Correlation of bony ingrowth to the distribution of stress and strain parameters surrounding a porous-coated implant

Bone ingrowth in randomly distributed porous interbody cage during lumbar spinal fusion

Porous interbody cages are often used in spinal fusion surgery since they allow bone ingrowth which facilitates long-term stability. However, the extent of bone ingrowth in and around porous interbody cages has scarcely been investigated. Moreover, tissue differentiation might not be similar around the superior and inferior cage-bone interfaces. Using mechanobiology-based numerical framework and physiologic loading conditions, the study investigates the spatial distribution of evolutionary bone ingrowth within randomly distributed porous interbody cages, having varied porosities. Finite Element (FE) microscale models, corresponding to cage porosities of 60 %, 72 %, and 83 %, were developed for the superior and inferior interfacial regions of the cage, along with the macroscale model of the implanted lumbar spine. The implant-bone relative displacements of different porosity models were mapped from macroscale to microscale model. Bone formation of 10-40 % was predicted across the porous cage models, resulting in an average Young's modulus ranging between 765 MPa and 915 MPa. Maximum bone ingrowth of ∼34 % was observed for the 83 % porous cage, which was subject to low implant-bone relative displacements (maximum 50μm). New bone formation was found to be greater at the superior interface (∼34 %) as compared to the inferior interface (∼30 %) for P83 model. Relatively greater volume of fibrous tissue was formed at the implant-bone interface for the cage with 60 % and 72 % porosities, which might lead to cage migration and eventual failure of the implant. Hence, the interbody cage with 83 % porosity appears to be most favorable for bone ingrowth, provided sufficient mechanical strength is offered.

Influence of sequential opening/closing of interface gaps and texture density on bone growth over macro-textured implant surfaces using FE based mechanoregulatory algorithm

Intramedullary implant fixation is achieved through a press-fit between the implant and the host bone. A stronger press-fit between the bone and the prosthesis often introduces damage to the bone canal creating micro-gaps. The aim of the present investigation is to study the influences of simultaneous opening/closing of gaps on bone growth over macro-textured implant surfaces. Models based on textures available on CORAIL and SP-CL hip stems have been considered and 3D finite element (FE) analysis has been carried out in conjunction with mechanoregulation based tissue differentiation algorithm. Additionally, using a full-factorial approach, different combinations (between 5 µm to 15 µm) of sliding and gap distances at the bone-implant interface were considered to understand their combined influences on bone growth. All designs show an elevated fibrous tissue formation (10.96% at 5 µm to 29.38% at 40 µm for CORAIL based textured model; 11.45% at 5 µm to 32.25% at 40 µm for SP-CL based textured model) and inhibition of soft cartilaginous tissue (75.64% at 5 µm to 53.94% at 40 µm for CORAIL based model; 76.02% at 5 µm to 53.60% at 40 µm SP-CL based model) at progressively higher levels of normal micromotion, leading to a fragile bone-implant interface. These results highlight the importance of minimizing both sliding and gap distances simultaneously to enhance bone growth and implant stability. Further, results from the studies with differential texture density over CORAIL based implant reveal a non-linear complex relationship between tissue growth and texture density which might be investigated in a machine learning framework.

The Bone Buttress Theory: The Effect of the Mechanical Loading of Bone on the Osseointegration of Dental Implants

Simple Summary

The bone, as a vertebrate support tissue, is capable of adapting its structure and function to the mechanical demands resulting from the loads that are produced during the performance of its activity. This regulatory action also occurs during the healing processes of a fracture. The purpose of this study was to determine to what extent a dynamic load was capable of modulating the bone healing response around a titanium implant. The study was carried out on experimental rabbits, to which dental implants were placed in the tibiae and there were two test groups, one in which they did not undergo exercise during healing period and another that ran daily during this process on a treadmill. The trail results showed an improvement in the osseointegration process of the implant in the group in which it was subjected to load. The importance of these results is that it opens the door to a better understanding of the mechanisms that can modulate bone healing, especially around dental implants, supporting implant loading protocols that are based on efficiency.

Abstract

Objectives: To determine the effect of mechanical loading of bone on the stability and histomorphometric variables of the osseointegration of dental implants using an experimental test in an animal model. Materials and Methods: A total of 4 human implants were placed in both tibiae of 10 New Zealand rabbits (n = 40). A 6-week osseointegration was considered, and the rabbits were randomly assigned to two groups: Group A (Test group) included 5 rabbits that ran on a treadmill for 20 min daily during the osseointegration period; Group B (Controls) included the other 5 that were housed conventionally. The monitored variables were related to the primary and secondary stability of the dental implants (implant stability quotient—ISQ), vertical bone growth, bone to implant contact (BIC), area of regenerated bone and the percentage of immature matrix. Results: The results of the study show a greater vertical bone growth (Group A 1.26 ± 0.48 mm, Group B 0.32 ± 0.47 mm, p < 0.001), higher ISQ values (Group A 11.25 ± 6.10 ISQ, 15.73%; Group B 5.80 ± 5.97 ISQ, 7.99%, p = 0.006) and a higher BIC (Group A 19.37%, Group B 23.60%, p = 0.0058) for implants in the test group, with statistically significant differences. A higher percentage of immature bone matrix was observed for implants in the control group (20.68 ± 9.53) than those in the test group (15.38 ± 8.84) (p = 0.108). A larger area of regenerated bone was also observed for the test implants (Group A 280.50 ± 125.40 mm², Group B 228.00 ± 141.40 mm²), but it was not statistically significant (p = 0.121). Conclusions: The mechanical loading of bone improves the stability and the histomorphometric variables of the osseointegration of dental implants.

The influence of macro-textural designs over implant surface on bone on-growth: A computational mechanobiology based study

The longerterm secondary stability of an uncemented implant depends primarily on the quality and extent of bone in-growth or on-growth at the bone-implant interface. Investigations are warranted to predict the influences of implant macro-textures on bone on-growth pattern. Mechanoregulatory tissue differentiation algorithms can predict such patterns effectively. There is, however, a dearth of volumetric in silico study to assess the influence of macro-textures on bone growth. The present study investigated the influence of macro-textural grooves/ribs on changes in tissue formation at the bone-implant interface by carrying out a 3D finite element (FE) analysis. Three distinct macro-textures, loosely based on commercially viable hip stem models, were comparatively assessed for varying levels of interfacial micromotion. The study predicted elevated fibrogenesis and chondrogenesis, followed by a suppressed osteogenesis for higher levels of micromotion (60 μm and 100 μm), resulting in weak bone-implant interface strength. However, small judicious modifications in implant surface texture may enhance bone growth to a considerable extent. The numerical scheme can further be used as a template for more rigorous parametric and multi-scale studies.

Pulsed electromagnetic fields modify the adverse effects of glucocorticoids on bone architecture, bone strength and porous implant osseointegration by rescuing bone-anabolic actions

Long-term glucocorticoid therapy is known to induce increased bone fragility and impaired skeletal regeneration potential. Growing evidence suggests that pulsed electromagnetic fields (PEMF) can accelerate fracture healing and increase bone mass both experimentally and clinically. However, how glucocorticoid-treated bone and bone cells respond to PEMF stimulation remains poorly understood. Here we tested the effects of PEMF on bone quantity/quality, bone metabolism, and porous implant osseointegration in rabbits treated with dexamethasone (0.5 mg/kg/day, 6 weeks). The micro-CT, histologic and nanoindentation results showed that PEMF ameliorated the glucocorticoid-mediated deterioration of cancellous and cortical bone architecture and intrinsic material properties. Utilizing the new porous titanium implant (Ti2448) with low toxicity and low elastic modulus, we found that PEMF stimulated bone ingrowth into the pores of implants and enhanced peri-implant bone material quality during osseous defect repair in glucocorticoid-treated rabbits. Dynamic histomorphometric results revealed that PEMF reversed the adverse effects of glucocorticoids on bone formation, which was confirmed by increased circulating osteocalcin and P1NP. PEMF also significantly attenuated osteocyte apoptosis, promoted osteoblast-related osteocalcin, Runx2 and Osx expression, and inhibited osteocyte-specific DKK1 and Sost expression (negative regulators of osteoblasts) in glucocorticoid-treated skeletons, revealing improved functional activities of osteoblasts and osteocytes. Nevertheless, PEMF exerted no effect on circulating bone-resorbing cytokines (serum TRAcP5b and CTX-1) or skeletal gene expression of osteoclast-specific markers (TRAP and cathepsin K). PEMF also significantly upregulated skeletal gene expression of canonical Wnt ligands (Wnt1, Wnt3a and Wnt10b), whereas PEMF did not alter non-canonical Wnt5a expression. This study demonstrates that PEMF treatment improves bone mass, strength and porous implant osseointegration in glucocorticoid-treated rabbits by promoting potent bone-anabolic action, which is associated with canonical Wnt-mediated improvement in osteoblast and osteocyte functions. This study provides a new treatment alternative for glucocorticoid-related bone disorders in a convenient and non-invasive manner.

Nanomechanical Assessment of Bone Surrounding Implants Loaded for 3 Years in a Canine Experimental Model

PURPOSE

This work evaluated the nanomechanical properties of bone surrounding submerged and immediately loaded implants after 3 years in vivo. It was hypothesized that the nanomechanical properties of bone would markedly increase in immediately and functionally loaded implants compared with submerged implants.

MATERIALS AND METHODS

The second, third, and fourth right premolars and the first molar of 10 adult Doberman dogs were extracted. After 6 months, 4 implants were placed in 1 side of the mandible. The mesial implant received a cover screw and remained unloaded. The remaining 3 implants received fixed dental prostheses within 48 hours after surgery that remained in occlusal function for 3 years. After sacrifice, the bone was prepared for histologic and nanoindentation analysis. Nanoindentation was carried out under wet conditions on bone areas within the plateaus. Indentations (n = 30 per histologic section) were performed with a maximum load of 300 μN (loading rate, 60 μN per second) followed by a holding and unloading time of 10 and 2 seconds, respectively. Elastic modulus (E) and hardness (H) were computed in giga-pascals. The amount of bone-to-implant contact (BIC) also was evaluated.

RESULTS

The E and H values for cortical bone regions were higher than those for trabecular bone regardless of load condition, but this difference was not statistically significant (P > .05). The E and H values were higher for loaded implants than for submerged implants (P < .05) for cortical and trabecular bone. For the same load condition, the E and H values for cortical and trabecular bone were not statistically different (P > .05). The loaded and submerged implants presented BIC values (mean ± standard deviation) of 57.4 ± 12.1% and 62 ± 7.5%, respectively (P > .05).

CONCLUSION

The E and H values of bone surrounding dental implants, measured by nanoindentation, were higher for immediately loaded than for submerged implants.

Effects of external stress on biodegradable orthopedic materials: A review

Biodegradable orthopedic materials (BOMs) are used in rehabilitation and reconstruction of fractured tissues. The response of BOMs to the combined action of physiological stress and corrosion is an important issue in vivo since stress-assisted degradation and cracking are common. Although the degradation behavior and kinetics of BOMs have been investigated under static conditions, stress effects can be very serious and even fatal in the dynamic physiological environment. Since stress is unavoidable in biomedical applications of BOMs, recent work has focused on the evaluation and prediction of the properties of BOMs under stress in corrosive media. This article reviews recent progress in this important area focusing on biodegradable metals, polymers, and ceramics.

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The response of biodegradable orthopedic materials to the combined action of physiological stress and corrosion was reviewed.

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Physiological function stress to bone formation was reported.

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Factors influencing the effects of stress and corrosion on biodegradable metals were discussed.

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The response of biodegradable polymers to different stress mode was reported.

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Degradation prediction of biodegradable biopolymers under stress was mentioned.

The mechanical consequences of load bearing in the equine third metacarpal across speed and gait: the nonuniform distributions of normal strain, shear strain, and strain energy density

Distributions of normal strain, shear strain, and strain energy density (SED) were determined across the midshaft of the third metacarpal (MCIII, or cannon bone) of 3 adult thoroughbred horses as a function of speed and gait. A complete characterization of the mechanical demands of the bone made through the stride and from mild through the extremes of locomotion was possible by using three 3-element rosette strain gauges bonded at the diaphyseal midshaft of the MCIII and evaluating the strain output with beam theory and finite element analysis. Mean ± sd values of normal strain, shear strain, and SED increased with speed and peaked during a canter (-3560±380 microstrain, 1760±470 microstrain, and 119±23 kPa, respectively). While the location of these peaks was similar across animals and gaits, the resulting strain distributions across the cortex were consistently nonuniform, establishing between a 73-fold (slow trot) to a 330-fold (canter) disparity between the sites of maximum and minimum SED for each gait cycle. Using strain power density as an estimate of strain history across the bone revealed a 154-fold disparity between peak and minimum at the walk but fell to ~32-fold at the canter. The nonuniform, minimally varying, strain environment suggests either that bone homeostasis is mediated by magnitude-independent mechanical signals or that the duration of stimuli necessary to establish and maintain tissue integrity is relatively brief, and thus the vast majority of strain information is disregarded.

Enhancement of implant osseointegration by high-frequency low-magnitude loading

Background

Mechanical loading is known to play an important role in bone remodelling. This study aimed to evaluate the effect of high- and low-frequency axial loading, applied directly to the implant, on peri-implant bone healing and implant osseointegration.

Methodology

Titanium implants were bilaterally installed in rat tibiae. For every animal, one implant was loaded (test) while the other one was not (control). The test implants were randomly divided into 8 groups according to 4 loading regimes and 2 experimental periods (1 and 4 weeks). The loaded implants were subject to an axial displacement. Within the high- (HF, 40 Hz) or low-frequency (LF, 8 Hz) loading category, the displacements varied 2-fold and were ranked as low- or high-magnitude (LM, HM), respectively. The strain rate amplitudes were kept constant between the two frequency groups. This resulted in the following 4 loading regimes: 1) HF-LM, 40 Hz-8 µm; 2) HF-HM, 40 Hz-16 µm; 3) LF-LM, 8 Hz-41 µm; 4) LF-HM, 8 Hz-82 µm. The tissue samples were processed for resin embedding and subjected to histological and histomorphometrical analyses. Data were analyzed statistically with the significance set at p<0.05.

Principal Findings

After loading for 4 weeks, HF-LM loading (40 Hz-8 µm) induced more bone-to-implant contact (BIC) at the level of the cortex compared to its unloaded control. No significant effect of the four loading regimes on the peri-implant bone fraction (BF) was found in the 2 experimental periods.

Conclusions

The stimulatory effect of immediate implant loading on bone-to-implant contact was only observed in case of high-frequency (40 Hz) low-magnitude (8 µm) loading. The applied load regimes failed to influence the peri-implant bone mass.

Involvement of Wnt activation in the micromechanical vibration-enhanced osteogenic response of osteoblasts

BACKGROUND

Low-magnitude vibration has been widely used as a tool for rehabilitation, enhancing physical performance, and stimulating bone development. Although mechanical stimulation generated by vibrations is regarded as important factor in bone remodeling, the underlying cellular and molecular regulatory mechanisms of this response, which may be important in the development of new mechanobiological strategies, currently remain unclear.

METHODS

In this study, to investigate the mechanobiological mechanisms of vibration-enhanced osteogenic responses in osteoblasts, MC3T3-E1 cells were subjected to vibrations of different amplitude (0.06, 0.14, 0.32, 0.49, 0.66, and 0.8 × g) at 40 Hz for 30 min/day over 3 days. The osteogenesis-related transcription factors Wnt10B, Sclerostin, OPG, and RANKL were analyzed for mRNA and protein expression.

RESULTS

The results revealed that protein expression of Wnt10B and OPG was increased in a magnitude-dependent manner by mechanical vibrations at amplitudes of 0.06, 0.14, 0.32, and 0.49 × g; the maximum increases were 2.4-fold (p < 0.001) and 7.9-fold (p < 0.001), respectively, at 0.49 × g. Sclerostin and RANKL levels were reduced at all amplitudes. On the basis of mRNA levels, the reduced expression of RANKL was further downregulated (p < 0.05) whereas OPG expression was further increased (p < 0.01) when the MC3T3-E1 cells were treated with LiCl compared with the effects of vibration alone.

CONCLUSIONS

The findings may indicate that Wnt signaling is involved in mechanotransduction at low-magnitude vibration; this may provide a cellular basis, and impetus for further development of, biomechanically based intervention for enhancing bone strength and accelerating implant osseointegration.

Local bone formation due to combined mechanical loading and intermittent hPTH-(1-34) treatment and its correlation to mechanical signal distributions

We evaluated the local response of cortical bone in the rat tibia due to combined treatment with synthetic parathyroid hormone, hPTH-(1-34), and mechanical stimulation by four-point bending. Forty-eight female retired breeder Sprague-Dawley rats were divided into six groups. Mechanically stimulated animals included the following groups: (1) Bend+PTH, (2) Sham+PTH, (3) Bend+Vehicle, (4) Sham+Vehicle. Non-mechanically stimulated animals included a (5) Control group that received neither loading nor injections, and a (6) PTH group that received only hPTH-(1-34) injections. The right limbs of mechanically loaded animals were exposed to a peak force of 50 N for 36 cycles at 2 Hz, three days per week for four weeks, and PTH-treated animals received injections equivalent to 50 microg/kg BW. Fluorochrome labeling was used to measure local formation at 12 sectors about the endocortical periphery. The distributions of endocortical bone formation were compared to the local formation differences between treatment groups and to a variety of potential mechanical stimuli signals. Results indicated that hPTH-(1-34) exerted a potent anabolic effect with near-uniform formation about the endocortical surface, and that localized formation peaks due to bending were further augmented in the presence of hPTH-(1-34) treatment. Correlation of formation patterns to mechanical signal distributions highlighted several candidate signals including the mid-principal stress, the dilatational strain, and the radial gradient of the local radial strain.

Bone attachment to glass-fibre-reinforced composite implant with porous surface

A method has recently been developed for producing fibre-reinforced composites (FRC) with porous surfaces, intended for use as load-bearing orthopaedic implants. This study focuses on evaluation of the bone-bonding behaviour of FRC implants. Three types of cylindrical implants, i.e. FRC implants with a porous surface, solid polymethyl methacrylate (PMMA) implants and titanium (Ti) implants, were inserted in a transverse direction into the intercondular trabeculous bone area of distal femurs and proximal tibias of New Zealand White rabbits. Animals were sacrificed at 3, 6 and 12 weeks post operation, and push-out tests (n=5-6 per implant type per time point) were then carried out. At 12 weeks the shear force at the porous FRC-bone interface was significantly higher (283.3+/-55.3N) than the shear force at interfaces of solid PMMA/bone (14.4+/-11.0 N; p<0.001) and Ti/bone (130.6+/-22.2N; p=0.001). Histological observation revealed new bone growth into the porous surface structure of FRC implants. Solid PMMA and Ti implants were encapsulated mostly with fibrous connective tissue. Finite element analysis (FEA) revealed that porous FRC implants had mechanical properties which could be tailored to smooth the shear stress distribution at the bone-implant interface and reduce the stress-shielding effect.

Bone ingrowth into a porous coated implant predicted by a mechano-regulatory tissue differentiation algorithm

Bone ingrowth into a porous surface is one of the primary methods for fixation of orthopaedic implants. Improved understanding of bone formation and fixation of these devices should improve their performance and longevity. In this study predictions of bone ingrowth into an implant porous coating were investigated using mechano-reculatory models. The mechano-regulatory tissue differentiation algorithm proposed by Lacroix et al., and a modified version that enforces a tissue differentiation pathway by transitioning from differentiation to bone adaptation were investigated. The modified algorithm resulted in nearly the same behavior as the original algorithm when applied to a fracture-healing model. The algorithms were further compared using micromechanical finite element model of a beaded porous scaffold. Predictions of bone and fibrous tissue formation were compared between the two algorithms and to clinically observed phenomena. Under loading conditions corresponding to a press-fit hip stem, the modified algorithm predicted bone ingrowth into approximately 25% of the pore space, which is similar to that reported in experimental studies, while the original algorithm was unstable. When micromotion at the bone-implant interface was simulated, 20 microm of transverse displacement resulted in soft tissue formation at the bone-implant interface and minimal bone ingrowth. In contrast, 10 and 5 microm of micromotion resulted in bone filling 40% of the pore space and a stable interface, again consistent with clinical and experimental observations.

ERK 1/2 activation in enhanced osteogenesis of human mesenchymal stem cells in poly(lactic‐glycolic acid) by cyclic hydrostatic pressure

The aim of this study was to identify the signal transduction pathways and mechano-transducers that play critical roles in the processes induced by changes in cyclic hydrostatic pressure and fluid shear in 3-dimensional (3D) culture systems. Mesenchymal stem cells were loaded into a polymeric scaffold and divided into three groups according to the stress treatment: static, fluid shear, and hydrostatic pressure with fluid shear. Cells were exposed daily to a hydrostatic pressure of 0.2 MPa for 1 min followed by 14 min rest with fluid flow at 30 rpm. Protein extracts were analyzed by Western blot for extracellular signal-regulated kinase 1/2 (ERK1/2). The complexes were cultured under the mechanical stimuli for 21 days with or without phospho-ERK1/2 inhibitor (U0126) and evaluated by RT-PCR, calcium contents, and immunohistochemistry. Under conditions of mechanical stimulation, the activation of ERK1/2 was sustained or increased with time. U0126 suppressed mechanical stimuli-induced expression of osteocalcin. In addition, calcium contents and the degrees of osteocalcin and osteopontin staining were decreased by this inhibitor. These results demonstrate that mechanical stimuli, particularly hydrostatic pressure with fluid shear, enhance osteogenesis in 3D culture systems via ERK1/2 activation.

Biomechanical comparison of component position and hardware failure in the reverse shoulder prosthesis

There has been renewed interest in reverse shoulder arthroplasty for the treatment of glenohumeral arthritis with concomitant rotator cuff deficiency. Failure of the prosthesis at the glenoid attachment site remains a concern. The purpose of this study was to examine glenoid component stability with regard to the angle of implantation. This investigation entailed a biomechanical analysis to evaluate forces and micromotion in glenoid components attached to 12 polyurethane blocks at -15 degrees, 0 degrees, and +15 degrees of superior and inferior tilt. The 15 degrees inferior tilt had the most uniform compressive forces and the least amount of tensile forces and micromotion when compared with the 0 degrees and 15 degrees superiorly tilted baseplate. Our results suggest that implantation with an inferior tilt will reduce the incidence of mechanical failure of the glenoid component in a reverse shoulder prosthesis.

Modulation of bone ingrowth of rabbit femur titanium implants by in vivo axial micromechanical loading

Titanium implants commonly used in orthopedics and dentistry integrate into host bone by a complex and coordinated process. Despite increasingly well illustrated molecular healing processes, mechanical modulation of implant bone ingrowth is poorly understood. The objective of the present study was to determine whether micromechanical forces applied axially to titanium implants modulate bone ingrowth surrounding intraosseous titanium implants. We hypothesized that small doses of micromechanical forces delivered daily to the bone-implant interface enhance implant bone ingrowth. Small titanium implants were placed transcortically in the lateral aspect of the proximal femur in 15 New Zealand White rabbits under general anesthesia and allowed to integrate with the surrounding bone for 6 wk. Micromechanical forces at 200 mN and 1 Hz were delivered axially to the right femur implants for 10 min/day over 12 consecutive days, whereas the left femur implants served as controls. The average bone volume 1 mm from mechanically loaded implants (n = 15) was 73 +/- 12%, which was significantly greater than the average bone volume (52 +/- 21%) of the contralateral controls (n = 15) (P < 0.01). The average number of osteoblast-like cells per endocortical bone surface was 55 +/- 8 cells/mm(2) for mechanically loaded implants, which was significantly greater than the contralateral controls (35 +/- 6 cells/mm(2)) (P < 0.01). Dynamic histomorphometry showed a significant increase in mineral apposition rate and bone-formation rate of mechanically stressed implants (3.8 +/- 1.2 microm/day and 2.4 +/- 1.0 microm(3).microm(-2).day(-1), respectively) than contralateral controls (2.2 +/- 0.92 microm/day and 1.2 +/- 0.60 microm(3).microm(-2).day(-1), respectively; P < 0.01). Collectively, these data suggest that micromechanical forces delivered axially on intraosseous titanium implants may have anabolic effects on implant bone ingrowth.

Modeling Deformation-Induced Fluid Flow in Cortical Bone?s Canalicular?Lacunar System

To explore the potential role that load-induced fluid flow plays as a mechano-transduction mechanism in bone adaptation, a lacunar-canalicular scale bone poroelasticity model is developed and implemented. The model uses micromechanics to homogenize the pericanalicular bone matrix, a system of straight circular cylinders in the bone matrix through which bone fluids can flow, as a locally anisotropic poroelastic medium. In this work, a simplified two-dimensional model of a periodic array of lacunae and their surrounding systems of canaliculi is used to quantify local fluid flow characteristics in the vicinity of a single lacuna. When the cortical bone model is loaded, microscale stress, and strain concentrations occur in the vicinity of individual lacunae and give rise to microscale spatial variations in the pore fluid pressure field. Furthermore, loading of the bone matrix containing canaliculi generates fluid pressures in the contained fluids. Consequently, loading of cortical bone induces fluid flow in the canaliculi and exchange of fluid between canaliculi and lacunae. For realistic bone morphology parameters, and a range of loading frequencies, fluid pressures and fluid-solid drag forces in the canalicular bone are computed and the associated energy dissipation in the models compared to that measured in physical in vitro experiments on human cortical bone. The proposed model indicates that deformation-induced fluid pressures in the lacunar-canalicular system have relaxation times on the order of milliseconds as opposed to the much shorter times (hundredths of milliseconds) associated with deformation-induced pressures in the Haversian system.

Fixation to the canine bone of artificial implant with new surface structure

Screw and laser (SL) column by making screw threads and forming small holes using laser irradiation on the base metal and conventional beads coating (BC) columns were embedded into the shaft of canine femurs, and compared the implant fixation to the host bone. The interfacial strength in SL columns was almost equivalent as BC columns, and bone-column contact rate was higher than BC columns significantly at twelve weeks after implantation. The newly devised SL surface had almost equivalent bone fixation strength comparable to the conventional BC surface. Also, this surface should provide a useful porous surface for use in artificial joints since there is no risk of surface structure detachment.

Micromechanically based poroelastic modeling of fluid flow in Haversian bone

To explore the hypothesis that load-induced fluid flow in bone is a mechano-transduction mechanism in bone adaptation, unit cell micro-mechanical techniques are used to relate the microstructure of Haversian cortical bone to its effective poroelastic properties. Computational poroelastic models are then applied to compute in vitro Haversian fluid flows in a prismatic specimen of cortical bone during harmonic bending excitations over the frequency range of 10(0) to 10(6) Hz. At each frequency considered, the steady state harmonic response of the poroelastic bone specimen is computed using complex frequency-domain finite element analysis. At the higher frequencies considered, the breakdown of Poisueille flow in Haversian canals is modeled by introduction of a complex fluid viscosity. Peak bone fluid pressures are found to increase linearly with loading frequency in proportion to peak bone stress up to frequencies of approximately 10 kHz. Haversian fluid shear stresses are found to increase linearly with excitation frequency and loading magnitude up until the breakdown of Poisueille flow. Tan delta values associated with the energy dissipated by load-induced fluid flow are also compared with values measured experimentally in a concurrent broadband spectral analysis of bone. The computational models indicate that fluid shear stresses and fluid pressures in the Haversian system could, under physiologically realistic loading, easily reach the level of a few Pascals, which have been shown in other works to elicit cell responses in vitro.

Low-magnitude mechanical loading becomes osteogenic when rest is inserted between each load cycle

Strategies to counteract bone loss with exercise have had fairly limited success, particularly those regimens subjecting the skeleton to mild activity such as walking. In contrast, here we show that it is possible to induce substantial bone formation with low-magnitude loading. In two distinct in vivo models of bone adaptation, we found that insertion of a 10-s rest interval between each load cycle transformed a locomotion-like loading regime that minimally influenced osteoblast activity into a potent anabolic stimulus. In the avian ulna model, the minimal mean (+SE) periosteal labeled surface (Ps.LS) observed in the intact contralateral bones (1.6 +/- 1.5%) was doubled after 3 consecutive days of low-magnitude loading (3.8 +/- 1.5%; p = 0.03). However, modifying the regimen by inserting 10 s of rest between each load cycle significantly enhanced the periosteal response (21.9 +/- 4.5%; p = 0.03). In the murine tibia model, 5 consecutive days of 100 low-magnitude loading cycles did not significantly alter mean periosteal bone formation rate (BFR) compared with contralateral bones (0.011 +/- 0.005 microm3/microm2 per day vs. 0.021 +/- 0.013 microm3/microm2 per day). In contrast, separating each of 10 of the same loading cycles with 10 s of rest significantly elevated periosteal BFR (0.167 +/- 0.049 microm3/microm2 per day; p = 0.01). Endocortical bone formation parameters were not altered by any loading regimen in either model. We conclude that 10 s of rest between each load cycle of a low-magnitude loading protocol greatly enhances the osteogenic potential of the regimen.

Does the mechanical milieu associated with high-speed running lead to adaptive changes in diaphyseal growing bone?

Exercise during growth can be important for attaining optimal bone mass. High-intensity long-duration protocols, however, can have detrimental effects on immature bone morphology and mechanics. The underlying mechanisms are poorly understood. Here, we quantified the mechanical environment of the middiaphyseal rooster tarsometatarsus during high-speed running and examined whether short bouts of this exercise-related mechanical milieu can induce positive changes in cortical bone morphology, mechanics, and mineral ash content. At 9 weeks of age, roosters were assigned to controls (n = 9) and runners (n = 8). Treadmill running was applied in loading sessions of 5 min, three times per day (approximately 2600 cycles/day) for 8 weeks. Both controls and runners received double-fluorochrome labels during weeks 3 and 8 of the protocol. Middiaphyseal distributions of tarsometatarsal longitudinal normal strain, strain rate, and strain gradients engendered by walking and running were determined via in vivo strain gauges. Compared with walking, running elevated mean peak strain magnitude by 19%, peak strain rates by 136%, and peak strain gradients by approximately 18%. After 8 weeks of running, middiaphyseal areal and mechanical properties and normalized ash weight were no different between runners and controls. Transient and focal reductions in periosteal mineral apposition rates occurred during the exercise protocol. Our current data suggest that reducing the number of loading cycles can mitigate the adverse response previously observed in this model with long-duration running. This study also supports the tenet that the exercise-generated mechanical milieu must differ substantially from the habitual milieu to induce significant adaptations.

Biomechanical and functional behavior of implants

The ability to achieve a long-term stable implant interface is not a significant clinical issue when sufficient uni- or bi-cortical stabilization is available. Clinical outcomes studies suggest that the higher-risk implants are those placed in compromised cortical bone (thin, porous, etc.) in anatomic sites with minimal existing trabecular bone (characterized as type IV bone). In establishing and maintaining an implant interface in such an environment, one needs to consider the impact of masticatory forces. These forces, in turn, have the potential to create localized changes in interfacial stiffness through the viscoelastic properties of bone. Changes in these properties will alter the communication between osteocytes and osteoblasts, leading to an increase in new bone growth, a maintenance of established bone, or a loss (potentially catastrophic) of either cortical or trabecular bone. Therefore, a key to understanding the biomechanical and functional behavior at an implant interface is to control the extent of anticipated modeling and remodeling behavior through an optimal implant design combined with a thorough understanding of how tissues respond to the mechanically active environment.

Mechanotransduction and functional response of the skeleton to physical stress: the mechanisms and mechanics of bone adaptation

The skeleton's primary mechanical function is to provide rigid levers for muscles to act against as they hold the body upright in defiance of gravity. Many bones are exposed to thousands of repetitive loads each day. During growth and development, the skeleton optimizes its architecture by subtle adaptations to these mechanical loads. The mechanisms for adaptation involve a multistep process of cellular mechanotransduction including: mechanocoupling - conversion of mechanical forces into local mechanical signals, such as fluid shear stresses, that initiate a response by bone cells; biochemical coupling - transduction of a mechanical signal to a biochemical response involving pathways within the cell membrane and cytoskeleton; cell-to-cell signaling from the sensor cells (probably osteocytes and bone lining cells) to effector cells (osteoblasts or osteoclasts) using prostaglandins and nitric oxide as signaling molecules; and effector response - either bone formation or resorption to cause appropriate architectural changes. These architectural changes tend to adjust and improve the bone structure to its prevailing mechanical environment. Structural changes can be predicted, to some extent, by mathematical formulas derived from three fundamental rules: (1) bone adaptation is driven by dynamic, rather than static, loading; (2) extending the loading duration has a diminishing effect on further bone adaptation; (3) bone cells accommodate to a mechanical loading environment, making them less responsive to routine or customary loading signals.

Three rules for bone adaptation to mechanical stimuli

The primary mechanical function of bones is to provide rigid levers for muscles to pull against, and to remain as light as possible to allow efficient locomotion. To accomplish this bones must adapt their shape and architecture to make efficient use of material. Bone adaptation during skeletal growth and development continuously adjusts skeletal mass and architecture to changing mechanical environments. There are three fundamental rules that govern bone adaptation: (1) It is driven by dynamic, rather than static, loading. (2) Only a short duration of mechanical loading is necessary to initiate an adaptive response. (3) Bone cells accommodate to a customary mechanical loading environment, making them less responsive to routine loading signals. From these rules, several mathematical equations can be derived that provide simple parametric models for bone adaptation.

Nonlinear dependence of loading intensity and cycle number in the maintenance of bone mass and morphology

The daily stress stimulus theory of bone adaptation was formulated to describe the loading conditions necessary to maintain bone mass. This theory identifies stress/strain magnitude and loading cycle number as sufficient to define an appropriate maintenance loading signal. Here, we extend the range over which loading cycle number has been evaluated to determine whether the daily stress stimulus theory can be applied to conditions of very high numbers of loading cycles at very low strain magnitudes. The ability of a relatively high-frequency (30-Hz) and moderate-duration (60-minute) loading regimen to maintain bone mass in a turkey ulna model of disuse osteopenia was evaluated by correlating the applied strain distributions to site-specific remodeling activity. Changes in morphology were investigated following 8 weeks of disuse compared with disuse plus daily exposure to 108,000 applied loading cycles sufficient to induce peak strains of approximately 100 microstrain. A strong correlation was observed between the preservation of bone mass and longitudinal normal strain (R = 0.91) (p < 0.01). The results confirm the strong antiresorptive influence of mechanical loading and identify a threshold near 70 microstrain for a daily loading cycle regimen of approximately 100,000 strain cycles. These results are not consistent with the daily stress stimulus theory and suggest that the frequency or strain rate associated with the loading stimulus must also play a critical role in the mechanism by which bone responds to mechanical strain.

Whole-body vibration in the skeleton: development of a resonance-based testing device

Whole-body vibration (WBV) has been demonstrated to have a strong influence on physiological systems, ranging from severely destructive to potentially beneficial. Unfortunately, the study of WBV in a controlled manner is commonly constrained by space and budgetary factors, particularly where vibration in the low frequency range is considered. In the work presented here, a small, low-cost device for performing WBV of the human skeleton is developed to assist in studies of vertical acceleration in a clinical setting. The device design consists of a spring-supported plate driven by an 18 N peak-force electromagnetic actuator, and the associated driving and monitoring electronics. Animal and human lumped-mass models have been coupled with a model of the loading device to seek a resonance response in the vicinity of 30 Hz. This approach minimizes the loading requirements of such a device, and thus a major component of the cost, yet can provide peak accelerations of 0.15 g at a frequency of 30 Hz in a small, lightweight package capable of use in a clinical or laboratory setting.