RGP MULTIFOCAL

Clinical Experience With a Multiaspheric RGP

By TIMOTHY T. MCMAHON, OD and S. BARRY EIDEN, OD
November 1998

These practitioners have worked with the new Boston MultiVision lens for over two years. Their experience can help you learn the patient factors and fitting techniques required for success.

Designed for emerging to early presbyopes (<52 years old), the Boston MultiVision rigid gas permeable (RGP) multifocal contact lens is new to the contact lens market and is the first multifocal design from Polymer Technology, Inc. This article will introduce you to the Boston MultiVision contact lens and relate the more than two years of experience that we gained working with this lens throughout its development.

Lens Design

The Boston MultiVision RGP multifocal contact lens is a complex, multiaspheric, posterior curve design (Fig. 1). The central region is elliptical in shape, changing to a hyperbola moving outward. There is a small spherical fillet curve in the midperiphery which is used to smooth the hyperbola to an inverted (negative e-value) aspheric curve for fitting purposes, and the periphery is finished as a hyperbola. The front surface of the lens is spherical, and the diameter is fixed at 9.6mm to simplify fitting parameters.

Fundamentally, the MultiVision contact lens is a simultaneous vision class multifocal. Like many of its aspheric cousins, MultiVision's near vision is substantially gaze-independent. Unlike some members of this class of contact lenses, however, the MultiVision also utilizes translation to enhance plus power for near-viewing tasks. As the eye translates with inferior gaze, the visual axis moves from the central elliptical curve into the higher eccentricity hyperbolic curve, thus generating greater plus power for near. Most importantly, the MultiVision lens is fit differently than its aspheric multifocal relatives. Manufactured in Boston ES material, Polymer Technology delivers the lens to local laboratories with both the posterior surface and the edge finished (Fig. 2). The laboratories then add the front power curve as indicated and any necessary edge treatment, such as lenticulation. The resultant product is a contact lens that has a thin edge and a relatively low mass.

Study Protocol

When the manufacturer asked us to evaluate this lens design several years ago, the fitting protocols and performance characteristics were unknown and there were already a multitude of aspheric contact lenses on the market.

During our initial evaluation, we consciously selected relatively difficult patients, such as marginal successes, prior RGP multifocal failures and critical observers. We recruited 18 subjects, ranging in ages from 43 to 57 years, with a mean age of 50 years. All subjects were healthy, presbyopic and current RGP lens wearers. Eleven of the 18 subjects were female. Average length of lens wear was 24 years, with a range of 0.5 to 35 years.

The average length of RGP contact lens wear was eight years, ranging from 0.5 to 15 years. Nine participants were using RGP multifocals, eight of which were aspheric in design, and one was a segmented translating design. Nine subjects were using single-vision RGP lenses, one was using an aspheric design with readers, five were using spherical monovision and three were using spherical single-vision lenses corrected for distance acuity with reading spectacles for near tasks.

At baseline exam and at each subsequent visit, we measured high and low contrast distance acuity and high contrast near acuity under wearing conditions as well as best corrected distance and near high contrast acuity. We recorded central keratometry and corneal topography readings and performed a thorough slit lamp exam at each visit. We also assessed manifest refraction and best spectacle corrected visual acuity, and we gave all subjects a visual analog scale of comfort (0 to 100) at each visit.

We used a trial set of 11 lenses in base curve steps of 0.10mm over the range 7.30 to 8.30mm. The diameter was a fixed 9.6mm and the distance power was -3.00D.

We initially trial fitted the patients quite steeply, as most posterior aspheres require, but performance was actually superior with this lens when an alignment fit was achieved. Therefore, fitting the lens near the flat keratometry reading turned out to be the most common fitting relationship. We trial fitted patients empirically and subjectively, and we ordered the best performing cornea-to-base curve relationship lens with the best distance overcorrection applied. We dispensed the lenses and scheduled follow-up visits for one week, one month, three months and nine months.

Results

All 18 subjects successfully enrolled and completed the initial three months of the study, though three subjects discontinued use of the lenses by the end of the three-month period. Two of the participants who dropped out returned to their previous lenses due to inadequate near vision; the third participant quit RGP lens wear altogether due to discomfort and is now using single-vision soft lenses. After the nine-month follow-up visit, 15 patients continued to use the study lenses. We conducted data analysis of lens performance utilizing data from one eye from each subject, randomly selected.

The mean prestudy (habitual) lens base curve was 43.58D, the standard deviation was 1.48D and the range was from 40.75 to 46.00D. The mean pre-study base curve to flat K relationship was +0.72D, with a standard deviation of 1.14D, ranging from -0.62 to +4.00D. The study lens-to-cornea fitting relationship resulted in a mean final base curve of 43.50D with a standard deviation of 1.25D and a range of 40.75 to 45.62D. The study lens base curve to flat K fitting relationship was +0.56D with a standard deviation of 0.63D and a range of -0.50 to +1.75D. These figures demonstrate a fitting relationship slightly steeper than the flat K reading.

We measured high contrast visual acuity with the dispensed lenses using a Bailey-Lovie logMAR chart which revealed a mean visual acuity of 20/37 at baseline dispensing with a range of 20/20 to 20/200. At one month, the mean visual acuity was 20/30 with a range of 20/15 to 20/120, and at three months, the mean visual acuity was 20/24 with a range of 20/15 to 20/48. The difference was not statistically significant (p>0.05).

The mean best corrected high contrast visual acuity with dispensed lenses was 20/25 at baseline (range 20/20 to 20/30), 20/20 at one month (range 20/15 to 20/25) and 20/20 at three months (range 20/15 to 20/40). The difference between baseline and the one-month visit was statistically significant to the p=0.01 level.

The low contrast visual acuity using a 10 percent contrast Bailey-Lovie logMAR chart with dispensed lenses was 20/40 at one month (range 20/24 to 20/120) and 20/45 at three months (range 20/24 to 20/120). There was no statistically significant difference between these groups (p>0.05).

Mean near visual acuity with dispensed lenses using a near point logMAR chart in M units was 0.63M at baseline (range 0.4 to 1.60), 0.61M at one month (range 0.4 to 1.25), and 0.64M at three months (range 0.4 to 1.60). The difference between the groups was not statistically significant.

Mean keratometry was 43.30D at baseline and showed a clinically and statistically significant flattening at one and three months. The corneal flattening occurred in both the flat and steep meridians, and averaged slightly less than 0.50D (Table 1). Mean manifest refraction at baseline was -5.52D sphere and 0.96D of cylinder. The cylinder remained unchanged throughout the trial, whereas the sphere decreased to -4.97D at one month and -5.13D at three months (p<0.05). This represents a refractive change of slightly less than 0.50D. The keratometric change and the refraction change corroborate the influence of this lens design on corneal curvature.

Discussion

Our study indicates that clinicians should consider fitting this lens in an alignment manner such that the base curve approximates the flat keratometry reading or is a little steeper. Virtually all patients ended up
+0.50D of the flat keratometry meridian in our study, which is notably flatter than practitioners would normally fit posterior aspheric multifocal lens designs. Corneas with higher cylinders require a steeper relationship, and flat corneas require flatter base curves.

The fluorescein patterns demonstrated by this lens are rather striking and may seem to reveal excessive edge lift to those who are accustomed to the low edge clearance profiles popularized over the past 10 years (Fig. 3). Clinicians who have experience with PMMA lenses will see a familiar pattern that was common in the 1960s and 1970s. The patterns appear to show slight apical clearance to slight apical touch. Aim for a central alignment fit, but slight variations from this pattern will work. There will commonly be midperipheral bearing acting as a fulcrum for lens rocking.

In our adapted lens-wearing group, this was the most comfortable rigid lens that 16 of the 18 participants had ever worn. We suspect that the peripheral posterior design is responsible for these reports (Table 2). The high retention rate in this group of difficult cases was impressive to us. Other than the three subjects who dropped out, the remaining participants continue to wear the MultiVision lens. If we consider continued use to be a measure of success, then 83 percent of our subjects have met that criteria. This is a higher rate than patients have achieved in the past with multifocals or bifocal rigid or soft contact lenses of any design.

The changes found in central keratometry, corneal topography and refraction are important. There appears to be a trend toward mild central flattening which is reflected by the reduced keratometric readings, which in turn reduces spectacle-corrected myopia by a small but visually important amount. Corneal topography maps frequently reveal central to superior flattening with an inferior arcuate smile-shaped area of steepening approximating the location of the inferior edge of the lens. We feel that it is important to consider this issue early in the fitting process and to advise your patients about the potential need for a change in spectacles once a wearing schedule has been established. Previous corneal molding tendencies were a predictor of corneal and refractive changes. However, for the majority of subjects, we were unable to discern who would change. This phenomenon is not unique to the MultiVision lens, as it occurs in virtually all other posterior aspheric multifocal designs. However, unlike many other high eccentricity aspheric multifocal RGP designs we have used, best corrected spectacle visual acuity is not significantly reduced following wear of MultiVision. Though we report on a nine-month study, our experience has also shown that these changes are transient, at least up to the two years that we have followed these subjects.

Our experience has led us to believe that MultiVision represents an excellent entry-level multifocal rigid gas permeable lens for the emerging to early presbyope. The fitting characteristics are similar to single-vision aspheric lens fitting, so there appears to be little additional skills required of the fitter to use this lens design effectively. Its greatest value will be for the current rigid lens wearer who is experiencing presbyopic symptoms. Additionally, the surprising degree of comfort found by our subjects suggests that clinicians should not hesitate in considering this lens for the interested spectacle wearing or soft lens wearing candidate as well. CLS

The authors have no proprietary interest in this product nor in Polymer Technology, Inc. or Bausch and Lomb, Inc., Polymer's parent company.

This study was supported in part by grants from Polymer Technology to North Suburban Vision Consultants, LTD, and the University of Illinois at Chicago.

Dr. McMahon is an associate professor at the University of Illinois in the Department of Ophthalmology and Visual Sciences. He is a fellow of the AAO and is a diplomate in the Cornea and Contact Lens Section.

Dr. Eiden is president of North Suburban Vision Consultants, Ltd., a group optometric practice which emphasizes primary eye care and contact lens services. He is also a member of the advisory board of the RGP Lens Institute and president of United Vision Associates, Ltd., which provides continuing education for the ophthalmic professions.

Timothy T. McMahon, OD and S. Barry Eiden, OD

FIG. 1: A schematic diagram of the posterior surface demonstrating a central ellipse that changes to a hyperbola for a near add effect in a seamless (junctionless) manner. A spherical fillet curve connects this segment to an inverted (- e value) fitting curve in the periphery.

TABLE 1 

Keratometry

* p<0.005 (from baseline)

Keratometry: Mean Change From Baseline

*p<0.005 (from baseline)

Manifest Refraction

Manifest Refraction: Mean Change From Baseline

*p=0.002 sphere-decrease (less) in minus

**p=0.0 15 cylinder-decrease in power

 

"Corneas with higher
cylinders require a steeper relationship, and flat corneas require flatter base curves."

FIG. 2: Example of a lens blank provided to finishing laboratories. Note the precut posterior surface and edge.

TABLE 2

Comfort Scores:

Visual Analog Scale - 1 (bad) to 100 (good)

(difference not statistically significant)

 

FIG. 3: Fluorescein pattern representing a typical fitting relationship. Note the high peripheral clearance.

"If we consider continued use to be a measure of success, then 83 percent of our subjects have
met that criteria."