Characterization of Ocular Tissues Using Micro-Indentation and Hertzian Viscoelastic Models

Abstract

Purpose: : Accurate biomechanical properties of ocular tissues must be known to understand their mechanical interactions, yet classical tensile testing techniques are impractical for the small and irregular specimens typically available. We applied a novel micro-indentation technique to characterize biomechanical properties of small ocular and orbital tissue specimens using the Hertzian viscoelastic formulation, which defines material viscoelasicity in terms of contact pressure imposed during indention by a hard sphere.

Methods: : We employed a load cell with 100 nm displacement and 100 µg force precision to indent, with fine stainless steel spheres, fresh (<2 hrs post-mortem) specimens of bovine sclera, iris, crystalline lens, kidney fat, orbital pulley tissue, and orbital fat. We also studied fresh (<30 min post-excision) specimens of normal human orbital fat, eyelid fat, and dermal fat, and orbital fat associated with thyroid eye disease. Stress relaxation testing was performed using multiple initial indentation rates. Results for single indentations were used to fit quantitative Hertzian viscoelastic models that were in turn compared with behavior for other indentations and indentation rates. Findings in orbital tissues were correlated with quantitative histological determinations of connective tissue content.

Results: : Viscoelastic properties of small specimens of orbital and ocular tissues were reliably characterized over a wide range of rates and displacements by microindentation using the Hertzian formulation. While orbital fatty tissues exhibited highly similar elastic and viscous behaviors whether from bovine or human sources, different orbital tissues within species exhibited widely varying biomechanical properties. Since stiffness of fatty tissues was highly correlated with connective tissue content, orbital pulley tissues were much stiffer than orbital fat.

Conclusions: : Relaxation testing by microindentation is a powerful method for characterization of time-dependent behaviors of a wide range of ocular and orbital tissues using small specimens, and provides data suitable to define finite element models of a wide range of mechanical interactions within ocular tissues.

Keywords: eye movements • sclera • iris