A low-angle X-ray diffraction analysis of osteonic inorganic phase using synchrotron radiation

Theoretical mathematics, polarized light microscopy and computational models in healthy and pathological bone

The needs of everyday life, such as counting and measuring, are roots of theoretical mathematics. I believe these roots are why mathematical ideas ground research so amazingly well within many scientific fields. Initially trained as a theoretical mathematician and having collaborated with non-mathematicians in the field of bone research, I address the advantages and challenges of collaborations across fields of research among investigators trained in different disciplines. I report on the mathematical ideas that have guided my research on the mechanics of bone tissue. I explain how the mathematical ideas of local vs. global properties influence my research. Polarized light microscopy (PLM) is a tool that I use consistently, in association with other microscopy techniques, to investigate bone in its healthy state and in the presence of bone disease, in humans and in animal models. I review the results that I and investigators around the world have obtained with PLM. Applied to thin bone sections, PLM yields extinct (black) and bright (white) signals that are interpreted in terms of the orientation of collagen type I, by means of other microscopy techniques. Collagen type I is an elementary component of bone tissue. Its orientation is important for the mechanical function of bone. Images obtained by PLM at a specific bone site yield big data sets regarding collagen orientation. Multiple data sets in respect of multiple sites are often needed for research because the bone tissue differs by location in response to the distinct forces acting on it. Mathematics, defined by philosophers as the theory of patterns, offers the backdrop for pattern identification in the big data sets regarding collagen orientation. I also discuss the computational aspect of the research, pursuant to which the patterns identified are incorporated in simulations of mechanical behaviors of bone. These mathematical ideas serve to understand the role of collagen orientation in bone fracture risk.

Techniques to assess bone ultrastructure organization: orientation and arrangement of mineralized collagen fibrils

Bone's remarkable mechanical properties are a result of its hierarchical structure. The mineralized collagen fibrils, made up of collagen fibrils and crystal platelets, are bone's building blocks at an ultrastructural level. The organization of bone's ultrastructure with respect to the orientation and arrangement of mineralized collagen fibrils has been the matter of numerous studies based on a variety of imaging techniques in the past decades. These techniques either exploit physical principles, such as polarization, diffraction or scattering to examine bone ultrastructure orientation and arrangement, or directly image the fibrils at the sub-micrometre scale. They make use of diverse probes such as visible light, X-rays and electrons at different scales, from centimetres down to nanometres. They allow imaging of bone sections or surfaces in two dimensions or investigating bone tissue truly in three dimensions, in vivo or ex vivo, and sometimes in combination with in situ mechanical experiments. The purpose of this review is to summarize and discuss this broad range of imaging techniques and the different modalities of their use, in order to discuss their advantages and limitations for the assessment of bone ultrastructure organization with respect to the orientation and arrangement of mineralized collagen fibrils.

Retrieving the spatially resolved preferred orientation of embedded anisotropic particles by small-angle X-ray scattering tomography

Experimental nondestructive methods for probing the spatially varying arrangement and orientation of ultrastructures in hierarchical materials are in high demand. While conventional computed tomography (CT) is the method of choice for nondestructively imaging the interior of objects in three dimensions, it retrieves only scalar density fields. In addition to the traditional absorption contrast, other contrast mechanisms for image formation based on scattering and refraction are increasingly used in combination with CT methods, improving both the spatial resolution and the ability to distinguish materials of similar density. Being able to obtain vectorial information, like local growth directions and crystallite orientations, in addition to scalar density fields, is a longstanding scientific desire. In this work, it is demonstrated that, under certain conditions, the spatially varying preferred orientation of anisotropic particles embedded in a homogeneous matrix can be retrieved using CT with small-angle X-ray scattering as the contrast mechanism. Specifically, orientation maps of filler talc particles in injection-moulded isotactic polypropylene are obtained nondestructively under the key assumptions that the preferred orientation varies slowly in space and that the orientation of the flake-shaped talc particles is confined to a plane. It is expected that the method will find application inin situstudies of the mechanical deformation of composites and other materials with hierarchical structures over a range of length scales.

Non-osteoporotic women with low-trauma fracture present altered birefringence in cortical bone

Areal bone mineral density (BMD) by DXA, although an important index, does not accurately assess risk of fragility fracture. Another bone structural parameter, the orientation of type I collagen, is known to add to risk determination, independently of BMD. Accordingly, we investigated the Haversian systems of transiliac crest biopsies from non-osteoporotic women with low-trauma fractures, matched to healthy women without fracture by age and BMD. We employed circularly polarized light (CPL) microscopy because 1) each of the extinct and bright birefringent signals of CPL corresponds to a specific collagen arrangement; and 2) CPL can employ magnification suitable to provide data, of manageable size, from the whole cortical component of a section of biopsy. Under CPL, the coaxial layers of osteons, called lamellae, appear either birefringent extinct or bright. On a section transverse to the Haversian system, the extinct lamella comprises mainly collagen forming small angles, and the bright lamella comprises mainly collagen forming large angles, relative to the general orientation of the Haversian system. We performed semi-automatic morphometry for birefringent and structural parameters for which we computed intra- and inter-observer errors. The statistical analysis used a linear mixed model to compare fracturing and non-fracturing groups while addressing pairing of fracturing and non-fracturing subjects, and linear regression to assess differences between matched subjects. We found significant reduction in 1) lamellar width and area for extinct lamella and bright lamella; 2) percentage of extinct birefringence in osteons, and 3) single osteon area; in the fracturing group; and in lamellar width in the fracturing subject of all pairs. Our results evidence the need to investigate, in a larger sample of subjects, the distribution of collagen orientation as a parameter diagnostic of increased fracture risk.

The Mineral-Collagen Interface in Bone

The interface between collagen and the mineral reinforcement phase, carbonated hydroxyapatite (cAp), is essential for bone's remarkable functionality as a biological composite material. The very small dimensions of the cAp phase and the disparate natures of the reinforcement and matrix are essential to the material's performance but also complicate study of this interface. This article summarizes what is known about the cAp-collagen interface in bone and begins with descriptions of the matrix and reinforcement roles in composites, of the phases bounding the interface, of growth of cAp growing within the collagen matrix, and of the effect of intra- and extrafibrilar mineral on determinations of interfacial properties. Different observed interfacial interactions with cAp (collagen, water, non-collagenous proteins) are reviewed; experimental results on interface interactions during loading are reported as are their influence on macroscopic mechanical properties; conclusions of numerical modeling of interfacial interactions are also presented. The data suggest interfacial interlocking (bending of collagen molecules around cAp nanoplatelets) and water-mediated bonding between collagen and cAp are essential to load transfer. The review concludes with descriptions of areas where new research is needed to improve understanding of how the interface functions.

3D scanning SAXS: a novel method for the assessment of bone ultrastructure orientation

The arrangement and orientation of the ultrastructure plays an important role for the mechanical properties of inhomogeneous and anisotropic materials, such as polymers, wood, or bone. However, there is a lack of techniques to spatially resolve and quantify the material's ultrastructure orientation in a macroscopic context. In this study, a new method is presented, which allows deriving the ultrastructural 3D orientation in a quantitative and spatially resolved manner. The proposed 3D scanning small-angle X-ray scattering (3D sSAXS) method was demonstrated on a thin trabecular bone specimen of a human vertebra. A micro-focus X-ray beam from a synchrotron radiation source was used to raster scan the sample for different rotation angles. Furthermore, a mathematical framework was developed, validated and employed to describe the relation between the SAXS data for the different rotation angles and the local 3D orientation and degree of orientation (DO) of the bone ultrastructure. The resulting local 3D orientation was visualized by a 3D orientation map using vector fields. Finally, by applying the proposed 3D scanning SAXS method on consecutive bone sections, a 3D map of the local orientation of a complete trabecular element could be reconstructed for the first time. The obtained 3D orientation map provided information on the bone ultrastructure organization and revealed links between trabecular bone microarchitecture and local bone ultrastructure. More specifically, we observed that trabecular bone ultrastructure is organized in orientation domains of tens of micrometers in size. In addition, it was observed that domains with a high DO were more likely to be found near the surface of the trabecular structure, and domains with lower DO (or transition zones) were located in-between the domains with high DO. The method reproducibility was validated by comparing the results obtained when scanning the sample under different sample tilt angles. 3D orientation maps such as the ones created using 3D scanning SAXS will help to quantify and understand structure-function relationships between bone ultrastructure and bone mechanics. Beyond that, the proposed method can also be used in other research fields such as material sciences, with the aim to locally determine the 3D orientation of material components.

In situ mechanical behavior of mineral crystals in human cortical bone under compressive load using synchrotron X-ray scattering techniques

It is of great interest to delineate the effect of orientation distribution of mineral crystals on the bulk mechanical behavior of bone. Using a unique synergistic approach combining a progressive loading scheme and synchrotron X-ray scattering techniques, human cortical bone specimens were tested in compression to examine the in situ mechanical behavior of mineral crystals aligned in different orientations. The orientation distribution was quantitatively estimated by measuring the X-ray diffraction intensity from the (002) plane in mineral crystals. In addition, the average longitudinal (c-axis), transverse (a-axis), and shear strains of the subset of mineral crystals aligned in each orientation were determined by measuring the lattice deformation normal to three distinct crystallographic planes (i.e. 002, 310, and 213) in the crystals. The experimental results indicated that the in situ strain and stress of mineral crystals varied with orientations. The normal strain and stress in the longitudinally aligned mineral crystals were markedly greater than those in the transversely oriented crystals, whereas the shear stress reached a maximum for the crystals aligned in ±30° with respect to the loading direction. The maximum principal strain and stress were observed in the mineral crystals oriented along the loading axis, with a similar trend observed in the maximum shear strain and stress. By examining the in situ behavior, the contribution of mineral crystals to load bearing and the bulk behavior of bone are discussed.

Internal strains and stresses measured in cortical bone via high-energy X-ray diffraction

High-energy synchrotron X-ray diffraction was used to study internal stresses in bone under in situ compressive loading. A transverse cross-section of a 12-14 year old beagle fibula was studied with 80.7 keV radiation, and the transmission geometry was used to quantify internal strains and corresponding stresses in the mineral phase, carbonated hydroxyapatite. The diffraction patterns agreed with tabulated patterns, and the distribution of diffracted intensity around 00.2/00.4 and 22.2 diffraction rings was consistent with the imperfect 00.1 fiber texture expected along the axis of a long bone. Residual compressive stress along the bone's longitudinal axis was observed in the specimen prior to testing: for 22.2 this stress equaled -95 MPa and for 00.2/00.4 was between -160 and -240 MPa. Diffraction patterns were collected for applied compressive stresses up to -110 MPa, and, up to about -100 MPa, internal stresses rose proportionally with applied stress but at a higher rate, corresponding to stress concentration in the mineral of 2.8 times the stress applied. The widths of the 00.2 and 00.4 diffraction peaks indicated that crystallite size perpendicular to the 00.1 planes increased from t=41 nm before stress was applied to t=44 nm at -118 MPa applied stress and that rms strain epsilon(rms) rose from 2200 muepsilon before loading to 4600 muepsilon at the maximum applied stress. Small angle X-ray scattering of the unloaded sample, recorded after deformation was complete, showed a collagen D-period of 66.4 nm (along the bone axis).

Biologically inspired synthesis of bone-like composite: self-assembled collagen fibers/hydroxyapatite nanocrystals

Replacement of bone tissue by graft materials and products of tissue engineering having composition, structure, and biological features that mimic natural tissue is a goal to be pursued. A biomimetic synthesis was performed to prepare new bone-like composites constituted of hydroxyapatite nanocrystals and self-assembled type I collagen fibers. We used a biological inspired approach that proved that the biological systems stored and processed information at the molecular level. Two different methodologies were used: dispersion of synthetic hydroxyapatite in telopeptides free collagen molecules solution and direct nucleation of hydroxyapatite into reconstituted collagen fibers during their assembling. The different preparation techniques were experimented then the composites thoroughly characterized and compared. Composite obtained by direct nucleation showed an intimated interaction of the inorganic and proteic components, which modified the apatitic phase and made its composition, morphology and structure similar to the mineral component of natural bone.

Structural differences between "dark" and "bright" isolated human osteonic lamellae

This investigation presents new insights into the structure of human secondary lamellae. Lamellar specimens that appear dark and bright on alternate osteon transverse sections under circularly polarizing light were isolated using a new technique, and examined by polarizing light microscopy, synchrotron X-ray diffraction, and confocal microscopy. A distribution of unidirectional collagen bundles and of two overlapping oblique bundles appears on circularly polarizing light microscopy images in relation to the angle between the specimen and the crossed Nicols' planes. The unidirectional collagen bundles observed at 45 degrees run parallel to the osteon axis in the dark lamellar specimens and perpendicular to it in the bright ones. Small and wide-angle micro-focus X-ray diffraction indicates that the dark lamellae are structurally quite homogeneous, with collagen fibers and apatite crystals preferentially oriented parallel to the osteon axis. Bright lamellar specimens exhibit different orientation patterns with the dominant ones bidirectional at +/-45 degrees with respect to the osteon axis. Accordingly, confocal microscopy evidences the presence of longitudinal bundles in dark lamellar specimens and oblique bundles in the bright ones. Radial bundles are evidenced in both lamellar types. The alternate osteon structure is described by a rather continuous multidirectional pattern, in which dark and bright lamellae display different mechanical and possibly biological functions.

Twisted plywood pattern of collagen fibrils in teleost scales: an X-ray diffraction investigation

The distribution and orientation of collagen fibrils, and apatite crystals, in the scales of a bony fish (Leuciscus cephalus) were investigated by X-ray diffraction. The small-angle diffraction patterns obtained with a microfocus scanning setup from most of the examined areas exhibit a distribution of intensity of the collagen reflections according to five preferential orientations, at 36 degrees from one another. It is suggested that the peculiar small-angle X-ray diffraction pattern is due to a plywood arrangement of collagen fibrils in successive layers parallel to the surface of the scale. The fibrils are strictly aligned in each layer and the alignment rotates by 36 degrees in successive layers, according to a discontinuous twist that generates a symmetric plywood pattern. The large spread of the wide-angle reflections does not allow one to distinguish the five directions of orientation in the intensity distribution of the 002 reflection of apatite. However, the patterns recorded from the less ordered regions of the scales display two different orientations of the 002 reflection and allow one to infer a preferential distribution of the apatite crystals with their c-axes parallel to the collagen fibrils. Although much electron microscopic evidence of plywood arrangements in calcified, as well as uncalcified, tissues has been reported, these are the very first diffraction data which unambiguously confirm the presence of these peculiar structures and suggest that this kind of investigation represents a powerful tool with which to study plywood arrangements in biological tissues.

Structural aspects of the calcification process of lower vertebrate collagen

In order to investigate the structural relationship between inorganic phase and collagen fibrils in the calcified tissues of lower vertebrates we have carried out a wide and small angle X-ray diffraction investigation on carp scales and bone samples. The small angle patterns from decalcified bone and scales, as well as uncalcified tendon samples from carp are very similar to that of type I collagen from higher vertebrates. The D-axial period, 67 nm, is the same as that of higher vertebrate type I collagen, while the most significant difference is the relatively low intensity of the first order reflection, which is, however, the most intense. The relative intensity distributions of the meridional reflections recorded from fish bone and scales are in agreement with an electron density distribution according to a step function. The calculated step length is very close to the values previously reported for calcified tissues from higher vertebrates. The small angle reflections from calcified, as well as decalcified, scales display different directions of orientation, which could be in agreement with a plywood arrangement of collagen fibrils in successive sheets parallel to the plane of the scale.

Hydroxyapatite Granule/Carrier Composites Promote New Bone Formation in Cortical Defects

BACKGROUND

A great deal of interest has been focused on finding substitutes for autogenous bone grafts. Among the most interesting materials are different calcium phosphate compositions (e.g., hydroxyapatite [HA]), due to their biocompatible properties in hard and soft tissue.

PURPOSE

The bone response to porous ceramic HA granules in combination with two lipid and one polysaccharide carrier was evaluated in an experimental bone defect model in rabbits.

MATERIALS AND METHODS

Circular defects (Ø 4 mm) were made in both tibias of 32 rabbits. The 64 defects were divided into four groups. Group A was augmented with a composite of HA granules and a phospholipid-diacetyl-glycerol carrier, group B with HA granules and a phospholipid carrier, group C received HA granules and a sodium hyaluronan carrier, and group D served as control. The animals were killed after 6 weeks and ground sections were evaluated using light microscopic morphometry. X-ray microfluorescence (XRF) was applied in order to evaluate the suitability of this method to examine bone-biomaterial interfaces. Calcium distribution was studied using x-ray fluorescence line scans at selected interface regions of two sections in group B.

RESULTS

The HA/phospholipid composites were easier to shape and handle than the HA/hyaluronan composite. Group A had 36% newly formed bone area within the defect. Groups B and C showed significantly more newly formed bone within the defect (47% and 49%, respectively) compared to the control group (31%). The XRF analysis revealed that the amount of calcium in the newly formed bone was similar to that observed for the HA granules and slightly lower when compared to the mature, lamellar bone.

CONCLUSIONS

Synchrotron radiation may be a new, suitable technique to study the interface between bone and biomaterials with regard to mineral content. The results suggest that HA granule/lipid and HA granule/hyaluronan composites have interesting properties as bone-substitute materials.

X-ray diffraction and polarizing optical microscopy investigation of the structural organization of rabbit tibia

X-ray diffraction and polarized optical microscopy investigations were carried out on thin sections of rabbit tibia in order to study the morphological organization of the structural components of this tissue, which often is utilized to test bone response to implants. In the optical microscope, the lateral face as well as the lateral portion of the caudal face exhibit a lamellar structure with an alternation of dark and bright lamellae running parallel to the long axis of the tibia. In contrast, both in the medial face and in the medial portion of the caudal face there are numerous osteonic structures. In spite of the complexity of this morphological organization, the results of small- and high-angle X-ray diffraction analyses indicate that the structural relationship between collagen fibrils and inorganic crystals is quite similar to that observed in single osteons and allows evaluation of the orientation of the two main structural components. Both collagen fibrils and apatitic crystallites are preferentially oriented parallel to the long axis of the tibia. The degree of orientation is greater in the thickness than in the plane of the lamellae, suggesting that collagen fibrils and inorganic crystallites lie preferentially in the plane of the lamellae, where they follow an oblique course. The degree of orientation of the apatitic crystallites is higher in the lateral face than in the medial and caudal faces, in agreement with the optical microscopic images. The results provide information that must be taken into account when evaluating the structural modifications of bone due to the insertion of a prosthetic device.

In vitro calcified tendon collagen: an atomic force and scanning electron microscopy investigation

Atomic force microscopy (AFM), scanning electron microscopy and X-ray energy dispersive spectroscopy have been performed on decalcified turkey tendons submitted to in vitro calcification in order to investigate the morphology and the surface relationships between the inorganic phase and the collagen fibres during deposition and compare with those found for physiologically calcified samples. 'Tapping mode' AFM was used to reduce the vertical force applied to the samples, which were examined without any preparation. A further characterization has been carried out by means of X-ray diffraction, infrared absorption and chemical analyses. The observations indicate that the inorganic phase deposited on collagen fibres during in vitro calcification is poorly crystalline B carbonated apatite. The composition, structure and dimensions of apatitic crystallites, as well as their orientation with respect to collagen fibrils, are very similar to those characteristic of physiologically calcified tissues. However, the crystallites seem to be nucleated on the fibril surface, without appreciably affecting the molecular packing of collagen.

X-ray diffraction study of in vitro calcification of tendon collagen

Decalcified samples of turkey leg tendon were submitted to in vitro calcification in the presence of metastable solutions of calcium phosphate at different concentrations. The structural relationship between apatitic deposits and collagen fibrils was examined by high- and small-angle X-ray diffraction using conventional and synchrotron radiation sources. At high supersaturation the apatitic crystallites were deposited on the collagen fibrils with their crystallographic c-axis preferentially oriented parallel to the fibril axis. At lower supersaturation, a fraction of the apatitic crystallites also grew with the c-axis preferentially oriented parallel to the collagen fibril axis, whereas other exhibited a preferential orientation perpendicular to the fibril axis. The analysis of the small-angle X-ray diffraction data indicates that the deposition of the apatitic phase in the sample stored in solution at lower supersaturation induced modifications of the collagen electron density distribution in the axial direction, which can be attributed to the deposition of the inorganic crystallites inside the gap region of the collagen structure.

A new theory of bone lamellation

A comparative polarized light (PLM), scanning (SEM), and transmission (TEM) electron microscopy study was carried out on cross- and longitudinal sections of human lamellar bone in the tibiae of four male subjects aged 9, 23, 45, and 70 years. SEM analysis was also performed on rectangular-prismatic samples in order to observe each lamella sectioned both transversely and longitudinally. The results obtained do not confirm the model hitherto suggested to explain the lamellar appearance of bone. In particular, the classic description by Gebhardt (still accepted by the majority of bone researchers), which suggests that collagen fibers alternate between longitudinal and transversal in successive lamellae, or that they have spiral paths of different pitches, appears to be no longer acceptable in the light of our findings. In fact, SEM and TEM observations here reported agree in demonstrating that lamellar bone is made up of alternating collagen-rich (dense lamellae) and collagen-poor (loose lamellae) layers, all having an interwoven arrangement of fibers. No interlamellar cementing substance was observed between the lamellae, and collagen bundles form a continuum throughout lamellar bone. Preliminary measurements of lamellar thickness indicate that dense lamellae are significantly (P < 0.001) thinner than loose lamellae. Compared with the classic model of Gebhardt, the dense lamellae correspond to the transverse lamellae and are birifringent under PLM, whereas the loose lamellae correspond to the longitudinal lamellae and are extinguished. Collagen-fiber organization in dense and loose lamellae is discussed in terms of bone biomechanics and osteogenesis.

The role of magnesium on the structure of biological apatites

X-ray diffraction, infrared absorption spectroscopy, and chemical investigation have been carried out on deproteinated samples of turkey leg tendon at different degrees of calcification. The inorganic phase consists of poorly crystalline B carbonated apatite. On increasing calcification, the apatite crystal size, as well as its thermal stability, increase while the relative magnesium content is reduced. On the other hand, synchrotron X-ray diffraction data clearly indicate that apatite lattice parameters do not change as the crystals get larger. At the last stage of calcification the crystal size, chemical composition, and thermal conversion of the apatite crystallites approximate those of bone samples, which have been examined for comparison. The results provide a quantitative relationship between relative magnesium content and extent of apatite conversion into B-tricalcium phosphate by heat treatment. Furthermore, they suggest that the smaller crystallites laid down inside the gap region of the collagen fibrils are richer in magnesium than the longer ones that fill the space between collagen fibrils.

Mineral crystals in calcified tissues: a comparative study by SAXS

The shape, the typical orientation, and the average size of mineral crystals in different types of mineralized tissues were investigated by means of small-angle x-ray scattering (SAXS). To rule out eventual artifacts due to sample preparation, four different standard preparation techniques were used and a comparison showed that the SAXS results were identical for all four methods. In mineralized turkey leg tendon, a frequently used model system for bone, the crystals were found to be typically plate-like with a thickness of the order of 2 nm. This stands in contrast to the case of bone (calvaria, femur, and iliac crest) from mouse, rat, and dog, where mainly needle-like crystals were found. The thickness of these crystals ranged from 3 to 4 nm but was remarkably constant for different bones of a given animal. The preferred orientation of the needle-like crystals was along the main axis of the femur and within the surface of the calvaria (for mouse, rat, and dog). The mineral plates in turkey leg tendon were located inside the hole zone and oriented along the fibril axis. Finally, no periodic arrangement of the crystals inside the hole zone of the collagen could be found.

Nucleation and growth of mineral crystals in bone studied by small-angle X-ray scattering

The mechanism of calcification in bone and related tissues is a matter of current interest. The mean size and the arrangement of the mineral crystals are important parameters difficult to obtain by electron microscopy. Furthermore, most studies have been carried out on poorly calcified model systems or chemically treated samples. In the work presented here, native bone was studied as a function of age by a quantitative small-angle X-ray scattering method (SAXS). Bone samples (calvariae and ulnae) from rats and mice were investigated. Measurements were performed on native bone immediately after dissection for samples up to 1 mm thick. The size, shape, and predominant orientation of the mineral crystals in bone were obtained for embryonal, young, and adult animals. The results indicate that the mineral nucleates as thin layers of calcium phosphate within the hole zone of the collagen fibrils. The mineral nuclei subsequently grow in thickness to about 3 nm, which corresponds to maximum space available in these holes.

Structural analysis of turkey tendon collagen upon removal of the inorganic phase

Calcified leg flexor tendons in which the inorganic phase content had been lowered by progressive demineralization were studied by small angle X-ray diffraction and thermogravimetry. The X-ray diffraction results agree very well with the data previously obtained on calcified turkey tendon indicating that the method used to decalcify tendons provides good correspondence with the process of calcification. Up to five thermal processes can be detected in the thermogravimetric scans: (1) water release; (2) collagen decomposition; (3 and 4) combustion of the residual organic components; (5) carbonate removal from the apatitic phase. The temperature of collagen decomposition decreases at lower inorganic phase content in agreement with the higher thermal stability of calcified collagen fibrils compared with uncalcified ones. The decrease of collagen thermal stability upon decalification is paralleled by a decrease of the structural order of the collagen fibrils as indicated by small angle X-ray diffraction data. Decalcification down to about 40% wt of inorganic phase does not significantly alter the inorganic blocks that are regularly arranged inside the gap zone of the collagen. Further removal of inorganic phase down to about 15% wt provokes a variation of the intensity distribution of the small angle meridional reflections that can be ascribed to a reduction of the mean height of the inorganic blocks. At inorganic phase contents below 15% wt the gap region is more free to contract upon air drying as a result of the reduction of the mean length of the inorganic blocks.

Collagen structural organization in uncalcified and calcified human anterior longitudinal ligament

Collagen structure and collagen-apatite structural relationship has been investigated in human anterior ligament, where the mineral deposition occurs on collagen fibrils morphologically different from those of bone and tendons. Ultrastructural observations made on replicas of cryoprotected and freeze fractured uncalcified samples display a "helicoidal" morphology of the collagen fibrils. X-ray diffraction analysis carried out using conventional and synchrotron radiation sources revealed that the D-axial spacing is 65.0 nm and the electron density distribution inside the repeating period is very similar to those of tendon collagen in the same conditions of hydration. The short D-period can be interpreted as due to a greater angle of molecular crimping and/or molecular tilt compared to that of tendon. Air drying does not cause any appreciable variation in the D-axial period and induces an increase of the gap/overlap ratio that can be ascribed to telopeptide disorder. In spite of the different morphology of the collagen fibrils, the structural relationship between collagen and the mineral phase in calcified ligament is very close to that observed in bone and tendons. The apatitic phase is laid down in blocks along the collagen fibrils with the same axial periodicity, D = 65.0 nm, as that of uncalcified collagen fibrils. The mean height of the mineral blocks, which are 0.45D long, is even higher than in bone and masks any further fluctuation of the electron density due to the organic matrix.

On the adaptive structures of the collagen fibrils of bone and cartilage

In order to investigate how bone and cartilage respond at the molecular level to changing demands of the skeleton, the influence of endogenous and external chemical stresses on collagen fibrillar structure as a function of locale in the femoral articular cartilage from 2 to 14 year old cows has been studied by x-ray diffraction. The fibrils were found to be osmotically compressed by vicinal proteoglycans. For most locales, the molecular packing density was less at the articular surface and increased to a maximum adjacent to the bone. This gradient in structure became more accentuated with animal age. For all ages, the packing density gradients could be almost completely eliminated upon enzymatic removal of the proteoglycans. However, in the regions of cartilage which had experienced the greatest stress in locomotion, the fibrils had a hyperswollen structure. This tendency towards hyperswelling increased with animal age. We have concluded that the collagen fibrils in articular cartilage adapt a structure that is in response to their respective mechanico-chemical histories.

Ultrastructure, morphology and crystal growth of biogenic and synthetic apatites

The morphology, structure and crystal growth of apatite crystals isolated from calcified turkey tendon and synthetic carbonated apatites have been examined using high resolution transmission electron microscopy. The biogenic apatite consisted of small (35 x 20 x 5 nm) platelike crystals. Despite their irregular shape and ill-defined edges, individual particles were single domain crystals. Lattice images recorded from isolated turkey tendon crystals indicated that the crystallographic c-axis (0001) of apatite lies in the plane of the plate and parallel to the length of the crystallites. Lattice images suggested that the top face corresponds to the (1100) face of carbonated apatite. Lattice fringes observed in platelike crystallites viewed from the side corresponded to the projection of the apatite structure viewed along the [1120] direction. Thus, it can be argued that crystal growth is constrained along the [1100] direction, extends laterally along the [1120] direction, and is maximal along the [0001] direction. This latter direction is aligned with the collagen fiber axis. A mean length to width ratio (1.7) was determined by systemically measuring the maximum distances parallel and perpendicular to the c-axis identified from lattice images of the crystals. Similar information was obtained from lattice images of crystals located in collagen fibres. This confirmed that the morphological and structural features of isolated turkey tendon apatite crystals correlate directly with the in vivo crystallochemical characteristics of apatite. Crystals of synthetic carbonated apatite prepared at 37 degrees C were also platelike and, although generally much larger, had length to width ratios comparable with the turkey tendon apatite. The synthetic carbonated apatites were noticeably more sensitive to radiolytic damage than the turkey tendon crystals. The crystallographic c-axis of the inorganic particles was aligned parallel with the long, physical axis of the plate and the top face was identified as (1100). Similar data were also obtained from noncarbonated synthetic apatite samples. The results of the present study offer critical information about the crystal growth of individual carbonated apatite crystals in calcified turkey tendon and its relationship to the morphology of the crystallites. As similar growth characteristics are expressed in synthetic analogues, the data bring into question the putative regulatory role of the collagen-based matrix upon the nucleation and growth of biogenic apatite.(ABSTRACT TRUNCATED AT 400 WORDS)

The locus of mineral crystallites in bone

The organic content of mineralized tissues has been found to decrease with increasing tissue density, from about 60% of the mineral weight in light bone like deer antler to 1 to 2% in hyperdense bone like porpoise petrosal. The ratio of the weight of mineral that can fill the collagen hole zones to the total mineral content can be no greater than 20% for deer antler and decreases to less than 5% for hyperdense bone. Moreover, the dimensions of hydroxyapatite crystallites have been determined by various investigators to be larger than the intermolecular spacing of collagen molecules. Such crystallites can only be fitted within the collagen fibril if collagen molecules are packed differently from the accepted models. Electron micrographs of fish dentin, at a very early stage of mineralization, show the needle-like crystallites lying in dense strips between collagen fibrils and practically no crystallites within the fibrils. A similar pattern of dense strips of crystallites between fibrils can be identified in examples from more advanced stages of mineralization, taken from fish dentin, cat dentin and cow tibia, even though some of the needle-like crystallites are superimposed on the fibril banded pattern. In every instance there are regions of the fibrils where there are no visible needle-like crystallites. Examination of the work of others shows a similar distribution of the mineral component, except that none exactly resemble the micrograph of the earliest stage of fish dentin provided in this report. The collagen banding is observed to be in spatial phase over many fibrils. The needle-like crystallites may be observed to be bunched in phase with the collagen banding and with the same spatial periodicity. The bunching is most obvious in the least densely mineralized specimens. This observation can account for the x-ray and neutron diffraction patterns which shown the axial period of the mineral to be like that of the collagen axial macroperiod and to be in phase with the hole zones of collagen fibrils. These prior studies were interpreted to show that the crystallites must be within the hole zones. Our images are interpreted to show that most of the mineral is outside of the collagen fibrils in the extrafibrillar volume. The interpretation is in agreement with neutron diffraction studies of various mineralized tissues as well as with earlier diffraction studies of mineralized turkey leg tendon and with the calculations of the amount of mineral that can be contained within the collagen of mineralized tissue.