Effects of loading frequency on mechanically induced bone formation.

The anabolic effect of mechanical loading on bone tissue is modulated by loading frequency. The objective of this study was to characterize the new bone formation on the periosteal and endocortical surfaces of the ulnar diaphysis in adult, female rats in response to controlled dynamic loading and to examine the interactions between strain magnitude, loading frequency, and bone formation rate (BFR/BS) for frequencies ranging from 1 to 10 Hz. Cyclic, compressive loading was applied to the ulnas of 60 adult, female rats divided into 12 loading groups. Loading was applied for 360 cycles/day with peak loads ranging from 4.3 to 18N at frequencies of 1, 5, and 10 Hz. After 2 weeks of loading, bone formation on the periosteal and endocortical surfaces of the ulna was quantified using double-label histomorphometry on transverse sections obtained at the middiaphysis. Periosteal bone formation increased in a dose-response manner with peak load at each of the three loading frequencies tested. Loading frequency significantly affected the x intercepts and slopes of the peak strain versus BFR/BS (p < 0.001) and peak strain versus mineralizing surface (MS/BS; p < 0.001) curves. Periosteal osteogenesis was best predicted by a mathematical model that assumed: (1) bone cells are activated by fluid shear stresses and (2) that stiffness of the bone cells and the extracellular matrix near the cells increases at higher loading frequencies because of viscoelasticity. Consequently, mechanotransduction appears to involve a complex interaction between extracellular fluid forces and cellular mechanics.     

Suppression of prostaglandin synthesis with NS-398 has different effects on endocortical and periosteal bone formation induced by mechanical loading

Prostaglandins mediate adaptive bone formation induced by mechanical loading. Inhibition of cyclooxygenase-2 (COX-2) with NS-398 effectively blocks loading-induced osteogenesis on the endocortical bone surface of the tibia. In this study, we compared the effects of selective inhibition of COX-2 with NS-398 on mechanically induced osteogenesis at the endocortical surface (tibia) with that on the periosteal surface (ulna). We further tested the effect of NS-398 administered at different times before (3 hrs or 30 min) or after (30 min) mechanical loading. Mechanical loading induced lamellar bone formation on the endocortical surface of the tibia and the periosteal surface of the ulna. Oral administration of either indomethacin or NS-398 3 hrs before loading significantly decreased loading-induced bone formation rate (BFR) and mineralizing surface (MS/BS), but not mineral apposition rate (MAR), at the endocortical surface of the tibia and the periosteal surface of the ulna. NS-398 reduced loading-induced MS/BS by 96% on the endocortical surface of the tibia, but only by 37% on the periosteal surface of the ulna (significantly different from endocortical, P <0.05). Indomethacin reduced MS/BS and BFR to a lesser extent than NS-398 and did not have different effects on the periosteal and endocortical surfaces. These data suggest that the endocortical bone adaptive response to mechanical loading is more dependent upon COX-2 activity than is the periosteal bone response. Intraperitoneal injection of NS-398 3 hrs before loading suppressed load-induced bone formation rate at the endocortical surface of the tibia significantly more (27%) than when administered 30 min before loading. When NS-398 was given 30 min after loading, bone formation was not significantly suppressed. These data suggest that a primary cellular mechanism of bone formation following brief bouts of mechanical loading involves release of prostaglandins from cells at the time mechanical loading is applied, rather than new prostaglandin synthesis associated with a mechanically induced COX-2 expression.     

Mice lacking thrombospondin 2 show an atypical pattern of endocortical and periosteal bone formation in response to mechanical loading

Thrombospondin 2 (TSP2) is an extracellular matrix (ECM) protein localized to bone. Since mice with a targeted disruption of the TSP2 gene (TSP2-null) have increased bone formation, we hypothesized that mice lacking TSP2 would show an enhanced osteogenic response to mechanical loading. We addressed our hypothesis by subjecting wild-type (WT) and TSP2-null mice to mechanical loading using the non-invasive murine tibia loading device, and statistical comparisons were made between loaded and unloaded bones within genotype, between genotypes, and between the periosteal and endocortical surfaces within genotype. Right tibiae of WT and TSP2-null mice received 5 days of a low-magnitude loading protocol. This low-magnitude loading (inducing approximately 900 and 500 muepsilon at periosteal and endocortical surfaces of WT bones, respectively) affected neither periosteal nor endocortical bone formation rate (BFR/BS) when comparing loaded to intact bones in either WT or TSP2-null mice, nor did it result in any significant differences between WT and TSP2-null. As well, there was no difference between loaded endocortical and periosteal surfaces in WT mice; however, endocortical BFR/BS in TSP2-null loaded tibia was significantly elevated relative to the periosteal BFR/BS-despite peak periosteal strains being significantly greater than endocortical strains in TSP2-null mice (690 versus 460 muepsilon). To confirm this counterintuitive surface-specific response in TSP2-null mice and to induce significant periosteal bone formation, osteogenic potency of the loading protocol was amplified by doubling the number of loading bouts (10 loading days) and loading magnitude (1 Hz, resulting in 1400 and 900 muepsilon peak strain at the periosteal and endocortical surfaces, respectively). Under load, both WT and TSP2-null mice showed significantly increased periosteal mineralizing surface (by nearly three-fold and five-fold, respectively), but mineral apposition rate (MAR) was not statistically changed. The increased MS/BS resulted in a five-fold increase in WT periosteal BFR/BS, but the TSP2-null periosteal BFR/BS was unchanged. Furthermore, this increase in WT loaded periosteal BFR/BS was statistically greater than the WT endocortical BFR/BS. At the endocortical surface of WT mice, loading did not significantly increase bone formation parameters (versus intact). In contrast, at the endocortical surface of TSP2-null mice, loading induced a significant two-fold increase in BFR/BS (versus intact), that was also significantly greater than the endocortical BFR/BS of loaded WT mice. Thus, exogenous loading of TSP2-null mice resulted in highly variable responses that did not reflect the induced strains at the periosteal and endocortical surfaces. While in WT mice, loading resulted in increased periosteal BFR/BS that was greater than the endocortical BFR/BS, in TSP2-null mice loading resulted in endocortical (not periosteal) BFR/BS that was elevated. This reversal in envelope-specific bone formation in TSP2-null mice occurred despite periosteal strains being significantly greater than endocortical (1290 versus 775 muepsilon) and strain distributions being similar to that of WT. These results show that the disruption of a single gene can lead to a reversal in normal pattern of load induced bone formation, and more specifically, that the functional interaction of TSP2 with mechanical loading is highly contextual and specific to the cortical bone envelope examined.     

Low-dose estrogen treatment suppresses periosteal bone formation in response to mechanical loading

Estrogen and exercise influence cortical bone formation. Both affect bone during growth, but with complex interactions. We hypothesized that estrogen reduces the osteogenic response caused by exercise at the periosteal surface of bone, while it enhances bone formation on the endocortical surface. To test our hypothesis, 16 young (8 weeks old) male Sprague-Dawley rats were randomized into two groups: (1) low-dose 17- ethynylestradiol treatment + bone loading (EE₂) or (2) vehicle-treated + bone loading (vehicle). We applied controlled loading to the right ulna at a peak force of 17 N, 2 min/day, 3 days/week for 5 weeks to simulate exercise. The left nonloaded ulna served as an internal control for loading. Mechanical loading increased cortical area (7.7%) and bone mineral content (8%) in the vehicle-treated group (P < 0.05) but only slightly increased cortical area in the EE₂ group (P = 0.08). Histomorphometry showed 1 week of mechanical loading increased periosteal bone formation rate by 29% in the vehicle group and this response was reduced (P < 0.05) to only 15% in the EE₂ group. At the endocortical surface, there were no differences in the loading response between the vehicle and EE₂-treated groups. We conclude low-dose EE₂ suppresses the mechanical loading response on the periosteal surface of long bones, but had no effect on the loading response at the endocortical bone surface in growing male rats.     

Bone Response to In Vivo Mechanical Loading in C3H/HeJ Mice

Bone, being sensitive to mechanical stimulus, adapts to mechanical loads in response to bending or deformation. Although the signal/receptor mechanism for bone adaptation to deformation is still under investigation, the mechanical signal is related to the amount of bone deformation or strain. Adaptation to changes in physical activity depends on both the magnitude of increase in strain above average daily levels for maintaining current bone density and the Minimum Effective Strain (MES) for initiating adaptive bone formation. Given the variation of peak bone density that exists in any human population, it is likely that variation in levels for MES is, to a considerable degree, inherited and varies among animal species and breeds. This study showed a dose-related periosteal response to loading in C3H/HeJ mice. The extent of active formation surface, the rate of periosteal bone formation, and area of bone formation increased with increasing peak periosteal strain. In these mice, the loaded tibia consistently showed lower endocortical formation surface and mineral apposition rate than the nonloaded bones at every load level. Although periosteal expansion is the most efficient means of increasing moment of inertia in adaptation to bending, a dose response increase in endocortical formation would have been predicted. Our characterization of the mouse bone formation response to increasing bending loads will be useful in the design of experiments to study the tibial adaptive response to known loads in different mouse breeds. Received: 17 February 1998 / Accepted: 9 December 1998     

32 wk old C3H/HeJ mice actively respond to mechanical loading

Numerous studies indicate that C3H/HeJ (C3H) mice are mildly responsive to mechanical loading compared to C57BL/6J (C57) mice. Guided by data indicating high baseline periosteal osteoblast activity in 16 wk C3H mice, we speculated that simply allowing the C3H mice to age until basal periosteal bone formation was equivalent to that of 16 wk C57 mice would restore mechanoresponsiveness in C3H mice. We tested this hypothesis by subjecting the right tibiae of 32 wk old C3H mice and 16 wk old C57 mice to low magnitude rest-inserted loading (peak strain: 1235 mu epsilon) and then exposing the right tibiae of 32 wk C3H mice to low (1085 mu epsilon) or moderate (1875 mu epsilon) magnitude cyclic loading. The osteoblastic response to loading on the endocortical and periosteal surfaces was evaluated via dynamic histomorphometry. At 32 wk of age, C3H mice responded to low magnitude rest-inserted loading with significantly elevated periosteal mineralizing surface, mineral apposition rate and bone formation compared to unloaded contralateral bones. Surprisingly, the periosteal bone formation induced by low magnitude rest-inserted loading in C3H mice exceeded that induced in 16 wk C57 mice. At 32 wk of age, C3H mice also demonstrated an elevated response to increased magnitudes of cyclic loading. We conclude that a high level of basal osteoblast function in 16 wk C3H mice appears to overwhelm the ability of the tissue to respond to an otherwise anabolic mechanical loading stimulus. However, when basal surface osteoblast activity is equivalent to that of 16 wk C57 mice, C3H mice demonstrate a clear ability to respond to either rest-inserted or cyclic loading.     

Modulation of appositional and longitudinal bone growth in the rat ulna by applied static and dynamic force

Appositional and longitudinal growth of long bones are influenced by mechanical stimuli. Using the noninvasive rat ulna loading model, we tested the hypothesis that brief-duration (10 min/day) static loads have an inhibitory effect on appositional bone formation in the middiaphysis of growing rat ulnae. Several reports have shown that ulnar loading, when applied to growing rats, results in suppressed longitudinal growth. We tested a second hypothesis that load-induced longitudinal growth suppression in the growing rat ulna is proportional to time-averaged load, and that growth plate dimensions and chondrocyte populations are reduced in the loaded limbs. Growing male rats were divided into one of three groups receiving daily 10 min bouts of static loading at 17 N, static loading at 8.5 N, or dynamic loading at 17 N. Periosteal bone formation rates, measured 3 mm distal to the ulnar midshaft, were suppressed significantly (by 28-41%) by the brief static loading sessions despite normal (dynamic) limb use between the daily loading bouts. Static loading neither suppressed nor enhanced endocortical bone formation. Dynamic loading increased osteogenesis significantly on both surfaces. At the end of the 2 week loading experiment, loaded ulnae were approximately 4% shorter than the contralateral controls in the 17 N static and dynamic groups, and approximately 2% shorter than the control side in the 8.5 N static group, suggesting that growth suppression was proportional to peak load magnitude, regardless of whether the load was static or dynamic. The suppressed growth in loaded limbs was associated with thicker distal growth plates, particularly in the hypertrophic zone, and a concurrent retention of hypertrophic cell lacunae. Negligible effects were observed in the proximal growth plate. The results demonstrate that, in growing animals, even short periods of static loading can significantly suppress appositional growth; that dynamic loads trigger the adaptive response in bone; and that longitudinal growth suppression resulting from compressive end-loads is proportional to load magnitude and not average load.     

Effect of parathyroid hormone on cortical bone response to in vivo external loading of the rat tibia

Cortical bone responses following administration of parathyroid hormone (PTH) were evaluated using a four-point bending device to clarify the relationship between the effect of PTH and mechanical loading. Female Wistar rats, 36-months-old, were used. Rats were randomized into three groups (n = 10/group), namely PTH-5 (5 μg PTH/kg body weight), PTH-30 (30 μg PTH/kg body weight), and PTH-v (vehicle). PTH (human PTH (1–34)) was injected subcutaneously three times/week for 3 weeks. Loads on the right tibia were applied in vivo at 29.1 ± 0.3 N for 36 cycles at 2 Hz 3 days/week for 3 weeks using four-point bending. The administration of PTH and tibial mechanical loading were performed on the same day. After calcein double labeling, rats were killed and tibial cross-sections were prepared from the region with maximal bending at the central diaphysis. Histomorphometry was performed over the entire periosteal and endocortical surfaces of the tibiae, dividing the periosteum into lateral and medial surfaces. The in vivo average peak tibial strains (predicted) on the lateral periosteal surface were 1392.4, 1421.8 and 1384.7 μstrain in PTH-v, PTH-5 and PTH-30 groups, respectively, showing no significant difference among the three groups. Significant loading-related increases in the bone formation surface, mineral apposition rate, and bone formation rate were observed at the periosteal and endocortical surfaces. Significant differences between PTH groups were also seen. Interaction between mechanical loading and PTH was significant at both periosteal and endocortical surfaces. It is concluded that PTH has a synergistic effect on the cortical bone response to mechanical loading. Received: October 4, 2000 / Accepted: January 12, 2001     

Viscoelastic response of the rat loading model: implications for studies of strain-adaptive bone formation.

Studies of the adaptive skeletal response to mechanical loading require appropriate animal models. Two new approaches involve the nonsurgical application of loads to either the ulna or tibia of rats. Both of these approaches require the loading of bone through adjacent soft tissues, and thus the tissue viscoelasticity might affect the way load is transferred to the bone. The objective of this study was to characterize the mechanical strain in the rat tibia or ulna during applied loading at different frequencies. For the rat ulna model, loading was applied to the ulnae of four adult, female rats as a haversine waveform at frequencies of 1, 2, 5, 10, and 20 Hz and peak loads of 5, 10, 15, and 20 N. Mechanical strain was measured on the medial and lateral ulnar surfaces using single element strain gauges. For the rat tibia model, four-point bending loads were applied to the right tibiae of seven rats at frequencies of 0.5, 1, 2, 5, 10, and 20 Hz and peak loads of 30, 40, 50, and 60 N. Mechanical strain was measured on the lateral tibial surface at 5 mm proximal to the tibiofibular junction. We found that peak strains were linearly proportional to applied load, but decreased logarithmically as loading frequency was increased, indicating a significant viscoelastic effect in the soft tissues surrounding the ulnocarpal joint and in the soft tissues surrounding the tibia shaft. The viscoelastic response of the ulna and tibia tends to "filter out" high-frequency loading components and, as a result, the rat loading systems act as a low-pass filter. Consequently, any experiment designed to test the effect of loading frequency on bone formation in the rat ulna and tibia should employ progressively larger loads at higher loading frequencies to guarantee a consistent peak strain magnitude in the bone. The filtering effect of the ulna loading system is illustrated by an analysis of the strain waveforms from the recent study by Mosley and Lanyon (Bone 23:313-318; 1998) that was designed to evaluate the effect of strain rate on bone formation.     

Validation of a technique for studying functional adaptation of the mouse ulna in response to mechanical loading

Functional adaptation of the mouse ulna in response to artificial loading in vivo was assessed using a technique previously developed in the rat. Strain gauge recordings from the mouse ulnar midshaft during locomotion showed peak strains of 1680 muepsilon and maximum strain rates of 0.03 sec(-1). During falls from 20 cm these reached 2620 muepsilon and 0.10 sec(-1). Axial loads of 3.0 N and 4.3 N, applied through the olecranon and flexed carpus, engendered peak strains at the lateral ulnar midshaft of 2000 muepsilon and 3000 muepsilon, respectively. The left ulnae of 17, 17-week-old female CD1 mice were loaded for 10 min with a 4 Hz trapezoidal wave engendering a strain rate of 0.1 sec(-1) for 5 days/week for 2 weeks. The mice were killed 3 days later. The response of the cortical bone of the diaphysis was assessed histomorphometrically using double calcein labels administered on days 3 and 12 of the loading period. Loading to peak strains of 2000 muepsilon stimulated lamellar periosteal bone formation, but no response endosteally. The greatest increase in cortical bone area was 4 mm distal to the midshaft (5 +/- 0.4% compared with 0.1 +/- 0.1% in controls [p < 0.01]). Periosteal bone formation rate (BFR) at this site was 0.73 +/- 0.06 microm(2)/microm per day, compared with 0.03 +/- 0.02 microm(2)/microm per day in controls (p < 0.01). Loading to peak strains of 3000 muepsilon induced a mixed woven/lamellar periosteal response and lamellar endosteal bone formation. Both of these were greatest 3-4 mm distal to the ulnar midshaft. At this level, the loading-induced periosteal response increased cortical bone area by 21 +/- 4% compared with 0.03 +/- 0.02% in controls, and resulted in a BFR of 2.84 +/- 0.42 microm(2)/microm per day, compared with 0.01 +/- 0.01 microm(2)/microm per day in controls (p < 0.05). Endosteal new bone formation resulted in a 2 +/- 0.4% increase in cortical bone area, compared with 0.4 +/- 0.3% in controls, and a BFR of 1.05 +/- 0.23 microm(2)/microm per day, compared with 0.22 +/- 0.15 microm(2)/microm per day in controls (p < 0.05). These data show that the axial ulna loading technique developed in the rat can be used successfully in the mouse. As in the rat, a short daily period of loading results in an osteogenic response related to peak strain magnitude. One important advantage in using mice over rats involves the potential for assessing the effects of loading in transgenics.     

Development of an in vivo rabbit ulnar loading model

Ulnar and tibial cyclic compression in rats and mice have become the preferred animal models for investigating the effects of mechanical loading on bone modeling/remodeling. Unlike rodents, rabbits provide a larger bone volume and normally exhibit intracortical Haversian remodeling, which may be advantageous for investigating mechanobiology and pharmaceutical interventions in cortical bone. Therefore, the objective of this study was to develop and validate an in vivo rabbit ulnar loading model. Ulnar tissue strains during loading of intact forelimbs were characterized and calibrated to applied loads using strain gauge measurements and specimen-specific finite element models. Periosteal bone formation in response to varying strain levels was measured by dynamic histomorphometry at the location of maximum strain in the ulnar diaphysis. Ulnae loaded at 3000 microstrain did not exhibit periosteal bone formation greater than the contralateral controls. Ulnae loaded at 3500, 4000, and 4500 microstrain exhibited a dose-dependent increase in periosteal mineralizing surface (MS/BS) compared with contralateral controls during the second week of loading. Ulnae loaded at 4500 microstrain exhibited the most robust response with significantly increased MS/BS at multiple time points extending at least 2 weeks after loading was ceased. Ulnae loaded at 5250 microstrain exhibited significant woven bone formation. Rabbits required greater strain levels to produce lamellar and woven bone on periosteal surfaces compared with rats and mice, perhaps due to lower basal levels of MS/BS. In summary, bone adaptation during rabbit ulnar loading was tightly controlled and may provide a translatable model for human bone biology in preclinical investigations of metabolic bone disease and pharmacological treatments.     

Low-intensity,high-frequency vibration appears to prevent the decrease in strength of the femur and tibia associated with ovariectomy of adult rats

The effect of low-intensity, high-frequency vibration on bone mass, bone strength, and skeletal muscle mass was studied in an adult ovariectomized (OVX) rat model. One-year-old female rats were allocated randomly to the following groups: start control, sham OVX, OVX without vibration, OVX with vibration at 17 Hz (0.5g), OVX with vibration at 30 Hz (1.5g), OVX with vibration at 45 Hz (3.0g). Vibrations were given 30 min/day for 90 days. During vibration each group of rats was placed in a box on top of the vibration motor. The amplitude of the vibration motor was 1.0 mm. The animals were labeled with calcein at day 63 and with tetracycline at day 84. The tibia middiaphysis was studied by mechanical testing and dynamic histomorphometry and the femur distal metaphysis by mechanical compression. OVX without vibration increased the periosteal bone formation rate and increased the medullary cross-sectional area, i.e., increased the endocortical resorption and outward anteromedial and lateral drifts of cortical bone at the tibia middiaphysis. OVX also resulted in a reduced maximum bending stress of the tibia diaphysis and a reduced compressive stress of the femur distal metaphysis. Vibration at the highest intensity, i.e., 45 Hz, of OVX rats induced a further increase in periosteal bone formation rate and inhibited the endocortical resorption seen in OVX rats. Furthermore, vibration at 45 Hz inhibited the decline in maximum bending stress and compressive stress induced by OVX. Neither OVX nor OVX with vibration influenced skeletal muscle mass. In conclusion, the results support the idea of a possible beneficial effect of passive physical loading on the preservation of bone in OVX animals.     

In vivo fatigue loading of the rat ulna induces both bone formation and resorption and leads to time-related changes in bone mechanical properties and density.

Fatigue loading triggers bone resorption and is associated with stress fractures. Neither the osteogenic response nor the changes in bone mechanical properties following in vivo fatigue loading have been quantified. To further characterize the skeletal response to fatigue loading, we assessed bone formation, mechanical properties, density and resorption in the ulnae of 72 adult rats subjected to a single bout of in vivo loading followed by 0, 6, 12 or 18 days of recovery. Axial, compressive loading (peak force 13.3 N, 2 Hz) was applied to the right forelimb until the ulna was fatigued to a pre-determined level. The left forelimb served as a contralateral control. The primary osteogenic response to fatigue loading was woven bone formation that occurred on the periosteal surface of the ulnar diaphysis and was significantly greater in loaded limbs versus controls at 6, 12 and 18 days (p <.0.05). Ultimate force of the ulna in three-point bending decreased by 50% and stiffness decreased by 70% on day 0 (p < 0.01 vs. control), indicative of acute fatigue damage. By day 12, ultimate force and stiffness had returned to control levels (p > 0.05) and by day 18 had increased 20% beyond controls (p < 0.01). Bone cross-sectional area, moment of inertia, and mineral content increased with recovery time (p < 0.01), consistent with the increases in woven bone formation and mechanical properties. Intracortical resorption space density and osteoclast density also increased with recovery time (p < 0.05), indicating activation of intracortical remodeling. In summary, our findings demonstrate the remarkable ability of the adult skeleton to rapidly form periosteal woven bone and thereby offset the negative structural effects of acute fatigue damage and subsequent intracortical resorption.     

The effects of altered strain environments on bone tissue kinetics

This study defines the alteration in bone tissue kinetics responsible for the "adaptive remodeling" response to altered strain environments. Adult beagle dogs were separated into three experimental groups: ulnar osteotomy, ulnar osteotomy with fracture fixation plate spanning the gap and sham surgery. Four sets of double fluorochrome labels were administered. Prior to sacrifice at 1, 3, and 6 months, strains were measured through rosette strain gages on the cranial and caudal surfaces of the intact radius. Histomorphometric analysis indicated that the increased bone mass in response to elevated strain results from increased activation frequency of modeling with more sites undergoing formation processes than resorption processes on periosteal and endocortical surfaces. Increased remodeling activation did not lead to increased bone mass. There was no evidence that elevated strain changes the individual vigor of osteoclasts or osteoblasts, or that the sigma period was altered by elevated strain.     

In vivo static creep loading of the rat forelimb reduces ulnar structural properties at time-zero and induces damage-dependent woven bone formation

Periosteal woven bone forms in response to stress fractures and pathological overload. The mechanical factors that regulate woven bone formation are poorly understood. Fatigue loading of the rat ulna triggers a woven bone response in proportion to the level of applied fatigue displacement. However, because fatigue produces damage by application of cyclic loading it is unclear if the osteogenic response is due to bone damage (injury response) or dynamic strain (adaptive response). Creep loading, in contrast to fatigue, involves application of a static force. Our objectives were to use static creep loading of the rat forelimb to produce discrete levels of ulnar damage, and subsequently to determine the bone response over time. We hypothesized that 1) increases in applied displacement during loading correspond to ulnae with increased crack number, length and extent, as well as decreased mechanical properties; and 2) in vivo creep loading stimulates a damage-dependent dose-response in periosteal woven bone formation. Creep loading of the rat forelimb to progressive levels of sub-fracture displacement led to progressive bone damage (cracks) and loss of whole-bone mechanical properties (especially stiffness) at time-zero. For example, loading to 60% of fracture displacement caused a 60% loss of ulnar stiffness and a 25% loss of strength. Survival experiments showed that woven bone formed in a dose-dependent manner, with greater amounts of woven bone in ulnae that were loaded to higher displacements. Furthermore, after 14 days the mechanical properties of the loaded limb were equal or superior to control, indicating functional repair of the initial damage. We conclude that bone damage created without dynamic strain triggers a woven bone response, and thus infer that the woven bone response reported after fatigue loading and in stress fractures is in large part a response to bone damage.     

Aged mice have enhanced endocortical response and normal periosteal response compared with young‐adult mice following 1 week of axial tibial compression

With aging, the skeleton may lose its ability to respond to positive mechanical stimuli. We hypothesized that aged mice are less responsive to loading than young‐adult mice. We subjected aged (22 months) and young‐adult (7 months) BALB/c male mice to daily bouts of axial tibial compression for 1 week and evaluated cortical and trabecular responses using micro–computed tomography (µCT) and dynamic histomorphometry. The right legs of 95 mice were loaded for 60 rest‐inserted cycles per day to 8, 10, or 12 N peak force (generating mid‐diaphyseal strains of 900 to 1900 µε endocortically and 1400 to 3100 µε periosteally). At the mid‐diaphysis, mice from both age groups showed a strong anabolic response on the endocortex (Ec) and periosteum (Ps) [Ec.MS/BS and Ps.MS/BS: loaded (right) versus control (left), p < .05]. Generally, bone formation increased with increasing peak force. At the endocortical surface, contrary to our hypothesis, aged mice had a significantly greater response to loading than young‐adult mice (Ec.MS/BS and Ec.BFR/BS: 22 months versus 7 months, p < .001). Responses at the periosteal surface did not differ between age groups (p > .05). The loading‐induced increase in bone formation resulted in increased cortical area in both age groups (loaded versus control, p < .05). In contrast to the strong cortical response, loading only weakly stimulated trabecular bone formation. Serial (in vivo) µCT examinations at the proximal metaphysis revealed that loading caused a loss of trabecular bone in 7‐month‐old mice, whereas it appeared to prevent bone loss in 22‐month‐old mice. In summary, 1 week of daily tibial compression stimulated a robust endocortical and periosteal bone‐formation response at the mid‐diaphysis in both young‐adult and aged male BALB/c mice. We conclude that aging does not limit the short‐term anabolic response of cortical bone to mechanical stimulation in our animal model. © 2010 American Society for Bone and Mineral Research     

Estrogen receptor beta: the antimechanostat?

We have known for sometime that sex hormones influence the growth, preservation, and loss of bone tissue in the skeleton. However, we are only beginning to recognize how estrogen influences the responsiveness of the skeleton to exercise. Frost's mechanostat theory proposes that estrogen reduces the mechanical strain required to initiate an osteogenic response, but this may only occur at the endocortical and trabecular bone surfaces. The discovery of estrogen receptors alpha and beta may help us to understand the bone surface-specific effects of exercise. Findings from estrogen receptor knockout mice suggest that the activity of ERalpha may explain the positive interaction between estrogen and exercise on bone formation near marrow, that is, endocortical and trabecular bone surfaces. Estrogen inhibits the anabolic exercise response at the periosteal surface, and this we propose is due to the activation of ERbeta. Signaling through this receptor retards periosteal bone formation and suppresses gains in bone size and bone strength, and for these reasons it behaves as an antimechanostat.     

Noninvasive loading of the rat ulna in vivo induces a strain-related modeling response uncomplicated by trauma or periostal pressure

Adaptive changes in bone modeling in response to noninvasive, cyclic axial loading of the rat ulna were compared with those using 4-point bending of the tibia. Twenty cycles daily of 4-point bending for 10 days were applied to rat tibiae through loading points 23 and 11 mm apart. Control bones received nonbending loads through loading points 11 mm apart. As woven bone was produced in both situations, any strain-related response was confounded by the response to direct periosteal pressure. Four-point bending is not, therefore, an ideal mode of loading for the investigation of strain-related adaptive modeling. The ulna's adaptive response to daily axial loading over 9 days was investigated in 30 rats. Groups 1–3 were loaded for 1200 cycles: Group 1 at 10 Hz and 20 N, Group 2 at 10 Hz and 15 N, and Group 3 at 20 Hz and 15 N. Groups 4 and 5 received 12,000 cycles of 20 N and 15 N at 10 Hz. Groups 1 and 4 showed a similar amount of new bone formation. Group 4 showed the same pattern of response but in reduced amount. The responses in Groups 2 and 3 were either small or absent. Strains were measured with single-element, miniature strain gauges bonded around the circumference of dissected bones. The 20 N loading induced peak strains of 3500–4500 strain. The width of the periosteal new bone response was proportional to the longitudinal strain at each point around the bone's circumference. It appears that when a bone is loaded in a normal strain distribution, an osteogenic response occurs when peak physiological strains are exceeded. In this situation the amount of new bone formed at each location is proportional to the local surface strain. Cycle numbers between 1200 and 12,000, and cycle frequencies between 10 and 20 Hz have no effect on the bone's adaptive response.     

Bone resorption and formation on the periosteal envelope of the ilium: a histomorphometric study in healthy women.

Continuation of net periosteal bone gain after cessation of longitudinal growth has been inferred from sequential radiographic morphometry. Accordingly, we performed histomorphometry of the periosteal surfaces of transilial bone biopsies from 57 healthy women aged 24-74 years, 29 premenopausal and 28 postmenopausal. Compared to the endocortical surface, the extents of eroded and osteoid surfaces were very similar, but the extents of osteoclast- and osteoblast-covered surfaces were 80-90% smaller, and both wall thickness and osteoid thickness were about 30% lower. Double tetracycline labels were present in only 11 cases. The second (demethylchlortetracycline) label was almost four times as long as the first (oxytetracycline) label, a much greater difference than on the endocortical surface, so that the extent of mineralizing surface was based only on the second label. Even so, adjusted apposition rates and bone formation rates were only about 20% of the endocortical values, and unlike the endocortical surface, formation rates were not higher in the postmenopausal than in the premenopausal women. Resorption, reversal, and formation periods were each much longer than on the endocortical surface. There was no correlation between periosteal and endocortical values for any variable. At least 54% of total cement line length was scalloped, implying reversal of remodeling direction from resorption to formation, and at least 18% of total cement line length was smooth, implying temporary arrest of bone formation. Convincing evidence of modeling, related to growth or mechanical stimulation, was not observed.(ABSTRACT TRUNCATED AT 250 WORDS)     

Skeletal change in response to altered strain environments: is woven bone a response to elevated strain?

Studies demonstrate that geometric changes in bone architecture in response to altered mechanical strain occur through the formation of woven bone. The goal of this study was to test the hypothesis that these changes are partly the result of surgical manipulation rather than a true adaptive response to altered strain. Beagle dogs were subjected to either an ulnar osteotomy, an osteotomy with plate fixation, or sham operation. Strains on the radius were measured just prior to sacrifice 1, 3 or 6 months after surgery. Our results support the idea that woven bone can be a normal response to an abnormal strain environment if the mechanical challenge is intense enough; that elevated mechanical strains can cause the endocortical bone envelope to revert to a state of net formation; and that "adaptive remodeling" in adults in response to a change in mechanical strain may be a special case of modeling in which resorption is not required prior to formation at a particular skeletal site.