Intraosseous pressure and strain generated potential of cylindrical bone samples in the drained uniaxial condition for various loading rates

Cortical bone is a composite material consisting of a porous elastic solid and viscous fluid. It is well known that the intraosseous fluid circulates as a result of a bone fluid pressure gradient in the porous space of the cortical bone. When a time-dependent mechanical load is applied to the bone, intraosseous fluid flow occurs through the interconnected pore space in the bone. Bone fluid flow leads to a strain generated streaming potential (SGP). However, there is no experimental study on the relationship between the generation of intraosseous pressure and the SGP. The purpose of this study was to obtain the relationship between SGP and intraosseous pressure generations in cortical bone. In order to understand the issue, a drained, one-dimensional experimental setup for fluid-filled cortical bone samples with four different strain rates was used to simultaneously measure the intraosseous pressure and SGP. The results revealed a significant correlation (r = 0.98, p = 0.02) between the generation of the SGP and the intraosseous pressure, which indicates that an intraosseous pressure gradient produces a SGP in cortical bone.

Rate dependence of hydraulic resistance in human lumbar vertebral bodies

STUDY DESIGN

Twenty-one intact human lumbar vertebral bodies (L3 and L4) were used to determine the changes in measured intraosseous pressure for 2 volumetric flow rates and to calculate hydraulic resistance in both cases.

OBJECTIVE

To evaluate changes in hydraulic resistance in intact vertebral bodies under different rates of flow.

SUMMARY OF BACKGROUND DATA

Hydraulic resistance has been implicated in the creation of high-speed vertebral injuries, such as burst fracture, but no previous study has measured hydraulic resistance under high-speed loading conditions. Previous work in whole bone preparations showed that hydraulic resistance was constant under low-speed conditions. The authors hypothesized that: (1) measured pressure would increase with increasing input flow rates, and (2) hydraulic resistance would remain constant at increased input flow rates.

METHODS

Using 2 input velocity conditions (10 mm/s and 2500 mm/s), resultant pressures were measured and hydraulic resistance calculated. Trabecular architecture was determined using micro-computerized tomography after testing.

RESULTS

Measured pressure increased with increasing input flow rates. However, average hydraulic resistance decreased significantly when comparing low-speed (3.40 +/- 1.58kPa*s/mL) and high-speed (0.16 +/- 0.08kPa*s/mL) groups.

CONCLUSIONS

Current hydraulic resistance results contradict previous findings. The observed decrease in hydraulic resistance suggests that, during high-speed injury events, marrow flow may damage the trabeculae and thereby weaken the vertebra.

Hydraulic strengthening affects the stiffness and strength of cortical bone

A nonlinear, interstitial fluid flow constitutive model for cortical bone was developed to study the strain-rate dependency of cortical bone apparent modulus (Ea). Nine representative volume element (RVE) structural models of cortical bone spanning an effective pore volume fraction P range of 1-40% were examined. Dynamic loading conditions were used to study the fluid flow contribution or hydraulic strengthening (HS) effect on Ea for each RVE model. The model indicated that there is an upper and lower asymptotic bound of strain-rate (10(+/-3) sec(-1)) above or below which there are no further HS effects on Ea. At certain strain-rates (10(-1) to 10(0) sec(-1)) variations in cortical bone porosity had little or no influence on Ea. At lower and higher frequencies, the loss tangent, hence the magnitude of viscoelastic effects is greater. For strain-rates less than 10(-1) sec(-1), lower porosity RVE models were always stiffer than higher porosity RVE models. A generalized power law model is proposed to account for the fact that HS in cortical bone exhibits an upper and lower asymptotic bound and is bi-modal in terms of strain-rate.

Modeling of phosphate ion transfer to the surface of osteoblasts under normal gravity and simulated microgravity conditions

We have modeled the transport and accumulation of phosphate ions at the remodeling site of a trabecular bone consisting of osteoclasts and osteoblasts situated adjacent to each other in straining flows. Two such flows are considered; one corresponds to shear levels representative of trabecular bone conditions at normal gravity, the other corresponds to shear level that is representative of microgravity conditions. The latter is evaluated indirectly using a simulated microgravity environment prevailing in a rotating wall vessel bioreactor (RWV) designed by NASA. By solving the hydrodynamic equations governing the particle motion in a RWV using a direct numerical simulation (DNS) technique, the shear stress values on the surface of the microcarriers are found. In our present species transfer model, osteoclasts release phosphate ions (Pi) among other ions at bone resorption sites. Some of the ions so released are absorbed by the osteoblast, some accumulate at the osteoblast surface, and the remainder are advected away. The consumption of Pi by osteoblasts is assumed to follow Michaelis-Menten (MM) kinetics aided by a NaPi cotransporter system. MM kinetics views the NaPi cotransporter as a system for transporting extracellular Pi into the osteoblast. Our results show, for the conditions investigated here, the net accumulation of phosphate ions at the osteoblast surface under simulated microgravity conditions is higher by as much as a factor of three. Such increased accumulation may lead to enhanced apoptosis and may help explain the increased bone loss observed under microgravity conditions.

Hydraulic resistance and permeability in human lumbar vertebral bodies

Hydraulic resistance (HR) was measured for ten intact human lumbar vertebrae to further understand the mechanisms of fluid flow through porous bone. Oil was forced through the vertebral bodies under various volumetric flow rates and the resultant pressure was measured The pressure-flow relationship for each specimen was linear. Therefore, HR was constant with a mean of 2.22 +/- 1.45 kPa*sec/ml. The mean permeability of the intact vertebral bodies was 4.90x10(-10) +/- 4.45x10(-10) m2. These results indicate that this methodology is valid for whole bone samples and enables the exploration of the effects of HR on the creation of high-speed fractures.

Linear poroelastic cancellous bone anisotropy: trabecular solid elastic and fluid transport properties

The mechanical performance of cancellous bone is characterized using experiments which apply linear poroelasticity theory. It is hypothesized that the anisotropic organization of the solid and pore volumes of cancellous bone can be physically characterized separately (no deformable boundary interactive effects) within the same bone sample. Due to its spongy construction, the in vivo mechanical function of cancellous or trabecular bone is dependent upon fluid and solid materials which may interact in a hydraulic, convective fashion during functional loading. This project provides insight into the organization of the tissue, ie., the trabecular connectivity, by defining the separate nature of this biphasic performance. Previous fluid flow experiments [Kohles et al., 2001, Journal of Biomechanics, 34(11), pp. 1197-1202] describe the pore space via orthotropic permeability. Ultrasonic wave propagation through the trabecular network is used to describe the solid component via orthotropic elastic moduli and material stiffness coefficients. The linear poroelastic nature of the tissue is further described by relating transport (fluid flow) and elasticity (trabecular load transmission) during regression analysis. In addition, an empirical relationship between permeability and porosity is applied to the collected data. Mean parameters in the superior-inferior (SI) orientation of cubic samples (n=20) harvested from a single bovine distal femur were the largest (p<0.05) in comparison to medial-lateral (ML) and anterior-posterior (AP) orientations: Apparent elastic modulus (2,139 MPa), permeability (4.65x10(-10) m2), and material stiffness coefficient (13.6 GPa). A negative correlation between permeability as a predictor of structural elastic modulus supported a parametric relationship in the ML (R2=0.4793), AP (R2=0.3018), and SI (R2=0.6445) directions (p<0.05).

A Porous Media Approach to Finite Deformation Behaviour in Soft Tissues

The present work presents a porous medium formulation for the biomechanical analysis of soft tissues. An updated Lagrangian approach is developed to study the coupled effects of low speed flows of fluid phases, in partially or fully saturated conditions, and the finite deformation occurring in the solid matrix. The procedure developed allows both for the evaluation of coupled geometric and material non-linearities. The main theoretical and computational aspects of this multiphase formulation are discussed. The finite element method is used for the numerical solution of the resulting coupled system of equations. A reference case is reported with regard to healthy and degenerative phases of intervertebral segment. Results reported allow for a detailed interpretation of the formulation reliability, also by comparison with existing experimental data. In particular, the role played by the fluid on the load carrying mechanism is pointed out, thus stressing the importance of a multiphase approach to the overall behaviour of the spinal motion segment in time.

Tensile stress induces bone morphogenetic protein 4 in preosteoblastic and fibroblastic cells, which later differentiate into osteoblasts leading to osteogenesis in the mouse calvariae in organ culture

Mechanical stress is an important factor controlling bone remodeling, which maintains proper bone morphology and functions. However, the mechanism by which mechanical stress is transduced into biological stimuli remains unclear. Therefore, the purpose of this study is to examine how gene expression changes with osteoblast differentiation and which cells differentiate into osteoblasts. Tensile stress was applied to the cranial suture of neonatal mouse calvaria in a culture by means of helical springs. The suture was extended gradually, displaying a marked increase in cell number including osteoblasts. A histochemical study showed that this osteoblast differentiation began in the neighborhood of the existing osteoblasts, which can be seen by 3 h. The site of osteoblast differentiation moved with time toward the center of the suture, which resulted in an extension of osteoid. Scattered areas of the extended osteoid were calcified by 48 h. Reverse-transcription polymerase chain reaction (RT-PCR) revealed that tensile stress increased bone morphogenetic protein 4 (BMP-4) gene expression by 6 h and it remained elevated thereafter. This was caused by the induction of the gene in preosteoblastic cells in the neighborhood of osteoblasts and adjacent spindle-shaped fibroblastic cells. These changes were evident as early as 3 h and continued moving toward the center of the suture. The expression of Cbfa1/Osf-2, an osteoblast-specific transcription factor, followed that of BMP-4 and those cells positive with these genes appeared to differentiate into osteoblasts. These results suggest that BMP-4 may play a pivotal role by acting as an autocrine and a paracrine factor for recruiting osteoblasts in tensile stress-induced osteogenesis.

Poroelastic properties of bovine vertebral trabecular bone

The theory of poroelasticity has been used to study bone mechanics without directly measuring poroelastic properties. In this study, we developed an experimental protocol and measured the poroelastic properties of bovine vertebral trabecular bone. Mean (+/-SD) values for drained shear modulus, drained Poisson's ratio, undrained Poisson's ratio, Skempton's coefficient, and permeability coefficient were, respectively, 90.85 (+/-59.59) MPa and 0.242 (+/-0.099), 0.399 (+/-0.083), 0.851 (+/-0.144), and 16.31 (+/-8.02) x 10(-8) m2/Pa/sec, respectively. The experimental protocol can be used generally for the measurement of poroelastic properties of bone when cylindrical specimens are available. Measured poroelastic properties can be used directly or converted to Biot's coefficient and modulus, without assuming the incompressibility of solid and fluid constituents, for the poroelastic modeling of bone.

A uniform strain criterion for trabecular bone adaptation: do continuum-level strain gradients drive adaptation?

In this paper, it is postulated that the apparent density of trabecular bone adapts so that continuum-level strains within the bone are uniform and, as a consequence, spatial strain gradients within the bone/marrow continuum are minimized. The feasibility of a uniform strain criterion was tested using computational finite-element analysis of the proximal femur. We demonstrated that (1) this criterion produced a realistic apparent density distribution in the proximal femur, (2) the solutions for apparent density were convergent and unique, (3) predicted apparent densities compared well to experimental measurements, and (4) strain gradients within the bone/marrow continuum were reduced substantially. Thus, a possible goal of trabecular bone adaptation may be the reduction of strain gradients within the bone/marrow continuum. Osteocytes within the bone tissue and bone cells on the surface of a trabeculum are mechanosensitive and play a role in bone adaptation. In addition, the bone marrow is rich in osteoprogenitor cells near the bone surface that are mechanosensitive. Strain gradients within bone/marrow continuum cause pressure gradients in the marrow, causing extracellular fluid flow which could stimulate osteoprogenitor cells and contribute to bone adaptation.

In vivo observations of hydraulic stiffening in the canine femoral head

The role that intertrabecular contents and their boundary conditions have on the dynamic mechanical response of canine femoral heads was investigated in vivo. Femoral heads from paired intact hind limbs of canine specimens were subjected to a sinusoidal strain excitation at physiologic frequencies, in the cranio-caudal direction. The fluid boundary conditions for the contralateral limbs were changed by predrilling through the lateral femoral cortex and into the femoral neck. The drilling procedure did not invade the head itself. This femoral head fluid boundary alteration reduced the stiffness by 19 percent for testing at 1 Hz. The results of this study demonstrate that fluid stiffening occurs in vivo as previously observed ex vivo.

Mechanotransduction and the functional response of bone to mechanical strain

Mechanotransduction plays a crucial role in the physiology of many tissues including bone. Mechanical loading can inhibit bone resorption and increase bone formation in vivo. In bone, the process of mechanotransduction can be divided into four distinct steps: (1) mechanocoupling, (2) biochemical coupling, (3) transmission of signal, and (4) effector cell response. In mechanocoupling, mechanical loads in vivo cause deformations in bone that stretch bone cells within and lining the bone matrix and create fluid movement within the canaliculae of bone. Dynamic loading, which is associated with extracellular fluid flow and the creation of streaming potentials within bone, is most effective for stimulating new bone formation in vivo. Bone cells in vitro are stimulated to produce second messengers when exposed to fluid flow or mechanical stretch. In biochemical coupling, the possible mechanisms for the coupling of cell-level mechanical signals into intracellular biochemical signals include force transduction through the integrin-cytoskeleton-nuclear matrix structure, stretch-activated cation channels within the cell membrane, G protein-dependent pathways, and linkage between the cytoskeleton and the phospholipase C or phospholipase A pathways. The tight interaction of each of these pathways would suggest that the entire cell is a mechanosensor and there are many different pathways available for the transduction of a mechanical signal. In the transmission of signal, osteoblasts, osteocytes, and bone lining cells may act as sensors of mechanical signals and may communicate the signal through cell processes connected by gap junctions. These cells also produce paracrine factors that may signal osteoprogenitors to differentiate into osteoblasts and attach to the bone surface. Insulin-like growth factors and prostaglandins are possible candidates for intermediaries in signal transduction. In the effector cell response, the effects of mechanical loading are dependent upon the magnitude, duration, and rate of the applied load. Longer duration, lower amplitude loading has the same effect on bone formation as loads with short duration and high amplitude. Loading must be cyclic to stimulate new bone formation. Aging greatly reduces the osteogenic effects of mechanical loading in vivo. Also, some hormones may interact with local mechanical signals to change the sensitivity of the sensor or effector cells to mechanical load.

Effects of loading rate on strength of the proximal femur

Results from previous quasi-static mechanical tests indicate that femurs from elderly subjects fail in vitro at forces 50% below those available in a fall from standing height. However, bone is a rate-dependent material, and it is not known whether this imbalance is present at rates of loading which occur in a fall. Based on recent data on time to peak force and body positions at impact during simulated falls, we designed a high rate test of the femur in a loading configuration meant to represent a fall on the hip. We used elderly (mean age 73.5 +/- 7.4 (SD) years) and younger adult (32.7 +/- 12.8 years) cadaveric femurs to investigate whether (1) the strength, stiffness, and energy absorption capacity of the femur increases under high rate loading conditions; (2) elderly femurs have reduced strength, stiffness, and energy absorption capacity compared with younger adult femurs at this loading rate; and (3) densitometric and geometric measures taken at the hip correlate with the measured fracture loads. Femurs were scanned using dual-energy X-ray absorptiometry (DXA) and then tested to failure in a fall loading configuration at a displacement rate of 100 mm/second. The fracture load in elderly and younger adult femurs increased by about 20% with a 50-fold increase in displacement rate. However, energy absorption did not increase with displacement rate because of a twofold increase in stiffness at the higher loading rate.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanical consequences of core drilling and bone-grafting on osteonecrosis of the femoral head

We employed an anatomically realistic three-dimensional finite-element model to explore several biomechanical variables involved in coring or bone-grafting of a segmentally necrotic femoral head. The mechanical efficacy of several variants of these procedures was indexed in terms of their alteration of the stress:strength ratio in at-risk necrotic cancellous bone. For coring alone, the associated structural compromise was generally modest, provided that the tract did not extend near the subchondral plate. Cortical bone-grafting was potentially of great structural benefit for femoral heads in which the graft penetrated deeply into the superocentral or lateral aspect of the lesion, ideally with abutment against the subchondral plate. By contrast, central or lateral grafts that stopped well short of the subchondral plate were contraindicated biomechanically because they caused marked elevations in stress on the necrotic cancellous bone. Calculated levels of stress were relatively insensitive to variations in the diameter of the graft.