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H. A. Swarbrick, J. H. Yoon; Posterior Corneal Shape Changes in Overnight Orthokeratology. Invest. Ophthalmol. Vis. Sci. 2008;49(13):4847.
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To investigate posterior corneal shape changes in overnight orthokeratology (OK) over 14 nights wear of reverse geometry rigid gas-permeable (RGP) contact lenses.
Eighteen young adult subjects (19-32 yrs) with low myopia (<-4.00D) and astigmatism (<1.50D) were fitted with OK lenses (BE; Capricornia Contact Lens, Brisbane, Australia) in Boston XO material (nominal Dk/t 46). Lenses were worn overnight only for 14 days. Another group of 10 subjects (19-32 yrs) with low astigmatism (<1.50D) wore J-Contour conventional RGP lenses (Capricornia Contact Lens) in Boston XO material (nominal Dk/t 56) for one night. Corneal topographic data using the Medmont E300 topographer (Medmont, Camberwell, Australia), and total corneal thickness using the Holden-Payor optical pachometer across the horizontal meridian were measured at baseline and 8-10 hours after lens removal on Days 1, 4, 7 and 14 of overnight OK lens wear, and after one night of RGP lens wear. Posterior corneal apical radius of curvature Ro and asphericity Q were calculated using an in-house program based on the anterior corneal ellipsoid curve and corneal thickness. Repeated measures ANOVA with protected post hoc paired t-tests was used to compare changes from baseline with a critical p-value of 0.05.
Myopia reduced from -2.64 ± 0.99D (mean ± SD) to -0.39 ± 0.49D over 14 days of overnight OK lens wear. In the OK lens-wearing eyes, there were no statistically significant changes in posterior Ro (p>0.141) over the lens-wearing period. However there were significant increases in posterior Q on days 4 (p=0.007) and 7 (p=0.002). In the conventional RGP lens-wearing eyes, there were no significant changes of either posterior Ro or Q after overnight lens wear.
The results of this study demonstrate that overnight OK lens wear does not cause flattening of the central posterior corneal curvature, at least in the first two weeks of lens wear. This supports the current hypothesis that the OK refractive effect is achieved primarily through remodelling of the anterior corneal layers. Changes in posterior corneal asphericity towards an oblate shape implicates mid-peripheral corneal changes in the response to OK lens wear.
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