BY CRAIG W. NORMAN, FCLSA
Orthokeratology, ortho-k, accelerated orthokeratology (AOK), corneal refractive therapy, corneal reshaping. These buzzwords have many in the RGP contact lens field excited today. But it seems that many of these descriptions have been around for quite some time. Is this field that much different today? Are the lens designs that much improved? Are fitting philosophies and nomograms significantly more predictable than in the past? Over the next few months many of these questions will be addressed in this column. To begin, let's review how the field of orthokeratology has evolved.
In the 1960s, George Jessen first attempted to deliberately change refractive error using rigid contact lenses using a technique he named "Orthofocus." Names such as Ziff, May, Grant, Fontana, Tabb, Carter, Siviglia and Kerns are other early proponents of ortho-k fitting. Their work lead to the thinking that in controlled situations, the cornea could be manipulated in a planned manner with rigid lenses.
For more than two decades, orthokeratology did not gain widespread acceptance, mainly due to resistance from the scientific community that altering the central cornea would not be safe. But introducing corneal mapping provided a more scientific approach to employing this procedure.
Studies in the efficacy of this technique found that a reduction in myopia of up to about 1.50D could occur from three to 10 months after treatment with varied myopia reduction rates among patients. Most myopia reduction occurred during the first six months, but fitting progressively flatter lenses also led to an increase of with-the-rule corneal astigmatism by as much as 0.75D.
The reasons for early failures and a lack of acceptance of orthokeratology were obvious. Early ortho-k lenses were designed with peripheries flatter than the central base curvessimply fit flatter than "conventional" rigid lenses, which resulted in designs that were flat fitting, commonly de-centering up or down causing corneal distortion and increased astigmatism (Figure 1.1)
The second generation of ortho-k lenses addressed the problem of controlling and increasing the amount of myopic reduction. Stoyan and others developed and patented the first "reverse geometry" ortho-k designs. They utilized three distinct zones to effect a more controlled and profound flattening of the central cornea, while shortening the time in which myopia reduction could be achieved. These designs offered a vast improvement over the older system of fitting (Figure 1.2).
Figure 1. 1. Original ortho-k design, spherical lens fit progressively flatter than K;
2. 3-curve design, 3. Modern 4-curve lens.
This method of making four to five small incremental (flatter) base curve changes allowed better control of lens centration, but the three- to six-month treatment period to reach a maximum of 2.00D to 3.00D of myopic reduction could be cumbersome.
Today's four- or five-zone lenses are a significant improvement (Figure 1.3). They consist of a base curve, reverse (steeper) curve, fitting (alignment) curve(s) and peripheral curve. These parameters can be manipulated to reach an optimal fit, providing a more predictable result.
Studies are being performed with high Dk FSA materials in these designs for overnight wear. These fitting methods will be addressed in future columns.
www.clspectrum.com for information on the Global Orthokeratology Symposium in Toronto.
Craig W. Norman is Director of the Contact Lens Section at the South Bend Clinic in South Bend, Indiana. He is a fellow of the Contact Lens Society of America and an advisor to the RGP Lens
Contact Lens Spectrum, Issue: January 2002