發表於：2001年10月
Fitting Orthokeratology Contact Lenses
By Belinda M.W. Luk, OD, Edward S. Bennett, OD, MSEd, and Joseph T. Barr, OD, MS, FAAO
October 2001
This comprehensive look at fitting corneal reshaping lenses will help you become a more confident ortho-k practitioner.
In the late 1950s and early 1960s, many practitioners observed that patients were slightly less nearsighted upon removing their lenses when corneal contact lenses were being fit slightly flatter than K. In the early 1960s, several optometrists developed their own orthokeratology techniques by using large, flat lenses with large optical zones. Unfortunately, their only choice of material was PMMA. Rigid gas permeable (RGP) lens materials, developed in the late 1970s, greatly improved the safety and efficacy of orthokeratology. These materials achieved an overall improvement in performance, in part the result of the ability to use larger overall diameters, which greatly enhanced centration of the orthokeratology lenses.
The first reverse zone lens design for orthokeratology was developed in the 1980s by Dr. Richard Wlodyga and manufactured by Mr. Nick Stoyan. These first lenses consisted of three curves, including a secondary curve often two to three diopters steeper than the base curve radius to accelerate the process. Currently, there are numerous four curve/zone or similar designs to further accelerate this process.
Of interest today is overnight orthokeratology, in which orthokeratology lens designs are manufactured in high Dk materials, such that the experimental lenses are worn during sleep and removed upon awakening. Numerous laboratories and all major RGP button manufacturers are currently very active in orthokeratology. It represents one of the fastest developing areas of contact lenses, and it is imperative for every optometrist who is interested in orthokeratology to keep updated on the available information and the different orthokeratology lens systems present.
General Fitting of Reverse Curvature lenses
Most modern orthokeratology lenses are designed with secondary curves steeper than their base curves. The steeper secondary curves serve the following purposes: to provide space for the cornea to move as the central cornea is flattened; to help lens centration, thereby reducing induced astigmatism; and to create a reservoir for tear exchange. Although there are many unique designs of orthokeratology lenses, most are variations of this form of reverse zone lens.
The first reverse zone lenses produced were of the three-zone design. The general design of these lenses consists of an optical zone, a reverse curve (the steep secondary curve) and a peripheral curve. The optical zone is typically 6.0mm in diameter, and the initial base curve is fit 1.00D to 1.50D flatter than the flat K reading. As the fitting relationship of this lens changes as a result of the flattening of the central cornea, a new lens with a flatter base curve will have to be dispensed promptly to prevent distortion and/or physiological problems of the cornea. The initial change occurs rather quickly; therefore, many fitters will order a second pair of lenses at the time of the initial order. The width of the reverse curve can vary from 1.0mm to 1.4mm, and the initial reverse curve radius is best determined through diagnostic fitting. The ideal fluorescein pattern should show a ring of at least 270 degrees due to with-the-rule corneal toricity (to 360 degrees in nearly spherical corneas) of midperipheral touch with an area of clearance to either side. The peripheral curve is approximately 0.4mm wide, with a radius from 10.5 to 12.25 mm. Centration is very important to the fit of these lenses and can be improved by steepening the base curve or the reverse curve.
With the four-zone reverse zone lenses, the expected maximum amount of myopia reduction is 3.50D to 4.25D. This lens design is similar to the three-curve design, except for the addition of an intermediate curve between the reverse curve and the peripheral curve, often referred to as the alignment curve or zone. The alignment zone is fit such that it is in alignment with the peripheral cornea, and it plays a very important role in lens centration and movement.
Typically, with four-zone lenses, the initial pair of lenses is expected to be used for the entire course of the treatment, as well as for retainer wear. These lenses are large, from 10.0mm to 11.0mm in overall diameter, with a small optical zone, often 6.0mm in diameter. In several designs the initial base curve radius is fit flatter than K reading by an amount just greater than the refractive change desired, often equal to +0.50D. For example, if the amount of myopia is 2.00D, then the base curve radius should be 2.50D to 2.75D flatter than K; and the lens power will be +0.50D or +0.75D. This power allows for regression of myopic refractive error during the day. Accurately perform keratometry and preferably corneal topography in addition to a manifest refraction, and regularly calibrate all instruments.
The second curve is the reverse or return zone which is approximately 0.6mm wide and steeper than the base curve radius by two to 2.6 times the amount the base curve is flatter than K, and can range from 6.00D to 12.00D steeper than the base curve radius. In our example, if the base curve radius is 2.50D flatter than K, then the reverse curve radius should be 5.00 to 6.50D steeper than the base curve radius.
The third zone in the lens is the alignment curve which is typically about 1.0mm in width and fit in alignment with the midperipheral cornea. Determine an alignment curve radius using the central keratometry reading, a temporal keratometry reading or information from topography. The alignment curve can be ordered as 0.25D flatter than the central flat K reading or as the temporal keratometry reading measured by asking the patient to fixate on the edge of the keratometer target. Using the eccentricity
- The alignment curve radius should be equal to the central flat K reading if the eccentricity
value is between 0 and 0.30; - The alignment curve radius should be 0.25D flatter than the central flat K reading if the eccentricity
value is 0.31 to 0.55; - The alignment curve radius should be 0.50D flatter than the central flat K reading if the eccentricity
value is 0.56 to 0.70. The alignment zone specifications may also be determined by using trial lens sets. Adjustments will have to be made according to the fluorescein pattern and lens centration. The outer curve of the lens is the peripheral curve, which is very similar to the peripheral curve of the three-zone design; it is most commonly 0.4mm wide with a radius of curvature of 10.50mm to 12.50mm.
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| Figure 1. Fluorescein pattern of a four-zone orthokeratology lens. |
As with the three zone designs, a well-centered fitting relationship with limited lens movement with the blink is important. The fluorescein pattern should exhibit central touch, paracentral clearance, midperipheral touch and minimal peripheral clearance (Figure 1). The most important component of the fit is lens centration, which is primarily influenced by the alignment curve. As lens wear occurs while the patient is sleeping, lid-lens interaction may not have as much effect on lens centration as with standard RGP lenses, and on-eye evaluation of the lens may not be as valuable in assessing the lens position. Evaluate lens position during sleep by reviewing corneal topography and evaluating the centration of the treatment area in relation to the pupil. If the lens is sitting superiorly, the lens may be loose, causing it to move to the flatter superior cornea. Steepen the alignment curve if it appears flat from the fluorescein pattern, or increase center thickness, decrease edge thickness or add a prism to increase the mass of the lens if the fluorescein pattern appears to be ideal. For an inferiorly-fitting lens with heavy touch in the alignment zone, flatten the alignment curve. However, if the fluorescein pattern shows alignment in the midperiphery, then decrease the center thickness or increase the edge thickness. Again, depending on the fluorescein pattern in the alignment zone, the solution to a laterally-decentered lens may be to flatten the alignment curve or increase the diameter of the lens.
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| Figure 2. "Smiley face" corneal topography. |
Problem-Solving
Most successful patients obtain 20/20 to 20/25 visual acuity that is maintained throughout the day. However, a major cause of poor unaided vision is an inadequately-fitted lens, which can often be diagnosed from topography. If the treatment area is decentered, the topography map will exhibit an arcuate area of steepening in the cornea within or very near the pupil margin. An excessively flat or apical touch fitting lens design will often result in a "smiley face" topography map (Figure 2). In this case, refit the lens to create a more centered treatment area. Central islands in the topography are small areas of incomplete treatment or corneal distortion at or near the visual axis, which can be caused by either poor centration or a steep reverse curve (Figure 3). Poor unaided vision may also be due to under-treatment or a too-steep base curve. However, if the lens fits well and the initial base curve radius calculation was performed correctly, then the best strategy is patience, as some patients may be slower to respond or respond to a lesser degree than expected. Re-evaluate the patient in two to three weeks. If the changes are still insufficient, try fitting the base curve radius 0.50D to 0.75D flatter to cause more corneal epithelial movement. Remember some patients may not respond to their satisfaction. Another common visual problem with orthokeratology lenses is flare and glare at night, which may be due to a decentered lens or large pupil size.
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| Figure 3. Central islands corneal topography. |
Potential physiological problems include central corneal staining and persistent lens adhesion. Corneal staining is often the result of either build-up of debris on the back surface of the lens or mechanical irritation from an excessively flat lens. Solve this problem by instilling lubricants, polishing the back surface of the lens, re-educating the patient on the importance of lens cleaning and reordering the lens with a steeper base curve radius. Excessive lens adhesion, like corneal staining, could also be due to deposit build-up on the back surface. A steep alignment or peripheral curve causing an insufficient tear reservoir could also result in lens adhesion. Flattening and/or widening the alignment or peripheral curve will prevent future lens adhesion.
FDA Approval for Overnight Orthokeratology
Overnight orthokeratology is an off-label use of the reverse curve lens design and is not FDA approved. Many fitters prescribe these lenses to patients for orthokeratology with informed consent, though they may not advertise them. The FDA does prohibit RGP laboratories and manufacturers from promoting such a design to practitioners.
Orthokeratology Lens Systems
Each orthokeratology lens system has its own design and fitting philosophy. In some of these systems, lenses are ordered empirically from the patient's keratometry readings and spectacle refraction; in others, lenses are ordered only after the diagnostic fitting is complete. Whether a practitioner chooses to order lenses empirically or from diagnostic fitting depends upon the present comfort level in fitting and evaluating these lenses. Each practitioner should learn more about each system to decide which one most closely approximates his or her own fitting philosophy. You may need to fit several patients in each lens system to fully appreciate the differences and to find the system with which you are most comfortable. Remember that, although the following systems are not associated with specific RGP consulting companies, all laboratories will have consultants on staff to assist practitioners in fitting.
Orthokeratology Lens Design Tools
For the practitioner who would like to begin fitting orthokeratology lenses, some useful tools are available. EyeQuip developed the WAVE Contact Lens Design Software, which enables practitioners to design single vision, front or back surface multifocal or orthokeratology (RGP) lenses. The program utilizes the mathematics of wavelet theory from topographic data and creates a digital signal to describe the cornea, which is used to create the lens design with an emphasis to match the periphery of the lens to the peripheral cornea. This results in a lens with excellent centration, according to the manufacturer. The final lens design can be e-mailed directly to the Optiform lathe at Custom Craft contact lens laboratory. The WAVE program is already integrated into the Scout topographer software and is included with the Keratron topographer as a stand-alone software.
OrthoTool 2000, developed by EyeDeal Software & Design, is an RGP design, tear film modeling and manufacturing software. OrthoTool 2000 performs the optical calculations from keratometry readings and spectacle refraction to display complete lens parameters, manufacturing data, the cross section of the lens, thickness profile and the tear film thickness across the lens diameter. The contact lens practitioner can choose from 12 different contact lens designs such as standard spherical designs, thin, ultra thin, aspheric or bitoric lenses, as well as several reverse geometry lens designs.
Orthokeratology Lens Design Consulting
Tabb developed the Nightmove lenses, which are manufactured with Boston Equalens II material. The Nightmove lens is of a reverse geometry back surface construction with up to nine curves, including the base curve, the reverse curve, a variable number of alignment curves and the peripheral curve. Nightmove lenses can be ordered through Advanced Corneal Engineering, Inc..
Many More Systems
Reversible Corneal Therapy by ABBA Optical is a standard four-curve reverse curve lens manufactured from the Paragon HDS 100 material which is FDA approved for overnight wear. Base curve radius determination is accomplished by fitting the lens flatter than K by the amount of desired refractive change +0.75D. Lens diameter is 10.6mm; alignment curve radius is 0.25D flatter than central flat K reading. The reverse curve radius is calculated by a consultant when the lens order is placed.
Contex was the first company to receive FDA approval for its orthokeratology lens for daily wear. The Contex OK Lens is fit based upon the central K reading, manifest refraction and corneal eccentricity
Correctech, Inc., is currently conducting FDA investigational studies on its overnight accelerated orthokeratology lenses. For the study, patient data is forwarded to the laboratory and lenses are empirically designed for the patient. The Correctech, Inc., orthokeratology lenses are four-zone reverse geometry lens designs in Boston Equalens II material.
The DreimLens, designed by Dr. Thomas Reim, is also currently involved in orthokeratology overnight wear FDA studies. These lenses, in Boston XO material, are of the four-zone design. The base curve radius of the central zone is calculated from the flat central K reading and the amount of refraction to be corrected. The standard fitting zone, alignment zone and peripheral zone parameters have been clinically and theoretically determined to work together to provide the desired results in the majority of cases. Lenses can be ordered empirically with patient's keratometry
The Emerald and Jade designs are available from Euclid Systems Corporation. Both are manufactured with the EPT manufacturing system, which offers polish-free lens finish, helping to eliminate inconsistencies in posterior sagittal depth from polishing. The Emerald design is based on a four-curve reverse curve design. The Jade design is more advanced and uses a conic model of the cornea and information about corneal eccentricity and patient's refraction to calculate the proper reverse curve. The Euclid system includes the lens designs and the Euclid ET-800 Corneal Topographer. Overnight lenses are manufactured with the Boston Equalens II material. The company has completed the first clinical study for an overnight wear orthokeratology lens, and the data is currently under review by the FDA.
Gelflex Laboratories in Australia manufactures the EZM orthokeratology lenses. EZM lenses are made of the Boston XO material, if used for overnight orthokeratology, and are available in 10.6mm or 11.2 mm overall diameters depending upon the patient's intrapalpebral aperture size. They are fenestrated at 120-degree intervals to prevent lens adhesion during overnight wear. Gelflex developed a computer calculator program to aid practitioners in determining the initial trial EZM lenses by incorporating corneal topography data and the overall lens diameter requested. Once the initial trial lenses are determined, Gelflex recommends performing an overnight trial to determine whether the patient is a fast or slow responder and whether the initial trial lens choices were correct. If, on the following morning, the response seems to be poor and due to an inadequately fitted lens, the patient will have to return for another trial with different lenses.
Paragon Vision Sciences is currently conducting investigational studies and applying to the FDA for approval of its Corneal Refractive Therapy (CRT) lens system. These lenses are different from the traditional reverse curve lenses. In standard reverse curve lenses, the four zones consist of curves of different widths and radii of curvature; in the CRT lenses, the reverse zone and the alignment zone are different from other designs. The reverse zone, called the return zone in CRT, is a sigmoid; it is not a curve that can be defined by a radius of curvature. The width of the return zone is kept constant; it is the depth of the sigmoid that is varied to change the fit of the lens. The alignment zone, called the landing zone in CRT, is a flat section (a straight line) defined by the negative angle that it makes with a horizontal line. The landing zone in CRT, like the alignment zone in reverse curve, is meant to be fit in tangent or alignment with the midperipheral cornea and aids in lens centration. Paragon requires practitioners interested in fitting CRT lenses to attend a fitting seminar. Fitters can select from two different diagnostic systems: a 24-lens diagnostic set with a Palm Pilot calculator program or a 65-lens diagnostic set.
Precision Technology Services is the only RGP laboratory in North America to produce Dr. John Mountford's BE lens design for orthokeratology. BE lens fitting is based on the theory that the sagittal height of the contact lens must match the sagittal height of the cornea, allowing for the tear film layer. Sagittal height of the cornea is determined with an equation that requires the following pieces of information: apical radius of the cornea, elevation of the cornea and chord length (the total diameter of the lens to be fitted). Also, as positive pressure from the lens is applied to the central cornea, due to the flat base curve, negative pressure is exerted on the paracentral cornea, in the area of the tear reservoir; this is called the squeeze film force, which can be manipulated by altering the base curve and the depth of the tear reservoir. The squeeze film force is the factor that determines how much epithelial movement will occur; therefore, it determines the amount of refractive change that will occur. Mountford developed the BE computer program which simplifies the calculations. One needs to input only the apical radius of the cornea and the corneal elevation from topographical data, the lens diameter desired and the desired amount of refractive change; the program will calculate the initial trial lens. Then, the patient should try the lenses overnight. On the following day, by inputting the resultant topographical and refractive information, the computer program will calculate the final lens order.
R & R Lens Design creators Drs. James Reeves and John Rinehart conduct training seminars for interested practitioners. By participating in such a training session, fitters learn about the lens designing process, the purpose of each curve and how the curves affect lens fit and lens performance. With this information, the practitioner will be able to design his or her own lenses and troubleshoot when complications arise. Practitioner may order lenses from their preferred RGP contact lens laboratory in their preferred material. Lenses are fit using a 14-lens diagnostic kit.
See Table 1 for a listing of the designs and design tools previously described. Another option for practitioners interested in learning more about orthokeratology is attending the Global Orthokeratology Symposium, sponsored by Contact Lens Spectrum, to be held August 2002 in Toronto. Visit www.healthcareconferencegroup.com for more information.
Orthokeratology is an exciting new frontier which provides a potentially valuable and reversible alternative for low myopic refractive error individuals who are not interested in or are poor candidates for refractive surgery. The advances in lens design and corneal topography instrumentation complemented by an overnight wearing schedule have resulted in a much shorter time frame for myopia reduction to occur as well as patient satisfaction.
References are available upon request to the editors of Contact Lens Spectrum. To receive references via fax, call (800) 239-4684 and request document #75. (Have a fax number ready.)
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