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Technical Briefs

Microindentation of the Young Porcine Ocular Lens

[+] Author and Article Information
Matthew Reilly

 Department of Veterans Affairs, 915 North Grand Boulevard, St. Louis, MO 63106; Department of Energy, Environmental, and Chemical Engineering, Washington University in St. Louis, St. Louis, MO 63110

Nathan Ravi1

 Department of Veterans Affairs, 915 North Grand Boulevard, St. Louis, MO 63106; Department of Energy, Environmental, and Chemical Engineering, and Department of Ophthalmology and Visual Sciences, Washington University in St. Louis, St. Louis, MO 63110nathan.ravi@med.va.gov

1

Corresponding author.

J Biomech Eng 131(4), 044502 (Jan 30, 2009) (4 pages) doi:10.1115/1.3072891 History: Received May 22, 2008; Revised October 10, 2008; Published January 30, 2009

Debate regarding the mechanisms of how the eye changes focus (accommodation) and why this ability is lost with age (presbyopia) has recently been rejoined due to the advent of surgical procedures for the correction of presbyopia. Due to inherent confounding factors in both in vivo and in vitro measurement techniques, mechanical modeling of the behavior of the ocular lens in accommodation has been attempted to settle the debate. However, a paucity of reliable mechanical property measurements has proven problematic in the development of a successful mechanical model of accommodation. Instrumented microindentation was utilized to directly measure the local elastic modulus and dynamic response at various locations in the lens. The young porcine lens exhibits a large modulus gradient with the highest modulus appearing at the center of the nucleus and exponentially decreasing with distance. The loss tangent was significantly higher in the decapsulated lens and the force waveform amplitude decreased significantly upon removal of the lens capsule. The findings indicate that localized measurements of the lens’ mechanical properties are necessary to achieve accurate quantitative parameters suitable for mechanical modeling efforts. The results also indicate that the lens behaves as a crosslinked gel rather than as a collection of individual arched fiber cells.

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Copyright © 2009 by American Society of Mechanical Engineers
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Figures

Grahic Jump Location
Figure 3

Results for dynamic indentation of both capsulated (◆) and decapsulated (◼) lenses. (a) Amplitude of the force waveform as a function of frequency. (b) Loss tangent as a function of frequency. The force amplitude decreased significantly when the capsule was removed, while the loss tangent increased dramatically. Both of these findings reinforce the notion that the lens capsule is a stiff, elastic membrane relative to the lens matrix, which is a soft viscoelastic solid.

Grahic Jump Location
Figure 2

Dependence of the lens’ elastic modulus on distance from the center of the nucleus. The modulus monotonically decreases from the center of the nucleus outwards. (a) Modulus fitted using Fisher’s method with distinct nuclei for the nucleus and cortex. (b) Simple exponential fit of the modulus.

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Figure 1

Three-dimensional schematic of the lens matrix architecture. Note the arching of the lens fiber cells from pole to pole in the optical axis section and the hexagonal cross section of the cells in the equatorial section. The size of the cells is greatly exaggerated for clarity. The true dimensions of the fiber cells are on the order of 5 μm(22), while the porcine lens radius is on the order of 5 mm.

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