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Skeletal Responses to Physical Loading
Abstract
The principal functions of most bones are to provide shape and withstand repetitive load-bearing. The function of bone tissue is to provide both a load-bearing material and an available repository for mineral. The functional responsibilities of bone and bones can only be discharged through the agency of populations of cells whose activity must be regulated, coordinated, and controlled accordingly.
... Ce tissu osseux est remodelé en permanence par une succession de phases de destruction (ostéoclasie) et de phases de synthèse osseuse (ostéogenèse) [30,135,264]. Et c'est aussi grâce à ces phénomènes que l'os peut libérer ou fixer du calcium et du phosphore et donc jouer un rôle dans la régulation du métabolisme phosphocalcique [11,30,60,226] ...
... Une autre hypothèse avancée est que comme l'ostéocyte est une cellule mécanosensible, elle pourrait synthétiser des facteurs de régulations localement qui permettraient un remaniement osseux sous l'effet de contraintes mécaniques auxquelles l'os est exposé [135]. ...
... Au sein de ce tissu non haversien, on observe des canaux vasculaires à l'aspect dilaté et flexueux. L'espace laissé libre par les trabécules est comblé par de la moelle osseuse, composée de cellules-souches des différentes lignées sanguines, contenues dans un treillis conjonctivo-vasculaire .L'architecture du système, la taille et l'orientation des trabécules s'organisent selon la valeur et la direction des contraintes mécaniques s'exerçant sur le tissu osseux spongieux[30,135,264] . ...
Les pertes de substance osseuse chez les Carnivores domestiques sont la conséquence de traumatismes ou de resections thérapeutiques (tumeur, infection, correction orthopédique). L'os est un tissu vivant hautement spécialisé dont l'activité est influencée par des facteurs internes (hormones, signaux biologiques) et externes (contraintes mécaniques). L'auteur présente tout d'abord les différents éléments constitutifs de l'os. Puis il décrit les étapes de la cicatrisation osseuse et précise les limites de cette cicatrisation lorsqu'elle est spontanée. L'auteur étudie ensuite les différents moyens disponibles pour combler une perte de substance osseuse par de l'os (greffe osseuse, allongement). Il y donne les définitions, les sites et les méthodes de prélèvement (pour les greffes) puis les indications et les limites de chaque technique. La dernière partie présente les méthodes alternatives de comblement (substituts minéraux, composés synthétiques non minéraux, substances ostéoinductives) et les nouvelles perspectives en matière de comblement.
... Consequently, management strategies with increased floor space at mixing therefore emerges as a viable option [11,12]. Adequate floor space also supports exercise, maintain muscle tone, and improve bone composition and strength [10,13,14]. However, it is difficult to draw clear conclusions on the minimum floor space requirements as the quality of the pen (e.g., space for subordinate sows to avoid conflict, flooring etc.), the total space shared by the sows, and management strategies are as important. ...
... The use of deep straw bedding and the larger total area of 3.9 m 2 per sow in System I, might have contributed to fewer injuries to the locomotor apparatus as compared to the pens in System II, where straw served solely as enrichment and the floor was partly slatted. The flooring and bedding material in System I might have mitigated potential injuries and the opportunity for exercise and avoiding aggression was greater in System I. Exercise, increased bone density, improved piglet survival, and altered lying behaviour among gilts have been linked to good leg health, enhancing animal welfare and productivity [13,14,31]. Despite System I including more gilts than System II, the instances of arthritis treatment and removal for fractures, injuries, and weakness were lower in System I which may be reflected in the higher number of total and liveborn piglets from sows in System I. ...
Background
Group housing of sows has been extensively studied since the EU banned gestation crating. Well-managed group-housing promotes sow welfare, but the impact varies based on factors such as feeding, group characteristics, and environmental features. Adequate floor space, particularly directly post-mixing, is crucial for social interactions, natural behaviours, and to reduce injuries caused by aggression. The aim of this study was to compare two group-housing systems for gestating sows with respect to productivity, treatment frequency, and removal of sows. Both systems were static but differed in space allowance, quantity of enrichment material and feeding management. System I comprised of large sized pens with deep litter straw bedding, housing in total 40 sows, and System II of smaller sized pens with permanent access to straw, housing 8 to 10 sows.
Results
The mean parity number was 3.1 ± 1.3 in both groups. Sows housed in System I with large groups ( n = 40) in large pens with deep litter straw gave birth to 16.8 ± 0.33 (Least Squares Means, LSM) piglets, compared to 15.4 ± 0.31 (LSM) for sows in System II kept in smaller groups ( n = 8–10) in smaller pens ( p = 0.0005). Medical treatments of sows were more frequent ( p < 0.001) in System II. The incidence of replacement of sows was comparable in both systems, and there was a high occurrence of sows becoming pregnant during the subsequent insemination in both groups.
Conclusions
This study indicated that sows kept in larger groups provided with a larger floor space (a total area of 156 m ² , corresponding to 3.9 m ² per sow) and housed on deep straw had a higher number of liveborn and weaned piglets and lower incidence of antibiotic treatments than sows with less floor space (a total area of 24.5 m ² , corresponding to 2.5–3.1 m ² per sow) and less bedding/manipulable material.
... Consequently, management strategies with increased oor space at mixing therefore emerges as a viable option [11,12]. Adequate oor space also supports exercise, maintain muscle tone, and improve bone composition and strength [10,13,14]. However, it is di cult to draw clear conclusions on the minimum oor space requirements as the quality of the pen (e.g., space for subordinate sows to avoid con ict, ooring etc), the total space shared by the sows and management strategies are as important. ...
... The ooring and bedding material in System I might have mitigated potential injuries. The opportunity for exercise and avoiding aggression was greater in System I. Exercise, increased bone density, improved piglet survival, and altered lying behaviour among gilts have been linked to good leg health, enhancing animal welfare and productivity [13,14,31]. Despite System I including more gilts than System II, the instances of arthritis treatment and removal for fractures, injuries, and weakness were lower in System I which may be re ected in the higher number of total and liveborn piglets from sows in System I. ...
Background
Group housing of sows has been extensively studied since the EU banned gestation crating. Well-managed group housing promotes sow welfare, but the impact varies based on factors such as feeding, group characteristics, and environmental features etc. Adequate floor space, particularly directly post-mixing, is crucial for social interactions, innate behaviours, and to reduce injuries caused by aggression. The aim of this study was to compare two group housing systems for pregnant sows with respect to productivity, treatment frequency, and removal of sows.
Results
The mean parity number was 3.1 ± 1.3 in both groups. Sows housed in System I with large groups (n = 40) in large pens with deep litter straw gave birth to 16.8 ± 0.33 (Least Squares Means, LSM) piglets, compared to 15.4 ± 0.31 (LSM) for sows in System II kept in smaller groups (n = 8–10) in smaller pens (p = 0.0005). Medical treatments of sows were more frequent (p < 0.001) in System II. The incidence of replacement of sows was comparable in both systems, and there was a high occurrence of sows becoming pregnant during the subsequent insemination in both groups.
Conclusions
This study indicated that sows kept in larger groups provided with a larger floor space (a total area of 156 m², corresponding to 3.9 m² per sow) and housed on deep straw had a higher number of liveborn and weaned piglets and lower incidence of antibiotic treatments than sows with less floor space (a total area of 24.5 m², corresponding to 2.5–3.1 m² per sow) and less bedding/manipulable material. Increased total floor space appeared to alleviate the negative effects of mixing, even in larger groups.
... Leg disorders are exacerbated by characteristic inactivity (Weeks et al., 2000), and lack of exercise hinders leg skeletal development and weight-bearing ability (Lanyon, 1993;Rath et al., 2000). In other words, skeletal disorders partially caused by lack of exercise cause lameness, which itself causes inactivity. ...
... These walking difficulties are primarily caused by rapid growth rates, in conjunction with lack of activity, leading to skeletal disorders of the legs (Knowles et al., 2008;Bassler et al., 2013). It has been wellestablished that exercise and weight-bearing during early growth are critical for successful bone development (Lanyon, 1993), and although collagen cross-linking contributing to bone strength increases with age, this may not play a meaningful role in broilers due to the 6-7 wk harvest age (Rath et al., 2000). ...
Genetic selection for fast growth rate and high breast muscle yield in modern broilers has unintended effects on animal welfare and behavior, namely in terms of inactivity and leg disorders. We hypothesized that exercise stimulated through environmental enrichment could positively stimulate pen-wide activity and improve bird welfare. The study objectives were to implement a laser enrichment device to motivate active and feeding behaviors throughout the pen. Twelve hundred Ross 708 broilers were randomly assigned to enrichment (LASER; laser enrichment, or CON; no laser enrichment) for 49 d. Seventy focal birds were randomly assigned to 14 video-recorded pens for behavioral analysis, including focal bird home pen behavior and walking distance. Pen-wide activity was also measured during the 4-min laser periods, four times daily, d0–8, and 1 day weekly, wk 1–6. Focal birds were gait scored wk 1–6, and were euthanized on d42 for tibia bone mineral content, density, and bone breaking strength analysis. Time spent active was increased in LASER-enriched birds compared to CON on wk 3–5 by up to 214% (wk 4), and percent of time at the feeder was increased in LASER-enriched birds by 761% on wk 4 ( P < 0.05). Peak percent of birds following the laser (LASER-enriched pens only) was observed on d0 (8.52%). Over wk 1–6, peak laser-following behavior was observed on wk 3 (3.07% of birds). Percent of birds moving during laser periods was increased in LASER-enriched pens on d0, 1, 2, 6, 7, and 8, with a percent increase of 68.7% observed on d1 ( P < 0.05). Percent of birds moving (laser-following or not) was increased on wk 1, 3, and 4 in LASER-enriched pens, with an increase of 69.7% observed on wk 4 ( P < 0.05). No differences were found in tibia measures. These data indicate that laser enrichment stimulated voluntary locomotion through wk 5 and laser-following behavior through wk 6, and that the relatively small percent of birds actively following the laser stimulated pen-wide movement above the level of the CON through wk 4 on study.
... The same authors attributed higher tibia ash with increased activity of the birds walking to the feeder. Lanyon (1993) has shown similar findings in turkeys, with physical loading being essential in the maintenance of cortical bone. The findings of Nester and Emmerson (1990) and Lanyon (1993) are consistent with Wolff's law, that states bones in a healthy animal will adapt to the loads under which it is placed. ...
... Lanyon (1993) has shown similar findings in turkeys, with physical loading being essential in the maintenance of cortical bone. The findings of Nester and Emmerson (1990) and Lanyon (1993) are consistent with Wolff's law, that states bones in a healthy animal will adapt to the loads under which it is placed. If loading on a particular bone increases, the bone will remodel itself over time to become stronger. ...
A total of 600 Ross 308-day-old male broiler chicks were used in a 28 day digestibility study to investigate the interaction between dietary calcium (Ca) and non-phytate phosphorus (nPP) on the digestibility of minerals and amino acids. Diets were formulated to be nutritionally adequate except for Ca and nPP. Fifteen mash diets based on corn and soya bean meal with varying concentrations of Ca (6.4 to 12.0 g/kg) and nPP (2.4 to 7.0 g/kg) were used. Diets were clustered around total densities of Ca and nPP of 12, 13.5 or 15.0 (g/kg) and within each density, a range of five Ca : nPP ratios (1.14 : 1, 1.5 : 1, 2.0 : 1, 2.75 : 1 and 4.0 : 1) were fed. Birds had free access to feed and water throughout the study. At day 28, birds were euthanised for the determination of apparent ileal mineral and amino acid digestibility. Data were modelled in R version 2.15 using a linear mixed-effects model and interrogation of the data was performed by fitting a low order polynomial function. At high Ca concentrations, increasing nPP led to an increase in the apparent digestibility of minerals. Apparent ileal digestibility of phosphorus (P) was enhanced with increasing dietary nPP up to 5.5 g/kg beyond which no improvements were found. Maximal Ca digestibility was found in diets with >8.0 g/kg Ca with concomitant low concentrations of nPP. Diets with a broader Ca : nPP ratio improved the digestibility of Ca but were deleterious to the digestibility of P. In this study, apparent digestibility of amino acids was broadly unaffected by dietary Ca and nPP concentrations. However, interactions between Ca and nPP were observed for the digestibility of glutamine, tyrosine and methionine (all P
... The same authors attributed higher tibia ash with increased activity of the birds walking to the feeder. Lanyon (1993) has shown similar findings in turkeys, with physical loading being essential in the maintenance of cortical bone. The findings of Nester and Emmerson (1990) and Lanyon (1993) are consistent with Wolff's law, that states bones in a healthy animal will adapt to the loads under which it is placed. ...
... Lanyon (1993) has shown similar findings in turkeys, with physical loading being essential in the maintenance of cortical bone. The findings of Nester and Emmerson (1990) and Lanyon (1993) are consistent with Wolff's law, that states bones in a healthy animal will adapt to the loads under which it is placed. If loading on a particular bone increases, the bone will remodel itself over time to become stronger. ...
The interaction between calcium (Ca) and non-phytate phosphorus (nPP) in broiler nutrition and skeletal health is highly complex with many factors influencing their digestion, absorption and utilisation. The use of an investigative model such as the geometric framework allows a graphical approach to explore these complex interactions. A total of 600 Ross 308-day-old male broiler chicks were allocated to one of 15 dietary treatments with five replicates and eight birds per replicate. Dietary treatments were formulated to one of three total densities of total Ca+nPP; high (15 g/kg), medium (13.5 g/kg) and low (12 g/kg) and at each density there were five different ratios of Ca : nPP (4, 2.75, 2.1, 1.5 and 1.14 : 1). Weekly performance data was collected and at the end of the experiment birds were individually weighed and the right leg removed for tibia ash analysis. Skeletal health was assessed using the latency to lie (LTL) at day 27. At low Ca and high nPP as well as high Ca and low nPP diets, birds had reduced feed intake, BW gain, poorer feed efficiency and lower tibia ash, resulting in a significant interaction between dietary Ca and nPP (P
... At the same time, although radiogrammetric approaches have been shown to be biased in their estimation of bone mass and strength, it is not unreasonable to ask what import such bias might have for comparative study of bone growth, aging, asymmetry, and other questions to which data so acquired have been applied. It is well known that bone geometry is responsive to functional input, and will remodel its configuration ac- cording to changes in its biomechanical environment (Lanyon, 1993). Indeed, it is this relationship which permits biobehavioral interpretation from bone geometry (e.g., Ruff [1987] and the association of sexual dimorphism in lower limb geometry, subsistence and division of labour). ...
Radiogrammetry of the second metacarpal has long served as a measure of normal and abnormal bone growth and aging, functional asymmetry, and fracture risk in osteoporosis. The method relies on algebraic interpretation (circular or elliptical models) of uni- or biplanar radiographic images. This paper tests the conformance of these models with actual measures of metacarpal geometric variation in a sample of 356 bones from an historic archaeological sample. Both the circular and elliptical models significantly over-estimate actual values for all variables (e.g., total area (3.05% and 9.42%, respectively) and cortical area (7.25% and 12.25%), bending rigidity about the mediolateral (13.88% and 20.92%) and anteroposterior axes (17.35% for the elliptical model). The greater degree of systematic bias found for the elliptical model is contrary to results of an earlier study (Lazenby, 1997), and suggests that sample composition can influence the method error associated with a particular approach. The import of radiogrammetric bias will depend on the degree of shape variation among samples compared (e.g., left vs right, male vs female, etc.), which in most cases cannot be predetermined. Consequently, reduced major axis equations were derived from regressing actual on estimated values for total area (TA), cortical area (CA) and two measures of bending rigidity (Ix, Iy) in order to permit adjustment for radiogrammetric error associated with these algebraic models. Application of these formulae to a hold-out sample showed no significant differences between actual and predicted values. Am. J. Hum. Biol. 10:747–756, 1998. © 1998 Wiley-Liss, Inc.
It is generally accepted that the earliest human ancestors grew more like apes than like humans today. If they did so, and we are now different, when, how and why did our modern growth patterns evolve? Originally published in 2003, this book focuses on species within the genus Homo to investigate the evolutionary origins of characteristic human patterns and rates of craniofacial and postcranial growth and development, and to explore unique ontogenetic patterns within each fossil species. Experts examine growth patterns found within available Plio-Pleistocene hominid samples, and analyse variation in ontogenetic patterns and rates of development in recent modern humans in order to provide a comparative context for fossil hominid studies. Presenting studies of some of the newer juvenile fossil specimens and information on Homo antecessor, this book will provide a rich data source with which anthropologists and evolutionary biologists can address the questions posed above.
Modern commercial broilers have been genetically selected for fast growth and heavy breast muscling, contributing to a top-heavy phenotype and increased leg lameness. A quick-growing phenotype coupled with poor leg health fosters inactivity. The objective of this study was to stimulate broiler movement using novel environmental enrichment and determine the impact of movement on production, leg health, and environmental parameters. A total of 1,200 Ross 308 broilers were housed in 40 pens with 30 birds/pen for 6 wk in 2 separate rooms (laser enrichment or control). Each enrichment device was mounted above 2 adjoining pens, projected 2 independent, randomly moving laser beams at the floor to stimulate innate predatory behavior, and was active 4 times daily in 4-min periods. Performance outcomes were calculated by pen and averaged per bird for each performance period and overall days 0 to 42. A total of 70 randomly selected focal birds were examined for breast blisters and footpad dermatitis each week and euthanized on day 42 for tibia quality measures. Air quality and litter moisture were sampled by week. Laser-enriched pens had greater average bird feed intake in starter (P < 0.001), grower (P = 0.004), finisher periods (P = 0.004), and overall days 0 to 42 (0.19 kg/bird; P = 0.0003). Average bird weight gain was also increased in enriched pens in each performance period: starter (P = 0.043), grower (P = 0.001), finisher (P < 0.001), and overall days 0 to 42 (0.24 kg/bird; P < 0.001). Enriched pens had improved feed conversion ratio (FCR) vs. control with a decrease of 3 FCR points in the grower (P = 0.031), 18 points in the finisher (P < 0.001), and 7 points overall (P < 0.001). Enriched pens had higher ADG during starter (P = 0.048), finisher (P < 0.001), and overall (5.7 g/bird/d; P < 0.001). No differences were found in breast blister, footpad dermatitis, tibia, air, or litter quality measures (P > 0.05). In summary, a novel enrichment device based on bird visual feeding and predatory instincts positively affected performance through decreased FCR and increased ADG without sacrificing external animal-based measures, tibia quality, or air or litter quality.
Radiogrammetry of the second metacarpal has long served as a measure of normal and abnormal bone growth and aging, functional asymmetry, and fracture risk in osteoporosis. The method relies on algebraic interpretation (circular or elliptical models) of uni- or biplanar radiographic images. This paper tests the conformance of these models with actual measures of metacarpal geometric variation in a sample of 356 bones from an historic archaeological sample. Both the circular and elliptical models significantly over-estimate actual values for all variables (e.g., total area (3.05% and 9.42%, respectively) and cortical area (7.25% and 12.25%), bending rigidity about the mediolateral (13.88% and 20.92%) and anteroposterior axes (17.35% for the elliptical model). The greater degree of systematic bias found for the elliptical model is contrary to results of an earlier study (Lazenby, 1997), and suggests that sample composition can influence the method error associated with a particular approach. The import of radiogrammetric bias will depend on the degree of shape variation among samples compared (e.g., left vs right, male vs female, etc.), which in most cases cannot be predetermined. Consequently, reduced major axis equations were derived from regressing actual on estimated values for total area (TA), cortical area (CA) and two measures of bending rigidity (Ix, Iy) in order to permit adjustment for radiogrammetric error associated with these algebraic models. Application of these formulae to a hold-out sample showed no significant differences between actual and predicted values. Am. J. Hum. Biol. 10:747-756, 1998. © 1998 Wiley-Liss, Inc.
Our aim was to study the suitability of the ex-vivo human trabecular bone bioreactor ZetOS to test the biocompatibility of calcium phosphate bone cement composites modified with spray dried, drug loaded microspheres. We hypothesized, that this bone bioreactor could be a promising alternative to in vivo assessment of biocompatibility in living human bone over a defined time period. Composites consisting of tetracycline loaded poly(lactic-co-glycolic acid) microspheres and calcium phosphate bone cement, were inserted into in vitro cultured human femora head trabecular bone and incubated over 30 days at 37°C in the incubation system. Different biocompatibility parameters, such as lactate dehydrogenase activity, alkaline phosphatase release and the expression of relevant cytokines, IL-1β, IL-6, and TNF-α, were measured in the incubation medium. No significant differences in alkaline phosphatase, osteocalcin, and lactate dehydrogenase activity were measured compared to control samples. Tetracycline was released from the microspheres, delivered and incorporated into newly formed bone. In this study we demonstrated that ex vivo biocompatibility testing using human trabecular bone in a bioreactor is a potential alternative to animal experiments since bone metabolism is still maintained in a physiological environment ex vivo.
Abuse of alcohol has been found to be an important risk factor for fractures and osteoporosis, and tissue culture experiments have indicated that low concentrations of ethanol can affect bone formation and resorption. This study investigated direct effects of ethanol on bone cells using an in vitro resorption assay. Osteoclasts from long bones of 19-day prehatch chicks were seeded onto slices of dentine and cultured with control medium alone, or medium containing 0.001%, 0.01% or 0.1% ethanol, at 37 degrees C with 5% CO2 for 24 h before being removed. The volumes and areas of resorption pits made in the dentine were measured using confocal laser reflection microscopy (Lasertech 1LM21 system) and the pits counted. An increase in the pit numbers and mean pit areas, volumes and volume/area ratios was observed with 0.001% and 0.01% ethanol, with a dose-related, bell-shaped curve of resorption. Greatest mean volume resorbed per pit (p < 0.05), mean area resorbed per pit (p < 0.01) and number of pits was at 0.01% ethanol. Volume/area (mean depth) per pit was greatest at 0.001% ethanol (p < 0.05). This study has shown that ethanol, even at blood concentrations experienced by the social drinker, has an immediate direct effect on bone cells in vitro, resulting in increased resorption by osteoclasts.
Study of bone mass at the second metacarpal midshaft has contributed to our understanding of skeletal growth and aging within and between populations and has relied extensively on noninvasive techniques and in particular radiogrammetric data. This study reports age, sex, and side variation in size and shape data acquired from direct measurement of cross-sections obtained from a large (n = 356), homogeneous skeletal sample. Correlation analysis and three-way ANOVA of size-adjusted data confirm general impressions of patterned variation in this element: males have absolutely but not necessarily relatively larger bones than females; the right side is larger than the left, though a larger than expected proportion (approximately 25%) of left metacarpals exhibits greater values than the right; and bone mass but not strength (in males) declines with age. Contrary to the widely accepted assumption of circularity for this location, direct measurement of cross-sectional geometry confirms previous biplanar radiogrammetric conclusions regarding the noncircularity of the second metacarpal midshaft and identifies a significant difference between males and females, with the latter having a more cylindrical diaphysis. Deviation of the axes of maximum and minimum bending strength associated with noncircularity suggests a distribution of bone mass to resist bending moments perpendicular to the distal palmar arch, though this conclusion awaits more robust study of the functional anatomy of the metacarpal diaphysis.
Adolescent meat-type poultry and cage layers exhibit a high incidence of bone problems that include bone weakness, deformity, breakage, and infection and osteoporosis-related mortalities. These problems include economic and welfare issues. To improve bone quality in poultry, it is essential to understand the physiological basis of bone maturity and strength in poultry. A complex array of factors that include structural, architectural, compositional, physiological, and nutritional factors interactively determine bone quality and strength. Bone is approximately 70% mineral, 20% organic, and 10% water. Collagen is the major organic matrix that confers tensile strength to the bone, whereas hydroxyapatite provides compressional strength. In recent years, the roles of different collagen crosslinks have been shown to be important in the increase of bone mechanical strength. Similarly, age-related glyco-oxidative modifications of collagen have been shown to increase the stiffness of collagen. These posttranslational modifications of matrix can affect bone quality as it would be affected by the changes in the mineralization process. Our studies show that the growth in the tibia continued until 25 wk of age, which correlated with the increase in the content of hydroxylysylpridinoline (HP) and lysylpyridinoline (LP), the collagen crosslinks. The tibia from 5-wk-old chicks were strong but brittle because of low collagen crosslinks and high mineral content. Bone maturity may relate to its crosslink content. Compared to crosslink content, bone density and ash content showed moderate increases during growth. The bones from younger turkeys were more susceptible to corticosteroid-induced stunting of growth, which also resulted in decreased bone strength. This review discusses how different factors can compromise bone strength by reducing growth, altering shape, affecting mineralization, and affecting collagen crosslinking.
The purpose of this study was to determine the effect of 2 different bedding types, sand and wood shavings, on the behavior of broiler chickens. In experiment 1, 6 pens were divided down the center and bedded half with sand and half with wood shavings. Male broilers (10/pen) were observed by scan sampling at 5- or 12-min intervals throughout the 6-wk growth period during the morning (between 0800 to 0900 h), afternoon (1200 to 1500 h), and night (2300 to 0600 h). There was a significant behavior x substrate x week interaction during the day (P < 0.0001) and at night (P < 0.0002). Drinking, dustbathing, preening, and sitting increased in frequency on the sand side but decreased on the wood shavings side during the day, as did resting at night. In general, broilers performed a greater proportion of their total behavioral time budget on the sand (P < 0.0001) as they aged. Broilers used the divider between the 2 bedding types to perch; perching behavior peaked during wk 4. In experiment 2, male broilers were housed in 8 pens (50 birds/pen) bedded only in sand or wood shavings. Bedding type had no effect on behavioral time budgets (P = 0.8946), although there were age-related changes in behavior on both bedding types. These results indicate that when given a choice, broilers increasingly performed many of their behaviors on sand, but if only one bedding type was provided they performed those behaviors with similar frequency on sand or wood shavings.
Changes in the midshaft cross-sectional area of the ulna were measured in egg-laying turkeys on a diet insufficient in calcium. Left:right comparisons were used to assess the bone loss over a six-week period due to 1) calcium insufficiency, 2) calcium insufficiency plus disuse, and 3) calcium insufficiency and disuse interrupted by a short daily period of intermittent loading applied from an external device. Calcium insufficiency alone in the intact ulna resulted in a 15% reduction in cross-sectional area. In the functionally deprived bones this loss was increased to 32%. In bones where the disuse was interrupted by a single short daily period of loading, the degree of bone loss was significantly modified (P less than 0.006) to 25%. No significant difference in the modulating effect of loading was achieved by varying the peak strain from 0.0015 to 0.003, the strain rate from 0.01 to 0.05, or the duration of the single loading period from 100 sec per day to 25 minutes. All the loading regimes employed had been demonstrated to be osteogenic in mature male birds on a diet sufficient in calcium.
The effect of two structured exercise programmes on the bone mass of 48 healthy postmenopausal white women aged 50-62 was studied after one year. Volunteers were randomised to group 1 (control), group 2 (aerobic exercise), or group 3 (aerobic and strengthening exercises). Before and after the training programme each subject had evaluations of bone mass (determined by neutron activation analysis and expressed as calcium bone index) and maximum oxygen uptake attained on a multistage exercise treadmill test. After one year both exercise groups had higher levels of fitness and greater bone mass than controls. Mean values (2 SEM) for changes in the calcium bone index were -0.011 (0.037), 0.039 (0.035), and 0.066 (0.036) for groups 1, 2, and 3, respectively. Analysis of variance on the observed data and analysis of covariance adjusting changes to the initial mean value for the whole group showed significant differences between each exercise group and the controls but no difference between the exercise groups themselves. Both exercise groups showed a significant improvement in maximum oxygen uptake. This study suggests that exercise may modify bone loss in healthy postmenopausal women.
Both ulnas of skeletally mature roosters (Gallus domesticus) were deprived of functional load bearing by proximal and distal submetaphyseal osteotomies. Twenty-four hours later the animals were injected with 1.5 mCi of 3H-uridine and the ulna on one side was subjected to a single period of a cyclical load engendering physiological strain levels at 1 Hz for 6 min. Twenty-four hours after loading the animals were killed. Autoradiographic examination of comparable regions of cortex in sections from the bone's midshafts showed that in the loaded bones, 72 +/- 2.7% of osteocytes were labeled compared with 12 +/- 3.5% in the corresponding areas of their contralateral nonloaded pair (P less than 0.001). The number of grains per labeled osteocyte was also higher in the loaded side (6 +/- 0.5 compared with 4 +/- 0.5, P less than 0.01). There was no obvious correlation between the longitudinal strain distribution during artificial loading and the distribution of labeled osteocytes throughout the bone cross-section. However, previous long-term experiments using a similar loading preparation had consistently shown the site of most periosteal new bone formation to also not be directly related to the local strain magnitude. Perhaps it is significant that the greatest percentage of labeled cells were found in the cortex where the long-term experiments had shown most new bone formation to subsequently occur.
The in vivo remodeling behavior within a bone protected from natural loading was modified over an 8-week period by daily application of 100 consecutive 1 Hz load cycles engendering strains within the bone tissue of physiological rate and magnitude. This load regime resulted in a graded dose:response relationship between the peak strain magnitude and change in the mass of bone tissue present. Peak longitudinal strains below 0.001 were associated with bone loss which was achieved by increased remodeling activity, endosteal resorption, and increased intra-cortical porosis. Peak strains above 0.001 were associated with little change in intra-cortical remodeling activity but substantial periosteal and endosteal new bone formation.
Remodelling activity in the avian ulna was assessed under conditions of disuse alone, disuse with a superimposed continuous compressive load, and disuse interrupted by a short daily period of intermittent loading. The ulnar preparation consisted of the 110mm section of the bone shaft between two submetaphyseal osteotomies. Each end of the preparation was transfixed by a stainless steel pin and the shaft either protected from normal functional loading with the pins joined by external fixators, loaded continuously in compression by joining the pins with springs, or loaded intermittently in compression for a single 100s period per day by engaging the pins in an Instron machine. Similar loads (525 N) were used in both static and dynamic cases. The strains engendered were determined by strain gauges, and at their maximum around the bone's midshaft were -0.002. The intermittent load was applied at a frequency of 1 Hz as a ramped square wave, with a rate of change of strain during the ramp of 0.01 s-1. Peak strain at the midshaft of the ulna during wing flapping in the intact bone was recorded from bone bonded strain gauges in vivo as -0.0033 with a maximum rate of change of strain of 0.056 s-1. Examination of bone sections from the midpoint of the preparation after an 8 week period indicated that in both non-loaded and statically loaded bones there was an increase in both endosteal diameter and intra cortical porosity. These changes produced a decrease in cross sectional area which was similar in the two groups (-13%).(ABSTRACT TRUNCATED AT 250 WORDS)
Unlabelled:
In studies on a functionally isolated avian-bone preparation to which external loads could be applied in vivo, we determined the following information. Removal of load-bearing resulted in substantial remodeling endosteally, intracortically, and, to a lesser extent, periosteally. Since the balance of this remodeling was negative, bone mass declined. It therefore appears that functional load-bearing prevents a remodeling process that would otherwise lead to disuse osteoporosis. Four consecutive cycles a day of an externally applied loading regimen that engendered physiological strain magnitudes but an altered strain distribution prevented remodeling and was thus associated with no change in bone mass. A small exposure to, or the first effect of, a suitable dynamic strain regimen appears to be sufficient to prevent the negatively balanced remodeling that is responsible for disuse osteoporosis. Thirty-six 0.5-hertz cycles per day of the same load regimen also prevented intracortical resorption but was associated with substantial periosteal and endosteal new-bone formation. Over a six-week period, bone-mineral content increased to between 133 and 143 per cent of the original value. Physiological levels of strain imposed with an abnormal strain distribution can produce an osteogenic stimulus that is capable of increasing bone mass. Neither the size nor the character of the bone changes that we observed were affected by any additional increase in the number of load cycles from thirty-six to 1800.
Clinical relevance:
The results of this experiment must be considered in relation to the type and duration of the non-physiological loads that were imposed.(ABSTRACT TRUNCATED AT 250 WORDS)
The general form of each bone, its attachments, and its anatomical relationships are all genetically determined and will develop in the absence of functional influences. However, the particular features on which each bone’s load-bearing competence depends (ie, its mass, girth, cortical thickness, curvature, and the density and arrangement of its cancellous bone) are all achieved, and will only persist in the presence of continued functional load bearing. Normality of skeletal structure, and the load-bearing competence it reflects, is therefore not a predetermined state but rather the cumulative achievement of local adaptation to load bearing throughout the skeleton. Skeletal fragility may be viewed as a failure or insufficiency in the process which normally matches skeletal structure to its load-bearing requirements.
Excerpt
MRC Unit of Reproductive Biology, 2 Forrest Road, Edinburgh EH1 2QW
(Received 2nd July 1974)
The antlers of red deer, Cervus elaphus, are cast in the spring and a new set develop during the summer months. This cycle of antler renewal is controlled by seasonal changes in testosterone secretion by the testes (Lincoln, Youngson & Short, 1970; Lincoln, 1971). Low levels of circulating testosterone are associated with the casting and regrowth of the antlers, while the high androgen levels in late summer and autumn lead to the maturation of the new antler and the associated shedding of velvet.
During the period of antler growth, testosterone is present in the peripheral blood of stags in very low concentrations ( >200 pg/ml), but the hormone is physiologically active at this time and is associated with the normal development of the fully formed antler. If the testosterone levels are artificially modified by castration or
Part of the reduction in bone density observed in older people is due to disuse rather than the aging process itself. While some mechanical stress is needed to maintain optimal bone density, it is not clear just which types of exercise are most valuable or whether appropriate exercise might reduce the need for estrogen therapy in postmenopausal women.
Cross-sectional studies. Physical activity, aerobic fitness, and strength have all been correlated with bone density. Young people who use a specific part of the body in vigorous exercise exhibit enhanced bone density in that part of the body, but not necessarily in other regions. Older people who have been active for many years seem to exhibit generally enhanced bone density.
Prospective studies. Most regimens which used vigorous aerobic and strength training enhanced bone density, but walking is relatively ineffective for prevention of postmenopausal bone loss. Most studies using specific bone-loading exercise have shown substantial increases in bone density at the specific sites loaded. Elderly people seem capable of responding favorably to vigorous exercise. No direct comparisons of exercise and estrogen therapy have been reported.
Excessive exercise. Extremely high volumes of exercise may overwhelm a person's adaptive capacity, leading to stress fractures. For example, young women athletes who suffer from menstrual dysfunction exhibit reduced bone density and musculoskeletal disorders.
Clinical implication. Although the evidence is far from conclusive, an exercise regimen should probably include vigorous total body exercise, including strength and aerobic training.
Previous studies of Alcian blue-induced birefringence in adult avian cortical bone showed that a short period of intermittent loading rapidly produces an increased level of orientation of proteoglycans within the bone tissue. In the absence of further loading, this persists for over 24 hours. We have proposed that this phenomenon could provide a means for "capturing" the effects of transient strains, and so provide a persistent, constantly updated strain-related influence on osteocyte populations related to the bones' averaged recent strain history, in effect, a "strain memory" in bone tissue. In our present study, we use the Alcian blue-induced birefringence technique to demonstrate that proteoglycan orientation also occurs after intermittent loading of both cortical and cancellous mammalian bone in vivo and in vitro. We also show that the change in birefringence is proportional to the magnitude of the applied strain, and that the reorientation occurs rapidly, reaching a maximal value after only 50 loading cycles. Examination of electron micrographs of bone tissue after staining with cupromeronic blue allows direct visualization and quantification of the change in proteoglycan orientation produced by loading. This shows that intermittent loading is associated with a realignment of the proteoglycan protein cores, bringing them some 5 degrees closer to the direction of collagen fibrils in the bone matrix.
This study investigated the efficacy of 4 years of exercise intervention in deterring bone loss in middle-aged women, and is a correction and extension of previously published data. Sixty-two control subjects (mean age 50.8) and 80 exercise subjects (mean age 50.1) completed a 4-year study. Subjects exercised three times a week, 45 minutes per session. Bilateral radius, ulna, and humerus bone mineral content (BMC) and width (W) were measured on each subject 11 times over the 4-year period. The two groups did not differ initially in age, height, or weight, but the control group had a greater maximum VO2 (ml/kg/min) than the exercise group. Slopes and intercepts of the bone variables vs. time were determined for each subject, and these values were used for between-group comparisons of loss. The control group BMC and BMC/W declined significantly in all three bones in both arms. The exercise group rate of decline was significantly less than that of the control group for 12 of the 18 bone variables. The greatest effect of the exercise intervention was on the ulna and radius. Exercise subjects lost significantly less than control subjects in left and right ulna and radius BMC and BMC/W, and left ulna and radius W. Lesser differences between groups were observed in the humerus. BMC and W loss rates of the left humerus were reduced in the exercise group, with no difference between exercise and control subjects in the other humerus variables. To determine if menopausal status influenced the response to exercise, we analyzed the difference between groups for premenopausal and postmenopausal subjects separately. Regardless of menopausal status, exercise subjects had lower bone loss rates than control subjects. In both premenopausal and postmenopausal subjects, exercise reduced bone loss significantly for 10 of the 18 bone variables. It can be concluded that physical activity significantly reduces bone loss in the arms of middle-aged women.
The capacity of bone to organize and reorganize its structure in response to changing mechanical demands is well recognized. However, the mechanism by which the changing mechanical environment is detected, and the means by which this information is translated into a stimulus for structural modification, are not understood. A group of substances suggested to be involved in the initial transduction of strain information are the prostaglandins. In this experiment we used a single period of dynamic loading to stimulate an adaptive osteogenic response in vivo. Loading was performed in the presence and absence of indomethacin. Measurements of the periosteum 5 days after loading showed that the presence of indomethacin at the time of loading reduced the osteogenic response. Though consistent with the hypothesis that prostaglandins are involved in the initial transduction of tissue strain into a biochemical response, this result is not sufficient to demonstrate this conclusively because reduced prostaglandin levels during the 24 hours immediately after the period of loading may affect many other points in the cascade of events between strain transduction and adaptive new bone formation. Furthermore, indomethacin at the relatively high levels we used (40 mg/kg) may have effects other than those on prostaglandin synthesis.
We previously demonstrated that muscle-building exercise is associated with increases in serum Gla-protein, serum 1,25(OH)2D, and urinary cyclic AMP. These studies were interpreted to mean that this form of exercise increases bone formation and modifies the vitamin D-endocrine system to provide more calcium for bone. The present investigation was carried out in normal young adult white men to determine the effects of exercise on bone mineral density at weight-bearing and nonweight-bearing sites. Twelve men who had regularly engaged in muscle-building exercises (use of weights, exercise machines, or both) for at least 1 year and 50 age-matched controls (aged 19-40 years) were studied. The body weights of the two groups were not different from each other (78 +/- 2 vs. 74 +/- 1 kg, NS). Bone mineral density (BMD) of the lumbar spine, trochanter, and femoral neck was measured by dual-photon absorptiometry, and BMD of the midradius was measured by single photon absorptiometry. It was found that muscle-building exercise was associated with increased BMD at the lumbar spine (1.35 +/- 0.03 vs. 1.22 +/- 0.02 g/cm2, P less than 0.01), trochanter (0.99 +/- 0.04 vs. 0.86 +/- 0.02 g/cm2, P less than 0.01), and femoral neck (1.18 +/- 0.03 vs. 1.02 +/- 0.02 g/cm2, P less than 0.001) but not at the midradius (0.77 +/- 0.02 vs. 0.77 +/- 0.01 g/cm2, NS). These studies provide additional evidence that muscle-building exercise is associated with increases in BMD at weight-bearing sites but not at nonweight-bearing sites.
To determine whether low dietary calcium intake and physical inactivity are risk factors for hip fracture among subjects aged 65 and over.
Fifteen year follow up study of a large cohort of randomly selected elderly people living in the community, who had taken part in the 1973-4 survey of the Department of Health and Social Security, and for whom dietary and other data were recorded at initial interview and medical assessment.
Eight areas in Britain (England (five), Wales (one), and Scotland (two].
1688 Subjects living in the community, of whom 1419 subjects (720 men and 699 women) agreed to participate. 1356 Subjects completed a seven day dietary record and 983 (542 men and 441 women) agreed to be assessed by a geriatrician.
Incidence of hip fracture increased with age and was higher in women than men. Comparison with matched controls showed no evidence that the risk of hip fracture was related to calcium intake: the odds ratio for the lowest third of dietary calcium compared with the highest was 0.7 (95% confidence interval 0.1 to 3.9) after adjustment for smoking and body mass index. The adjusted odds ratio for the lowest third of outdoor activity compared with the highest was 4.3 (0.7 to 26.8), and that for grip strength was 3.9 (0.7 to 23.0).
Reduced intake of dietary calcium does not seem to be a risk factor for hip fracture. Further evidence is provided that physical activity in the elderly protects against hip fracture.
Thirty-seven patients with postmenopausal crush fracture osteoporosis were randomized to oral cyclic estrogen/gestagen (n = 20) or oral calcium (2000 mg elemental calcium per day) (n = 17). Fourteen in each group completed 1 year of treatment. Iliac crest bone biopsies were obtained after intravital double labeling with tetracycline before and after treatment in 10 patients on estrogen/gestagen and 11 patients on calcium. In the estrogen/gestagen group the activation frequency in trabecular bone decreased from 0.52 + 0.11 (SEM) year-1 to 0.27 + 0.08 year-1 (p less than 0.01). No significant changes were found in resorption or formation periods. The osteoid surfaces and the mineralizing surfaces decreased (p less than 0.05), whereas the decrease in eroded surfaces was insignificant. Furthermore, no significant changes were observed in final resorption depth, wall thickness or bone balance per remodeling cycle. Serum alkaline phosphatase and renal hydroxyproline excretion decreased during treatment (p less than 0.002), whereas the lumbar bone mineral content (BMC) increased (p less than 0.01). In the calcium group the extent and thickness of osteoid surfaces decreased (p less than 0.05) without significant changes in activation frequency. Serum alkaline phosphatase and renal hydroxyproline excretion decreased during treatment (p less than 0.02). No significant changes were observed in lumbar BMC or the other histomorphometric parameters. The study supports that the positive effect of estrogen/gestagen on BMC can be explained by a reduction in the activation frequency of new remodeling cycles leading to a decreased remodeling space and an increase in mean bone age. There is no evidence of a positive balance per remodeling cycle.(ABSTRACT TRUNCATED AT 250 WORDS)
A group of 223 military recruits aged 18-21 years underwent strenuous physical training for a period of 14 weeks. The absolute bone density of the distal tibia in both lower limbs was measured before the training period and at its end. The density was determined by the Compton scattering technique, which has been developed in our laboratory; this method provides the bone mass of all bone constituents per unit volume. The distribution curve of the bone density in both tibiae shifted to higher values at the end of the training period. The mean bone density in the right and left tibia increased significantly by 7.5%. This study indicates that following an intensive physical exercise regime, a significant increase in the mass density of bone can be obtained in young adults within a short period.
A program of diverse and dynamic loading exercises of the distal forearm, a common site of osteoporotic fractures, was applied three times a week for 5 months to 14 postmenopausal osteoporotic women. Two parameters were used to assess the effect of the exercises on bone mass. The mass density of bone (g/cm3) was measured by the Compton scattering technique. The bone mineral content (g/cm2) was measured by single photon absorptiometry. Both measurements were taken at the same location in the distal radius 1 year prior to the exercise period, at its beginning, and at its end, in both the exercised group and a matched control group of 26 osteoporotic women. During the exercise period the mean bone density decreased by 1.9% in the control group whereas it increased by 3.8% in the exercise group. The bone mineral content results did not demonstrate any significant trend over the whole period of the study. It is concluded that the trabecular bone tissue in the distal radius of postmenopausal osteoporotic women responds favorably to dynamic and diverse bone stressing exercises even in the seventh decade of life.
Regular exercise and high calcium intake possibly help to preserve bone mass. Little is known, however, about their role in preventing hip fracture. The physical activity and calcium intake of 300 elderly men and women with hip fractures were compared with those of 600 controls matched for age and sex. In both sexes increased daily activity, including standing, walking, climbing stairs, carrying, housework, and gardening protected against fracture. This was independent of other known risk factors, including body mass, cigarette smoking, and alcohol consumption. Strength of grip correlated with activity and was inversely related to the risk of fracture. Calcium intake was not related to the risk of fracture in women. Men with daily calcium intakes above 1g had lower risks. These findings point to the importance of elderly people in Britain maintaining physical activity in their day to day lives.
The load-carrying capacity of the skeleton is achieved and maintained as the result of a continued functional stimulus to the cell populations responsible for bone remodeling. Although some bone cells have been assumed to be influenced by the load-induced changes in strain throughout the matrix, no evidence is available to indicate which cells are susceptible to such strain change or how such transient events provide a sustained influence on cell behaviour. In the present study, we showed that a short period of dynamic loading in vivo affects the orientation of proteoglycan within bone tissue. This reorientation declines only slowly, thus providing a persistent record of the tissue's recent strain history. Such a record has the ability not only to "capture" strain transients but also to "update" and "average" them. In this way, the bone cells could be presented with a sustained and coherent stimulus directly related to dynamic strain transients. These transients are the tissue's principal function variable.
In osteoporosis it is postulated that while the amount of bone is diminished, the quality of the bone is unaltered. Recently relatively novel methods of analysis have shown that, at the fracture site of osteoporotic subcapital fractures, there is a marked change in the molecular orientation of components of the non-collagenous bone matrix. These procedures, now applied to iliac crest biopsies, confirm earlier findings of altered orientation of the proteoglycans at the subcapital fracture site but show that very similar changes occur even in the iliac crests from patients with both types of osteoporotic proximal femoral fracture. Thus, whereas the amount of these acidic moieties of the non-collagenous bone matrix was unchanged, the molecular orientation was markedly altered, albeit not to the same extent as that found at the fracture site. These results imply that the quality of the bone, as well as the quantity, may be generally affected in osteoporosis.
The potential for the recovery of bone lost during the active phase of disuse osteoporosis, both in the diaphyseal compacta and metaphyseal spongiosa was tested in young adult and old Beagle dogs. Immobilization for up to 60 weeks was achieved by placing the forelimb in a spica cast and remobilization by removing it. Bone volume was estimated in the third metacarpus, radius, ulna and humerus at the mid-diaphysis and at the level of distal metaphyseal spongiosa in both forelimbs by radiography and histomorphometry. Measurements carried out on animals remobilized showed considerable recovery of the original bone loss. In both age groups, the residual deficits increased, however, with the duration of immobilization and were similar in the metaphyseal spongiosa and in the diaphyseal compacta. The old dogs which began the study with 10% less bone than the younger dogs, showed smaller proportional losses than the younger dogs but greater residual deficits, most evident in the diaphysis. In both age groups the distal, weight-bearing bones tended to show greater losses and also greater recovery both in diaphyseal compacta and the metaphyseal spongiosa. Thus, 28 weeks after cast removal following 32 weeks of immobilization the following findings were noted: In the third metacarpal diaphyseal compacta in the younger dogs, a 53.6% loss (mostly from the periosteal envelope) decreased to 16.3% (a 70% recovery) while in the older dogs a 37.6% loss (mostly from the endosteal envelope) decreased to 23% (a 40% recovery).(ABSTRACT TRUNCATED AT 250 WORDS)
We describe the SEM appearance of the rat endosteal bone lining cell ( BLC ) population, and the sequence of morphological changes of these cells as they self-incorporate into unmineralized bone matrix (osteoid), establish intercellular connections, and construct lacunae. The osteoblast/nascent osteocyte series was progressively unsheathed by gentle digestion of the osteoid with 0.25% collagenase. The osteoblasts which leave the polygonally packed BLC compartment rapidly develop numerous complexly branched processes that contact the processes elaborated by previous generations of maturing and mature osteocytes. As osteoblasts mature and approach the mineralization front, they appear to lose processes. The mature cells begin to form osteocyte lacunae by depositing an asymmetric perimeter of woven collagen fibrils, such that as the cells roof-over, the lacunae appear as pocketlike constructions. The collagen fibrils on the perilacunar matrix are oriented in a tangential or circular pattern, while those in the more distal matrix are arranged in a parallel pattern. With the completion of a lacuna, its wall appears to mineralize quickly, for lacunae could be recognized only when they are forming.
Bone density was studied in intercollegiate athletes and older athletic women. Single-photon densitometry was used to assess bone density parameters at a new distal radial site, the midradius, and the first metatarsus. Dual-photon densitometry assessed bone density of the lumbar spine. Eleven intercollegiate tennis players, 23 swimmers, and 86 older "athletic" women from 23 to 75 years of age were compared with age-matched nonathletic controls. "Athletic" describes adult women who exercised at least three times per week, 8 or more months of the year, for a minimum of 3 years. The radius and metatarsus bone content of intercollegiate athletes was significantly above control values. Lumbar spine density was significantly higher only in tennis players. Mean bone density values for adult "athletic" women were also significantly greater than in age-matched controls. In the oldest athletic group (55-75 years of age) bone measurement values in radius and lumbar spine were in the same range as for younger "athletic" women. In contrast, after 50 years of age, these values in the control population decreased by 0.7%/year. Therefore the largest variance (increase) from age-matched controls occurred in the oldest "athletic" group. Also, we have established a distal radial density value (using our modified site) below which we consider women "at risk" and recommend further bone health evaluation. Only two adult athletic women greater than 55 years of age fell into this category. It is concluded from this cross-sectional study that a regularly maintained athletic program for adult women may reduce the rate of "normal" bone mass loss accompanying age, particularly postmenopausally.(ABSTRACT TRUNCATED AT 250 WORDS)
Mechanical function has always been acknowledged to have a significant, continuing but hitherto unquantified influence on bone remodeling. The structural objective of this relationship is presumably to ensure that, at each location throughout the skeleton, there is sufficient bone tissue, appropriately placed, to withstand functional load-bearing without damage. The architectural modifications necessary to achieve and maintain this structural competence are made by the coordinated remodeling activity of populations of osteoblasts and osteoclasts. The specific structure-function objectives at each location remain undefined, as are the mechanisms by which tissue loading is transduced into cellular control. The remodeling responses following a variety of experimental alterations in bones' strain environment are presented. Their significance to the process of remodeling control is discussed, and a scheme for the interaction of mechanical and hormonal influences proposed.
Cell membrane specializations occur at contact sites between adjacent osteoblasts and osteoblasts and osteocytes. These junctions have been described by other investigators as being important in preventing the extracellular movement of material around bone cells. Previously we described how certain small proteins circumvented the osteoblast population and rapidly penetrated the canalicular-osteocyte system. In the present study we used lanthanum colloid as an extracellular marker, the lanthanum readily penetrated the bone cell junctions and the extracellular space of bone. Morphologically, these junctions were not "tight" or "occluding" structures, but resembled "gap" junctions. These gap junctions contained elements which formed intercellular bridges between adjacent cells but also maintained a 2 nm space between cells that contained extracellular fluid. These gap junctions may have an important function in the control or coordination of bone cell activity throughout a given volume of bone.
Removal of the ulna in mature sheep causes a slight increase in peak principal walking strains in the radius which can be recorded by rosette strain gauges. The overstrain on the cranial surface of the radius (20%) was more than twice that on the caudal surface (8%) yet over the 50 weeks following ulnar osteotomy new bone was deposited predominantly on the bone's caudal periosteal surface. The total amount of new bone deposited on the radius replaced the area of bone in the removed ulna, thus equilibrating strains due to compression between osteotomised and non-osteomised limbs. Strains due to bending, and consequently total strains, were reduced to below normal suggesting that mechanically adaptive bone remodelling may not be related to absolute strain levels but to the relative distribution of strain. New bone formation can therefore be stimulated as the result of a mechanical reorganization in which total strains are lower than those which normally occur. The new bone deposited on the caudal cortex of the radius became intensively remodelled with secondary osteons while that on the cranial surface remained in its primary form. This suggests that osteonal remodelling may not always be a simple reparative process but may be one influenced by the strain situation possibly to improve the structure and physical properties of the tissue.
Bending and compressive loads were applied intermittently at 0.5 Hz through implants chronically inserted into the radius and ulna of experimental sheep. The plane of bending was the same as that imposed during locomotion. The principal variables of the artificial strain regime were the peak strains and strain rates, which never exceeded the range attainable during normal locomotion. The bones were loaded artificially for one hour per day for six weeks. Post mortem, the parameters of the bones' remodelling response were assessed by measuring the change in gross geometry and the amount of new bone formation which was marked with fluorescent label given during the experimental period. The strain-related variable, which had the greatest influence on every remodelling parameter investigated, was the ratio between the maximum strain rate of the artificial regime and the maximum strain rate during walking. The variation in this ratio could be shown to account for between 68 and 81% of the variation in the measures of surface bone deposited. The strain parameter which caused the greatest additional increase in the proportion of total variation explained was most commonly the ratio between the proportion of peak strain due to axial loading in the normal and artificial situations. The effect of this axial strain was far less marked than that of strain rate, however, and only increased the percentage of the total variance which could be explained by between 6 and 12%. The direction of bending and axial loading (tension or compression) appeared to have no effect on the course of the remodelling observed. The most effective influence on the amount of intracortical secondary osteal remodelling was also the maximum strain rate ratio. Its effect on this process seemed less marked than that on surface remodelling, however, since it could only explain 43% of the variance in the total number of secondary osteons formed. The addition of other strain-related variables did not significantly increase this proportion.
Cellular response to mechanical loading in explants of adult cancellous bone
- A J El Haj
- S Minter
- M J Pead
- T M Skerry
- L E Lanyon
- AJ Haj
Proteoglycan orientation in bone; its relationship to loading, disuse and clinical osteoporosis
- R Dodds
- Tm Skerry
- Mj Pead
- Le Lanyon
Pathways of àctivation in adult adaptive bone remodelling
- A J El Haj
- S Minter
- L E Lanyon
- AJ Haj
Use of in vitro techniques to investigate cellular responses to mechanical load in bone
- S C Minter
- SC Minter
Prostacyclin production in osteocytes may be an early strain-related step in mechanically adaptive bone remodelling
- Scf Rawlinson
- Aj El-Haj
- Sc Minter
- Ia Tavares
- A Bennet
- Le Lanyon
Load-related release of prostaglandins in cores of cancellous bones in culture-A role for prostacyclin in adaptive remodelling?
- Scf Rawlinson
- Aj El-Haj
- Sc Minter
- A Bennett
- Ia Tavares
- Le Lanyon
Effects of inactivity and exercise on bone
- E L Smith
- C Gilligan
- EL Smith