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JL Semmlow, S Strenk, L Strenk, AG Krudy; Mechanical Properties of the In Vivo Human Lens . Invest. Ophthalmol. Vis. Sci. 2002;43(13):407.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Presbyopia is a reduction in the ability to focus close and is an inevitable consequence of aging. Though of longstanding interest, the mechanisms underlying this loss of function are still in doubt due largely to a lack of information on the in vivo characteristics of the lens and its support structures. Our laboratory has pioneered the use of high resolution magnetic resonance imaging of the eye to obtain in vivo information regarding the lens and ciliary muscle. Here we describe an experimental protocol that provides a quantitative measure of lens elastic properties and, though a straightforward mechanical analysis, the forces applied to the lens in vivo. Methods: High resolution MR images of the eye are acquired under normal conditions and when the lens is loaded by an external weight. This force produces a compression of the lens (i.e., a change in lens volume) which can be measured from the MRI image. The change in lens shape can be used to estimate lens compressibility. Since accommodative forces also produce a change in lens volume, it is possible to estimate these internal forces from the external load experiments. Results: Results from four subjects 22 to 59 yrs. of age show an increase in lens bulk modulus (i.e., a decrease in compressibility) of from 246 to 387 x 103 dynes/cm2. Estimates of zonule force in a 22 and 36 year old were 1039 and 1240 dynes, respectively. Conclusion: The estimates of internal force are comparable with those derived by Fisher through an extensive series of in vitro experiments. The decrease in lens compressibility has been quantitatively documented, albeit in a limited number of subjects. Estimates of internal force in two subjects show that the force used to generate maximal accommodation also increases with age.
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