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Technical Briefs

Finite Element Modeling of Inverted (Inside Out) Soft Contact Lenses

[+] Author and Article Information
Fabian Conrad, Klaus Ehrmann

 Brien Holden Vision Institute, Sydney, Australia; Vision Cooperative Research Centre, Sydney, Australia; School of Optometry and Vision Science, UNSW, Sydney, NSW 2052 Australia

Jennifer D. Choo

 Brien Holden Vision Institute, Sydney, NSW 2052 Australia

Brien A. Holden

 Brien Holden Vision Institute, Sydney, NSW 2052 Australia; Vision Cooperative Research Centre, Sydney, Australia; School of Optometry and Vision Science, UNSW, Sydney, NSW 2052 Australia

J. Med. Devices 4(2), 024501 (Aug 04, 2010) (6 pages) doi:10.1115/1.4001519 History: Received September 13, 2009; Revised March 14, 2010; Published August 04, 2010; Online August 04, 2010

Soft contact lenses (SCLs) can be inserted inside out with consequences for optical, mechanical, and on-eye comfort performance. Wearing lenses inside out may also cause corneal deformation especially with silicone hydrogel lenses. Since inside out insertion of SCLs cannot always be avoided, it is important to study their effects, and it may even be feasible to use these inside out forces to reshape the cornea. To study these possible scenarios, a finite element (FE) based model capable of simulating the inversion of soft contact lenses was developed and validated by comparing modeled results with laboratory measurements of lenses in right side and inside out conformations. In this study, the front surface contour of five SCLs (four commercially available and one custom design) was determined using a profile projector. The lenses were turned inside out, and the front surface contour was remeasured. The thickness profile obtained by a profilometer was “added” to the front surface shape in both orientations to derive the back surface shape. A detailed nonlinear 2D axisymmetric FE model of each lens in its right side in state was created, and the lens was inverted by applying a rigid probe. The modeled and measured inverted lens shapes were compared with respect to parameter alterations (sagittal depth (sag) and diameter) and overall geometry changes using a Procrustes analysis. Measured and modeled results revealed very substantial geometry changes when turning the lens inside out; however, the maximum sagittal deviation between measured and modeled inside out lens shapes was less than 0.05 mm over the central 6 mm half chord. Overall, the modeled results matched the inverted geometries for both parameter changes as well as overall shape changes. The developed FE model is able to predict the geometry of soft contact lenses when they are inverted.

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Copyright © 2010 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Inverted BF lens using 564 (left) and 1176 (right) quadrilateral elements. Contour bands depict equivalent von Mises stress (MPa).

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Figure 2

Initial model setup. Boundary conditions preventing the lens edge from moving along the x-axis (a), the symmetry condition (b) and the imposed displacement on the probe (c) are shown as arrows.

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Figure 3

Incremental application of displacement in the global x direction via a rigid probe leading to eversion of the BF lens. Image 0: initial position (increment 0), no contact between lens and probe; images 1–6: probe displacement in increments of 1 mm (1–6 mm displacement); note that separation occurred in image 6; image 7: lens settled in its final position with probe completely removed. Contour maps depict equivalent von Mises stress (MPa).

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Figure 4

Custom lens nominal and measured geometries. Sagittal depth for front and back surfaces (left y axis) and sagittal difference of the midsurfaces (right y axis).

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Figure 5

Sag difference when lens is turned inside out (measured)

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Figure 6

Sag difference when lens is turned inside out (modeled)

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Figure 7

BF lens geometry in RSI and ISO states (measured)

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Figure 8

Equivalent of elastic strain in the inverted lens. For demonstration purposes, contours are drawn on the undeformed shape.

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Figure 9

Equivalent von Mises stress (MPa) in the inverted lens; contours are drawn on the inverted shape

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Figure 10

Difference between modeled and measured ISO lens sag and CR range of measurement. Deviations within the CR range are random and may be caused by a measurement error.

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