Biomedical Engineering Reference
In-Depth Information
Fig. 6. SF apparent viscosity as related to time (velocity gradient 2000s
-1
)
Due to the fact that the lubrication abilities of SF strongly depend on the magnitude of
viscosity, and SF viscosity depends on the SF flow velocities, the effects of the magnitudes
and directions of shifting forces or shear stresses respectively on the distributions of the
magnitudes and directions of SF flow velocity vectors in the space between the opposite AC
surfaces had to be analyzed.
The kinematics of the limb motion (within one cycle) shows that during a step the leg
continuously passes through the phases of flexion - extension - flexion (Fig. 8.). The effect of
shifting forces (or shear stresses respectively) is predominantly manifested in the phases of
flexion, while normal forces representing the effects of the gravity (weight) of each
individual mostly apply in the phases of extension, Fig. 8. The distributions of the
magnitudes of SF flow velocity vectors depend on the shifts of the tibial and femoral part of
the knee joint, Fig. 9.
,
reaching their peaks in places on the interface of SF with the upper
and lower AC surface, Fig. 10. The velocities of SF flows very substantially affect the SF
behavior contributing to the lubrication of AC surfaces and their protection.
At rest the bonds are created among the macromolecules of hyaluronic acid (HA) leading to
the creation of associates.
By associating molecular chains of HA (at rest) into a continuous
structure, a spatial macromolecular grid is created in SF which contributes to the growth in viscosity
and also to the growth in elastic properties.
The associations of HA molecules are the manifestation of cohesive forces among HA
macromolecular chains. SF represents a dispersion system (White, 1963) in which the
dispersion rate is dominantly formed by snakelike HA macromolecules. The dispersion
environment is formed by water. Cohesive forces among NaHA polymer chains in SF are of
physical nature. The density (number) of bonds among HA macromolecules is dominantly
controlled by mechanical effects. Fig. 9. In relation to the magnitudes of velocity gradients,
NaHA macromolecules are able to form “thick” synovial gel which possesses elastic
properties characteristic of solid elastic materials, even though the dispersion environment
of synovial gel is liquid.
