Bone Modeling and Remodeling

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C H A P T E R

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Bone Modeling and RemodelingMatthew R. Allen and David B. Burr

Department of Anatomy and Cell Biology, Indiana University School of Medicine, Indianapolis, Indiana, USA

SKELETAL DEVELOPMENT

Skeletal development begins during the first trimes-

ter of gestation and continues into the postnatal years.Development occurs through two distinct processes:intramembranous ossification and endochondralossification. These modes of development differ in theenvironment in which ossification is initiated and inthe cells that produce the matrix. Intramembranousossification occurs in a collection or condensation of mesenchymal cells that differentiate directly into osteo-

blasts, whereas endochondral ossification occurs on acartilage template produced by chondrocytes. Bothprocesses occur during embryogenesis, as well aspostnatally.

Intramembranous Ossification

Most bones of the skull, as well as certain other bones (such as the scapula and clavicle), are formedembryonically through intramembranous ossification.The process of intramembranous ossification is initi-ated and takes place within a sea of mesenchyme, anembryonic or primitive connective tissue comprisedprimarily of mesenchymal cells. Although intramem-

branous ossification is most often associated withembryonic development, it can also occur postnatally(during bone healing, for example). The initial step in

intramembranous ossification is the consolidation of mesenchymal cells, commonly referred to as a bone

blastema. Cells within the blastema differentiate intoosteoblasts and begin to produce matrix (Fig. 4.1). Thetranscription factor RUNX2 plays an indispensablerole in driving cells within the blastema toward theosteoblastic lineage. The initial production of bonematrix by osteoblasts establishes a primary ossificationcenter, i.e. a spatial location where the processes of

bone growth take place. As the osteoblasts within theindividual ossification centers produce more and morematrix, some of the osteoblasts become encapsulated,

at which point they become osteocytes. The bonematrix produced by these initial osteoblasts is knownas woven bone, an unorganized collagen structureresulting from rapid production. Once sufficient bonematrix is produced to form a small island of bone,additional osteoblasts are recruited to the surface,where there is continued production of either woven

bone or more organized primary lamellar bone.During development, some bones form through themerging of several small bony islands. Some bonesformed through intramembranous ossification, such asthe jaw, develop marrow cavities. These cavities formonce the bone becomes so large that the central osteo-

cytes are too distant from an adequate blood supply,thus stimulating the invasion of blood vessels into themiddle of the ossification center to form a marrow cav-ity. Other bones formed through this process, such asthe scapula and clavicle, do not form a marrow cavity.

Endochondral Ossification

The remainder of the bones in the skeleton areformed through endochondral ossification, a processin which a hyaline cartilage template is formed andover time replaced by mineralized bone tissue

(Fig. 4.2). Endochondral ossification is not limited toembryonic development; it also has a significant rolein fracture healing. Similar to intramembranousossification, endochondral ossification begins with acondensation of mesenchymal cells. Instead of differ-entiating into osteoblasts, however, these cells differen-tiate into chondroblasts. This process is driven by thetranscription factor SOX-9. These chondroblasts pro-duce a cartilage matrix that eventually envelops some

75Basic and Applied Bone Biology.

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cells, which then become chondrocytes. The hyalinecartilage is surrounded by perichondrium, a cellular/

fibrous membrane found on the surface of the cartilagemodel that serves to provide cells for cartilage growth.This cartilage template strongly resembles the shape of the mineralized bone that will eventually be formed,and so it is sometimes called the cartilage model (oranlage).

Early during development, cells of the perichon-drium differentiate into osteoblasts and begin forming

bone on the surface of the cartilage template. As inintramembranous ossification, this process of osteo-

blast differentiation is governed by the transcriptionfactor RUNX2. Bone formation is initially localized tothe circumference of the midshaft (diaphysis) of the

long bone and results in a structure called the bone col-lar. The bone collar is lamellar bone and, once formed,the adjacent fibrous tissue transitions from perichon-drium to periosteum, becoming populated with osteo-genic precursor cells. Formation of the bone collarlimits the ability of nutrients to diffuse into the nearbycartilage resulting in calcification of the local matrixand, eventually, death of the chondrocytes. These pro-cesses signal recruitment of a primary blood vessel to

penetrate the bone collar (with the help of osteoclasts)and enter the region of calcified cartilage. This vessel

delivers nutrients to the surviving cells and transportsosteoclasts that remove the calcified matrix. The resultof this vascular invasion is the formation of a marrowspace and a primary ossification center—the site forcoordinated cell activity for further development. Asthe bone marrow is slowly formed and populated withcells, additional bone continues to be formed on theperiosteal surface of the bone collar. Secondary ossifi-cation centers eventually form at the ends of the long

bones (epiphyses) through a similar process. Othervessels penetrate into the region to supply the cellsand nutrients necessary for additional development.

As the primary ossification center grows, it eventu-

ally makes up roughly the middle third of the hyalinecartilage template. This results in the template havingtwo cartilaginous ends with a central diaphysealregion that includes a marrow cavity. At the interface

between the marrow and cartilage at each end of the bone is a structure called the growth plate (or epiphy-seal plate). The growth plate is responsible for longitu-dinal bone growth. It is comprised of fivemorphologically distinct zones that are conveniently

A B C

D

FIGURE 4.1 The process of intramembranous ossification begins within a sea of mesenchymal cells. (A). mesenchymal cells begin toconsolidate into a blastema and transform into osteoblasts (B), eventually producing bone matrix (C), which over time will be remodeled. (D)Photomicrograph of island of bone forming through intramembranous ossification in the jaw (hematoxylin and eosin stain).

76 4. MODELING AND REMODELING

1. BASIC BONE BIOLOGY AND PHYSIOLOGY


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classified according to the main cellular processes thatoccur at each location (Fig. 4.3). In reality, they exist asa continuum of cells that gradually transition from onezone to another as growth occurs.

The region most distant from the primary ossifica-tion center (nearest the ends of the cartilage tem-plate) is called the resting zone and is comprised of

hyaline cartilage matrix with embedded chondro-cytes. New resting zone matrix, rich in type II colla-gen, is continually produced by the chondroblastsnear the perichondrium. The chondroblasts thatembed themselves in this matrix differentiate intomature chondrocytes. The chondrocytes embedded inmatrix also produce new matrix. This region of active production is sometimes referred to as thereserve zone, since the term resting zone implies that

the cells are inactive. The chondrocytes within theresting zone have similar morphological and physio-logic characteristics as those in hyaline cartilage inother regions of the body.

The second region is called the proliferative zoneand, as the name implies, it is a site of active chondro-cyte mitosis (Fig. 4.3B,C). This region is readily identi-

fiable histologically by its stacked coin appearance,which results from cell division in columns along thelongitudinal axis. These cells produce modestamounts of matrix rich in type II collagen. The zoneof proliferation is regulated by a number of growthfactors. Somatotropin/growth hormone (GH), insulin-like growth factors (IGFs), Indian hedgehog protein(IHH), bone morphogenic proteins (BMPs), and theWnt-β-catenin signaling pathway all play important

A

GH

I

J

BC

DE

Cartilage

Diaphyseo-

epiphyseal

junction

Epiphyseal

ossification

center

(secondary)

Diaphysealossification

center

(primary)

Epiphysis

Epiphyseal

plate

DiaphysisCalcified cartilage

Bone

Arteries

F

FIGURE 4.2 Endochondral ossification. The process of endochondral ossification begins with a cartilage template that transforms into bone through a series of stages involving the coordinated activity of chondrocytes, osteoblasts, and osteoclasts.

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roles in stimulating chondrocyte proliferation.Fibroblast growth factor (FGF) is one of the few fac-tors shown to inhibit proliferation of chondrocytes inthis region.

The third region is the hypertrophic zone (Fig. 4.3).Growth of the long bones is driven by cells in the upperregions of this zone (or prehypertrophic zone). In thelower hypertrophic zone, cells begin to enlarge and die.

A B

C

D

Proliferative

zone

Prehypertrophic

zone

Hypertrophic

zone

Bone

Invading vasculature

Calcification of cartilage

Resting (reserve)

zone

FIGURE 4.3 The epiphyseal growth plate is classified into five zones (A and B) based on the different histologic appearance of the cellsand matrix: resting (reserve) zone; proliferative zone; hypertrophic zone; zone of calcified cartilage; and zone of ossification. As long bonegrowth occurs, cells progress through the different zones. In the low power photomicrograph (A), the individual delineations between zonesare not obvious, although the overall structure of the region can be appreciated. (B) The schematic more clearly shows the cell/tissue morphol-ogy of the zones within the growth plate. (C) At the top of this photomicrograph is the zone of proliferation, where the chondrocytes rapidlydivide and become stacked in a longitudinal orientation. In the zone of hypertrophy the cells grow in size and the matrix produced by the cells

begins to change. Toward the bottom of the field is the zone of calcified cartilage, which can be identified by the slightly darker-stained matrix.(D) This photomicrograph shows the hypertrophic, calcified cartilage, and ossification zones. Chondrocytes in the zone of hypertrophy grow insize, while the region in which the matrix begins to calcify (noted by the dark blue-stained matrix) delineates the transition to the zone of calci-fied cartilage. The zone of ossification begins where bone matrix is produced (pink stain).




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Thyroxine appears to be the main promoter of chon-drocyte hypertrophy, although other factors (e.g. com-ponents of the Wnt-β-catenin pathway) also areimportant for promoting hypertrophy. IHH and para-thyroid hormone-related protein (PTHrP) are two fac-tors known to inhibit chondrocyte hypertrophy in thisregion. Hypertrophy enhances cell growth and the pro-duction of extracellular matrix, which is initially rich in

type II collagen. However, this region also features atransition from the production of type II collagen totype X collagen. Type X collagen is exclusive to hyper-trophic chondrocytes in the developing bone (althoughit can also be found in the fracture callus and damagedarticular cartilage in adults). It differs from type II col-lagen in that it contains fibers, which are absent in thetype II collagen matrix. Although the fibers of type Xcollagen provide stiffness to the region, they create amatrix less able to diffuse nutrients to the cells. Type Xcollagen is intimately connected to vascular invasion,which occurs in the adjacent regions. If no type X colla-gen is present in the hypertrophic zone, then vascularinvasion does not occur and growth in disrupted.

The condensed matrix around the hypertrophiedchondrocytes eventually begins to mineralize (or cal-cify; Fig. 4.3D). Mineralization of the matrix does notoccur in the absence of hypertrophy. The region wherecartilage calcification can be observed is the fourthregion of the growth plate, the calcified cartilage zone.The chondrocytes in this region are either dead, or inthe process of dying, due to lack of nutrient diffusionor cellular waste removal. The signal for apoptosisappears to be related to cellular hypoxia, as cells in thelower proliferative zone and upper hypertrophic zone

have been shown to be more hypoxic than those in themore superficial regions of the growth plate. Matrixcalcification is an active process directed by the chon-drocytes, the specifics of which are not well under-stood. Chondrocytes release vesicles into theextracellular matrix. These vesicles contain alkalinephosphatase (a key contributor to matrix mineraliza-tion), ATPase (to provide energy to transport calciumions into the vesicles), and enzymes that cleave

calcium and phosphate from the surrounding envi-ronment. The increase in local mineral concentrationleads to formation of calcium-phosphate aggregatesand calcification of the matrix. As the region

becomes more calcified and loses more cells, localsignaling leads to vascular invasion. Chondroclasts,cells that are similar to osteoclasts but specialized forthe removal of calcified cartilage, come to the site to

begin cartilage resorption.The final zone of the growth plate is the zone of ossi-

fication, where bone is initially formed (Fig. 4.3). Thisskeletal tissue is formed by osteoblasts that arerecruited to the calcified tissue surface to produce newwoven bone. There are also osteoclasts in the zone of ossification, working to remove both the calcified carti-lage and the newly produced woven bone—the latter

being replaced by more mature lamellar bone through bone remodeling.

Longitudinal growth via endochondral ossificationoccurs until the epiphyseal plate becomes ossified. Inhumans, for most bones this occurs in the late teensand early twenties. As the skeleton matures, activitywithin the zone of ossification exceeds chondrocyterepopulation in the reserve zone. This leads to a slowreduction in the size of the growth plate until the pro-cess ceases completely, leaving behind a mineralizedregion of bone separating the epiphysis and metaphy-sis. This thin plate of bone is called the epiphyseal line.Epiphyseal closure is accelerated by estrogen, whichcauses a more rapid senescence of chondrocytes. Thisis the reason for earlier growth plate closure in womenthan in men.

BONE MODELING

Bone modeling is defined as either the formationof bone by osteoblasts or resorption of bone byosteoclasts on a given surface. This contrasts with

bone remodeling (discussed below), in which osteoblast and osteoclast activity occur sequentially in acoupled manner on a given bone surface (Table 4.1).

TABLE 4.1 Important Characteristics of Modeling and Remodeling

Modeling Remodeling

Goal Shape bone, increase bone mass Renew bone

Cells Osteoclasts or osteoblasts and precursors Osteoclasts, osteoblasts, and precursors

Bone envelope Periosteal, endocortical, trabecular Periosteal, endocortical, trabecular, intracortical

Mechanism Activation-formation or activation-resorption Activation-resorption-formation

Timing Primarily childhood but continues throughout life Throughout life

Net effect on bone mass Increase Maintain or slight decrease

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Modeling by osteoblasts is called formation modeling;modeling by osteoclasts is called resorptive modeling.The primary function of bone modeling is to increase

bone mass and maintain or alter bone shape.Although formation and resorption modeling arelocally independent, they are not globally indepen-dent since both processes occur simultaneouslythroughout the skeleton and must be coordinated to

shape bone. Modeling always occurs on a preexisting bone surface, which is why the initial stages of intra-membranous and endochondral ossification are notconsidered modeling. Modeling activity can occur onperiosteal, endocortical, and trabecular bone surfacesor envelopes.

Events that Signal Modeling

The principal signal for bone modeling is local tis-sue strain (see Chapters 6 and 9 for more detail). If thelocal strains exceed a certain threshold, then formation

modeling is initiated to add new bone matrix. If strainsare low, then resorptive modeling is stimulated and bone is removed.

Cellular Processes

The process of modeling occurs in two stages: acti-vation and either formation or resorption. Activationinvolves recruitment of precursor cells that differenti-ate into mature osteoblasts or osteoclasts. Additionally,

bone lining cells can be stimulated to differentiate intomature, active osteoblasts that begin producing matrix.Once the appropriate cells are activated, the processes

of formation or resorption take place until sufficient bone mass is added or removed to normalize localstrains.

Modeling During the Life Cycle

Bone modeling is most prominent during growthand development and primarily serves to reshape the

bone or change the position of the cortex relative to itscentral axis (called bone drift). The adult skeleton doesundergo modeling but, in the absence of pathology, itis less prominent.

Longitudinal Growth

Both formation and resorption modeling play anessential role in maintaining bone shape duringgrowth associated with endochondral ossification. Inorder to maintain the proper shape of the long bones,

both types of modeling are coordinated in the meta-physeal region (Fig. 4.4). As the bone lengthens,

resorption modeling removes bone on periosteal sur-faces, while formation modeling adds new bone toendocortical surfaces. While these processes are highlycoordinated, they are distinct from remodeling becauseformation and resorption occur on different surfaces.Disruption of modeling during growth results in

abnormal metaphyseal morphology. This is most evi-dent in cases of osteoclast inhibition (examples includegenetic dysfunction or pharmaceutical intervention),which results in the metaphysis developing anErlenmeyer flask or club-shaped morphology.

Radial Growth

Formation modeling is the major mechanism of radial bone growth throughout life, beginning with theinitial formation of the bone collar on the cartilaginousdiaphysis. The rate of periosteal modeling is highest

during growth and then slows during adulthood.The rapid periosteal modeling during growth is coun-tered by resorptive modeling on the endocortical sur-face, resulting in a relatively consistent corticalthickness over time (Fig. 4.5). The modeling activity onthe diaphyseal cortices is sexually dimorphic, bothduring puberty and with aging. Estrogen acts to inhibitperiosteal modeling such that at puberty the amountof formation modeling is decreased in girls relative to

boys. Conversely, boys have a spike in growth hor-mone and IGF-I during puberty and this, along withincreasing levels of testosterone, stimulates periostealgrowth. These gender-specific hormonal differences

result in men having larger bone diameters when peak bone mass is attained. The estrogenic inhibition of for-mation modeling is released when women go throughmenopause, resulting in a brief but measurable stimu-lation of formation modeling and an increase in bonediameter. A number of other factors, such as mechani-cal loading, parathyroid hormone, and sclerostin, playmajor roles in dictating radial bone growth throughperiosteal formation modeling.

Formation modeling

Resorption modeling

FIGURE 4.4 As the bone grows longitudinally, there is coordi-nated modeling activity on the metaphyseal bone surfaces that serveto preserve the bone shape.




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Bone Drift

Bone drift is a process related to radial growth butit has the distinct goal of changing the position of thecortex relative to its central axis. This process is mostactive during growth, although it can occur in theadult skeleton in the presence of extreme alterationsin mechanical loading patterns. Bone drift occursthrough the coordinated action of formation andresorption modeling on distinctive bone surfaces. Indiaphyseal bone, formation modeling occurs on one

periosteal and one endocortical surface, while resorp-tive modeling occurs on opposing periosteal andendocortical surfaces (Fig. 4.6). Bone drift can be illus-trated using the example of bone curvature correc-tion. Children with rickets have long bones that areconsiderably bowed. Following correction of theunderlying condition that causes rickets, the bonewill straighten to some extent through the process of cortical drift. Cortical bone drift also represents themain mechanism for orthodontic tooth movement,where application of spatially specific loads can beutilized to move teeth through space. The trabecularnetwork can also undergo modeling, in which indi-

vidual trabecular struts are moved in order to moreeffectively accommodate local strain environments(Fig. 4.7).

BONE REMODELING

Bone remodeling involves sequential osteoclast-mediated bone resorption and osteoblast-mediated

bone formation at the same location. Remodeling isoften referred to as a quantum concept, whereby discretelocations of the skeleton are replaced by quantumpackets of bone through the coupled activity of osteo-

blasts and osteoclasts. This is the mechanism by whichthe replacement of bone matrix and the repair of smalldefects (e.g. microdamage) occur, thereby renewingthe skeleton over time. Remodeling can occur upon/within any of the four bone surfaces/envelopes: peri-

osteal, endocortical, trabecular, and intracortical.During intracortical remodeling, teams of osteoclastsand osteoblasts burrow through the matrix. This groupof cells, and its associated blood vessels, is called a

bone multi-cellular unit (BMU). The final product of remodeling within the cortex is an osteon, a structurecomposed of concentric layers of bone enclosed by acement line with a central (haversian) canal (seeChapter 1, Fig. 1.7). New BMUs can be initiated from

Resorption

modeling

Formation

modeling

FIGURE 4.5 Radial bone growth involves formation modeling onperiosteal surfaces and resorption modeling on the endocortical sur-face. Over time, these processes preserve cortical thickness, whileincreasing the width of the bone.

FIGURE 4.6 Using multiple fluorochrome labels, bone drift can be observed in a growing mouse. Several different fluorochromeswere administered, roughly 1 week apart, to a growing mouse fromage 3.5 weeks to 13 weeks, and then the radius and ulna were sec-tioned for histologic analysis. Radial growth occurred in the ulna(left bone) as the periosteal surfaces expanded outward, while therewas also some formation modeling on the endocortical surface. Theradius (right bone) underwent significant drift (down and to theright in the picture) by formation modeling on one periosteal surfaceand the opposite endocortical surface, along with resorption model-ing on the other two surfaces. The schematic below the photomicro-graph conceptualizes what the bone probably looked like at

3.5 weeks of age and how it was transformed through modeling tothe eventual geometry.

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within the bone marrow, the periosteum, or the vascu-lature within an existing osteon. When remodelingtakes place on endocortical/cancellous and periostealsurfaces, it does not incorporate a blood vessel, and itsresulting structure is called a hemiosteon (see Chapter 1,Fig. 1.7). Both endocortical/cancellous remodeling andintracortical remodeling are common during growthand development, as well as in adulthood, while remo-deling on the periosteal surface is far less frequent atall stages of life.

Events that Signal Remodeling

Bone remodeling can be classified as targeted or sto-chastic. In targeted remodeling, there is a specific localsignaling event that directs osteoclasts to a given loca-

tion to begin remodeling. The two most acceptedsignaling events for targeted remodeling are micro-damage and osteocyte apoptosis, although these maynot be independent. Stochastic remodeling is consid-ered a random process, with osteoclasts resorbing

bone without a location-specific signaling event.Targeted remodeling serves to repair bone matrix thatis mechanically compromised, while stochastic remo-deling is thought to play more of a role in calciumhomeostasis.

The concept of microdamage serving as a signal forremodeling was first theorized in the 1960s, whenHarold Frost suggested microdamage needed to be

actively remodeled by the bone in order to prevent cat-astrophic failure. Over 20 years later, the theory wasexperimentally tested using a model in which micro-damage was induced in canine bone using supraphy-siologic mechanical loads. Damage was imparted inone animal limb and, after one week, the same loadswere applied to the contralateral limb. Histologic anal-yses documented that the levels of microdamage in

both limbs were similar and significantly higher than

in the limbs of nonloaded animals. Most importantly,the limb initially loaded had significantly more resorp-tion cavities than did the contralateral limb, and thesecavities were spatially associated with the microdam-age. These results provided the first evidence thatmicrodamage serves as a signal for targeted boneremodeling.

Subsequent to these initial large animal studies, sev-eral experiments have advanced our understanding of the mechanism underlying microdamage-inducedremodeling. These experiments have taken advantageof the fact that rats and mice do not normally exhibitintracortical remodeling. Thus, through various inter-ventions (such as supraphysiologic mechanical loadingto induce microcracks or genetic manipulations toablate osteocytes), intracortical remodeling can belinked to specific causative factors. It is important to

realize that although rodents do not normally undergointracortical remodeling, they still have remodeling onthe other bone envelopes similar to that of other ani-mals and humans.

These rodent studies have shown that microdamageresults in localized disruption to the osteocyte networkvia physical breakage of the cytoplasmic connections

between cells. These osteocytes are then cut off fromthe remainder of the network and begin to undergoapoptosis. Prior to dying they begin to actively pro-duce factors, such as tumor necrosis factor ligandsuperfamily member 11 [receptor activator of theNF-κB ligand (RANKL)], a key factor in osteoclast

development. In addition, the cells more distant fromthe microdamage produce strong antiapoptotic signals[such as/tumor necrosis factor receptor superfamilymember 11B (OPG)]. This pattern of signaling by theviable and dying osteocytes probably serves as a targetfor the osteoclasts to begin their remodelingactivity (Fig. 4.8). When microdamage is produced andosteocyte apoptosis is inhibited through pharmacologi-cal intervention, remodeling is also inhibited.

Load

Formation modeling

Resorption modeling

FIGURE 4.7 Modeling occurs on trabecular surfaces in order to normalize loading-induced strains. Areas of high strain undergo forma-tion modeling, while regions of low strain are resorbed. These activities reorient individual trabecular structures in the direction of theprincipal loads.




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Alternatively, when the osteocyte network is disruptedin the absence of microdamage, such as through loss of estrogen, mechanical disuse, or glucocorticoid excess,intracortical remodeling is enhanced in association withosteocyte apoptosis. Collectively, these studies illustratethat although microdamage leads to targeted remodel-ing, osteocyte apoptosis is the critical event in theprocess.

Historically, calculations estimating the balance between targeted and stochastic remodeling have usedthe assumption that microdamage was the targetingevent. Mathematical models based on experimentaldata have calculated that about 30% of remodeling istargeted to microdamage. The recent evidence thatosteocyte apoptosis is probably the key event suggeststhat targeted remodeling, albeit to something otherthan microdamage, is likely to form an even greaterpercentage of total remodeling. Indeed, osteocyte apo-ptosis may be a key precondition for remodeling tooccur on any surface. This considerably blurs the dis-tinction between targeted and stochastic remodeling,which are probably better demarcated by the initiating

event (local versus systemic) and by function (repairversus mineral homeostasis).

Remodeling Cycle

Whether or not the remodeling is targeted or sto-chastic, the cellular events are similar. The process of remodeling is divided into five stages: activation,resorption, reversal, formation, and quiescence. This is

often referred to collectively as the remodeling cycle(Fig. 4.9). At any given time, there are thousands of remodeling cycles taking place throughout the body.These cycles are at various stages, depending on whenthey were initiated. The entire remodeling process nor-mally takes roughly 46 months in humans, althoughthis can be highly altered by disease (see Chapter 7).

ActivationThe activation stage represents the recruitment of

osteoclast precursors to the bone surface followed bytheir differentiation and fusion to become fully func-tional osteoclasts. The process of osteoclast differentia-tion and maturation is outlined in Chapter 2.

Resorption

Once mature osteoclasts are present, bone liningcells retract from the surface to expose the mineralizedmatrix to osteoclasts. This appears to be an active pro-cess and is stimulated either by the osteoclasts them-selves as they approach the surface or by the samesignals that initiated the remodeling. Without retrac-tion of the bone lining cells, osteoclasts are unable to

bind to the bone and begin resorption. Upon attach-ment, the osteoclasts actively dissolve the mineral andliberate collagen fragments. These fragments can bemeasured in the blood and urine, thus providing use-ful biomarkers for the assessment of bone remodeling.

As resorption proceeds, new osteoclasts can berecruited to the remodeling site to either support existingosteoclasts or replace those that die. There is significantvariability in the size of individual remodeling sites onendocortical and trabecular surfaces. Regulation of this

variability is well not understood. Intracortical radialresorption spaces, which can be quantified by measuringosteon diameter, are relatively consistent in size. Osteonlength, on the other hand, ranges from several hundredmicrometers to several millimeters.

Reversal

The reversal phase is characterized by the cessationof osteoclast resorption and the initiation of bone for-mation. The signal for reversal within BMUs isunknown, although several theories exist. Direct cell-cell interaction between osteoclasts and osteoblasts (ortheir precursors) may induce signaling for cessation of

one cell type and activation of another. The discoveryof ephrin extracellular proteins on both osteoblasts(EphB4) and osteoclasts (ephrin-B2) supports this the-ory, although experimental evidence of direct contact

between cells is lacking. Another plausible mechanism(also theoretical) is the presence of factors releasedfrom the bone matrix during resorption [e.g. trans-forming growth factor beta (TGF-β) and BMPs] thatstimulate osteoblast migration and differentiation.

FIGURE 4.8 Regions around microdamage have both pro- andantiremodeling signals that are involved in targeted remodeling.Osteocytes near the microcracks express high levels of RANKL andlow levels of OPG, thus favoring osteoclast recruitment. The osteo-cytes farther away express low levels of RANKL and high levels of

OPG. This is believed to serve as a target for the osteoclasts to knowwhich bone to remodel.

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However, this theory fails to explain how osteoblastsare specifically signaled to the remodeling site, whenthe liberation of these molecules would probably be

widespread in the nearby marrow. Recent descriptionsof the remodeling canopy, which creates an underlying bone remodeling compartment (BRC) and provides amechanism to localize these liberated factors, seem toaddress this limitation. The remodeling canopy is aphysical structure, made up of bone lining cells, underwhich the remodeling unit exists (see Chapter 2, Fig.2.10). The junctional complexes that link adjacent bonelining cells may allow some factors to be exchanged

between the remodeling compartment and the outsideenvironment, while maintaining appropriate molecularconcentrations within the compartment.

Once osteoclasts have finished resorbing bone, the

remaining collagen fragments on the exposed surfacemust be removed. It is currently thought that this isdone by a specialized form of bone lining cell. If suchfragments are not removed, then bone formation byosteoblasts does not proceed. These specialized cellsare also thought to deposit a thin layer of new bonematrix (the cement or reversal line), a clear histologicfeature that delineates the boundaries of osteons andhemiosteons from the surrounding, older matrix. The

cement line is rich in proteoglycans, such as osteopon-tin. Controversy exists regarding the composition of the cement line; specifically, whether it is highly or

minimally mineralized. Regardless, it is widelyaccepted that cement line mineralization differs fromthe surrounding bone and that this plays an importantrole in its mechanical properties.

Formation

During the bone formation stage osteoblasts laydown an unmineralized organic matrix (osteoid), whichis primarily composed of type I collagen fibers andserves as a template for inorganic hydroxyapatite crys-tals. Osteoid mineralization occurs in two distinctphases. Primary mineralization, the initial incorporationof calcium and phosphate ions into the collagen matrix,

occurs rapidly over 2

3 weeks and accounts forroughly 70% of the final mineral content. Secondarymineralization, the final addition and maturation of mineral crystals, occurs over a much longer time frame(up to a year or more) (see Chapter 1, Fig. 1.6).

The osteoblasts participating in new bone forma-tion undergo one of three fates. The majority (90%)die through apoptosis. These are replaced by newosteoblasts as long as formation is still necessary at

A

B

C

D

E

FIGURE 4.9 The remodeling cycle. The remodeling cycle involves five stages: (A) activation; (B) resorption; (C) reversal; (D) formation;and (E) quiescence. At any one time, remodeling cycles throughout the body are at various stages.




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the local site. Another fraction of osteoblasts is incor-porated into the osteoid matrix and eventually

become osteocytes. The osteoblasts remaining at theconclusion of formation remain at the bone surface asinactive bone lining cells. These cells retain the capac-ity to become activated and begin producing bonematrix again.

Quiescence (Resting)

At the completion of a bone remodeling cycle, theresulting bone surface is covered with bone liningcells. The matrix within the remodeling unit will con-tinue to mineralize over time. At any given time, themajority of bone surfaces within the bone are in a stateof quiescence.

Bone Remodeling Cycle Duration

Absent of pathology, a complete remodeling cycletakes about 46 months from the time of activation tothe time that osteoblasts finish producing matrix

(Fig. 4.10). Mineralization of the matrix continues formonths after production. This time is not routinelytaken into account when assessing remodeling cycleduration. The duration of a remodeling cycle is notevenly divided between resorption and formation.Osteoclasts typically resorb bone for 36 weeks (at agiven site), with the remainder of the cycle comprising

bone formation. Consequently, when one looks at boneunder the microscope, it is much more common tofind formation sites than resorption sites (by a ratio of about 4:1). The duration of a remodeling cycle isaltered in a number of diseases (some of which aredetailed in Chapter 7).

Bone Remodeling Rate

The rate of bone remodeling is very high duringgrowth and then slowly decreases until peak bonemass is attained. In adulthood, the rate is highly vari-able and is influenced by age and genetics, as well as anumber of modifiable factors such as physical activity,nutrition, hormonal activity, and medications. Infemales, remodeling increases at menopause due tothe loss of circulating estrogen, which normally acts tosuppress remodeling through direct effects on osteo-clasts and the suppression of osteoclast apoptosis. Theincrease in remodeling is progressive in the years fol-

lowing menopause. Individuals who take hormonereplacement therapy can offset this increase in remo-deling, as can those who take antiresorptive pharma-ceutical agents (see Chapter 17). Men experience lessdramatic increases in remodeling, and these typically

begin to occur about a decade later than the increaseobserved in women. Eventually, with age (around the

Activation Resorption Reversal Formation* and mineralization**

10 days 21 days 5 days 90 days* ~1 year**

FIGURE 4.10 The stages of bone remodeling occur over different time frames, with the formation phase taking 45 times longer than theresorption phase. Final mineralization of the newly formed bone can take up to 1 year.

85BONE REMODELING



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eighth decade and beyond), the remodeling rates in both women and men begin to decline.

Bone Remodeling Balance

Although resorption and formation are coupled inremodeling, they are often not balanced. Couplingrefers to the sequential cellular processes of osteoclastand osteoblast activity in each BMU, while balancerefers to the amount of tissue resorbed and formed ateach site (Fig. 4.11). Coupling explains why, in condi-

tions of low bone resorption, as with antiresorptivetherapy, bone formation is also low. Natural condi-tions in which activity at the BMU level becomesuncoupled have not been described in the literature,although this does occur transiently with some osteo-porosis treatments, such as intermittent rhPTH(134)(recombinant human PTH(134), or teriparatide).In some cases, an argument is made for remodeling

being uncoupled, based on differences in resorption

and formation biomarkers, but this represents a sys-temic imbalance between the two processes, not BMU-level uncoupling.

In a healthy individual, bone remodeling is alwayscoupled at the individual BMU level. Bone balance atthe BMU level is slightly negative in healthy indivi-duals; more so within cortical bone than trabecular

bone, in order to accommodate the central canal withinthe osteon. In common conditions of bone loss such aspostmenopausal osteoporosis, bone remodelingremains coupled but bone balance becomes even morenegative. A net negative bone balance at each BMU

plays an important role in bone loss. This is magnifiedin cases such as postmenopausal osteoporosis or otherconditions in which the number of remodeling sites isincreased, resulting in an accelerated rate of bone loss(Fig. 4.12). A positive BMU bone balance, where more

bone is formed than is resorbed at the individual BMUlevel, has been shown to occur with intermittentrhPTH(134) treatment (Fig. 4.11).

BONE REPAIR

The process of bone healing is covered in depth inChapter 10. Bone remodeling accounts for most of theactivity associated with primary bone healing. In sec-ondary bone healing, both modeling and remodelingactivities take place, although these events occurduring the later stages. During the early stages of sec-ondary bone healing, intramembranous and/or endo-chondral ossification are recapitulated in order toprovide initial stability to the fracture site. This

Normal BMU balance

Nagative BMU balance

Positive BMU balance

FIGURE 4.11 The amount of bone formed relative to the amountof bone resorbed by an individual bone remodeling unit is referredto as bone (or BMU) balance. In normal situations, BMU balance isslightly negative. In certain diseases, such as postmenopausal osteo-porosis, BMU balance becomes even more negative, resulting in sig-nificant bone loss at each BMU. Pharmacological treatment withintermittent teriparatide/rhPTH has been shown to produce a posi-tive bone balance at the BMU level.

FIGURE 4.12 The rate of bone remodeling is positively associ-ated with bone loss. High rates of remodeling result in more BMUsexisting at any one time and a slower rate of formation relative toresorption (see Fig. 4.10). This, together with the slightly negativeBMU balance, results in reduced bone mass in most conditions of high remodeling.




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stability is provided by a combination of woven boneand cartilage. Bone modeling and remodeling thentake place in order to replace the tissue with normallamellar bone and achieve the original bone shape.

EFFECTS OF MODELING ANDREMODELING ON BONE STRUCTUREAND MATERIAL PROPERTIES

Bone Structure Effects of Modeling andRemodeling

The processes of bone modeling and remodelingalter the skeleton in distinct ways. Modeling plays anessential role in shaping skeletal structure duringgrowth, yet its influence also exists in mature bone. Incortical bone, formation modeling on the periostealsurface plays an essential role in mechanical adapta-tion. Exercise induces a potent modeling response onthe periosteal surface of long bones (see Chapter 9).Periosteal modeling also plays an important role in off-setting bone loss with age. Increases in endocorticalremodeling during the postmenopausal years result inthe loss of cortical shell thickness and, if not compen-sated, bone strength decreases. However, loss of estro-gen stimulates periosteal modeling. Due to themechanical advantage of adding bone to a surface fur-ther from the central bending axis, the addition of onlya small amount of bone on the periosteal surface is suf-ficient to offset larger losses of bone from the endocor-tical surface (see Chapter 6, Fig 6.8). Even so, the

periosteal expansion in postmenopausal women is stillinsufficient to offset the drastic loss of bone on theendocortical surface. Formation modeling also benefitstrabecular bone structure by reorienting trabeculae inthe direction of primary stresses.

Although remodeling has an essential role in renew-ing the skeleton through the replacement of damagedtissue, its net negative bone balance produces an over-all loss of bone at each remodeling site. Thus, remodel-ing gradually reduces bone mass at a small rate untillater in life (or if pathology exists). A more dramaticeffect occurs when a large number of remodelingcycles are initiated within a short time frame. Referred

to as the remodeling transient, the resorption phaseleaves a void that is slowly filled by formation. One of the best examples of the remodeling transient occurswith the pharmaceutical treatment teriparatide/rhPTH(134). Daily injections of teriparatide stimulate boneremodeling, and studies in both humans and animalshave shown that this rapid and significant stimulationof remodeling is associated with acute reductions in

bone mass, particularly at cortical bone sites. This is

the reason for the initial loss of bone at the femoralneck in osteoporotic patients treated with this drug.

In cortical bone, increased remodeling results inincreased cortical porosity. Even in situations of nor-mal BMU balance, a significant amount of bone is lostwith every remodeling event due to the production of a new central (haversian) canal. When osteoblast func-tion is compromised, either in old age or in pathology,

the size of the central canal is increased, resulting inlarger pores within cortical bone. In trabecular bone,the net negative BMU balance slowly thins trabeculaeover time and, at some point, the struts become so thinthat a remodeling unit can completely pierce throughthe bone causing a disconnection. Once trabeculae aredisconnected, the normal remodeling process cannotreconnect them. Reestablishment of trabecular connec-tions necessitates woven bone bridging through denovo formation.

Bone Material Property Effects of Remodeling

Bone material properties are those properties of theskeletal tissue, independent of the size and shape of the bone (see Chapter 1). The primary bone materialproperties are mineralization; collagen content, matu-rity, and cross-linking; and microdamage. These prop-erties are each influenced by the rate of boneremodeling.

Mineralization

Bone mineralization can be assessed at several levelsand these are important to distinguish when determin-

ing the effects of remodeling. At the tissue level, bonemineralization is often described in terms of the degreeand heterogeneity of mineralization, which are inde-pendent of the amount of tissue present. These areroutinely referred to using terms such as bone mineraldensity distribution (BMDD) or the mean degree of miner-alization of bone (MDMB). It is important to differentiateBMDD and MDMB from the more traditionally usedterm, bone mineral density (BMD). BMD provides a mea-sure of density that is dependent on both the amountof bone present and its mineralization. Therefore, BMDcannot distinguish changes in tissue-level mineraliza-tion from changes in the amount of bone tissue.

Conversely, BMDD (and MDMB) provide measuresspecific to tissue mineralization regardless of theamount of bone. These measures provide two keyproperties of the material mineralization: average tis-sue mineralization and the degree of mineralizationheterogeneity. Bone remodeling has a significant influ-ence on these two parameters. In general, when boneremodeling is high, average tissue mineralization isreduced and heterogeneity is increased. This is because

87EFFECTS OF MODELING AND REMODELING ON BONE STRUCTURE AND MATERIAL PROPERTIES



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older, more highly mineralized bone is being replaced by newer bone tissue that has lower mineralization.Conversely, in situations in which bone remodeling islow, average tissue mineralization is increased andheterogeneity is decreased because less bone is beingreplaced and more regions have completed secondarymineralization.

Collagen Cross-LinksCollagen cross-links take two forms. Enzymatic

cross-links form through a highly regulated processduring matrix formation and mature quickly. The levelsof immature and mature enzymatic cross-links aredetermined at the time of matrix formation and do notappear to be significantly affected by the rate of remo-deling (although the ratio of immature to mature cross-links will increase with increased rates of remodeling).In situations of rapid remodeling, a large number of cross-links are removed, but a large number are alsoproduced in the new tissue. In low remodeling states,few enzymatic cross-links are removed and few areformed. The second type of collagen cross-link is nonen-zymatic and occurs spontaneously. Levels of nonenzy-matic cross-links within the bone matrix are affected by

bone remodeling in much the same way as propertiesof mineralization. High levels of bone remodelingreduce levels of nonenzymatic collagen cross-linking,while suppression of remodeling allows levels toincrease.

Microdamage

Microdamage accumulation in bone is a normalphysiologic process, the consequence of repeated

cycles of loading during activities of daily living. Thegeneration of microdamage serves to dissipate energyand, therefore, the formation of microcracks is animportant mechanism for preventing overt fracture.Yet microdamage, specifically its coalescence into amacrocrack, is detrimental to mechanical properties.Hence, there is a need for minimizing microdamageaccumulation. Under normal physiologic conditions,the microscopic cracks that are formed in bone do notaccumulate to levels that negatively affect bone’smechanical properties because they are targeted by theremodeling process. In most circumstances, changes inremodeling are probably driven by levels of micro-

damage such that high remodeling occurs in part because microdamage is high, and its repair by remo-deling reestablishes mechanical equilibrium. As such,the levels of microdamage are relatively stable duringthe middle of life. The relationship between microdam-age and remodeling rate becomes disconnected withage and in response to pharmacological intervention.There is an exponential increase in the levels of micro-damage beyond about 70 years, which roughly

coincides with the period that remodeling begins todecrease. There is also a significant accumulation of microdamage associated with the use of antiremodel-ing agents. The mechanism underlying both age- anddrug-related increases in damage is twofold. First,when remodeling is suppressed, microdamage accu-mulates because the damage that forms is not remo-deled. Second, because lower levels of remodeling

make the tissue more brittle (from increased minerali-zation and collagen cross-linking), mechanical forcesare more likely to generate damage.

LABORATORY ASSESSMENT OFMODELING AND REMODELING

The rate of bone remodeling is an independent riskfactor for fracture. When matched for bone mineraldensities, individuals with high bone remodeling ratesare at a greater risk of fracture than are those with lowremodeling rates. Individuals with high remodeling,even if they have high BMD, have fracture rates equiv-alent to those of patients with significantly lowerBMD. As a consequence of this relationship, the assess-ment of bone remodeling is a useful diagnostic tool.

Bone histomorphometry, i.e. the assessment of histologic sections from bone biopsy, is an essentialtechnique for understanding the tissue-level me-chanisms of bone remodeling (see Chapter 7).Histomorphometry is the gold standard for clinicalassessment of bone remodeling. However, due to theinvasiveness of sample collection, such procedures aremost often reserved for assessing pathology.

More commonly, biochemical markers are obtained.Several bone formation and resorption markers exist inthe blood and urine (Table 4.2). Bone resorption mar-kers include mature collagen fragments (N-telopeptideor C-telopeptide) or cross-links (pyridinoline anddeoxypyridinoline) that are released from the bonematrix during osteoclastic resorption. Formation mar-kers include products secreted from osteoblasts duringactive matrix secretion (alkaline phosphatase andosteocalcin) or fragments of collagen that are cleavedduring collagen synthesis (e.g. procollagen N-terminalextension peptide). The clinical value of biochemicalmarkers is that they are relatively inexpensive and

easy to assess, thus allowing individual patients to betracked over time. On a population basis, biomarkerscorrelate bone loss and fracture risk. However, bio-chemical markers are limited by their large variabilitywithin an individual and between assays.

Biomarkers provide a whole-body assessment of bone formation or resorption, taking into accountactivity on all bone surfaces (both cortical and trabecu-lar) at a given moment in time. This can be seen as an




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advantage because it provides insight into the wholeskeleton; yet it could be a misleading assessment if interest lies in a specific skeletal site. The alternativeassessment, histomorphometry on an iliac crest biopsy,provides data on a single bone site, and within that

bone site cortical and trabecular bone properties can be assessed separately. These parameters provide an

advantage over biomarkers, but are limited by theassumption that properties of the iliac crest apply toother skeletal sites of clinical interest. Animal studies,along with limited human data, suggest that trabecu-lar bone remodeling rate in the iliac crest is similarto some, but not all, clinically relevant sites(Fig. 4.13).

40

30

20

10

0

B o n e f o r m a t i o n r a t e , % / y e a r

I l i a c c r e s t b i o p s y

4 t h t h o r a c i c v e r t e b r a



1 0 t h t h o r a c i c v e r t e b r a

1 2 t h t h o r a c i c v e r t e b r a

4 t h l u m b a r v e r t e b r a

H u m e r u s

D i s t a l R a d i u s

F e m o r a l n e c k

T i b i a c o n d y l e

C a l c a n e o u s

P a r i e t a l b o n e

O c c i p i t a l b o n e

FIGURE 4.13 Bone remodeling is highly heterogeneous across skeletal sites. These dynamic histomorphometry data, collected from asingle individual who died suddenly within 2 weeks following double tetracycline labeling for an iliac crest biopsy, represent the most com-prehensive analysis of the human skeleton for remodeling rates and demonstrate the variability in bone formation rate throughout the skele-ton. Adapted from Pødenphant et al., 1987; 40: 184188.

TABLE 4.2 Whole Body Formation and Resorption Can be Measured by Various Urine and Serum Biomarkers

Abbreviation Matrix

FORMATION

Osteocalcin OC Serum

Bone-specific alkaline phosphatase BSAP Serum

Procollagen type I N propeptide PINP Serum

RESORPTION

Pyridonoline PYD Urine

Deoxypyridinoline DPD Urine and serum

N-terminal cross-linking telopeptide of type I collagen NTX-I Urine and serum

C-terminal cross-linking telopeptide of type I collagen CTX-I Urine and serum

Tartrate-resistant acid phosphatase TRACP Serum

89LABORATORY ASSESSMENT OF MODELING AND REMODELING



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STUDY QUESTIONS

1. Compare and contrast intramembranous and

endochondral ossification, making special reference to

which cells are involved in each process, and how the

final product of each process differs.

2. Describe the process of skeletal maturation at the

growth plate. What are the major characteristics of each

region, and how does this facilitate transition from the

original cartilage model to mature skeletal tissue?

3. Differentiate between bone modeling and bone

remodeling. List and describe the various functions of

each.

4. Describe the five major stages of bone remodeling.

Which cells are involved and how do they

communicate with one another? What events lead to

the transition from one stage to the next?

5. How does bone remodeling affect the material

properties of the skeleton?

6. What methods are used to assessbone remodeling in a

clinical setting, and what aspect of remodeling do theydetect? List the advantages and disadvantages of each

method.

Suggested Readings

Frost, H.M., 1963. Bone Remodeling Dynamics. CC Thomas,Springfield, IL.

Frost, H.M., 1986. Intermediary Organization of the Skeleton, vols. Iand II. CRC press.

Hall, B., 2005. Bones and Cartilage: Developmental and EvolutionarySkeletal Biology. Academic Press.

Henriksen, K., Neutzsky-Wulff, A.V., Bonewald, L.F., Karsdal, M.A.,2009. Local communication on and within bone controls boneremodeling. Bone. 44, 1026

1033.

Parfitt, A.M., 1994. Osteonal and hemiosteonal remodeling: the spa-tial and temporal framework for signal traffic in adult bone.

J. Cell Biochem. 55, 273286.Parfitt, A.M., 2002. Targeted and nontargeted bone remodeling: rela-

tionship to basic multicellular unit origination and progression.Bone. 30 (1), 57.

Recker, R.R., 1983. Bone Histomorphometry: Techniques andInterpretation. CRC Press.

Robling, A.G., Castillo, A.B., Turner, C.H., 2006. Biomechanical andmolecular regulation of bone remodeling. Annu. Rev. Biomed.Eng. 8, 455498.


1 BASIC BONE BIOLOGY AND PHYSIOLOGY
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Bone Modeling and Remodeling