HLA-DPA1 gene polymorphism in primary glaucoma.

OBJECTIVE: To explore the association between human leukocyte antigen (HLA)-DPA1 gene polymorphism and primary glaucoma. PATIENTS AND METHODS: Six single nucleotide polymorphisms (SNPs) were genotyped in 51 patients and 51 healthy controls through Polymerase Chain Reaction (PCR). The possible association between HLA-DPA1 gene mutation and primary glaucoma was detected using the t-test and the Chi-square test. RESULTS: Rs1676486 genotype had a significant genetic correlation. Rs3753841 and rs12138977 genotypes had a higher minor-allele frequency in control group. The CT + CC genotype frequency of rs12138977 showed a significant genetic correlation in both case group and control group. Moreover, the rs12138977 polymorphism and corneal thickness had little influence on the occurrence of primary angle-closure glaucoma (PACG). Also, the main risk factors for PACG were intraocular hypertension and short axial length. CONCLUSIONS: The HLA-DPA1 gene polymorphism may be related to the severity of PACG.

[The research of the innate defense regulator peptide on the effects of methicillin resistant staphylococcus aureus biofilm].

Objective: To investigate the destruction of the mature biofilm and the inhibitory effect of the biofilm formation of methicillin-resistant Staphylococcus aureus (MRSA) by different concentrations of the innate defense regulatory peptide (IDR-1018). Methods: 1 ×10(5)CFU /ml MRSA was inoculated uniformly into 96 well plates, the biofilm model would be completed after 48 h. Given the different concentration of IDR-1018 solution as the experimental group double diluted with tryptic soy broth (TSB), the concentration in bacteria suspension reached 3.75-1 000 mg/L respectively. Erythromycin is double diluted into different concentration gradient, combined with low concentration (15 mg/L) of IDR-1018 as the mixed group.The same amount of TSB treated as the blank control group. The growth of the biofilm was measured through the measurement of the value of absorbance (A)by the semi-quantitative method of crystal violet staining at 24 h. Using SPSS 18.0 as statistical software to analyze the data. Results: Compared with the control group (A(595)=1.764 ± 0.026), IDR-1018 significantly damaged the mature MRSA biofilm, and function was worked in a dose-dependent method. With decreasing drug concentration, the destruction of the biofilm decreased correspondingly. When the concentration was as low as 15 mg/L, A(595) = 0.946 ± 0.047(t=32.955, P<0.01). When the concentration was 7.5 mg/L, A(595) = 1.211±0.054 (t=12.731, P<0.05). When the concentration was 3.75 mg/L, A(595)=1.360±0.066(t=4.843, P<0.05), the difference was still statistically significant compared with the control group. For the immature biofilm, compared with the control group(A(595)=1.689±0.068), IDR-1018 still had a significant inhibitory effect on the formation process of MRSA biofilm when the concentration was as low as 15 mg/L (A(595)=0.846±0.057, t=34.127, P<0.01). The inhibition of biofilm had a certain decline, when the concentration was 7.5 mg/L (A(595)=1.402 ± 0.181, t=5.240, P<0.05). But the difference was still statistically significant compared with the control group. However, the inhibitory effect was significantly decreased when the concentration was 3.75 mg/L (A(595)=1.631±0.190, t=0.913, P>0.05). When the low concentration (15 mg/L) of IDR-1018 and different concentrations of erythromycin were used together, the destruction and inhibition of MRSA biofilm was significantly higher than using erythromycin or IDR-1018 alone. Conclusion: IDR-1018 can play a good inhibitory role in the formation process of MRSA biofilm, and can play a good role in destroying MRSA biofilm.

Comparison of morphological changes of muscle fibers in response to dynamic electrical muscle contraction and dynamic hydraulic stimulation in a rat hindlimb disuse model.

This study attempted to compare the muscle fiber morphological responses to dynamic electrical muscle stimulation (DEMS) and dynamic hydraulic stimulation (DHS) in rats under hindlimb suspension (HLS). DEMS at 1 Hz, 50 Hz and 100 Hz for 10 min/day, 5 days/week were introduced to the animals' right quadriceps. Static and 2 Hz DHS were introduced to the right tibiae of other animal groups on a "10 min on - 5 min off - 10 min on" loading regime for 5 days/week. In the end of the 4-week experiments, histological changes in the corresponding soleus, gastrocnemius and quadriceps of the stimulated sites were examined. Compared to age-matched, HLS led to muscle atrophy and strongly reduced muscle wet weights and averaged cross-sectional fiber areas. Among the tested DEMS frequencies, the averaged cross-sectional quadriceps fiber area in the 50 Hz group was 29 % larger than the 100 Hz group. In contrast, difference in the muscle fiber response to the static and 2 Hz DHS was not observed in either soleus or gastrocnemius. Muscle fiber morphological responses to the active DEMS was in a load frequency dependent manner under disuse condition. Relatively passive compressions, either via static or 2Hz DHS, were unable to induce any difference in the muscle fiber responses under functional disuse.

[Investigation of molecular epidemiology of methicillin-resistant staphylococcus aureus in department of critical care medicine].

Objective: To investigate the feature of antimicrobial resistance, homology and other molecular epidemiology of methicillin-resistant staphylococcus aureus(MRSA) in Department of Critical Care Medicine(ICU). Methods: From October 2010 to December 2011, 149 strains of MRSA were collected and identified through sputum culture of patients from 10 ICUs of 10 teaching hospitals distributed in 9 chinese central city of China. Susceptibility testing to 18 kinds of antibiotic was performed, the method of pulsed field gel electrophoresis (PFGE) was used to analyze the homology, and the technique of multilocus sequencing typing (MLST) was used to identify the sequence type (ST). Results: Antibiotic susceptibility testing implied that vancomycin, daptomycin and linezolid are 100% sensitive to collected 149 strains of MRSA. Cotrimoxazole resistance rate is about 0-11.1%. Rifampicin resistant rate was less than 25% in 2 hospitals; the resistance rate of gentamicin and moxifloxacin were more than 80% besides of 50% to70% in 3 hospitals; beta lactam resistant rate was 100%. In 149 strains of MRSA, the main types of PFGE were J (28.9%), C (19.5%), G (10.7%), F (8%)types. J, C, G types mainly distributed in the North, while the F type only distributed in the Guiyang region. The MLST type: 8 ST types were determined ultimately. In which, was dominated by ST-239(67 strains, 45%), distributed in the South and North; followed by ST-5 (54 strains, 36.2%), mainly in the Northeast region (χ2=26.42, P<0.01). Conclusions: Vancomycin, daptomycin and linezolid are 100% sensitive drugs to MRSA in ICU; Higher regional homology for MRSA were observed and it is probably that homologic disseminated infection exited in ICU. It is necessary to enhance continuous monitoring and take effective nosocomial infection control action to avoid MRSA homologic outbreak.

Chondro-protective effects of low intensity pulsed ultrasound.

OBJECTIVES: Cartilage is a highly mechano-responsive tissue. Chondrocytes undergo a series of complex changes, including proliferation and metabolic alteration as the target of external biomechanical and biochemical stimuli. IL-1ß is known to regulate chondrocyte metabolism and plays an important role in the pathogenesis of osteoarthritis (OA). The objective of this study was to employ low-intensity pulsed ultrasound (LIPUS) as a localized mechanical stimulus and assess its effects on chondrocyte migration, proliferation, metabolism, and differentiation, as well as its ability to suppress IL-1ß mediated catabolism in cartilage. METHODS: Human cartilage explants and chondrocytes were stimulated by LIPUS in the presence and absence of IL-1ß to asses cartilage degradation, chondrocytes metabolism, migration, and proliferation. Western blot analyses were conducted to study IL-1ß the associated NFκB pathway in chondrocytes. RESULTS: LIPUS stimulation increased the proteoglycan content in human cartilage explants and inhibited IL-1ß induced loss of proteoglycans. LIPUS stimulation increased rates of chondrocyte migration and proliferation, and promoted chondrogenesis in mesenchymal stem cells (MSC). Further, LIPUS suppressed IL-1ß induced activation of phosphorylation of NFκB-p65 and IĸBα leading to reduced expression of MMP13 and ADAMT5 in chondrocytes. CONCLUSIONS: Collectively, these data demonstrate the potential therapeutic effects of LIPUS in preventing cartilage degradation and treating OA via a mechanical stimulation that inhibits the catabolic action of IL-1ß and stimulates chondrocyte migration, proliferation, and differentiation.

Hemerythrin is required for Aeromonas hydraphlia to survive in the macrophages of Anguilla japonica.

Survival in host phagocytes is an effective strategy for pathogenic microbes to spread. To understand the mechanisms of Aeromonas hydrophila survival within host macrophages, a library of mini-Tn10 transposon insertion mutants was constructed. The M85 mutant, whose survival in host macrophages was only 23.1% of that of the wild-type (WT) strain, was utilized for further study. Molecular analysis showed that a 756-bp open reading frame (ORF) (GenBank accession No. CP007576) in the M85 mutant was interrupted by mini-Tn10. This ORF encodes for a 183-amino acid protein and displays the highest sequence identity (99%) with the hemerythrin (Hr) protein of A. hydrophila subspecies hydrophila ATCC 7966. The survival of the WT, M85 mutant, and complemented M85 (Hr) strains were compared in host macrophages in vitro, and the results showed that M85 exhibited defective survival, while that of M85 (Hr) was restored. To investigate the possible mechanisms of A. hydrophila survival in host macrophages, the expression of Hr under hyperoxic and hypoxic conditions was evaluated. The results revealed that the expression of this protein was higher under hyperoxic conditions than under hypoxic conditions, which indicates that Hr protein expression is sensitive to O2 concentration. Hydrogen peroxide sensitivity tests further suggested that the M85 mutant was more sensitive to oxidative stress than the WT and M85 (Hr) strains. Taken together, these results suggest that the Hr protein may act as an O2 sensor and as a detoxifier of reactive oxygen species, and is required for A. hydrophila survival within host macrophages.

FlgN plays important roles in the adhesion of Aeromonas hydrophila to host mucus.

Adhesion to the host mucus is a crucial step in the early infection stage of pathogenic bacteria. To investigate the mechanisms of the adhesion of Aeromonas hydrophila to its host mucus, a mutant library was constructed using the mini-Tn10 transposon mutagenesis system. Of 276 individual colonies, the mutant strain with the most attenuated adhesion ability in this study was screened out and designated A77. Molecular analysis showed that a 414-bp sequence flanking mini-Tn10 in A77 had the highest identity (97%) with the bacterial flagellar protein gene flgN. A complemented strain flgN+ was constructed and the biological characteristics of the wild-type, mutant A77, and complemented flgN+ strains were investigated. The results showed that the decreased abilities of motility, adhesion to mucus, and biofilm formation in the mutant strain were partially recovered in the complemented flgN+ strain, which suggested that flgN plays an important role in the adhesion of A. hydrophila to its host.

Role of MshQ in MSHA pili biosynthesis and biofilm formation of Aeromonas hydrophila.

Biofilm formation of pathogen bacterium is currently one of the most widely studied topics; however, little is known regarding pathogen bacteria biofilms in aquaculture. Aeromonas hydrophila is a representative species of the genus Aeromonas, which has been recognized as a common pathogen, is associated with many diseases in aquatic animals, and causes significant mortality. The objectives of this study are i) to confirm that A. hydrophila can form biofilms on abiotic substrates and construct a biofilm growth curve for this bacterium; ii) to identify the genes that play crucial roles in A. hydrophila biofilm formation. The biofilm growth curve of A. hydrophila was constructed using a crystal violet assay, which showed that biofilm formation for this bacterium is a dynamic process. Next, a mutant library of pathogenic A. hydrophila B11 was constructed using the mini-Tn10 transposon mutagenesis system. A total of 861 mutants were screened, and 5 mutants were stably deficient in biofilm formation. Molecular analysis of the mutant B112 revealed that the open reading frame that encodes the protein MshQ was disrupted. Comparison of biological characteristics including growth, motility, and adhesion between the mutant B112 and the wild-type strain B11 suggested that MshQ is necessary for mannose-sensitive hemagglutinin pilus biosynthesis of A. hydrophila, and that these pili play crucial roles in A.hydrophila adherence to a solid surface during the early stages of biofilm formation.

Characterization of yellow grouper Epinephelus awoara (Serranidae) karyotype by chromosome bandings and fluorescence in situ hybridization.

The cytogenetics of yellow grouper Epinephelus awoara was studied using multiple cytogenetic markers [Giemsa staining, C-banding, Ag-NORs and fluorescence in situ hybridization (FISH)]. Giemsa staining results showed that the karyotypic formula of E. awoara was 2n = 48a, FN (fundamental number) = 48. Faint C-bandings were only detected at the centromeric regions of chromosome pair number 24, being almost indiscernible on the other chromosome pairs. After Ag-NOR staining, one pair of nucleolar organizer regions (NOR) was observed in the subcentromeric region of pair number 24. FISH results showed that 5S rDNA was located at a pair of medium-sized chromosomes, while 18S rDNA appeared at the same location in the subcentromeric region of pair number 24 where Ag-NORs were detected. The telomeric sequence (TTAGGG)(n) detected by FISH was located at both ends of each chromosome. The results suggested that E. awoara has retained general karyotypic structure stability during the evolutionary diversification process.

Dynamic skeletal muscle stimulation and its potential in bone adaptation.

To identify mechanotransductive signals for combating musculoskeletal deterioration, it is essential to determine the components and mechanisms critical to the anabolic processes of musculoskeletal tissues. It is hypothesized that the interaction between bone and muscle may depend on fluid exchange in these tissues by mechanical loading. It has been shown that intramedullary pressure (ImP) and low-level bone strain induced by muscle stimulation (MS) has the potential to mitigate bone loss induced by disuse osteopenia. Optimized MS signals, i.e., low-intensity and high frequency, may be critical in maintaining bone mass and mitigating muscle atrophy. The objectives for this review are to discuss the potential for MS to induce ImP and strains on bone, to regulate bone adaptation, and to identify optimized stimulation frequency in the loading regimen. The potential for MS to regulate blood and fluid flow will also be discussed. The results suggest that oscillatory MS regulates fluid dynamics with minimal mechanical strain in bone. The response was shown to be dependent on loading frequency, serving as a critical mediator in mitigating bone loss. A specific regimen of dynamic MS may be optimized in vivo to attenuate disuse osteopenia and serve as a biomechanical intervention in the clinical setting.

Bone surface topology mapping and its role in trabecular bone quality assessment using scanning confocal ultrasound.

INTRODUCTION: Quantitative ultrasound (QUS) has been used to assess non-invasively bone quality, in which ultrasound velocity (UV) is a primary acoustic property. METHODS: While UV calculation requires the tissue thickness in the ultrasound path, a bone surface topology mapping (STM) method was developed in this study for enhancing the accuracy of the UV measurement. STM accuracy was verified by both aluminum and a QUS heel phantom, indicating that the STM can determine the phantom thickness within 0.02 mm thickness error and the aluminum calibration step within 0.1 mm thickness error. STM performance was further evaluated using 25 cadaveric human calcanei samples. RESULTS: The UV calculations using STM had a significant better correlation to bone mineral density (BMD) (r = 0.75, p < 0.05), volume fraction (r = 0.72, p < 0.05) and modulus (r = 0.69, p < 0.05) than the UV with fixed thickness. The later correlation coefficients were r = 0.64 for BMD, r = 0.65 for volume fraction, and r = 0.58 for modulus. The nBUA value determined using STM was also highly correlated to BMD (r(2) = 0.74) and modulus (r(2) = 0.62). This was comparable to the correlation result for BUA (BMD: r(2) = 0.76; Modulus: r(2) = 0.64). CONCLUSION: These results suggested that STM technique in scanning ultrasound is capable of determining calcaneus bone thickness and hence enhancing the accuracy of UV measurement.

Rapid establishment of chemical and mechanical properties during lamellar bone formation.

The development of prophylaxes and treatments of bone diseases that can effectively increase the strength of bone as a structure necessitates a better understanding of the time course by which chemical properties define the stiffness of the material during primary and secondary mineralization. It was hypothesized that these processes would be relatively slow in the actively growing skeleton. Seven-week-old Sprague-Dawley female rats (n = 8) were injected with multiple fluorochrome labels over a time span of 3 weeks and killed. Chemical and mechanical properties of the tibial mid-diaphysis were spatially characterized between the endocortical and periosteal surface by in situ infrared microspectroscopy and nanoindentation. The phosphate-to-protein ratio of bone 2-6 days old was 20% smaller at the periosteal surface and 22% smaller at the endocortical surface (P < 0.05 each) compared to older intracortical regions. The ratios of carbonate to protein, crystallinity, type A/type B carbonate, collagen cross-linking, and bone elastic modulus did not differ significantly between bone 2-6, 10-14, and 8-22 days old and intracortical regions. Intracortical properties of 10-week-old rats, except for the carbonate-to-protein ratio which was 23% smaller (P < 0.01), were not significantly different from intracortical matrix properties of young adult rats (5 months, n = 4). Spatially, the phosphate-to-protein ratio (R(2) = 0.33) and the phosphate-to-carbonate ratio (R(2) = 0.55) were significantly correlated with bone material stiffness, while the combination of all chemical parameters raised the R(2) value to 0.83. These data indicate that lamellar bone has the ability to quickly establish its mechanical and chemical tissue properties during primary and secondary mineralization even when the skeleton experiences rapid growth.

On the relationship between streaming potential and strain in an in vivo bone preparation.

Transcortical streaming potentials were measured at each of two cortical-surface sites with respect to a reference electrode in the medullary canal, in the left ulnae of six live, adult (2 yr-old), male 18.2 +/- 1.4 kg domestic turkeys, under general anesthesia, for each of two loading conditions. We observed that the relationship among streaming potential magnitude, surface strain, and strain gradient is not as simple as anticipated. Under predominantly axial and bending load conditions, significantly different strain and strain gradients were generated at the two recording sites. However, no significant differences were detectable in transcortical streaming potentials for one of the loading conditions, and only a slight difference was detected in the other. Conversely, correlations of streaming potential magnitude to strain at both sites show robust relationships (r2 = 0.45, P - 0.02), albeit with different slopes for the two sites. These findings may have implications for the contribution of streaming potentials, or at least, fluid flow to the stimulation of bone cells.

Patterns of strain in the macaque tibia during functional activity.

The strain environment of the tibial midshaft of two female macaques was evaluated through in vivo bone strain experiments using three rosette gauges around the circumference of the bones. Strains were collected for a total of 123 walking and galloping steps as well as several climbing cycles. Principal strains and the angle of the maximum (tensile) principal strain with the long axis of the bone were calculated for each gauge site. In addition, the normal strain distribution throughout the cross section was determined from the longitudinal normal strains (strains in the direction of the long axis of the bone) at each of the three gauge sites, and at the corresponding cross-sectional geometry of the bone. This strain distribution was compared with the cross-sectional properties (area moments) of the midshaft. For both animals, the predominant loading regime was found to be bending about an oblique axis running from anterolateral to posteromedial. The anterior and part of the medial cortex are in tension; the posterior and part of the lateral cortex are in compression. The axis of bending does not coincide with the maximum principal axis of the cross section, which runs mediolaterally. The bones are not especially buttressed in the plane of bending, but offer the greatest strength anteroposteriorly. The cross-sectional geometry therefore does not minimize strain or bone tissue. Peak tibial strains are slightly higher than the peak ulnar strains reported earlier for the same animals (Demes et al. [1998] Am J Phys Anthropol 106:87-100). Peak strains for both the tibia and the ulna are moderate in comparison to strains recorded during walking and galloping activities in nonprimate mammals.

Ultrasonic wave propagation in trabecular bone predicted by the stratified model.

The objective of this study was to investigate ultrasound propagation in trabecular bone by considering the wave reflection and transmission in a multilayered medium. The use of ultrasound to identify those at risk of osteoporosis is a promising diagnostic method providing a measure of bone mineral density (BMD). A stratified model was proposed to study the effect of transmission and reflection of ultrasound wave within the trabecular architecture on the relationship between ultrasound and BMD. The results demonstrated that ultrasound velocity in trabecular bone was highly correlated with the bone apparent density (r=0.97). Moreover, a consistent pattern of the frequency dependence of ultrasound attenuation coefficient has been observed between simulation using this model and experimental measurement of trabecular bone. The normalized broadband ultrasound attenuation (nBUA) derived from the simulation results revealed that nBUA was nonlinear with respect to trabecular porosity and BMD. The curve of the relationship between nBUA and BMD was parabolic in shape, and the peak magnitude of nBUA was observed at approximately 60% of bone porosity. These results agreed with the published experimental data and demonstrated that according to the stratified model, reflection and transmission were important factors in the ultrasonic propagation through the trabecular bone.

Inhibition of osteopenia by low magnitude, high-frequency mechanical stimuli.

The identification of anabolic agents for the treatment of metabolic bone disease is a highly prized, and elusive, goal. In searching for the osteogenic (bone-producing) constituents within mechanical stimuli, it was determined that high frequency (10-100 Hz) and low magnitude (<10 microstrain) stimuli were capable of augmenting bone mass and morphology, thereby benefiting both bone quantity and quality. Using animal models, it is shown that these mechanical signals can double bone-formation rates, inhibit disuse osteoporosis and increase the strength of trabecular bone by 25%. Considering that the magnitude of these mechanical signals are several orders of magnitude below those which cause damage to the bone tissue, it is proposed that this modality could be useful in the treatment of metabolic bone diseases.

Encoding of location and intensity of noxious indentation into rat skin by spatial populations of cutaneous mechano-nociceptors.

The ability of a spatial population of cutaneous, Adelta, and C mechano-nociceptors to encode the location and intensity of a noxious, cutaneous indentation was examined using an isolated preparation in a rat model. Skin and its intact innervation were harvested from the medial thigh of the rat hindlimb and placed in a dish, with the corium side down, containing synthetic interstitial fluid. The margins of the skin were coupled to an apparatus that could stretch and apply compression to the skin. The skin was suspended on top of a deformable platform whose bulk, nonlinear, compressive compliance emulated that found in vivo. The isolated preparation facilitated examination of the spatial population response by eliminating the nonlinear geometry and inhomogeneous compressive compliance present in-vivo. Spatial population responses (SPR) were formed from recordings of single neurons that were stimulated by compressing the skin with an indenter (flat cylinder, 3-mm diam) at discrete intervals from the center of their receptive fields. SPR were composed of the neural responses (z axis) at each indentation location (x, y plane), and were analyzed quantitatively using nonlinear regression to fit an equation of a Gaussian surface. Both Adelta and C SPR accurately encoded the location and intensity of noxious indentation. The intensity of the stimulus was encoded in the peak neural response of the SPR, which had a nonlinear relationship to the compressive force. The location of the stimulus was encoded in the x, y position of the peak of the SPR. The position of the peak remained constant with increasing magnitudes of compressive force. The overall form of the SPR also remained constant with changes of compressive load, suggesting a possible role for encoding in the SPR some aspects of shape of a noxious stimulus.

Does bone perfusion/reperfusion initiate bone remodeling and the stress fracture syndrome?

Stress fractures have been proposed to arise from repetitive activity of training inducing an accumulation of microfractures in locations of peak strain. However, stress fractures most often occur long before accumulation of material damage could occur; they occur in cortical locations of low, not high, strain; and intracortical osteopenia precedes any evidence of micro-cracks. We propose that this lesion arises from a focal remodeling response to site-specific changes in bone perfusion during redundant axial loading of appendicular bones. Intramedullary pressures significantly exceeding peak arterial pressure are generated by strenuous exercise and, if the exercise is maintained, the bone tissue can suffer from ischemia caused by reduced blood flow into the medullary canal and hence to the inner two-thirds of the cortex. Site specificity is caused by the lack, in certain regions of the cortex, of compensating matrix-consolidation-driven fluid flow which brings nutrients from the periosteal surface to portions of the cortex. Upon cessation of the exercise, re-flow of fresh blood into the vasculature leads to reperfusion injury, causing an extended no-flow or reduced flow to that portion of the bone most strongly denied perfusion during the exercise. This leads to a cell-stress-initiated remodeling which ultimately weakens the bone, predisposing it to fracture.

Skeletal cell stresses and bone adaptation.

There is no tissue in which mechanical stresses have been studied in more detail than the skeletal system, this focus arising primarily because bone plays a clear structural role in the body. However, the hypothesis that the skeleton represents an optimally designed structure has contributed remarkably little to our understanding of the development and adaptive capabilities of bone tissue. Recent investigations on the consequences of mechanical, hydrostatic, and electrical stresses on the cells of bone tissue have served to redirect the discussion of bone modeling and remodeling processes. These studies have refocused attention on the importance of chronic low-level dynamic stresses in mediating the physiologic response of bone tissue. Important recent observations suggest that an approach premised on the self-organizational properties of bone tissue may lead to significant improvements in our understanding and control of bone morphologic development, adaptation, and healing.

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

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