Bone as a Dynamic Biomaterial

Like most biological tissues, bone is a dynamic material facilitating growth of the individual, fracture healing, repair mechanisms of micro fractures and adaption to mechanical loading.

The major actors for these remodeling and modeling processes are the osteoblasts (bone build-ing cells) and osteoclasts (bone resorbbuild-ing cells). The interplay of these cells is controlled by various communication mechanisms like the RANK - RANKL (Receptor Activator of NFkB Ligand) pathway or WNT signaling, which are extensively described in the literature [48, 49].

The overall bone volume balance can be neutral, positive or negative depending on osteoblast and osteoclast activity [50]. Obviously, during a growth phase a positive balance is needed while it remains almost neutral in healthy adult people. Hormonal changes during aging or in bone diseases often cause a negative balance (e.g. in Osteoporosis). This results in an overall loss of trabecular bone and thinning of cortical bone increasing the susceptibility for fragility fractures [33]. Therefore, to medicate patients with fragile bone, much eort is made to de-velop drugs to shift bone remodeling from a negative to a neutral or even a positive balance which requires an essential understanding of the remodeling cycle. Bone remodeling appears on the endosteal surface and on trabecular surfaces (both surfaces together are termed the endosteum). Bone modeling occurs throughout life in murine bones on the outer periosteal surface (periosteum). In bigger mammals a remodeling process also occurs inside the cortical, thus forming a system of cannels occupied by blood vessels (Haversian system). Sims et al.

lists ve phases occurring during remodeling [51]:

After a resting period, where no bone formation or resorption occurs, the activation and resorbtion phase (1) starts. Initiated by systemic hormones (Parathyroid hormone (PTH) or 1,25-dihydroxyvitamin D3) or by mechanical stress sensed by osteocytes, osteoclasts

dif-Figure 2.2: Schematic illustration of various anatomical sites of a long bone and the ve phases of the bone remodeling cycle; (1) activation and resorbtion phase, (2) reversal phase, (3) bone apposition and osteoblast maturation, (4) osteoblast apoptosis or dierentiation, (5) osteocyte maturation; Reprinted from Reference [51] with permission from Elsevier.

ferentiate into multinucleated cells. During bone remodeling osteoclasts attach to the bone surface, solute the mineral phase of the bone matrix, while simultaneously resorbing the or-ganic matrix leaving behind resorption pits or Howship's lacunae and release coupling factors (cardiotrophin-1 and sphingosine-1-phosphate) that stimulate osteoblast dierentiation on the endosteal surface. These coupling factors also signal to periosteal osteoblasts, perhaps through the osteocyte canalicular network.

In the following (reversal phase (2)) osteoclasts undergo apoptosis (programmed cell death).

Mononuclear cells are then forming a layer rich of Proteoglycans, which will later become the so-called cement line.

After the reversal phase pre-osteoblasts mature, attach to the bone surface and place non-mineralized bone matrix, which mainly consists of type I collagen. This purely organic matrix, called osteoid, has a thickness of approximately10µm and rests for about10days before the primary mineralization process starts. In this phase (3), HAP crystals are formed through various transient phases, which are yet not fully understood. Within a few days the degree of mineralization reaches about 70 % of the maximum value. That corresponds to roughly 18 weight percent Ca (wt%Ca) [14]. This period of rapid mineralization is followed by the phase of secondary mineralization, that lasts for several months. Finally theCaconcentration saturates (at about23 to24 wt% reported for humans [14]).

In the next phase (4) mature osteoblasts, when their task of producing osteoid is completed,

Figure 2.3: Calibrated backscattered elec-tron image (qBEI) showing dierently miner-alized bone packets. The numbers represent theCaconcentration inwt%as determined by qBEI. Modied from Reference [25] with per-mission of the Royal Society of Chemistry.

become lining cells or become encased in the bone matrix dierentiating into preosteocytes.

The preosteocytes become osteocytes and develop processes which connect to their neighbors and to the bone surface. They start expressing proteins characteristic for the dierentiation into mature osteocytes (Sclerostin, RANKL) ) during matrix mineralization seems to be spe-cic and highly-controlled (phase 5) [5]. Proteins (e.g. IL-6 family cytokines) are released by the osteoblast lineage and act to stimulate osteoblast dierentiation and bone matrix produc-tion on endosteal surfaces, but limit osteoblast activity on the periosteum.

As a result of the gradual remodeling processes, bone tissue consists of various bone structural units (BSU) with dierent tissue age and therefore also of various degree of mineralization as depicted in Figure 2.3. Since remodeling is a continuous process, the entire human skeleton is replaced by new bone matrix in about 10 years [52]. Consequently, it needs to be strictly distinguished between the individual age (depicting the age of the animal or human) and tissue age (corresponding to the age of a dened region of bone material).

Instantly, the question arises how tissue age can be measured. Fortunately an elegant routine was developed to label time points of bone formation using uorescent dyes like Tetracycline (humans), or Alizerin or Calcein (animals). These uorochromes are administered typically for 2 times for 3 days in 14 days interval (in adult humans) or are injected with a1−10days break (mice) before bone examination[18, 53, 54]. Since these dyes bind to the apatite crystal during the very early stage of mineralization, regions of new bone apposition can be identied using a uorescent microscope, and even more interestingly they label regions of well known tissue age in the mineralized matrix. A Raman micro spectroscopy routine was developed to set measurements with respect to these labels and also recently backscatter electron microscopy was combined with the tissue age information to gain bone material properties at regions with dened tissue ages [53, 55, 56]. Thus combinations of these methods provide material information independent of bone turnover.