Spectral analysis of viscoelasticity of the human lens.

PURPOSE: In the aging lens, two different structural changes become progressively apparent: loss of accommodation and formation of cataract. These changes are due to different alterations in lens structure taking place on a molecular scale. METHODS: For the characterization of liquids, gels, elastic bodies and solids, dynamic mechanical analysis (DMA) represents a non-destructive macroscopic analysis method with considerable sensitivity to microscopic structural changes. Spectra of the shear compliance (reciprocal shear modulus) of human lenses at different ages were measured in a wide frequency range (1 mHz to 1 kHz). By means of computer analysis, the parameters of the involved relaxation processes and the viscous flow were determined. RESULTS: The increase in cross-linking density was correlated with the disappearance of viscous flow in the measured spectra. The accumulation of high-molecular-weight aggregates was held responsible for the shift of the lower frequency relaxation process to lower frequencies with increasing stiffness, which is likely to be correlated with a prolonged accommodation time. CONCLUSION: Dynamic mechanical analysis represents a useful method for the spectroscopic characterization and quantification of the changes in the macroscopic viscoelastic properties caused by molecular changes in cataract formation and loss of accommodation.

Dynamic mechanical spectroscopy of the cornea for measurement of its viscoelastic properties in vitro.

Cylindric samples (3 mm in diameter) of explanted human corneas were biomechanically characterized in the frequency range from 0.1 mHz to 100 Hz with a specially modified viscoelasticity spectrometer. Such spectra of the shear compliance J (the reciprocal value of the shear modulus G) were measured as a function of corneal hydration and temperature. Variation of the hydration from 0.20- to 1.00-mm sample thickness (5-fold) changed the shear compliance 600-fold. Such a strong effect means that the shear-compliance spectra are highly sensitive to changes in the biomechanical properties of the cornea. This is demonstrated by three examples. A myopic cornea (-3D) was significantly softer (by a factor of 7 at lower frequencies and a factor of 4 at higher frequencies) than an emmetropic cornea. An increasing post mortem interval decreased the shear-compliance values (stiffening) obtained at higher frequencies (by a factor of 0.7 per day), whereas the values obtained at lower frequencies were reproduced. The biomechanics of thermal coagulation were studied in detail. The temperature was increased step by step, and at 48 degrees C a first irreversible decrease in the compliance was recorded at lower frequencies (0.8-fold). A further stiffening by a factor of 1/6 at lower frequencies and 1/5 at higher frequencies ended at 64 degrees C with a minimum in the compliance. To provide a new diagnostic tool, the biomechanical spectra of the cornea have to be measured in vivo. Equipment suitable for the megahertz range has been developed.(ABSTRACT TRUNCATED AT 250 WORDS)