Birefringence of articular cartilage and the distribution on collagen fibril orientations

Ultraviolet-Visible-Near Infrared Spectroscopy May Aid in the Qualitative Assessment of Early-Stage Cartilage Degradation

Purpose

To assess the potential of ultraviolet-visible near-infrared spectroscopy to provide quantitative information on the cartilage surface at early osteoarthritis.

Methods

We used a similar source and optical path to a standard arthroscope and constraining input to the range available to a standard detector/camera, further capturing and analyzing spectral information quantitatively in terms of specific electronic absorbance bands and scattering from the cartilage surface, with a focus on the early stages of degradation.

Results

The ratio of the 320-nm and longer than 500-nm absorbances produced a distinct change from the normal to diseased states. The slopes between the wavelengths of 600 and 980 nm may show the transition of the single fibril to fibril bundles that occurs during early stages disease.

Conclusions

Ultraviolet-visible near-infrared spectroscopy has good potential for use in integrated arthroscopic assessment.

Clinical Relevance

This raises the possibility of advancing arthroscopy from a qualitative to a quantitative tool, without requiring modification of either the radiation (the light source and path) or instrumentation (the arthroscope itself) delivered to the patient, thus allowing a low-cost yet potentially high-value technology.

Microstructural differences in the osteochondral unit of terrestrial and aquatic mammals

During evolution, animals have returned from land to water, adapting with morphological modifications to life in an aquatic environment. We compared the osteochondral units of the humeral head of marine and terrestrial mammals across species spanning a wide range of body weights, focusing on microstructural organization and biomechanical performance. Aquatic mammals feature cartilage with essentially random collagen fiber configuration, lacking the depth-dependent, arcade-like organization characteristic of terrestrial mammalian species. They have a less stiff articular cartilage at equilibrium with a significantly lower peak modulus, and at the osteochondral interface do not have a calcified cartilage layer, displaying only a thin, highly porous subchondral bone plate. This totally different constitution of the osteochondral unit in aquatic mammals reflects that accommodation of loading is the primordial function of the osteochondral unit. Recognizing the crucial importance of the microarchitecture-function relationship is pivotal for understanding articular biology and, hence, for the development of durable functional regenerative approaches for treatment of joint damage, which are thus far lacking.

Quantification method to objectively evaluate the fibrous structural status of tendons based on polarization-sensitive OCT

Histological analysis is widely used to evaluate injured tendons; however, it has the limitation of being semi-quantitative. Hence, we developed a quantification method to objectively evaluate the fibrous structure of tendons, exhibiting the optical property of birefringence, using polarization-sensitive optical coherence tomography (PS-OCT). We used a partial-rupture rat model in which the middle 0.75 cm of the Achilles tendon was cut with a blade. Rats were sacrificed at 2, 4 or 6 weeks after the injury, and PS-OCT and histological analyzes were performed. The PS-OCT phase retardation images and score well represented the structural changes of the injured tendon according to the wound healing state. Therefore, the proposed novel quantification method using PS-OCT can be used to evaluate the fibrous structural status of tendons.

Walking on water: revisiting the role of water in articular cartilage biomechanics in relation to tissue engineering and regenerative medicine

The importance, and the difficulty, of generating biosynthetic articular cartilage is widely recognized. Problems arise from obtaining sufficient stiffness, toughness and longevity in the material and integration of new material into existing cartilage and bone. Much work has been done on chondrocytes and tissue macromolecular components while water, which comprises the bulk of the tissue, is largely seen as a passive component; the ‘solid matrix’ is believed to be the main load-bearing element most of the time. Water is commonly seen as an inert filler whose restricted flow through the tissue is believed to be sufficient to generate the properties measured. We propose that this model should be turned on its head. Water comprises 70–80% of the matrix and has a bulk modulus considerably greater than that of cartilage. We suggest that the macromolecular components structure the water to support the loads applied. Here, we shall examine the structure and organization of the main macromolecules, collagen, aggrecan and hyaluronan, and explore how water interacts with their polyelectrolyte nature. This may inform the biosynthetic process by identifying starting points to enable developing tissue properties to guide the cells into producing the appropriate macromolecular composition and structure.

Effects of body mass on microstructural features of the osteochondral unit: A comparative analysis of 37 mammalian species

Since Galileo's days the effect of size on the anatomical characteristics of the structural elements of the body has been a subject of interest. However, the effects of scaling at tissue level have received little interest and virtually no data exist on the subject with respect to the osteochondral unit in the joint, despite this being one of the most lesion-prone and clinically relevant parts of the musculoskeletal system. Imaging techniques, including Fourier transform infrared imaging, polarized light microscopy and micro computed tomography, were combined to study the response to increasing body mass of the osteochondral unit. We analyzed the effect of scaling on structural characteristics of articular cartilage, subchondral plate and the supporting trabecular bone, across a wide range of mammals at microscopic level. We demonstrated that, while total cartilage thickness scales to body mass in a negative allometric fashion, thickness of different cartilage layers did not. Cartilage tissue layers were found to adapt to increasing loads principally in the deep zone with the superficial layers becoming relatively thinner. Subchondral plate thickness was found to have no correlation to body mass, nor did bone volume fraction. The underlying trabecular bone was found to have thicker trabeculae (r=0.75, p<0.001), as expected since this structure carries most loads and plays a role in force mitigation. The results of this study suggest that the osteochondral tissue structure has remained remarkably preserved across mammalian species during evolution, and that in particular, the trabecular bone carries the adaptation to the increasing body mass.

Noninvasive assessment of articular cartilage surface damage using reflected polarized light microscopy

Articular surface damage occurs to cartilage during normal aging, osteoarthritis, and in trauma. A noninvasive assessment of cartilage microstructural alterations is useful for studies involving cartilage explants. This study evaluates polarized reflectance microscopy as a tool to assess surface damage to cartilage explants caused by mechanical scraping and enzymatic degradation. Adult bovine articular cartilage explants were scraped, incubated in collagenase, or underwent scrape and collagenase treatments. In an additional experiment, cartilage explants were subject to scrapes at graduated levels of severity. Polarized reflectance parameters were compared with India ink surface staining, features of histological sections, changes in explant wet weight and thickness, and chondrocyte viability. The polarized reflectance signal was sensitive to surface scrape damage and revealed individual scrape features consistent with India ink marks. Following surface treatments, the reflectance contrast parameter was elevated and correlated with image area fraction of India ink. After extensive scraping, polarized reflectance contrast and chondrocyte viability were lower than that from untreated explants. As part of this work, a mathematical model was developed and confirmed the trend in the reflectance signal due to changes in surface scattering and subsurface birefringence. These results demonstrate the effectiveness of polarized reflectance microscopy to sensitively assess surface microstructural alterations in articular cartilage explants.

Fibre Reinforcement and Mechanical Stability in Articular Cartilage

The gel phase of articular cartilage is reinforced by collagen fibrils. These fibrils have low flexural and torsional stiffness, but are able to provide reinforcement if deformation of the tissue increases their tensile stress. An estimate suggests that the lengths of collagen fibrils in articular cartilage are at least of the same order as their critical length so that tensile stress in the tissue will increase the stress in the fibrils rather than simply pull them out of the gel. In the surface zone the collagen fibrils are oriented so that the efficiency of reinforcement, η, is about 0.6 tangential to the surface; tension in the fibrils is thus able to withstand swelling pressure within the tissue whose condition for stability resembles that of a pressure vessel. Swelling pressure allows the tissue to support applied pressure. An intermediate zone has a roughly isotropic η value of about 0.2, while in the deep zone collagen fibrils appear to tie the cartilage to the subchondral bone; in this deep zone η has a value of about 0.6 perpendicular to the surface direction. There is also some preferred orientation of collagen fibrils in the plane of the articular surface within the surface zone; in patellar cartilage the preferred orientations can be related to the direction of stress which could be generated by movement of the joint.

Quantitative mapping of collagen fiber alignment in thick tissue samples using transmission polarized-light microscopy

Immersion optical clearing makes it possible to use transmission polarized-light microscopy for characterization of thick (200 to 2000  μm) layers of biological tissues. We discuss polarization properties of thick samples in the context of the problem of characterization of collagen fiber alignment in connective tissues such as sclera and dermis. Optical chirality caused by azimuthal variations of the macroscopic (effective) optic axis of the medium across the sample thickness should be considered in polarization mapping of thick samples of these tissues. We experimentally evaluate to what extent the optical chirality affects the measurement results in typical situations and show under what conditions it can be easily taken into account and does not hinder, but rather helps, in characterization of collagen fiber alignment.

QUANTIFICATION AND THE UNDERLYING MECHANISM OF SKIN THERMAL DAMAGE: A REVIEW

Skin thermal damage is the most common thermal trauma in civilian and military communities. Besides, advances in laser, microwave, and similar technologies have led to recent developments of thermal treatments for diseases involving skin tissue aiming at inducing damage precisely within targeted tissue structures without affecting the surrounding healthy tissue. Pain sensation accompanying thermal damage is also a serious problem for burn patients. Therefore, it is of great importance to quantify the thermal damage in skin tissue. In this review, we detail the progress of the state-of-the-art mathematical models and experimental methods for the quantification of thermal damage (both heat damage and cold damage) and the general development of thermal treatments in tissue engineering. This could enable better understanding of the underlying mechanisms of skin thermal damage and the optimization of clinical thermal therapies.

Quantitative determination of morphological and territorial structures of articular cartilage from both perpendicular and parallel sections by polarized light microscopy

In order to investigate the three-dimensional structure of the collagen fibrils in articular cartilage, full-thickness canine humeral cartilage was microtomed into perpendicular sections that included both the articular surface and the subchondral bone and approximately 100 successive parallel sections that were each 6 microm thick and from a different cartilage depth. Each section was imaged using polarized light microscopy with a 5x objective (2.0 microm pixel size), generating two quantitative images (angle and retardation). Selected sections were also imaged using a 40x objective (0.25 microm pixel size). At an increased depth from the articular surface, the angle and retardation results in the perpendicular sections showed the well-known 90 degrees change in fibril orientation between the surface and the deep cartilage. In contrast, the retardation results of the parallel sections decreased from the articular surface and remained approximately 0 through most of the radial zones, while the angle results of the parallel sections only changed about 30 degrees. The territorial matrix morphology surrounding 61 chondrocyte clusters was quantified by its length, aspect ratio, and orientation. The cellular clusters in the surface cartilage were ellipsoidal in both parallel and perpendicular sections. In the radial zone, the cellular clusters were oriented in vertical columns in the perpendicular sections and as circular groupings in the parallel sections. This orthogonal imaging technique could provide a better understanding of the three-dimensional territorial and interterritorial fibrils in articular cartilage, the disturbance of which could signify the onset of degenerative cartilage diseases such as osteoarthritis.

A polarized light microscopy method for accurate and reliable grading of collagen organization in cartilage repair

OBJECTIVES

Collagen organization, a feature that is critical for cartilage load bearing and durability, is not adequately assessed in cartilage repair tissue by present histological scoring systems. Our objectives were to develop a new polarized light microscopy (PLM) score for collagen organization and to test its reliability.

DESIGN

This PLM score uses an ordinal scale of 0-5 to rate the extent that collagen network organization resembles that of young adult hyaline articular cartilage (score of 5) vs a totally disorganized tissue (score of 0). Inter-reader reliability was assessed using Intraclass Correlation Coefficients (ICC) for Agreement, calculated from scores of three trained readers who independently evaluated blinded sections obtained from normal (n=4), degraded (n=2) and repair (n=22) human cartilage biopsies.

RESULTS

The PLM score succeeded in distinguishing normal, degraded and repair cartilages, where the latter displayed greater complexity in collagen structure. Excellent inter-reader reproducibility was found with ICCs for Agreement of 0.90 [ICC(2,1)] (lower boundary of the 95% confidence interval is 0.83) and 0.96 [ICC(2,3)] (lower boundary of the 95% confidence interval is 0.94), indicating the reliability of a single reader's scores and the mean of all three readers' scores, respectively.

CONCLUSION

This PLM method offers a novel means for systematically evaluating collagen organization in repair cartilage. We propose that it be used to supplement current gold standard histological scoring systems for a more complete assessment of repair tissue quality.

Changes in collagen fibril network organization and proteoglycan distribution in equine articular cartilage during maturation and growth

The aim of this study was to record growth-related changes in collagen network organization and proteoglycan distribution in intermittently peak-loaded and continuously lower-level-loaded articular cartilage. Cartilage from the proximal phalangeal bone of the equine metacarpophalangeal joint at birth, at 5, 11 and 18 months, and at 6-10 years of age was collected from two sites. Site 1, at the joint margin, is unloaded at slow gaits but is subjected to high-intensity loading during athletic activity; site 2 is a continuously but less intensively loaded site in the centre of the joint. The degree of collagen parallelism was determined with quantitative polarized light microscopy and the parallelism index for collagen fibrils was computed from the cartilage surface to the osteochondral junction. Concurrent changes in the proteoglycan distribution were quantified with digital densitometry. We found that the parallelism index increased significantly with age (up to 90%). At birth, site 2 exhibited a more organized collagen network than site 1. In adult horses this situation was reversed. The superficial and intermediate zones exhibited the greatest reorganization of collagen. Site 1 had a higher proteoglycan content than site 2 at birth but here too the situation was reversed in adult horses. We conclude that large changes in joint loading during growth and maturation in the period from birth to adulthood profoundly affect the architecture of the collagen network in equine cartilage. In addition, the distribution and content of proteoglycans are modified significantly by altered joint use. Intermittent peak-loading with shear seems to induce higher collagen parallelism and a lower proteoglycan content in cartilage than more constant weight-bearing. Therefore, we hypothesize that the formation of mature articular cartilage with a highly parallel collagen network and relatively low proteoglycan content in the peak-loaded area of a joint is needed to withstand intermittent stress and shear, whereas a constantly weight-bearing joint area benefits from lower collagen parallelism and a higher proteoglycan content.

Practical considerations in the use of polarized light microscopy in the analysis of the collagen network in articular cartilage

Polarized light microscopy is a traditional method for visualizing the collagen network architecture of articular cartilage. Articular cartilage repair and tissue engineering studies have raised new demands for techniques capable of quantitative characterization of the scar and repair tissues, including properties of the collagen network. Modern polarized light microscopy can be used to measure collagen fibril orientation, parallelism, and birefringence. New commercial instruments are computer controlled and the measurements are easy to perform. However, often the interpretation of results causes difficulties, even errors, because the theoretical aspects of the technique are demanding. The aim of this study was to describe the instrumentation and properties of a modern polarized light microscope, to point out some sources of error in the interpretation of the results, and to recall the theoretical background of the polarized light microscopy.

Attenuated total reflection Fourier transform infrared spectroscopy analysis of human hair fiber structure

The structure of human hair was studied by attenuated total reflection Fourier transform infrared (ATR-FT-IR) spectroscopy. The use of Ge, ZnSe, and Si internal reflection elements, various incident light angles, and difference spectra allowed detailed analysis of the cuticle, cortex, and cuticle-cortex intercellular regions without physically or mechanically removing the cuticle of the hair fiber. The ATR-FT-IR data showed the cuticle to be composed of protein having predominantly beta-sheet and disorder and beta-turn configurations. In contrast, the cortex spectra showed alpha-helical structures due to the presence of intermediate filaments of alpha keratin plus beta-sheet, beta-turn and disorder structures. In the cuticle-cortex interface region the protein structures were primarily disorder and beta-turn with small amounts of beta-sheet configurations. The spectral analyses are consistent with the general information on hair fiber structure proposed in the literature.

The Biophysical Effects of a Single Impact Load on Human and Bovine Articular Cartilage

Impact injury to a joint is a known risk factor for the subsequent development of secondary osteoarthritis. An in vitro model, employing a drop-tower loading machine, was used to examine the effect of an impact load on isolated articular cartilage explants from human and bovine femoral heads. Two different types of impact experiment were performed. In the first, 4 mm diameter explants were loaded using a plane-ended impactor. In the second, a modified impactor was developed that had a central 4 mm diameter plane-ended indentor which was used to load the centre of 8 mm diameter explants. This enabled the unloaded outer ring of each explant to be compared with the loaded central core. The modulus values measured using the impactor were found to be higher, compared with the indentor in both species. Scanning electron microscopy showed that cartilage surrounding the loaded central region of the 8 mm explants protected the indented tissue, and these explants showed less damage than the 4 mm samples that were fully impacted. In addition, human cartilage was found to be less damaged than bovine, possibly as a consequence of the different structure as well as of a greater thickness. Both the source of the tissue and the nature of the impact affected the type of damage observed.

Characterizing of tissue microstructure with single-detector polarization-sensitive optical coherence tomography

Assessing tissue birefringence with imaging modality polarization-sensitive optical coherence tomography (PS-OCT) could improve the characterization of in vivo tissue pathology. Among the birefringent components, collagen may provide invaluable clinical information because of its alteration in disorders ranging from myocardial infarction to arthritis. But the features required of clinical imaging modality in these areas usually include the ability to assess the parameter of interest rapidly and without extensive data analysis, the characteristics that single-detector PS-OCT demonstrates. But beyond detecting organized collagen, which has been previously demonstrated and confirmed with the appropriate histological techniques, additional information can potentially be gained with PS-OCT, including collagen type, form versus intrinsic birefringence, the collagen angle, and the presence of multiple birefringence materials. In part I, we apply the simple but powerful fast-Fourier transform (FFT) to both PS-OCT mathematical modeling and in vitro bovine meniscus for improved PS-OCT data analysis. The FFT analysis yields, in a rapid, straightforward, and easily interpreted manner, information on the presence of multiple birefringent materials, distinguishing the true anatomical structure from patterns in image resulting from alterations in the polarization state and identifying the tissue/phantom optical axes. Therefore the use of the FFT analysis of PS-OCT data provides information on tissue composition beyond identifying the presence of organized collagen in real time and directly from the image without extensive mathematical manipulation or data analysis. In part II, Helistat phantoms (collagen type I) are analyzed with the ultimate goal of improved tissue characterization. This study, along with the data in part I, advance the insights gained from PS-OCT images beyond simply determining the presence or absence of birefringence.

Depth-resolved phase retardation measurements for laser-assisted non-ablative cartilage reshaping

Since polarization-sensitive optical coherence tomography (PS-OCT) is emerging as a new technique for determining phase retardation in biological materials, we measured phase retardation changes in cartilage during local laser heating for application to laser-assisted cartilage reshaping. Thermally-induced changes in phase retardation of nasal septal cartilage following Nd:YAG laser irradiation were investigated using a PS-OCT system. A PS-OCT system and infrared imaging radiometer were used to record, respectively, depth-resolved images of the Stokes parameters of light backscattered from ex vivo porcine nasal septal cartilage and radiometric temperature changes following laser irradiation. PS-OCT images of cartilage were recorded before (control), during and after laser irradiation. From the measured Stokes parameters (I, Q, U and V), an estimate of the relative phase retardation between two orthogonal polarizations was computed to determine birefringence in cartilage. Phase retardation images of light backscattered from cartilage show significant changes in retardation following laser irradiation. To investigate the origin of retardation changes in response to local heat generation, we differentiated two possible mechanisms: dehydration and thermal denaturation. PS-OCT images of cartilage were recorded after dehydration in glycerol and thermal denaturation in heated physiological saline. In our experiments, observed retardation changes in cartilage are primarily due to dehydration. Since dehydration is a principal source for retardation changes in cartilage over the range of heating profiles investigated, our studies suggest that the use of PS-OCT as a feedback control methodology for non-ablative cartilage reshaping requires further investigation.

Monitoring osteoarthritis in the rat model using optical coherence tomography

OBJECTIVE

A need exists for an animal model to assess therapeutics for osteoarthritis (OA) without sacrificing the animal. Our goal is to assess the progression of experimentally induced osteoarthritis in the rat knee joint by monitoring articular cartilage thickness, surface abnormalities, and collagen organization using a new technology known as optical coherence tomography (OCT).

DESIGN

OA was generated in Wistar Hanover rats via injection of sodium iodoacetate into the left articular joint of the knee while normal saline was injected as a control in the contralateral right knee. Rats were sacrificed at 1-, 2-, 3-, 4-, and 8-week intervals and the knee joints were subsequently harvested and imaged using normal and polarization sensitive OCT (PS-OCT). Treated knees were compared to normal counterparts in the contralateral leg. Following imaging, knees underwent both routine histological processing and picrosirus staining for organized collagen.

RESULTS

OCT images indicate that injection of sodium iodoacetate resulted in a progressive decrease in cartilage thickness and loss of the bone-cartilage interface which correlated with histology. In addition, PS-OCT was able to detect collagen disorganization, an early indicator of OA.

CONCLUSIONS

The use of OCT in combination with the induction of OA in rats is a promising new animal model for assessing articular changes with the goal of monitoring therapeutics longitudinally. Future work will extend the model to in vivo assessments.

Vascular graft healing. III. FTIR analysis of ePTFE graft samples from implanted bigrafts

A bigraft, composed of a 30-microm internodal distance expanded polytetrafluoroethylene (ePTFE) arterial graft segment and a preclotted polyethylene terephthalate arterial graft segment, was used to study the healing process of two different materials in the same dog. Healing was followed by FTIR-ATR spectroscopy and correlated with ELISA analyses of selected growth factors and matrix proteins. The FTIR analyses of the ePTFE explants from 1 and 3 h; 3, 5, 7, 10, and 14 days; and 4 and 8 weeks showed that the ePTFE grafts did not heal (endothelialize), but appeared to remain in overlapping inflammation and proliferation stages of wound healing. Although FN was found in the fibrin layer coating the luminal surface of the ePTFE, VEGF was not present. The inability of VEGF to complex with FN may be the result of FN binding sites being blocked or hidden by the conformation of the fibrin clot that forms on ePTFE. Also, TGF-beta was not present beyond the initial clot formation at 3 h postimplantation. The absence of these two growth factors may be an important factor in the lack of healing of 30-microm ePTFE grafts, because both were found to be present during the early stages of healing for the preclotted polyester grafts that did endothelialize.

Anaylsis of birefringence during wound healing and remodeling following alkali burns in rabbit cornea

The use of synthetic inhibitors of metalloproteinases (SIMP) or medroxyprogesterone (MP) can prevent or significantly delay the ulceration of alkali-injured corneas by influencing collagen degradation. We have examined the remodeling of rabbit corneal stroma following alkali injury and have assessed the effect of SIMP and MP treatment. Following a defined alkali injury to the rabbit cornea, animals were divided into three subgroups, one group treated with topical beta-mercaptomethyl tripeptide (SIMP), one treated by subconjunctival injection of MP and one treated with a control solution. The corneal tissue was taken at 3 days, 1, 2, 3, 4, 9 and 26 weeks after alkali injury and prepared for light microscopy and transmission electron microscopy (TEM). A quantitative measurement of birefringence, in terms of the optical path difference (OPD), was made using a modified polarized microscopy technique based on the analysis of interference colours. The results showed that SIMP effectively prevented deep corneal ulceration. MP could delay the ulceration and the corneas treated with MP appeared to have better transparency than the other groups. There was a significant difference of the OPD between the anterior (5.8 +/-0.3 nm) and posterior (7.8 +/-0.4 nm) stroma of the normal cornea (P<0.001). The OPD values from the central corneas from alkali-injured eyes were generally lower than normal during the first 4 weeks and then gradually recovered to the normal level or above, except for the posterior stroma of the MP-treated eyes. We found that the OPD changes were very dependent on the presence of corneal lesions. The stroma near corneal ulceration, scar tissue, calcified stroma and the retro-corneal collagen layer showed a significant reduction of birefringence (lower OPD values). These OPD values remained much lower than normal up to the end of the experiment. TEM showed disrupted corneal stroma in all three groups, with thinner scar tissue in the MP group. The fibril diameters did not change significantly 3 days and 1 week after the alkali burns (27.1+/-2.3 nm in the control group, 27.3+/-2.2 nm in the SIMP group and 27.7+/-2.1 nm in the MP group) and there were no differences compared with 29.7+/-1.7 nm of the normal cornea (P>0.05). After 2 weeks of tissue remodeling, the fibril diameters in alkali-injured corneas showed a large variation (the range was between 11.5 and 80 nm) with a bimodal distribution, especially in the control group. The technique presented here for birefringence evaluation can provide an alternative way to monitor wound healing and tissue remodeling, both visually and quantitatively.

Vascular graft healing: I. FTIR analysis of an implant model for studying the healing of a vascular graft

FTIR analysis of sequential biopsy samples of the primary and replacement segments obtained from 70 to 80-cm long carotid-femoral bypass grafts implanted bilaterally in the dog indicate that the healing pattern is similar over the entire length of these grafts. Preliminary analysis of the spectra also indicates that the major developments in the formation of the biological layer appear to occur during the first 4 weeks after implantation. Collagen IV could be detected by both FTIR and antibody staining in the 8-week samples. Thus, this study supports the application of FTIR attenuated total reflectance to determine the components of the biological tissue that forms on an implanted vascular graft surface.

Light scattering from cells: finite-difference time-domain simulations and goniometric measurements

We have examined the light-scattering properties of inhomogeneous biological cells through a combination of theoretical simulations and goniometric measurements. A finite-difference time-domain (FDTD) technique was used to compute intensity as a function of scattering angle for cells containing multiple organelles and spatially varying index of refraction profiles. An automated goniometer was constructed to measure the scattering properties of dilute cell suspensions. Measurements compared favorably with FDTD predictions. FDTD and experimental results indicate that scattering properties are strongly influenced by cellular biochemical and morphological structure.

The structure of avian cartilage: a combined X-ray and biochemical analysis

The antitrochanter is a cartilaginous extension of the avian hip joint that is susceptible to degenerative changes. This study consisted of X-ray diffraction and complementary biochemical examination of collagen in the turkey antitrochanter from the articulating surface through to the bone. X-ray diffraction was performed by means of synchrotron radiation with a 200 microns collimated beam to examine the antitrochanter from three 24-week-old, large, male turkeys. The arced distribution of intensity from equatorial reflections was used to determine the parameter g(psi), the probability distribution of fibril angular orientation. Analysis of five distinct regions showed a variety of fibril orientation profiles. The surface of the cartilage exhibited a near isotropic profile. The superficial fibrous region contained well-aligned fibrils parallel to the surface. The middle hyaline region was less well organized, but fibrils were orientated in a direction similar to that of fibrils in the superficial fibrous region. The deep hyaline region exhibited a bimodal distribution of fibril orientation. The ossified front (decalcified) showed an alignment similar to that seen in the middle hyaline region. Biochemically, two main regions were found. The first, 1.2 mm from the surface, consisted of type I collagen and the deeper region (1.26-3.0 mm) consisted of type II collagen. The change from type I to type II was abrupt (less than 80 microns).

Relation between structure and mechanical behaviour of fibre-reinforced composite materials at large strains

A theory is presented for the mechanical behaviour of fibre-reinforced composite materials at large strains. It is based on the distribution of fibres within the material and uses only basic mechanical properties of the fibres and matrix. The angular distribution of fibres is described by an orientation distribution function, g ( θ, ϕ ), which changes as the material is stretched, becoming greater along the directions of greatest tensile strain. No assumption is made about stress transfer to the fibres, or their length, except that they are long enough for this to happen effectively. The analysis predicts the reorientation of the fibres with strain and shows good agreement with preliminary experimental results from spinal ligaments. The predicted stress-strain behaviour shows qualitative agreement with experiment but further analysis is needed to take account of the crimping of the collagen fibres which occurs in this material. The theory may also have application to reorientation effects observed in drawn polymers and this is demonstrated with reference to polyethylene and polyethylene terephthalate.

Collagen organization in the crural interosseous membrane and its relationship to fibular osteotomy

The organization of collagen fibres in the crural interosseous membrane was measured using X-ray diffraction. The fibres were found to be highly aligned and steeply angled in a distal direction from tibia to fibula with a mean angle of 13 degrees. This organization will lead to a very stiff, ligament-like structure which will transmit forces to the tibia generated by the muscles attached to the fibula. It also suggests an optimum site for a fibular osteotomy about 4 cm distal to the tibial fracture site.

Cellular shape and pressure may mediate mechanical control of tissue composition in tendons

In vivo studies have suggested that mechanical factors are involved in the regulation of the morphology and biochemical composition of tendons that wrap around bones. In these tendons, fibrocartilage is found in the segment wrapped around the bone, and tendon far from the bone displays normal tendon histomorphology. Recent in vitro studies have shown that intermittently loaded connective tissue cells are sensitive to changes in cellular shape and hydrostatic pressure: stretching and distortion of the cells enhances production of fibrous matrix and hydrostatic pressure enhances production of cartilaginous matrix. We used finite-element analysis to determine whether the regions of increased development of cartilaginous matrix in tendons that wrap around bones correspond to regions in which tendon cells are subjected to higher pressures, and whether the maintenance and rearrangement of fibrous extracellular matrix in these tendons is associated with regions of stretching and distortion of cells. We found that regions of cartilaginous matrix and fibrous matrix formation and turnover correlate well with patterns of hydrostatic compressive stress and distortional strain in the tendon. Although further experiments clearly are needed to establish the predictive value of our approach, hydrostatic stress and distortional strain history--parameters intimately related to changes in cellular pressure and shape, respectively--appear to be important tissue-level mechanical stimuli that regulate cartilaginous and fibrous matrix composition of connective tissues.

A methodology for the systematic and quantitative study of cell contact guidance in oriented collagen gels. Correlation of fibroblast orientation and gel birefringence

Despite the likely role of contact guidance in every physiological process involving cell migration, its study in a three-dimensional tissue-equivalent environment has been precluded, heretofore, by inherent difficulties in systematically preparing well-defined contact guidance fields and quantifying the resultant contact guidance. Here, we describe a novel use of a magnetic field to orient collagen fibrils during fibrillogenesis, entrapping cells dispersed in the collagen solution. Using computer-controlled staging and image analysis, we show from automated birefringence measurements of the resultant slab of cell-populated gel contained in a specially designed observation chamber that the fibril orientation is biased along the long axis of the chamber uniformly throughout the chamber. Further, we show that the degree of fibril orientation, and consequently the elicited contact guidance, can be controlled by independently varying the magnetic field strength or temperature during fibrillogenesis. We characterize the contact guidance response to the imposed contact guidance field by measuring cell orientation relative to the axis of fibril orientation from still images obtained in time-lapse via automated image analysis. We present the first quantitative correlation of contact guidance (based on cell orientation) with collagen fibril orientation (based on birefringence) for human foreskin fibroblasts cultured in a collagen gel, by using gels of varying orientation resulting from different magnetic field strengths and temperatures during fibrillogenesis, and by using sufficiently low cell concentrations and early observation times.

Stress in collagen fibrils of articular cartilage calculated from their measured orientations

Articular cartilage may be considered as a form of pressure vessel in which the internal swelling pressure is balanced by tensile stress in the collagen fibrils. This stress is calculated by analysing the tissue as a series of microscopically small pressure vessels. The previously measured orientations of the collagen fibrils describe the structure necessary for this calculation. The stresses and strains developed in the fibrils are shown to be well within physiological limits.

X-ray diffraction studies of tibial plateau cartilage using synchrotron radiation

X-ray diffraction studies of articular cartilage have been performed using X-rays from two sources: a conventional generator and synchrotron radiation. Blocks of tissue from the tibial plateaux of pigs and dogs were examined to find the orientations of collagen fibrils throughout their depth. Analysis of the resulting diffraction patterns revealed a zonal arrangement of fibrils in both pig and dog tissue. At the articular surface the preferred orientation of fibrils was parallel to the surface, below this was a zone with no obvious preferred orientation and the remaining tissue had fibrils oriented mainly perpendicular to the surface. Using a conventional source of X-rays the exposure time for a single diffraction pattern was a number of days but this was reduced to about 10 min using the more intense synchrotron radiation. This improvement in exposure time facilitates the examination of small areas of tissue. X-ray diffraction using the synchrotron source offers a quick method of determining collagen fibril orientations in small areas of articular cartilage.