CORNEAL RESHAPING 101
Corneal Reshaping Made Easy
This technology is gaining popularity -- do you know how to use it to its full potential?
By Kathryn McCampbell, Edward S. Bennett, OD, MSEd, & Marjorie J. Rah, OD, PhD
Corneal reshaping (orthokeratology, corneal refractive therapy) temporarily reduces myopic and astigmatic refractive errors by using a contact lens to reshape the cornea. Becoming familiar with corneal reshaping has required a learning curve, but has recently been simplified so that any practitioner wanting a modality that can potentially improve the quality of life for patients -- young to more mature -- has the opportunity.
As this article will emphasize, the introduction of state-of-the-art and dynamic educational resources has made the simplification of this process possible. In addition, as the CRT (corneal refractive therapy) lens was initially introduced with the use of new and potentially confusing terminology such as landing zone and return zone depth, often made more complicated during the certification seminar, the fitting process has become streamlined, certification more straightforward and understandable, and the end result has been that CRT has been a profitable modality for a large and growing number of U.S. practitioners.
Picking Suitable Subjects
The best candidates for CRT are patients who have parameters that fit the following basic criteria:
- Their flat keratometry readings should range between 42.00D and 45.00D
- They should have a spherical refractive error of 4.00D or less, a cylindrical refractive error of 1.00D or less and their sphere-to-cylinder ratio should be 2:1
- Candidates should have adequate tear quantity and quality
However, if patients don't meet the above criteria because of greater spherical or cylindrical refractive error, then we can still fit them with CRT lenses as long as they're aware that they may not attain full treatment, will most likely need a series of different lenses and will require more follow-up visits than a patient who has less refractive error. All candidates should appear highly motivated and educated about the CRT process.
Figure 1. Paragon says its Initial Lens Selector makes fitting easy.
Open Lines of Communication
Patient communication is essential to the success of CRT and the most important aspect of communication is helping the patient set realistic expectations. Obtain a contact lens history, along with an approximate age and refractive status for new patients. Inform new or established patients who currently wear GP lenses that they must discontinue lens wear for a minimum of three weeks before they return for a CRT evaluation. Also inform patients about the cost of CRT and the follow-up schedule that you recommend. Patients must realize that the goal of CRT is to provide functional vision for most daily tasks and that, depending on the situation, you can't ensure 20/20 unaided vision although this is your goal. Remember to explain that the first two weeks of CRT are the most challenging.
The process usually takes about 10 nights of sleeping in the lenses to achieve full treatment and 28 nights before the treatment will last through full waking hours. During this time, the patient's vision may fluctuate and he may need to wear the CRT lenses, soft contact lenses or spectacles during the day, especially for distance tasks. Make patients aware that they may need a pair of spectacles for driving at night, reading the computer or performing near work, even after they have reached full treatment. Let patients know (and remind them) that to maintain the effect of CRT, they must continue to wear the lenses.
CRT Fitting Pearls
The fitting process for the CRT lens is much easier when you use the 100-lens diagnostic dispensing set because it provides a variety of lens parameters with which to fit a patient. Paragon Vision Sciences' Initial Lens Selector (Figure 1) uses the patient's flat keratometry reading and manifest refraction sphere to select the initial lens. To use the Initial Lens Selector, slide the tab on it until the arrow points at the correct flat keratometry reading. You can then locate the manifest refraction sphere in one of the boxes to the right and the initial diagnostic lens parameters will appear in the window next to the manifest refraction sphere. Repeat the process for the other eye and retrieve the corresponding lenses from the diagnostic dispensing system and place on the patient's eyes. After approximately one minute, apply fluorescein and assess the lens-to-cornea fitting relationship.
Centration is the most important variable for successful CRT. The other variables are important, but it's okay if they're less-than-ideal as long as the lens is centering on the pupil. If the lens doesn't center, then the CRT process won't be successful. It's important to ensure that there's a deep enough return zone depth for proper lens centration. Adequate edge lift is also important and a too-large landing zone area reduces the likelihood of full treatment. A lens that fits properly will show a bulls-eye pattern, centered lens position, 3mm to 4mm treatment zone, mid-peripheral tangential tear film touch, and acceptable edge lift.
Once you've evaluated the lens-to-cornea fitting relationship under the biomicroscope and you've made adjustments to achieve the proper fit, perform a sphero-cylindrical over-refraction. The over-refraction should range between plano and +0.50D. Next, dispense the lenses, educate the patient on lens insertion, removal and care techniques, and instruct him to insert his lenses around 10 minutes before bedtime. Finally, remind the patient to return to your office the next morning wearing his CRT lenses.
The recommended follow-up schedule includes visits at one day, seven to 14 days, one month and three months. The 24-hour visit is the most important visit. It allows you to evaluate the patient's reaction to the therapy and make any changes, if indicated, to enhance centration and power.
At the first visit, as mentioned above, have the patient visit the office wearing his CRT lenses. Assess visual acuity, perform an over-refraction (values should still range between plano and +0.50D) and confirm centration using the biomicroscope. Next, have the patient remove his lenses so you can perform visual acuity, a manifest refraction, biomicroscopy and corneal topography.
After you've completed the assessments, make any necessary centration and base curve adjustments. Finally, discuss the recommended follow-up schedule with the patient, informing him that it's not necessary to wear the CRT lenses upon arrival for his follow-up examinations; however, he needs to bring the lenses to each visit. At the remaining follow-up visits, perform the aforementioned procedures.
Figure 2. Certified fitters of Paragon CRT can use the Quick Start Reference card.
Make and keep detailed notes regarding the observed centration and fluorescein patterns on the day of the initial fitting visit. This information is essential for evaluation and comparison during the treatment and maintenance process (and these data are difficult to collect at a later date). During follow-up visits, if topography shows lens decentration, withdraw the treatment until the corneas return to baseline and you can refit the patient. Lens changes are common when treating patients who have pretreatment refractive error of more than 3.50D. Commonly, after two to three weeks of wearing the lenses, the patient will experience loss of treatment effectiveness or a need for increased cylindrical power.
Lens changes may be necessary throughout the treatment and maintenance process. Common changes include modification of the base curve and the landing zone area.
If the patient exhibits under or over treatment, the problem may be because of use of an inaccurate treatment base curve, existence of residual astigmatism or excessive sagittal depth. Inaccurate base curve calculation may be because of inaccurate manifest refraction sphere, inaccurate measurement of the central corneal curvature or a combination of both. Excessive sagittal lens depth causes insufficient applanation of the central cornea and may create a small or nonexistent corneal treatment zone. This causes inadequate change in refractive error, longer treatment duration, variable vision and central islands on topography.
To treat under or overcorrection, perform an over-refraction. If the over-refraction isn't between plano and +0.50D, then the base curve of the treatment lens is incorrect. If the over-refraction indicates that the patient is under-corrected and he requires more minus power, then flatten the base curve by 0.10mm for every 0.50D. If the over-refraction indicates that the patient is overcorrected and that he needs more plus power, then steepen the base curve by 0.10mm for every +0.50D. And if the over-refraction is between plano and +0.50D, then this treatment base curve is accurate and the likely cause of under treatment is excessive sagittal depth.
To reduce the sagittal depth, reduce the landing zone area by one degree. If this doesn't improve the vision, then return the landing zone area to its original value and then decrease the return zone depth in 25-µm steps until you achieve centration. When making changes to the landing zone area, monitor edge lift and remember: As the landing zone angle decreases, edge lift increases and vice versa.
Absence of lens centration can lead to reduced results with CRT. If the patient exhibits a central island, then the treatment zone of the lens doesn't sufficiently applanate the central cornea and may indicate that the central cornea is under-treated. To remedy central islands, reduce the landing zone area by one degree. If this doesn't improve vision, then return the landing zone area to its original value and decrease the return zone depth in 25-µm steps until you achieve centration.
Induced astigmatism can also cause the patient to exhibit a central island. Inadequate central applanation, lens decentration, too large of a landing zone area or a "Z" axis tilt can induce astigmatism. "Z" axis tilt is one of the most common causes of undertreatment and central islands and is usually a result of a return zone depth that's too shallow. To resolve "Z" axis tilt, increase the return zone depth in 25-µm steps until you achieve a proper fit. If needed, increase the landing zone area to accomplish adequate treatment.
If the patient exhibits inferior decentration, then gently move the lens back to the center of the pupil and evaluate the fluorescein pattern. Also evaluate the edge lift and, if appropriate, decrease the landing zone area by one degree. Then evaluate centration and central applanation of the new lens. If the lens decenters superiorly or temporally after the one degree reduction of the landing zone area, then increase the return zone depth by 25µm.
If the lens continues to exhibit inferior decentration following a one-degree reduction of the landing zone area, then decrease the landing zone area by another degree. A two-degree reduction in landing zone area usually requires an increase in return zone depth of 25µm to achieve centration. You can increase the return zone depth in increments of 25µm until the lens centers properly. When a patient exhibits superior decentration, move the lens to the center of the pupil and evaluate the fluorescein pattern. You can increase the return zone depth by 25µm and re-evaluate the lens centration. If the lens still doesn't center, then increase the landing zone area by one degree and evaluate centration.
Nudge a lens that exhibits lateral decentration back to the center and evaluate the fluorescein pattern. If sufficient edge lift exists, then increase the landing zone area by one degree and re-evaluate centration, applanation and edge lift. If edge lift is limited or nonexistent and nasal/inferior or nasal/central positioning is evident, then follow the same steps as you would to correct inferior decentration.
If adequate central applanation exists and slight superior decentration is occurring, then increase the return zone depth by 25µm and evaluate the centration and central applanation of the new lens. Increase the return zone depth in 25µm steps until the lens centers. Consider reducing the landing zone area by one degree to accomplish better treatment if, to achieve centration, you increased the return zone depth by 50µm. Often increasing overall diameter of the lens will aid in troubleshooting lateral decentration. It may be necessary to increase the landing zone angle one degree when you increase the overall diameter by 0.5mm.
|TABLE 1 Valuable Fitting Resources|
Researching the Resources
Many valuable CRT fitting resources are available. Paragon offers an Initial Lens Selector and a Quick Start Reference card (Figure 2) for use by certified fitters of Paragon CRT. The Initial Lens Selector helps practitioners determine the correct lens parameters with which to begin the fitting process. The Quick Start Reference card offers tips on achieving the correct fit and troubleshooting.
Paragon provides practitioners with two e-mail newsletters each month: "The Return Zone" and "The Landing Zone." "The Return Zone" primarily contains practice management solutions. Paragon also provides teleconferences/webinars at least once each week. The subjects range in information from basic to advanced and once a month they have an "ask the experts" session. These sessions involve outside practitioners who actively fit CRT lenses and these practitioners field questions regarding CRT fitting and/or practice management.
Corneal reshaping is an increasingly successful alternative to refractive surgery for patients who have a low myopic refractive error and who either do not desire or who aren't good candidates for refractive surgery. Corneal reshaping can also prove quite profitable and represents an excellent potential practice builder.
As this article emphasizes, whereas careful patient selection and monitoring is essential, the process of corneal reshaping itself has been streamlined and simplified to represent a modality that all practitioners should consider implementing into their practices.
The authors would like to acknowledge Jim Slightom, Dr. John Mark Jackson, Dr. Jerry Legerton and Kathy Shafer for their assistance with this article.
|Ms. McCampbell graduated with a Bachelor of Science Degree in Biology from Northwest Missouri State University in Maryville, Missouri. She's currently a fourth-year student at the University of Missouri-St. Louis College of Optometry.|
|Dr. Bennett, a 1979 graduate of Indiana University School of Optometry, is director of student services at University of Missouri -- St. Louis College of Optometry. He's also executive director of the GP Lens Institute. Contact him at EBennett@umsl.edu.|
|Dr. Rah is an assistant professor at the New England College of Optometry where she works primarily in the Cornea and Contact Lens Service in patient care, teaching and research.|