Dynamic compressive behavior of human meniscus correlates with its extra-cellular matrix composition.

The menisci of the knee play a significant role in the complex biomechanics of the joint and are critically important in maintaining articular cartilage health. While a general form-function relationship has been identified for the structural orientation of the extra-cellular matrix of the meniscus, the role of individual biochemical components has yet to be fully explored. To determine if correlations exist between the dynamic and static compressive modulus of human menisci and their major extra-cellular matrix constituents (collagen, glycosoaminoglycan and water content), 12 lateral and 11 medial menisci from 13 adult donors were examined. The results showed that in dynamic compression at high loading frequencies (0.1-1 Hz) the menisci behave as a rubber-like elastic material while at lower frequencies (0.01-0.03 Hz) significant viscous dissipation occurs. While regional variations in compressive moduli and extra-cellular matrix composition were observed, the magnitude of both dynamic and static compressive moduli were found to be insensitive to collagen content (p>0.4). However, this magnitude was found to significantly increase with increasing glycosaminoglycan content (p<0.001) and significantly decrease with increasing water content (p<0.001). The results of this study identify significant relationships between the viscoelastic behavior of the meniscus and its extra-cellular matrix composition.

IGF-I and mechanical environment interact to modulate engineered cartilage development.

Bovine calf articular chondrocytes were seeded onto biodegradable polyglycolic acid scaffolds and cultured for four weeks using in vitro systems providing different mechanical environments (static and mixed Petri dishes, static and mixed flasks, and rotating vessels) and different biochemical environments (medium with and without supplemental insulin-like growth factor I, IGF-I). Under all conditions, the resulting engineered tissue histologically resembled cartilage and contained its major constituents: glycosaminoglycans, collagen, and cells. The mechanical environment and supplemental IGF-I (a) independently modulated tissue morphology, growth, biochemical composition, and mechanical properties (equilibrium modulus) of engineered cartilage as previously reported; (b) interacted additively or in some cases nonadditively producing results not suggested by the independent responses, and (c) in combination produced tissue superior to that obtained by modifying these factors individually.

A microstructural model of elastostatic properties of articular cartilage in confined compression.

A microstructural model of cartilage was developed to investigate the relative contribution of tissue matrix components to its elastostatic properties. Cartilage was depicted as a tensed collagen lattice pressurized by the Donnan osmotic swelling pressure of proteoglycans. As a first step in modeling the collagen lattice, two-dimensional networks of tensed, elastic, interconnected cables were studied as conceptual models. The models were subjected to the boundary conditions of confined compression and stress-strain curves and elastic moduli were obtained as a function of a two-dimensional equivalent of swelling pressure. Model predictions were compared to equilibrium confined compression moduli of calf cartilage obtained at different bath concentrations ranging from 0.01 to 0.50 M NaCl. It was found that a triangular cable network provided the most consistent correspondence to the experimental data. The model showed that the cartilage collagen network remained tensed under large confined compression strains and could therefore support shear stress. The model also predicted that the elastic moduli increased with increasing swelling pressure in a manner qualitatively similar to experimental observations. Although the model did not preclude potential contributions of other tissue components and mechanisms, the consistency of model predictions with experimental observations suggests that the cartilage collagen network, prestressed by proteoglycan swelling pressure, plays an important role in supporting compression.

Confined and unconfined stress relaxation of cartilage: appropriateness of a transversely isotropic analysis.

Previous studies have shown that stress relaxation behavior of calf ulnar growth plate and chondroepiphysis cartilage can be described by a linear transverse isotropic biphasic model. The model provides a good fit to the observed unconfined compression transients when the out-of-plane Poisson's ratio is set to zero. This assumption is based on the observation that the equilibrium stress in the axial direction (deltaz) is the same in confined and unconfined compression, which implies that the radial stress deltar = 0 in confined compression. In our study, we further investigated the ability of the transversely isotropic model to describe confined and unconfined stress relaxation behavior of calf cartilage. A series of confined and unconfined stress relaxation tests were performed on calf articular cartilage (4.5 mm diameter, approximately 3.3 mm height) in a displacement-controlled compression apparatus capable of measuring delta(z) and delta(r). In equilibrium, delta(r) > 0 and delta(z) in confined compression was greater than in unconfined compression. Transient data at each strain were fitted by the linear transversely isotropic biphasic model and the material parameters were estimated. Although the model could provide good fits to the unconfined transients, the estimated parameters overpredicted the measured delta(r). Conversely, if the model was constrained to match equilibrium delta(r), the fits were poor. These findings suggest that the linear transversely isotropic biphasic model could not simultaneously describe the observed stress relaxation and equilibrium behavior of calf cartilage.

Dynamic cell seeding of polymer scaffolds for cartilage tissue engineering.

Cell seeding of three-dimensional polymer scaffolds is the first step of the cultivation of engineered tissues in bioreactors. Seeding requirements of large scaffolds to make implants for potential clinical use include: (a) high yield, to maximize the utilization of donor cells, (b) high kinetic rate, to minimize the time in suspension for anchorage-dependent and shear-sensitive cells, and (c) high and spatially uniform distribution of attached cells, for rapid and uniform tissue regeneration. Highly porous, fibrous polyglycolic acid scaffolds, 5-10 mm in diameter and 2-5 mm thick, were seeded with bovine articular chondrocytes in well-mixed spinner flasks. Essentially, all cells attached throughout the scaffold volume within 1 day. Mixing promoted the formation of 20-32-micron diameter cell aggregates that enhanced the kinetics of cell attachment without compromising the uniformity of cell distribution. The kinetics and possible mechanisms of cell seeding were related to the formation of cell aggregates by a simple mathematical model that can be used to optimize seeding conditions for cartilage tissue engineering.

Mass transfer studies of tissue engineered cartilage.

Tissue engineered cartilage can be grown in vitro using isolated cartilage cells and biodegradable polyglycolic acid scaffolds. In the present study, the kinetics of mass transfer and the regeneration of tissue components (i.e., cells, glycosaminoglycan, and collagen) were studied for cell-polymer constructs cultured in orbitally mixed petri dishes. Over 6 weeks of cultivation, the composition and morphology of the constructs changed toward an increasingly compact tissue structure as a result of cell proliferation, regeneration of the cartilaginous matrix, and scaffold degradation. Overall rates of mass transfer were determined for disk shaped constructs (10 mm in diameter x 5 mm thick) exposed to a small volume of a well mixed solution of tracer molecules, i.e., glucose and dextran with molecular weights of 180 Da and 4400 Da, respectively. The kinetics of mass transfer between the construct and the solution was assessed from measured time profiles of tracer concentration in the solution and physical construct properties. Mass transfer parameters (e.g., kinetic constants of tracer uptake, partition coefficients) were calculated numerically by solving the material balance equations over the time of tracer diffusion. Mass transfer rates of glucose and dextran decreased over cultivation time in parallel with the accumulation of tissue components in cell-polymer constructs.

[Electron microscopy of blood vessels in alopecia areata]. / Elektronska mikroskopija krvnih sudova kod alopecije areate.

The use of electron microscope in the study of the diseased skin of four patients with Alopecia areata has detected changes in the endothelial cells with narrowed capillary lumen. The authors have brought these findings in connection with the decreased blood circulation in lesions of patients with Alopecia areata which necessiates the use of vasodilatator agents in their therapy. Etiopathogenesis of the disease has been discussed.