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)