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Abstract
The structure-function relations of cartilage proteoglycans are examined from nonlinear viscoelastic flow data on precisely prepared and characterized proteoglycan solutions at high concentrations. Both steady-state and transient flow properties are determined using a cone-on-plate viscometer. All proteoglycan solutions tested possessed: (1) linear viscoelastic properties--as measured by the dynamic complex modulus under small amplitude oscillatory conditions, (2) nonlinear shear-rate dependent apparent viscosity and primary normal stress difference under steady shearing conditions. Proteoglycan aggregate solutions of high concentrations exhibited transient stress overshoot, stress relaxation and hysteresis effects. Link-stable aggregates form more elastic (lower tan$\delta$) and stiffer (higher $\vert$G*$\vert$) networks than link-free aggregates. The influence of aging on flow properties is mainly due to the changes in molecular size and the portion of proteoglycans present as aggregates in solutions. Increased chondroitin sulfate/keratan sulfate ratio does not appear to have significant influence on flow properties.
An extension of the De Kee and Carreau network theory is made to model the responses of proteoglycan solutions to various flow conditions. The results from the model indicate that link protein stabilization in aggregates does not change the total amount of interactions formed in networks but increases the strength of network interactions resulting in greater structural rigidity. The total number of network interactions is directly correlated with molecular size and concentration. Fewer interaction sites are formed by proteoglycans from mature articular cartilage in networks but they are of greater strength when compared with those formed by proteoglycans from fetal epiphyseal cartilage.
The function of proteoglycans is further examined in the extracellular matrix of articular cartilage by testing a cylindrical disc specimen under torsional shear condition. Experimental results show that collagen-proteoglycan fiber-reinforced composite solid matrix exhibit viscoelastic behavior in shear. This intrinsic viscoelastic nature can be described by the quasilinear viscoelastic theory proposed by Fung. These viscoelastic properties are very different from concentrated proteoglycan solutions indicating that the collagen network and collagen-proteoglycan interactions make significant contributions to cartilage properties in shear. The shear stiffness of cartilage is greatly reduced when the structure of proteoglycans is biochemically altered by chondroitinase ABC and Streptomyces hyaluronidase.
