Orthokeratology Overview
Roger L. Tabb,
O.D., F.A.A.O., F.I.O.S.
October 2000
An alternative to refractive surgery, the non-permanent ortho-k procedure involves wearing special design lenses.
People have attempted to correct refractive error for a long time. Hundreds of years ago, the Chinese laid sandbags on closed eyelids, creating pressure to flatten the cornea to improve unaided visual acuity. More recently, a device was invented and marketed in 1905 by a mail order catalogue, claiming to improve near- sightedness. It was a spring-loaded device which, when held against the closed eye and triggered, would produce a thump on the cornea, causing the cornea to flatten temporarily, supposedly improving unaided visual acuity (and probably causing a sharp spike in IOP). Great strides have been made since those times.
Modern Orthokeratology
Orthokeratology has existed since the 1960s. It is a non-invasive procedure in which contact lenses change the shape of the cornea to reduce refractive error. Since its advent, orthokeratology has experienced variable results. Some of the lens designs used initially produced unpredictable and fleeting changes. Opponents immediately claimed ortho-k was not permanent, predictable or effective. Most successful reports were anecdotal, and the scientific community understandably claimed to need more scientific studies. Until recently, industry, professional organizations and other professionals have offered little support. This increased interest has largely been due to the advent of well-designed reverse geometry lenses.
Proponents of orthokeratology will emphasize that the changes in refraction today are more predictable and long lasting due to improvements in lens designs. Lenses can effect change without weakening the cornea. The lack of permanency is actually an advantage. Ortho-k has a high safety record because it is almost always reversible. Patients such as pilots, police officers and fire fighters not desiring refractive surgery can now meet unaided visual acuity levels.
Ortho-k lenses fall into two categories: daywear and nightwear. Daywear consists of wearing a specially- designed lens (spherical, aspherical or complex reverse geometry) under daytime conditions to improve unaided visual acuity when the lenses are removed. This improved unaided vision will generally last fewer hours than that created by nightwear lenses. Daywear lenses have been available for 40 years and have been successful to varying degrees.
Nightwear involves wearing a reverse geometry lens while sleeping to allow unaided vision during waking hours. This procedure is not yet FDA approved. These specific designs can truly be referred to as a mold because they create a living visco-elastic mold for the cornea. I believe properly-designed nightwear orthokeratology lenses produce a more accurate, rapid, safe and lasting unaided visual acuity result than daywear lenses.
Daywear Ortho-k
Advantages of Daywear
- Patients wear the lenses during the day and enjoy good vision during wear. The lenses, when fitted properly, are comfortable and easy to wear.
- Daywear helps myopes who desire improved unaided visual acuity for a few hours, either for sports or to function around the house at night.
- This modality has the benefits of reversibility and safety. About 1.2 million people have worn ortho-k lenses over the past 40 years with an excellent safety track record.
- A low Dk material is safe for daywear lenses.
- RGPs have clinically demonstrated that they can stop and reverse myopia progression in some children. Studies are now in progress to verify this, such as the Singapore Study sponsored by Polymer Technology and The Ohio State University CLAMP (Contact Lenses and Myopia Progression) Study.
Disadvantages of Daywear
- Reducing refractive error with daywear lenses takes longer than the nightwear modality.
- These lenses require daily maintenance (wearing of the lenses during the day), which some patients label inconvenient.
- Daywear seldom provides functional unaided vision all day except with powers below 2.00D; therefore, the results may be less effective than nightwear.
- Comfort may be reduced as a result of foreign material under the lens in varied climatic conditions. Environmental conditions still have to be considered as with conventional RGP contact lenses.
- Patients may experience flare or ghosting without the lenses in place if the lenses frequently decenter during wear.
- Centering can be more difficult with daywear because the lens must be designed to move. With movement, the lens becomes less capable of true molding as a result of lost stability. This reduces the amount of corneal change, rate of change and holding time of change produced compared to nightwear.
- Resulting unaided visual acuity diminishes if wear of the lenses is stopped for any length of time.
Nightwear Ortho-k
Advantages of Nightwear
- This modality is more effective in myopes with up to 6.00D of myopia and 2.00D of astigmatism.
- Patients can function during the day without the inconveniences of contact lenses or spectacles.
- According to an unpublished study performed at Pacific University by Neil, Neil and Tabb, nightwear has an average holding time of 16.6 hours per day. Limits of the study were 4.00D with no more than 2.00D astigmatism.
- The lenses are worn only at night, primarily during sleep, eliminating environmental conditions such as dust, wind, sunlight and dry atmosphere.
- Comfort is high as the lenses are designed not to move, and patients are asleep during the majority of the wearing time. Many patients who have never been able to wear any type of contact lens are able to wear well-designed nightwear lenses.
- Practitioners can be easily trained to use this therapy, provided they use only the most efficient and effective nightwear designs.
- In my opinion, children may be fit with nightwear lenses with little fear of problems beyond normal contact lens wear.
- The process causes a rapid, safe response.
Disadvantages of Nightwear
- In some cases, corneal shape, lid structure or sleeping habits may not allow good centration. This can result in flare or ghosting if the lens doesn't center properly in relation to the pupil and optical axis, or if the patient has very large pupils.
- The lenses can stick because they are designed not to move. This can cause them to be more difficult to remove in the mornings. Such occurrences should not be viewed the same as adhesion in daytime wear because the resultant problems are dramatically different and can create different outcomes. Doctors must understand nighttime wear conditions in order to fit nightwear systems, which are different from daytime wear and extended wear conditions.
- Nightwear is a semi-static (designed not to move) system, so a higher Dk material is necessary. My opinion is that the Dk should not drop below 60 with the ISO-Fatt system.
- The lenses should be resurfaced every three months to keep the mold bed at its maximum. Any particles or aberrations of the lens or mold surface can lower the outcome.
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12-year old male student/athlete |
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40-year old female secretary/receptionist |
Dr. Tabb is in private group practice in Portland, Ore., and has been involved in ortho-k for the past 34 years. He is president of the consulting company Advanced Corneal Engineering and has a patent pending on his NightMove ortho-k lens design.