An In-Office Evaluation of a Multifocal RGP Lens Design

by Stephen P. Byrnes, O.D., F.A.A.O., & Alex Cannella, RN, F.C.L.S.A.
November 1999

These two professionals performed their own evaluation of a multifocal lens and ended up with some pleasing results.

We performed an in-office evaluation of an RGP multifocal contact lens design (Boston MultiVision/Polymer Technology) to determine how well it would perform in an everyday office setting.

This progressive add lens design uses back surface aspheric curves and a spherical front surface to create a contact lens with a nominal (+1.50D) add effect. The elliptical base curve has an eccentricity value that creates moderate flattening of the posterior optical zone. Using the simultaneous vision principle, this central zone provides distance and intermediate vision for the wearer. The mid-peripheral area of the lens is a hyperbolic curve that employs greater flattening to produce an effective near add power of +1.50D. To access this higher add area, the lens must be repositioned or translate relative to the eye's optical axis.

An elliptical curve at the extreme periphery of the lens creates an "inverse geometry" which provides both lens stabilization and appropriate edge lift (clearance). A "fillet" curve, a spherical curve mathematically calculated to create a smooth, junctionless transition, joins the hyperbolic and the "inverse geometry" elliptical peripheral curve.

The MultiVision lens is available in 0.10mm (0.50D) base curve increments from 7.30mm to 8.30mm. It is available in one diameter: 9.6mm. The 9.6mm lens diameter was not a limiting factor for our potential wearers. We found that changing the lens base curve solved lens positioning or lens-cornea relationship problems. During the initial diagnostic lens fitting, if the lens could not be fit by changing lens base curve, we knew immediately that it would not work for that particular patient.

Because our practice has a high RGP patient population, a large number of whom are becoming presbyopic, we felt that these adapted RGP wearers would be the most logical candidates to try this lens. A summary of the individuals who participated in our study is shown in Table 1. Our intention was to determine how well this RGP lens functioned in an everyday setting.

The Fitting Process

The actual fitting process was easy since the majority of the patients were successfully adapted RGP wearers. However, this meant more time could be spent qualifying potential wearers, discussing wearer expectations and options, and educating the wearer on how to use this lens.

We elected to use diagnostic lenses during the fitting process for two reasons. First, we wanted to get the fit right the first time, minimizing the need for reorders to improve lens fit and lens power. Second, we used trial contact lenses to provide the fitter with information on the patient's reaction to the lens and to allow the wearer to experience the optical dynamics of the new lens design. During our study, we found that the average number of lenses required to achieve an acceptable fit (for the patient and the fitter) was 3.04 lenses per patient. Eleven of the 21 patients required lens parameter changes for fit, lens power or both.

As expected in fitting these types of lenses, the majority of the lens parameter changes (88%) related to lens power for improvement of near and distance visual acuity. The number of parameter changes decreased as we became more accustomed to working with this design. However, the one recommendation we would make to the manufacturer would be to offer several steeper base curves. The patient who required the most changes wore a MultiVision lens, incorporating a front toric design to correct residual astigmatism. This required additional changes to adjust front surface lens cylinder.

The manufacturer recommends that the ideal candidate for this design would be 50 years old or younger and that near (add) power requirements should be +1.50 or less. We challenged these recommendations since 12 of our 21 patients had near add power requirements in excess of +1.50D (See Table 1). As such, more time was spent and a few more lens changes were required to provide an acceptable balance of near and distance visual acuity for the wearer.

In order for this contact lens to provide optimal near vision power, lens position and lens translation (vertical movement) were critical to the success of the fit. We followed the manufacturer's recommendation for initial base curve selection, but we relied on the fluorescein pattern as the ultimate indicator of best lens-cornea fitting relationship (Table 2). Final base curve dispensed referenced to the flattest corneal curvature (flat "K") showed that on average, lenses were fit approximately 0.15mm or 0.75D steeper than "K." We found that this contact lens design worked equally well on both near spherical and moderately astigmatic corneas.

Diagnostic Lens Evaluation

Lenses that positioned centrally, superior-central, superior and even slightly temporal all worked very well. Lenses that centered low or nasal were not successful. In terms of lens movement, the customary 1mm to 2mm movement with a blink was observed with eyes in the primary position. To evaluate vertical lens movement, we had each patient look downward, while elevating the upper lid. In each case, good lens translation was confirmed when we saw that the lens moved easily across the superior corneal limbus with downward gaze. In primary gaze, the well-fit lens dropped back to a central position.

Using fluorescein pattern evaluation, we attempted to create an alignment fit as best as possible. We realized later that erring toward a slightly steeper lens when a choice was presented resulted in a better lens fit. However, the lens still had to make a full translation vertically with no impingement evident.

We feel that overrefraction is most critical to the success of the lens fit. Overrefraction is best performed using loose trial lenses. Monocular overrefractions are performed for distance and near. It's important that the overrefraction be refined binocularly to give the wearer a "real world" feel for distance and near vision. The phoropter does not allow the patient to position their head properly to gaze easily from distance to near. We have spoken to some fitters who use the phoropter to complete the distance overrefraction, then revert to loose lenses for near, but we feel using loose lenses throughout allows us to train the patient to find the "sweet spot" in the lens that provides the best distance, intermediate and near vision quickly and easily.

Distance and Near Vision Results

Since the majority (57%) of the patients we fit were outside the recommended +1.50D add limitations, it was essential to "push" as much plus power as possible. This is also the reason why the majority of our lens parameter changes were for distance power adjustment. We simply had to be "stingy" with giving these wearers more minus power for distance in order to provide full near power for reading. Despite these compromises, 20 of the 21 (95.2%) wearers in our study were able to achieve distance visual acuities of 20/25 or better (Table 3).

At the time we conducted our study, the one limitation of the MultiVision design we found was that the only nominal add (near) power available is +1.50D. Since then, higher add powers are now available in this design. Many of the candidates we selected required higher near powers with their spectacle corrections. Options and alternatives regarding near vision were discussed with each wearer prior to and during the fitting process. Since our study group came from varying backgrounds and occupations, and each had different vision requirements, we discussed options such as half-glasses over their MultiVision lenses for extreme or prolonged near work and modified monovision when either distance or near demands required that we "stretch" the nominal add power. This worked especially well where the MultiVision lenses were found to meet most of the wearer's distance and near visual requirements.

Although the majority of our patients were already adapted to RGP lens wear, no remarkable negative slit lamp changes were noted with this lens fit compared to their previous lens fit. A positive observation that was noted was the absence or reduction of 3 and 9 o'clock corneal epithelial staining in patients who were refit with the MultiVision lens.

The edge design of this lens utilizes a constant edge lift of 120�m (0.12mm). The edge thickness is also held constant at 0.14mm (through lenticulation). A generous peripheral tear lake is created by the peripheral inverse elliptical design. Our impression is that the combination of these factors produces positive pressure under the lens edge to pump the tear film around the surface of the cornea, bathing it with tears, thereby reducing corneal desiccation. We also feel that this edge configuration and the manner in which it functions are major contributing factors to the unsolicited responses of excellent lens comfort volunteered by the patients in our study.

Post-Fitting Follow-Up

At the time of writing this article, we are 6 months post-fitting with these patients. We recently surveyed all 21 patients who participated in our study to determine if they were still wearing their lenses and to assess the current level of satisfaction. Those who did not respond were contacted by our staff. We were able to receive feedback from 16 patients.

We estimate that our fitting success rate exceeded 75 percent. We found that this lens was both easy to fit and had a very high acceptance rating among our study patients. The lens comfort ratings were excellent, and the visual acuity for both distance and near was acceptable to us as well as to the patients. Our patients are now beyond 6 months of lens wear, and all of the patients who responded to our survey are still successfully wearing their MultiVision lenses.

Interviewing, educating and overrefrating candidates for presbyopic lenses can result in a mutually successful and rewarding experience.

Dr. Byrnes is in private practice in Londonderry, New Hamp., and is a clinical consultant to Bausch & Lomb.

Alex Cannella is employed by Polymer Technology as International Professional Services Manager and is consultant to the practice of Byrnes and Riley OD's.