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Biomedical subjects

M R Forwood

Publications and source records attributed to M R Forwood.

At least 37 records · Page 2Linked to original sources
PubMed1998

Localisation of prostaglandin endoperoxide H synthase (PGHS)-1 and PGHS-2 in bone following mechanical loading in vivo.

Recent data suggests that induction of prostaglandin endoperoxide H synthase-2 (PGHS-2) is critical for the anabolic response of lamellar bone elicited by mechanical strain in vivo. The aim of the present study was to localise PGHS-1 and PGHS-2 in rat tibiae following four-point bending in vivo. Right tibiae of 19 adult female rats were subjected to 300 cycles of bending or sham loading at 2.0 Hz with an applied load of 65 N. At 0, 6, and 24 hr postloading, rats were anaesthetised and perfused with Bouin's fixative. Left and right tibiae were dissected, postfixed for 4 hr at 4 degrees C, decalcified in EDTA, and embedded in paraffin. Serial 5 pM sections were stained for PGHS-1 and PGHS-2 using standard immunoperoxidase procedures. For the first time, immunoreactivity for both PGHS-1 and PGHS-2 was localised in bone cells in situ, in the rat tibia. PGHS-1 was distributed widely in all tibiae, while PGHS-2 showed sparse localisation. At the endocortical surfaces (EcS), osteoblasts, lining cells, and osteocytes close to the surface reacted strongly for PGHS-1, as did intracortical osteocytes. At the periosteal surface (PsS), osteoblasts and cells of the osteogenic region were immunopositive. Immediately after loading, the numerical density (n.mm(-2)) of osteocytes labeled with PGHS-1 was significantly greater in loaded tibiae compared to controls. This increase was not seen after sham loading. At 6 and 24 hr postloading, this difference was no longer evident. Staining for PGHS-2 was sparse compared to PGHS-1. Light to moderate reactivity was observed in osteocytes and canaliculae, but the numerical density of labeled cells was significantly less than that for PGHS-1. Moderate staining was seen in lining cells and osteoblasts at the EcS and PsS of some tibiae. Osteoclasts at the PsS reacted strongly for both PGHS-1 and PGHS-2. There was a similar load-related increase in the density of PGHS-2-labeled osteocytes 0 hr postloading. The labeled osteocyte density had decreased at 6 hr, but remained significantly greater in loaded bones. These results show that both forms of PGHS can be localised in bone cells, with PGHS-1 expressed to a greater extent than PGHS-2. The data also suggest that both PGHS-1 and PGHS-2 may play important roles in the early response of bone to mechanical loading in vivo.

Adaptation, Physiological↗
PubMed1998

En bloc staining of bone under load does not improve dye diffusion into microcracks.

Microdamage accumulation in bone has been implicated in the pathogenesis of some bone fractures, and in implant loosening. Standard techniques for staining microcracks may not allow all cracks to be stained. We tested the hypothesis that crack closure in bone cortices after removal of a bending load may prevent diffusion of stain to sites of microcrack nucleation. Following cyclic loading, 26 canine femurs were divided into a group stained en bloc while applying a four point bending load, and another group stained without an applied load. No differences in number or length of microcracks were observed, indicating that crack closure does not prevent diffusion of stain to the crack location. Staining under load is unnecessary.

Animals↗
PubMed1998

Does microdamage accumulation affect the mechanical properties of bone?

It has never been demonstrated that microcrack accumulation in bone leads to impaired mechanical properties. We hypothesized that microdamage accumulation is positively and linearly correlated with a reduction in bone's elastic modulus. We also tested the hypothesis that damage accumulates more rapidly in tensile cortices, but crack growth is greater in compressive cortices. Canine femurs (n = 26) were tested in four-point cyclic bending under load control until they had lost between 5 and 43% of their stiffness. Ten femurs were used as nonloaded controls. The loaded portion of the bone was stained en bloc with basic fuchsin to detect the presence of microdamage. The number of stained microcracks, their lengths and the area of damaged bone were measured under the microscope. Crack numerical density, surface density, mean crack length, and the percentage of damaged area were calculated. Significant microdamage accumulation was not detected until the bone had lost 15% of its elastic modulus. The relationship between crack density and stiffness loss was approximately quadratic, but the relationship between damaged area and stiffness loss was linear. There were significantly more microcracks in tensile cortices, but on average cracks were significantly longer in compressive cortices. We conclude that microcrack accumulation impairs the mechanical properties of bone by reducing its elastic modulus. We also conclude that damage accumulates more rapidly in tensile cortices, but crack growth is greater in compressive cortices.

Animals↗
PubMed1998

A new method to measure elastic properties of plastic-viscoelastic connective tissue.

An experimental protocol was tested to measure elastic properties of connective tissue displaying viscoelastic as well as plastic properties. The protocol consisted of a slow rate, linear elongation (0.88 mms(-1), 8 mm) in combination with a superimposed sinusoidal vibration of small amplitude (50 Hz, 0.1 mm). Using digital filters and mathematical algorithms, the force responses to linear elongation and to vibration were obtained. The method was tested on excised fibromuscular tissue of the vaginal wall obtained from women who suffered a vaginal prolapse. The force-stiffness and force-elongation relationships based on the vibration response were unaffected by any long-term deformation of the specimens. The directly measured force-elongation curves were strongly affected by these deformations. It was therefore concluded that with the new method, it is possible to determine the elastic properties accurately. Furthermore, this method seems more sensitive to small changes in elastic properties than the classic tensile test.

Adult↗
PubMed1998

Assessment of cancellous bone quality in severe osteoarthrosis: bone mineral density, mechanics, and microdamage.

The role of bone microdamage (microscopic cracks or microcracks and ultrastructural collagen matrix and bone mineral damage) in diseases such as osteoarthrosis and osteoporosis is poorly understood. Microdamage accumulation in vivo is influenced by age and cyclic loading, therefore, it would be useful if the burden of microdamage in bone could be assessed by noninvasive measures such as the radiological measurement of bone mineral density (BMD). The aim of this study was to investigate the relationship between BMD, compressive strength and stiffness, and microdamage in the cancellous bone of the proximal femur in patients with severe osteoarthrosis. Trabecular bone core samples, from the intertrochanteric region of the femur, were obtained from 34 patients, with a mean age of 70.3 +/- 11.1 years, undergoing total hip arthroplasty for osteoarthrosis. Cores selected from contact X-ray images were used for BMD measurement, compressive mechanical testing or left untested (uncrushed), en bloc staining for microdamage, and bone histomorphometry. The study shows a strong dependence of both the elastic modulus and ultimate failure stress of the bone samples on BMD and a significant relationship between the elastic modulus and trabecular anisotropy (Tr. An). In multiple linear regression, BMD and Tr. An together account for about 70% of the variance in the elastic modulus. Then including microcrack crack density (Cr.Dn) and damage volume fraction (DxV/BV) variables, Tr. An alone accounts for a relatively small amount of the variation (8.5%) in ultimate failure stress and elastic modulus. The Cr.Dn accounts for more of the variation in the ultimate failure stress than in the elastic modulus (50% vs. 7%). In this experiment, data for Cr.Dn provide a measure of damage associated with the ultimate failure of cancellous bone. In specimens that were not mechanically tested, in vivo microcrack accumulation increases exponentially with age. In conclusion, data from this study suggest that BMD and Cr.Dn are the major determinants of cancellous bone strength, whereas BMD and Tr. An are major determinants of cancellous bone stiffness. In bone specimens subjected to compressive testing there was no relationship between microdamage and BMD, suggesting that BMD cannot be used to monitor changes in the mechanical properties of bone due to microdamage accumulation.

Absorptiometry, Photon↗
PubMed1998

Modification of the in vivo four-point loading model for studying mechanically induced bone adaptation.

We modified the noninvasive, in vivo technique for strain application in the tibiae of rats (Turner et al., Bone 12:73-79, 1991). The original model applies four-point bending to right tibiae via an open-loop, stepper-motor-driven spring linkage. Depending on the magnitude of applied load, the model produces new bone formation at periosteal (Ps) or endocortical surfaces (Ec.S). Due to the spring linkage, however, the range of frequencies at which loads can be applied is limited. The modified system replaces this design with an electromagnetic vibrator. A load transducer in series with the loading points allows calibration, the loaders' position to be adjusted, and cyclic loading completed under load control as a closed servo-loop. Two experiments were conducted to validate the modified system: (1) a strain gauge was applied to the lateral surface of the right tibia of 5 adult female rats and strains measured at applied loads from 10 to 60 N; and (2) the bone formation response was determined in 28 adult female Sprague-Dawley rats. Loading was applied as a haversine wave with a frequency of 2 Hz for 18 sec, every second day for 10 days. Peak bending loads were applied at 33, 40, 52, and 64 N, and a sham-loading group was included at 64 N. Strains in the tibiae were linear between 10 and 60 N, and the average peak strain at the Ps.S at 60 N was 2664 +/- 250 microstrain, consistent with the results of Turner's group. Lamellar bone formation was stimulated at the Ec.S by applied bending, but not by sham loading. Bending strains above a loading threshold of 40 N increased Ec lamellar bone formation rate, bone forming surface, and mineral apposition rate with a dose response similar to that reported by Turner et al. (J Bone Miner Res 9:87-97, 1994). We conclude that the modified loading system offers precision for applied loads of between 0 and 70 N, versatility in the selection of loading rates up to 20 Hz, and a reproducible bone formation response in the rat tibia. Adjustment of the loader also enables study of mechanical usage in murine tibia, an advantage with respect to the increasing variety of transgenic strains available in bone and mineral research.

Adaptation, Physiological↗
PubMed1998

Three-dimensional confocal images of microdamage in cancellous bone.

The accumulation of microdamage in bone may contribute to loss of bone quality in osteoporosis, and the role of microdamage in the etiology of fatigue fractures is unknown. Microdamage created during testing, ex vivo, can increase the fragility of bone by decreasing the load necessary to cause fracture. Microdamage can also accumulate in vivo, but its influence on bone fragility is unknown. To date, stained microcracks are the only criteria to have been correlated with bone mechanics, leaving the influence of ultrastructural damage on bone fragility open for scrutiny. Staining en bloc has identified three morphological features in the tissue, discrete microcracks, cross-hatch staining, and diffuse staining. The relationship between these features and their identification as microdamage remains equivocal. The purpose of this study was to investigate the three-dimensional nature of microdamage in cancellous bone and also to describe stained microcracks, cross-hatch staining, and diffuse staining and to determine whether they all relate to microdamage in bone. Laser scanning confocal microscopy that provides improved spatial resolution over bright-field microscopy was used to visualize bone damage. It was found that crack surface density was highly correlated with crack density (r = 0.95, p < 0.0001), suggesting that the crack surface of preexisting cracks increases as new cracks are formed or submicroscopic cracks become visible under bright-field microscopy. Cross-hatch staining and diffuse staining included ultra-microcracks about 10 microm in length. The ultra-microcracks in cross-hatch staining were organized in bands and surrounded by diffuse staining. This study demonstrates that damage in bone occurs over a wide range and that discrete microcracks, cross-hatch staining, and diffuse staining are all indicative of bone damage. The diffuse staining still evident in association with the ultra-microcracks seen in cross-hatch staining and diffuse staining is probably due to damage at a still smaller scale than we have been able to investigate.

Adult↗
PubMed1997

Bone microdamage and skeletal fragility in osteoporotic and stress fractures.

The accumulation of bone microdamage has been proposed as one factor that contributes to increased skeletal fragility with age and that may increase the risk for fracture in older women. This paper reviews the current status and understanding of microdamage physiology and its importance to skeletal fragility. Several questions are addressed: Does microdamage exist in vivo in bone? If it does, does it impair bone quality? Does microdamage accumulate with age, and is the accumulation of damage with age sufficient to cause a fracture? The nature of the damage repair mechanism is reviewed, and it is proposed that osteoporotic fracture may be a consequence of a positive feedback between damage accumulation and the increased remodeling space associated with repair.

Aged↗
PubMed1996

Inducible cyclo-oxygenase (COX-2) mediates the induction of bone formation by mechanical loading in vivo.

In vivo, indomethacin blockade of bone formation has been used to illustrate the role of prostaglandins. Indomethacin blocks the constitutive (COX-1) and inducible (COX-2) forms of cyclo-oxygenase, and is therefore nonspecific in its action. To test the hypothesis that COX-2 mediates the bone formation response to loading, rats were treated with vehicle, NS-398 (a specific COX-2 inhibitor) or indomethacin at 0.02, 0.2, or 2.0 mg/kg p.o. 3 h before loading the right tibia in four-point bending. Bending or sham loads of 65 N were applied for one bout of 300 cycles and bone formation assessed 5-8 days after loading. Mechanically induced bone formation at the endocortical surface was calculated by subtracting formation indices of the left leg (control) from those of the right (loaded), and woven bone surface and area were measured at the periosteal surface. Endocortical bone formation was significantly increased by bending but not sham loading (p < 0.05). The increase in the endocortical bone formation rate and mineralizing surface caused by bending was only partially inhibited by indomethacin, even at the highest dose, whereas NS-398 completely blocked bone formation at all doses (p < 0.05). The mineral apposition rate was depressed in a dose-response fashion by NS-398 (p < 0.05), but not by indomethacin. Woven bone formation at the periosteal surface was not prevented by treatment with indomethacin nor NS-398, suggesting that its formation is not dependent on prostaglandin production. These data suggest that induction of COX-2 is important for lamellar bone formation elicited by mechanical strain.

Analysis of Variance↗
PubMed1996

Increased bone formation in rat tibiae after a single short period of dynamic loading in vivo.

Based on our quantum concept for mechanically adaptive bone formation, we hypothesized that a single bout of loading would increase bone formation at the endosteal surface in rat tibiae, with a maximal response 4-8 days after loading and a stimulus-response relationship for load magnitude. Bending loads were applied to right tibiae of rats at 31, 43, 53, or 65 N for a single bout of 36 or 360 cycles; bone formation was assessed 1-4, 5-8, or 9-12 days after loading. A single loading episode increased lamellar bone formation rate (BFR) in all groups (P<0.05) and was maximal 5-8 days after loading. A distinct dose-response relationship was not evident among all load magnitudes or for duration, but 65 N was significantly more osteogenic than loads of 31-53 N (P<0.05), consistent with a threshold response to loading. There was also evidence for a significant increase in BFR (P<0.05) and double-labeled surface (P<0.01) within 4 days of loading, suggesting that bone-lining cells were activated directly by the stimulus. Thus subtle changes in BFR may occur by modulating the activity of surface cells, but large modeling drifts and anabolic responses require recruitment and differentiation of osteoprogenitor cells near the bone surface.

Analysis of Variance↗
PubMed1995

High frequency components of bone strain in dogs measured during various activities.

Rubin et al. [J. Biomechanics 23, 43-54 (1990)] proposed that high frequency bone strains in the 15-30 Hz range might have significant influence on the morphological adaptation of bone tissue. We sought to further their findings by determining the magnitude of high frequency strains during various activities. We measured strains in the forelimbs of dogs during walking, standing and with their limbs unweighted. As negative controls, we measured strains while the dogs were tranquilized and after sacrifice. Strains in the 15-30 Hz range were not significantly greater than controls for any activity except walking, in which they were less than 4% as large as in the 0-15 Hz range. The high frequency strains observed during walking seemed to occur prior to footfall, suggesting that they were not the result of foot strike. The biological significance of high frequency bone strains has yet to be determined, but may be less than originally proposed because of the relatively small magnitude of these strains.

Adaptation, Physiological↗
PubMed1995

Risedronate treatment does not increase microdamage in the canine femoral neck.

Cortical and trabecular bone from the femoral neck of 24 adult female beagle dogs was examined for microdamage following 2 years of treatment with risedronate (NE-58095). Specimens of the femoral neck, sectioned between the femoral head and the intertrochanteric groove, were bulk stained in 1% basic fuchsin in graded alcohols and embedded in methylmethacrylate. Five transverse sections of 100 microns from each specimen were examined for microdamage and measurement of cortical and trabecular area, and three sections from each specimen were measured for calculation of trabecular and cortical bone activation frequency (Ac.f) and bone formation rate (BFR/BV) in the superior and anterior regions of the femoral neck. Although no statistical differences were observed among groups for numerical density or length of microcracks, Kruskal-Wallis analysis showed differences among groups for both cortical and trabecular bone area (p < 0.05). Ac.f was significantly lower in both cortical bone (p < 0.05) and trabecular bone (p < 0.005) of the femoral neck at all dosage levels. No significant difference was observed among groups for trabecular mean wall thickness. The hypothesis that microdamage accumulation increases following reduction in Ac.f was not supported for the canine femoral neck in this experiment. This result could be explained by the fact that microdamage does not accumulate following treatment; that transient increases in microdamage at the beginning of the study period had been repaired; or finally, that the canine femoral neck does not reflect weight-bearing conditions of clinical relevance to humans for assessment of microdamage.

Animals↗
PubMed1995

Skeletal adaptations to mechanical usage: results from tibial loading studies in rats.

Wolff's law defines a static relationship between stress trajectories and trabecular architecture. More recent theories have attempted to describe the dynamic relationship between the form of bone and its mechanical environment. Frost's mechanostat theory is unique among these in its distinction between modeling and remodeling processes, lamellar and woven bone formation, mechanical usage windows for activation and its application to disorders of bone and mineral metabolism. Our studies suggest that lamellar and woven bone formation are very different not only in histological appearance, but in the temporal characteristics of their formation. Thus, it is important to distinguish these two histological types when interpreting studies of adaptive bone formation. Studies using the in vivo 4-point bending model in rat tibiae show that static loads do not play a role in mechanotransduction and that bone formation is threshold-driven and dependent on strain rate, amplitude and duration of loading. They have also provided strong indirect evidence that mechanical strains cause interstitial fluid flow that, in turn, activates the bone cell response. Based on these observations, we hypothesize that strain rate determines the vigor of osteoblastic activity and the regularity of loading bouts determines osteoblast recruitment in a "quantum" fashion.

Adaptation, Physiological↗
PubMed1995

The influence of hand guards on forces and muscle activity during giant swings on the high bar.

To investigate the influence of hand guards on the loads experienced by gymnasts during giant swings on the high bar, forces applied to the bar by each hand and muscle activity of the extrinsic finger flexor and wrist extensor muscle groups were measured in 10 male gymnasts as they completed a minimum of three backward giant swings on the high bar. Measurements were made under four conditions of performance: bare hands, with webbing loops, with doweled hand guards (DHG), and a wind-up swing using DHG. Peak reaction forces at the hands were of the order of 2.2 times body weight (BW) on each hand, and were significantly (P < 0.05) lower when swinging bare-handed, compared with the other three conditions. By contrast, the integrated electromyograms showed that both wrist flexor and extensor muscle activity was unchanged across conditions. These results indicate that the use of hand guards allows greater tensile forces to act across the wrist without a measureable increase in forearm muscle activity. Thus, it is possible that there is extra stress on the ligaments of the wrist or at the epiphyseal plates. In adolescent and preadolescent gymnasts, the additional tension on the distal epiphyses of the radius and ulna may have implications for bone growth.

Adult↗
PubMed1994

Mechanical loading thresholds for lamellar and woven bone formation.

Bone formation was measured in rat tibiae after 12 days of applied loading. Bending forces were applied using a four-point loading apparatus. Sham loads were applied at the same magnitudes as bending forces but the loading pads were arranged so that bending was minimized. Bending and sham loading were applied to the right tibiae of rats and the left tibiae served as contralateral controls. Loading was applied as a sine wave with a frequency of 2 Hz for 18 s (36 cycles) per day. The peak magnitude of applied load was 27, 33, 40, 52, and 64 N. Woven bone was observed on the periosteal surface in all animals subjected to loads of 40 N or greater. Periosteal woven bone formation occurred in both bending and sham loading groups. Woven bone formation on the periosteal surface was either absent or responded at a maximal rate if the stimulus threshold was surpassed. The amount of new woven bone and the woven bone-forming surface were independent of the magnitude of applied strain. Bone formation on the endocortical surface was exclusively lamellar. Lamellar bone formation was stimulated by applied bending of the tibia but not by sham loading. Bending strains above a loading threshold of 40 N or about 1050 mu strain increased both bone-forming surface and the mineral apposition rate and subsequently increased the bone formation rate as much as sixfold. No evidence of increased bone formation was seen for applied strains below 1050 mu strain. Examination of bulk stained sections from animals exposed to the highest applied loads showed no evidence of microcracks.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1994

Mechanotransduction in bone: do bone cells act as sensors of fluid flow?

When compact bone is subjected to bending loads, interstitial fluid in the bone matrix flows away from regions of high compressive stress. The amount of interstitial fluid flow is strongly influenced by the loading rate in a dose-dependent fashion. We hypothesize that interstitial fluid flow affects bone formation, and we tested this hypothesis indirectly by measuring the effect of different loading frequencies on bone formation rate in vivo. The right tibiae of adult female rats were subjected to applied bending at frequencies of 0.05, 0.1, 0.2, 0.5, 1.0, and 2.0 Hz for a 2-wk period. The rats were then killed and histomorphometric measurements of bone formation were made of the midshaft of the tibia. Bending of the tibia increased bone formation rate in the higher-frequency (0.5 to 2.0 Hz) loading groups as much as fourfold, yet no increase in bone formation rate was observed for loading frequencies below 0.5 Hz. In a separate experiment, we found stress-generated potentials (SGP) in the rat tibia to increase monotonically with increasing loading frequency. The dose-response relationship between loading frequency and the bone formation response closely resembles the relationship between loading frequency and SGP within bone. The qualitative similarity between these two relationships suggests that increased bone formation is associated with increased SGP, which are caused by interstitial fluid flow. Bone cells are known to be sensitive to electric fields and may respond directly to SGP. Also, fluid shear forces have been shown to stimulate bone cells in culture, so it is possible that increased interstitial fluid flow directly affects bone formation.

Animals↗