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A. Calossi, M. Romano, F. Romano, G. Ferraioli; Overnight Orthokeratology Flattening the Cornea Without Direct Compression of the Center . Invest. Ophthalmol. Vis. Sci. 2006;47(13):100.
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One of the concerns in orthokeratology is central epithelial thinning induced by a direct compression of the optical zone of the cornea. We think it is possible to produce central flattening, by affecting the periphery of the cornea, and we designed a lens geometry that would aid the displacement of peripheral epithelium towards the optical zone. Our biomechanical hypothesis is that the central flattening might be secondary to a mid–peripheral steepening, induced by a displacement of the epithelium that results from a proper compression in the peripheral alignment zone of this lens.
We fitted 28 eyes of 14 myopic patients (ranging from –1.00 to –4.25 D sph, and astigmatism up to 1.00 D) with an hexa–curve reverse geometry design that attempts to mold the periphery of the cornea with a minimum compression in the centre (hexafocon–A material). Assessment criteria included UCVA, BCVA, manifest refraction, ultrasound pachymetry, corneal topography, and biomicroscopy. These data were collected at baseline, and then after one night, one week, two weeks, one month, and three months of overnight lens wear. All the examinations were performed in the morning immediately after lens removal and repeated in the evening of the same day.
The cornea responded rapidly with significant (p<0.05) central corneal flattening and improvement in visual acuity after the first night of contact lens wear. By the end of one week, all corneal and visual changes had reached a maximal level and remained stable during the day. These changes were sustained at the following visits. After the first molding, the fluorescein pattern showed a clearance under the center of the lens that demonstrated a minimal central touch. Biomicroscopy showed no significant ocular adverse events.The average pre–treatment CCT was 533 ±31 µm. During the period of the study, ultrasound pachymetry did not show any significant change in the central thickness of the cornea (repeated measures ANOVA: p=0.978), both in the morning and in the evening (Bonferroni/Dunn post–hoc test: p>0.414).
The absence of change found in the central pachymetry suggests that this overnight contact lens design can successfully flatten the cornea without direct compression of the center. The absence of change in CCT during the day seems to exclude a masking effect due to edema. Contrary to our finding, the majority of previous studies reported that orthokeratology caused epithelial and total central corneal thinning. This difference could be caused by differing lens geometry and differing lens behavior during overnight.
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