Is Orthokeratology OK in the New Millennium? Part II
BY ED BENNETT, OD, MSED
Many practitioners need to know if orthokeratology really is a viable alternative to refractive surgery, or if it's a brief shooting star in the contact lens universe. This column, along with Part I, helps answer this question.
How do you fit the current generation of lenses?
Orthokeratology lens designs have changed often in recent years, resulting in some confusion to the novice practitioner interested in fitting these lenses. Over 10 years ago reverse geometry lenses were introduced, with a secondary curve 2.00D to 4.00D steeper than the base curve, accompanied by a much flatter peripheral curve. The reverse curve allowed for more effective redistribution of corneal tissue while allowing the desired refractive error reduction to occur at an accelerated rate. More recent designs, however, have four zones:
- Base curve radius (fit as much as 4.00D flatter than K)
- Reverse curve (often 6.00D to 12.00D steeper than the base curve)
- Wider alignment or anchor curve (fit close to on K)
- Flat peripheral curve
These designs create a reduction in refractive error rapidly accompanied by large changes in corneal topography which stabilize over time. With the four-zone lens, it is possible to obtain the desired endpoint without the need for a series of lenses. These lenses are typically large in diameter, approximately 10mm, with a small optical zone, approximately 6mm. Characteristics of a good fitting relationship would include good centration, lens lag of no greater than 1mm with the blink and a fluorescein pattern with a broad area of central bearing (approximately 5mm), a narrow dense surrounding area of pooling, a wide alignment curve bearing area and moderate edge clearance. Diagnostic lens fitting is essential for a successful fit, and practitioners must understand the lens design, fitting and, most importantly, what changes to make to achieve the desired lens-to-cornea fitting relationship. As these lens designs become FDA approved, manufacturers must educate interested practitioners.
Four-zone lenses replace the need for a series of lenses to reach endpoint.
What does the future hold for orthokeratology?
Orthokeratology is a more viable option today than in the past. Improvements in lens designs have been supplemented by sophisticated instrumentation for monitoring corneal topography change and improvements in the ability to predict individual change. Multicenter research studies currently in progress should ultimately provide more answers to how practitioners should approach orthokeratology. In addition, Corneaplasty, where an intrastromal injection of an enzyme makes the cornea more pliable, has great potential to bring orthokeratology to a more permanent endpoint. Orthokeratology will grow in popularity and success as lens designs become FDA approved, night wear becomes standard protocol, practitioners become more educated on fitting and troubleshooting these designs and patients become more aware of this as a non-invasive, nonpermanent means of myopia reduction.
Acknowledgement: I would like to thank Drs. Cary Herzberg, John Rinehart, Roger Tabb, and John Mountford for their contributions to this column.
Dr. Bennett is an associate professor of optometry at the University of Missouri-St. Louis College of Optometry and executive director of the RGP Lens Institute.