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G. S. Wilde, S. J. Judge, H. J. Burd; Measurement of Lens Stiffness Using a Spinning Lens Test. Invest. Ophthalmol. Vis. Sci. 2008;49(13):3785. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
To collect new data on the stiffness of the human lens using an improved form of the spinning lens test originally developed by Fisher (1971), and to investigate the causes of differences between Fisher’s original stiffness measurements and the results of more recent tests using indentation methods (Heys 2004; Weeber 2007).
Lens spinning tests were performed using a custom-built rig. Digital images of the lens were acquired with a 6 megapixel DSLR camera. Exposure was controlled by a short duration flash triggered electronically to capture eight equally-spaced angular orientations. Initially, each lens was tested with the capsule intact. Subsequently, the capsule was removed and the test was repeated. Tests were conducted at rotational speeds of 700 and 1000rpm. Reference images of the lens, rotating slowly, were captured before and after each test. A hyperelastic finite element inverse analysis was used to compute values of shear modulus for each lens. Each inverse analysis was based on an axisymmetric mesh generated on the average outline obtained from the images of the slowly rotating lens. Several models for the spatial variation of shear modulus within the lens were investigated.
The equatorial displacements of encapsulated lenses spun at 1000rpm declined with age from around 0.07mm in the third decade of life to less than 0.01mm by the seventh; the corresponding axial thickness changes were approximately 0.24mm and 0.04mm. Deformations in the decapsulated lenses were systematically greater than those in the encapsulated lenses. The lenses exhibited irreversible deformations after spinning at 700 and 1000rpm; these were more pronounced in the decapsulated lenses. A typical result from the inverse analysis for a 23 year-old decapsulated lens, assuming material homogeneity, suggested a shear modulus of 0.3kPa.
Equatorial and polar displacements measured for encapsulated lenses are broadly consistent with those reported by Fisher (1971). On the other hand, initial results indicate that the presence of the capsule has an important effect on the deformations generated in the spinning lens test, in contrast to Fisher’s assertion that the capsule has only a minor effect.
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