Expand Your Knowledge of Orthokeratology
Perfect your fitting skills with this information on
ortho-K lens designs and materials.
BY JOHN M. RINEHART, O.D., F.A.A.O., AND JAMES W. REEVES,
O.D., F.I.O.S., Peoria, Ariz., and Great Falls, Mont.
In part one of this article
2002), we explained how orthokeratology (ortho-K) can benefit your practice. In this concluding article, we'll tell you what you need to know about
ortho-K lens designs and materials because understanding the lenses is vital to applying your fitting skills correctly.
Reviewing the FDA's role
The main responsibility of the Food and Drug Administration (FDA) is to protect the public health. The FDA's concern is that legally marketed devices (such as reverse geometry lenses used in
ortho-K) are safe and effective for their labeled and intended use. It doesn't approve or regulate procedures or training. In fact, the FDA Notification of September 25, 1998 states, "A licensed practitioner may individually design and prescribe an RGP orthokeratology lens for a particular patient within the scope of his/her practice." Three companies presently have FDA approval for daily wear
ortho-K materials and/or lens designs and two other companies are awaiting approval. They are:
ILLUSTRATION BY ANTHONY
Contex Inc. has a Reverse Geometry OK Lens for
Polymer Technology Corporation is involved in an overnight FDA study involving 340 subjects regarding lens design and materials. Roughly 25% of the subjects are juveniles.
Paragon Vision Sciences
received FDA approval for over-night ortho-K using its CRT lens.
Correctech, Inc. and Euclid Systems Corp. are also involved in overnight FDA
ortho-K studies of lens designs and lens materials. Robert Breece, O.D., president, states,
"Correctech has completed the clinical investigation of its overnight wear
ortho-K lens and anticipates FDA marketing approval later this year." Correctech used Polymer Technology's Equalens II for its FDA overnight
Joann Simonsen, director of Professional Services at Euclid, reports that the company submitted its overnight
ortho-K study results and that the FDA is actively reviewing its PMA Application.
Many gas permeable lens laboratories can make reverse geometry lenses to your exact specifications. If you're interested, a list of labs is available on the Web page of the Rigid Gas Permeable Lens Institute
(RGPLI) at www.rgpli.org.
Learning about lens materials
The ideal material for an overnight
ortho-K lens is stable, wettable and has a Dk of at least 100 ISO/Fatt. A great resource for information on lens stability and wettability is your gas permeable lab. Assuming your lab finds a material that's easy to lathe and has minimal breakages, you can expect it to be stable for extended periods of time.
Ortho-K lenses come in such unique colors (e.g., pale yellow, pale red, violet or yellow) because Polymer Technology Corporation
(PTC), manufacturer of the Boston materials, entered into an agreement with several holders of
ortho-K lens patents.
These patent holders agreed not to bring patent infringement suits against
PTC, authorized Boston Manufacturers or each other. The agreement requires labs to use unique lens tints developed especially for
ortho-K lenses. Under the agreement, daily and overnight ortho-K lenses made in Boston XO and Equalens II must either use the unique red or yellow tint. A lab that makes a lens in a different material or color exposes itself to possible litigation.
According to Tim Koch of Paragon Vision Sciences, "Paragon has entered into licensing agreements with the known patent owners for reverse geometry contact lens designs for corneal reshaping. It's Paragon's plan to collect the royalties due to these patent owners from any lab that employs the inventions outlined in the licensed patents in the manufacture and/or sale of reverse geometry contact lenses for corneal shaping in the USA."
Figure 1: A cross-section representation of a typical reverse geometry lens.
Figure 1 shows a cross-section representation of a typical reverse geometry lens. (The area from the outer edge of the optical zone to the inside edge of the reverse curve is shown to create a deep tear layer. This is for ease of demonstration only.)
Each of the zones has a specific purpose and is designed to create a specific effect in the
ortho-K process. The zones are:
The base curve zone is used to create the forces necessary to cause corneal changes. Determine the base curve radius in one of two ways:
- The base curve radius is 0.50D to 0.75D flatter than the flattest corneal radius. This method varies the tear layer profile depending on the amount of desired myopia reduction. The greater the myopia reduction, the deeper the tear layer at the edge of the back optical zone.
- The base curve radius is determined so as to create a specific tear layer profile. This technique uses the same profile regardless of the amount of myopia you want to reduce.
The peripheral curve. This zone provides a tear reservoir and serves the same function as in a traditional gas permeable lens design.
The alignment curve or cone angle. This region of a reverse geometry lens functions just like the base curve of a traditional gas permeable design, controlling the centration and movement of the lens. If the reverse geometry lens moves excessively or centers high, the alignment curve is
steepened; conversely, if the lens fit appears too tight, then the alignment curve is flattened. These are the same changes that would be made to the base curve of a traditional (non-reverse geometry) lens design.
The reverse/relief curve. This is the steepest region of any reverse geometry lens for
ortho-K. Use this curve to connect the base curve to the alignment curve while maintaining the ideal relationship of these zones to the underlying cornea. Diagnostic lens fitting determines the optimum lens sagittal depth. In the final design process, adjust the reverse curve to create the exact sagittal depth that you determined during the diagnostic fitting.
The overall lens diameter of reverse geometry lenses used in
ortho-K ranges from 9.5 mm to 11.5 mm. Most frequently, the lens diameter will be 10.6 mm to 11.0 mm. These larger diameters create better centering lenses and a well-centered treatment zone.
|Figure 2: A smiley-face topographic map.
||Figure 3: An example of a central island
Fitting the ortho-K lens
ortho-K diagnostic fitting consists of these parts:
Evaluation of lens movement and centration with fluorescein pattern evaluation. This determines the lens that provides near-perfect centration with minimal movement. Once you determine this open-eye lens, use topography to verify and modify the fit to closed-eye night wear.
Evaluation of the resultant topography. The diagnostic fitting uses lenses of known sagittal depth to determine the sagittal depth of the cornea. In this step, perform corneal topography after one night of lens wear and compare to pretreatment topography using subtractive plots.
Protected by the FTC
The main responsibility of the Federal Trade Commission (FTC) is to protect consumers from unfair and deceptive acts or practices in advertising or promotion. Regarding
ortho-K, the FTC is concerned with false or unsubstantiated claims that may mislead consumers about the purchase or use of the lenses.
If topography shows a treatment zone that's decentered superiorly (called a "smiley face," Fig. 2), then you need to increase the sagittal depth of the lens. The presence of a central island indicates that you should reduce the sagittal depth (Fig. 3).
What you know will help you
Orthokeratologists should emphasize their skill, not the lens design. Patients aren't purchasing "special" lenses, but your service, expertise and skill. Every orthokeratologist should know all phases of this therapy. By having a full working knowledge of
ortho-K materials and design, you'll be able to either design your own lenses and/or best use an existing design to meet your patients' needs.
Dr. Rinehart is in private practice in Peoria, Ariz., and has been lecturing on orthokeratology since 1996. Dr. Reeves is in private practice in Great Falls, Mont., and has lectured on
ortho-K since 1999. Both doctors are graduates of Pacific University of Optometry and have been performing accelerated
ortho-K since 1995.
Optometric Management, Issue: October 2002