Purpose:
To compare the biomechanical response to controlled inflation of the sclera between normal and glaucoma human post-mortem eyes accounting for the effects of age and glaucoma damage.
Methods:
Scleral specimens from 22 donors with no history of glaucoma (35 eyes) and 11 donors with a history of glaucoma (22 eyes) were inflated in a series of pressure-controlled load-unload tests to 30 mmHg and creep tests to 15 and 30 mmHg. Optic nerve cross-sections were graded in masked fashion to determine the presence of axon loss. Circumferential and meridional strains were computed from the Digital Image Correlation displacements and mid-posterior stresses were determined from pressure and deformed geometry. Strains and material parameters were compared between diagnosed glaucoma specimens without axon loss (undamaged), diagnosed glaucoma specimens with axon loss (damaged), and normal specimens using multivariate analyses.
Results:
Among normal specimens, older age was predictive of a thinner sclera (p=0.017) and a stiffer response (Fig. B). A 3-fold increase in the circumferential fiber stiffness was observed between age 40 and 80 (p=0.0007). Compared to normal eyes, glaucoma eyes had a stiffer meridional strain response (Fig. C) and slower circumferential creep rates (p=0.014) in the peripapillary sclera. In the age group 75-93, peripapillary sclera meridional strains were on average 1.34±0.76% for normal specimens, compared to 0.58±0.46% for undamaged glaucoma specimens and 0.59±0.61% for damaged glaucoma specimens (p=0.007 and p=0.057, for the comparison between normal and undamaged and normal and damaged respectively). Glaucoma eyes were not significantly different from normal eyes in stresses and strains in the mid-posterior sclera.
Conclusions:
The observed differences in the biomechanical response of normal and glaucoma sclera may represent baseline properties that contribute to axon damage, or may be characteristics that result from glaucomatous disease.
Keywords: intraocular pressure • sclera • stress response