Purpose:
To compare the mechanical response to controlled inflation and the regional thickness variations of the sclera between normal and glaucoma human post-mortem eyes and between normal specimens accounting for the effects of age.
Methods:
25 glaucoma eyes (15 donors) and 30 normal human eyes (18 donors) were tested within 48 hours post-mortem. The sclera was incised 3 mm posterior to the equator, glued to a plastic ring, and mounted on an inflation chamber. Prior to testing, scleral thickness was measured with an ultrasonic pachymeter at 16 sites. The specimen was inflated by pressure controlled injection of balanced salt solution through a series of load-unload tests from 1 mmHg to 30 mmHg at 2 pressure rates (0.05, 1 mmHg/s) separated by a recovery period and creep tests to 15 and 30 mmHg. Two cameras imaged the deforming scleral surface at 0.5 Hz to permit three dimensional displacement mapping using a Digital Image Correlation program. After testing, an axonal count was performed to evaluate glaucoma damage. Outcome measures were compared by analysis of variance (ANOVA).
Results:
For normal specimens, the thickness significantly decreased with age (p-value<0.05) at a rate of 40 µm/decade. The superior-temporal quadrant of the peripapillary sclera was the thickest and the nasal-inferior quadrant was the thinnest. Older eyes were stiffer than younger eyes (p-value<0.05). The stiffness of the pressure-displacement response doubled between age 40 and age 90 from 15 to 30 kPa/mm. Both creep rates to 15 and 30 mmHg significantly decreased with age. No difference was found between male and female eyes.The stiffness of glaucoma specimens exhibited a different statistical dependence on age than normal specimens. Younger glaucoma specimens (age 70-80) were stiffer than normal eyes.
Conclusions:
The biomechanical properties of the human sclera were found to be age-dependent. Preliminary statistical studies showed that glaucoma diagnosis has an effect on the stiffness. We are now examining the effects of glaucoma on the viscoelastic scleral properties and the relationship between stiffness and degree of nerve axon loss.
Keywords: sclera • aging • stress response