Biomedical Engineering Reference
In-Depth Information
AC is composed of cells (chondrocytes), of extracellular composite material representing a
reinforcing component - collagen 2
nd
type (Benninghoff, 1925) and of a non reinforcing,
molecularly complex matrix (Bjelle, 1975)
.
A matrix is dominantly composed of glycoprotein
molecules and firmly bonded water. In the peripheral zone, there is synovial fluid unbound
by ions.
The principal construction components of the matrix are glycoproteins. They possess a
saccharide component (80-90 %) and a protein component (ca 20 - 10 %). Polysaccharides
are composed of molecules of chondroitin-4-sulphate, chondtroitin-6-sulphate and
keratansulphate. They are bonded onto the bearing protein, which is further bonded onto
the hyaluronic acid macromolecule by means of two binding proteins. Keratansulphates and
chondroitinsulphates are proteoglycans which, through bearing and binding proteins and
together with the supporting macromolecule of hyaluronic acid, constitute the proteoglycan
(or glycosaminoglycan) aggregate. As the saccharide part contains spatial polyanion fields,
the presence of a large number of sulphate, carboxyl and hydroxyl groups results in the
creation of
extensive fields of ionic bonds with water molecules.
The proteoglycan aggregate, together with
bonded water
, creates an amorphous extracellular
material (matrix) of cartilage, which is bonded onto the reinforcing component - collagen
2
nd
type. Glycosaminoglycans are connected onto the supporting fibres of collagen type II by
means of electrostatic bonds. In articular cartilage, nature took special efforts in
safeguarding the biomechanical protection of chondrocytes in the peripheral zone. In the
biomechanical perspective, chondrocytes are protected by glycocalix (i.e. a spherical
saccharide envelope with firmly bonded water). Glycocalix is composed of a saccharide
envelope bonded onto chondrocytes via transmembrane proteoglycans, transmembrane
glycoproteins and adsorbed glycoproteins.
The glycocalix envelopes create gradually the
incompressible continuous layer during the loading in peripheral zone of AC
(Fig. 12.).
Our research has been focussed on analyses of viscoelastic strains of the upper peripheral
cartilage zone, on the residual strains arising at cyclic loading, on the analyses of strain rate
and on the creation of a peripheral incompressible cartilage cushion.
The peripheral cartilage zone consists of chondrocytes packaged in proteoglycans (GAGs)
with firmly bonded molecules of water. In the intercellular space, there is unbound synovial
fluid which contains water, hyaluronic acid, lubricin, proteinases and collagenases. Synovial
fluid exhibits non-Newtonian flow characteristics. As was pointed out above, under a load
the synovial fluid is relocated on the surfaces of AC.
Fig. 12. Peripheral zone of articular cartilage without (a)/with (b) loads. The peripheral
incompressible zone is integrated with the incompressible zone in the middle (transitional)
zone and low (radial) zone
