Lance E. Lanyon

Regulation of bone mass by mechanical strain magnitude

SummaryThein 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.

Static vs dynamic loads as an influence on bone remodelling.

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 bones 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)

Mechanically adaptive bone remodelling

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 bones 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.

Endocrinology - Bone adaptation requires oestrogen receptor-alpha

The strain imposed by mechanical loading on bone tissue normally stimulates a response by bone cells that results in an adjustment of bone architecture that enables the bone to withstand reasonable loads. But it is unclear why this process should become less effective in some 50 per cent of postmenopausal women, who suffer fractures as a result. Here we show that bone in vivo undergoes an adaptive response to loading that is less effective in the absence of the α-form of the oestrogen receptor (ER-α) and that osteoblast-like cells require ER-α to proliferate in response to mechanical strain in vitro. As ER-α expression in osteoblasts and osteocytes depends on oestrogen concentration, a failure to maintain bone strength after the menopause might be due to a reduction in the activity of ER-α in bone cells, thereby limiting their anabolic response to mechanical loading and allowing a loss of bone tissue comparable to that associated with disuse.

Wnt/beta-catenin signaling is a component of osteoblastic bone cell early responses to load-bearing and requires estrogen receptor alpha.

The Wnt/β-catenin pathway has been implicated in bone cell response to their mechanical environment. This response is the origin of the mechanism by which bone cells adjust bone architecture to maintain bone strength. Osteoporosis is the most widespread failure of this mechanism. The degree of osteoporotic bone loss in men and women is related to bio-available estrogen. Here we report that in osteoblastic ROS 17/2.8 cells and primary osteoblast cultures, a single short period of dynamic mechanical strain, as well as the glycogen synthase kinase-3β (GSK-3β) inhibitor LiCl, increased nuclear accumulation of activated β-catenin and stimulated TCF/LEF reporter activity. This effect was blocked by the estrogen receptor (ER) modulators ICI 182,780 and tamoxifen and was absent in primary osteoblast cultures from mice lacking ERα. Microarray expression data for 25,000 genes from total RNA extracted from tibiae of wild-type mice within 24 h of being loaded in vivo showed differential gene regulation between loaded and contralateral non-loaded bones of 10 genes established to be involved in the Wnt pathway. Only 2 genes were involved in loaded tibiae from mice lacking ERα (ERα-/-). Together these data suggest that Wnt/β-catenin signaling contributes to bone cell early responses to mechanical strain and that its effectiveness requires ERα. Reduced effectiveness of bone cell responses to bone loading, associated with estrogen-related decline in ERα, may contribute to the failure to maintain structurally appropriate bone mass in osteoporosis in both men and women.

Bone stress in the horse forelimb during locomotion at different gaits: A comparison of two experimental methods

Longitudinal stresses acting in the cranial and caudal cortices of the radius and the dorsal and palmar cortices of the metacarpus in the horse were determined using two independent methods simultaneously. One approach involved the use of rosette strain gauges to record in vivo bone strain; the other involved filming the position of the horses forelimb as it passed over a force plate. Agreement between the two analyses was better for the radius than for the metacarpus. Both methods showed the radius to be loaded primarily in sagittal bending, acting to place the caudal cortex in compression and the cranial cortex in tension. At each gait the magnitude of peak stress in each cortex based on the film/force analysis was 1.5-2 times higher than that determined from the bone strain recordings. In the metacarpus, the magnitude of stress in each cortex calculated from the film/force method was 2-3 times greater at each gait than that shown by the bone strain recordings. However, whereas the film/force analysis indicated that the metacarpus was loaded in sagittal bending (acting to place the palmar cortex in compression and the dorsal cortex in tension), the bone strain recordings showed the metacarpus to be loaded primarily in axial compression at each gait. Because the film/force method depends on an accurate measure of limb segment orientation relative to the direction of ground reaction force, comparatively small errors in calculations of bending moments may lead to a significant difference in the level and distribution of stress determined to act in the bones cortices. The discrepancy in metacarpal loading obtained by the two methods may be explained in part by the simplicity of the biomechanical model which, for instance, neglected the force exerted by the sesamoids on the distal end of the metacarpus. The records of stress determined from the in vivo bone strain recordings showed that each bone was subjected to a consistent loading regime despite changes of gait. Such a consistent strain distribution should allow these bones to maximize economy in the use of tissue required to support the dynamic loads applied. Peak stresses measured from the bone strain recordings in the radius during locomotion at constant speed (-40.8 +/- 4.1 MN m-2) were significantly larger than those in the metacarpus (-25.1 +/- 2.8 MN m-2), regardless of speed and gait. During acceleration and deceleration, however, peak stress rose dramatically in the metacarpus (-40.6 +/- 3.4 MN m-2) but remained constant in the radius (-37.8 +/- 5.8 MN m-2).(ABSTRACT TRUNCATED AT 400 WORDS)

The relationship of functional stress and strain to the processes of bone remodelling. An experimental study on the sheep radius

Abstract Changes in bone strain were recorded from the cranial (longitudinally convex) and caudal, (longitudinally concave) surfaces of the radial mid-shaft in a number of sheep. These recordings confirmed that during locomotion the cranial surface of this bone was subjected to longitudinal tension and the caudal surface to longitudinal compression. During walking, the peak compression strain on the caudal surface was 1.8 times as great as the peak tension strain on the cranial surface. In both growing and adult animals the more highly strained caudal cortex had a lower ash content and a lower elastic modulus than the less highly strained cranial cortex. Values of elastic modulus and ash content were highly correlated both between cortices and between age groups. The differences between cortices in modulus and ash appeared to be closely related to the greater degree of osteonal remodelling in the more highly strained cortex. The ratio of peak walking stress between the caudal and cranial cortices was calculated to be 1.6:1. The stress values suggest that at every stride during walking, the cranial and caudal cortices of the sheep radius are loaded to within the same proportion of their respective yield stress.

Bones' adaptive response to mechanical loading is essentially linear between the low strains associated with disuse and the high strains associated with the lamellar/woven bone transition

There is a widely held view that the relationship between mechanical loading history and adult bone mass/strength includes an adapted state or “lazy zone” where the bone mass/strength remains constant over a wide range of strain magnitudes. Evidence to support this theory is circumstantial. We investigated the possibility that the “lazy zone” is an artifact and that, across the range of normal strain experience, features of bone architecture associated with strength are linearly related in size to their strain experience. Skeletally mature female C57BL/6 mice were right sciatic neurectomized to minimize natural loading in their right tibiae. From the fifth day, these tibiae were subjected to a single period of external axial loading (40, 10‐second rest interrupted cycles) on alternate days for 2 weeks, with a peak dynamic load magnitude ranging from 0 to 14 N (peak strain magnitude: 0–5000 µε) and a constant loading rate of 500 N/s (maximum strain rate: 75,000 µε/s). The left tibiae were used as internal controls. Multilevel regression analyses suggest no evidence of any discontinuity in the progression of the relationships between peak dynamic load and three‐dimensional measures of bone mass/strength in both cortical and cancellous regions. These are essentially linear between the low‐peak locomotor strains associated with disuse (∼300 µε) and the high‐peak strains derived from artificial loading and associated with the lamellar/woven bone transition (∼5000 µε). The strain:response relationship and minimum effective strain are site‐specific, probably related to differences in the mismatch in strain distribution between normal and artificial loading at the locations investigated.

Analysis of surface bone strain in the calcaneus of sheep during normal locomotion. Strain analysis of the calcaneus.

Abstract An investigation is reported in which single element semi-conductor and triple element 45° foil rosette strain gauges were attached to the lateral side of the calcaneus in live sheep. Results from the two types of gauge were similar but those from the rosettes allowed calculations to be made of the changing direction and magnitude of the principal strains and the maximum shear strain during locomotion. This represents a considerable increase in the value and amount of information available from these direct measuring techniques. Each stride imposed a definite and characteristic deformation cycle on the bone. On its dorsal side this was primarily compressive and identical in direction and timing in all animals. Strain amplitude and rate of change were comparable to values obtained from other regions in the same species. This agreement tends to support the hypothesis that bone deformation per se may be at least one of the governing stimuli for the remodelling necessary in the maintenance of bones structure and mechanical strength.

Estrogen Receptor α Mediates Proliferation of Osteoblastic Cells Stimulated by Estrogen and Mechanical Strain, but Their Acute Down-regulation of the Wnt Antagonist Sost is Mediated by Estrogen Receptor β

Background: Strain and estrogens down-regulate Sost/sclerostin and stimulate osteoblastic proliferation. Results: ERα inhibition prevents proliferation. ERβ inhibition prevents Sost down-regulation by strain or estradiol. Sclerostin prevents proliferation following strain and not estradiol. Conclusion: ERα promotes proliferation, and ERβ mediates Sost down-regulation following estradiol ligand stimulation and ligand independently following strain. Significance: Selective ER modulators could promote osteogenesis through differential regulation of Sost and proliferation. Mechanical strain and estrogens both stimulate osteoblast proliferation through estrogen receptor (ER)-mediated effects, and both down-regulate the Wnt antagonist Sost/sclerostin. Here, we investigate the differential effects of ERα and -β in these processes in mouse long bone-derived osteoblastic cells and human Saos-2 cells. Recruitment to the cell cycle following strain or 17β-estradiol occurs within 30 min, as determined by Ki-67 staining, and is prevented by the ERα antagonist 1,3-bis(4-hydroxyphenyl)-4-methyl-5-[4-(2-piperidinylethoxy)phenol]-1H-pyrazole dihydrochloride. ERβ inhibition with 4-[2-phenyl-5,7-bis(trifluoromethyl)pyrazolo[1,5-β]pyrimidin-3-yl] phenol (PTHPP) increases basal proliferation similarly to strain or estradiol. Both strain and estradiol down-regulate Sost expression, as does in vitro inhibition or in vivo deletion of ERα. The ERβ agonists 2,3-bis(4-hydroxyphenyl)-propionitrile and ERB041 also down-regulated Sost expression in vitro, whereas the ERα agonist 4,4′,4″-[4-propyl-(1H)-pyrazol-1,3,5-triyl]tris-phenol or the ERβ antagonist PTHPP has no effect. Tamoxifen, a nongenomic ERβ agonist, down-regulates Sost expression in vitro and in bones in vivo. Inhibition of both ERs with fulvestrant or selective antagonism of ERβ, but not ERα, prevents Sost down-regulation by strain or estradiol. Sost down-regulation by strain or ERβ activation is prevented by MEK/ERK blockade. Exogenous sclerostin has no effect on estradiol-induced proliferation but prevents that following strain. Thus, in osteoblastic cells the acute proliferative effects of both estradiol and strain are ERα-mediated. Basal Sost down-regulation follows decreased activity of ERα and increased activity of ERβ. Sost down-regulation by strain or increased estrogens is mediated by ERβ, not ERα. ER-targeting therapy may facilitate structurally appropriate bone formation by enhancing the distinct ligand-independent, strain-related contributions to proliferation of both ERα and ERβ.