Physicochemical study of articular cartilage from healthy and osteo-arthritic human hips. Orientation and thermal stability of collagen fibres

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.

Shear deformations of human articular cartilage: Certain mechanical anisotropies apparent at large but not small shear strains

Articular cartilage has pronounced through-the-thickness heterogeneity in both ultrastructure and mechanical function. The tissue undergoes a combination of large deformations in vivo, where shear is critical in both failure and chondrocyte death. Yet the microstructure mechanical response of cartilage to multi-axial large shear deformations is unknown. We harvested a total of 42 cartilage specimens from seven matched locations across the lateral femoral condyles and patellofemoral grooves of six human male donors (30.2±8.8yrs old, M±SD). With each specimen we applied a range of quasi-static, multi-axial large (simple) shear displacements both parallel and perpendicular to the local split-line direction (SLD). Shear stresses in cartilage specimens from the patellofemoral grooves were higher, and more energy was dissipated, at all applied strains under loading parallel to the local SLD versus perpendicular, while specimens from the lateral condyles were mechanically anisotropic only under larger strains of 20% and 25%. Cartilage also showed significant intra-donor variability at larger shear strains but no significant inter-donor variability. Overall, shear strain-energy dissipation was almost constant at 5% applied shear strain and increased nonlinearly with increasing shear magnitude. Our results suggest that full understanding of cartilage mechanics requires large-strain analyses to account for nonlinear, anisotropic and location-dependent effects not fully realized at small strains.

Cartilage collagen matrix reorientation and displacement in response to surface loading

An investigation of collagen fiber reorientation, as well as fluid and matrix movement of equine articular cartilage and subchondral bone under compressive mechanical loads, was undertaken using small angle X-ray scattering measurements and optical microscopy. Small angle X-ray scattering measurements were made on healthy and diseased samples of equine articular cartilage and subchondral bone mounted in a mechanical testing apparatus on station ID18F of ESRF, Grenoble, together with fiber orientation analysis using polarized light and displacement measurements of the cartilage matrix and fluid using tracers. At surface pressures of up to approximately 1.5 MPa, there was reversible compression of the tangential surface fibers and immediately subjacent zone. As load increased, deformation in these zones reached a maximum and then reorientation propagated to the radial deep zone. Between surface pressures of 4.8 MPa and 6.0 MPa, fiber orientation above the tide mark rotated 10 deg from the radial direction, with an overall loss of alignment. With further increase in load, the fibers "crimped" as shown by the appearance of subsidiary peaks approximately +/-10 deg either side of the principal fiber orientation direction. Failure at higher loads was characterized by a radial split in the deep cartilage, which propagated along the tide mark while the surface zone remained intact. In lesions, the fiber organization was disrupted and the initial response to load was consistent with early rupture of fibers, but the matrix relaxed to an organization very similar to that of the unloaded tissue. Tracer measurements revealed anisotropic solid and fluid displacement, which depended strongly on depth within the tissue.

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.

Birefringence of articular cartilage and the distribution on collagen fibril orientations

Birefringence of the deep zone of pig tibial plateau cartilage has been measured by polarized light microscopy using monochromatic light and a Sénarmont compensator. Thin (20 micrometers) sections of the tissue were imbibed with mixtures of physiological saline and glycerol to separate the effects of form and intrinsic birefringence. Measured birefringence was plotted against liquid refractive index and the experimental points defined a parabola whose minimum corresponded to a refractive index for collagen of 1.48. When the tissue was imbibed with glycerol, the birefringence measurements provided a good estimate of intrinsic birefringence. The orientation distribution function, g(phi), of collagen fibrils in this zone was then determined by X-ray diffraction. Thus X-ray diffraction yielded the form of g(phi) which corresponded to the measured intrinsic birefringence. Using this result the intrinsic birefringence of a hypothetical cartilage site with perfectly oriented collagen fibrils was calculated to be (6 +/- 1) x 10(-4). Birefringence measurements on further sites could then be converted into standard deviations of g(phi), assuming a Gaussian distribution.

Structure of collagen in cartilage of intervertebral disk

Small-angle x-ray and neutron diffraction patterns have been obtained from the annulus fibrosus of porcine intervertebral disk. These show that the collagen in this tissue is modified compared with that in tendon.

Amianthoid change: orientation of normal collagen fibrils during aging

High-angle x-ray diffraction provides direct evidence that amianthoid change, occurring during aging of costal cartilage, corresponds to a transformation from an isotropic to a marked anisotropic distribution of collagen fibrils. Low-angle x-ray diffraction and electron microscopy show that the fibrils have the customary 67-nanometer axial periodicity. Electron microscopy shows that wide amianthoid collagen fibrils consist of smaller parallel fibrils fused together. Similarities between amianthoid change and tendon morphogenesis are briefly discussed. Amianthoid change is remarkable in that aging is accompanied by increased order.

High-angle electron diffraction of frozen hydrated collagen

By using the techniques developed by Taylor et al. [(1975) J. Mol. Biol. 92, 165-167] (freezing of the hydrated specimen before its insertion into the electron microscope and keeping it frozen throughout the diffraction experiment), it was possible to obtain a high-angle electron-diffraction pattern from collagen fibrils. This pattern is in good agreement with that obtained by high-angle X-ray diffraction. Electron diffraction will be very useful to study collagen, because the diffraction pattern from a carefully selected area of one fibril is now feasible.