Effects of Age and Diabetes on Scleral Stiffness

Abstract

The effects of diabetes on the collagen structure and material properties of the sclera are unknown but may be important to elucidate whether diabetes is a risk factor for major ocular diseases such as glaucoma. This study provides a quantitative assessment of the changes in scleral stiffness and collagen fiber alignment associated with diabetes. Posterior scleral shells from five diabetic donors and seven nondiabetic donors were pressurized to 30 mm Hg. Threedimensional surface displacements were calculated during inflation testing using digital image correlation (DIC). After testing, each specimen was subjected to wideangle Xray scattering (WAXS) measurements of its collagen organization. Specimenspecific finite element models of the posterior scleras were generated from the experimentally measured geometry. An inverse finite element analysis was developed to determine the material properties of the specimens, i.e., matrix and fiber stiffness, by matching DICmeasured and finite element predicted displacement fields. Effects of age and diabetes on the degree of fiber alignment, matrix and collagen fiber stiffness, and mechanical anisotropy were estimated using mixed effects models accounting for spatial autocorrelation. Older age was associated with a lower degree of fiber alignment and larger matrix stiffness for both diabetic and nondiabetic scleras. However, the agerelated increase in matrix stiffness was 87% larger in diabetic specimens compared to nondiabetic controls and diabetic scleras had a significantly larger matrix stiffness (p = 0.01). Older age was associated with a nearly significant increase in collagen fiber stiffness for diabetic specimens only (p = 0.06), as well as a decrease in mechanical anisotropy for nondiabetic scleras only (p = 0.04). The interaction between age and diabetes was not significant for all outcomes. This study suggests that the agerelated increase in scleral stiffness is accelerated in eyes with diabetes, which may have important implications in glaucoma.