contact lens case reports
Ortho-k = Stable Unaided Acuity
BY PATRICK J. CAROLINE, FAAO, & MARK P. ANDRÉ, FAAO
Orthokeratology temporarily reduces or eliminates myopia and astigmatism through the use of specially designed rigid lenses. Today, orthokeratology is accomplished with a single or series of lenses, which flatten the central radius of the cornea thereby reducing myopia and improving unaided visual acuity. Our current understanding of orthokeratology indicates that the flatter central fitting relationship results in a thin layer sheer force beneath the lens. The flat lens-to-cornea fitting relationship results in a redistribution and/or compression of the corneal tissue, resulting in a flattening of the central cornea with some degree of mid-peripheral steepening.
The topographical changes created by ortho-k lenses result from the unique configuration of four curves on the posterior lens surface, base curve, steeper reverse curve, alignment or fitting curve and peripheral curve. This shape is commonly referred to as a reverse geometry lens design. The radii and widths of each curve can vary, and the various radii can be spherical, aspherical or in some cases, a tangent (Figure 1).
Figure 1. Anatomy of a modern four curve lens design
Ortho-k in Practice
Patient E.L. was a 28-year-old male with a longstanding history of congenital unilateral myopia. His spectacle Rx was OD 3.75 sphere 20/20 and OS 0.25 sphere 20/20. We suggested ortho-k for his right eye, and he enthusiastically agreed.
Upon dispensing, the patient's lens exhibited the optimum fitting criteria of:
- Centered position
- 1 to 4 mm of central bearing.
- Fluorescein pooling in the reverse curve.
- 1 to 1.5 mm mid-peripheral alignment in the area of the alignment/fitting curve.
- 360 degrees of peripheral lens clearance
- 1.0 mm of blink-induced lens movement (Figure 2)
Figure 2. Patient E. L.'s well centered orthokeratology lens
The goal of modern ortho-k is to provide the patient with lenses that can be worn overnight to create the desired refractive change, providing clear unaided vision during the day. Our patient elected to pursue (non-FDA approved) overnight ortho-k. Within seven days his uncorrected visual acuity was 20/20, and he enjoys that level of stable uncorrected acuity for 14 hours a day (Figure 3).
Figure 3. Patient's pre and post fitting corneal topography
Modern ortho-k has dramatically changed the contemporary practitioner's view of the technique. It has evolved from a specialty practiced by only a handful of clinicians to a technique that can be successfully incorporated into almost any practice.
Patrick Caroline is an associate professor of optometry at Pacific University and an assistant professor of ophthalmology at the Oregon Health Sciences University.
Mark André is director of contact lens services at the Oregon Health Sciences University.