Point/Counterpoint: Laser Refractive Surgery Versus Orthokeratology (PART 2)

Orthokeratology (OK) procedures using flat-fitting rigid contact lenses to reduce or eliminate myopia have been in use for the past 50 years. A number of studies in the 1970s and 1980s demonstrated the safety of the procedure regarding the integrity of the cornea, but its effectiveness was limited and the procedure never gained popularity. The conventionally designed rigid contact lenses, when fitted flat, often decentered on the cornea, resulting in oblique residual astigmatism. With later developments in high-oxygen transmission rigid gas permeable (RGP) materials, new lathing technologies, and improved clinical instrumentation such as corneal topographers, interest in OK is resurging. Our center has been studying the procedures since the early 1990s as a part of our corneal topography investigations.

For many years, we have known that the new reverse geometry lens designs that incorporate a secondary curve steeper than the central back curve of a rigid gas permeable contact lens, and steeper than the corneal curvature on which it rests, will center better (Figure 1A) than the conventionally designed lens. The latter progressively flattens toward the periphery of the lens to match the progressively flattening corneal curvature (Figure 1B). The “reverse geometry” lens when centered about the corneal apex can flatten the central corneal curvature, and in turn will reduce or eliminate myopia. We have demonstrated that the cornea can be remodeled within hours and that the reversal is generally slower. For example, a cornea that is flattened by 1.0 D in 4 hours will take 8 hours to reverse.

The major drawback with OK has been that the lenses can only be used during waking hours. This requires patients to wear their OK lenses for 6 to 8 hours early in the day to achieve unaided visual acuity of 20/40 or better for the remainder of the day, offering very little advantage over conventional RGP lenses. It has been argued that if a patient is able to wear RGP lenses for 6 to 8 hours, then he or she may as well wear the lenses for the remainder of the day. However, if OK lenses could be worn overnight with minimal discomfort, and the cornea could be flattened during sleep to achieve acceptable (20/40) unaided visual acuity, then the myopic patient could very well have an exciting option for vision correction. This applies if the change can be retained for all waking hours.

There are currently four different reverse geometry RGP designs that are in the various stages of consideration by the FDA for overnight OK. Paragon CRT, CRT100, Quadra RG, and Quadra RG100 (Paragon Vision Science, Mesa, Arizona) received conditional approval (PMA P870024/S043) from the Ophthalmic Devices Advisory Panel in January 2002. BE (Precision Technologies Vancouver, Canada), Contex and the Dreim (DreimLens International, Alhambra, CA) lenses are currently in an FDA clinical trial. All these lenses attempt to flatten central corneal curvature during overnight wear. Although fitting differs for each lens type, in general, the lenses are 10 to 11 mm in diameter and have a central back radius that is flatter than the central corneal curvature. The secondary curve is steeper than the central back radius and the curvature of the midperipheral cornea. The fluorescein pattern of an acceptable lens fit is similar to that shown in Figure 1A. The lenses can be worn all night starting the first day of lens wear. The lens fit and corneal and refractive changes are closely monitored starting with the first follow-up on the morning after the first night of wear. Visual acuity must be evaluated later in the day to determine the stability of the procedure throughout the day.

In a recent overnight OK investigation (FDA IDE# G000059 and G000059/A001) using Contex lenses (Contex, Inc., Sherman Oaks, CA), we have demonstrated significant changes in refractive error, unaided visual acuity and corneal curvature following only 1 week of lens use. The data at 1 week were stable throughout the day with no significant differences noted in any of the three variables evaluated (Table 1). Eight of the 10 subjects were able to achieve and retain 0.0 LogMAR or 20/20 unaided visual acuity after overnight wear of reverse geometry OK lenses. The two unsuccessful subjects had pupil diameters greater than 7 mm in normal light. Over a 3-month follow-up, central diffuse punctate keratitis was noted in 50% of the cases at the morning visits, but the condition was completely resolved at the 4-hour visit. Larger diameter ?reverse geometry? lenses used in this investigation moved minimally, centered well on the cornea, and were reported by the subjects to be considerably more comfortable than the small movable conventional lenses used prior to dispensing of OK lenses

This controlled, yet temporary correction of mild-to- moderate refractive error (we only attempted up to 3.0 D) is an attractive option for myopes who do not wish to wear either a corrective device or undergo laser correction. In comparison to the permanence of laser refractive correction, a major advantage of overnight OK is that each patient has an option to evaluate the process without any long term or permanent damage to the cornea or visual acuity. Should the process prove unsuccessful, the patient can discontinue lens wear and allow his or her cornea to return to its original shape.

P. Sarita Soni, OD, MS is a Professor of Optometry and Vision Science at Indiana University, School of Optometry, and the Co-Director of the Borish Center for Ophthalmic Research in Bloomington, Indiana. She does not hold a financial interest in any of the products mentioned herein. Dr. Soni may be reached at 812-855-4703; sonip@indiana.edu. The author thanks Tracy Nguyen, OD, for her assistance.