Biological basis of bone strength : Anatomy, physiology and measurement

This is a self-archived version of an original article. This version may differ from the original in pagination and typographic details. Author(s): Title: Year: Version: Copyright: Rights: Rights url: Please cite the original version: CC BY-NC-SA 4.0 https://creativecommons.org/licenses/by-nc-sa/4.0/ Biological basis of bone strength : Anatomy, physiology and measurement © Authors, 2020 Published version Hart, Nicolas H.; Newton, Robert U.; Tan, Jocelyn; Rantalainen, Timo; Chivers, Paola; Siafarikas, Aris; Nimphius, Sophia Hart, N. H., Newton, R. U., Tan, J., Rantalainen, T., Chivers, P., Siafarikas, A., & Nimphius, S. (2020). Biological basis of bone strength : Anatomy, physiology and measurement. Journal of Musculoskeletal and Neuronal Interactions, 20(3), 347-371. http://www.ismni.org/jmni/pdf/81/jmni_20_347.pdf 2020 347 Review Article Biological basis of bone strength: anatomy, physiology and measurement Nicolas H. Hart1,2,3,4, Robert U. Newton1,4, Jocelyn Tan2,3,5, Timo Rantalainen1,2,3,4,6, Paola Chivers1,2,3,4, Aris Siafarikas1,2,3,7,8, Sophia Nimphius3,4 1Exercise Medicine Research Institute, Edith Cowan University, Perth, W.A., Australia; 2Institute of Health Research, The University of Notre Dame Australia, Fremantle, W.A., Australia; 3Western Australian Bone Research Collaboration, Perth, W.A., Australia; 4School of Medical and Health Sciences, Edith Cowan University, Perth, W.A., Australia; 5School of Health Sciences, The University of Notre Dame Australia, Perth, W.A., Australia; 6Gerontology Research Center, University of Jyväskylä, Jyväskylä, Finland; 7Department of Endocrinology and Diabetes, Perth Childrens Hospital, Perth, W.A., Australia; 8School of Paediatrics and Child Health, University of Western Australia, Perth, W.A., Australia Introduction Bone is a remarkable and exquisite biomaterial. It is highly adaptive, structurally dynamic and metabolically active, and is superior to all other biomaterials in terms of strength and toughness1-4. In particular, bone structure, size and strength are reliant upon and responsive to the routine physiological and mechanical demands placed upon it5-12. Mechanical stimuli thus initiate or inhibit bone modelling and remodelling processes in response to variations in internal or external forces or as a consequence of immobilisation13-17. More specifically, bone continuously modifies and regenerates itself in the presence or absence of mechanical loading, which subsequently leads to the accrual (formation), maintenance (homeostasis) or degradation (resorption) of bone mass18-24. This is achieved through a sophisticated process involving the careful cellular regulation and coordination of osteoblasts (bone matrix deposit) and osteoclasts (bone matrix resorption) in order to remove damaged or extraneous bone material and subsequently replace it with new robust material19-21,25-30. As bone remodelling is a continuous process, even a slight perturbation or imbalance in either of these regulatory cells can lead to osteopenia or osteoporosis; such is the importance of bone health to load tolerance capabilities29-35. In particular, the mechanical integrity and performance of bone under various loading conditions is directly affected by its mechanical properties and geometric characteristics1,7,12,13,18,36 which are both Abstract Understanding how bones are innately designed, robustly developed and delicately maintained through intricate anatomical features and physiological processes across the lifespan is vital to inform our assessment of normal bone health, and essential to aid our interpretation of adverse clinical outcomes affecting bone through primary or secondary causes. Accordingly this review serves to introduce new researchers and clinicians engaging with bone and mineral metabolism, and provide a contemporary update for established researchers or clinicians. Specifically, we describe the mechanical and non-mechanical functions of the skeleton; its multidimensional and hierarchical anatomy (macroscopic, microscopic, organic, inorganic, woven and lamellar features); its cellular and hormonal physiology (deterministic and homeostatic processes that govern and regulate bone); and processes of mechanotransduction, modelling, remodelling and degradation that underpin bone adaptation or maladaptation. In addition, we also explore commonly used methods for measuring bone metabolic activity or material features (imaging or biochemical markers) together with their limitations. Keywords: Cortical, Imaging, Modelling, Remodelling, Trabecular The authors have no conflict of interest. Corresponding author: Dr. Nicolas H. Hart – PhD, AES, CSCS, ESSAM, Senior Research Fellow, Edith Cowan University, 270 Joondalup Drive, JOONDALUP, Perth, WA, Australia, 6027 Edited by: G. Lyritis Accepted 24 April 2020 Journal of Musculoskeletal and Neuronal Interactions Published under Creative Common License CC BY-NC-SA 4.0 (Attribution-Non Commercial-ShareAlike) J Musculoskelet Neuronal Interact 2020; 20(3):347-371 348http://www.ismni.org N.H. Hart et al.: Biological basis of bone strength indicators of bone health and underpin bone strength. The ability of bone to withstand forces and moments (mechanical loads) differs substantially across the loading spectrum under various loading conditions, specific to the mode, magnitude, direction, rate and frequency of load applied3,12,16,17,37-39. As bone is anisotropic in nature, it has different thresholds of load tolerability across different planes of action2,18,40,41. Indeed, habitual human behaviours routinely expose bones to various, often unpredictable loading patterns spanning from cyclical low-grade forces when walking or running, to sudden high-grade forces when jumping, landing or changing direction. As a result, compressive, torsional, transverse and tensile loads in combination and isolation are routinely applied to bone, exposing the skeleton to stimuli that can lead to positive bone-specific and site-specific adaptations16,42-49, or in the absence of suitable conditioning, recovery and nutrition, an increased likelihood of injury50-57. Despite the complex and multidimensional relationship between various loading schemes and bone mechanical properties (beyond the scope of this review, and published earlier12), bone strength and stiffness are greatest in the direction by which loads are most commonly expressed13,44,49,58. The prevaling bone structure reflects an appropriate adaptation to mechanical loading highlighting a specificity of adaptation (site-specific) as force transmission regulates osteogenic (anabolic) bone formation outcomes concomitantly with other stochastic (spatially non-specific) adaptations2,16,20,21,59. In particular, the regulation and coordination of bone to physically adapt to loading demands is initiated and managed at the cellular level by osteocytes through mechanotransduction59-62. Proportionate to mechanical stimulation, osteocytes biochemically promote osteogenesis by coordinating osteoblast and osteoclast activity so that overall bone morphology and bone shape positively adapts in favour of greater bone strength63-65. Within this process, older osteoblasts make way for new osteoblasts by transforming into osteocytes which become embedded into the bone-matrix. As osteocytes form 95% of bone-matrix composition, this increase in osteocyte concentration leads to an increase in bone mass while maintaining regulatory osteoblast-to-osteoclast homeostasis7,19-21,66,67. As reviewed below, bone loss and bone accrual are not necessarily co-located and occur in a targeted or sitespecific manner around bone circumference and along its length, additional to observable coadaptive bone morphological traits. A thorough understanding of these cellular and physiologic processes and their contribution to determining and maintaining bone strength will facilitate clinical diagnostics, designing appropriate interventions, and evaluating clinical musculoskeletal outcomes of pharmacological and non-pharmacological interventions68. Accordingly, this review aims to provide a comprehensive update of current scientific literature and our understanding of these processes for clinicians and researchers, in companionship with the mechanical basis of bone strength12 published earlier. Bone strength Bone strength explicitly refers to the ability of bone to withstand force prior to catastrophic failure1,24,69-72, and is inextricably linked with fatigue resistance to repetitive loads73-78. Given the complex and multidimensional nature of bone, its strength is ultimately determined by the interaction and adjustment of its material and structural properties evident at macroscopic, microscopic and nanoscopic levels1,70,72,79-82. At the material level, the collagenous extracellular matrix of bone provides resistance to tension, whereas the mineral inorganic phase of bone provides resistance to compression. Indeed, variations in collagen (such as osteogenesis imperfecta) or mineralisation (such as anti-resorptive drugs) can weaken or strengthen bone. Microscopically, the trabeculae in trabecular meshwork have implications on bone stuctural strength, and macroscopically, varying the shape of the bone will increase or decrease the amount of bending and torsion a bone can withstand given a particular amount of total mineral mass. The adaptability, modulation and regulation of bone to mechanical and non-mechanical stimuli provides practitioners with the ability to directly influence and target bone strength through numerous interdependent mechanisms. Specifically, deterministic site-specific bone strength adaptations are driven by habitual mechanical loading, whereas general and non-specific bone strength adaptations are predominantly driven through endocrinological variations, responsive to physical, pharmacological and nutritional interventions1,32,33,83-86. As all forms of bone adaptation collaboratively determine structural integrity and mechanical competency, it is desirable to optimise and preserve bone strength during growth, development, maturity and advanced age through multi-disciplinary and holistic approaches which importantly address all bone strength determinants. The biological basis of bone strength is determined by its structure and function through its anatomy and physiology. Bone anatomy Skeletal function Our skeletons are responsible for several important mechanical and non-mechanical functions22,36,87. Mechanically, they provide a structural framework and stable foundation for human movement and locomotion to occur, generating mechanical rigidity and kinematic connectivity within the body22,36,88-90. It specifically achieves this by providing skeletal muscle with attachment sites to use as leverage points and platforms with which to act, contract and produce force, and serves to protect the brain, spinal cord and internal organs2,18,26,36,91,92. Non-mechanically, bone provides a reservoir for mineral deposition and blood regulation of calcium and phosphorous, supports haematopoiesis, defends against acidosis, and absorbs or captures potentially toxic minerals22,26,36,91,93. In order to fulfil these many functions 349http://www.ismni.org N.H. Hart et al.: Biological basis of bone strength simultaneously, bone has unique structural, morphological and mechanical properties that are highly dynamic, metabolically active and physiologically adaptive to the environment in which they’re exposed21,23,88,94. Bone is also highly vascular, facilitating the perfusion of oxygenated blood to enable the removal of metabolites and provision of nutrient availability required by bone to constantly model (form new bone) and remodel (recycle damaged bone) in response to routinely imposed mechanical demands, subsequently altering its configuration and material properties to preserve or increase strength in order to meet its functional requirements18,19,24,79,89. In its adult form, the human skeleton consists of approximately 200 distinguishable bones, with 74 located in the axial skeleton, and 126 located in the appendicular skeleton22,95. Long bones, however, are the most commonly loaded structures and therefore strongest load-bearing bones in the body, predominantly in the appendicular skeleton. They comprise of a hollow cylindrical shaft known as the diaphysis, a cone-shaped proximal and distal metaphysis, and rounded proximal and distal epiphysis22,96-98, each portion has different architectural features which are organised and configured to withstand and manage different physical loads during regular activities of daily living79,80,88,99. Macroscopic architecture Bone is a structurally complex and sophisticated biomaterial1,2,4,33. It must be rigid and stiff to withstand forces and accommodate loading, yet be flexible and elastic to deform and absorb energy24,80,100,101. It must shorten and widen under compression, yet lengthen and narrow under tension, whilst also withstanding torsional and shear forces in isolation and in combination without experiencing catastrophic failure24,79. In order to manage these contradictory and paradoxical requirements, the skeleton contains two macroscopic osseous tissues (trabecular and cortical bone) which are architecturally and functionally different33,81,102-105. In its entirety, skeletal mass consists of approximately 20% trabecular tissue and 80% cortical tissue, which co-exists at various proportions in all bones through-out the body in accordance with the functional and regional demands of each individual bone18,22,79,80,105,106. The structural intricacies and interactions between these two osseous tissues, enable long bones to be remarkably light yet durable and strong in order to facilitate locomotion24,79,82,107,108. Trabecular bone Trabecular bone, also known as cancellous bone, is encapsulated beneath cortical bone. It is most prominently found in weight-bearing skeletal structures, specifically the proximal and distal ends of long-bones (epiphyseal and metaphyseal regions), the carpals and tarsals of the extremities, and vertebrae22,79,81,109,110. Texturally, trabecular tissue presents as a meshwork of bone (trabeculae) with many interconnecting spaces through-out which contain red bone marrow8 8 ,10 2 ,111-114 . The three-dimensional latticelike structure of trabecular bone is primarily organised in the direction from which the greatest stresses are most commonly experienced, a design best suited for the mechanical loading of bone7,89,101,109,114-116. The spongy and porous architecture of trabecular bone enables it to store large amounts of energy prior to yielding18,23,105,117,118, thus allowing it to routinely tolerate cyclical low-grade forces. Cortical bone Cortical bone, also known as compact bone, forms the thin superficial layer of all bones, though is most prominently found in the thick central cortex (diaphysis) of long bones through-out the appendicular skeleton2,22,95,119. Cortical bone encapsulates trabecular bone, however the relative co-existence and composition of each tissue varies between bones through-out the skeleton1,18,99,102. In long bones, cortical tissue is arranged in a cylindrical fashion with concentric layers across two primary surfaces: the periosteum (a dense fibrous membrane forming the outside layer) and endosteum (a thin membrane forming the inner layer) of the diaphyseal shaft7 9, 9 5 ,9 7,111,119 -12 2 . Both surfaces contain important cells (osteoclasts, osteoblasts and osteocytes) responsible for modelling and remodelling processes essential to bone adaptation and osteogenesis17,24,25,97,123. The endosteum additionally lines the central cavity with yellow marrow8 8 , 95 ,111,112 ,12 2 . Structurally, cortical bone is highly organised, densely packed, rigid, and texturally smooth18 , 2 3 ,111,12 0 , with mineralized lamellar bone and collagen fibre matrix most prominently arranged in the direction of routine mechanical stress69,101,119,120,124,125. This provides cortical bone with an increased capability to tolerate sudden, high impact forces i.e. a sample of cortical bone is ~25% stronger than a sample of trabecular bone1,18,23, 119,126. Microscopic architecture Bone also has microscopic and sub-microscopic levels which, together with the macroscopic level, form a multidimensional architectural biomaterial with a deliberate mass (size, geometry and density) aimed at achieving optimal structural strength1,33,70,73,80. Microscopically, bone presents in the form of woven and lamellar bone at the tissue level81,98,127-129, and consists of organic and inorganic components at the material level26,33,59,130-132. Tissue level Bone presents in the form of immature (woven) and mature (lamellar) tissue at different stages of the modelling and remodelling processes at the microscopic level22,100,127,129,133-135. Woven tissue is an immature form of bone characterised by a random and spontaneous collagen arrangement, a large volume of cells, and relatively low tissue density100,104. It is formed rapidly, producing a highly unorganised and porous structure22,127,128. Woven bone features primarily throughout development, exclusively forming the entire skeleton at 350http://www.ismni.org N.H. Hart et al.: Biological basis of bone strength birth prior to a gradual transformation into mature lamellar bone during growth and physical maturation22,98,100,136. At any other time, woven bone formation occurs only following an injury or extreme structural overload which is thought to be a rapid, protective and restorative response to significantly damaged or weakened hard tissue structures2,127,137-139. It is therefore considered a premature and provisional material. Lamellar tissue, however, is a mature form of bone, which eventually replaces woven tissue in the form of trabecular or cortical bone formations. Lamellar tissue is characterised by a precise and deliberate parallel and concentric arrangement of lamellae sheets produced slowly due to a low turnover rate2,81,98,134. Lamellae sheets are formed in alternating directions that vary in rotational position and thickness in order to optimally withstand mechanical loads, in particular torsional stress1,81,95,128,134. Lamellar bone is therefore denser and stronger than woven bone22,100,101,140. Material level Bone is a specialised, bi-phasic connective tissue consisting of extracellular organic material coupled with a uniquely high content of mineralised inorganic material1,18,33,124,130,141. The organic portion provides bone with one-third of its mass and two-thirds of its volume; whereas the inorganic portion provides bone with the remaining two-thirds of its mass and one-third of its volume59,70,132. The extracellular organic component is mostly collagenous, conferring flexibility and resilience to bone by solidifying in tension as a protection against stretching, twisting and torsion142-146. Conversely, the mineralised inorganic component is primarily calcium and phosphate in the form of an insoluble salt known as hydroxyapatite130,147-152, giving bone its hardness and rigidity, particularly in compression153-155. As a result, the overall structural strength of bone relies upon the joint contribution and inter-play of these organic and inorganic material properties1,2,24,148,153, such that variations of inorganic mineral density will potentially adjust stiffness and flexibility arrangements in bone24,130,156, the optimal balance of which remains largely unknown. That is, highly mineralised bone can become brittle (e.g. atypical femoral fractures), whereas less mineralised bone will be tougher yet less stiff (e.g. greenstick fracture). Fortunately, this can be somewhat examined as elements held within the mineralised (inorganic) portion of bone provide considerable resistance to X-ray beams, forming the theoretical basis underpinning the use of bone densitometry devices. Bone physiology Historically, bone has been regarded as the domain of anatomical study. However mechanically receptive, biologically adaptive and metabolically active features of bone have since solidified it as a biomaterial well-suited for physiological and biomechanical investigation2,12,69,89,157. In particular, the skeleton is able to construct (model) and reconstruct (remodel) itself through cellular processes in response to developmental and mechanical loading demands through tightly controlled cellular activities20,21,24,25,91,93,158. Cellular mechanisms Bone is generated, regulated and maintained by an interaction of four key cells: osteoblasts, osteoclasts, osteocytes and extra-cellular lining cells13,19,26-28,159. Osteoblasts are anabolic in nature, producing new bone material by synthesizing and calcifying newly generated collagen2,21,23,141. Osteoblasts are uniquely adaptable and compatible, transforming into bone lining cells (surrounding the extra-cellular matrix) and osteocytes (embedded within the bone matrix) during the osteogenic process25,160-162. Conversely, osteoclasts are a catabolic cell which degrades, dissolves and resorbs bone material, often as a response to material damage or disuse21,29,123,163. Osteoclasts have a limited lifespan, undergoing apoptosis (programmed cell death) within 2 to 4 weeks of osteoclastogenesis25,123,164. Osteoblasts and osteoclasts work independently during bone creation and formation (modelling), and co-operatively via a basic multi-cellular unit (BMU) during bone maintenance and homeostasis (remodelling). Osteocytes are central to bone development and renewal as the most abundant residential cell in bone, accounting for approximately 90% to 95% of all bone cells66,141,162,165,166. Specifically, osteocytes are descendants of osteoblasts produced during osteogenesis, which subsequently become entombed within the mineralised collagen matrix25,27,66,109,162. Osteocytes form a well-connected network of sensory channels to detect environmental alterations and communicate reactionary processes to osteoblasts, bone lining cells and fellow osteocytes13,136,165,167,168. This network is explicitly formed by dendritic connections (~60 to 80 per osteocyte) which proliferate through canaliculated passages to provide a functional and mechanosensitive platform integral to the detection of mechanical load and associated microdamage13,66,158,165,167. This mechanically sensitive function, known as mechanotransduction, enables bone to physiologically detect and convert mechanical energy into proportionate biochemical signals in order to promote growth and repair processes59,60,65,158,168. The process of mechanotransduction, including how bones sense mechanical changes, are described further under the Bone Adaptation section of this review. Hormonal mechanisms Bone growth, development and preservation is largely reliant upon hormonal regulation, globally controlling skeletal homeostasis somewhat independently of mechanical loads through-out the lifespan in order to facilitate non-mechanical functions of bone33,169-173. Specifically, the endocrine system serves to maintain bone mineral deposition and homeostatic cellular balance through continual, non-mechanically induced generation and regeneration of bone during biological growth and maturation24,174-177. While the endocrine system does 351http://www.ismni.org N.H. Hart et al.: Biological basis of bone strength not explicitly strive to optimise bone strength, endocrine status can have a profound, indirect and negative impact on structural integrity and mechanical competency when irregular hormonal environments arise172,173,178-183. Endocrine activity therefore forms a central component of a complex biological system that mediates calcium-phosphate balance, energy metabolism and bone mineralisation in response to dynamic and volatile physiological requirements179,184-190. In this regard, endocrine function majorly influences bone health and metabolism, ascending into domination through adulthood and advanced ageing169,175,178,182,183,191,192. Endocrinological regulation of bone metabolism is highly influenced and tightly controlled by sub-categories of growth, gonadal and calcitropic hormones (Table 1), with varying levels of contribution and relative dominance through-out life170,174,175,178,187-206. Specifically, growth hormones exert formative effects; gonadal hormones exert formative and antiresorptive effects; and calcitropic hormones exert homeostatic effects; co-operatively acting to promote bone mass accrual during growth and maturation171,178,179,183-186,189,192,207-213. However, hormonal activity begins to decline following the establishment of peak bone mass, as bone formation and resorption shifts from net formation during ontogeny, to equilibrium during early-to-middle adulthood, and net resorption during advanced and older age24,34,71,173,214. This imbalance in bone metabolism is primarily driven by altered endocrine-paracrine activity, and confounded by multi-dimensional, synergistic and antagonistic hormonal interactions necessary to achieve and maintain metabolic homeostasis21,23,123,191,215. As a result, hormonal imbalances and environmental irregularities underpinning deficient endocrine function form the nutritional and pharmacological basis of bone preservation strategies34,214,216-218, utilising natural and artificial suppression and stimulation of bone Table 1. Endocrine regulation of bone metabolism. Hormones General Description Bone Metabolism Growth Regulators hGH Peptide hormone secreted from the anterior pituitary; influences muscle, liver, kidney and bone; promotes longitudinal growth of bone. Stimulates Formation IGF-1 Polypeptide with an essential role in growth and development; primarily circulated by liver; also paracrine delivered by non-hepatic tissues. Stimulates Formation Glucocorticoids Produced by adrenal glands, inhibits synthesis of IGF-1, supresses BMP-2 and calcium absorption. Inhibits Formation Stimulates Resorption Ghrelin Gut-derived peptide hormone; secretagogue of growth hormone; modulates energy homeostasis. Stimulates Formation Inhibits Resorption Leptin Adipocyte peptide hormone; proportional to fat stores; modulates energy homeostasis. Inhibits Formation Stimulates Resorption Thyroxin (T3 and T4)Tyrosine-based hormones produced by thyroid gland; regulates energy metabolism through thyroid stimulation hormone (TSH) activity. Stimulates Formation Stimulates Resorption Net Effect: Homeostatic ACTH Peptide hormone secreted from the anterior pituitary; stimulates cortisol production; dosedependent proliferation of osteoblast activity. Stimulates Formation Stimulates Resorption Net Effect: Homeostatic Oxytocin Peptide hormone secreted from the posterior pituitary; modulated by estrogen; autocrineparacrine osteoblast regulator of formation. Stimulates Formation Stimulates Resorption Net Effect: Homeostatic Gonadal Regulators Androgens Sex steroid secreted from testes (men) and adrenals (men and women); also converts to estrogen; acts in presence of hGH. Stimulates Formation Estrogen Synthesised from androgens in ovaries (women) and extra-glandular tissue (men and women); dominant role in bone metabolism. Permits Formation Inhibits Resorption Calcitropic Regulators PTH Polypeptide secreted by parathyroid gland, tightly controls calcium and phosphate; acts to maintain bone mineral homeostasis. Stimulates Formation Stimulates Resorption Net Effect: Formation Calcitonin Secreted by thyroid gland when plasma calcium is elevated; lowers plasma calcium; deposits into bone; relatively weak in comparison to PTH. Stimulates Formation Inhibits Resorption Vitamin D3 Activated in the liver and kidney; essential for intestinal absorption of calcium and phosphate; deficiency results in bone demineralisation. Permits Formation Stimulates Resorption 352http://www.ismni.org N.H. Hart et al.: Biological basis of bone strength resorption and formation to prevent and manage pathogenic conditions through-out the life-span. Bone adaptation Mechanotransduction Bone modelling and remodelling paradigms pioneered by Julius Wolff, improved by Wilhelm Roux (Wolff’s Law), and expanded upon by Harold Frost (Mechanostat Theory), remain the central focus of emerging and contemporary research11,89,219-233. Their meritorious work collectively describes the ability of bone to alter its mass and structure in response to routine mechanical loads15,69,92,106,234-238. However, scientific understanding of this mechanobiological relationship remains elusive and poorly understood. The conceptual basis of mechanical events stimulating and mediating bone formation, adaptation, maintenance and repair is widely accepted2,15,61,141,239. However, the cellular mechanisms and structural framework which underpins this observed phenomenon is not yet fully understood and forms the basis of current-day research15,59,62,67,240,241. In principle, mechanotransduction (Figure 1) refers to the conversion of biophysical forces (mechanical load) into cellular responses which drive morphological change at the tissue level, a functional adaptation of bone which purposely improves structural integrity and strength13,63-65,158,242,243. This biologic detection of mechanical force and their conferred cellular responses primarily involve four key activities: 1) mechanical coupling, 2) biochemical coupling, 3) signal transmission, and 4) effector response60,63,98,133,244. Specifically, forces which lead to bone deformation create interstitial fluid movement within canaliculi, stimulating biochemical activity via mechanosensory cells64,245-251. Piezoelectric signals are then transmitted through comprehensive lacuno-canalicular networks of osteocytes, lining cells and osteoblasts to determine the format and magnitude of cellular response relative to the perceived dose of mechanical load59,65,98,113,141,252-255. This fundamental dose-response relationship between mechanical load and structural bone adaptation provides the foundation of bone modelling and re-modelling theory63-65,158,240,243,256. Modelling Modelling is a dynamic and constructive process which adjusts the size, shape and strength of bone in order to Figure 1. Mechanotransduction (adapted from 14,15): illustrating the hierarchical structure of bone and the organizational structure of osteocytes within (left); and the mechanically induced fluid flow from hydrostatic pressure and osteoprogenitors through which biochemical signals proliferate (right). 353http://www.ismni.org N.H. Hart et al.: Biological basis of bone strength achieve its structural potential during ontogeny, specifically in response to physiological and mechanical influences through-out physical maturation2 2 ,79,111,12 2 , 2 57-2 59. It comprises of a complex and multifarious array of cellular and material activity which interact to position and configure cells and matrices during growth and development7,69,239. At the cellular level, osteoblasts work independently from osteoclasts to create an environment where matrix deposition exceeds matrix resorption11,15 , 22 ,111, 26 0 , 261. At the tissue level, this is expressed through periosteal apposition and simultaneous yet slower endocortical resorption22,73,82,97,107,111,122,261,262, leading to the formation of new bone material and partial preservation of old bone material to deliver a net increase in bone mass15,24,79,243,263,264. Longitudinal and radial growth are developmental features of depositional modelling during ontogeny. In particular, collagen is synthesised and deposited onto the extracellular matrix in order to elongate, thicken and widen the periosteum, while endocortical resorption expands the marrow cavity to concurrently increase the diameter of the endosteum together with the periosteum22,69,79,82,97,107,122,265. These morphological alterations structurally enhance bone strength through two key mechanisms: 1) increasing the bony (i.e. excluding any cavities) cross-sectional area, and 2) by placing the material farther from the centre of the bone, which increases the polar moment of inertia1,22,69,73,82,258. Increasing the amount of bone material in a given crosssection improves bone strength in compression and tension, whereas distributing bone material farther from the centre of the bone improves strength in bending and torsion. For further details on bone mechanics, refer to our companion review12. Ultimately, these morphological alterations keep stresses and strains of applied mechanical loads within a desired range by distributing compressive forces over a larger area, while also resisting bending and twisting forces at the mid-shaft69,72,73,107,266-268. Bone formation is presently thought to be limited to the first three-decades of human life, achieving maturity at this time to establish peak bone mass269-271. The potential of bone to develop during growth is influenced by a range of non-modifiable (gender, ethnicity, genetics) and modifiable (nutrition, hormones, lifestyle, physical activity) factors which ultimately determine skeletal development73,82,97,257,262,267,272-277. However, the accrual of bone is not a linear process, with bone developing most rapidly in adolescent years, acquiring ~50 to 60% of total adult bone mass within this short and critical period of time216,278282. Given the heightened sensitivity and responsiveness of bone during its premature stage of life, a considerable opportunity (window of adaptation) is provided to improve skeletal robustness and resilience through maximising bone mass during early-stage development83,267,283-290. Despite this apparent ceiling of bone mass augmentation (Figure 2), bone strength is able to increase through other spatially relevant mechanisms in maturity using a regulatory process known as re-modelling33,73,79,91,269,291,292. Remodelling Remodelling is an on-going, homeostatic and restorative process which replaces old and damaged bone with new and healthy material (Figure 3) to maintain and improve structural integrity and mechanical competency19-21,23,26,29,82,107,159,293. The regulatory nature of Figure 2. Bone mineral density accrual, maintenance and loss through-out the life-span as indication of bone mass alterations; with approximately 50–60% of total adult bone mass gained during adolescent years preceding peak bone mass and skeletal maturity at ~30 years of age. Bone mass deteriorates gradually following peak bone mass into older age to within normal (green), osteopaenic (yellow) or osteoporotic (red) bone density ranges. 354http://www.ismni.org N.H. Hart et al.: Biological basis of bone strength re-modelling relies upon integrated sensory signals in order to provide a feedback-controlled modulation of skeletal structure; a mechanism designed to sustain current and future functional requirements2 0 - 24 , 79 , 8 0 , 91,111. This complex and multidimensional process is essential to ensure bone structure remains balanced between excessive bone mass and excessive bone fragility (a continuum of robustness to slenderness) in order to optimise bone strength without sacrificing mobility; one of many paradoxical expressions of bone adaptation17,25,29,82,107,123. Remodelling occurs through stochastic and deterministic mechanisms19,20,59,80,91,294. Stochastic remodelling describes randomly delivered and spatially non-specific forms of regeneration via the endocrine system, whereas deterministic remodelling forms the morphological and mechanosensitive basis of bone strength adaptation through-out the lifespan15,17,123,293,295. Specifically, deterministic remodelling represents a precisely assigned, targeted and site-specific form of remediation to repair damaged bone or initiated as a consequence of mechanical behaviour2,19,237,292,293,296,297. In particular, bone acutely and accumulatively incurs microdamage in response to mechanical loading (gravitational and muscular forces), requiring coordinated cellular-level and tissue-level activity in order to manage and prevent structural failure and bone fracture21,59,79,80,297. As a result, bone is resorbed in regionally and temporally distinct locations, detected and driven at the cellular level by osteocytes through mechanotransduction in order to target, repair and replace damaged material at the tissue-level19,20,24,29,79,293,296. Figure 3. A graphical representation of the remodelling cycle (adapted from 24). Bone resorption (left) is stimulated by a micro-crack which severs canaliculi channels between osteocytes leading to osteocytic apoptosis. Lining cells and osteocytes release signals attracting cells from blood and marrow reservoirs into the damaged area leading to osteoclastogenesis. Bone formation (right) commences with successive streams of osteoblastic activity depositing new lamellar bone. Osteoblasts then transform into new lining cells (extra-cellular layer) or osteocytes (embedded in osteoid and bone matrix). Table 2. Adult bone remodelling (adapted from 96,109110,123). • Lifespan of BMU: ~6-9 months • Duration of remodelling: ~4-6 months • Speed of remodelling: ~25 µm/day • Bone volume replaced by a single BMU: ~0.025 mm3 • Lifespan of osteoclasts: ~2 weeks • Lifespan of osteoblasts (active): ~3 months • Interval between successive remodelling events at the same location: ~2-5 years. • Rate of turnover of whole skeleton: ~10% per year a a 10% per year approximation assumes 4% turnover per year of cortical bone (75% of the skeleton), and 28% turnover per year of trabecular bone (25% of the skeleton): Calculated as [0.75 x 4] + [0.25 x 28] = 10%; BMU = basic multicellular unit. 361http://www.ismni.org N.H. Hart et al.: Biological basis of bone strength 74. Popp KL, Hughes JM, Smock AJ, Novotny SA, Stovitz SD, Koehler SM, et al. Bone geometry, strength, and muscle size in runners with a history of stress fracture. Med Sci Sports Exerc 2009;41(12):2145-50. 75. Warden SJ, Hurst JA, Sanders MS, Turner CH, Bur DB, Li J. Bone adaptation to a mechanical loading program significantly increases skeletal fatigue resistance. J Bone Miner Res 2005;20(5). 76. Tommasini SM, Nasser P, Hu B, Jepsen KJ. Biological co-adaptation of morphological and composition traits contributes to mechanical functionality and skeletal fragility. J Bone Miner Res 2008;23(2). 77. Franklyn M, Oakes B, Field B, Wells P, Morgan D. Section modulus is the optimum geometric predictor for stress fractures and medial tibial stress syndrome in both male and female athletes. Am J Sports Med 2008;36(6):1179-89. 78. Tommasini SM, Nasser P, Schaffler MB, Jepsen KJ. Relationship between bone morphology and bone quality in male tibias: Implications for stress fracture risk. J Bone Miner Res 2005;20(8):1372-80. 79. Seeman E. Ageand menopause-related bone loss compromise cortical and trabecular microstructure. J Gerontol A Biol Sci Med Sci 2013;68(10):1218-25. 80. Brandi ML. Microarchitecture, the key to bone quality. Rheumatology 2009;48:8. 81. Rho J-Y, Kuhn-Spearing L, Zioupos P. Mechanical properties and the hierarchical structure of bone. Med Eng Phys 1998;20(2):92-102. 82. Seeman E. Bone quality: The material and structural basis of bone strength. J Bone Miner Metab 2008; 26(1):1-8. 83. Nikander R, Sievänen H, Heinonen A, Daly RM, Uusi-Rasi K, Kannus P. Targeted exercise against osteoporosis: A systematic review and meta-analysis for optimising bone strength throughout life. BMC Med 2010;8(1):47. 84. Sandhu SK, Hampson G. The pathogenesis, diagnosis, investigation and management of osteoporosis. J Clin Pathol 2011;64:1042-50. 85. Karinkanta S, Heinonen A, Sievänen H, Uusi-Rasi K, Pasanen M, Ojala K, et al. A multi-component exercise regimen to prevent functional decline and bone fragility in home-dwelling elderly women: randomized, controlled trial. Osteoporos Int 2007;18(4):453-62. 86. Kannus P, Uusi-Rasi K, Palvanen M, Parkkari J. Nonpharmacological means to prevent fractures among older adults. Ann Med 2005;37(4):303-10. 87. Banfi G, Lombardi G, Colombini A, Lippi G. Bone metabolism markers in sports medicine. Sports Med 2010;40(8):697-714. 88. Taichman RS. Blood and bone: two tissues whose fates are intertwined to create the hematopoietic stem-cell niche. Blood 2005;105(7):2631-9. 89. Frost HM. Bone’s mechanostat: a 2003 update. Anat Rec 2003;275(2):1081-101. 90. Burr DB. Muscle strength, bone mass, and age-related bone loss. J Bone Miner Res. 1997;12(10):1547-51. 91. Harada S-I, Rodan GA. Control of osteoblast function and regulation of bone mass. Nature 2003; 423(6937):349-55. 92. Turner CH, Pavalko FM. Mechanotransduction and functional response of the skeleton to physical stress: The mechanisms and mechanics of bone adaptation. J Orthop Sci 1998;3(6):346-55. 93. Schwab P, Scalapino K. Exercise for bone health: Rationale and prescription. Curr Opin Rheumatol 2011;23(2):137-41. 94. Karlsson MK, Rosengren BE. Physical activity as a strategy to reduce the risk of osteoporosis and fragility fractures. Int J Clin Endocrinol Metab 2012;10(3):52736. 95. Marieb E, Hoehn K. Human Anatomy & Physiology. 9th ed: Pearson Education; 2013. 96. White TD, Black MT, Folkens PA. Human osteology. 3rd ed. San Francisco: Elsevier; 2012. 97. Orwoll ES. Toward an expanded understanding of the role of the periosteum in skeletal health. J Bone Miner Res 2003;18(6):949-54. 98. Sikavitsas VI, Temenoff JS, Mikos AG. Biomaterials and bone mechanotransduction. Biomaterials 2001;22(19):2581-93. 99. Beaupied H, Lespessailles E, Benhamou C-L. Evaluation of macrostructural bone biomechanics. Joint Bone Spine 2007;74(3):233-9. 100. Currey JD. How well are bones designed to resist fracture? J Bone Miner Res 2003;18(4):591-8. 101. Currey JD. The many adaptations of bone. J Biomech 2003;36(10):1487-95. 102. Zebaze RMDD, Ghasem-Zadeh A, Bohte AP, IulianoBurns SP, Mirams MP, Price RIP, et al. Intracortical remodelling and porosity in the distal radius and postmortem femurs of women: a cross-sectional study. Lancet 2010;375(9727):1729-36. 103. Mosekilde L, Ebbesen E, Tornvig L, Thomsen J. Trabecular bone structure and strength-remodelling and repair. J Musculoskelet Neuronal Interact 2000;1(1):25-30. 104. Weiner S, Wagner HD. The material bone: Structuremechanical function relations. Annu Rev Mater Sci 1998;28(1):271-98. 105. Keaveny TM, Hayes WC. A 20-year perspective on the mechanical properties of trabecular bone. J Biomech Eng 1993;115(4B):534-42. 106. Huiskes R. If bone is the answer, then what is the question? J Anat 2000;197(02):145-56. 107. Seeman E. Structural basis of growth-related gain and age-related loss of bone strength. Rheumatology 2008;47(4):iv2-iv8. 108. Yeni YN, Brown CU, Wang Z, Norman TL. The influence of bone morphology on fracture toughness of the human femur and tibia. Bone 1997;21(5):453-9. 109. Huiskes R, Ruimerman R, Van Lenthe GH, Janssen JD. Effects of mechanical forces on maintenance and adaptation of form in trabecular bone. Nature 2000;405(6787):704-6. 362http://www.ismni.org N.H. Hart et al.: Biological basis of bone strength 110. Parfitt AM. Osteonal and hemi-osteonal remodeling: The spatial and temporal framework for signal traffic in adult human bone. J Cell Biochem 1994a;55(3):27386. 111. Szulc P, Seeman E, Duboeuf F, Sornay-Rendu E, Delmas PD. Bone fragility: Failure of periosteal apposition to compensate for increased endocortical resorption in postmenopausal women. J Bone Miner Res 2006;21(12). 112. Travlos GS. Normal structure, function and histology of bone marrow. Toxicol Pathol 2006;34(5):548-65. 113. Ruimerman R, Hilbers P, Rietbergen Bv, Huiskes R. A theoretical framework for strain-related trabecular bone maintenance and adaptation. J Biomech 2005; 38:931-41. 114. Jacobs CR. The mechanobiology of cancellous bone structural adaptation. J Rehabil Res Dev 2000;37(2):209-16. 115. Ruimerman R, Rietbergen BV, P.Hilbers, Huiskes R. The effects of trabecular-bone loading variables on the surface signaling potential for bone remodeling and adaptation. Ann Biomed Eng 2005;33(1):71-8. 116. Ruimerman R, Huiskes R, Van Lenthe G, Janssen J. A computer-simulation model relating bone-cell metabolism to mechanical adaptation of trabecular architecture. Comput Methods Biomech Biomed Engin 2001;4(5):433-48. 117. Kopperdahl DL, Keaveny TM. Yield strain behavior of trabecular bone. J Biomech 1998;31(7):601-8. 118. Ding M, Dalstra M, Danielsen CC, Kabel J, Hvid I, Linde F. Age variations in the properties of human tibial trabecular bone. J Bone Joint Surg Br 1997; 79(6):995-1002. 119. Augat P, Schorlemmer S. The role of cortical bone and its microstructure in bone strength. Age Ageing 2006;35(S2):ii27–ii31. 120. Carnelli D, Vena P, Dao M, Ortiz C, Contro R. Orientation and size-dependent mechanical modulation within individual secondary osteons in cortical bone tissue. J R Soc Interface 2013;10(81). 121. Techawinboonwong A, Song HK, Leonard MB, Wehrli FW. Cortical bone water: In vivo quantification with ultrashort echo-time MR Imaging 1. Radiology 2008;248(3):824-33. 122. Seeman E. The periosteum - A surface for all seasons. Osteoporos Int 2007;18(2):123-8. 123. Manolagas SC. Birth and death of bone cells: basic regulatory mechanisms and implications for the pathogenesis and treatment of osteoporosis 1. Endocr Rev 2000;21(2):115-37. 124. Burr D. The contribution of the organic matrix to bone’s material properties. Bone 2002;31(1):8-11. 125. Sevostianov I, Kachanov M. Impact of the porous microstructure on the overall elastic properties of the osteonal cortical bone. J Biomech 2000;33:881-8. 126. Bayraktar HH, Morgan EF, Niebur GL, Morris GE, Wong EK, Keaveny TM. Comparison of the elastic and yield properties of human femoral trabecular and cortical bone tissue. J Biomech 2004;37(1):27-35. 127. Liu XS, Zhang XH, Sekhon KK, Adams MF, McMahon DJ, Bilezikian JP, et al. High-resolution peripheral quantitative computed tomography can assess microstructural and mechanical properties of human distal tibial bone. J Bone Miner Res 2010;25(4):746-56. 128. Su X, Sun K, Cui FZ, Landis WJ. Organization of apetite crystals in human woven bone. Bone 2003; 32(2):150 -62. 129. Turner CH, Forwood M, Rho JY, Yoshikawa T. Mechanical loading thresholds for lamellar and woven bone formation. J Bone Miner Res 1994;9(1):87-97. 130. Bala Y, Farlay D, Boivin G. Bone mineralization: from tissue to crystal in normal and pathological contexts. Osteoporos Int 2013;24(8):2153-66. 131. Yeni Y, Brown C, Norman T. Influence of bone composition and apparent density on fracture toughness of the human femur and tibia. Bone 1998;22(1):79-84. 132. Hangartner TN, Gilsanz V. Evaluation of cortical bone by computed tomography. J Bone Miner Res 19 9 6 ;11(10 ) :1518 -2 5 . 133. Shapiro F. Bone development and its relation to fracture repair. The role of mesenchymal osteoblasts and surface osteoblasts. Eur Cell Mater 2008;15:53-76. 134. Weiner S, Traub W, Wagner HD. Lamellar bone: Structurefunction relations. J Struct Biol 1999;126(3):241-55. 135. Forwood M, Turner C. Skeletal adaptations to mechanical usage: results from tibial loading studies in rats. Bone 1995;17(4):S197-S205. 136. Kusuzaki K, Kageyama N, Shinjo H, Takeshita H, Murata H, Hashiguchi S, et al. Development of bone canaliculi during bone repair. Bone 2000;27(5):655-9. 137. McBride SH, Silva MJ. Adaptive and injury response of bone to mechanical loading. BoneKEy Rep 2012;1:192. 138. Marsell R, Einhorn TA. The biology of fracture healing. Injury 2011;42:551-5. 139. Fazzalari NL. Bone fracture and bone fracture repair. Osteoporos Int 2011;22(6):2003-6. 140. Zioupos P, Currey JD. The extent of microcracking and the morphology of microcracks in damaged bone. J Mater Sci 1994;29(4):978-86. 141. Burger EH, Klein Nulend J. Mechanotransduction in bone—role of the lacuno-canalicular network. FASEB J 1999;13(9001):S101-S12. 142. Martin E, Shapiro JR. Osteogenesis Imperfecta: Epidemiology and pathophysiology. Curr Osteoporos Rep 2007;5(3):91-7. 143. Viguet-Carrin S, Garnero P, Delmas P. The role of collagen in bone strength. Osteoporos Int 2006;17(3):319-36. 144. Fratzl P, Gupta HS, Paschalis EP, Roschger P. Structure and mechanical quality of the collagen-mineral, nanocomposite in bone. J Mater Chem 2004;14(14):2115-23. 145. Wang X, Puram S. The toughness of cortical bone and its relationship with age. Ann Biomed Eng 2004; 32(1):123-35. 146. Yamashita J, Li X, Furman BR, Rawls HR, Wang X, 363http://www.ismni.org N.H. Hart et al.: Biological basis of bone strength Agrawal CM. Collagen and bone viscoelasticity: A dynamic mechanical analysis. J Biomed Mater Res 2002;63(1). 147. Golub EE. Biomineralization and matrix vesicles in biology and pathology. Semin Immunopathol 2011;33(5):409-17. 148. Farlay D, Panczer G, Rey C, Delmas PD, Boivin G. Mineral maturity and crystallinity index are distinct characteristics of bone mineral. J Bone Miner Metab 2010;28(4):433-45. 149. Golub EE. Role of matrix vesicles in biomineralization. Biochimica et Biophysica Acta (BBA)-General Subjects 2009;1790(12):1592-8. 150. Bouxsein ML. Bone quality: Where do we go from here? Osteoporos Int 2003;14(S5):118-27. 151. Boivin G, Meunier PJ. Changes in bone remodeling rate influence the degree of mineralization of bone. Connect Tissue Res 2002;43(2-3):535-7. 152. Boivin G, Meunier PJ. The degree of mineralization of bone tissue measured by computerized quantitative contact microradiography. Calcif Tissue Int 2 0 0 2 ;7 0 ( 6) : 5 0 3 -11. 153. Boivin G, Bala Y, Doublier A, Farlay D, Ste-Marie L, Meunier P, et al. The role of mineralization and organic matrix in the microhardness of bone tissue from controls and osteoporotic patients. Bone 2008;43(3):532-8. 154. Allen MR, Burr DB. Three years of alendronate treatment results in similar levels of vertebral microdamage as after one year of treatment. J Bone Miner Res 2007;22(11):1759-65. 155. Follet H, Boivin G, Rumelhart C, Meunier PJ. The degree of mineralization is a determinant of bone trength: a study on human calcanei. Bone 2004;34(5):783-9. 156. Bala Y, Farlay D, Delmas PD, Meunier PJ, Boivin G. Time sequence of secondary mineralization and microhardness in cortical and cancellous bone from ewes. Bone 2010;46(4):1204-12. 157. Taylor D, Hazenberg JG, Lee TC. Living with cracks: Damage and repair in human bone. Nat Mater 2007;6(4):263-8. 158. Stern AR, Nicolella DP. Measurement and estimation of osteocyte mechanical strain. Bone 2013;54(2):191-5. 159. Parfitt AM. The two faces of growth: Benefits and risks to bone integrity. Osteoporos Int 1994b;4(6):382-98. 160. Karsenty G, Kronenburg HM, Settembre C. Genetic control of bone formation. Annu Rev Cell Dev Biol 2009;25:629-48. 161. Karsenty G. Transcriptional control of skeletogenesis. Annu Rev Genom Hum 2008;9:183-96. 162. Franz-Odendaal TA, Hall BK, Witten PE. Buried alive: How osteoblasts become osteocytes. Dev Dyn 2006;235(1):176-90. 163. Mizoguchi T, Muto A, Udagawa N, Arai A, Yamashita T, Hosoya A, et al. Identification of cell cycle–arrested quiescent osteoclast precursors in vivo. J Cell Biol 2009;184(4):541-54. 164. Yavropoulou MP, Yovos JG. Osteoclastogenesis - Current knowledge and future perspectives. J Musculoskelet Neuronal Interact 2008;8(3):204-16. 165. Lu XL, Huo B, Chiang V, Guo XE. Osteocytic network is more responsive in calcium signaling than osteoblastic network under fluid flow. J Bone Miner Res 2012;27(3):563-74. 166. Marotti G. The structure of bone tissues and the cellular control of their deposition. Ital J Anat Embryol 1996;101(4):25. 167. Bonewald LF. Generation and function of osteocyte dendritic processes. J Musculoskelet Neuronal Interact 2005;5(4):321-4. 168. Aarden EM, Burger EH, Nijweide PJ. Function of osteocytes in bone. J Cell Biochem 1994;55(3):287. 169. Sapir-Koren R, Livshits G. Bone mineralization and regulation of phosphate homeostasis. IBMS BoneKEy 2011;8(6):286-300. 170. Leppanen M, Aaltonen S, Parkkari J, Heinonen A, Kujala UM. Interventions to prevent sports related injuries: A systematic review and meta-analysis of randomised controlled trials. Sports Med 2014;44(4):473-86. 171. Venken K, Callewaert F, Boonen S, Vanderschueren D. Sex hormones, their receptors and bone health. Osteoporos Int 2008;19(11):1517-25. 172. Lindsay R. Hormones and bone health in postmenopausal women. Endocrine 2004;24(3):223-30. 173. Rizzoli R, Bonjour J, Ferrari S. Osteoporosis, genetics and hormones. J Mol Endocrinol 2001;26(2):79-94. 174. Imai Y, Youn M-Y, Inoue K, Takada I, Kouzmenko A, Kato S. Nuclear receptors in bone physiology and diseases. Physiol Rev 2013;93(2):481-523. 175. Manolagas SC, O’Brien CA, Almeida M. The role of estrogen and androgen receptors in bone health and disease. Nat Rev Endocrinol 2013;9(12):699-712. 176. Karsenty G, Yadav V. Regulation of bone mass by serotonin: Molecular biology and therapeutic implications. Annu Rev Med 2011;62:323-31. 177. Fukumoto S, Martin TJ. Bone as an endocrine organ. Trends Endocrinol Metab 2009;20(5):230-6. 178. Khosla S, Oursler MJ, Monroe DG. Estrogen and the skeleton. Trends Endocrinol Metab 2012;23(11):576-81. 179. Ducy P. The role of osteocalcin in the endocrine crosstalk between bone remodelling and energy metabolism. Diabetologia 2011;54(6):1291-7. 180. Hamilton EJ, Ghasem-Zadeh A, Gianatti E, Lim-Joon D, Bolton D, Zebaze R, et al. Structural decay of bone microarchitecture in men with prostate cancer treated with androgen deprivation therapy. J Clin Endocrinol Metab 2010;95(12):E456-E63. 181. Ribot CA, Trémollieres FA. Effect of estrogens on bone and other systems and hormonal substitute treatment. Curr Opin Orthop 1997;8(5):45-52. 182. Lanyon L. Using functional loading to influence bone mass and architecture: objectives, mechanisms, and relationship with estrogen of the mechanically adaptive process in bone. Bone 1996;18(1):S37-S43. 183. Britto JM, Fenton AJ, Holloway WR, Nicholson GC. 364http://www.ismni.org N.H. Hart et al.: Biological basis of bone strength Osteoblasts mediate thyroid hormone stimulation of osteoclastic bone resorption. Endocrinology 1994;134(1):169-76. 184. Fuqua JS, Rogol AD. Neuroendocrine alterations in the exercising human: Implications for energy homeostasis. Metabolism 2013;62(7):911-21. 185. Sinnesael M, Claessens F, Boonen S, Vanderschueren D. Novel insights in the regulation and mechanism of androgen action on bone. Curr Opin Endocrinol Diabetes Obes 2013;20(3):240-4. 186. Colaianni G, Sun L, Di Benedetto A, Tamma R, Zhu L-L, Cao J, et al. Bone marrow oxytocin mediates the anabolic action of estrogen on the skeleton. J Biol Chem 2012;287(34):29159-67. 187. Karsenty G. Bone endocrine regulation of energy metabolism and male reproduction. CR biologies 2011;334(10):720-4. 188. Karsenty G. Convergence between bone and energy homeostases: Leptin regulation of bone mass. Cell Metab 2006;4(5):341-8. 189. Godfrey RJ, Madgwick Z, Whyte GP. The exerciseinduced growth hormone response in athletes. Sports Med 2003;33(8):599-613. 190. Ohlsson C, Bengtsson B-Ak, Isaksson OG, Andreassen TT, Slootweg MC. Growth hormone and bone 1. Endocr Rev 1998;19(1):55-79. 191. Agas D, Sabbieti MG, Marchetti L. Endocrine disruptors and bone metabolism. Arch Toxicol 2013;87(4):735-51. 192. Esbrit P, Alcaraz MJ. Current perspectives on parathyroid hormone (PTH) and PTH-related protein (PTHrP) as bone anabolic therapies. Biochem Pharmacol 2013;85(10):1417-23. 193. Colaianni G, Tamma R, Di Benedetto A, Yuen T, Sun L, Zaidi M, et al. The oxytocin–bone axis. J Neuroendocrinol 2014;26(2):53-7. 194. Csakvary V, Puskas T, Oroszlan G, Lakatos P, Kalman B, Kovacs GL, et al. Hormonal and biochemical parameters correlated with bone densitometric markers in prepubertal Hungarian children. Bone 2013;54(1):106-12. 195. Dhanwal DK. Thyroid disorders and bone mineral metabolism. Indian J Endocrinol Metab 2011;15:S107. 196. Isales CM, Zaidi M, Blair HC. ACTH is a novel regulator of bone mass. Ann N Y Acad Sci 2010;1192(1):110-6. 197. Quarles LD. Endocrine functions of bone in mineral metabolism regulation. J Clin Invest 2008;118:3820-8. 198. Fukushima N, Hanada R, Teranishi H, Fukue Y, Tachibana T, Ishikawa H, et al. Ghrelin directly regulates bone formation. J Bone Miner Res 2005;20(5):790-8. 199. Elmquist JK, Strewler GJ. Physiology: Do neural signals remodel bone? Nature 2005;434(7032):447-8. 200. Misra M, Miller KK, Bjornson J, Hackman A, Aggarwal A, Chung J, et al. Alterations in growth hormone secretory dynamics in adolescent girls with anorexia nervosa and effects on bone metabolism. J Clin Endocrinol Metab 2003;88(12):5615-23. 201. Olney RC. Regulation of bone mass by growth hormone. Med Pediatr Oncol 2003;41(3):228-34. 202. Pfeifer M, Begerow B, Minne H. Vitamin D and muscle function. Osteoporos Int 2002;13(3):187-94. 203. Neer RM, Arnaud CD, Zanchetta JR, Prince R, Gaich GA, Reginster J-Y, et al. Effect of parathyroid hormone (1-34) on fractures and bone mineral density in postmenopausal women with osteoporosis. N Engl J Med 2001;344(19):1434-41. 204. Langdahl BL, Eriksen EF. The influence of thyroid hormones on bone turnover in health and osteopetrosis. Eur J Endocrinol 1998;139(1):10-1. 205. Gallagher JC, Kinyamu HK, Fowler SE, Dawson-Hughes B, Dalsky GP, Sherman SS. Calciotropic hormones and bone markers in the elderly. J Bone Miner Res 1998;13(3):475-82. 206. Uzzan B, Campos J, Cucherat M, Nony P, Boissel JP, Perret GY. Effets on bone mass of long term treatment with thyroid hormones: A meta-analysis. J Clin Endocrinol Metab 1996;81(12):4278-89. 207. Delhanty PJ, Eerden BC, Leeuwen JP. Ghrelin and bone. Biofactors 2014;40(1):41-8. 208. Legiran S, Brandi ML. Bone mass regulation of leptin and postmenopausal osteoporosis with obesity. Clin Cases Miner Bone Metab 2012;9(3):145. 209. Williams GR. Actions of thyroid hormones in bone. Endokrynol Pol 2009;60(5):380-8. 210. Grote FK, Van Suijlekom-Smit LW, Mul D, Hop WC, Ten Cate R, Oostdijk W, et al. Growth hormone treatment in children with rheumatic disease, corticosteroid induced growth retardation, and osteopenia. Arch Dis Child 2006;91(1):56-60. 211. Yakar S, Rosen CJ, Beamer WG, Ackert-Bicknell CL, Wu Y, Liu J-L, et al. Circulating levels of IGF-1 directly regulate bone growth and density. J Clin Invest 2002;110(6):771-81. 212. Kroll MH. Parathyroid hormone temporal effects on bone formation and resorption. Bull Math Biol 2000;62(1):163-88. 213. MacDonald B, Gallagher J, Russell R. Parathyroid hormone stimulates the proliferation of cells derived from human bone. Endocrinology 1986;118(6):2445-9. 214. Bone HG, Hosking D, Devogelaer J-P, Tucci JR, Emkey RD, Tonino RP, et al. Ten years’ experience with alendronate for osteoporosis in postmenopausal women. N Engl J Med 2004;350(12):1189-99. 215. Takeda S, Karsenty G. Central control of bone formation. J Bone Miner Metab 2001;19:195-8. 216. Weaver CM. The role of nutrition on optimizing peak bone mass. Asia Pac J Clin Nutr 2008;17(S1):135-7. 217. Palacios C. The role of nutrients in bone health, from A to Z. Crit Rev Food Sci Nutr 2006;46:621-8. 218. Levy R. Costs and benefits of pharmaceuticals: The value equation for older Americans. Care Manage J 2002;3(3):135-43. 219. Frost HM. From Wolff’s law to the Utah paradigm: insights about bone physiology and its clinical applications. Anat Rec 2001;262(4):398-419. 365http://www.ismni.org N.H. Hart et al.: Biological basis of bone strength 220. Frost HM. Why do bone strength and “mass” in aging adults become unresponsive to vigorous exercise? Insights of the Utah paradigm. J Bone Miner Metab 1999;17(2):90-7. 221. Frost HM. From Wolff’s law to the mechanostat: a new “face” of physiology. J Orthop Sci 1998;3(5):282-6. 222. Frost HM. Perspectives: A proposed general model of the “mechanostat”(suggestions from a new skeletalbiologic paradigm). Anat Rec 1996;244(2):139-47. 223. Frost HM. Wolff’s Law and bone’s structural adaptations to mechanical usage: an overview for clinicians. Angle Orthod 1994;64(3):175-88. 224. Frost HM. Skeletal structural adaptations to mechanical usage (SATMU): 1. Redefining Wolff’s law: the bone modeling problem. Anat Rec 1990;226(4):403-13. 225. Frost HM. Skeletal structural adaptations to mechanical usage (SATMU): 2. Redefining Wolff’s law: the remodeling problem. Anat Rec. 1990;226(4):414-22. 226. Wolff J, Maquet P, Furlong R. The law of bone remodeling. Berlin: Springer-Verlag; 1986. 227. Frost HM. A determinant of bone architecture: the minimum effective strain. Clin Orthop Relat Res 1983;175:286-92. 228. Frost HM. Tetracycline-based histological analysis of bone remodeling. Calcif Tissue Res 1969;3(1):211-37. 229. Roux W. Beitrage zur morphologie der funktionnellen anspassung. Arch Anat Physiol Anat Abt 1885;9:12058. 230. Wolff J. Das gesetz der transformation der knochen. Berlin: A. Hirchwild; 1892. 231. Roux W. Der kampf der teile im organismus. Leipzig: Engelmann; 1881. 232. Wolff J. Uber die bedeutung der architektur der spongiosen Substanz. Zentralblatt fur die medizinische Wissenschaft 1869;6:223-34. 233. Wolff J. Uber die innere architektur der knocken und ihre bedeutung fur die frage vom knochenwachstum. Archi fur pathologische Anatomie und Physiologie und Kinische Medizin 1870;50:389-453. 234. Hammer A. The paradox of Wolff’s theories. Ir J Med Sci 2014:1-10. 235. Stoltz JF. Response of cells and tissues to mechanical stimulation. Series Biomech 2012;27(1-2):17-32. 236. Ruff C, Holt B, Trinkaus E. Who’s afraid of the big bad Wolff?: “Wolff’s law” and bone functional adaptation. Am J Phys Anthropol 2006;129(4):484-98. 237. Skerry TM. One mechanostat or many? Modifications of the site-specific response of bone to mechanical loading by nature and nurture. J Musculoskelet Neuronal Interact 2006;6(2):122-7. 238. Lee T, Taylor D. Bone remodelling: Should we cry wolff? Ir J Med Sci. 1999;168(2):102-5. 239. Turner CH. Three rules for bone adaptation to mechanical stimuli. Bone 1998;23(5):399-407. 240. Wu Q, Sample SJ, Baker TA, Thomas CF, Behan M, Muir P. Mechanical loading of a long bone induces plasticity in sensory input to the central nervous system. Neurosci Lett 2009;463(3):254-7. 241. Turner CH. Toward a mathematical description of bone biology: The principle of cellular accommodation. Calcif Tissue Int 1999;65:466-71. 242. van Oers RF, Ruimerman R, Tanck E, Hilbers PA, Huiskes R. A unified theory for osteonal and hemi-osteonal remodeling. Bone 2008;42(2):250-9. 243. Järvinen T, Kannus P, Pajamäki I, Vuohelainen T, Tuukkanen J, Järvinen M, et al. Estrogen deposits extra mineral into bones of female rats in puberty, but simultaneously seems to suppress the responsiveness of female skeleton to mechanical loading. Bone 2003;32(6):642-51. 244. Duncan RL, Turner CH. Mechanotransduction and the functional response of bone to mechanical strain. Calcif Tissue Int 1995;57(5):344-58. 245. Bacabac RG, Smit TH, Mullender MG, van Loon JJWA, Klein Nulend J. Initial stress-kick is required for fluid shear stress-induced rate dependent activation of bone cells. Ann Biomed Eng 2005;33(1):104-10. 246. Ciani C, Doty SB, Fritton SP. Mapping bone interstitial fluid movement: Displacement of ferritin tracer during histological processing. Bone 2005;37(3):379-87. 247. Bacabac RG, Smit TH, Mullender MG, Dijcks SJ, Van Loon JJ, Klein-Nulend J, editors. Fluid shear stressinduced activation of bone cells is rate dependent and requires an initial stress kick. 50th Annual Meeting of the Orthopaedic Research Society; 2004; Anaheim, CA. 248. Han Y, Cowin SC, Schaffler MB, Weinbaum S. Mechanotransduction and strain amplification in osteocyte cell processes. Proceedings of the National Academy of Sciences 2004;101(47):16689-94. 249. Knoth Tate ML, Adamson JR, Tami AE, Bauer TW. The osteocyte. Int J Biochem Cell Biol 2004;36(1):1-8. 250. Bacabac RG, Dijcks SJ, Mullender MG, Smit TH, Van Loon JJ, Klein-Nulend J, editors. Rapid nitric oxide production by fluid flow treated bone cells is fluid shear rate dependent. 49th Annual Meeting of the Orthopaedic Research Society; 2003; New Orleans, LA. 251. Knoth Tate ML. “Whither flows the fluid in bone?” An osteocyte’s perspective. J Biomech 2003; 36(10):1409-24. 252. Nicolella DP, Lankford J. Microstructural strain near osteocyte lacuna in cortical bone in vitro. J Musculoskelet Neuronal Interact 2002;2(3):261-3. 253. Martin RB. Toward a unifying theory of bone remodeling. Bone 2000;26(1):1-6. 254. Klein-Nulend J, van der Plas A, Semeins CM, Ajubi NE, Frangos JA, Nijweide PJ, et al. Sensitivity of osteocytes to biomechanical stress in vitro. FASEB J 1995;9(5):441. 255. Cowin SC, Moss-Salentijn L, Moss ML. Candidates for the mechanosensory system in bone. J Biomech Eng 1991;113(2):191-7. 256. Judex S, Gupta S, Rubin C. Regulation of mechanical signals in bone. Orthod Craniofac Res 2009;12(2):94104. 366http://www.ismni.org N.H. Hart et al.: Biological basis of bone strength 257. Heino TJ, Hentunen TA, Vaananen HK. Conditioned medium from osteocytes stimulates the proliferation of bone marrow mesenchymal stem cells and their differentiation into osteoblasts. Exp Cell Res 2004;294(2):458-68. 258. Prendergast PJ. Mechanics applied to skeletal ontogeny and phylogeny. Meccanica 2002;37(4-5):317-34. 259. Luo Y. Bone mineral density averaged over a region of interest on femur is affected by age-related change of bone geometry. Osteoporos Int 2018;29(6):1419-25. 260. Martin TJ, Sims NA. Osteoclast-derived activity in the coupling of bone formation to resorption. Trends Mol Med 2005;11(2):76-81. 261. Seeman E. Periosteal bone formation - A neglected determinant of bone strength. N Engl J Med 2003;349(4):320-3. 262. Seeman E. Growth in bone mass and size--are racial and gender differences in bone mineral density more apparent than real? J Clin Endocrinol Metab 1998;83(5):1414-9. 263. Kukuljan S, Nowson CA, Sanders KM, Nicholson GC, Seibel MJ, Salmon J, et al. Independent and combined effects of calcium-vitamin D3 and exercise on bone structure and strength in older men: an 18-month factorial design randomized controlled trial. J Clin Endocrinol Metab 2011;96(4):955-63. 264. Uusi-Rasi K, Kannus P, Cheng S, Sievanen H, Pasanen M, Heinonen A, et al. Effect of alendronate and exercise on bone and physical performance of postmenopausal women: A randomized controlled trial. Bone 2003;33(1):132-43. 265. Raab-Cullen D, Thiede M, Petersen D, Kimmel D, Recker R. Mechanical loading stimulates rapid changes in periosteal gene expression. Calcif Tissue Int 1994;55(6):473-8. 266. Beck BR, Snow CM. Bone health across the lifespan: Exercising our options. Exerc Sport Sci Rev 2003;31(3):117-22. 267. Cullen D, Smith R, Akhter M. Time course for bone formation with long-term external mechanical loading. J Appl Physiol 2000;88(6):1943-8. 268. Turner CH, Burr DB. Basic biomechanical measurements of bone: A tutorial. Bone 1993;14(4):595-608. 269. Horcajada M-N, Offord E. Naturally plant-derived compounds: Role in bone anabolism. Curr Mol Pharmacol 2012;5(2):205-18. 270. Nilsson M, Ohlsson C, Odén A, Mellström D, Lorentzon M. Increased physical activity is associated with enhanced development of peak bone mass in men: A five-year longitudinal study. J Bone Miner Res 2012;27(5):1206-14. 271. Huuskonen J, Väisänen S, Kröger H, Jurvelin J, Alhava E, Rauramaa R. Regular physical exercise and bone mineral density: a four-year controlled randomized trial in middle-aged men. The DNASCO study. Osteoporos Int 2001;12(5):349-55. 272. Duan Y, Wang XF, Evans A, Seeman E. Structural and biomechanical basis of racial and sex differences in vertebral fragility in Chinese and Caucasians. Bone 2005;36(6):987-98. 273. Wang XF, Duan Y, Beck TJ, Seeman E. Varying contributions of growth and ageing to racial and sex differences in femoral neck structure and strength in old age. Bone 2005;36(6):978-86. 274. Stegen S, Stockmans I, Moermans K, Thienpont B, Maxwell PH, Carmeliet P, et al. Osteocytic oxygen sensing controls bone mass through epigenetic regulation of sclerostin. Nat Commun 2018;9(1):2557. 275. Choksi P, Jepsen KJ, Clines GA. The challenges of diagnosing osteoporosis and the limitations of currently available tools. Clinical diabetes and endocrinology 2018;4(1):12. 276. Reider L, Beck T, Alley D, Miller R, Shardell M, Schumacher J, et al. Evaluating the relationship between muscle and bone modeling response in older adults. Bone 2016;90:152-8. 277. Santos L, Elliott-Sale KJ, Sale C. Exercise and bone health across the lifespan. Biogerontology 2017;18(6):931-46. 278. Laudermilk MJ, Manore MM, Thomson CA, Houtkooper LB, Farr JN, Going SB. Vitamin C and zinc intakes are related to bone macroarchitectural structure and strength in prepubescent girls. Calcif Tissue Int 2012;91(6):430-9. 279. Pitukcheewanont P, Safani D. Extremely low-level, shortterm mechanical stimulation increases cancellous and cortical bone density and muscle mass of children with low bone density: A pilot study. The Endocrinologist 2006;16(3):128-32. 280. Hartman C, Hochberg Z, Shamir R. Osteoporosis in paediatrics. Isr Med Assoc J 2003;5(7):509-15. 281. Bonjour J-P, Theintz G, Buchs B, Slosman D, Rizzoli R. Critical years and stages of puberty for spinal and femoral bone mass accumulation during adolescence. J Clin Endocrinol Metab 1991;73(3):555-63. 282. Ilich JZ, Kerstetter JE. Nutrition in bone health revisited: A story beyond calcium. J Am Coll Nutr 2000;19(6):715-37. 283. Ireland A, Rittweger J, Schonau E, Lamberg-Allardt C, Vijakainen H. Time since onset of walking predicts tibial bone strength in early childhood. Bone 2014;68:76-84. 284. Warden SJ, Roosa SM. Physical activity completed when young has residual bone benefits at 94 years of age: A with-in subject controlled case study. J Musculoskelet Neuronal Interact 2014;14(2):239. 285. Pettersson U, Nilsson M, Sundh V, Mellström D, Lorentzon M. Physical activity is the strongest predictor of calcaneal peak bone mass in young Swedish men. Osteoporos Int 2010;21(3):447-55. 286. Janz KF, Gilmore JM, Burns TL, Levy SM, Torner JC, Willing MC, et al. Physical activity augments bone mineral accrual in young children: The Iowa Bone Development Study. J Pediatr 2006;148(6):793-9. 287. Ruff C. Growth in bone strength, body size, and muscle size in a juvenile longitudinal sample. Bone 367http://www.ismni.org N.H. Hart et al.: Biological basis of bone strength 2003;33(3):317-29. 288. MacKelvie KJ, Khan KM, McKay HA. Is there a critical period for bone response to weight-bearing exercise in children and adolescents? A systematic review. Br J Sports Med 2002;36(4):250-7. 289. Modlesky CM, Lewis RD. Does exercise during growth have a long-term effect on bone health? Exerc Sport Sci Rev 2002;30(4):171-6. 290. MacKelvie KJ, McKay HA, Khan KM, Crocker PR. A school-based exercise intervention augments bone mineral accrual in early pubertal girls. J Pediatr 2001;139(4):501-8. 291. Ural A, Vashishth D. Interactions between microstructural and geometrical adaptation in human cortical bone. J Orthop Res 2006;24(7):1489-98. 292. Neu CM, Rauch F, Manz F, Schoenau E. Modeling of cross-sectional bone size, mass and geometry at the proximal radius: A study of normal bone development using peripheral quantitative computed tomography. Osteoporos Int 2001;12(7):538-47. 293. Burr D. Why bones bend but don’t break. J Musculoskelet Neuronal Interact 2011;11(4):270-85. 294. Heaney RP. The bone-remodeling transient: Implications for the interpretation of clinical studies of bone mass change. J Bone Miner Res. 1994;9(10):1515-23. 295. Hillam RA, Skerry TM. Inhibition of bone resorption and stimulation of formation by mechanical loading of the modeling rat ulna in vivo. J Bone Miner Res 1995;10(5):683-9. 296. Herman BC, Cardoso L, Majeska RJ, Jepsen KJ, Schaffler MB. Activation of bone remodeling after fatigue: differential response to linear microcracks and diffuse damage. Bone 2010;47(4):766-72. 297. Li J, Mashiba T, Burr D. Bisphosphonate treatment suppresses not only stochastic remodeling but also the targeted repair of microdamage. Calcif Tissue Int 2001;69(5):281-6. 298. Jilka RL. Biology of the basic multicellular unit and the pathophysiology of osteoporosis. Med Pediatr Oncol 2003;41(3):182-5. 299. Lassen NE, Andersen TL, Pløen GG, Søe K, Hauge EM, Harving S, et al. Coupling of bone resorption and formation in real time: New knowledge gained from human haversian BMUs. J Bone Miner Res 2017;32(7):1395-405. 300. Thiel A, Reumann MK, Boskey A, Wischmann J, von Eisenhart-Rothe R, Mayer-Kuckuk P. Osteoblast migration in vertebrate bone. Biological Reviews 2018;93(1):350-63. 301. Clansey AC, Hanlon M, Wallace ES, Lake MJ. Effects of fatigue on running mechanics associated with tibial stress fracture risk. Med Sci Sports Exerc 2012;44(10):1917-23. 302. Bloomfield SA. Disuse osteopenia. Curr Osteoporos Rep 2010;8:91-7. 303. Sievänen H. Immobilization and bone structure in humans. Arch Biochem Biophys 2010;503(1):146-52. 304. Bennell K, Matheson G, Meeuwisse W, Brukner P. Risk factors for stress fractures. Sports Med 1999;28(2):91122. 305. Khosla S. Pathogenesis of age-related bone loss in humans. J Gerontol A Biol Sci Med Sci 2013;68(10):1226-35. 306. Bergmann P, Body JJ, Boonen S, Boutsen Y, Devogelaer JP, Goemaere S, et al. Loading and skeletal development and maintenance. J Osteoporos 2011;2011:1-15. 307. Lau RY-c, Guo X. A review on current osteoporosis research: With special focus on disuse bone loss. J O s t e o p o r o s 2 011; 2 011:16 . 308. Riggs BL, Melton LJ, Robb RA, Camp JJ, Atkinson EJ, McDaniel L, et al. A population-based assessment of rates of bone loss at multiple skeletal sites: Evidence for substantial trabecular bone loss in young adult women and men. J Bone Miner Res 2008;23(2):205-14. 309. Cervinka T, Rittweger J, Hyttinen J, Felsenberg D, Sievänen H. Anatomical sector analysis of load-bearing tibial bone structure during 90-day bed rest and 1-year recovery. Clin Physiol Funct Imaging 2011;31(4):249-57. 310. Landrigan MD, Li J, Turnbull TL, Burr DB, Niebur GL, Roeder RK. Contrast-enhanced micro-computed tomography of fatigue microdamage accumulation in human cortical bone. Bone. 2011;48(3):443-50. 311. Macione J, Kavukcuoglu N, Nesbitt R, Mann A, Guzelsu N, Kotha S. Hierarchies of damage induced loss of mechanical properties in calcified bone after in vivo fatigue loading of rat ulnae. J Mech Behav Biomed Mater. 2011;4(6):841-8. 312. Berg HE, Eiken O, Miklavcic L, Mekjavic IB. Hip, thigh and calf muscle atrophy and bone loss after 5-week bedrest inactivity. Eur J Appl Physiol. 2007;99(3):283-9. 313. Danova N, Colopy S, Radtke C, Kalscheur V, Markel M, Vanderby Jr R, et al. Degradation of bone structural properties by accumulation and coalescence of microcracks. Bone. 2003;33(2):197-205. 314. Ehrlich PJ, Lanyon LE. Mechanical strain and bone cell function: A review. Osteoporos Int 2002;13(9):688700. 315. LeBlanc AD, Schneider VS, Evans HJ, Engelbretson DA, Krebs JM. Bone mineral loss and recovery after 17 weeks of best rest. J Bone Miner Res. 1990;5(8):843-50. 316. Bauer JJ, Snow CM. What is the prescription for healthy bones? J Musculoskelet Neuronal Interact 2003;3(4):352-5. 317. Ellman R, Spatz J, Cloutier A, Palme R, Christiansen BA, Bouxsein ML. Partial reductions in mechanical loading yield proportional changes in bone density, bone architecture and muscle mass. J Bone Miner Res 2013;28(4):875-85. 318. Gaudio A, Pennisi P, Bratengeier C, Torrisi V, Lindner B, Mangiafico RA, et al. Increased sclerostin serum levels associated with bone formation and resorption markers in patients with immobilization-induced bone loss. J Clin Endocrinol Metab 2010;95(5):2248-53. 368http://www.ismni.org N.H. Hart et al.: Biological basis of bone strength 319. Giangregorio L, Blimkie CJR. Skeletal adaptations to alterations in weight-bearing activity: A comparison of models of disuse osteoporosis. Sports Med 2002;32(7):459-76. 320. Gislason MK, Coupaud S, Sasagawa K, Tanabe Y, Purcell M, Allan DB, et al. Prediction of risk of fracture in the tibia due to altered bone mineral density distribution resulting from disuse: A finite element study. Proc Inst Mech Eng 2014;228(2):165-74. 321. Lloyd SA, Lang CH, Zhang Y, Paul EM, Laufenberg LJ, Lewis GS, et al. Interdependence of muscle atrophy and bone loss induced by mechanical unloading. J Bone M i n e r R e s 2 014 ; 2 9 ( 5 ) :1118 -3 0 . 322. Torcasio A, Jähn K, Van Guyse M, Spaepen P, Tami AE, Sloten JV, et al. Trabecular bone adaptation to lowmagnitude high-frequency loading at micro-gravity. J Biomech 2012;45:S531. 323. Wall BT, Dirks ML, Snijders T, Senden JMG, Dolmans J, van Loon LJ. Substantial skeletal muscle loss occurs during only 5 days of disuse. Acta Physiol 2014;210(3):600-11. 324. Klein-Nulend J, Bacabac RG, Bakker AD. Mechanical loading and how it affects bone cells: the role of the osteocyte cytoskeleton in maintaining our skeleton. Eur Cell Mater 2012;24:278-91. 325. Armbrecht G, Belavý DL, Backström M, Beller G, Alexandre C, Rizzoli R, et al. Trabecular and cortical bone density and architecture in women after 60 days of bed rest using high-resolution pQCT: WISE 2005. J Bone Miner Res 2011;26(10):2399-410. 326. Rittweger J, Beller G, Armbrecht G, Mulder E, Buehring B, Gast U, et al. Prevention of bone loss during 56 days of strict bed rest by side-alternating resistive vibration exercise. Bone 2010;46(1):137-47. 327. Rittweger J, Felsenberg D. Recovery of muscle atrophy and bone loss from 90 days bed rest: Results from a one-year follow-up. Bone 2008;44(2):214-24. 328. Rittweger J, Simunic B, Bilancio G, Gaspare De Santo N, Cirillo M, Biolo G, et al. Bone loss in the lower leg during 35 days of bed rest is predominantly from the cortical compartment. Bone 2009;44(4):612-8. 329. LeBlanc AD, Spector ER, Evans HJ, Sibonga JD. Skeletal responses to space flight and the bed rest analog: A review. J Musculoskelet Neuronal Interact 2007;7(1):33. 330. Rittweger J, Frost HM, Schiessl H, Ohshima H, Alkner B, Tesch P, et al. Muscle atrophy and bone loss after 90 days’ bed rest and the effects of flywheel resistive exercise and pamidronate: Results from the LTBR study. Bone 2005;36(6):1019-29. 331. Baecker N, Tomic A, Mika C, Gotzmann A, Platen P, Gerzer R, et al. Bone resorption is induced on the second day of bed rest: results of a controlled crossover trial. J Appl Physiol 2003;95(3):977-82. 332. Klein-Nulend J, Bacabac R, Veldhuijzen J, Van Loon J. Microgravity and bone cell mechanosensitivity. Adv Space Res 2003;32(8):1551-9. 333. Shields RK, Dudley-Javoroski S, Boaldin KM, Corey TA, Fog DB, Ruen JM. Peripheral quantitative computed tomography: Measurement sensitivity in persons with and without spinal cord injury. Arch Phys Med Rehabil 2006;87(10):1376-81. 334. Lang T, LeBlanc A, Evans H, Lu Y, Genant H, Yu A. Cortical and trabecular bone mineral loss from the spine and hip in long-duration spaceflight. J Bone Miner Res 2004;19(6):1006-12. 335. Vico L, Collet P, Guignandon A, Lafage-Proust MH, Thomas T, Rehailia M, et al. Effects of long-term microgravity exposure on cancellous and cortical weight-bearing bones of cosmonauts. Lancet 2000; 3 5 5 ( 9215 ) :16 0 7-11. 336. Collet P, Uebelhart D, Vico L, Moro L, Hartmann D, Roth M, et al. Effects of 1and 6-month spaceflight on bone mass and biochemistry in two humans. Bone 1997;20(6):547-51. 337. Del Puente A, Pappone N, Mandes MG, Mantova D, Scarpa R, Oriente P. Determinants of bone mineral density in immobilization: A study on hemiplegic patients. Osteoporos Int 1996;6(1):50-4. 338. Bettis T, Kim BJ, Hamrick MW. Impact of muscle atrophy on bone metabolism and bone strength: implications for muscle-bone crosstalk with aging and disuse. Osteoporos Int 2018;29(8):1713-20. 339. Einhorn TA. Bone Strength: The bottom line. Calcif Tissue Int 1992;51(5):333-9. 340. Nagaraja MP, Jo H. The role of mechanical stimulation in recovery of bone loss-high versus low magnitude and frequency of force. Life 2014;4(2):117-30. 341. Ju Y-I, Sone T, Okamoto T, Fukunaga M. Jump exercise during remobilization restores integrity of the trabecular architecture after tail suspension in young rats. J Appl Physiol 2008;104(6):1594-600. 342. Eser P, Schiessl H, Willnecker J. Bone loss and steady state after spinal cord injury: a cross-sectional study using pQCT. J Musculoskelet Neuronal Interact 2004;4(2):197-8. 343. Birkhold AI, Razi H, Duda GN, Weinkamer R, Checa S, Willie BM. The Periosteal Bone Surface is Less Mechano-Responsive than the Endocortical. Sci Rep 2016;6(1):23480. 344. Demontiero O, Vidal C, Duque G. Aging and bone loss: new insights for the clinician. Ther Adv Musculoskelet Dis 2012;4(2):61-76. 345. Dionyssiotis Y, Lyritis GP, Mavrogenis AF, Papagelopoulos PJ. Factors influencing bone loss in paraplegia. Hippokratia 2011;15(1):54-9. 346. Dionyssiotis Y, Trovas G, Galanos A, P. R, Papaioannou N, Papagelopoulos P, et al. Bone loss and mechanical properties of tibia in spinal cord injured men. J Musculoskelet Neuronal Interact. 2007;7(1):62-8. 347. Jiang S-D, Dai L-Y, Jiang L-S. Osteoporosis after spinal cord injury. Osteoporos Int 2006;17(2):180-92. 348. Lisková M, Hert J. Reaction of bone to mechanical stimuli. 2. Periosteal and endosteal reaction of tibial 369http://www.ismni.org N.H. Hart et al.: Biological basis of bone strength diaphysis in rabbit to intermittent loading. Folia Morphol (Praha) 1971;19(3):301-17. 349. Mellon SJ, Tanner KE. Bone and its adaptation to mechanical loading: a review. International Materials Reviews 2012;57(5):235-55. 350. Ruff C, Holt B, Trinkaus E. Who’s afraid of the big bad Wolff?: “Wolff’s law” and bone functional adaptation. Am J Phys Anthropol 2006;129(4):484-98. 351. Rantalainen T, Duckham RL, Suominen H, Heinonen A, Alén M, Korhonen MT. Tibial and Fibular Mid-Shaft Bone Traits in Young and Older Sprinters and Non-Athletic Men. Calcif Tissue Int 2014;95(2):132-40. 352. Cheng S, Sipilä S, Taaffe DR, Puolakka J, Suominen H. Change in bone mass distribution induced by hormone replacement therapy and high-impact physical exercise in post-menopausal women. Bone 2002;31(1):126-35. 353. Lespessailles E, Hambli R, Ferrari S. Osteoporosis drug effects on cortical and trabecular bone microstructure: a review of HR-pQCT analyses. BoneKEy Rep 2016;5:836. 354. Tsai JN, Uihlein AV, Burnett-Bowie S-AM, Neer RM, Zhu Y, Derrico N, et al. Comparative Effects of Teriparatide, Denosumab, and Combination Therapy on Peripheral Compartmental Bone Density, Microarchitecture, and Estimated Strength: the DATA-HRpQCT Study. J Bone Miner Res 2015;30(1):39-45. 355. Zebaze RM, Libanati C, Austin M, Ghasem-Zadeh A, Hanley DA, Zanchetta JR, et al. Differing effects of denosumab and alendronate on cortical and trabecular bone. Bone 2014;59:173-9. 356. Beck TJ, Ruff CB, Mourtada FA, Shaffer RA, MaxwellWilliams K, Kao GL, et al. Dual-energy X-ray absorptiometry derived structural geometry for stress fracture prediction in male U.S. Marine Corps recruits. J Bone Miner Res 1996;11(5):645. 357. McCormick F, Nwachukwu BU, Provencher MT. Stress fractures in runners. Clin Sports Med 2012;31(2):291306. 358. Warden SJ, Burr DB, Brukner PD. Stress fractures: Pathophysiology, epidemiology, and risk factors. Curr Osteoporos Rep 2006;4(3):103-9. 359. Jones BH, Thacker SB, Gilchrist J, Kimsey CD, Sosin DM. Prevention of lower extremity stress fractures in athletes and soldiers: a systematic review. Epidemiol Rev 2002;24(2):228-47. 360. Tommasini SM, Nasser P, Jepsen KJ. Sexual dimorphism affects tibia size and shape but not tissue-level mechanical properties. Bone 2007;40(2):498-505. 361. Milgrom C, Simkin A, Eldad A, Nyska M, Finestone A. Using bone’s adaptation ability to lower the incidence of stress fractures. Am J Sports Med 2000; 28(2):245-51. 362. Anliker E, Toigo M. Functional assessment of the muscle-bone unit in the lower leg. J Musculoskelet Neuronal Interact 2012;12(2):46-55. 363. Ashe MC, Liu-Ambrose T, Khan KM, White N, McKay HA. Optimizing results from pQCT: reliability of operatordependent pQCT variables in cadavers and humans with low bone mass. J Clin Densitom 2005;8(3):335. 364. Lai YM, Qin L, Yeung HY, Lee KKH, Chan KM. Regional differences in trabecular BMD and micro-architecture of weight-bearing bone under habitual gait loading—A pQCT and microCT study in human cadavers. Bone 2005;37(2):274-82. 365. Griffin LV, Gibeling JC, Martin RB, Gibson VA, Stover SM. The effects of testing methods on the flexural fatigue life of human cortical bone. J Biomech. 1999;32(1):105-9. 366. Snyder SM, Schneider E. Estimation of mechanical properties of cortical bone by computed tomography. J Orthop Res 1991;9(3):422-31. 367. Nazer RA, Lanovaz J, Kawalilak C, Johnston JD, Kontulainen S. Direct in vivo strain measurements in human bone: A systematic literature review. J Biomech 2012;45:27-40. 368. Lester ME, Urso ML, Evans RK, Pierce JR, Spiering BA, Maresh CM, et al. Influence of exercise mode and osteogenic index on bone biomarker responses during short-term physical training. Bone 2009;45(4):768-76. 369. Popp AW, Buffat H, Eberli U, Lippuner K, Ernst M, Richards RG, et al. Microstructural parameters of bone evaluated using HR-pQCT correlate with the DXAderived cortical index and the trabecular bone score in a cohort of randomly selected premenopausal women. PLoS One 2014;9(2):e88946. 370. Wehrli FW, Song HK, Saha PK, Wright AC. Quantitative MRI for the assessment of bone structure and function. NMR Biomed 2006;19(7):731-64. 371. Kang C, Paley M, Ordidge R, Speller R. In vivo MRI measurements of bone quality in the calcaneus: a comparison with DXA and ultrasound. Osteoporos Int 1999;9(1):65-74. 372. Ferretti JL. Perspectives of pQCT technology associated to biomechanical studies in skeletal research employing rat models. Bone. 1995;17(4):S353-S64. 373. Sheu Y, Zmuda JM, Boudreau RM, Petit MA, Ensrud KE, Bauer DC, et al. Bone strength measured by peripheral quantitative computed tomography and the risk of nonvertebral fractures: The osteoporotic fractures in men (MrOS) study. J Bone Miner Res 2011;26(1):63-71. 374. Petit MA, Beck TJ, Kontulainen SA. Examining the developing bone: What do we measure and how do we do it? J Musculoskelet Neuronal Interact 2005;5(3):213-24. 375. Cross TM, Smart RC, Thomson JE. Exposure to diagnostic ionizing radiation in sports medicine: assessing and monitoring the risk. Clin J Sport Med2003;13(3):164-70. 376. Nijs J, Westhovens R, Joly J, Cheng X, Borghs H, Dequeker J. Diagnostic sensitivity of peripheral quantitative computed tomography measurements at ultradistal and proximal radius in postmenopausal women. Bone 1998;22(6):659-64. 377. Desforges JF, Johnston Jr CC, Slemenda CW, Melton III LJ. Clinical use of bone densitometry. N Engl J Med 370http://www.ismni.org N.H. Hart et al.: Biological basis of bone strength 1991;324(16):1105-9. 378. Rogers RS, Dawson AW, Wang Z, Thyfault JP, Hinton PS. Acute response of plasma markers of bone turnover to a single bout of resistance training or plyometrics. J A p p l P h y s i o l 2 011;111:135 3 - 6 0 . 379. Rantalainen T, Heinonen A, Linnamo V, Komi PV, Takala TES, Kainulainen H. Short-term bone biochemical response to a single bout of high-impact exercise. J Sports Sci Med 2009;8(4):553-9. 380. Durkin JL, Dowling JJ, Andrews DM. The measurement of body segment inertial parameters using dual energy X-ray absorptiometry. J Biomech 2002;35(12):1575-80. 381. Toombs RJ, Ducher G, Shepherd JA, Souza MJ. The impact of recent technological advances on the trueness and precision of DXA to assess body composition. Obesity 2012;20(1):30-9. 382. Pietrobelli A, Wang Z, Formica C, Heymsfield SB. Dualenergy X-ray absorptiometry: fat estimation errors due to variation in soft tissue hydration. Am J Physiol Endocrinol Metab 1998;274(5):E808-E16. 383. Cointry G, Capozza R, Negri A, Roldan E, Ferretti J. Biomechanical background for a noninvasive assessment of bone strength and muscle-bone interactions. J Musculoskelet Neuronal Interact 2004;4(1):1. 384. Licata A. Bone density vs bone quality: What’s a clinician to do? Cleve Clin J Med 2009;76(6):331-6. 385. Cummings SR, Bates D, Black DM. Clinical use of bone densitometry: Scientific review. JAMA 2002;288(15):1889-97. 386. Bilsborough JC, Greenway K, Opar D, Livingstone S, Cordy JT, Coutts AJ. The accuracy and precision of DXA for assessing body composition in team sport athletes. J Sports Sci 2014;20(1):1-8. 387. Rothney MP, Martin FP, Xia Y, Beaumont M, Davis C, Ergun D, et al. Precision of GE Lunar iDXA for the measurement of total and regional body composition in nonobese adults. J Clin Densitom 2012;15(4):399-404. 388. Santos DA, Silva AM, Matias CN, Fields DA, Heymsfield SB, Sardinha LB. Accuracy of DXA in estimating body composition changes in elite athletes using a four compartment model as the reference method. Nutr Metab (Lond) 2010;7(1):22-31. 389. Hart NH, Nimphius S. Segmental musculoskeletal examinations using dual-energy X-Ray absorptiometry (DXA): Positioning and analysis considerations. J Sports Sci Med 2015. 390. Burkhart TA, Arthurs KL, Andrews DM. Manual segmentation of DXA scan images results in reliable upper and lower extremity soft and rigid tissue mass estimates. J Biomech 2009;42(8):1138-42. 391. Chen Z, Wang Z, Lohman T, Heymsfield SB, Outwater E, Nicholas JS, et al. Dual-energy X-ray absorptiometry is a valid tool for assessing skeletal muscle mass in older women. J Nutr 2007;137(12):2775-80. 392. Stewart AD, Hannan J. Total and regional bone density in male runners, cyclists and controls. Med Sci Sports Exerc 2000;32(8):1373-7. 393. De Lorenzo A, Andreoli A, Candeloro N. Within-subject variability in body composition using dual-energy X-ray absorptiometry. Clin Physiol 1997;17(4):383-8. 394. Trevisan C, Gandolini GG, Sibilla P, Penotti M, Caraceni MP, Ortolani S. Bone mass measurement by DXA: influence of analysis procedures and interunit variation. J Bone Miner Res 1992;7(12):1373-82. 395. Bianchi ML, Baim S, Bishop NJ, Gordon CM, Hans DB, Langman CB, et al. Official positions of the International Society for Clinical Densitometry (ISCD) on DXA evaluation in children and adolescents. Pediatr Nephrol 2010;25(1):37-47. 396. Suman VB, Subbalakshmi NK, Pai SR, Shaila MD. Risk factors associated with osteoporosis - A population based study using p-DXA technique. Int J Sci Res 2013;3(2):1-5. 397. Gurlek AM, Bayraktar M, Ariyurek M. Inappropriate reference range for peak bone mineral density in dual-energy X-ray absorptiometry: implications for the interpretation of T-scores. Osteoporos Int. 2000;11(9):809-13. 398. Chen Q, Bao N, Yao Q, Li Z-Y. Fractal dimension: A complementary diagnostic indicator of osteoporosis to bone mineral density. Med Hypotheses. 2018;116:136-8. 399. Sievänen H, Koskue V, Rauhio A, Kannus P, Heinonen A, Vuori I. Peripheral quantitative computed tomography in human long bones: Evaluation of in vitro and in vivo precision. J Bone Miner Res 1998;13(5):871-82. 400. Ammann P, Rizzoli R. Bone strength and its determinants. Osteoporos Int. 2003;14(S3):13-8. 401. Kröger H, Vainio P, Nieminen J, Kotaniemi A. Comparison of different models for interpreting bone mineral density measurements using DXA and MRI technology. Bone 1995;17(2):157-9. 402. Ho-Pham LT, Ho-Le TP, Mai LD, Do TM, Doan MC, Nguyen TV. Sex-difference in bone architecture and bone fragility in Vietnamese. Sci Rep. 2018;8(1):7707. 403. Boutroy S, Bouxsein ML, Munoz F, Delmas PD. In vivo assessment of trabecular bone microarchitecture by high-resolution peripheral quantitative computed tomography. J Clin Endocrinol Metab 2005; 90(12):6508-15. 404. Popp AW, Windolf M, Senn C, Tami A, Richards RG, Brianza S, et al. Prediction of bone strength at the distal tibia by HR-pQCT and DXA. Bone 2012;50(1):296-300. 405. Di Iorgi N, Maruca K, Patti G, Mora S. Update on bone density measurements and their interpretation in children and adolescents. Best Pract Res Clin Endocrinol Metab 2018;32(4):477-98. 406. Stagi S, Cavalli L, Cavalli T, De Martino M, Brandi ML. Peripheral quantitative computed tomography (pQCT) for the assessment of bone strength in most of bone affecting conditions in developmental age: A review. Ital J Pediatr 2016;42(1):88.