BIOMECHANICAL ALTERATION OF CORNEAL MORPHOLOGY AFTER CORNEAL REFRACTIVE THERAPY
<strong>Purpose:</strong> Although orthokeratology (non-surgical corneal reshaping, Corneal Refractive Therapy, CRT®) has been used for almost a half century, contemporary CRT's outcomes and mechanisms still require investigation. A series of studies was designed to examine different aspects of non-surgical corneal reshaping for myopic and hyperopic corrections, including the efficacy and stability of this procedure, the effect of the lens material characteristics (Dk/t), and the corneal or superficial structural change (e. g. corneal/epithelial thickness) in corneal reshaping. <br /> <strong>Methods:</strong> In the CRT® for myopia (CRT1) study, 20 myopes wore CRT® lenses on one eye and control lenses on the contralateral eye (eye randomized) for one night while sleeping. Corneal topography and refractive error were measured the night prior to lens insertion, immediately after lens removal on the following morning and at 20 and 60 minutes and 3, 6 and 12 hours later. In the CRT® for hyperopia (CRTH) study, 20 ametropes wore CRT®H lenses on one eye for one night while sleeping, the contralateral eye (no lens wear) served as control (eye randomized). Corneal topography, aberrations and refractive error were measured the night prior to lens insertion, immediately after lens removal on the following morning and at 1 and 3, 6, 12 and 28 hours later. In the relatively long term (4 weeks) CRT® for myopia (CRT2) study, 23 myopes wore CRT® lenses overnight and removed their lenses on awakening. Visual Acuity (VA), subjective vision, refractive error, aberrations, and corneal topography were measured at baseline, immediately after lens removal on the first day and 14 hours later, and these measurements were repeated on days 4, 10, and 28. The treatment zone size was demarcated by the change in corneal curvature from negative to positive and vice versa, using tangential difference maps from the corneal topographer. In the study of effects of Dk/t on CRT® for myopia (CRTHDK), 20 myopic subjects were fit with Menicon Z (MZ) lenses (Dk/t=90. 6, Paragon CRT®) on one eye and an Equalens II (EII) CRT® lenses (Dk/t=47. 2) on the contralateral eye (eye randomized). Corneal topography, refractive error and aberrations were measured before lens insertion (baseline), and the following day after overnight lens wear, on lens removal and 1, 3, 6, 12 hours later. In the study of short term effects of CRT® for myopia and hyperopia (STOK), 20 ametropes wore CRT® and CRT®H lenses in a random order on one eye (randomly selected). The lenses were worn for 15, 30 and 60 minutes (randomly ordered, with each period taking place on a different day). Refractive error, aberrations, corneal topography, and corneal/epithelial thickness (using OCT) were measured before and after lens wear. The measurements were performed on the control eyes at 60 minutes only. <br /> <strong>Results:</strong> In the CRT1 study, after one night of CRT® for myopia, the central cornea flattened and the mid-periphery steepened, and myopia reduced. In the CRTH study, after one night of CRT® for hyperopia, the central cornea steepened and the para-central region flattened, myopia was induced or hyperopia was reduced, all aberrations except for the astigmatism increased and signed spherical aberration (SA) shifted from positive to negative. In the CRT2 study, after 4 weeks of CRT® lens wear, in general, the treatment zones stabilized by day 10, vision improved, myopia diminished, total aberration and defocus decreased and higher order aberrations (HOAs) including coma and SA increased. The visual, optical and subjective parameters became stable by day 10. In the CRTHDK study, after one night of CRT® (MZ vs. EII) lens wear, the central corneal curvature and aberration were similar with a slight exception: The mid-peripheral corneal steepening was greater in the EII (lower Dk/t) lens-wearing eyes compared to the MZ (higher Dk/t) eyes. In the STOK study, after brief CRT® and CRT®H lens wear, significant changes occurred from the 15 minutes time point: The corneal shape and optical performance changed in a predictable way; the central cornea swelled less than the mid-periphery after CRT® lens wear, whereas the central cornea swelled more than the para-central region after CRT®H lens wear; the central epithelium was thinner than the mid-periphery after CRT® lens wear and was thicker than the para-central region after CRT®H lens wear. <br /> <strong>Conclusion:</strong> After one night of lens wear, CRT® and CRTH® lenses were effective for myopia and hyperopia correction, respectively. In the 4 week CRT study, the treatment zone size changed during the first 10 days. Its size was associated with VA, refractive error, aberrations, and subjective vision. In the CRTHDK study, after one night of lens wear, changes in corneal shape were slightly different, with more mid-peripheral steepening in the lower Dk lens-wearing eyes compared to the higher Dk lens-wearing eyes. Changes in central corneal shape and optical performance were similar in both eyes. In the STOK study, CRT® lenses for myopia and hyperopia induced significant structural and optical changes in as little as 15 minutes. The cornea, particularly the epithelium, is remarkably moldable, with very rapid steepening and flattening possible in a small amount of time.
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