The Basis for Corneal Shape Change During Contact Lens Wear

Nancy Keir - BSc OD Centre for Contact Lens Research, University of Waterloo

Nancy Keir is currently a Research Associate at the Centre for Contact Lens Research at the University of Waterloo in Ontario, Canada, where she is responsible for conducting clinical research in the areas of contact lenses and refractive surgery. She graduated with honours in Optometry from the University of Waterloo and is currently working towards her PhD Degree in Vision Science on a part-time basis.

Carney, Leo G. Am J Optom & Physiol Optics. 1975;52: 445-454.

There have been numerous publications investigating the effect of contact lens wear on the cornea. Efforts to determine the relationship between changes in corneal curvature and changes in refraction following the use of hydrogel contact lenses are as important today as they were 30 years ago. Reports of induced myopia following the use of low-Dk hydrogel lenses during extended wear have been made (1-4), however it is still unclear as to the exact cause of this increase in refractive error. It’s also been shown that a reversal of this myopic shift can occur during extended wear when someone is switched from a low-Dk hydrogel lens to a high-Dk silicone hydrogel lens (3).

In the July 1975 issue of the American Journal of Optometry and Physiological Optics, Leo G. Carney discusses the basis for corneal shape change during contact lens wear and takes us back to the basics. This paper investigates the effects of hydrogel contact lenses and oxygen tension on the cornea using a very methodological approach.

This is a prospective study involving six non-contact lens wearers. The primary outcome variables are changes in corneal thickness and topography across different corneal meridians under varying levels of oxygen both with and without hydrogel lens wear. Corneal thickness was measured using a Haag-Streit pachometer attached to a slit lamp for both central and peripheral readings (up to 40 degrees both nasally and temporally). Corneal topography was measured using a Clark autocollimating photokeratoscope and an average of all semi-meridians was determined at various distances from the ophthalmometric axis. Each participant had corneal thickness and topographical measurements taken under different conditions. Gases of known concentration were passed through rubber goggles using a tubing connection on the side in order to vary the amount of oxygen tension at the anterior corneal surface. Zero, 100% and 50% oxygen tension mixtures of nitrogen and oxygen were used. These conditions were tested with and without methylmethacrylate lenses, fit either 0.2 mm steeper or 0.2 mm flatter than the flattest corneal meridian, and worn for two hours.

Results showed that both central and peripheral corneal thickness increased when the oxygen tension across the anterior cornea was less than 100%, but not when oxygen tension was 100%. With no contact lens in place and with a contact lens and 0% oxygen (100% nitrogen), this increase was uniformly distributed across the horizontal corneal meridian. When a contact lens was worn on its own or with 50% oxygen tension, the increase in corneal thickness was unevenly distributed over the horizontal meridian. This was regardless of lens fit and the greatest increase in thickness occurred centrally.

Corneal topography results reflect the changes found in corneal thickness. When a contact lens was worn for two hours, there was a marked steepening of the topography, regardless of fit. The addition of 50% oxygen tension caused less overall steepening, and resulted in little change from baseline for the flat-fitted lens and reduced steepening for the steep-fitted lens. This response was likely due to a greater increase in corneal thickness centrally, which was reduced by the addition of oxygen. Also, a negligible change in corneal topography occurred when gas of 0% oxygen tension was passed over the corneal surface both with and without a contact lens in place. Again this can be explained by the even change in corneal thickness across the horizontal meridian under the same conditions. Interestingly, however, 100% oxygen tension caused marked flattening of the cornea with the flat-fitted lens and marked steepening of the cornea with a steep-fitted lens. This cannot be explained by changes in corneal thickness and is likely due to a mechanical molding effect by the lens.

The author concludes that the changes in corneal topography result from two possible causes: an unevenly distributed change in corneal thickness and/or a mechanical molding effect by the lens.

If we specifically look at the effect when the oxygen tension across the cornea is 100%, there is little change in corneal thickness for both the flat and steep lens. Topographical changes still occur, however, with flattening for a flat-fitted lens and steepening for a steep-fitted lens.

With silicone hydrogel lenses, where the oxygen transmissibility to the cornea is far superior to a hydrogel lens, the lens-cornea fitting relationship might be more of a factor for refractive change, compared to changes in corneal thickness. This would mean that lens design and modulus of elasticity, etc could play a very important role. These are questions that still need to be determined and it might be that we need to go back to the basics to fully appreciate how these innovative lenses differ from hydrogels.

1. Jalbert I, Stretton S, Naduvilath T, Holden B, Keay L, Sweeney D. Changes in Myopia with Low-Dk Hydrogel and High-Dk Silicone Hydrogel Extended Wear. Optom Vis Sci 2004;81(8):591-596.
2. Fonn D, MacDonald KE, Richter D, Pritchard N. The Ocular Response to Extended Wear of a High-Dk Silicone Hydrogel Contact Lens. Clin Exp Optom 2002;85:3:176-182.
3. Dumbleton KA, Chalmers RL, Richter DB, Fonn D. Changes in Myopic Refractive Error with Nine Months’ Extended Wear of Hydrogel Lenses with High and Low Oxygen Permeability. Optom Vis Sci 1999;76:845-849.
4. Harris MG, Sarver MD, Polse KA. Corneal Curvature and Refractive Error Changes Associated with Wearing Hydrogel Contact Lenses. Am J Optom & Physiol Optics. 1975;52:313-319.