The "Duozone" Lens: Fitting the Proud Nebula

By Bezalel Schendowich, O.D., F.I.A.C.L.E.
DECEMBER 1999

Keratoconus sometimes requires a better contact lens design than what you have on hand. Find out how to design a successful lens for this condition.

Advanced keratoconus is characterized by the structural disintegration of a circumscribed area of the cornea. As this circular or oval region steepens, distorts and thins, the disease gradually progresses. In some cases, the weakening that results allows the cone to become extremely steep, occasionally in very small zones.

Typically, contact lenses are fit in order to restore the quality of visual function. Management with contact lenses is begun when spectacle correction is either no longer practical because of the high astigmatism or anisometropia, or when achievable visual acuity no longer reaches acceptable levels. Contact lenses are prescribed and worn until they become intolerable to wear, vision is no longer significantly improvable or the structure of the cornea can no longer support the contact lens.

Contact Lenses for Keratoconic Eyes

Contact lenses for incipient keratoconic eyes may not vary greatly in design from those used for normal eyes. As the disease develops, we change the lens parameters to those more generally recognized as lenses specially designed for keratoconic corneas.

Traditionally, keratoconus cone shapes have been described as oval, nipple or globus. Several of the more advanced cases, which we have seen recently, have topography profiles that can be described as "corneal protuberances." Clinically, we measure a cone tip (with or without scarring) of between 4.0mm and 5.0mm radius of curvature with an absolute diameter of less than 1.0mm. Not farther than 2.0mm from the center of the protuberance, the cone slope surrounding the protuberance rapidly flattens to between 6.0mm and 7.0mm. These pinnacles can be located either centrally on the optic axis or decentered from the corneal axis (typically infero-nasally).

Designing Lens Background

The problem in designing lenses for these corneas is generating curves which are extremely steep centrally and which flatten radically to minimize the tendency of the immediate "peri-protuberance" zone to admit air bubbles and to produce dimple-veiling. This structure is necessary to minimize bearing on the tip of the protuberance, while attempting to achieve near alignment over the middle and peripheral cornea with a flat enough periphery to keep the lens from binding. It's also important to find a lens which will not fly off the cornea.

In other words, a lens that is simply steep will either rock or provide insufficiently sharp visual acuity if it is small and is likely to bind if it is too big, while a flat lens will either fly off the eye or abrade the corneal protuberance.

Another aspect of these advanced cones is that in many cases they appear in our clinic after having been dismissed by other practitioners with a "proud nebula" or a scarred protuberance. This heaped-up, epithelial leukoma may present along the patient's visual axis and stands ready to be further abraded unless a lens is designed to avoid rubbing.

Keratoconic Lens Design Wanted

A modern, innovative lens design is required to help these patients. The advanced cases which I have seen clearly need a lens which uses an off-center area of their corneas for vision, while the central zone of the lens needs to be designed to protect and perhaps, even to permit their nebulas to heal to some extent while wearing the lenses.

The lens design which can accomplish these goals was described some time earlier by Mandell, et al. The Mandell lens, dubbed the "Duozone" lens, has an extremely steep and small central zone for vaulting peaks of corneal nebulas or protuberances, but for vision, it relies on the optics of a secondary zone that is wide enough to cover enough of the pupil and not to produce diplopia.

Since my first attempt to design a Duozone lens, I have ordered 34 for 20 patients, 11 of whom are still using the lens. These patients are summarized below in Table 1.

Fine Tuning

My experience working with the Duozone lens has helped me pinpoint various ways to fine-tune the fitting process. An absolute pre-requisite is a well-focused and finely tuned corneal-topography study of the cornea to be fit.

Fine tuning is accomplished by reading the curvature of the steepest point and the curvature of the flattest point, calculating the mean and setting the mean as the mid-range value. Then, the extreme range of corneal curvatures is divided by the number of colors available in the scale (our Eye-Sys topographer has 15). This procedure provides the scale step size. With these settings, the most steeply curved areas and the flattest areas are minimized and the mid-range curves are more accurately defined.

From the topography, three vital parameters are learned:

1. Central vault radius -- Roughly that of the protuberance peak.

2. Central vault diameter -- Slightly larger than that of the peak but not practically greater than 2.5mm in order not to disturb acuity. The further decentered the peak, the larger the vault can be.

3. A first choice for the back optic zone radius -- based on the intermediate band curvature (bright green to second light blue zone).

A second aid in fitting these lenses is a set of trial lenses. Table 2 reflects my recommendation for a six lens diagnostic kit. The peripheral curve system should be consistent with the base curve radius and the degree of the progression of the keratoconus.

I order the Duozone as a standard lens with the vault specified as a final parameter. One specifies both the curvature and the diameter of the vault zone. The lens looks like a relatively large RGP for the steep keratometry measured (of the lenses I have ordered, the smallest was 9.0mm OAD with the majority between 9.3mm-9.8mm). For its size, the OZD is also relatively quite large, to allow enough area for both the non-optical vault zone and the secondary zone to be used for viewing.

The intermediate and peripheral curve systems are not unlike those used for a standard multi-curve design for the chosen base curve radius. For example, a base curve radius of 7.50mm might come with a secondary curve of 8.9mm out to 9.0mm, and finish with 12.3mm to the edge of a 9.3mm overall diameter lens. On the other hand, a base curve radius of 6.50mm can be finished with any set of keratoconus curves, allowing that the final curve is flat enough to avoid binding and not so flat as to cause lid irritation and blink inhibition.

Table 3 is a summary of lenses currently still worn by our patients. Note, the relative flatness of the BCR with respect to the sim-K values. Notice that the fit is not related to the cone apex, but rather to the intermediate zone of the cornea. All values except the power are expressed in millimeters.

Results

The range of contact lens designs available to the clinician for the keratoconic corneas and other specialty cases is only limited by the fitter's imagination and the technical capabilities of his manufacturing laboratory.

We have demonstrated the usefulness of a very special lens design and have detailed a facilitated fitting procedure, which for us, has shown a success rate of approximately 77 percent (11/14) when used in properly selected cases.

Realistic Views

No single contact lens design can satisfy the ocular health, and visual and comfort needs of every patient. While most keratoconic patients can be fit at least initially with an adapted tri-curve lens, some will do better with aspheric or bi-aspheric designs, while others will require lenses designed with very small optic zones and proprietary peripheral curve systems.

Finally, there are those keratoconic patients who have progressed to such advanced stages of the disease, that their cone apices are too steep to be reliable for vision. These patients' cones are so thin that contact lens bearing readily causes distortion and scarring. In these cases, the cone apices need to be vaulted and the secondary areas of their corneas need to be used for vision. That is the niche of the Duozone concept.

As a final note, I would like to add that in the past several months since I first submitted my article, we have in total, ordered 97 lenses of this design. I can account for 25, which now make up my trial set, and another handful, which have been reported lost or broken. I can still claim around a 65 percent success rate, and in many cases, improved comfort and corneal physiological response with this relatively easy-to-fit design. 

I am indebted to Shaul Sterman of Precision Contact Lens of Jerusalem, Israel for the lens manufacture.

References are available upon request to the editors at Contact Lens Spectrum. To receive references via fax, call (800) 239-4684 and request document #55. (Be sure to have a fax number ready.)