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Cement lines are the boundaries between secondary osteons and the surrounding interstitial bone matrix in cortical bone. The interfacial properties of cement lines have been determined by osteon pushout tests. However, distinctively different material properties were obtained when osteon pushout tests were performed under different test geometries. In the present study, an axisymmetric two-dimensional finite element model was used to simulate an osteon pushout test using the test geometry of actual experiments. The results indicated that shear failure within the osteonal lamellae would occur when the osteon pushout test was performed under the condition of a thick specimen and large supporting hole. On the other hand, cement line debonding occurred when the osteon pushout test was performed using a thin specimen and small supporting hole. The finite element results were consistent with previous experiments of osteon pushout tests under different test geometries. Furthermore, the finite-element results suggest that a smoothly curved punch would most likely cause debonding at the cement line instead of osteonal lamellae.

Numerous methods are currently being employed to estimate completed wall thickness and final erosion depth. Conflicting estimates of calculated bone balance have been obtained from the estimates of wall thickness and erosion depth using these various methods. To assess the utility of two specific methods to estimate wall thickness (polarized microscopy) and erosion depth (lamellar counts), we conducted a study in normal young adult beagle dogs, a model where bone balance should approximate 0. Dogs were administered multiple fluorochrome labels in vivo to label actively forming bone pockets. These labels were used to confirm the position of the cement line of the bone structural unit (BSU) in fluorescent light. Parallel measurements of wall thickness were then collected in polarized light. These estimates were compared to estimates of erosion depth obtained by lamellar counting and bone balance was calculated. Estimates of wall thickness correlated well with estimates of erosion depth with bone balance not differing significantly from 0. These data suggest that the combination of these two methods is a reasonable approach to obtaining estimates of bone balance at the level of the remodeling unit.

Bone is constantly remodeled throughout life, but whether the microstructure of the bone affects resorption is unknown. In this present study, longitudinal and transverse devitalized bone slices were cultured in-vitro with rat bone marrow cells to investigate the influence of bone microstructure in directing and regulating bone resorption by osteoclasts. After the 14-day culture period, the average size of resorption areas produced in the longitudinal slices was found to be three times larger than in the transverse slices (p < 0.05). In addition, bone resorption pro-files were consistent with the microstructure of the bone slices. In the longitudinal bone sections resorption areas proceeded more longitudinally than transversely and resorption areas in transverse bone sections were more circular, either contained inside or outside the cement line surrounding the osteon. These results show that the bone microstructure influences osteoclastic resorption and that the cement lines may be the microstructural element that directs and regulates osteoclastic resorption.

The length of the osteonal cement (reversal) line per unit area (/spl mu/m//spl mu/m/sup 2/) in human cadaveric tibias was quantified and correlated with overall tibial mechanical properties. In addition, the relationship between cement line quantity and gender, age, and location (left versus right limbs) was also evaluated. The study has shown that the cement line length per bone area within the anterior tibial quadrant to be significantly greater (p

Summary This study was designed to observe drifting molars of 70-day-old rats by light microscopy, and to elucidate whether there are similar attachment mechanisms at different kinds of adhesive lines in periodontal mineralized tissue of the rat molar region. Three kinds of adhesive lines — cement lines on resorbed alveolar bone, cement lines on resorbed roots, and cemento-dentinal junctions were examined. The two kinds of cement lines showed similar histological and histochemical features, they were proteoglycan-rich and fiber-poor. They appeared to form on the resorbed tissue before principal fiber reattachment. After covering by new bone or by reparative cementum, the cement lines retained the original features. The cemento-dentinal junction showed features very similar to those of the cement lines. Previous studies have suggested that the cemento-dentinal junctions bind the cementum and dentine by adhesion of proteoglycans. Structural similarities suggest that cement lines provide similar links between new bone and resorbed bone and between resorbed root and reparative cementum. In conclusion, this study suggests that there is one attachment mechanism for the different kinds of collagen based hard tissue in the rat molar region.

Previous investigations carried out in our laboratory on secondary osteons have shown that osteocyte lacunae decrease in size from the cement line towards the Haversian canal, and lamellar bone is made up of alternating nonosteocytic dense lamellae and osteocytic loose lamellae, all having an interwoven texture of collagen fibers. Such alternation of acellular and cellular lamellae was hypothesized to depend on osteocyte recruitment from osteogenic laminae in successive layers, assuming that the loose lamellae form because of alignment and fusion of the periosteocytic loosely arranged collagen fibers. In order to discover whether a correlation really exists between osteocyte lacunar size and lamellar thickness, as would be expected if the above-mentioned hypothesis were true, both these parameters were measured in completed secondary osteons in relation to their distance from the Haversian canal. The size of osteocyte lacunae was measured under light microscopy on undecalcified dry-mounted ground section of tibial compact bone from three adult males and three adult females not affected by metabolic bone disease. The measurement of the thickness of bony lamellae was carried out on the same samples under scanning electron microscopy. Statistical analyses of the results showed that the decrease in size of osteocytic lacunae from the outer to the inner osteonal wall is paralleled by a decrease in thickness of osteocytic loose lamellae. The fact that acellular dense lamellae do not follow such a decremental pattern, but remain of the same thickness throughout the osteonic wall, corroborates Marotti's view that the formation of lamellar bone depends on the orderly distribution of the osteocytes in alternating planes. The topographical distribution of osteocyte lacunar size and lamellar thickness is briefly discussed in relation to secondary osteon mechanical function.

Publisher Summary This chapter introduces a series of contributions dealing with osteoporosis in a wide variety of clinical settings. Its purpose is to consider the definition of osteoporosis and to discuss the nature of osteoporotic bone, its mass and distribution, its microscopic architecture, and other aspects of its quality and strength. Osteoporosis is a condition of generalized skeletal fragility in which bone strength is sufficiently weak for fractures to occur with minimal trauma—often no more than what is applied by routine daily activity. “Primary” osteoporosis is, by tradition, a skeletal disorder of postmenopausal women (“postmenopausal” osteoporosis) or of older men and women (“senile” osteoporosis). The term “secondary” osteoporosis refers to bone loss resulting from specific, defined clinical disorders, such as thyrotoxicosis or hyperadrenocorticism. This chapter discusses the limitations of a bone mass-based diagnosis of osteoporosis. A primary difficulty with such a definition is that its sensitivity to factors known collectively as “bone quality” has not been clarified, and it is tempting to attribute the diagnostic ambiguities of bone mineral density (BMD) measurements to their failure to account for these features. Although these concerns persist, the fact that information contained in the BMD estimate accounts, in part, for some of the important geometric, material, and micro-architectural properties solidifies its rationale as a diagnostic criterion. Certainly, any substantial degree of matrix under-mineralization would be reflected in a lower BMD, and data suggest that trabecular disruption of sufficient magnitude to be mechanically important would also register as a bone mineral deficit, and therefore as a lower BMD. Qualitative features that would not be included in a BMD assessment include cement lines, un-remodeled fatigue damage, and fluoride accumulation.

It is generally accepted that osteocytes derive from osteoblasts that have secreted the bone around themselves. Osteocytes are cells embedded in the lacunae in the bone, and they are characteristically in contact with other cells by many slender cytoplasmic processes in canaliculi. During bone remodeling, many osteocytes in the bone are released from their lacunae by osteoclasts; however it remains unclear what happens to these released osteocytes. The cortical bone of the rat mandibular body was used in this study. Mandibles were fixed, decalcified, and then embedded in Epon 812. Specimens were sectioned in the frontal direction into serial 0.5 microm-thick semithin or 0.1 microm-thick ultrathin sections, and then examined by light or transmission electron microscopy. Cells that fitted in the osteocytic lacunae with canaliculi extending to the bone were identified as osteocytes in this study. Among many osteocytes released by osteoclasts in cutting cones, there were osteocytes half-released from their lacunae. These cells fitted in their lacunae with canaliculi extending to the bone and showed developed cell organelles in the cytoplasm. In closing cones, many osteocytes were situated in the bone away from cement lines; however, there were half-embedded osteocytes in the bone formed on cement lines. These cells fitted in their lacunae with canaliculi extending to the bone formed below cement lines and showed developed cell organelles in the cytoplasm. These results show that half-embedded osteocytes in closing cones derive from half-released osteocytes in cutting cones. Osteocytes encircled by osteoclasts were sometimes observed on one section, but serial sections showed that these osteocytes fitted in their remaining lacunae in the bone on other sections. This shows that not all osteocytes released from their lacunae are engulfed by osteoclasts. Consequently, the present results suggests that some osteocytes released from their lacunae are embedded again in the bone and not engulfed by osteoclasts during bone remodeling.

Bone is remodeled by a sequence of cellular events which occur in discrete locations known as bone remodeling units. This process begins with the activation of mature osteoclasts adhering to the bone surfaces usually covered by lining osteoblasts and with an expansion of the osteoclastic pool. Through the interaction of preexisting and newly formed osteoclasts with osteoblasts, resorption is initiated in discrete areas. This phase, which lasts 2–4 weeks, leads to the formation of focal areas of bone resorption which reach a depth of about 30 μm [1]. Toward the end of the resorption phase mononuclear cells, an important source of cytokines, are typically found at the bottom of the resorption cavity [1]. The transition from resorption to formation is called reversal. This phase is characterized by the accumulation of osteoblast precursors and of a thin layer of inorganic matrix, known as cement line, at the bottom of the resorption pit. The cement line is rich in osteopontin, a RGD-rich protein which may be involved in signaling the cessation of osteoclastic activity. This is followed by the replacement of the removed bone by osteoblasts which accumulate at the base of the resorption lacunae and subsequently fill in the resorption cavity with newly formed bone.

Very few quantitative comparisons between mechanical test behavior of cortical bone and microscopic evidence of damage have been reported. In this study, the hypothesis that age-related degradation of mechanical properties in human cortical bone is associated with increases in damage in the form of microcracks was investigated. The initial modulus and yield stress were 6% (not significant) and 10% (p = 0.05) lower, respectively, in specimens from elderly femora than in specimens from young adult femora. However, both groups showed a 34% decrease in modulus after being loaded to 1% strain. Microcracks were observed in cement lines and between lamellae and were parallel to the loading direction. There were 50% more cracks in longitudinal sections of tested specimens than in controls from elderly femora; however, there were no more cracks in tested specimens than in controls from young adult femora. In addition, there were twice as many cracks in controls and three times as many cracks in tested specimens from elderly femora than in those from young adult femora (p0.01). A microstructurally based model was developed which supported the mechanical test results and indicated that damage began to develop at about 1500 mu epsilon. The results suggest that older bone may have reduced mechanical properties due to the presence of more microcracks, and that older bone is more susceptible to developing microcracks at a given strain level. However, the mechanical test data indicate that specimens from young adult femora also sustained some king of damage as a result of mechanical loading, which requires further investigation.

The prediction of bone fracture risk in individual patients based on Bone Mineral Density (BMD) measurements is of limited accuracy. This is due to the fact that bone is a complex hierarchically structured material consisting of more than four different hierarchical levels which span from the nanoup to the macro-scale [Fratzl; 2007]. Therefore the mechanical properties of bone derive from the behaviour that each individual hierarchical level exhibits during loading. Furthering our understanding of bone’s structure-function relationship [Uzel; 2010], i.e. the origin of bone material properties, is the first step towards more accurate fracture risk estimation. While several studies have dealt with the nanoand the micro-scale mechanical behaviour of the bone, little is known about the so-called sub-osteonal level. This level extends from some hundred of nanometres up to some tens of microns and includes bone single lamellae, the interlamellar areas and the cement lines. The aim of this study is to correlate nanomechanical properties of human cortical bone samples with more conventionally measured fracture toughness parameters. Using cantileverbased nanoindentation we measured cortical bone’s nanoelasticity in a spatially resolved fashion, and its distribution within the various sub-osteonal features (i.e. lamellar and interlamellar areas). First results from this study suggest a strong relationship between nanoelasticity distribution and fracture toughness within human cortical bone.

Bone is unique among the tissues of the body. Firstly, during life it is self-repairing and can alter its properties and configuration in response to mechanical demand; and, secondly, because of its biochemical make up and micromorphological structure it can withstand hundreds, if not thousands of years of internment in many different burial environments.

Screw implants of commercially pure titanium and titanium alloy together with rods of slip-cast dense hydroxyapatite were implanted transfemorally in young adult rats. The femora were harvested after fixation and freeze-fractured in liquid nitrogen to create two tissue fragments of each specimen. The first contained the implant while the second enabled examination of the tissue immediately adjacent to the implant. In all cases, the tissue abutting the implant was a cement line-like layer which separated the implant from the surrounding bone proper. Greater adherence of this interfacial tissue was demonstrated to hydroxyapatite samples compared with the titanium screws, while no adherence was demonstrated to the titanium alloy implants. The differences in adhesion of interfacial tissue to titanium and titanium alloy were attributed, in some areas, to localized increase in surface roughness, and in others to subtle chemical differences between the oxide-covered metal surfaces. The cement line-like layer was composed of globular accretions which were clearly visible on the apparently smooth grain surfaces of the retrieved hydroxyapatite rods. However, modification of the ceramic, during the implantation period, to produce a micropitted surface, also suggested that microtopographical features were implicated in interfacial tissue adhesion.

The purpose of this study was to investigate the rate of type I zinc phosphate cement solubility as it relates to the degree of marginal opening. Standardized test samples were constructed that would simulate clinically relevant marginal gaps of 25, 50, 75, and 150 microns and their subsequent cement lines. The study was divided into two phases. Phase 1 evaluated the effects of simple diffusion on cement solubility in a static environment, whereas phase 2 investigated the effects of convective forces on cement dissolution in a dynamic environment. Both the phase 1 and phase 2 studies demonstrated no significant difference in the rate of cement dissolution for the 25-, 50-, and 75-micron test groups. The 150-micron test groups for both studies, however, demonstrated an increase in the rate of cement dissolution. The results of the phase 1 and phase 2 studies should not be compared because different methodologies were used.

Cement lines provide a record of sites of past remodeling buried in the matrix of bone. A method is reported for application of Stains-all, a cationic carbocyanine dye, for demarcation of cement lines in bone. The method, which is simple, works well for both glycol methacrylate and methyl methacrylate undemineralized embedments and produces good concomitant staining of cytoplasm and nuclei of osteoblasts, osteoclasts and marrow cells.

The skeleton of the black coral Antipathes fiordensis is a laminar structure displaying major and minor growth rings. The skeleton is composed primarily of protein and chitin organized into successive microlayers. Each microlayer is tightly bound to the next by an organic cement that is periodically opaque in section, and occurs as clusters throughout the skeleton. In contrast, the skeletal microlayers themselves are never opaque. The apposition of multiple, opaque cement lines between skeletal layers is primarily responsible for the growth ring pattern.

Paget’s disease is a common affliction in Western society, being pathologically manifest in approximately 4% of older whites.1 Although rarely fatal, the disorder may be extremely disabling and painful and lead to marked skeletal distortion.

Histochemistry is a science in which chemical techniques are used to display the distribution of a substance within tissues. A perfect histochemical method would fulfill the following criteria: 1) It should be specific for the substance under study. 2) It should be sufficiently sensitive to detect any amount of the substance no matter how small. 3) There should be a direct relationship between the amount of final reaction product and the quantity of substance originally present in the tissue — so allowing accurate quantition.

Bone is a tough substance, but exactly how it resists cracks and breaks has been difficult to determine. In the March of Nature Materials, Nalla et al. take on this question. One way that bones stay tough is by bridging gaps using structures such as collagen fibrils) (shown in this scanning electron micrograph). An even more important contributor to bone toughness is deflection of cracks along so-called cement lines, the boundaries between osteons (cylinder-like bone structures). The authors made notches on the edges of bones and found that the cracks developed similiarly to those seen in ductile substances such as certain metal alloys—at the site of the notch, instead of ahead of it. The investigators are eager to examine bone properties in people of different ages and with diseases such as osteoporosis. Some osteoporosis drugs can inhibit bone remodeling, a process that counterbalances bone consumption with bone formation. Such treatments could result in disrepair and the accumulation of 'micro-cracks' as the body ages.

In order to clarify the coupling mechanism (or phenomena) of the bone resorption-formation in the remodeling process, this study was attempted to observe the histochemical location of acid phosphatase (ACPase) and tartrate-resistant ACPase (TRACPase) activity of the newly formed cement lines and the mononuclear cells in BMU (basic multicellular unit) using JB-4 embedding technique for the demonstration of ACPase and TRACPase activity on the tibial metaphysis of rat. The ACPase activity could principally be observed both at the sites where the onset of the newly cement line formation of the trabecular bone and the endosteal bone surfaces, and the resorbed bone surfaces. However, the ACPase reactive cement line was never observed at the erosion zone of the tibial metaphysis, where the osteoblastic bone formation does not occur. Other than osteoclasts and cement lines, the ACPase activity was also observed either in the mononuclear cells closely located to the ACPase positive resorbed bone surfaces or the osteocytes near the osteoclasts. They were also positive for the TRACPase reaction. It was concluded that the active osteoclasts are moving on the bone surface as secreting ACPase to form the cement line, which may influence to induce osteoblasts from adjacent preosteoblasts. It was also suggested that the mononuclear cells and the osteocytes may also relate to the bone remodeling.

We present a short term review of twenty-one De La Caffinière trapezio-metacarpal prostheses inserted into twenty patients with an average review time of fifteen months. Only one case was of rheumatoid arthritis and the main indication for operation was pain. From a functional and symptomatic point of view the results are similar to other series with surgery for trapezio-metacarpal arthritis but due to cup loosening 24% have needed revision, 24% have loose cups and a further 19% have lucent cement lines around the cup. This review draws attention to the worrying frequency of loosening with this prosthesis.

Adult human cortical bone was decalcified and stained with Harris’s hematoxylin, which has a strong affinity for cement lines, resting lines, hypercalcified rings and reversal lines. Small pieces of bone tissue containing these morphological entities were then dissected and manipulated mechanically so as to disrupt them partially. Scanning electron microscopy of these regions of interest revealed the presence of relatively large concentrations of amorphous ‘ground substance’ which was morphologically distinguishable from the fibrous collagen fiber bundles located in the adjoining collagen-rich lamellae. The methodology used in this study confirms the existence of the above-mentioned regions as true morphological entities and also makes possible enzymatic studies designed to identify the major biochemical components constituting the observed ‘ground substance’.

陶材焼付鋳造冠と支台歯と共に抜去した.試験例(試料1)と臨床例(試験2)の2試料より臨床的な適合度につき検した.1)試料1で,辺縁部セメント層の厚さは19〜98 μm(平均47.5 μm),冠辺縁の位置は-215〜+7 μm,オーバーハング量は-20〜+141 μmであった.2)試料2で,辺縁部セメント層の厚さは50〜261 μm(平均154.5 μm),冠辺縁の位置は-453〜+144 μm,オーバーハング量は-129〜+217 μmであった.試料1から,日常の臨床でも実験値と同程度の良好な適合度を得ることを確認した.試料2の適合度は試料1と比較しやや劣った

A gallocyanin method for demonstrating cement lines in thin, undecalcified sections of bone has been developed that is compatible with prestaining with osteochrome before plastic embedding. After sectioning at 5 microns on the Jung K heavy duty microtome, the sections are attached to a microslide using Haupt's adhesive mounting medium, placed on a slide warmer at 37 C until completely dry, and deplasticized in xylene at 45 C for 16-24 hr. Sections are stained with 0.15% gallocyanin-5% chrome alum solution for 30 min, followed by staining in buffered Villanueva blood stain for 1-1 1/2 hr, quickly dehydrated, differentiated in equal parts xylene and 100% ethanol, cleared, and mounted in Eukitt's medium. Reversal lines appear as thin, scalloped, blue or purple lines approximately 0.3 micron wide, and arrest lines as thick, homogeneous, straight or evenly curved, dark blue or purple lines approximately 2 microns wide. The method also demonstrates abnormal halo volumes around osteocytes, old and new bone matrix, osteoid seams, and the granular mineralization front at the osteoid-bone interface. It promises to be valuable in the study of age-related bone loss, osteoporosis, and metabolic bone disease.

Coefficients of correlation between certain physical properties and the histological components of the break area were calculated on an IBM 7090 computer for 56 femoral, 79 tibial and 37 fibular specimens of embalmed cortical bone of standardized size and shape. Strong positive correlations (0.01–0.02 significance level) were found between tensile strength and the percentage of interstitial lamellae in the break area; between hardness and the number of osteons/mm2; and between hardness and the percentage of osteons in the break area. Equally high negative correlations were found between tensile strength and percentage of osteons in the break area; between shearing strength and average area/osteon remnant; between elastic modulus and percentage of spaces in the break area; and an even higher correlation (0.001) between hardness and percentage of spaces in the break area. Negative correlations (at slightly more than 0.05 significance level) were found between shearing strength and modulus and average area/osteon. Osteons tend to reduce the tensile strength and elastic modulus of bone while interstitial lamellae tend to increase them. The probable reason is the relatively greater amount of cement lines, which are sites of weakness where failure can occur, in Haversian bone as compared with lamellar bone. The predominant orientation of collagen fibers and the amount and distribution of calcium may also be involved. These factors are now being investigated.

In cement lines and in adjacent Haversian systems from bone of orangutan and whale the content of mineral, organic matrix, and sulfur was determined microradiographically and the arrangement of the mineral part was studied by X-ray diffraction. Cement lines were often highly mineralized, maximum being 50% by volume. This value was a few per cent higher than the highest value for Haversian systems. Content of collagen varied to a certain extent, and low values were found in cement lines. Determinations of sulfur and X-ray diffraction studies of the organization of the mineral fraction did not reveal any significant differences between cement lines and adjacent osteons.

The fracture behavior of human bone is an interesting field of research for medical scientists, which are interested in predicting the fracture risk, but also for engineers, which are interested in mimicking bone structure in the design of new materials. To understand this phenomenon and to be able to make predictions, many numerical techniques are currently adopted, with the goal of reaching different length scales. In this paper we present a numerical study of the microstructure of human bone, via a cohesive-based X-FEM approach. A single osteon model is developed to investigate the mechanisms of crack propagation and the role played by the mechanical properties of the micro-structural constituents, and in particular by the cement line. The latter has shown to strongly affect the crack path, by acting as a barrier and arresting or preventing a continuous crack propagation, and causing a deviation, leading to an increase in the fracture energy.

Bone is a hierarchical material that involves fracture mechanisms at multiple scales ranging from nano- to macroscale. The fracture behavior of bone is altered by aging, diseases or therapeutic treatments, therefore, robust predictive methods are necessary to evaluate the changes affecting the fracture risk of bone. Although cohesive modeling has been widely used in various engineering disciplines, it has recently been applied to bone fracture. In this paper, the application of cohesive modeling to the assessment of macro- and microscale fracture mechanisms in bone is presented. The macroscale simulations focus on predicting the fracture risk at the whole bone level whereas the microscale simulations evaluate the influence of microstructural features on crack propagation behavior in bone. These studies demonstrate the strength of cohesive modeling in providing insight into bone's fracture behavior.

Compact bovine bone subjected to constant torsional load for long periods of time exhibits large anelastic effects. Displacements occur at the cement lines and are responsible for part or all of the long-term deformation. The absence of an asymptotic creep strain is consistent with an interpretation of the cement line as a viscous interface.

The theoretical considerations of the marginal fit of crowns are discussed and the conventional approach in tooth-cutting procedures is addressed. Gingival marginal surface irregularities described as “peak and valley” and “stepping” effects can leave a cement line in excess of 80 μm after cementation. A regimented method of tooth reduction for metal ceramic crowns, using specially designed rotary cutting instruments, is described. The suggested technique accomplishes the crown preparation more rapidly with a consistent standard of excellence.

The kinetics of the bone remodeling sequence in the rat has been studied using a system in which well-synchronized remodeling units were induced along the periosteum of rat mandibles. Remodeling of the periosteal surface of the mandibles was induced according to Tran Van (1979) by extraction of the opposing row of teeth; namely, the right maxillary molars were extracted under light ether anesthesia, therefore allowing the right mandibular molars to egress; this, in turn, induced a wave of remodeling activity on the buccal side of the periosteal surface of the alveolar bone. The quantification of the different cellular activities involved in bone remodeling has been performed up to 16 days after induction. This allowed us to demonstrate the sequential activity of the different cell types involved in bone remodeling, to study the cellular kinetics of this sequence of events, and to directly measure the duration of each phase of the bone remodeling sequence. A single wave of osteoclasts appeared 3 days after induction, reached a peak at 4–5 days, and then decreased sharply. This was followed by a single wave of mononuclear cells (Baron et al., 1980) within remodeling sites during the reversal phase; they appeared 4 days after induction, reached a peak by day 7, and then decreased sharply. This reversal activity was then followed by osteoblasts forming new bone on top of a reversal cement line in the remodeling sites, starting 6 days after induction and increasing until the end of the experiment. In addition, the synchronization of the system used in this study allowed direct measurement of the duration of the successive steps of the remodeling sequence. The directly measured values have then been compared to previous data calculated from other systems.

Human compact bone may be viewed as a fiber reinforced composite material in which the secondary osteons act as the fiber reinforcements. The cement line, which is the interface between the ‘fibers’ (osteons) and extraosteonal bone matrix, may impart important mechanical properties to compact bone. The nature of these properties is not known partly because the composition of the cement line is unknown. This analysis examines the constituents of the osteon cement line using scanning electron microscopy and X-ray microprobe analysis to address its biomechanical functions as a local interface. The analysis suggests that the cement line is a region of reduced mineralization which may contain sulfated mucosubstances. This composition is consistent with the hypothesis that the cement line provides a relatively ductile interface with surrounding bone matrix, and that it provides the point specific stiffness differences, poor ‘fiber’-matrix bonding and energy transfer qualities required to promote crack initiation but slow crack growth in compact bone.

The so-called ‘cement line’ that demarcates the edge of osteones is a well-documented locus minoris resistentiae in cortical bone (Dempster and Coleman, 1961; Piekarski, 1970). Unlike certain other histological features of bones and joints, it has not received rigorous anatomical scrutiny. It seems worthwhile to call attention to the structure of the cement line in view of the mechanical importance attributed to it in promoting or arresting crack propagation.

SummaryA heparin-like substance was demonstrated in the endothelial cement lines of the large vessels and cardiac valves of guinea pigs in vivo. Heparin present in blood increases staining of the lines, probably because of absorption of this substance from the circulating blood.

Summary An efficient technique is described for reducing incisal discoloration of abutment teeth without creating a cement line. One disadvantage is the need for both opaque and tooth-colored translucent resin cements, but the complex procedure can be facilitated by increasing the working time of the resin cements.

In this paper, a simple method has been developed which can combine the mechanical tests with histological techniques used in physiology.

CONTENTS Coarse-Fibered or Primary Bone Fine-Fibered or Lamellar Bone General Structure of Lamellar Bone The Compacta The Canals Haversian Systems Bone Cells The Fibrillar Structure of the Haversian Lamellae The Ground Lamellae The Interstitial Lamellae The Cement Lines Sharpey Fibers Elastic Fibers The Spongiosa Origin Structure The Relation of Function to the Structure of Bone: Wolff's Law The Origin of the Conception of Wolff's Law The Law Objections to the Law The microscopic anatomy of bone has been carefully studied by the older anatomists and histologists. All the controversial points have not been settled, and many questions still remain open. However, judging from the limited knowledge of bone structure that is prevalent now, except among those especially interested in the subject, one would think that this work had not been done. Perhaps there is a correlation between the dearth of recognized pathologic exposition of bone conditions and the lack of