Biomedical Engineering Reference
In-Depth Information
this way, the hydrodynamic biomechanism quickly presupplements the surface localities
with lubrication material. Shortly after unloading, the strain rate is high. During strain
relaxation, it slows down. This is the way how the articular cartilage tissue attempts to
retain the lubrication material between the articular plateaus of synovial joints as long as
possible during cyclic loading.
Analogically to the low and the middle zone of AC where an incompressible zone arises
under high loads whose dominant function is to bear high loads and protect chondrocytes
with the intercellular matrix from destruction, in the peripheral zone as well a partial
incompressible zone arises whose function is to bear high loads and protect the peripheral
tissue from mechanical failure. The appearance of the incompressible tissue in all zones is
synchronized aiming at the creation of a single (integrated)
incompressible cushion
. The
existence of an incompressible zone secures the protection of chondrocytes and extracellular
material from potential destruction.
3.1 Significance of results for clinical practice
Metabolic processes during the HA synthesis are very dynamic. The chondrocytes in AC
actively synthesize and catabolise HA so that its optimal “usability” is achieved (in a
relatively short time). The HA synthesis is usually in equilibrium with catabolic processes.
These processes result in the achievement of the optimum HA concentration. The studies of
metabolic processes (Schurz et al., 1987) implied that the half-life of the functional existence
of HA molecules are mere 2-3 weeks. The solved project makes it evident that the “short
lifecycle“ of HA is dominantly caused by dynamic (biomechanical) effects. During leg
movements, long snakelike NaHA/HA macromolecules are exposed to fast changes in
shape accompanied by permanently arising and vanishing physical (non-covalent) bonds.
To avoid the shortage of HA/NaHA, old polymer chains are replaced with new chains. The
disturbance of HA new formation processes may lead to initiations of pathological
processes. Mechanical effects during movements continuously initiate new groupings of HA
macromolecules and newly arising (and vanishing) bonds among them. Frequented
variations of kinetic energy transfers into HA molecular structures contribute to HA
fragmentations in the biophysical perspective. These fragmentations may also be
biochemically accelerated by hyaluronisades (Saari et al., 1993). HA fragments may initiate
the formation of macrophages and extensive inflammations of AC.
The above examples of the interrelated nature of the causes of some AC defects show the
key role of the rheological properties of non-Newtonian synovial fluid.
4. Acknowledgment
The contents presented in this chapter was supported by the Research Grant from MSMT
No.6840770012.
5. References
Akizuki, S.; Mow, V.C.; Muller, F.; Pita, J.C; Howell, D.S. & Manicourt, D.H. (1986). Tensile
properties of human knee joint cartilage: I. Influence of ionic conditions, weight
bearing, and fibrillation on the tensile modulus,
J. of orthopaedic research
, Vol. 4, No.
4, pp. 379-392
