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T. Sakuma, Y.–Y. Won, J. Sueda, N. Usumoto, D.A. Weitz, T. Hirose; Rheology of Vitreous : Effects of Enzymes . Invest. Ophthalmol. Vis. Sci. 2004;45(13):1948.
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Purpose: To study the rheology of the vitreous after incubation with enzyme to establish a method to evaluate liquefaction quantitatively. Methods: We used bulk rheology to determine how viscoelastic properties of the vitreous body in bovine eyes are affected by enzymes. The rheologic properties of the pristine vitreous and those treated with three enzymes (testicular hyaluronidase [HA], chondrotinase ABC [CA], and collagenase [CO]) were examined using a strain–controlled fluid rheometer. To evaluate the elastic and viscous moduli of the vitreous (G’ and G", respectively), the stress state of vitreous loaded into a gap between parallel plates was measured under applied oscillatory strain as functions of frequency (w). We also measured the vitreous gel weight to determine weight loss associated with enzyme digestion. Results: In all cases, the vitreous was predominantly elastic; G’ is only slightly dependent on w and much greater than G" overall measurement frequencies, confirming the space–filling interconnectivity of the collagen fibers. HA (100 and 500 U/gel) and CA (0.2 and 0.4 U/gel) treatment showed negligible viscoelasticity changes. CO digestion (300 U/gel) caused considerable reduction of elasticity, e.g., G’ decreased by a factor of 2 at 1 rad/s. There was a difference in rheologic effects between CO and the other enzymes, consistent with the different enzymatic activities: CO reacted with type II collagen, whereas HA and CA degraded only network junctions. Digestion with HA (to 72.30% of original wet weight vs 89.88% for controls incubated with buffer, 100 U/gel; 70.31% vs 89.88%, 500 U/gel) and CA (84.71% vs 93.85%, 0.2 U/gel; 82.82% vs 93.85%, 0.4 U/gel) resulted in significant weight decreases resulting from network loss caused by enzymatic degradation of the network junctions. However, CO digestion caused only a slight decrease in gel wet weight (90.70% vs 93.85%, 300 U/gel), despite affecting vitreous elasticity more than the other enzymes. Conclusions: Enzymatic liquefaction of the vitreous body is related to changes in the gel structure and viscoelasticity. Effective liquefaction can be achieved by breakdown in the collagen fiber structures rather than hyaluronan and chondrotinin sulfate.
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