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S.R. Evans, A. Ho, J.D. Choo; Orthokeratology–Like Effects of Everted Soft Contact Lenses: A Mechanical Model . Invest. Ophthalmol. Vis. Sci. 2005;46(13):2059.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Recent work has shown that soft contact lenses (SCLs) can produce corneal reshaping when worn everted (inside–out). Orthokeratology traditionally relies on rigid contact lenses to reshape the cornea. The purpose of this study is to examine the mechanisms by which everted SCLs produce similar effects. Methods: Finite–element analysis (FEA) was used to simulate the eversion of various SCLs, and their on–eye pressure profiles. The model uses 500–1500 axisymmetric planar elastic elements, depending on lens shape. The lenses were a simple generic design in a range of powers (–30, –20, –10, –6, plano and +6 dioptres) and elastic moduli (0.2, 1.0 and 2.0 MPa). First, the everted lenses are compared off–eye, since the internal stresses produced by eversion may be important on–eye. Secondly, each lens is pressed against a rigid cornea by imposing a uniform constant ‘eyelid pressure’ of 200 Pa, in order to derive an estimate for the on–eye pressure profile. The changes in internal stress produced by pressing the everted lens onto the eye are analysed, and the on–eye pressure profiles are compared to those of right–side–in lenses. Results: On eye, all high minus lenses showed a low–pressure area near the optic zone margin, and high pressures (>800 Pa) in the mid–periphery. In the central zone, pressure is nearly independent of power for minus–powered lenses (around 300 Pa), while the +6D lens showed central pressure of about 550 Pa. The pressure pattern compares favourably with images obtained clinically using Fluoresoft 0.35%. The everted pressure profiles are in dramatic contrast to those of non–everted lenses. The effect of lens modulus is to adjust the position and magnitude of peaks in the pressure profile for a given power. For example, a –10D lens with a modulus of 1.0 MPa gives a central treatment zone of 3 mm, while a modulus of 2.0 MPa gives a central treatment zone of 5.5 mm. The difference between the off–eye (equilibrium) and on–eye (non–equilibrium) stress states of everted lenses show an alternating pattern of ‘push–down’ and ‘spring–off’ forces which modify the pressure profile. On–eye performance of an everted lens is thus different to that of an identically shaped lens without prestressing. Conclusions: Both geometry and stress preload play a role in the on–eye performance of everted SCLs. The position of pressure peaks and stand–off zones compare well with clinical results. FEA provides a useful tool for the analysis of everted lenses, whose shape and preload states would be difficult to determine otherwise.
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