Depth-Dependent Biomechanics of the Anterior Sclera

Abstract

 

Purpose

 

Several ocular conditions including keratoconus, myopia and glaucoma are associated with mechanical changes in the eye. To properly characterize these changes, it is necessary to first quantify the baseline mechanical properties of the healthy eye. Among the key parameters governing the mechanical response of the eye is G_SI, the transverse shear modulus of the sclera. However, how this property varies with depth and location is unknown. Thus, our objective was to measure the depth-dependent transverse shear modulus of the superior, inferior, nasal and temporal regions of the human anterior sclera. Given that G_SI in the cornea is highest in the anterior stroma--possibly as an adaptation to shear stress from blinking--we hypothesized that G_SI in the sclera would also be highest near the outer surface.

 

Methods

 

A total of 24 specimens from the superior, inferior, nasal and temporal sclera on 6 human corneoscleral rims were subjected to 5% shear strain in the superior-inferior direction using a specialized microscope-mounted mechanical testing device. From the acquired images and measurements of the applied stress, the depth-dependent transverse shear modulus of each specimen was determined.

 

Results

 

In all locations, G_SI was generally highest near the outer surface and peaked at around 30% depth (Figure 1). However, only in the inferior sclera were differences in G_SI with depth statistically significant. As hypothesized, in every cardinal location, the correlation between G_SI in the sclera and G_SI in the cornea was significant (though G_SI was approximately 5 times higher in the sclera). A modest correlation (R² = 0.54-0.62) was found in the superior, temporal and nasal regions while a strong correlation (R² = 0.92) was found in the inferior region.

 

Conclusions

 

The heterogeneity of G_SI in the sclera across depth may be attributed to an adaptation to differences in endured shear stress across its depth. However, the correlation between G_SI in the cornea and sclera suggests an additional purpose—to minimize stress concentrations that occur at abrupt material interfaces. In general, the findings of this study suggest a promising method for quantifying differences in the depth- and location-dependent shear modulus of healthy and diseased scleras.  

 

Figure 1: Measured G_SI across scleral depth for each cardinal location. * Indicates p<0.05/9 versus the posterior edge and error bars are mean ± SEM.