Diaphyseal bone formation in murine tibiae in response to knee loading

Influence of intramedullary pressure on Lacuno-Canalicular fluid flow: A systematic review

While most of current models investigating bone remodelling are based on matrix deformation, intramedullary pressure also plays a role. Bone remodelling is orchestrated by the Lacuno-Canalicular Network (LCN) fluid-flow. The aim of this review was hence to assess the influence of intramedullary pressure on the fluid circulation within the LCN. Three databases (Science Direct, Web of Science, and PubMed) were used. The first phase of the search returned 731 articles, of which 9 respected the inclusion/exclusion criteria and were included. These studies confirm the association between intramedullary pressure and fluid dynamics in the LCN. Among the included studies, 7 experimental studies using animal models and 2 numerical models were found. The studies were then ranked according to the nature of the applied loading, either axial compression or direct cyclic intramedullary pressure. The current review revealed that there is an influence of intramedullary pressure on LCN fluid dynamics and that this influence depends on the magnitude and the frequency of the applied pressure. Two studies confirmed that the influence was effective even without bone matrix deformation. While intramedullary pressure is closely associated with LCN fluid, there is a severe lack of studies on this topic. STATEMENT OF SIGNIFICANCE: Since the 1990's, numerical models developed to investigate fluid flow in bone submicrometric porous network are based on the flow induced by matrix deformation. Bone fluid flow is known to be involved in cells stimulation and hence directly influences bone remodeling. Different studies have shown that intramedullary pressure is also associated with bone mechanosensitive adaptation. This pressure is developed in bone due to blood circulation and is increased during loading or muscle stimulation. The current article reviews the studies investigating the influence of this pressure on bone porous fluid flow. They show that fluid flow is involved by this pressure even without bone matrix deformation. The current review article highlights the severe lack of studies about this mechanism.

Effect of load-induced local mechanical strain on peri-implant bone cell activity related to bone resorption and formation in mice: An analysis of histology and strain distributions

The purpose of this study was to investigate the effect of load-induced local mechanical strain on bone cell activity of peri-implant bone in mice. Titanium implants were placed in the maxillae of 13-week-old male C57BL/6J mice and subjected to intermittent 0.15 N, 0.3 N, or 0.6 N loads for 30 min/day for 6 days. The animals were sacrificed 2 days after the final loading. Unloaded mice were used as controls. An animal-specific three-dimensional finite element model was constructed based on morphological data retrieved from in vivo microfocus computed tomography for each mouse to calculate the mechanical strain distribution. Strain distribution images were overlaid on corresponding histological images of the same site in the same animal. The buccal cervical region of the peri-implant bone was predetermined as the region of interest (ROI). Each ROI was divided by four strain intensity levels: 0-20 με, 20-60 με, 60-100 με, and ≥100 με, and the bone histomorphometric parameters were analyzed by the total area of each strain range for all loaded samples. The distance between the calcified front and calcein labeling as a parameter representing the mineral apposition rate was significantly greater in the areas with strain intensity ≥100 με than in the area with strain intensity <100 με, suggesting that the bone formation activity of osteoblasts was locally enhanced by a higher mechanical strain. However, the shrunken osteocytes and the empty osteocyte lacunae were significantly lower in the highest strain area, suggesting that osteoclastogenesis was more retarded in higher strain areas than in lower strain areas. The histomorphometric parameters were not affected geometrically in the unloaded animals, suggesting that the load-induced mechanical strain caused differences in the histomorphometric parameters. Our findings support the hypothesis that bone cell activity related to bone resorption and formation is local strain-dependent on implant loading.

Mechanical stimulations can inhibit local and remote tumor progression by downregulating WISP1

Mechanical stimulations can prevent bone loss, but their effects on the tumor-invaded bone or solid tumors are elusive. Here, we evaluated the effect of knee loading, dynamic loads applied to the knee, on metastasized bone and mammary tumors. In a mouse model, tumor cells were inoculated to the mammary fat pad or the proximal tibia. Daily knee loading was then applied and metabolic changes were monitored mainly through urine. Urine samples were also collected from human subjects before and after step aerobics. The result showed that knee loading inhibited tumor progression in the loaded tibia. Notably, it also reduced remotely the growth of mammary tumors. In the urine, an altered level of cholesterol was observed with an increase in calcitriol, which is synthesized from a cholesterol derivative. In urinary proteins, knee loading in mice and step aerobics in humans markedly reduced WNT1-inducible signaling pathway protein 1, WISP1, which leads to poor survival among patients with breast cancer. In the ex vivo breast cancer tissue assay, WISP1 promoted the growth of cancer fragments and upregulated tumor-promoting genes, such as Runx2, MMP9, and Snail. Collectively, the present preclinical and human study demonstrated that mechanical stimulations, such as knee loading and step aerobics, altered urinary metabolism and downregulated WISP1. The study supports the benefit of mechanical stimulations for locally and remotely suppressing tumor progression. It also indicated the role of WISP1 downregulation as a potential mechanism of loading-driven tumor suppression.

An Implanted Magnetic Microfluidic Pump for In Vivo Bone Remodeling Applications

Modulations of fluid flow inside the bone intramedullary cavity has been found to stimulate bone cellular activities and augment bone growth. However, study on the efficacy of the fluid modulation has been limited to external syringe pumps connected to the bone intramedullary cavity through the skin tubing. We report an implantable magnetic microfluidic pump which is suitable for in vivo studies in rodents. A compact microfluidic pump (22 mm diameter, 5 mm in thickness) with NdFeB magnets was fabricated in polydimethylsiloxane (PDMS) using a set of stainless-steel molds. An external actuator with a larger magnet was used to wirelessly actuate the magnetic microfluidic pump. The characterization of the static pressure of the microfluidic pump as a function of size of magnets was assessed. The dynamic pressure of the pump was also characterized to estimate the output of the pump. The magnetic microfluidic pump was implanted into the back of a Fischer-344 rat and connected to the intramedullary cavity of the femur using a tube. On-demand wireless magnetic operation using an actuator outside of the body was found to induce pressure modulation of up to 38 mmHg inside the femoral intramedullary cavity of the rat.

Signalling molecule transport analysis in lacunar-canalicular system

Mechanical loading-induced fluid flow in lacunar-canalicular space (LCS) of bone excites osteocyte cells to release signalling molecules which initiate osteo-activities. Theoretical models considered canaliculi as a uniform and symmetrical space/channel in bone. However, experimental studies reported that canalicular walls are irregular and curvy resulting in inhomogeneous fluid motion which may influence the molecular transport. Therefore, a new mathematical model of LCS with curvy canalicular walls is developed to characterize cantilever bending-induced canalicular flow behaviour in terms of pore-pressure, fluid velocity, and streamlines. The model also analyses the mobility of signalling molecules involved in bone mechanotransduction as a function of loading frequency and permeability of LCS. Inhomogeneous flow is observed at higher loading frequency which amplifies mechanotransduction; nevertheless, it also promotes trapping of signalling molecules. The effects of shape and size of signalling molecules on transport behaviour are also studied. Trivially, signalling molecules larger in size and weight move slower as compared to molecules small in size and weight which validates the findings of the present study. The outcomes will ultimately be useful in designing better biomechanical exercise in combination with pharmaceutical agents to improve the bone health.

Wnt3a involved in the mechanical loading on improvement of bone remodeling and angiogenesis in a postmenopausal osteoporosis mouse model

Osteoporosis is a major health problem, making bones fragile and susceptible to fracture. Previous works showed that mechanical loading stimulated bone formation and accelerated fracture healing. Focusing on the role of Wnt3a (wingless/integrated 3a), this study was aimed to assess effects of mechanical loading to the spine, using ovariectomized (OVX) mice as a model of osteoporosis. Two-week daily application of this novel loading (4 N, 10 Hz, 5 min/d) altered bone remodeling with an increase in Wnt3a. Spinal loading promoted osteoblast differentiation, endothelial progenitor cell migration, and tube formation and inhibited osteoclast formation, migration, and adhesion. A transient silencing of Wnt3a altered the observed loading effects. Spinal loading significantly increased bone mineral density, bone mineral content, and bone area per tissue area. The loaded OVX group showed a significant increase in the number of osteoblasts and reduction in osteoclast surface/bone surface. Though expression of osteoblastic genes was increased, the levels of osteoclastic genes were decreased by loading. Spinal loading elevated a microvascular volume as well as VEGF expression. Collectively, this study supports the notion that Wnt3a-mediated signaling involves in the effect of spinal loading on stimulating bone formation, inhibiting bone resorption, and promoting angiogenesis in OVX mice. It also suggests that Wnt3a might be a potential therapeutic target for osteoporosis treatment.-Li, X., Liu, D., Li, J., Yang, S., Xu, J., Yokota, H., Zhang, P. Wnt3a involved in the mechanical loading on improvement of bone remodeling and angiogenesis in a postmenopausal osteoporosis mouse model.

Mechanical loading mitigates osteoarthritis symptoms by regulating endoplasmic reticulum stress and autophagy

Osteoarthritis (OA) is a disease characterized by cartilage damage and abnormal remodeling of subchondral bone. Our previous study showed that in the early stage of OA, knee loading exerts protective effects by suppressing osteoclastogenesis through Wnt signaling, but little is known about loading effects at the late OA stage. Endoplasmic reticulum (ER) stress and autophagy are known to be involved in the late OA stage. We determined the effects of mechanical loading on ER stress and autophagy in OA mice. One hundred seventy-four mice were used for a surgery-induced OA model. In the first set of experiments, 60 mice were devoted to evaluation of the role of ER stress and autophagy in the development of OA. In the second set, 114 mice were used to assess the effect of knee loading on OA. Histologic, cellular, microcomputed tomography, and electron microscopic analyses were performed to evaluate morphologic changes, ER stress, and autophagy. Mechanical loading increased phosphorylation of eukaryotic translation initiation factor 2α (eIF2α) and regulated expressions of autophagy markers LC3II/I and p62. Osteoarthritic mice also exhibited an elevated ratio of calcified cartilage to total articular cartilage (CC/TAC), and synovial hyperplasia with increased lining cells was found. At the early disease stage, subchondral bone plate thinning and reduced subchondral bone volume fraction (B.Ar/T.Ar) were observed. At the late disease stages, subchondral bone plate thickened concomitant with increased B.Ar/T.Ar. Mice subjected to mechanical loading exhibited resilience to cartilage destruction and a correspondingly reduced Osteoarthritis Research Society International score at 4 and 8 wk, as well as a decrease in synovitis and CC/TAC. While chondrocyte numbers in the OA group was notably decreased, mechanical loading restored chondrogenic differentiation. These results demonstrate that mechanical loading can retard the pathologic progression of OA at its early and late stages. The observed effects of loading are associated with the regulations of ER stress and autophagy.-Zheng, W., Li, X., Liu, D., Li, J., Yang, S., Gao, Z., Wang, Z., Yokota, H., Zhang, P. Mechanical loading mitigates osteoarthritis symptoms by regulating endoplasmic reticulum stress and autophagy.

Effects of knee loading on obesity-related non-alcoholic fatty liver disease in an ovariectomized mouse model with high-fat diet

AIM

Hormonal and nutritional disorders are the main causes of obesity and non-alcoholic fatty liver disease, especially in the elderly and in postmenopausal women. Although physical activity might alleviate these disorders, the elderly may often have difficulty in carrying out physical exercise. The purpose of this study was to investigate the therapeutic effect of knee loading, a new form of physical stimulation, on the symptoms of obesity and fatty liver.

METHODS

Using ovariectomized mice fed a high-fat diet, we evaluated the effect of knee loading that applies gentle cyclic loads to the knee. Female C57BL/6 mice were divided into five groups: control (SCD), high-fat diet (HF), HF with loading (HF + L), HF with ovariectomy (HF + OVX), and HF + OVX with loading (HF + OVX + L). Except for SCD, mice underwent sham operation or ovariectomy and were maintained on HF diet. After 6 weeks, the mice in the HF + L and HF + OVX + L groups were treated with knee loading for 6 weeks.

RESULTS

Compared to the obesity groups (HF and HF + OVX), knee loading significantly decreased a gain in body weight, liver weight, and white adipose tissue (all P < 0.01). It also reduced the lipid level in the serum (P < 0.01) and histological severity of hepatic steatosis (P < 0.01). Furthermore, knee loading downregulated biomarkers related to endoplasmic reticulum (ER) stress (GRP78, p-eIF2α, and ATF4) and altered biomarkers in autophagy (LC3 and p62).

CONCLUSIONS

Knee loading suppressed obesity-associated metabolic alterations and hepatic steatosis. These effects with knee loading might be associated with suppression of ER stress and promotion of autophagy.

Bone mechanobiology in mice: toward single-cell in vivo mechanomics

Mechanically driven bone (re)modeling is a multiscale process mediated through complex interactions between multiple cell types and their microenvironments. However, the underlying mechanisms of how cells respond to mechanical signals are still unclear and are at the focus of the field of bone mechanobiology. Traditionally, this complex process has been addressed by reducing the system to single scales and cell types. It is only recently that more integrative approaches have been established to study bone mechanobiology across multiple scales in which mechanical load at the organ level is related to molecular responses at the cellular level. The availability of mouse loading models and imaging techniques with improved spatial and temporal resolution has made it possible to track dynamic bone (re)modeling at the tissue and cellular level in vivo. Coupled with advanced computational models, the (re)modeling activities at the tissue scale can be associated with the mechanical microenvironment. However, methods are lacking to link the molecular responses of different cell types to their local mechanical microenvironment and bone (re)modeling activities occurring at the tissue scale. With recent improvements in "omics" technologies and single-cell molecular biology, it is now possible to sequence the complete genome and transcriptome of single cells. These technologies offer unique opportunities to comprehensively investigate the cellular transcriptional profiles within their specific microenvironment. By combining single-cell "omics" technologies with well-established tissue-scale models of bone mechanobiology, we propose a mechanomics approach to locally analyze the transcriptome of single cells with respect to their local 3D mechanical in vivo environment.

Understanding Mechanobiology: Physical Therapists as a Force in Mechanotherapy and Musculoskeletal Regenerative Rehabilitation

Achieving functional restoration of diseased or injured tissues is the ultimate goal of both regenerative medicine approaches and physical therapy interventions. Proper integration and healing of the surrogate cells, tissues, or organs introduced using regenerative medicine techniques are often dependent on the co-introduction of therapeutic physical stimuli. Thus, regenerative rehabilitation represents a collaborative approach whereby rehabilitation specialists, basic scientists, physicians, and surgeons work closely to enhance tissue restoration by creating tailored rehabilitation treatments. One of the primary treatment regimens that physical therapists use to promote tissue healing is the introduction of mechanical forces, or mechanotherapies. These mechanotherapies in regenerative rehabilitation activate specific biological responses in musculoskeletal tissues to enhance the integration, healing, and restorative capacity of implanted cells, tissues, or synthetic scaffolds. To become future leaders in the field of regenerative rehabilitation, physical therapists must understand the principles of mechanobiology and how mechanotherapies augment tissue responses. This perspective article provides an overview of mechanotherapy and discusses how mechanical signals are transmitted at the tissue, cellular, and molecular levels. The synergistic effects of physical interventions and pharmacological agents also are discussed. The goals are to highlight the critical importance of mechanical signals on biological tissue healing and to emphasize the need for collaboration within the field of regenerative rehabilitation. As this field continues to emerge, physical therapists are poised to provide a critical contribution by integrating mechanotherapies with regenerative medicine to restore musculoskeletal function.

Knee loading protects against osteonecrosis of the femoral head by enhancing vessel remodeling and bone healing

Osteonecrosis of the femoral head is a serious orthopedic problem. Moderate loads with knee loading promote bone formation, but their effects on osteonecrosis have not been investigated. Using a rat model, we examined a hypothesis that knee loading enhances vessel remodeling and bone healing through the modulation of the fate of bone marrow-derived cells. In this study, osteonecrosis was induced by transecting the ligamentum teres followed by a tight ligature around the femoral neck. For knee loading, 5 N loads were laterally applied to the knee at 15 Hz for 5 min/day for 5 weeks. Changes in bone mineral density (BMD) and bone mineral content (BMC) of the femur were measured by pDEXA, and ink infusion was performed to evaluate vessel remodeling. Femoral heads were harvested for histomorphometry, and bone marrow-derived cells were isolated to examine osteoclast development and osteoblast differentiation. The results showed that osteonecrosis significantly induced bone loss, and knee loading stimulated both vessel remodeling and bone healing. The osteonecrosis group exhibited the lowest trabecular BV/TV (p b 0.001) in the femoral head, and lowest femoral BMD and BMC (both p b 0.01). However, knee loading increased trabecular BV/TV (p b 0.05) as well as BMD (pb 0.05) and BMC (p b 0.01). Osteonecrosis decreased the vessel volume (pb 0.001), vessel number (pb 0.001) and VEGF expression (p b 0.01), and knee loading increased them (pb 0.001, pb 0.001 and p b 0.01). Osteonecrosis activated osteoclast development, and knee loading reduced its formation, migration, adhesion and the level of “pit” formation (pb 0.001, pb 0.01, pb 0.001 and pb 0.001). Furthermore, knee loading significantly increased osteoblast differentiation and CFU-F (both p b 0.001). A significantly positive correlation was observed between vessel remodeling and bone healing (both p b 0.01). These results indicate that knee loading could be effective in repair osteonecrosis of the femoral head in a rat model. This effect might be attributed to promoting vessel remodeling, suppressing osteoclast development, and increasing osteoblast and fibroblast differentiation. In summary, the current study suggests that knee loading might potentially be employed as a non-invasive therapy for osteonecrosis of the femoral head.

Wolff's law in action: a mechanism for early knee osteoarthritis

There is growing interest in the role of bone in knee osteoarthritis. Bone is a dynamic organ, tightly regulated by a multitude of homeostatic controls, including genetic and environmental factors. One such key environmental regulator of periarticular bone is mechanical stimulation, which, according to Wolff’s law, is a key determinant of bone properties. Wolff’s law theorizes that repetitive loading of bone will cause adaptive responses enabling the bone to better cope with these loads. Despite being an adaptive response of bone, the remodeling process may inadvertently trigger maladaptive responses in other articular structures. Accumulating evidence at the knee suggests that expanding articular bone surface area is driven by mechanical stimulation and is a strong predictor of articular cartilage loss. Similarly, fractal analysis of bone architecture provides further clues that bone adaptation may have untoward consequences for joint health. This review hypothesizes that adaptations of periarticular bone in response to mechanical stimulation cause maladaptive responses in other articular structures that mediate the development of knee osteoarthritis. A potential disease paradigm to account for such a hypothesis is also proposed, and novel therapeutic targets that may have a bone-modifying effect, and therefore potentially a disease-modifying effect, are also explored.

Biomechanical aspects of the muscle-bone interaction

There is growing interest in the interaction between skeletal muscle and bone, particularly at the genetic and molecular levels. However, the genetic and molecular linkages between muscle and bone are achieved only within the context of the essential mechanical coupling of the tissues. This biomechanical and physiological linkage is readily evident as muscles attach to bone and induce exposure to varied mechanical stimuli via functional activity. The responsiveness of bone cells to mechanical stimuli, or their absence, is well established. However, questions remain regarding how muscle forces applied to bone serve to modulate bone homeostasis and adaptation. Similarly, the contributions of varied, but unique, stimuli generated by muscle to bone (such as low-magnitude, high-frequency stimuli) remains to be established. The current article focuses upon the mechanical relationship between muscle and bone. In doing so, we explore the stimuli that muscle imparts upon bone, models that enable investigation of this relationship, and recent data generated by these models.

Evaluating treatment of osteoporosis using particle swarm on a bone remodelling mathematical model

Bone loss in osteoporosis, commonly observed in postmenopausal women and the elderly, is caused by an imbalance in activities of bone-forming osteoblasts and bone-resorbing osteoclasts. To treat osteoporosis and increase bone mineral density (BMD), physical activities and drugs are often recommended. Complex systems dynamics prevent an intuitive prediction of treatment strategies, and little is known about an optimal sequence for the combinatorial use of available treatments. In this study, the authors built a mathematical model of bone remodelling and developed a treatment strategy for mechanical loading and salubrinal, a synthetic chemical agent that enhances bone formation and prevents bone resorption. The model formulated a temporal BMD change of a mouse's whole skeleton in response to ovariectomy, mechanical loading and administration of salubrinal. Particle swarm optimisation was employed to maximise a performance index (a function of BMD and treatment cost) to find an ideal sequence of treatment. The best treatment was found to start with mechanical loading followed by salubrinal. As treatment costs increased, the sequence started with no treatment and usage of salubrinal became scarce. The treatment strategy will depend on individual needs and costs, and the proposed model is expected to contribute to the development of personalised treatment strategies.

Physical weight loading induces expression of tryptophan hydroxylase 2 in the brain stem

Sustaining brain serotonin is essential in mental health. Physical activities can attenuate mental problems by enhancing serotonin signaling. However, such activity is not always possible in disabled individuals or patients with dementia. Knee loading, a form of physical activity, has been found to mimic effects of voluntary exercise. Focusing on serotonergic signaling, we addressed a question: Does local mechanical loading to the skeleton elevate expression of tryptophan hydroxylase 2 (tph2) that is a rate-limiting enzyme for brain serotonin? A 5 min knee loading was applied to mice using 1 N force at 5 Hz for 1,500 cycles. A 5-min treadmill running was used as an exercise (positive) control, and a 90-min tail suspension was used as a stress (negative) control. Expression of tph2 was determined 30 min – 2 h in three brain regions ––frontal cortex (FC), ventromedial hypothalamus (VMH), and brain stem (BS). We demonstrated for the first time that knee loading and treadmill exercise upregulated the mRNA level of tph2 in the BS, while tail suspension downregulated it. The protein level of tph2 in the BS was also upregulated by knee loading and downregulated by tail suspension. Furthermore, the downregulation of tph2 mRNA by tail suspension can be partially suppressed by pre-application of knee loading. The expression of tph2 in the FC and VMH was not significantly altered with knee loading. In this study we provided evidence that peripheral mechanical loading can activate central tph2 expression, suggesting that physical cues may mediate tph2-cathalyzed serotonergic signaling in the brain.

Experimental tooth movement-induced osteoclast activation is regulated by sympathetic signaling

Experimental tooth movement (ETM) changes the distribution of sensory nerve fibers in periodontal ligament and the bone architecture through the stimulation of bone remodeling. As the sympathetic nervous system is involved in bone remodeling, we examined whether ETM is controlled by sympathetic signaling or not. In male mice, elastic rubber was inserted between upper left first molar (M1) and second molar (M2) for 3 or 5 days. Nerve fibers immunoreactive for not only sensory neuromarkers, such as calcitonin gene-related peptide (CGRP), but also sympathetic neuromarkers, such as tyrosine hydroxylase (TH) and neuropeptide Y (NPY) were increased in the periodontal ligament during ETM. To elucidate the effect of the sympathetic signal mediated by ETM, mice were intraperitoneally injected with a β-antagonist, propranolol (PRO: 20 μg/g/day), or a β-agonist, isoproterenol (ISO: 5 μg/g/day) from 7 days before ETM. PRO treatment suppressed the amount of tooth movement by 12.9% in 3-day ETM and by 32.2% in 5-day ETM compared with vehicle treatment. On the other hand, ISO treatment increased it. Furthermore, ETM remarkably increased the osteoclast number on the bone surface (alveolar socket) (Oc.N/BS) in all drug treatments. PRO treatment suppressed Oc.N/BS by 39.4% in 3-day ETM, while ISO treatment increased it by 32.1% in 3-day ETM compared with vehicle treatment. Chemical sympathectomy using 6-hydroxydopamine (6-OHDA: 250 μg/g) showed results similar to those for PRO treatment in terms of both the amount of tooth movement and osteoclast parameters. Our data showed that blockade of sympathetic signaling inhibited the tooth movement and osteoclast increase induced by ETM, and stimulation of sympathetic signaling accelerated these responses. These data suggest that the mechano-adaptive response induced by ETM is controlled by sympathetic signaling through osteoclast activation.

Salubrinal promotes healing of surgical wounds in rat femurs

Phosphorylation of eukaryotic initiation factor 2α (eIF2α), transiently activated by various cellular stresses, is known to alleviate stress-induced cellular damage. Here, we addressed a question: does elevation of eIF2α phosphorylation by salubrinal (a pharmacological inhibitor of eIF2α dephosphorylation) enhance healing of bone wounds? We hypothesized that salubrinal would accelerate a closure of surgically generated bone holes by modifying expression of stress-sensitive genes. To examine this hypothesis, we employed a rat wound model. Surgical wounds were generated on anterior and posterior femoral cortexes, and salubrinal was locally administered on the anterior side. The results showed that, compared to a contralateral control, the size of surgical wounds was reduced by 10.8 % (day 10) and 18.0 % (day 20) on the anterior side (both p < 0.001), and 4.1 % (day 10; p < 0.05) and 11.1 % (day 20; p < 0.001) on the posterior side. In addition, salubrinal locally elevated cortical thickness and increased BMD and BMC. Pharmacokinetic analysis revealed that subcutaneous injection of salubrinal transiently increased its concentration in plasma followed by a rapid decrease within 24 h, and its half-life in plasma was 1.2 h. Salubrinal altered the phosphorylation level of eIF2α as well as the mRNA levels of ATF3, ATF4, and CHOP, and suppressed cell death induced by stress to the endoplasmic reticulum. In summary, the results herein demonstrate that subcutaneous administration of salubrinal accelerates healing of surgically generated bone holes through the modulation of eIF2α phosphorylation.

Skeletal adaptation to intramedullary pressure-induced interstitial fluid flow is enhanced in mice subjected to targeted osteocyte ablation

Interstitial fluid flow (IFF) is a potent regulatory signal in bone. During mechanical loading, IFF is generated through two distinct mechanisms that result in spatially distinct flow profiles: poroelastic interactions within the lacunar-canalicular system, and intramedullary pressurization. While the former generates IFF primarily within the lacunar-canalicular network, the latter generates significant flow at the endosteal surface as well as within the tissue. This gives rise to the intriguing possibility that loading-induced IFF may differentially activate osteocytes or surface-residing cells depending on the generating mechanism, and that sensation of IFF generated via intramedullary pressurization may be mediated by a non-osteocytic bone cell population. To begin to explore this possibility, we used the Dmp1-HBEGF inducible osteocyte ablation mouse model and a microfluidic system for modulating intramedullary pressure (ImP) to assess whether structural adaptation to ImP-driven IFF is altered by partial osteocyte depletion. Canalicular convective velocities during pressurization were estimated through the use of fluorescence recovery after photobleaching and computational modeling. Following osteocyte ablation, transgenic mice exhibited severe losses in bone structure and altered responses to hindlimb suspension in a compartment-specific manner. In pressure-loaded limbs, transgenic mice displayed similar or significantly enhanced structural adaptation to Imp-driven IFF, particularly in the trabecular compartment, despite up to ∼50% of trabecular lacunae being uninhabited following ablation. Interestingly, regression analysis revealed relative gains in bone structure in pressure-loaded limbs were correlated with reductions in bone structure in unpressurized control limbs, suggesting that adaptation to ImP-driven IFF was potentiated by increases in osteoclastic activity and/or reductions in osteoblastic activity incurred independently of pressure loading. Collectively, these studies indicate that structural adaptation to ImP-driven IFF can proceed unimpeded following a significant depletion in osteocytes, consistent with the potential existence of a non-osteocytic bone cell population that senses ImP-driven IFF independently and potentially parallel to osteocytic sensation of poroelasticity-derived IFF.

Elbow loading promotes longitudinal bone growth of the ulna and the humerus

Mechanical stimulation plays a critical role in bone development and growth. In view of recently recognized anabolic responses promoted by a joint-loading modality, we examined the effects of elbow loading on longitudinal growth of the ulna and the humerus. Using a custom-made piezoelectric loader, the left elbow of growing C57/BL/6 female mice was given daily 5-min bouts of dynamic loading for 10 days. The right forelimbs of those mice served as contralateral controls, and the limbs of non-treated mice were used as age-matched controls. The effects of elbow loading were evaluated through measurement of bone length, weight, bone mineral density (BMD), and bone mineral content (BMC), as well as mRNA expression levels of load-sensitive transcription factors such as c-fos, egr1, and atf3. The results revealed that the humerus was elongated by 1.2% compared to the contralateral and age-matched controls (both p < 0.001), while the ulna had become longer than the contralateral control (1.7%; p < 0.05) and the age-match control (3.4%; p < 0.001). Bone lengthening was associated with increases in bone weight, BMD and BMC. Furthermore, the mRNA levels of the selected transcription factors were elevated in the loaded ulna and humerus. Interestingly, the increase was observed not only at the elbow but also at the wrist and shoulder in the loaded limb. The present study demonstrates that joint loading is potentially useful for stimulating bone lengthening and treating limb length discrepancy.

Mouse tail vertebrae adapt to cyclic mechanical loading by increasing bone formation rate and decreasing bone resorption rate as shown by time-lapsed in vivo imaging of dynamic bone morphometry

It is known that mechanical loading leads to an increase in bone mass through a positive shift in the balance between bone formation and bone resorption. How the remodeling sites change over time as an effect of loading remains, however, to be clarified. The purpose of this paper was to investigate how bone formation and resorption sites are modulated by mechanical loading over time by using a new imaging technique that extracts three dimensional formation and resorption parameters from time-lapsed in vivo micro-computed tomography images. To induce load adaptation, the sixth caudal vertebra of C57BL/6 mice was cyclically loaded through pins in the adjacent vertebrae at either 8 N or 0 N (control) three times a week for 5 min (3000 cycles) over a total of 4 weeks. The results showed that mechanical loading significantly increased trabecular bone volume fraction by 20% (p<0.001) and cortical area fraction by 6% (p<0.001). The bone formation rate was on average 23% greater (p<0.001) and the bone resorption rate was on average 25% smaller (p<0.001) for the 8 N group than for the 0 N group. The increase in bone formation rate for the 8 N group was mostly an effect of a significantly increased surface of bone formation sites (on average 16%, p<0.001), while the thickness of bone formation packages was less affected (on average 5% greater, p<0.05). At the same time the surface of bone resorption sites was significantly reduced (on average 15%, p<0.001), while the depth of resorption pits remained the same. For the 8 N group, the strength of the whole bone increased significantly by 24% (p<0.001) over the loading period, while the strain energy density in the trabecular bone decreased significantly by 24% (p<0.001). In conclusion, mouse tail vertebrae adapt to mechanical loading by increasing the surface of formation sites and decreasing the surface of resorption sites, leading to an overall increase in bone strength. This new imaging technique will provide opportunities to investigate in vivo bone remodeling in the context of disease and treatment options, with the added value that both bone formation and bone resorption parameters can be nondestructively calculated over time.

Knee loading stimulates healing of mouse bone wounds in a femur neck

Healing of bone wounds is sensitive to various environmental stimuli. Using knee loading, which has been shown to stimulate bone formation in mouse femora and tibiae, we addressed a question: Does knee loading accelerate a closure of open wounds in a femur neck? A surgical wound (0.5 mm in diameter) was generated at the femur neck in the left and right femora of C57/BL/6 female mice, and knee loading was applied to the left knee for 3 min/day for 3 consecutive days. Surgical holes at the femoral midshaft were used as control. Animals were sacrificed 1, 2, and 3 weeks after surgery for analyses with μCT and pQCT as well as mechanical testing. The results showed load-driven acceleration of the closure of surgical holes. Compared to a sham-loaded contralateral control, knee loading reduced the size of surgical wounds in the femoral midshaft by 14% (p<0.05), 21% (p<0.01), and 32% (p<0.001) in 1, 2, and 3 weeks, respectively. It also decreased the wound size in the femur neck by 16% (p<0.001; 1 week), 18% (p<0.001; 2 weeks), and 21% (p<0.001; 3 weeks). Images with pQCT revealed that bone mineral density (BMD) was increased from 571±19 mg/cm(3) (control) to 686±19 mg/cm(3) (loaded) (p<0.01), and bone mineral content (BMC) from 3.05±0.12 mg/mm (control) to 3.42±0.11 mg/mm (loaded) (p<0.05). Furthermore, mechanical testing showed that stiffness of the femur was increased by knee loading (p<0.05). This study demonstrates that knee loading is capable of accelerating healing of surgical wounds throughout the femur including the femoral midshaft and neck.

WISE-2005: bed-rest induced changes in bone mineral density in women during 60 days simulated microgravity

To better understand the effects of prolonged bed-rest in women, 24 healthy women aged 25 to 40 years participated in 60-days of strict 6° head-down tilt bed-rest (WISE-2005). Subjects were assigned to either a control group (CON, n=8) which performed no countermeasure, an exercise group (EXE, n=8) undertaking a combination of resistive and endurance training or a nutrition group (NUT, n=8), which received a high protein diet. Using peripheral quantitative computed tomography (pQCT) and dual X-ray absorptiometry (DXA), bone mineral density (BMD) changes at various sites, body-composition and lower-leg and forearm muscle cross-sectional area were measured up to 1-year after bed-rest. Bone loss was greatest at the distal tibia and proximal femur, though losses in trabecular density at the distal radius were also seen. Some of these bone losses remained statistically significant one-year after bed-rest. There was no statistically significant impediment of bone loss by either countermeasure in comparison to the control-group. The exercise countermeasure did, however, reduce muscle cross-sectional area and lean mass loss in the lower-limb and also resulted in a greater loss of fat mass whereas the nutrition countermeasure had no impact on these parameters. The findings suggest that regional differences in bone loss occur in women during prolonged bed-rest with incomplete recovery of this loss one-year after bed-rest. The countermeasures as implemented were not optimal in preventing bone loss during bed-rest and further development is required.

A descriptive analysis of abnormal postural patterns in children with hemiplegic cerebral palsy

BACKGROUND

Functional classification systems generally divide children with cerebral palsy (CP) into mild, moderate, and severe types. Although depending on functional limitations, they do not seem to evaluate abnormal postural patterns in standing. Since the most asymmetrical patterns can be observed in hemiplegia, the goal of this case series study was to provide their objective analysis and to establish any potential clinical value for evaluation and management of CP.

MATERIAL/METHODS

A group of 36 children (aged 5-10 years) with spastic hemiplegic CP, who could stand and ambulate independently, were selected. The photogrammetric and pedobarographic studies were obtained for the postural analysis in standing.

RESULTS

Two different anti- and pro- gravitational postural patterns were identified. They seem not only to affect functional status and rehabilitation potential, but also clinical value for evaluation and management of CP hemiplegia.

CONCLUSIONS

The importance of strong study design cannot be overemphasized. The 2 different postural patterns indicate dissimilar compensatory tendencies, which may help in prognosis of deformity and functional outcomes of rehabilitation. The use of objective photogrammetric and the pedobarographic studies may also help to develop a more specific therapeutic intervention in order to facilitate the pattern leading towards better outcome (orthosis in the anti-gravitational postural pattern vs focal spasticity management in the pro-gravitational postural pattern).

Evidence for an additional effect of whole-body vibration above resistive exercise alone in preventing bone loss during prolonged bed rest

SUMMARY

The addition of whole-body vibration to high-load resistive exercise may provide a better stimulus for the reduction of bone loss during prolonged bed rest (spaceflight simulation) than high-load resistive exercise alone.

INTRODUCTION

Prior work suggests that the addition of whole-body vibration to high-load resistive exercise (RVE) may be more effective in preventing bone loss in spaceflight and its simulation (bed rest) than resistive exercise alone (RE), though this hypothesis has not been tested in humans.

METHODS

Twenty-four male subjects as part of the 2nd Berlin Bed Rest Study performed RVE (n = 7), RE (n = 8) or no exercise (control, n = 9) during 60-day head-down tilt bed rest. Whole-body, spine and total hip dual X-ray absorptiometry (DXA) measurements as well as peripheral quantitative computed tomography measurements of the tibia were conducted during bed rest and up to 90 days afterwards.

RESULTS

A better retention of bone mass in RVE than RE was seen at the tibial diaphysis and proximal femur (p ≤ 0.024). Compared to control, RVE retained bone mass at the distal tibia and DXA leg sub-region (p ≤ 0.020), but with no significant difference to RE (p ≥ 0.10). RE impacted significantly (p = 0.038) on DXA leg sub-region bone mass only. Calf muscle size was impacted similarly by both RVE and RE. On lumbar spine DXA, whole-body DXA and calcium excretion measures, few differences between the groups were observed.

CONCLUSIONS

Whilst further countermeasure optimisation is required, the results provide evidence that (1) combining whole-body vibration and high-load resistance exercise may be more efficient than high-load resistive exercise alone in preventing bone loss at some skeletal sites during and after prolonged bed rest and (2) the effects of exercise during bed rest impact upon bone recovery up to 3 months afterwards.

Evolving concepts in neurogenic osteoporosis

Convincing evidence has accumulated of regulation of bone by the central nervous system. The neural connection between brain and bone is mediated centrally by classic neurotransmitters and several neuropeptides, and peripherally by many of the same neurotransmitters and neuropeptides, albeit with actions opposite to their central effects. Pharmacologic blockade of ß2-adrenergic receptors or disruption of the gene encoding them increases bone mass, whereas increased activity of the sympathetic nervous system (SNS) contributes to bone loss. Brainstem serotonergic neurons regulate SNS activity and its modulation by leptin. Physiologic stimulation of osteoblastic nicotinic receptors results in proliferation and deposition of bone, whereas higher levels inhibit osteoblast function. Activation of sensory nerves has a centrally mediated action on bone, albeit poorly understood. The relative importance of, and interactions between autonomic, sensory, and peripheral nervous system actions on bone mass are also not clear in healthy individuals, and less so in pathologic states.

Systemic effects of ulna loading in male rats during functional adaptation

Functional skeletal adaptation is thought to be a local phenomenon controlled by osteoctyes. However, the nervous system also may have regulatory effects on adaptation. The aim of this study was to determine the effects of loading of a single bone on adaptation of other appendicular long bones and whether these responses were neuronally regulated. Young male Sprague-Dawley rats were used. The right ulna was loaded to induce a modeling response. In other rats, a second regimen was used to induce bone fatigue with a mixed modeling/remodeling response; a proportion of rats from each group received brachial plexus anesthesia to induce temporary neuronal blocking during bone loading. Sham groups were included. Left and right long bones (ulna, humerus, tibia, and femur) from each rat were examined histologically 10 days after loading. In fatigue- and sham-loaded animals, blood plasma concentrations of TNF-α, RANKL, OPG, and TRAP5b were determined. We found that loading the right ulna induced an increase in bone formation in distant long bones that were not loaded and that this effect was neuronally regulated. Distant effects were most evident in the rats that received loading without bone fatigue. In the fatigue-loaded animals, neuronal blocking induced a significant decrease in plasma TRAP5b at 10 days. Histologically, bone resorption was increased in both loaded and contralateral ulnas in fatigue-loaded rats and was not significantly blocked by brachial plexus anesthesia. In young, growing male rats we conclude that ulna loading induced increased bone formation in multiple bones. Systemic adaptation effects were, at least in part, neuronally regulated.

Microfluidic enhancement of intramedullary pressure increases interstitial fluid flow and inhibits bone loss in hindlimb suspended mice

Interstitial fluid flow (IFF) has been widely hypothesized to mediate skeletal adaptation to mechanical loading. Although a large body of in vitro evidence has demonstrated that fluid flow stimulates osteogenic and antiresorptive responses in bone cells, there is much less in vivo evidence that IFF mediates loading-induced skeletal adaptation. This is due in large part to the challenges associated with decoupling IFF from matrix strain. In this study we describe a novel microfluidic system for generating dynamic intramedullary pressure (ImP) and IFF within the femurs of alert mice. By quantifying fluorescence recovery after photobleaching (FRAP) within individual lacunae, we show that microfluidic generation of dynamic ImP significantly increases IFF within the lacunocanalicular system. In addition, we demonstrate that dynamic pressure loading of the intramedullary compartment for 3 minutes per day significantly eliminates losses in trabecular and cortical bone mineral density in hindlimb suspended mice, enhances trabecular and cortical structural integrity, and increases endosteal bone formation rate. Unlike previously developed modalities for enhancing IFF in vivo, this is the first model that allows direct and dynamic modulation of ImP and skeletal IFF within mice. Given the large number of genetic tools for manipulating the mouse genome, this model is expected to serve as a powerful investigative tool in elucidating the role of IFF in skeletal adaptation to mechanical loading and molecular mechanisms mediating this process.

Lengthening of mouse hindlimbs with joint loading

For devising clinical approaches to treating limb length discrepancies, strategies that will generate differential longitudinal growth need to be improved. This report addresses the following question: does knee loading increase bone length of the loaded hindlimb? Knee loading has been shown to induce anabolic responses on the periosteal and endosteal surfaces, but its effects on longitudinal bone growth have not yet been examined. In the present studies, loads were applied to the left hindlimb (5-min bouts at 0.5 N) of C57/BL/6 mice (21 mice, ~8 weeks old). Compared to the contralateral and age-matched control groups, knee loading increased the length of the femur by 2.3 and 3.5%, together with the tibia by 2.3 and 3.7% (all P < 0.001), respectively. In accordance with the length measurements, knee loading elevated BMD and BMC in both the femur and the tibia. Histological analysis of the proximal tibia revealed that the loaded growth plate elevated its height by 19.5% (P < 0.001) and the cross-sectional area by 30.7% (P < 0.05). Particularly in the hypertrophic zone, knee loading increased the number of chondrocytes (P < 0.01) as well as their cellular height (P < 0.001) along the length of the tibia. Taken together, this study demonstrates for the first time the potential effectiveness of knee loading in adjusting limb length discrepancy.

Is bone's response to mechanical signals dominated by muscle forces?

Skeletal loading in vertebrates controls modeling drifts, modulated remodeling rates, and affects growth trajectories. It is unclear whether the majority of the mechanical stimulus detected by bone cells originates from muscle contraction forces or from gravitational forces associated with substrate impact. Several clinical and basic science reports indicate that muscle forces play a dominant role in generating the mechanical stimulus in exercise-induced bone gain. Although it is, in most cases, difficult to separate the effects of gravitational forces acting on body mass from muscle contractions, several well-conceived experiments offer considerable insight into the propensity of muscle-derived forces per se to drive the adaptive response in bone. Load-induced osteogenesis requires that mechanical signals come packaged with particular characteristics, all of which can be generated from either gravitational or muscle forces. Neither of these two sources have been demonstrated empirically to be the source of bone's adaptive response, but a convincing body of data suggests that muscle contractions are present, significant, and capable of accounting for most of the adaptive responses.

Functional adaptation to mechanical loading in both cortical and cancellous bone is controlled locally and is confined to the loaded bones

In order to validate whether bones' functional adaptation to mechanical loading is a local phenomenon, we randomly assigned 21 female C57BL/6 mice at 19 weeks of age to one of three equal numbered groups. All groups were treated with isoflurane anesthesia three times a week for 2 weeks (approximately 7 min/day). During each anaesthetic period, the right tibiae/fibulae in the DYNAMIC+STATIC group were subjected to a peak dynamic load of 11.5 N (40 cycles with 10-s intervals between cycles) superimposed upon a static "pre-load" of 2.0 N. This total load of 13.5 N engendered peak longitudinal strains of approximately 1400 microstrain on the medial surface of the tibia at a middle/proximal site. The right tibiae/fibulae in the STATIC group received the static "pre-load" alone while the NOLOAD group received no artificial loading. After 2 weeks, the animals were sacrificed and both tibiae, fibulae, femora, ulnae and radii analyzed by three-dimensional high-resolution (5 mum) micro-computed tomography (microCT). In the DYNAMIC+STATIC group, the proximal trabecular percent bone volume and cortical bone volume at the proximal and middle levels of the right tibiae as well as the cortical bone volume at the middle level of the right fibulae were markedly greater than the left. In contrast, the left bones in the DYNAMIC+STATIC group showed no differences compared to the left or right bones in the NOLOAD or STATIC group. These microCT data were confirmed by two-dimensional examination of fluorochrome labels in bone sections which showed the predominantly woven nature of the new bone formed in the loaded bones. We conclude that the adaptive response in both cortical and trabecular regions of bones subjected to short periods of dynamic loading, even when this response is sufficiently vigorous to stimulate woven bone formation, is confined to the loaded bones and does not involve changes in other bones that are adjacent, contra-lateral or remote to them.

Mechanical stimulation suppresses phosphorylation of eIF2alpha and PERK-mediated responses to stress to the endoplasmic reticulum

Cellular perturbations such as stress to the endoplasmic reticulum induce an integrated stress response, which activates phosphorylation of eIF2alpha and leads to alleviation of cellular injury or apoptosis. This study investigated the role of mechanical stimulation in the regulation of eIF2alpha and cell death. Mechanical stimulation was applied to mouse ulnae, MC3T3 cells, and mesenchymal stem cells. The results demonstrate that mechanical stimulation reduces phosphorylation of eIF2alpha through inactivation of Perk. Furthermore, flow pre-treatment reduces thapsigargin-induced cell mortality through suppression of phosphorylation of Perk. However, H(2)O(2)-driven cell mortality, which is not mediated by Perk, is not suppressed by mechanical stimulation. Taken together, our observations suggest a pro-survival role of mechanical stimulation in Perk-mediated stress responses.

Mechanical loading of a long bone induces plasticity in sensory input to the central nervous system

Although the skeleton is extensively innervated by sensory nerves, the importance of this innervation to skeletal physiology is unclear. Neuronal connectivity between limbs is little studied and likely underestimated. In this study, we examined the effect of bone loading on spinal plasticity in young male Sprague-Dawley rats, using end-loading of the ulna and transynaptic tracing with the Bartha pseudorabies virus (PRV). PRV was inoculated onto the periosteum of the right ulna after 10 days of adaptation to a single period of cyclic loading of the right ulna (1,500 cycles of load at 4 Hz, initial peak strain of -3,750 micro epsilon). We found that neuronal circuits connect the sensory innervation of right thoracic limb to all other limbs, as PRV was detectable in the dorsal root ganglia (DRG) of left and right brachial and lumbosacral intumescences. We also found that mechanical loading of the right ulna induced plasticity in the spinal cord, with significant augmentation of the connectivity between limbs, as measured by PRV translocation. Within the spinal cord, PRV was predominantly found adjacent to the central canal and in the dorsal horns, suggesting that plasticity in cross-talk between limbs is likely a consequence of dendritic growth, and enhanced connectivity of propriospinal interneurons. In conclusion, the data clearly demonstrate that the innervation of the skeleton exhibits plasticity in response to loading events, suggesting the existence of a dynamic control system that may be of regulatory importance during functional skeletal adaptation.

Joint loading-driven bone formation and signaling pathways predicted from genome-wide expression profiles

Joint loading is a recently developed loading modality that induces anabolic responses by lateral loads applied to a synovial joint such as an elbow and a knee. The present study extended this loading modality to an ankle and addressed a question: does ankle loading promote bone formation in the tibia? If so, what signaling pathways are involved in the anabolic responses? Using C57BL/6 female mice as a model system, lateral loads of 0.5 N were applied to the ankle at 5 Hz for 3 min/day for 3 consecutive days and load-driven bone formation was evaluated at three tibial cross-sections (the proximal, middle, and distal diaphysis). Furthermore, total RNA was isolated for 3 pairs of microarray experiments as well as quantitative real-time PCR analyses. The histomorphometric results revealed that in all cross-sections ankle loading elevated the cortical area and thickness as well as the calcein-labeled surface. Signaling pathway analysis from microarray-derived whole-genome mRNA expression profiles and quantitative real-time PCR predicted that molecules in phosphoinositide 3-kinase (PI3K), ECM-receptor interactions, TGFbeta signaling, and Wnt signaling were involved in the joint-loading driven responses. Since ankle loading stimulates bone formation throughout the tibia both in the endosteum and the periosteum, it may provide a non-pharmacological approach to effectively activate molecular signaling necessary for preventing bone loss.

Potential applications of pulsating joint loading in sports medicine

Because bone is responsive to mechanical loading, pulsating joint loading (PJL), which laterally applies oscillatory mechanical loads to joints, can be explored for preventive conditioning and therapeutic treatments. Herein, the general features of PJL are reviewed, and its potential usage for sports medicine is discussed.

Fluid and Solute Transport in Bone: Flow-Induced Mechanotransduction

Much recent evidence suggests that bone cells sense their mechanical environment via interstitial fluid flow. In this review, we summarize theoretical and experimental approaches to quantify fluid and solute transport in bone, starting with the early investigations of fluid shear stress applied to bone cells. The pathways of bone interstitial fluid and solute movement are high-lighted based on recent theoretical models, as well as a new generation of tracer experiments that have clarified and refined the structure and function of the osteocyte pericellular matrix. Then we trace how the fluid-flow models for mechanotransduction have evolved as new ultrastructural features of the osteocyte lacunar-canalicular porosity have been identified and how more recent in vitro fluid-flow and cell-stretch experiments have helped elucidate at the molecular level the possible pathways for cellular excitation in bone.

Functional adaptation to loading of a single bone is neuronally regulated and involves multiple bones

Regulation of load-induced bone formation is considered a local phenomenon controlled by osteocytes, although it has also been hypothesized that functional adaptation may be neuronally regulated. The aim of this study was to examine bone formation in multiple bones, in response to loading of a single bone, and to determine whether adaptation may be neuronally regulated. Load-induced responses in the left and right ulnas and humeri were determined after loading of the right ulna in male Sprague-Dawley rats (69 +/- 16 days of age). After a single period of loading at -760-, -2000-, or -3750-microepsilon initial peak strain, rats were given calcein to label new bone formation. Bone formation and bone neuropeptide concentrations were determined at 10 days. In one group, temporary neuronal blocking was achieved by perineural anesthesia of the brachial plexus with bupivicaine during loading. We found right ulna loading induces adaptive responses in other bones in both thoracic limbs compared with Sham controls and that neuronal blocking during loading abrogated bone formation in the loaded ulna and other thoracic limb bones. Skeletal adaptation was more evident in distal long bones compared with proximal long bones. We also found that the single period of loading modulated bone neuropeptide concentrations persistently for 10 days. We conclude that functional adaptation to loading of a single bone in young rapidly growing rats is neuronally regulated and involves multiple bones. Persistent changes in bone neuropeptide concentrations after a single loading period suggest that plasticity exists in the innervation of bone.

Joint loading modality: its application to bone formation and fracture healing

Sports-related injuries such as impact and stress fractures often require a rehabilitation programme to stimulate bone formation and accelerate fracture healing. This review introduces a recently developed joint loading modality and evaluates its potential applications to bone formation and fracture healing in post-injury rehabilitation. Bone is a dynamic tissue whose structure is constantly altered in response to its mechanical environments. Indeed, many loading modalities can influence the bone remodelling process. The joint loading modality is, however, able to enhance anabolic responses and accelerate wound healing without inducing significant in situ strain at the site of bone formation or fracture healing. This review highlights the unique features of this loading modality and discusses its potential underlying mechanisms as well as possible clinical applications.

Knee loading accelerates bone healing in mice

Knee loading is an anabolic loading modality that applies lateral loads to the knee. This study shows that loads applied to the proximal tibial epiphysis stimulate healing of surgically generated wounds in the tibial diaphysis.

INTRODUCTION

Wound healing is sensitive to mechanical stimulation such as various forms of stress and different magnitudes of strain. Knee loading has been shown to induce anabolic responses to murine tibias and femora when a strain of 10-20 mustrain is applied at the site of new bone formation. The object of this study was to address a question: does knee loading accelerate closure of open wounds in the tibia?

MATERIAL AND METHODS

Fifty-three C57/BL/6 female mice were used. A surgical wound (0.5 mm in diameter) was generated in the left tibia (loaded) and the right tibia (sham-loaded control). From the fourth postoperative day, knee loading was performed to the left knee with a custom-made piezoelectric loader for 3 min/d for 3 consecutive days. The peak-to-peak force was 0.5 N. Animals were killed 1, 2, or 3 wk after surgery, and the healing process was evaluated with muCT, pQCT, and bone histomorphometry with calcein labeling.

RESULTS

The measured strain was <20 mustrain with 0.5-N force regardless of the presence or absence of surgical wounds. Compared with sham-loaded controls, the results showed load-driven acceleration of wound healing. First, muCT data revealed that knee loading reduced the size of surgical wounds by 13% (p < 0.01; 1 wk), 25% (p < 0.001; 2 wk), and 15% (p < 0.01; 3 wk). Second, pQCT data indicated that total BMD and BMC and cortical BMD and BMC were significantly increased in the third postoperative week. Last, bone histomorphometry revealed that bone formation was stimulated from the site proximal (close to the knee) to the wound.

CONCLUSIONS

The reparative and remodeling phases of wound healing were enhanced by loads applied to the knee without inducing significant in situ strain at the site of wounds. Noninvasive knee loading might therefore be useful clinically to stimulate bone healing in the entire tibia along its length (including cast immobilized wounds).

Measurement of Strain Distributions in Mouse Femora with 3D-Digital Speckle Pattern Interferometry

Bone is a mechanosensitive tissue that adapts its mass, architecture and mechanical properties to external loading. Appropriate mechanical loads offer an effective means to stimulate bone remodeling and prevent bone loss. A role of in situ strain in bone is considered essential in enhancement of bone formation, and establishing a quantitative relationship between 3D strain distributions and a rate of local bone formation is important. Digital speckle pattern interferometry (DSPI) can achieve whole-field, non-contacting measurements of microscopic deformation for high-resolution determination of 3D strain distributions. However, the current system does not allow us to derive accurate strain distributions because of complex surface contours inherent to biological samples. Through development of a custom-made piezoelectric loading device as well as a new DSPI-based force calibration system, we built an advanced DSPI system and integrated local contour information to deformation data. Using a mouse femur in response to a knee loading modality as a model system, we determined 3D strain distributions and discussed effectiveness and limitations of the described system.

Effects of surgical holes in mouse tibiae on bone formation induced by knee loading

Loads applied directly to the knee (knee loading) have recently been demonstrated to induce anabolic responses in femoral and tibial cortical bone. In order to examine the potential role of intramedullary pressure in generating those knee loading responses, we investigated the effects of drilling surgical holes that penetrated into the tibial medullary cavity and thereby modulated pressure alteration. Thirty-nine C57/BL/6 female mice in total were used with and without surgical holes, and the surgical holes were monitored with micro CT and histology. The left knee was loaded for 3 days, and the contralateral limb was treated as a sham-loaded control. Mice were sacrificed for bone histomorphometry 2 weeks after the last loading. Although the surgical hole induced bone formation in both loaded and non-loaded tibiae, due to regional and systemic acceleratory phenomenon the anabolic effect of knee loading was substantially diminished. Without the holes, knee loading significantly elevated cross-sectional cortical area, cortical thickness, mineralizing surface, mineral apposition rate, and bone formation rate on the periosteal surface. For example, the rate of bone formation was elevated 2.1 fold (p<0.001; middle diaphysis--50% site from the knee along the length of tibiae) and 2.7 fold (p<0.01; distal diaphysis--75% site). With the surgical holes, however, knee loading did not provide significant enhancement either at the 50% or 75% site in any of the histomorphometric measurements (p>0.05). The results support the idea that alteration of intramedullary pressure is necessary for knee loading to induce bone formation in the diaphysis.

Effect of fatigue loading and associated matrix microdamage on bone blood flow and interstitial fluid flow

Functional adaptation of bone to cyclic fatigue involves a complex physiological response that is targeted to sites of microdamage. The mechanisms that regulate this process are not understood, although lacunocanalicular interstitial fluid flow is likely important. We investigated the effect of a single period of cyclic fatigue on bone blood flow and interstitial fluid flow. The ulnae of 69 rats were subjected to cyclic fatigue unilaterally using an initial peak strain of -6000 muepsilon until 40% loss of stiffness developed. Groups of rats (n=23 per group) were euthanized immediately after loading, at 5 days, and at 14 days. The contralateral ulna served as a treatment control, and a baseline control group (n=23) that was not loaded was also included. After euthanasia, localization of intravascular gold microspheres within the ulna (n=7 rats/group) and tissue distribution of procion red tracer were quantified (n=8 rats/group). Microcracking, modeling, and remodeling (Cr.S.Dn, microm/mm(2), Ne.Wo.B.T.Ar, mm(2), and Rs.N/T.Ar, #/mm(2) respectively) were also quantified histologically (n=8 rats/group). Cyclic fatigue loading induced hyperemia of the loaded ulna, which peaked at 5 days after loading. There was an associated overall decrease in procion tracer uptake in both the loaded and contralateral control ulnae. Tracer uptake was also decreased in the periosteal region, when compared with the endosteal region of the cortex. Pooling of tracer was seen in microdamaged bone typically adjacent to an intracortical stress fracture at all time points after fatigue loading; in adjacent bone tracer uptake was decreased. New bone formation was similar at 5 days and at 14 days, whereas formation of resorption spaces was increased at 14 days. These data suggest that a short period of cyclic fatigue induces bone hyperemia and associated decreased lacunocanalicular interstitial fluid flow, which persists over the time period in which osteoclasts are recruited to sites of microdamage for targeted remodeling. Matrix damage and development of stress fracture also interfere with normal centrifugal fluid flow through the cortex. Changes in interstitial fluid flow in the contralateral ulna suggest that functional adaptation to unilateral fatigue loading may include a more generalized neurovascular response.

Knee loading dynamically alters intramedullary pressure in mouse femora

Dynamic mechanical loads have been known to stimulate bone formation. Many biophysical factors such as number of daily loading cycles, bone strain, strain-induced interstitial fluid flow, molecular transport, and modulation of intramedullary pressure have been considered as potential mediators in mechanotransduction of bone. Using a knee loading modality that enhances anabolic responses in mouse hindlimb, we addressed a question: Do oscillatory loads applied to the knee induce dynamic alteration of intramedullary pressure in the femoral medullary cavity? To answer this question, mechanical loads were applied to the knee with a custom-made piezoelectric loader and intramedullary pressure in the femoral medullary cavity was measured with a fiber optic pressure sensor. We observed that in response to sinusoidal forces of 0.5 Hz and 10 Hz, pressure amplitude increased up to 4-N loads and reached a plateau at 130 Pa. This amplitude significantly decreased with a loading frequency above 20 Hz. To confirm alteration of intramedullary pressure, real-time motion of microparticles in a glass tube inserted to the femoral medullary cavity ex vivo was visualized. Taken together, these data reveal that knee loading dynamically alters intramedullary pressure as a function of loading intensities and frequencies.

Frequency-dependent enhancement of bone formation in murine tibiae and femora with knee loading

Knee loading is a relatively new loading modality in which dynamic loads are laterally applied to the knee to induce bone formation in the tibia and the femur. The specific aim of the current study was to evaluate the effects of loading frequencies (in Hz) on bone formation at the site away from the loading site on the knee. The left knee of C57/BL/6 mice was loaded with 0.5 N force at 5, 10, or 15 Hz for 3 min/day for 3 consecutive days, and bone histomorphometry was conducted at the site 75% away from the loading site along the length of tibiae and femora. The results revealed frequency-dependent induction of bone formation, in which the dependence was different in the tibia and the femur. Compared with the sham-loading control, for instance, the cross-sectional cortical area was elevated maximally at 5 Hz in the tibia, whereas the most significant increase was observed at 15 Hz in the femur. Furthermore, mineralizing surface, mineral apposition rate, and bone formation rate were the highest at 5 Hz in the tibia (2.0-, 1.4-, and 2.7 fold, respectively) and 15 Hz in the femur (1.5-, 1.2-, and 1.8 fold, respectively). We observed that the tibia had a lower bone mineral density with more porous microstructures than the femur. Those differences may contribute to the observed differential dependence on loading frequencies.

Knee loading stimulates cortical bone formation in murine femurs

Background

Bone alters its architecture and mass in response to the mechanical environment, and thus varying loading modalities have been examined for studying load-driven bone formation. The current study aimed to evaluate the anabolic effects of knee loading on diaphyseal cortical bone in the femur.

Methods

Using a custom-made piezoelectric loader, 0.5-N loads were laterally applied to the left knee of C57/BL/6 mice at 5, 10, 15, and 20 Hz for 3 minutes per day for 3 consecutive days. Animals were sacrificed for examination 13 days after the last loading. The contralateral femur was used as a non-loading control, and the statistical significance of loading effects was evaluated with p < 0.05.

Results

Although diaphyseal strains were measured as small as 12 μstrains, bone histomorphometry clearly demonstrated frequency-dependent enhancement of bone formation. Compared to a non-loading control, bone formation on the periosteal surface was significantly enhanced. The loading at 15 Hz was most effective in elevating the mineralizing surface (1.7 x; p < 0.05), mineral apposition rate (1.4 x; p < 0.001), and bone formation rate (2.4 x; p < 0.01). The loading at 10 Hz elevated the mineralizing surface (1.4 x; p < 0.05), mineral apposition rate (1.3 x; p < 0.01), and bone formation rate (1.8 x; p < 0.05). The cross-sectional cortical area and the cortical thickness in the femoral diaphysis were significantly increased by loading at 10 Hz (both 9%) and 15 Hz (12% and 13%, respectively).

Conclusion

The results support the anabolic effects of knee loading on diaphyseal cortical bone in the femur with small in situ strain, and they extend our knowledge on the interplay between bone and joints. Strengthening the femur contributes to preventing femoral fractures, and the discovery about the described knee loading might provide a novel strategy to strengthen osteoporotic bones. Further analyses are required to understand the biophysical and molecular mechanism behind knee loading.