Managing Keratoconus with Proprietary Designs

By Shelley I. Cutler, O.D., F.A.A.O
OCTOBER 1999

Unsure of which proprietary lens design to use for keratoconic patients? Read this article to find out.

Fitting the keratoconic cornea may be the ultimate challenge for many contact lens practitioners. There is great satisfaction in helping someone who is visually handicapped recapture a more normal lifestyle. Other practitioners have shied away from these patients. This avoidance may be due to the fact that their own inexperience frightens them or because keratoconic patients require more chair time. With today's managed care systems, some practitioners feel it's not cost effective to treat keratoconic patients. To complicate matters, many keratoconic patients have an allergic history and seem to be much more sensitive than the average patient. Dry eye complaints from these patients are also not unusual.

It's easy to see why many practitioners don't want this kind of fitting challenge. So what about those who welcome these challenging fits? Even the most experienced keratoconic fitters have to start somewhere. Many begin under the guidance of another practitioner or take contact lens residencies. These "students" become experienced at looking at a spherical lens and at manipulating the curves to contour the irregular cornea and achieve a successful contact lens fit. Is there a way for the less experienced practitioner to achieve the same result?

Many excellent keratoconic lens designs are available, including patented, trademarked and other proprietary material designs. Is one design better than another design? Of course not. If one design worked for all corneas, there would be no need for other designs.

Keratoconus: The Disease

Keratoconus is defined as a noninflammatory corneal ectasia. It results in a thinned protrusion that is usually inferior and decentered off of the visual axis. The steepest part of the cornea can vary in its location since no eye is exactly the same. To begin the challenging task of contact lens fitting, it's important to obtain corneal curvature information and to familiarize yourself with the various lens designs that are available.

Corneal topography is not a requirement for fitting keratoconus patients, but it is certainly a good starting point. It establishes the position of the cone apex and a basic shape pattern.

Historically, there have been two main keratoconic patterns recognized. The nipple cone, where the steepest dioptric value is smaller in shape and somewhat central in location, and the sagging cone, where the steepest dioptric portion of the cone is larger, inferior and more decentered. With corneal topography and the multitude of patient variations, it can be difficult to decide exactly which type of cone your patient has. It's especially difficult to determine cone type when one type of cone seems to blend into another. Warpage from a previous lens fit can also make the job of cone identification harder. A topographical map can assist in the differential.

Designs for Keratoconus

Multicurve sets with specific parameters designed by experienced keratoconic contact lens fitters have been available for many years. The base curve should be approximately equal to the optic zone (OZ), and the secondary and tertiary curves should be progressively flatter to align with the cornea as best as possible.

The Soper Cone -- The Soper Cone is a bicurve contact lens that was introduced in the mid-1970s in which the two posterior curves are separately cut and polished. The fitting philosophy is based on sagittal depth: As the curvature of the central posterior curve (base curve) increases for a given diameter, or the diameter of a given curve increases, the vaulting effect of the lens also increases.

The peripheral curve is 7.5mm unless the central base curve is flatter than 6.49mm. Then the peripheral curve is 7.85mm. This allows for the intermediate curve(s) to be added by standard techniques if necessary. The trial set consists of 10 lenses, designated by the letters: A-H. There are three groups of a given diameter/optic zone (D/OZ) relationship. The steep central posterior curvature varies in order to fit the cone.

The McGuire keratoconic lens system -- This system was introduced in 1978 and is a modification of the Soper design. There are three groups: the Nipple cone (D/OZ 8.1/5.5), the Oval cone (D/OZ 8.6/6.0) and the "Globus" cone (D/OZ 9.1/6.5). There are a series of four peripheral curves that should be blended well to create an almost aspheric relationship. The secondary curve is 0.5mm flatter than the central base curve (0.3mm width). The third curve is 1mm flatter than the secondary curve - width of .3mm. The fourth curve is 1.5mm flatter than the tertiary curve for a width of 0.3mm and the fourth and final peripheral curve is 2mm flatter than the tertiary curve for a width of .4mm.

Aspheric Contact Lenses

There are a few laboratories that make true aspheric contact lenses designed specifically for the keratoconic cornea. The posterior apical radii can be very steep in some cases, thus becoming a limiting factor with this design. The goal is to vault the apex of the cone or lightly touch it, and align the more normal peripheral cornea. Because the nature of an aspheric lens allows for plus power in the periphery, presbyopic keratoconic patients may find this lens preferable if the fit is acceptable. The lens needs to center for the optics to work.

One such lens is the KAS lens by GBF Contact Lens Inc., which has recently been purchased by Aero Contact Lens, Inc. It has a paraboloidal ocular surface geometry with a spherical front surface. The edge lift is considered hyperboloidal and diameters range from 7.5mm-10.0mm. The lens furnishes up to a +2.25 add by the nature of the design.

Contex, Inc. has 8 aspherical contact lens designs available with varying eccentricity values. The less severe the cone, the lower the recommended E-value. The Aspheric-20 is the most common design for keratoconus. It has a hyperbolic design. Larger diameters are recommended and varying edge lifts are available.

Spherical OZ with Aspheric Peripheral Curves

This type of lens is similar to the true aspheric lens in fitting philosophy. With the spherical OZ, visual acuity can be sharper and more stable.

The ComfortKone lens by Metro Optics is a triaspheric lens with a 4.0mm optic zone. The lens flattens into the aspheric "A" curve, which is considered the fitting curve. The greater the "A" value, the greater the change from the base curve to the peripheral fitting curve. The fitting set consists of 24 lenses. The diameter is either 8.5mm or 9.0mm and there is a choice of base curves of several "A" values: 5, 10, 15, or 20.

The Infinity Cone lens by Infinity Optical, Inc., fits on the sagittal value principle by using larger diameters (9.0mm and 9.5mm). Fit the periphery of the lens on the peripheral area of the cornea as far away from the cone protrusion as practical.

The Valley K lens by Valley Contax is a unique design modified after the McGuire lens. Instead of the four blended peripheral curves, there are computer-lathed curves halfway in between the basic McGuire curves, creating an almost aspheric periphery. It is very reproducible and the peripheral curve system can be modified as needed by making the curves flatter or steeper for a given base curve.

Custom Designs

There are several custom designs that have appeared, each having their own advantages and disadvantages.

The Rose K lens is a lens design with a complex, computer-generated peripheral curve system that resulted after several hundred fittings by Dr. Paul Rose of Hamilton, New Zealand. The standard peripheral curve system claims to fit 60 percent of keratoconus patients. The ideal edge lift is 0.8mm wide. Flatter combinations are available (1.0, 1.5, 2.0, 2.5 and 3.0mm flatter than the standard peripheral curve system) to obtain this edge lift. A steeper peripheral curve system is available in 0.5mm and 1.0mm steeper than the standard edge lift. The optic zone decreases as the base curve steepens. Toric curves are available on both the front and rear surface as well as peripherally.

The NiCone lens, patented in 1986, is available from Lancaster Contact Lens, Inc. It is a multiple back surface vaulting system designed with scientific formulas. Each lens has three base curves and a peripheral curve of 12.25mm. There are 3 fitting sets (#1 for mild keratoconus, #2 for average keratoconus and # 3 for advanced keratoconus). There are designations of curvatures between Numbers 1, 2 and 3 base curves, depending on the selection of the primary base curve. Each time the central posterior radius is changed, regardless of whether it is a Number 1, 2 or 3 cone, all formulas of the second and third base curve change as well.

The second base curve acts as a buffer between the diseased and the non-diseased areas of the cornea. It is a .3mm transitional zone for lens diameters up to 10.5mm. A diameter larger than 10.5mm can have a second base curve of 0.4mm, 0.5mm or greater. The lens diameter and optic zone are not standard and thus, may vary. All base curves are lathe-cut and optically polished which, according to the manufacturer, does not create the optical distortion that is present in grounded curvatures.

The Decentered OZ design for Keratoconus, made by Menicon, has four spherical curves on the back surface of the contact lens with the optical zone decentered approximately 1mm. The central base curve is the fitting curve, however, there is a back surface reverse annular which seems to line up with the visual axis. This curve may end up being the optical curve. This is an excellent design for a low, sagging cone.

The initial lens design can be determined by their standard trial set (D/OZ: 9.2/7.0 Secondary: 7.5/0.6 Intermediate: 9.5/0.3 Peripheral: 12.0/0.2). Prism increases as the base curve steepens and more minus power is needed.

Initial lens selection is the base curve that corresponds to the keratometric reading if the patient is looking up or from the topographical map. The recommended procedure using corneal topography is to locate the ring that corresponds to the base of the cone. The OZ should be at 1mm larger than the cone and fairly close to the base curve.

The Porus K lens, made by Lens Mode, has a smaller lens diameter that fits over the apex of the cone, striving for total alignment. The average diameter is 8.0mm but can vary between 7.7mm-8.4mm. The steeper the cone, the smaller the diameter. The flatter the cone, the larger the diameter. There are five curves which are all computer-lathed and a complex trial set, with curves that are only known by the manufacturer. Parts from one lens can be combined with parts from another lens to create a custom fit.

Some laboratories offer computer-assisted contact lens designs. However, because they are designed from corneal topography, and the keratoconic cornea can have changes in curvature that are too detailed for the corneal map to reveal, this type of design may or may not work when the lens is placed on the eye.

What is a Successful Keratoconus Lens?

The criteria for a successful keratoconic lens fit is no different than it is for a standard successful RGP lens fit. The lens needs to be comfortable, wearing time should be all waking hours (if possible) and vision needs to be acceptable, as does the post-wear biomicroscopy. However, achieving these criteria can be quite a challenge.

Vision for the keratoconic patient may not be 20/20 even with contact lenses. If the vision is satisfactory for the patient's needs, then you have satisfied the first requirement.

The health of the cornea is also of prime importance, and of course, may relate to wearing time. Some practitioners feel that a poorly designed lens can cause scarring and facilitate the disease. Then again, even with no lens wear, keratoconus is a progressive disease.

The optimum cornea-lens relationship continues to be debated: Is apical bearing or apical clearance the ideal fit? Total alignment would be best, but only in very rare instances can this be obtained. In my opinion, the optimum cornea-lens relationship occurs when the weight-bearing forces of the contact lens are distributed as evenly as possible on the cornea. I prefer a light "kissing" touch over the cone because it seems to provide patients with maximum acuity. The fluorescein pattern should be your guide for fitting keratoconus patients. On occasion, the shape of the cornea will not allow for what you might consider to be an ideal keratoconic fluorescein pattern. Often, only time will tell if a certain lens is satisfying the required criteria.

Fitting the Keratoconic Cornea with Specialty Lens Designs

Once you have identified the general shape and location of the cone using corneal topography, it's time to select an initial lens design and lens. Some designs are more appropriate for a given cone than others.
Fitting the Nipple Type Cone -- For nipple cones, which have a smaller, steeper cone, a variety of options are available. Lenses that are in the category of a spherical OZ with an aspheric periphery, the Rose K lens, the Nicone lens, the Porus K lens or possibly an aspheric design lens are all appropriate initial selections. The apex of these cones is just minimally off the visual axis. Centration is usually good since these lenses tend to center over the steepest part of the cornea. The limitation of many of these designs is usually not the final fluorescein pattern, but patient vision. Sometimes, the OZ may not be large enough to maintain sharp, stable vision. If this occurs, a larger OZ lens such as the Nicone lens or the larger D/OZ Infinity lens is needed.

Fitting the Sagging Type Cone -- Larger, sagging cones usually prove to be more of a fitting challenge. A contact lens will always center over the steepest part of the cornea. With these cones, the lens will position inferiorly. If a smaller lens with a smaller optic zone is used, vision can fluctuate or be compromised all together. An ideal contact lens for this type of cone might be Menicon's lens with the decentered optic zone. Even though the visual axis is through the secondary curve, acuity can be exceptionally good.

The Nicone lens also works well with a sagging cone. The optic zone can be made larger to vault the cone as well as cross the visual axis. The larger diameter Infinity lenses can also be successful with larger sagging cones due to their larger optic zone.

An alternative to fitting the sagging cone is to attempt to fit over the more normal superior cornea and attach the lens to the lid. Care needs to be taken so that excessive pressure on the cone does not occur. Some of the lens designs with larger diameter/optic zones could be used for this approach.

Evaluating the Trial Fit

Insert your selected contact lens, allow at least 10 minutes for adaptation and evaluate the fluorescein pattern. As with all rigid lens evaluations, you need to look at the central area, the mid-peripheral area and the periphery.

Start centrally, with the cone itself. How is the base curve-optic zone diameter combination housing the cone? If the bearing is too heavy, more vaulting needs to occur. This can be accomplished by making the base curve steeper for a given OZ diameter. (This is the easiest way if working with one trial set of standard parameters.) If you have a set with variable OZ diameters (such as the Infinity Cone, the Nicone or the Porus K), increase the OZ and maintain the base curve. The OZ of the Rose K decreases as the base curve steepens, so you may have to make changes of larger increments.

When do you know that the base curve is correct? Continue to steepen the base curve until there is apical clearance and then work backwards. There should be just enough touch to maximize vision. Striving for a light or "kissing" touch is ideal.

Sometimes just going steeper will not provide a successful fit. In this case, the optic zone diameter may need to be altered or you may need to switch lens designs altogether. On occasion there may be air bubbles in the pooling area surrounding the cone. If the base curve is satisfactory, the best way to eliminate them is to decrease the OZ. To maintain the same sagittal depth and fluorescein pattern, steepen the base curve. Be aware that the smaller OZ may interfere with visual acuity.

The paracentral or mid-peripheral area under the contact lens needs to be evaluated next. The goal is to strive for alignment. You may think that achieving this will not be difficult, especially if you know what curves of the contact lens are, but be prepared to be disappointed. Sometimes no matter how much you alter the curves, the ideal alignment pattern never seems to appear. Accept this situation. Do not accept heavy bearing that causes seal off. Tears need to be exchanged. If you are using proprietary sets, you are at the mercy of the labs since you can only request flatter or steeper peripheral curve systems.

Finally, observe the peripheral portion of the lens, looking at the edge lift. You want enough to allow good tear pumping under the lens but not enough to cause the patient discomfort. Too little edge lift will predispose the adjacent corneal area to desiccation as well as inhibit good tear exchange.

If you are satisfied with the trial fit, order your lens. Dispense the lens and begin to follow the patient, closely at first. Be sure to alert your patient that a few changes or modifications in lens design will be needed during the fitting process.

If you are satisfied with the post-wear biomicroscopy and the patient is satisfied with the comfort level, wearing schedule and vision that the lens provides, have them return for follow-up at least two times a year. Remind them that keratoconus is a progressive ectasia and you can not predict when and if it will start to progress again. Inform them that they need to see you immediately if they experience any of the following: decreased vision, decreased comfort and wearing time, increased difficulty in removing the contact lens, removal of lenses becomes much too easy or lenses dislocating or ejecting from the eye (assuming that it was not a problem before).

If you see a change in the fluorescein pattern or corneal health of the patient in a routine follow-up, a refit may be necessary. Talking with your patient may elicit subjective signs that they didn't feel were relevant previously.

Case Illustration

J.G., a 50-year-old male, presented for an evaluation of his present keratoconic contact lenses. He had been wearing this pair of contact lenses successfully for 4 years, but was presently wearing his spare pair and had been for the previous 3 months. Even with his spare pair, he noticed discomfort at the end of the day, vision which he described as "warped" and the lens seemed to be too easy to remove. Past records were unavailable.

Figure 1 shows his present lens OD. Verified lens parameters are: BC: 6.52 BVP: -10.00 D/OZ: 9.3/7.8. Peripheral curves were unknown and visual acuity was a consistent 20/25-. The lens centers and moves well, but not excessively. Note the heavy bearing over the cone. In addition to the apical scarring that is visualized, there is pooling surrounding the cone in the mid-peripheral area. In the periphery, note the superior area of bearing while the remainder of the peripheral portion of the lens has excessive edge lift. The distribution of the contact lens' weight-bearing forces are poor since they are mainly over the cone, making this lens fit too flatly.

FIG. 1: J.G.'s right eye with habitual contact lens.

Figure 2 shows topography OD. This is an extremely large cone, and if you look carefully at the dioptric values of the blue and green colors, you'll note that they are in the 50-60D range. Although the auto alignment feature of the corneal topographer was on, this was a very difficult map to capture. The Y and Z axis are off from zero, so it's difficult to determine whether this is an extremely large nipple cone or a sagging cone because one cone seems to flow into the other cone. The flatter areas in the center region correspond to the scarring areas present at the cone apex.

FIG. 2: Corneal topography of J.G.'s right eye.

In J.G.'s case, I attempted several different proprietary lens designs. Even though his cone was exceptionally large, I initially thought it was more of a nipple cone than a sagging one. I first attempted to fit the Rose K lens, but the fit was not successful visually. Knowing that the topography might have been off center, I concluded that maybe this was more of a sagging type cone after all. I attempted Menicon's decentered OZ keratoconic lens and was also unsuccessful. On the third attempt, I used the Nicone design lens and was successful.

Figure 3 is the best Rose K fit. (Lens parameters: BC: 5.80 BVP: -16.00 D 8.7: Peripheral curves: standard - proprietary information)

FIG. 3: Fluorescein staining of Rose K lens 5.8024

There is a nice even distribution of weight over the apex of the cone. Tear exchange reveals a "kissing" type of touch even though the area is large. The area superior to the cone is total clearance, but no air bubbles are present. There doesn't seem to be a definitive mid-peripheral pattern. Some bearing in the superior quadrant was present, but is not visible in the figure. Overrefraction revealed an endpoint visual acuity of 20/25+, but varied with a blink.

Figure 4 is the best Menicon decentered OZ keratoconic design. (Lens parameters: BC: 5.82 BVP: -18.00 D/OZ: 9.2/7.0) This lens centers well. There is adequate movement and tear exchange. The central area over the cone displays almost an alignment pattern to minimal apical clearance over the cone. There is a small area of light bearing to alignment pattern in the mid-peripheral region. Good edge lift is seen until the superior portion of the peripheral lens is observed. Although the edge lift is not ideal, tear exchange is present in that area. This pattern reveals excellent distribution of weight-bearing forces. The lens is comfortable and doesn't dislodge during eye movement. End point acuity is clear, but again, very variable.

 

FIG. 4: Fluorescein staining of Menicon lens 5.8226

Figure 5 shows the final Nicone #3 design. (Material: OP3 BC: 5.80 BVP: -15.00 D/OZ: 8.7/6.5 Peripheral curves: proprietary information.) The Number 3 configuration was used since this is a more advanced cone. Centrally, note the even weight distribution over the cone. There is a large "kissing" area of touch with tear exchange on the blink. In the mid-peripheral portion of the lens, there is a balance of weight distribution. There may be a little too much touch in the superior quadrant than desired (not viewed), but you have to make compromises and see if there are any repercussions over time. Peripherally, there is adequate edge lift for tear pumping action. The patient's acuity is a consistent 20/20-.

FIG. 5: Final Nicon #3 lens design.

Although the fluorescein patterns of the Rose K lens and the Menicon lens looked quite acceptable, the patient complained that his vision was not as sharp with these contact lenses as it was with his present lens. I could only assume that the smaller optic zone of the Rose K lens, and visualizing the secondary curve with the Menicon lens were the cause. In my opinion, this did not satisfy the criteria of a successful fit, so the Nicone was attempted. Vision and fluorescein were quite acceptable with this design. J.G. has been wearing this present lens design successfully for a year now, with 6-month interval follow-up examinations.

If you are a less experienced fitter, using today's proprietary keratoconic designs can help you gain the knowledge and experience necessary to successfully fit these challenging patients. In time, you may want to move on and custom-design your own contact lenses.

The experienced fitter may like these specialty designs because they allow them to facilitate chair time for a final successful result. The material costs are more expensive than a standard RGP contact lens. If presented properly to the patient, it is usually not a problem. CLS

Dr. Cutler has no financial interest in any of the lens designs mentioned, however, she was one of the original investigators of the Nicone design. Various laboratories are starting to make their own designs to address keratoconus. Any designs that are not included in the author's evaluation were not omitted intentionally.

The photographs and text are modifications from material in her chapter: S. I. Cutler O.D., Keratoconus and Penetrating Keratoplasty, MM Hom, O.D. (ed.). Manual of Contact Lens Prescribing and Fitting with CD-Rom, Revised Edition. Butterworth-Heinemann (In press).

References are available upon request to the editors at Contact Lens Spectrum. To receive references via fax, call (800) 239-4684 and request document #53. (Be sure to have a fax number ready.)

Dr. Cutler is a contact lens consultant and adjunct faculty member at the Pennsylvania College of Optometry and specializes in fitting difficult contact lens cases. She has lectured both nationally and internationally on various contact lens related topics.

THE EYESSENTIALS Before beginning a contact lens fit, obtain corneal curvature information and familiarize yourself with the various lens designs that are available. Select an initial lens design and lens after you have identified the general shape and location of the cone using corneal topography. An RGP needs to be comfortable and provide wearing time for all waking hours, and vision and post-wear biomicroscopy must be acceptable. Once you and the patient are satisfied, have them eturn for follow-up at least two times a year.