Mechanical stress and osteogenesis in vitro

Calcium Phosphates: A Key to Next-Generation In Vitro Bone Modeling

The replication of bone physiology under laboratory conditions is a prime target behind the development of in vitro bone models. The model should be robust enough to elicit an unbiased response when stimulated experimentally, giving reproducible outcomes. In vitro bone tissue generation majorly requires the availability of cellular components, the presence of factors promoting cellular proliferation and differentiation, efficient nutrient supply, and a supporting matrix for the cells to anchor - gaining predefined topology. Calcium phosphates (CaP) are difficult to ignore while considering the above requirements of a bone model. Therefore, the current review focuses on the role of CaP in developing an in vitro bone model addressing the prerequisites of bone tissue generation. Special emphasis is given to the physico-chemical properties of CaP that promote osteogenesis, angiogenesis and provide sufficient mechanical strength for load-bearing applications. Finally, the future course of action is discussed to ensure efficient utilization of CaP in the in vitro bone model development field.

Harnessing mechanical cues in the cellular microenvironment for bone regeneration

At the macroscale, bones experience a variety of compressive and tensile loads, and these loads cause deformations of the cortical and trabecular microstructure. These deformations produce a variety of stimuli in the cellular microenvironment that can influence the differentiation of marrow stromal cells (MSCs) and the activity of cells of the MSC lineage, including osteoblasts, osteocytes, and chondrocytes. Mechanotransduction, or conversion of mechanical stimuli to biochemical and biological signals, is thus part of a multiscale mechanobiological process that drives bone modeling, remodeling, fracture healing, and implant osseointegration. Despite strong evidence of the influence of a variety of mechanical cues, and multiple paradigms proposed to explain the influence of these cues on tissue growth and differentiation, even a working understanding of how skeletal cells respond to the complex combinations of stimuli in their microenvironments remains elusive. This review covers the current understanding of what types of microenvironmental mechanical cues MSCs respond to and what is known about how they respond in the presence of multiple such cues. We argue that in order to realize the vast potential for harnessing the cellular microenvironment for the enhancement of bone regeneration, additional investigations of how combinations of mechanical cues influence bone regeneration are needed.

Hyperbaric air mobilizes stem cells in humans; a new perspective on the hormetic dose curve

Introduction

Hyperbaric air (HBA) was first used pharmaceutically in 1662 to treat lung disease. Extensive use in Europe and North America followed throughout the 19th century to treat pulmonary and neurological disorders. HBA reached its zenith in the early 20th century when cyanotic, moribund "Spanish flu pandemic" patients turned normal color and regained consciousness within minutes after HBA treatment. Since that time the 78% Nitrogen fraction in HBA has been completely displaced by 100% oxygen to create the modern pharmaceutical hyperbaric oxygen therapy (HBOT), a powerful treatment that is FDA approved for multiple indications. Current belief purports oxygen as the active element mobilizing stem progenitor cells (SPCs) in HBOT, but hyperbaric air, which increases tensions of both oxygen and nitrogen, has been untested until now. In this study we test HBA for SPC mobilization, cytokine and chemokine expression, and complete blood count.

Methods

Ten 34-35-year-old healthy volunteers were exposed to 1.27ATA (4 psig/965 mmHg) room air for 90 min, M-F, for 10 exposures over 2-weeks. Venous blood samples were taken: (1) prior to the first exposure (served as the control for each subject), (2) directly after the first exposure (to measure the acute effect), (3) immediately prior to the ninth exposure (to measure the chronic effect), and (4) 3 days after the completion of tenth/final exposure (to assess durability). SPCs were gated by blinded scientists using Flow Cytometry.

Results

SPCs (CD45dim/CD34+/CD133-) were mobilized by nearly two-fold following 9 exposures (p = 0.02) increasing to three-fold 72-h post completion of the final (10th) exposure (p = 0.008) confirming durability.

Discussion

This research demonstrates that SPCs are mobilized, and cytokines are modulated by hyperbaric air. HBA likely is a therapeutic treatment. Previously published research using HBA placebos should be re-evaluated to reflect a dose treatment finding rather than finding a placebo effect. Our findings of SPC mobilization by HBA support further investigation into hyperbaric air as a pharmaceutical/therapy.

The Digital Twin: A Potential Solution for the Personalized Diagnosis and Treatment of Musculoskeletal System Diseases

Due to the high prevalence and rates of disability associated with musculoskeletal system diseases, more thorough research into diagnosis, pathogenesis, and treatments is required. One of the key contributors to the emergence of diseases of the musculoskeletal system is thought to be changes in the biomechanics of the human musculoskeletal system. However, there are some defects concerning personal analysis or dynamic responses in current biomechanical research methodologies. Digital twin (DT) was initially an engineering concept that reflected the mirror image of a physical entity. With the application of medical image analysis and artificial intelligence (AI), it entered our lives and showed its potential to be further applied in the medical field. Consequently, we believe that DT can take a step towards personalized healthcare by guiding the design of industrial personalized healthcare systems. In this perspective article, we discuss the limitations of traditional biomechanical methods and the initial exploration of DT in musculoskeletal system diseases. We provide a new opinion that DT could be an effective solution for musculoskeletal system diseases in the future, which will help us analyze the real-time biomechanical properties of the musculoskeletal system and achieve personalized medicine.

Chromium Oxide Nanoparticle Impaired Osteogenesis and Cellular Response to Mechanical Stimulus

Background

Release of metallic wear particles from hip replacement implants is closely associated with aseptic loosening that affects the functionality and survivorship of the prostheses. Chromium oxide nanoparticles (CrNPs) are the dominant form of the wear particles found in the periprosthetic tissues. Whether CrNPs play a role in the clinically observed particle-induced osteolysis, tissue inflammatory reactions and functional activities of human mesenchymal stem cells (MSCs) remain unknown.

Methods

A tibia-defect rat model, cytotoxicity assays and flow cytometry were applied to study the effect of CrNPs on MSCs survival and macrophage inflammatory response. Also, oscillatory fluid flow stimulation was used to analyse the osteogenic differentiation of MSCs while treated by CrNPs. In addition, the influence of CrNPs on MSC biomechanical properties was determined via atomic force microscope (AFM) and fluorescence microscopy.

Results

It was found that implantation of CrNPs significantly decreased bone formation in vivo. CrNPs had no obvious effects on inflammatory cytokines release of U937 macrophages. Additionally, CrNPs did not interfere with MSCs osteogenic differentiation under static culture. However, the upregulated osteogenic differentiation of MSCs due to fluid flow stimulation was reduced by CrNPs in a dose-dependent manner. Moreover, osteogenic gene expression of OPN, Cox2 and Rnux2 after mechanical stimulation was also decreased by CrNPs treatments. Furthermore, cell elasticity and adhesion force of MSCs were affected by CrNPs over 3 days of exposure. We further verified that these effects of CrNPs could be associated with its interruption on cell mechanical properties.

Conclusion

The results demonstrated that CrNPs impaired cellular response to mechanical stimulus and osteogenesis without noticeable effects on the survival of the human MSCs.

Bone physiological microenvironment and healing mechanism: Basis for future bone-tissue engineering scaffolds

Bone-tissue defects affect millions of people worldwide. Despite being common treatment approaches, autologous and allogeneic bone grafting have not achieved the ideal therapeutic effect. This has prompted researchers to explore novel bone-regeneration methods. In recent decades, the development of bone tissue engineering (BTE) scaffolds has been leading the forefront of this field. As researchers have provided deep insights into bone physiology and the bone-healing mechanism, various biomimicking and bioinspired BTE scaffolds have been reported. Now it is necessary to review the progress of natural bone physiology and bone healing mechanism, which will provide more valuable enlightenments for researchers in this field. This work details the physiological microenvironment of the natural bone tissue, bone-healing process, and various biomolecules involved therein. Next, according to the bone physiological microenvironment and the delivery of bioactive factors based on the bone-healing mechanism, it elaborates the biomimetic design of a scaffold, highlighting the designing of BTE scaffolds according to bone biology and providing the rationale for designing next-generation BTE scaffolds that conform to natural bone healing and regeneration.

Chitin nanocrystals assisted 3D printing of polycitrate thermoset bioelastomers

Citrate-based thermoset bioelastomer has numerous tissue engineering applications. However, its insoluble and unmeltable features restricted processing techniques for fabricating complex scaffolds. Herein, direct ink writing (DIW) was explored for 3D printing of poly(1, 8-octanediol-co-Pluronic F127 citrate) (POFC) bioelastomer scaffolds considering that POFC prepolymer (pre-POFC) was waterborne and could form a stable emulsion. The pre-POFC emulsion couldn't be printed, however, chitin nanocrystal (ChiNC) could be as a rheological modifier to tune the flow behavior of pre-POFC emulsion, and thus DIW printing of POFC scaffolds was successfully realized; moreover, ChiNC was also as a supporting agent to prevent collapse of filaments during thermocuring, and simultaneously as a biobased nanofiller to reinforce scaffolds. The rheological analyses showed the pre-POFC/ChiNC inks fulfilled the requirements for DIW printing. The printed scaffolds exhibited low swelling, and good performances in strength and resilence. Furthermore, the entire process was easily performed and eco-friendly.

Age and Sex Differences in Load-Induced Tibial Cortical Bone Surface Strain Maps

Bone adapts its architecture to the applied load; however, it is still unclear how bone mechano‐adaptation is coordinated and why potential for adaptation adjusts during the life course. Previous animal models have suggested strain as the mechanical stimulus for bone adaptation, but yet it is unknown how mouse cortical bone load‐related strains vary with age and sex. In this study, full‐field strain maps (at 1 N increments up to 12 N) on the bone surface were measured in young, adult, and old (aged 10, 22 weeks, and 20 months, respectively), male and female C57BL/6J mice with load applied using a noninvasive murine tibial model. Strain maps indicate a nonuniform strain field across the tibial surface, with axial compressive loads resulting in tension on the medial side of the tibia because of its curved shape. The load‐induced surface strain patterns and magnitudes show sexually dimorphic changes with aging. A comparison of the average and peak tensile strains indicates that the magnitude of strain at a given load generally increases during maturation, with tibias in female mice having higher strains than in males. The data further reveal that postmaturation aging is linked to sexually dimorphic changes in average and maximum strains. The strain maps reported here allow for loading male and female C57BL/6J mouse legs in vivo at the observed ages to create similar increases in bone surface average or peak strain to more accurately explore bone mechano‐adaptation differences with age and sex. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC. on behalf of American Society for Bone and Mineral Research.

Comparison of ridge resorption and patient satisfaction in single implant-supported mandibular overdentures with conventional complete dentures: A randomised pilot study

Purpose

To compare ridge resorption (RR) and patient satisfaction in single implant-supported mandibular overdentures (SIMO) with conventional complete dentures (CCD) over a period of one year.

Material and methods

This prospective, randomized trial enrolled 30 completely edentulous participants following inclusion and exclusion criteria. The study was completed by 28 participants. Rehabilitation of 14 participants was done by using SIMO (group I) and CCD (group C) each according to randomization chart. For both the groups, RR was computed in millimeters from residual ridge height measured by using orthopantogram at 6 months (T1), 9 months (T2), and 12 months (T3) at 3 anatomic locations: maxillary posterior (L1), maxillary anterior (L2), and mandibular posterior (L3). Patient satisfaction was evaluated by using Geriatric Oral Health Assessment Index Hindi version (GOHAI-Hi) at 1week and 12 months after denture delivery.

Results

At 12 months, minimum RR was observed at L2 of group I (0.62 ​± ​0.20 ​mm) and maximum RR was observed at L3 of group C (1.04 ​± ​0.15 ​mm). Comparison of ridge resorption between group I and group C was statistically significant at T3 (P ​= ​.001 for L1, P ​= ​.006 for L2, and P ​= ​.028 for L3). At T3, in group I, RR was more at L3 than L2 region (P ​= ​.011) and L1 region (P ​= ​.015). Statistically significant difference of GOHAI-Hi scores was observed between group I and group C at end of 12 months (P ​= ​.003).

Conclusions

SIMO cause less RR and higher patient satisfaction as compared to CCD and can be recommended with higher predictability of success than CCD.

Fabrication of poly (1, 8-octanediol-co-Pluronic F127 citrate)/chitin nanofibril/bioactive glass (POFC/ChiNF/BG) porous scaffold via directional-freeze-casting

The citrate-based thermoset elastomer is a promising candidate for bone scaffold material, but the harsh curing condition made it difficult to fabricate porous structure. Recently, poly (1, 8-octanediol-co-Pluronic F127 citrate) (POFC) porous scaffold was creatively fabricated by chitin nanofibrils (ChiNFs) supported emulsion-freeze-casting. Thanks to the supporting role of ChiNFs, the lamellar pore structure formed by directional freeze-drying was maintained during the subsequent thermocuring. Herein, bioactive glass (BG) was introduced into the POFC porous scaffolds to improve bioactivity. It was found the complete replacement of ChiNF particles with BG particles could not form a stable porous structure; however, existing at least 15 wt% ChiNF could ensure the formation of lamellar pore, and the interlamellar distance increased with BG ratios. Thus, the BG granules did not contribute to the formation of pore structure like ChiNFs, however, they surely endowed the scaffolds with enhanced mechanical properties, improved osteogenesis bioactivity, better cytocompatibility as well as quick degradation rate. Reasonably adjusting BG ratios could balance the requirements of porous structure and bioactivity.

Age-related osteogenesis on lateral force application to rat incisor – Part I: Premaxilla suture remodeling

Objectives:

The suture is a fibrous tissue intervening two adjacent bone segments, existing only in the craniofacial region. In spite of wide use of palatal expansion in various ages, the age-dependent cellular mechanism for osteogenesis is largely unknown. The aim of this study was to examine the proliferation and differentiation pattern of the suture cells on lateral expansion in rats depending on the ages.

Materials and Methods:

Calibrated lateral tensile stress of 50 g was given to the male Sprague-Dawley rat incisors using a double helix in 30 young (10 weeks) and another 30 aged (52 weeks) group, respectively. Each group was subdivided into control, 1, 3, 7, 14, and 21 days, with five animals in each group. Premaxilla area was retrieved from each animal for further histologic analyses including H and E, Masson’s trichrome, and immunohistochemical staining using antibodies against phospho-extracellular signal-regulated kinase, proliferating cell nuclear antigen (PCNA), and fibroblast growth factor receptor-2 (FGFR2). Positive cell counts in the region of interest were conducted.

Results:

Gross suture separation and subsequent bone formation on the sutural side bone surface were observed in both groups, characterized as active collagen turnover, remarkable woven bone projection toward the sutural mesenchyme and subsequent maturation in 3 weeks. Increase in PCNA- and FGFR2-postive cell proportions were comparable in both groups, indicating similar time- and area-specific proliferation and osteogenic differentiation patterns in the stretched suture regardless of the age groups.

Conclusion:

According to the results, it can be implicated that the tensile stress applied to the suture in the adult group may induce active bone formation similar to that in young group, in associated with FGFR2 and Erk signaling cascade. Mesenchymal cells in the premaxillary suture appear to retain remarkable potential for further proliferation and differentiation even in aged subjects.

Regulation of endothelial cell arrangements within hMSC - HUVEC co-cultured aggregates

Background

Micro-mass culturing or cellular aggregation is an effective method used to form mineralised bone tissue. Poor core cell viability, however, is often an impeding characteristic of large micro-mass cultures, and equally for large tissue-engineered bone grafts. Because of this, efforts are being made to enhance large graft perfusion, often through pre-vascularisation, which involves the co-culture of endothelial cells and bone cells or stem cells.

Methods

This study investigated the effects of different aggregation techniques and culture conditions on endothelial cell arrangements in mesenchymal stem cell and human umbilical vein endothelial cell co-cultured aggregates when endothelial cells constituted just 5%. Two different cellular aggregation techniques, i.e. suspension culture aggregation and pellet culture aggregation, were applied alongside two subsequent culturing techniques, i.e. hydrostatic loading and static culturing. Endothelial cell arrangements were assessed under such conditions to indicate potential pre-vascularisation.

Results

Our study found that the suspension culture aggregates cultured under hydrostatic loading offered the best environment for enhanced endothelial cell regional arrangements, closely followed by the pellet culture aggregates cultured under hydrostatic loading, the suspension culture aggregates cultured under static conditions, and the pellet culture aggregates cultured under static conditions.

Conclusions

The combination of particular aggregation techniques with dynamic culturing conditions appeared to have a synergistic effect on the cellular arrangements within the co-cultured aggregates.

Pressure-induced mesenchymal stem cell osteogenesis is dependent on intermediate filament remodeling

Macroscale loading of bone generates a complex local mechanical microenvironment that drives osteogenesis and bone mechanoadaptation. One such mechanical stimulus generated is hydrostatic pressure (HP); however, the effect of HP on mesenchymal stem cells (MSCs) and the mechanotransduction mechanisms utilized by these cells to sense this stimulus are yet to be fully elucidated. In this study, we demonstrate that cyclic HP is a potent mediator of cytoskeletal reorganization and increases in osteogenic responses in MSCs. In particular, we demonstrate that the intermediate filament (IF) network undergoes breakdown and reorganization with centripetal translocation of IF bundles toward the perinuclear region. Furthermore, we show for the first time that this IF remodeling is required for loading-induced MSC osteogenesis, revealing a novel mechanism of MSC mechanotransduction. In addition, we demonstrate that chemical disruption of IFs with withaferin A induces a similar mechanism of IF breakdown and remodeling as well as a subsequent increase in osteogenic gene expression in MSCs, exhibiting a potential mechanotherapeutic effect to enhance MSC osteogenesis. This study therefore highlights a novel mechanotransduction mechanism of pressure-induced MSC osteogenesis involving the understudied cytoskeletal structure, the IF, and demonstrates a potential new therapy to enhance bone formation in bone-loss diseases such as osteoporosis.-Stavenschi, E., Hoey, D. A. Pressure-induced mesenchymal stem cell osteogenesis is dependent on intermediate filament remodeling.

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.

Exercise training differentially alters axial and appendicular marrow cellularity in old mice

Aging gradually renders bone marrow hematopoietically inactive. Endurance exercise reverses this phenotype in young mice. Here, we determine the effects in aged mice. Twenty-two month old mice (n = 6) underwent a progressive exercise training protocol. In appendicular bones, marrow cellularity increased by 51% (p < 0.05) and marrow CFU, CFU-GM, and CAFC increased by 12%, 71%, and 86%, respectively (p < 0.05). Vertebral cellularity remained unchanged. The mechanical forces associated with treadmill exercise training may be responsible for these observations.

Bone up: craniomandibular development and hard-tissue biomineralization in neonate mice

The presence of regional variation in the osteogenic abilities of cranial bones underscores the fact that the mechanobiology of the mammalian skull is more complex than previously recognized. However, the relationship between patterns of cranial bone formation and biomineralization remains incompletely understood. In four strains of mice, micro-computed tomography was used to measure tissue mineral density during perinatal development in three skull regions (calvarium, basicranium, mandible) noted for variation in loading environment, embryological origin, and ossification mode. Biomineralization levels increased during perinatal ontogeny in the mandible and calvarium, but did not increase in the basicranium. Tissue mineral density levels also varied intracranially, with density in the mandible being highest, in the basicranium intermediate, and in the calvarium lowest. Perinatal increases in, and elevated levels of, mandibular biomineralization appear related to the impending postweaning need to resist elevated masticatory stresses. Similarly, perinatal increases in calvarial biomineralization may be linked to ongoing brain expansion, which is known to stimulate sutural bone formation in this region. The lack of perinatal increase in basicranial biomineralization could be a result of earlier developmental maturity in the cranial base relative to other skull regions due to its role in supporting the brain's mass throughout ontogeny. These results suggest that biomineralization levels and age-related trajectories throughout the skull are influenced by the functional environment and ontogenetic processes affecting each region, e.g., onset of masticatory loads in the mandible, whereas variation in embryology and ossification mode may only have secondary effects on patterns of biomineralization. Knowledge of perinatal variation in tissue mineral density, and of normal cranial bone formation early in development, may benefit clinical therapies aiming to correct developmental defects and traumatic injuries in the skull, and more generally characterize loading environments and skeletal adaptations in mammals by highlighting the need for multi-level analyses for evaluating functional performance of cranial bone.

The Cranial Base in Craniofacial Development: a Gene Therapy Study

The etiology of midface retrusion remains largely unclear. We hypothesized that the cranial base synchondroses play a key role in the development of the craniofacial skeleton in the Sandhoff mouse model. We observed that developmental abnormalities of the cranial base synchondroses involving proliferative chondrocytes are important in craniofacial growth and development. Neonatal restitution of β-hexosaminidase in mutant mice by gene therapy successfully ameliorated the attendant skeletal defects and restored craniofacial morphology in vivo, suggesting this as a critical temporal window in craniofacial development. Analysis of our data implicates parathyroid-related peptide (PTHrP) and cyclo-oxygenase-2 (COX-2) as possible factors underlying the development of the aforementioned skeletal defects. Hence, timely restitution of a genetic deficiency or, alternatively, the restoration of PTHrP or cyclo-oxygenase activity by the administration of PTH and/or non-steroidal anti-inflammatory drugs or COX-2 selective inhibitors to affected individuals may prove beneficial in the management of midface retrusion.

The Frictional Coefficient of the Temporomandibular Joint and Its Dependency on the Magnitude and Duration of Joint Loading

In synovial joints, friction between articular surfaces leads to shear stress within the cartilaginous tissue, which might result in tissue rupture and failure. Joint friction depends on synovial lubrication of the articular surfaces, which can be altered due to compressive loading. Therefore, we hypothesized that the frictional coefficient of the temporomandibular joint (TMJ) is affected by the magnitude and duration of loading. We tested this by measuring the frictional coefficient in 20 intact porcine TMJs using a pendulum-type friction tester. The mean frictional coefficient was 0.0145 (SD 0.0027) after a constant loading of 50 N during 5 sec. The frictional coefficient increased with the length of the preceding loading duration and exceeded 0.0220 (SD 0.0014) after 1 hr. Application of larger loading (80 N) resulted in significantly larger frictional coefficients. In conclusion, the frictional coefficient in the TMJ was proportional to the magnitude and duration of joint loading.

Ex vivo loading of trussed implants for spine fusion induces heterogeneous strains consistent with homeostatic bone mechanobiology

A truss structure was recently introduced as an interbody fusion cage. As a truss system, some of the connected elements may be in a state of compression and others in tension. This study aimed to quantify both the mean and variance of strut strains in such an implant when loaded in a simulated fusion condition with vertebral body or contoured plastic loading platens ex vivo. Cages were each instrumented with 78 fiducial spheres, loaded between platens (vertebral body or contoured plastic), imaged using high resolution micro-CT, and analyzed for deformation and strain of each of the 221 struts. With repeated loading of a cage by vertebral platens, the distribution (variance, indicated by SD) of strut strains widened from 50N control (4±114με, mean±SD) to 1000N (-23±273με) and 2000N (-48±414με), and between 1000N and 2000N. With similar loading of multiple cages, the strain distribution at 2000N (23±389με) increased from 50N control. With repeated loading by contoured plastic platens, induced strains at 2000N had a distribution similar to that induced by vertebral platens (84±426με). In all studies, cages exhibited increases in strut strain amplitude when loaded from 50N to 1000N or 2000N. Correspondingly, at 2000N, 59-64% of struts exhibited strain amplitudes consistent with mechanobiologically-regulated bone homeostasis. At 2000N, vertically-oriented struts exhibited deformation of -2.87±2.04μm and strain of -199±133με, indicating overall cage compression. Thus, using an ex vivo 3-D experimental biomechanical analysis method, a truss implant can have strains induced by physiological loading that are heterogeneous and of amplitudes consistent with mechanobiological bone homeostasis.

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.

Mechanical Stretching of Mouse Calvarial Osteoblasts In Vitro Models Changes in MMP-2 and MMP-9 Expression at the Bone-Implant Interface

Bone to mechanical loading elicits a biological response that has clinical significance for several areas in dental medicine, including orthodontic tooth movement, tempromandibular joint disease, and endosseous dental implant osseointegration. Human orthopedic studies of failed hip implant sites have identified increased mRNA expression of several collagen-degrading matrix metalloproteinases (MMPs), while in vitro experiments have shown increases in MMP secretion after exposure to inflammatory mediators. This investigation evaluates the effects of mechanical deformation on in vitro osteoblasts by assessing changes in MMP gene expression and enzyme activity. We seeded mouse neonatal calvarial osteoblasts onto flexible 6-well plates and subjected to continuous cyclic mechanical stretching. The expression and activity of mRNA for several MMPs (2, 3, 9, and 10) was assessed. When subjected to mechanical stress in culture, only mRNA specific for MMP-9 was significantly increased compared to nonstretched controls (P < .005). Measurement of MMP activity by gelatin zymography demonstrated that none of the MMPs showed increased activity with stretching; however, MMP-2 activity decreased. Our results suggest that in response to stretch, MMP-2 responds rapidly by inhibiting conversion of a MMP-2 to the active form, while a slower up-regulation of MMP-9 may play a role in the long-term remodeling of extracellular matrix in response to continuous mechanical loading. This study suggests that the regulation of metalloproteinases at both the mRNA and protein level are important in the response of bone to mechanical stress.

The effect of mechanical loading on osteogenesis of human dental pulp stromal cells in a novel in vitro model

Tooth loss often results in alveolar bone resorption because of lack of mechanical stimulation. Thus, the mechanism of mechanical loading on stem cell osteogenesis is crucial for alveolar bone regeneration. We have investigated the effect of mechanical loading on osteogenesis in human dental pulp stromal cells (hDPSCs) in a novel in vitro model. Briefly, 1 × 10(7) hDPSCs were seeded into 1 ml 3% agarose gel in a 48-well-plate. A loading tube was then placed in the middle of the gel to mimic tooth-chewing movement (1 Hz, 3 × 30 min per day, n = 3). A non-loading group was used as a control. At various time points, the distribution of live/dead cells within the gel was confirmed by fluorescence markers and confocal microscopy. The correlation and interaction between the factors (e.g. force, time, depth and distance) were statistically analysed. The samples were processed for histology and immunohistochemistry. After 1-3 weeks of culture in the in-house-designed in vitro bioreactor, fluorescence imaging confirmed that additional mechanical loading increased the viable cell numbers over time as compared with the control. Cells of various phenotypes formed different patterns away from the reaction tube. The cells in the middle part of the gel showed enhanced alkaline phosphatase staining at week 1 but reduced staining at weeks 2 and 3. Additional loading enhanced Sirius Red and type I collagen staining compared with the control. We have thus successfully developed a novel in-house-designed in vitro bioreactor mimicking the biting force to enhance hDPSC osteogenesis in an agarose scaffold and to promote bone formation and/or prevent bone resorption.

Lysine-specific demethylase 1-mediated demethylation of histone H3 lysine 9 contributes to interleukin 1β-induced microsomal prostaglandin E synthase 1 expression in human osteoarthritic chondrocytes

Introduction

Microsomal prostaglandin E synthase 1 (mPGES-1) catalyzes the terminal step in the biosynthesis of PGE₂, a critical mediator in the pathophysiology of osteoarthritis (OA). Histone methylation plays an important role in epigenetic gene regulation. In this study, we investigated the roles of histone H3 lysine 9 (H3K9) methylation in interleukin 1β (IL-1β)-induced mPGES-1 expression in human chondrocytes.

Methods

Chondrocytes were stimulated with IL-1β, and the expression of mPGES-1 mRNA was evaluated using real-time RT-PCR. H3K9 methylation and the recruitment of the histone demethylase lysine-specific demethylase 1 (LSD1) to the mPGES-1 promoter were evaluated using chromatin immunoprecipitation assays. The role of LSD1 was further evaluated using the pharmacological inhibitors tranylcypromine and pargyline and small interfering RNA (siRNA)-mediated gene silencing. The LSD1 level in cartilage was determined by RT-PCR and immunohistochemistry.

Results

The induction of mPGES-1 expression by IL-1β correlated with decreased levels of mono- and dimethylated H3K9 at the mPGES-1 promoter. These changes were concomitant with the recruitment of the histone demethylase LSD1. Treatment with tranylcypromine and pargyline, which are potent inhibitors of LSD1, prevented IL-1β-induced H3K9 demethylation at the mPGES-1 promoter and expression of mPGES-1. Consistently, LSD1 gene silencing with siRNA prevented IL-1β-induced H3K9 demethylation and mPGES-1 expression, suggesting that LSD1 mediates IL-1β-induced mPGES-1 expression via H3K9 demethylation. We show that the level of LSD1 was elevated in OA compared to normal cartilage.

Conclusion

These results indicate that H3K9 demethylation by LSD1 contributes to IL-1β-induced mPGES-1 expression and suggest that this pathway could be a potential target for pharmacological intervention in the treatment of OA and possibly other arthritic conditions.

Anatomical, functional, physiological and behavioural aspects of the development of mastication in early childhood

Mastication efficiency is defined as the efficiency of crushing food between the teeth and manipulating the resulting particles to form a swallowable food bolus. It is dependent on the orofacial anatomical features of the subject, the coordination of these anatomical features and the consistency of the food used during testing. Different measures have been used to indirectly quantify mastication efficiency as a function of children's age such as observations, food bolus characterisation, muscle activity measurement and jaw movement tracking. In the present review, we aim to describe the changes in the oral physiology (e.g. bone and muscle structure, teeth and soft tissues) of children and how these changes are associated with mastication abilities. We also review previous work on the effect of food consistency on children's mastication abilities and on their level of texture acceptance. The lack of reference foods and differences in testing methodologies across different studies do not allow us to draw conclusions about (1) the age at which mastication efficiency reaches maturity and (2) the effect of food consistency on the establishment of mature mastication efficiency. The effect of food consistency on the development of children's mastication efficiency has not been tested widely. However, both human and animal studies have reported the effect of food consistency on orofacial development, suggesting that a diet with harder textures enhances bone and muscle growth, which could indirectly lead to better mastication efficiency. Finally, it was also reported that (1) children are more likely to accept textures that they are able to manipulate and (2) early exposure to a range of textures facilitates the acceptance of foods of various textures later on. Recommending products well adapted to children's mastication during weaning could facilitate their acceptance of new textures and support the development of healthy eating habits.

Cyclic hydrostatic pressure stimulates enhanced bone development in the foetal chick femur in vitro

Mechanical loading of bone and cartilage in vivo results in the generation of cyclic hydrostatic forces as bone compression is transduced to fluid pressure in the canalicular network and the joint synovium. It has therefore been suggested that hydrostatic pressure is an important stimulus by which osteochondral cells and their progenitors sense and respond to mechanical loading in vivo. In this study, hydrostatic pressure regimes of 0-279kPa at 0.005-2Hz were applied to organotypically cultured ex vivo chick foetal femurs (e11) for 1hour per day in a custom designed bioreactor for 14days and bone formation assessed by X-ray microtomography and qualified by histology. We found that the mineralised portion of the developing femur cultured under any cyclic hydrostatic pressure regime was significantly larger and/or denser than unstimulated controls but that constant (non-cycling) hydrostatic pressure had no effect on bone growth. Further experiments showed that the increase in bone formation was directly proportional to stimulation frequency (R(2)=0.917), but independent of the magnitude of the pressure applied, whilst even very low frequencies of stimulation (0.005Hz) had significant effects on bone growth. Expression of Type-II collagen in both epiphyses and diaphysis was significantly upregulated (1.48-fold and 1.95-fold respectively), together with osteogenic genes (osteonectin and osteopontin) and the osteocyte maturation marker CD44. This work demonstrates that cyclic hydrostatic pressure promotes bone growth and mineralisation in a developmental model and supports the hypothesis that hydrostatic forces play an important role in regulating bone growth and remodelling in vivo.

A mechanobiological model of epiphysis structures formation

Developing bone consists of epiphysis, metaphysis and diaphysis. The secondary ossification centre (SOC) appears and grows within the epiphysis, involving two histological stages. Firstly, cartilage canals appear; they carry hypertrophy factors towards the central area of the epiphysis. Canal growth and expansion is modulated by stress on the epiphysis. Secondly, the diffusion of hypertrophy factors causes SOC growth. Hypertrophy is regulated by biological and mechanical factors present within the epiphysis. The finite element method has been used for solving a coupled system of differential equations for modelling these histological stages of epiphyseal development. Cartilage canal spatial-temporal growth patterns were obtained as well as the SOC formation pattern. This model qualitatively agreed with experimental results reported by other authors.

Pneumatic pressure bioreactor for cyclic hydrostatic stress application: mechanobiology effects on periodontal ligament cells

A bioreactor system was developed to provide high-amplitude cyclic hydrostatic compressive stress (cHSC) using compressed air mixed commercially as needed to create partial pressures of oxygen and carbon dioxide appropriate for the cells under investigation. Operating pressures as high as 300 psi are achievable in this system at cyclic speeds of up to 0.2 Hz. In this study, ligamentous fibroblasts from human periodontal ligaments (n = 6) were compressed on two consecutive days at 150 psi for 3 h each day, and the mRNA for families of extracellular matrix protein and protease isoforms was evaluated by real-time PCR array. Several integrins were significantly upregulated, most notably alpha-3 (6.4-fold), as was SPG7 (12.1-fold). Among the collagens, Col8a1 was highly upregulated at 53.5-fold, with Col6a1, Col6a2, and Col7a1 also significantly upregulated 4.4- to 8.5-fold. MMP-1 was the most affected at 122.9-fold upregulation. MMP-14 likewise increased 17.8-fold with slight reductions for the gelatinases and a significant increase of TIMP-2 at 5.8-fold. The development of this bioreactor system and its utility in characterizing periodontal ligament fibroblast mechanobiology in intermediate-term testing hold promise for better simulating the conditions of the musculoskeletal system and the large cyclic compressive stresses joints may experience in gait, exertion, and mastication.

Effects of cyclic hydraulic pressure on osteocytes

Bone is able to adapt its composition and structure in order to suit its mechanical environment. Osteocytes, bone cells embedded in the calcified matrix, are believed to be the mechanosensors and responsible for orchestrating the bone remodeling process. Recent in vitro studies have shown that osteocytes are able to sense and respond to substrate strain and fluid shear. However the capacity of osteocytes to sense cyclic hydraulic pressure (CHP) associated with physiological mechanical loading is not well understood. In this study, we subjected osteocyte-like MLO-Y4 cells to controlled CHP of 68 kPa at 0.5 Hz, and investigated the effects of CHP on intracellular calcium concentration, cytoskeleton organization, mRNA expression of genes related to bone remodeling, and osteocyte apoptosis. We found that osteocytes were able to sense CHP and respond by increased intracellular calcium concentration, altered microtubule organization, a time-dependent increase in COX-2 mRNA level and RANKL/OPG mRNA ratio, and decreased apoptosis. These findings support the hypothesis that loading induced cyclic hydraulic pressure in bone serves as a mechanical stimulus to osteocytes and may play a role in regulating bone remodeling in vivo.

Effect of food consistency on the degree of mineralization in the rat mandible

A switch to a soft diet, associated with reduced forces applied to the mandible during mastication, may result in an alteration of the degree of mineralization in the mandible. This alteration may be regionally different. The aim of this study was to analyze this alteration by examination of the degree of mineralization in the mandible of growing rats fed with a hard or soft diet. Fifteen Wistar male rats were used in this investigation. After weaning, six rats were fed with a hard diet and the remaining nine rats with a soft diet. After 9 weeks, three-dimensional reconstructions of the cortical and trabecular bone of their mandibles were obtained using a microCT system. The degree of mineralization was determined for the trabecular bone in the condyle and for the cortical bone in the anterior and posterior areas of the mandibular body. In both diet groups the degree of mineralization was significantly (p < 0.01) lower in the trabecular than in the cortical bone. In the mandibular body, the anterior area showed a significantly (p < 0.01) higher degree of mineralization than the posterior area in both diet groups. In both areas the soft diet group had a significantly (p < 0.05 or 0.01) higher degree of mineralization than the hard diet group. The trabecular bone in the condyle of the hard diet group showed a significantly (p < 0.01) higher degree of mineralization than in the soft diet group. The present results indicate the importance of proper masticatory muscle function for craniofacial growth and development.

Endochondral ossification in vitro is influenced by mechanical bending

Bone development is influenced by the local mechanical environment. Experimental evidence suggests that altered loading can change cell proliferation and differentiation in chondro- and osteogenesis during endochondral ossification. This study investigated the effects of three-point bending of murine fetal metatarsal bone anlagen in vitro on cartilage differentiation, matrix mineralization and bone collar formation. This is of special interest because endochondral ossification is also an important process in bone healing and regeneration. Metatarsal preparations of 15 mouse fetuses stage 17.5 dpc were dissected en bloc and cultured for 7 days. After 3 days in culture to allow adherence they were stimulated 4 days for 20 min twice daily by a controlled bending of approximately 1000-1500 microstrain at 1 Hz. The paraffin-embedded bone sections were analyzed using histological and histomorphometrical techniques. The stimulated group showed an elongated periosteal bone collar while the total bone length was not different from controls. The region of interest (ROI), comprising the two hypertrophic zones and the intermediate calcifying diaphyseal zone, was greater in the stimulated group. The mineralized fraction of the ROI was smaller in the stimulated group, while the absolute amount of mineralized area was not different. These results demonstrate that a new device developed to apply three-point bending to a mouse metatarsal bone culture model caused an elongation of the periosteal bone collar, but did not lead to a modification in cartilage differentiation and matrix mineralization. The results corroborate the influence of biophysical stimulation during endochondral bone development in vitro. Further experiments with an altered loading regime may lead to more pronounced effects on the process of endochondral ossification and may provide further insights into the underlying mechanisms of mechanoregulation which also play a role in bone regeneration.

Bone regeneration in mandibular distraction osteogenesis combined with compression stimulation

PURPOSE

This study compared modified distraction osteogenesis (DO) protocol with conventional DO protocol on healing bone formation. Computer simulation was performed to understand the mechanical environment of modified DO protocol, which applies compression during the consolidation period.

MATERIALS AND METHODS

Fifty rats were used in this study. Twenty-five rats in the conventional DO (control) group were sacrificed at postoperative days 11, 21, 28, 35, and 49 after osteotomy. In the modified DO (experimental) group, compression was applied on day 7 after distraction (day 18 postoperatively) for 4 days during the early consolidation period and 25 rats were sacrificed on postoperative day 19, 28, 39, 46, and 53. The histologic and radiographic findings were used to compare the 2 groups. Further, computer simulation was used to predict the mechanical environment of healing bone under conventional and modified DO protocol.

RESULTS

Radiographic findings showed that the experimental group resulted in denser and wider healing bone. Histologically, the experimental group yielded more mature lamellar bone than the control group. Computer simulation showed that absolute values of tissue strains were nearly double in the control group because of the softer healing tissues. Both the experimental and control groups showed high strains at the ridge crest. Concentrated tensile strain along the distraction direction at the ridge crest might hinder bone formation at the interface, while compressive strain could facilitate the process.

CONCLUSION

This study proposed a modified DO protocol of adding compression during the early consolidation period of conventional DO protocol. This new technique appears to provide faster and denser bone regeneration.

Accelerated closure of biopsy-type wounds by mechanical stimulation

OBJECTIVE

To determine whether a device designed to provide low-intensity, low-frequency mechanical stimulation improves healing time of acute wounds.

DESIGN

Repeated measures using mechanical stimulation on one side of a rat and sham stimulation on the contralateral side.

SETTING

Academic animal facility.

PARTICIPANTS

Six male Sprague-Dawley rats, approximately 400 g.

INTERVENTION

Mechanical stimulation of 4-mm biopsy wounds in rats was produced through the use of permanent magnets cyclically attracted and repelled by activation of an electromagnet by a square wave generator at a frequency of 1 Hz and a force equivalent to 64 mm Hg pressure.

MAIN OUTCOME MEASURE

Days to complete closure of 4-mm biopsy punch wounds.

MAIN RESULTS

This form of stimulation reduced time to close the biopsy wounds by nearly 50%. Mechanically stimulated wounds closed in 3.8 +/- 1.6 days (mean +/- SD) compared with 6.8 +/- 1.9 days for sham-stimulated wounds (P = .0002).

CONCLUSION

Production of a mechanical stimulation device with a miniaturized controller and power source and trials on humans are needed to determine the efficacy and potential cost savings of such a device in the management of wounds.

Expandable vertebral body replacement in patients with thoracolumbar spine tumors

INTRODUCTION

The objectives of surgical interventions for tumoral lesions of the spine include the establishment and improvement of tumor-related symptoms. Anterior tumor resection followed by reconstruction indicated if surgical treatment allowed a marginal removal of the tumor or could extend the individual survival rate in combination with adjuvant therapy options. Sufficient re-stabilization depends on adequate anterior column reconstruction. The purpose of this retrospective study was to present our experiences and results after anterior tumor resection followed by reconstruction with the expandable vertebral body replacement device (VBR, Ulrich, Germany) based on clinical application over 4 degrees years.

PATIENTS AND METHODS

We carried out an anterior tumor resection followed by reconstruction using an anterior extendable device in 32 patients with different spine tumors between 1996 and 2000. A retrospective evaluation was executed considering the patients medical records and radiological findings. Additionally, a clinical and radiological investigation of still living postoperative patients was carried out.

RESULTS

The mean surgical time of all evaluated patients was 317.2 min. The average blood loss was 1,272.5 ml. According to the Tokuhashi score, patients with a postoperative survival time of at least 12 months demonstrated a score value > or = 9 points. According to our evaluated patients group metastatic lesions of the spine represented the largest group (78.1%). The average survival rate of this group amounted to 18.4 months postoperatively. Considering primary tumors the average survival rate at the time of last re-examination amounted to 34.8 months postoperatively. Preoperative neurological pathologies were present in 12 patients (Frankel stage C-D). During the postoperative monitoring period 58.3% of the patients demonstrated an improvement in initial neurological findings. There were no intraoperative complications or perioperative deaths. Implant dislocations were not observed.

CONCLUSION

On account of the underlying, the anterior tumor resection with supplementary instrumentation represented a sufficient procedure in spinal tumor surgery. Adjuvant therapy can influence the postoperative survival period positively in addition to the surgical procedure. Following anterior tumor resection, extendable vertebral body replacements like the VBR device provide immediate spine stability by excellent defect adaptation. With regard to their intraoperative flexibility, expandable cages are more advantageous in contrast to non-expandable implants or bone grafts.

The mechanical consequences of mineralization in embryonic bone

The purpose of this study was to examine the effect of mineralization on the mechanical properties of embryonic bone rudiments. For this purpose, four-point bending experiments were performed on unmineralized and mineralized embryonic mouse ribs at 16 and 17 days of gestational age. Young's modulus was calculated using force-displacement data from the experiment in combination with finite element analysis (FEA). For the unmineralized specimens, a calculated average for the Young's modulus of 1.11 (+/- 0.62) MPa was established after corrections for sticking to the four-point bending device and aspect ratio, which is the ratio between the length of the bone and its diameter. For the mineralized specimens, the value was 117 (+/- 62) MPa after corrections. Hence, Young's moduli of embryonic bone rudiments increase by two orders of magnitude within 1 day, during endochondral ossification. As an effect, the hypertrophic chondrocytes in the calcifying cartilage experience a significant change in their mechanical environment. The chondrocytes are effectively stress shielded, which means that they do not carry stresses since stresses are supported by the stiffest parts of the tissue, which are in this case the diaphyseal cortex and the calcified matrix. The deformability of the hypertrophic chondrocytes is, therefore, severely reduced. Since the transition is so sudden and enormous, it can be seen as a process of 'catastrophic' proportion for the hypertrophic chondrocytes. The subsequent resorption of calcified cartilage and the expansion of the marrow cavity could be consequential to stress shielding.

Pressure simulation of orthodontic force in osteoblasts: a pilot study

OBJECTIVES

To elucidate the RUNX2 gene expression induction in human osteoblasts after mechanical loading.

DESIGN

Using a stringent pulse-chase protocol human osteoblasts were exposed to centrifugal pressure force for 30 and 90 min. Untreated control cells were processed in parallel. Before, and at defined times after centrifugation, total RNA was isolated. RUNX2 gene expression was measured using real-time quantitative reverse transcriptase polymerase chain reaction. The stress/control ratio was used to illustrate possible stimulatory or diminishing effects of force application.

RESULTS

Immediately after 30 min of force application the RUNX2 gene expression was induced by a factor of 1.7 +/- 0.14 as compared with the negative control. This induction decreased rapidly and reached its pre-load levels within 30 min. Longer force applications (up to 90 min) did not change the RUNX2 gene expression.

CONCLUSION

In mature osteoblasts centrifugal pressure force stimulates RUNX2 gene expression within a narrow time frame: loading of mature cells results in a temporary increase of RUNX2 expression and a fast downregulation back to its pre-load expression level. With this pilot study the gene expression behavior after mechanical stimuli could be determined with a simple laboratory setup.

Repetitive, negligible force reaching in rats induces pathological overloading of upper extremity bones

Work-related repetitive motion disorders are costly. Immunohistochemical changes in bones resulting from repetitive reaching and grasping in 17 rats were examined. After 3-6 weeks, numbers of ED1+ macrophages and osteoclasts increased at periosteal surfaces of sites of muscle and interosseous membrane attachment and metaphyses of reach and nonreach forelimbs. These findings indicate pathological overloading leading to inflammation and subsequent bone resorption.

INTRODUCTION

Sixty-five percent of all occupational illnesses in U.S. private industry are attributed to musculoskeletal disorders arising from the performance of repeated motion, yet the precise mechanisms of tissue pathophysiology have yet to be determined for work-related musculoskeletal disorders. This study investigates changes in upper extremity bone tissues resulting from performance of a voluntary highly repetitive, negligible force reaching and grasping task in rats.

MATERIALS AND METHODS

Seventeen rats reached an average of 8.3 times/minute for 45-mg food pellets for 2 h/day, 3 days/week for up to 12 weeks. Seven rats served as normal or trained controls. Radius, ulna, humerus, and scapula were collected bilaterally as follows: radius and ulna at 0, 3, 4, 5, 6, and 12 weeks and humerus and scapula at 0, 4, and 6 weeks. Bones were examined for ED1-immunoreactive mononuclear cells and osteoclasts. Double-labeling immunohistochemistry was performed for ED1 (monocyte/macrophage lineage cell marker) and TRACP (osteoclast marker) to confirm that ED1+ multinucleated cells were osteoclasts. Differences in the number of ED1+ cells over time were analyzed by ANOVA.

RESULTS

Between 3 and 6 weeks of task performance, the number of ED1+ mononuclear cells and osteoclasts increased significantly at the periosteal surfaces of the distal radius and ulna of the reach and nonreach limbs compared with control rats. These cells also increased at periosteal surfaces of humerus and scapula of both forelimbs by 4-6 weeks. These cellular increases were greatest at muscle attachments and metaphyseal regions, but they were also present at some interosseous membrane attachments. The number of ED1+ cells decreased to control levels in radius and ulna by 12 weeks.

CONCLUSIONS

Increases in ED1+ mononuclear cells and osteoclasts indicate that highly repetitive, negligible force reaching causes pathological overloading of bone leading to inflammation and osteolysis of periosteal bone tissues.

Signaling in morphogenesis: transport cues in morphogenesis

Extracellular transport processes play critical roles in morphogenesis. While diffusive transport effects on morphogenesis are well illustrated in examples like blood capillary architecture and in cell morphogenetic responses to the local extracellular protein environment, the effects of fluid convection, although important in many developing and regenerating tissues, are not well understood. Convective forces are present whenever a hydrated tissue undergoes dynamic mechanical strain, and so convection could not only dominate the transport of large molecules like proteins, but might also serve as a mechanism for mechanosensing. The complex interdependence of mechanical forces, protein transport and extracellular morphogen gradients needs to be elucidated in a comprehensive way in order for the importance of transport on morphogenesis to be fully appreciated.

Dynamic compressive properties of porcine temporomandibular joint disc

This study aimed to evaluate the effect of the strain frequency and amplitude on the compressive properties of the porcine temporomandibular joint disc and to determine the time-dependent changes associated with energy dissipation. Seven discs were used for compressive cycle tests, including various frequencies and magnitudes of compressive strain. Each experiment consisted of 25 cycles of loading and unloading. Hysteresis and the instantaneous and steady moduli were calculated. All specimens showed a clear hysteresis and repeatable stress-strain relationships within 19 cycles. The hysteresis at the initial cycle ranged between 35% and 62%, and gradually decreased in subsequent cycles. The instantaneous modulus became larger when the strain frequency and the strain amplitude increased. The steady modulus was approximately one-third of the instantaneous one. It was concluded that the disc has an energy-dissipating function during dynamic compression.

Impulsive compression influences the viscous behavior of porcine temporomandibular joint disc

Traumatic joint injury is known to produce osteoarthritic degeneration in the temporomandibular joint (TMJ). However, little information, is available on its possible effect on the dynamic viscoelastic behavior of the disc. In the present study, impulsive compression was applied to the disc as a model for traumatic joint injury. This was tested in 32 porcine discs. Prior to a dynamic tensile test, impulsive compression with strain rates of 0.01, 0.1 and 1 s(-1) to a final strain of 30% was applied to these discs. Tensile stress was applied in the antero-posterior direction with a wide range of frequencies (0.1-100 Hz). The dynamic E-moduli increased as the loading frequency increased. The dynamic viscosity and elasticity decreased with an increase of strain rate, although the effect on viscosity was greater than on elasticity. The results suggest that strain rate and subsequent peak stress of impulsive compression affect primarily the dynamic viscosity of the disc, and that impulsive compression at high strain rates reduces the resistance of the TMJ disc to stress, resulting in permanent disc deformation and perforation.

Biomechanical behavior of the temporomandibular joint disc

The temporomandibular joint (TMJ) disc consists mainly of collagen fibers and proteoglycans constrained in the interstices of the collagen fiber mesh. This construction results in a viscoelastic response of the disc to loading and enables the disc to play an important role as a stress absorber during function. The viscoelastic properties depend on the direction (tension, compression, and shear) and the type of the applied loading (static and dynamic). The compressive elastic modulus of the disc is smaller than its tensile one because the elasticity of the disc is more dependent on the collagen fibers than on the proteoglycans. When dynamic loading occurs, the disc is likely to behave less stiffly than under static loading because of the difference of fluid flow through and out of the disc during loading. In addition, the mechanical properties change as a result of various intrinsic and extrinsic factors in life such as aging, trauma, and pathology. Information about the viscoelastic behavior of the disc is required for its function to be understood and, for instance, for a suitable TMJ replacement device to be constructed. In this review, the biomechanical behavior of the disc in response to different loading conditions is discussed.

A central role for the nuclear factor-kappaB pathway in anti-inflammatory and proinflammatory actions of mechanical strain

Mechanical signals play an integral role in bone homeostasis. These signals are observed at the interface of bone and teeth, where osteoblast-like periodontal ligament (PDL) cells constantly take part in bone formation and resorption in response to applied mechanical forces. Earlier, we reported that signals generated by tensile strain of low magnitude (TENS-L) are antiinflammatory, whereas tensile strain of high magnitude (TENS-H) is proinflammatory and catabolic. In this study, we examined the mechanisms of intracellular actions of the antiinflammatory and proinflammatory signals generated by TENS of various magnitudes. We show that both low and high magnitudes of mechanical strain exploit nuclear factor (NF)-kappaB as a common pathway for transcriptional inhibition/activation of proinflammatory genes and catabolic processes. TENS-L is a potent inhibitor of interleukin (IL)-1 beta-induced I-kappaBbeta degradation and prevents dissociation of NF-kB from cytoplasmic complexes and thus its nuclear translocation. This leads to sustained suppression of IL-1beta-induced NF-kappaB transcriptional regulation of proinflammatory genes. In contrast, TENS-H is a proinflammatory signal that induces I-kappaBbeta degradation, nuclear translocation of NF-kappaB, and transcriptional activation of proinflammatory genes. These findings are the first to describe the largely unknown intracellular mechanism of action of applied tensile forces in osteoblast-like cells and have critical implications in bone remodeling.

Biomechanical response of bovine temporomandibular joint disc to prolonged tensile stress

This study was designed to evaluate the influence of prolonged tensile stress on the viscoelasticity of the temporomandibular joint (TMJ) disc. Twenty discs from 10, 3-year-old cattle were used. Tensile stress of 1.5 MPa was applied to specimens from the discs for 10, 20, 40 and 60 min. Following the prescribed period of tension for creep, the specimens were removed from the tension device and any recovery observed for 20 min. In all specimens, strain increased at the onset of stress application and reached almost steady conditions after 5 min. Although, the strain became slightly larger when the creep time was longer, no significant differences were found in the strains between any two tests with different periods of creep. The residual strain increased significantly with creep duration, and similarly the degree of recovery decreased significantly. In 10- and 20-min creep tests, the residual strains were 0.1 and 1.0%, the specimens in 40- and 60-min tests revealed irreversible changes in length. It was concluded that continuous loading for >40 min causes creep damage in bovine TMJ disc, and that prolonged sustained tension affects the recovery of joint homeostasis.

Signaling by mechanical strain involves transcriptional regulation of proinflammatory genes in human periodontal ligament cells in vitro

Intracellular signals generated by mechanical strain profoundly affect the metabolic function of osteoblast-like periodontal ligament (PDL) cells, which reside between the tooth and alveolar bone. In response to applied mechanical forces, PDL cells synthesize bone-resorptive cytokines to induce bone resorption at sites exposed to compressive forces and deposit bone at sites exposed to tensile forces in an environment primed for catabolic processes. The intracellular mechanisms that regulate this bone remodeling remain unclear. Here, in an in vitro model system, we show that tensile strain is a critical determinant of PDL-cell metabolic functions. Equibiaxial tensile strain (TENS), when applied at low magnitudes, acts as a potent antagonist of interleukin (IL)-1beta actions and suppresses transcriptional regulation of multiple proinflammatory genes. This is evidenced by the fact that TENS at low magnitude: (i) inhibits recombinant human (rh)IL-1beta-dependent induction of cyclooxygenase-2 (COX-2) mRNA expression and production of prostaglandin estradiol (PGE2); (ii) inhibits rhIL-1beta-dependent induction matrix metalloproteinase-1 (MMP-1) and MMP-3 synthesis by suppressing their mRNA expression; (iii) abrogates rhIL-1beta-induced suppression of tissue inhibitor of metalloprotease-II (TIMP-II) expression; and (iv) reverses IL-1beta-dependent suppression of osteocalcin and alkaline phosphatase synthesis. Nevertheless, these actions of TENS were observed only in the presence of IL-1beta, as TENS alone failed to affect any of the aforementioned responses. The present findings are the first to show that intracellular signals generated by low-magnitude mechanical strain interfere with one or more critical step(s) in the signal transduction cascade of rhIL-1beta upstream of mRNA expression, while concurrently promoting the expression of osteogenic proteins in PDL cells.

Characteristics of in vitro osteoblastic cell loading models

Normal loading strains of 200-2000 (mu)epsilon to bone result in bending forces, generating mechanical stretch and pressure gradients in canaliculi that drive extracellular fluid flow, resulting in stress on the membranes of osteocytes, lining cells, and osteoblasts. Under excess loading, as well as during unloading (e.g., microgravity, bed rest), the fluid shift and resultant change in interstitial fluid flow may play a larger role in bone remodeling than mechanical stretch. The in vitro model systems used to investigate mechanical loading of bone generate either fluid shear, hydrostatic compression, biaxial stretch, uniaxial stretch, or a combination of two or more of these forces. The results of in vitro experiments suggest that fluid shear is a major factor affecting bone cell metabolism. Both the flow-loop apparatus (which produces pulsatile flow and uses fluid shear as its principal stimulus) and the uniaxial silicone plate stretching apparatus (which generates cyclic stretch) create a reproducible and consistent stimulus. Endpoints measured in flow experiments, however, are short term and usually short lived, and it is unknown whether these changes impact the function of differentiated osteoblasts. Endpoints measured in uniaxial stretch experiments are generally long-term-sustained effects of mechanical perturbation and more easily relatable to changes in osteoblastic activity. Biaxial stretch devices create both bending and compressive forces, resulting in different types of force on the cells, with the relative amount of each depending on the position of the cell in the device. Therefore, systems that incorporate pulsatile fluid flow or uniaxial stretch as the principal stimulus should be further developed and implemented in the study of the relationship between mechanical loading and bone response.

Viscoelastic properties and residual strain in a tensile creep test on bovine temporomandibular articular discs

This study was designed to evaluate the creep characteristics and residual strain of bovine temporomandibular joint (TMJ) discs in tension. Twenty discs were divided into three specimens each: central, lateral and medial regions. Tension of 1.0 MPa was applied and sustained for 20 min to the specimens from 10 right-side discs, and tension of 1.5 MPa to specimens from 10 left-side discs. After the period of tension for creep, the specimens were removed from the tension devices and restoration observed for 20 min. Time-dependent creep curves showed a marked change in strain during the initial 5s. The essential time delay in strain ceased after 2 min, and strain reached an almost steady level after 3 min. At a tensile stress of 1.5 MPa, a strain of 14.5% on average was produced after 20 min creep in the central specimens; peripheral specimens showed strains of 12.4% on average. There were significant differences in strain between the central and peripheral specimens. The residual strain after 20 min restoration was 0.93% on average and there were no significant regional differences. This creep feature could be well represented by a generalized linear viscoelastic model. It was concluded that the regional differences in viscoelasticity might be caused by the complicated articulating functions of the TMJ, and that the residual strain caused by sustained stress could be an important factor in disc deformation.