Purpose. To achieve an optimal fit with reverse geometry Contex OK lenses and to determine a time course for and the stability of visual and corneal changes in achieving maximal refractive, corneal curvature, and corneal thickness changes after overnight wear of OK B and D series lenses.
Methods. This investigation was conducted under an Food and Drug Administration IDE G000059. Both eyes of 10 subjects were fitted with the lenses, and uncorrected visual acuity, refractive correction, contrast sensitivity, corneal curvature, and corneal thickness were measured at baseline and at 1 day, 1week, 1 month, and 3 months after lenses were worn. Except for baseline, data were collected at four different times during the day, immediately following lens removal and 4, 8, and 12 hours after lens removal.
Results. The results from eight subjects showed that uncorrected visual acuity, refractive correction, contrast sensitivity, and corneal curvature all changed significantly (P =0.01) overnight. By the end of 1 week, all corneal and visual changes had reached a maximal level and remained fairly stable during the day. These changes were sustained at 3 months. The epithelial thickness data from four subjects showed that the corneal epithelial thickness was reduced by approximately 19 μm after 3 months of lens wear.
Conclusions. Successful fitting of OK B and D series lenses requires a thorough understanding of the lens–cornea relationship.
Full effect of overnight orthokeratology is achieved by the end of 1 week. The visual and corneal changes remain stable for all waking hours of the day and allow patients to enjoy excellent device-free vision (20/20).
Orthokeratology procedures using flat-fitting, hard (polymethylmethacrylate [PMMA]) contact lenses of conventional design to reduce or eliminate myopia was first introduced in the 1960s. 1–5 A number of studies in the 1970s and 1980s showed the safety of the procedure regarding the integrity of the cornea, but its effectiveness was limited and the procedure never gained much of a following. 6–17 Emerging technologies of the 1980s permitted contact lens practitioners and manufacturers to take a fresh look at the orthokeratology procedure. The new technologies included gas-permeable contact lens materials that would allow the possibility of overnight orthokeratology, improvements in lathe technology that presented possibilities to experiment with lens designs, and new corneal topography instruments that allowed investigators to directly observe effects of lenses on the entire corneal surface.
After considerable research and development, Contex, Inc. (Sherman Oaks, CA) designed and manufactured a series of reverse geometry orthokeratology contact lenses (OK lenses) that received Food and Drug Administration (FDA) approval in 1998 for daily-wear use. Whereas conventionally designed contact lenses use progressively flattening radii of curvature on the back surface, a reverse geometry lens uses a central base curve that has a flatter radius of curvature than the central corneal curvature, and the radius of curvature of the secondary curve is 3 to 17 diopters (D) steeper than the radius of curvature of the central base curve. The third curve beyond the secondary curve, referred to as the alignment zone closely matches the mid-peripheral cornea. The fourth and final curve has a radius of curvature that is flatter than all other curves and is referred to as the lens edge clearance zone. The function of the central base curve is to affect central curvature flattening. The secondary and the alignment curves provide stability for the central flat curve to be held in place. The presence of the steep secondary curve is also believed to enhance the central corneal flattening by allowing central epithelium to distribute to the midperiphery of the cornea. The edge clearance zone acts as a tear reservoir providing a ready exchange of oxygenated tears between the lens and cornea.
With the introduction of the reverse geometry design in the late 1980s, off-label use of gas-permeable contact lenses for the reduction of myopia started to emerge. A number of investigators 18–31 reported accelerated reduction in myopia after the use of reverse geometry lenses worn during the day or overnight. However, use of the lenses during the day was not considered to be an attractive option because patients still required days to weeks to adapt to the rigid lenses and the nonlens wearing time was limited to 8 to 10 hours per day. Other reports on the transient nature of the change outlining time course of corneal and refractive change followed by time taken for recovery suggested that the cornea could be flattened quite rapidly. 32 These pilot projects provided an insight into the possibility of wearing lenses overnight for 8 hours followed by no lens wear during the day.
In 1997 Mountford 25 provided retrospective data on 60 patients who had worn orthokeratology lenses (Contex OK series) overnight until corneal and refractive changes were documented to be stable over a minimum of 2 months. He reported that the overnight changes were sustained during the following 8 hours of no lens wear. Nichols et al. 26 confirmed Mountford’s results with regards to the predictability of refractive and corneal changes after overnight wear of reverse geometry lenses and went on to provide evidence that corneal and refractive changes after 1 week of lens wear remained fairly stable with minimal regression during the 8 hours of no lens wear. Whereas 8-hour stability data show promise, there is a small but steady decay over time in unaided visual acuity and autorefraction. Thus, with an additional 4 to 6 hours of waking time available, it is important to study the recovery data to at least 12 hours after lens removal.
In addition, Swarbrick et al. 29 and Nichols et al. 26 reported a reduction in central corneal thickness after the use of reverse geometry lenses. Using an optical pachometer, Swarbrick et al. showed a statistically significant thinning of the central corneal epithelium (7.8 ± 6.4 μm, 9.6%) after 28 days of treatment with lenses worn for a minimum of 6 hours per day during waking hours. The central cornea (total) was found to be thinner by 2.4 ± 8.0 μm (0.46%), but this thinning was found not to be statistically significant. Nicholas et al. 26 reported a statistically significant thinning of the central cornea of 2.6% as measured with an Orbscan after 60 nights of overnight lens wear. They did not report epithelial thickness. The thinning of the central cornea led Swarbrick et al. and Nicholas et al. to suggest that the corneal and refractive changes that accompany orthokeratology may be explained as a redistribution of the corneal tissue rather than the overall bending of the cornea. Nicholas et al. provided important insight into overnight orthokeratology, but the limited observation to 8 hours after lens removal leaves 6 to 8 hours unaccounted for. In addition, the difference in corneal thinning presented by Swarbrick et al. and Nichols et al. must be resolved. Furthermore, both reports provided little information on lens fitting. Therefore, we designed an investigation with the purpose of achieving optimal fit with reverse geometry Contex OK lenses, determining a time course over 3 months to achieving maximum refractive, corneal curvature, and thickness changes after overnight wear of Contex OK lenses, and determining the stability of these changes over the course of 12 hours of no lens wear.