Abstract
Purpose: :
To present new measurements on the stiffness of human lenses of a range of ages, obtained using an improved form of the spinning lens test devised by Fisher (1971).
Methods: :
Lens spinning tests were performed using the custom-built rig described in a previous ARVO abstract (Wilde, Judge and Burd 2008). Tests were conducted on fresh lenses supplied by the Bristol Eye Bank from donors aged 12 to 59 years. Lenses that had become swollen prior to testing were identified from their aspect-ratio, following Augusteyn (2008). Tests were conducted on lenses from which the capsule had been carefully removed immediately prior to testing. Measurements on the deformations induced in the lens were used (together with a hyperelastic finite element inverse analysis) to infer 2-parameter functional forms for the spatial variation of shear modulus in the lens. Two functional forms were investigated; (a) linear variation of shear modulus with distance from a point at the centre of the equatorial plane, and (b) a discrete nucleus and cortex.
Results: :
Spinning lens stiffness data are presented for lenses that were judged not to have been swollen at the time of testing. We compared our results with previous spinning lens data (Fisher 1971) and with data determined by others using indentation testing (Heys et al. 2004, Weeber et al. 2007). The four data sets all show that, for young lenses, the nucleus is less stiff than the cortex. As the lens ages, the nuclear stiffness increases more rapidly than the cortex stiffness until, at a ‘cross-over’ age, the stiffness of the lens becomes approximately uniform. Above this age, the nucleus becomes stiffer than the cortex. In quantitative terms, our data show a more marked increase in stiffness with age than the Fisher data. Our values of cortex stiffness, for young lenses, are consistently higher than those of Heys. Our spatial variations of stiffness show good agreement with the curves Weeber et al. fitted to their data.
Conclusions: :
Differences between our data and those of Fisher can be accounted for by the improved procedures adopted in the current study. The spinning test and indentation testing are fundamentally different experimental techniques. It is therefore noteworthy that our results are broadly consistent with previous indentation test data.
Keywords: presbyopia • accomodation • computational modeling