Biomechanical Properties of Synovial Fluid in/Between Peripheral Zones of Articular Cartilage - Biomaterials: Physics and Chemistry - page 218

During loading, the chondrocytes with GAGs encapsulation (in the peripheral zone) create a
continuous incompressible mezzo layer with protected chondrocytes. Simultaneously, an
incompressible peripheral zone arises in the middle of the transitional zone and in the low
(radial) zone of AC. There are dominantly hyperelastic properties in the transitional and the
low radial zone (Fig. 11.). Stress states can be simulated by the modified Cauchy stress
tensor for incompressible hyperelastic material.
Viscous properties in the peripheral zone of articular cartilage result from the interaction
between the molecules of the extracellular matrix and the molecules of free (unbound)
synovial fluid. The transport of SF molecules through the extracellular space and the lack of
bonding of these molecules onto glycosaminoglycans create the basic condition for the
viscous behaviors of cartilage. High dynamic forces are dominantly undertaken by the AC
matrix with firmly bonded water in its low and middle zone with a simultaneous creation of
an incompressible tissue, a cushion (Fig. 1.).
The articular cartilage matrix with viscoelastic properties functions dominantly as a
protective pump and a regulator of the amount of SF permanently maintained (during cyclic
loading) between articular plateaus. The importance of the protective pump is evident from
the function of retention of AC strains during cyclic loading. Due to slow down viscoelastic
strain, part of accumulated (i.e. previously discharged) SF from the preceding loading cycle
is retained in articular cartilage (Fig. 13.).
Fig. 13. Application of Kelvin Voigt viscoelastic model for the expression of step by step
increments of strains ε ti in the peripheral zone of AC during cyclic loading (e.g. while
walking or running)
Fig. 13. in its upper part (a) shows the loading cycles e.g. during walking, while in the lower
part (b) strains during the strain time growth and during strain relaxation are visible. The
strain time growth occurs during the first loading (see the first concave curve OA of the
strain growth). At the time t 1 after unloading strain relaxation occurs (see the convex shape
of the second curve AB). At the time t 2 the successive (second) loading cycle starts. The
strain time growth during the successive loading cycle, however, does not start at a zero