Great advances have occurred in contact lenses over the last several years. High-oxygen-transmissibility silicone hydrogel lenses such as Night & Day (CIBA Vision) have the comfort, movement and wettability of conventional hydrogel contact lenses with the additional benefit of nearly eliminating corneal hypoxia and its adverse effects on corneal tissue.

Today practitioners are increasingly prescribing and dispensing silicone hydrogel lenses and even switching existing contact lens patients into silicone hydrogel lenses. One advantage of the increased oxygen transmissibility of these materials is a decrease in the probability of developing corneal neovascularization or pannus. This increased oxygen supply may actually contribute to regression of already existing neovascularization.

In addition to their higher oxygen permeability, some silicone hydrogels have a higher material modulus. It's been postulated that a stiffer material may provide improved vision because of decreased lens flexure. How, then, would the quality of vision with a silicone hydrogel contact lens compare with the quality of vision in a conventional hydrogel lens?

The basis of evaluating visual acuity (VA) with traditional letter charts is that the smallest high-contrast letters that a patient can correctly read provides an accurate measurement of visual function. We should use some measure of low-contrast vision when a discrepancy seems to exist between relatively good high-contrast visual performance and the extent of reduced subjective visual function. Researchers have used high- and low-contrast VA measurements for evaluating modes of correction for refractive error including contact lenses, but with varying results.

We performed a clinical evaluation comparing vision with Night & Day planned replacement lenses and CIBA's Focus Monthly lenses using high- and low-contrast Bailey-Lovie charts under conditions of normal room illumination.

Both Night & Day and Focus Monthly lenses are intended for replacement every 30 days. However, their similarities end here as Night & Day is an aspheric silicone hydrogel lens whereas Focus Monthly is a spherical conventional hydrogel lens. Night & Day is made of lotrafilcon A (38 percent water content) and is FDA-approved for 30 nights of extended wear. Focus Monthly is made of vifilcon A (55 percent water content) and is approved for daily wear and up to six nights of extended wear. Night & Day has a 13.8mm diameter and comes in 8.4mm and 8.6mm base curves. Its listed optical zone is 8.0mm and its center thickness is 0.08mm (average). The Focus Monthly has a diameter of 14.0mm and comes in 8.6mm and 8.9mm base curves. Its optical zone is 8.0mm and its center thickness is 0.08mm (at -3.00D).

Methods

Twenty-five subjects who were current soft spherical contact lens wearers (19 women and six men, ages 23 to 50 years) from the faculty, staff and student body of SUNY College of Optometry completed the study. There were no restrictions regarding sex, race, gender, current contact lens type or current care system. We recruited only subjects who had 0.75D or less of refractive astigmatism and were correctable to 20/25 or better with spherical contact lenses. We obtained informed consent from all study patients and our IRB approved the study.

The study involved four visits. To determine that all eligibility criteria were met, on the first visit each candidate underwent a sphero-cylindrical refraction using maximum plus to best acuity, Snellen VA measurements, slit lamp evaluation and manual keratometry. We then diagnostically fit each patient with both Night & Day and Focus Monthly lenses in each eye, determining the appropriate base curve in accordance with the manufacturer's guidelines for initial base curve selection and criteria for optimal lens movement. We determined appropriate lens power by over-refraction using the criteria of maximum plus to best VA.

At the second visit, we randomly assigned the patients to begin lens wear with a Night & Day lens in one eye and a Focus Monthly lens in the fellow eye. We provided detailed instructions on application, removal, wearing schedule and cleaning regimen utilizing Aquify Multi-Purpose Solution (in accordance with CIBA's guidelines) at this visit. We instructed all patients to wear the lenses on a daily wear basis for two weeks for a minimum of five days each week and for a minimum of eight hours each day.

Two weeks later at the third visit, we used the slit lamp to re-evaluate each patient's ocular health and lens fit. We determined high- and low-contrast VA separately for right and left eyes using printed Bailey-Lovie charts under controlled room illumination (70cd/m² to 110cd/m²). We recorded visual acuity as the total number of letters correctly identified until the patient was unable to correctly identify three consecutive letters. Patients then completed a subjective evaluation of vision using a 0mm to 100mm scale (where 0 represents the worst possible vision and 100 the clearest) for each eye separately. The scale contained five descriptive markers at the 0mm, 25mm, 50mm, 75mm and 100mm marks to assist patients in marking their vision as accurately as possible (appendix A, available in the online version of this article at www.clspectrum.com ).

At this visit we provided an alternate pair of contact lenses such that the eye previously wearing a Night & Day lens was switched into a Focus Monthly lens and vice versa. Once again we confirmed fit and lens power with the new lenses. Two weeks later at the fourth and final visit we re-evaluated the fit of the lenses and ocular health in each eye. We measured high- and low-contrast VA using the criteria described above. The subjects again provided a subjective evaluation of vision for each eye separately.

Two subjects who initially screened for inclusion in the study chose to drop out because of poor lens comfort. The remaining 25 subjects completed all four visits. Each of them demonstrated a good soft lens fit with proper limbal to limbal centration with both the Night & Day and the Focus Monthly lenses. Tables 1 and 2 show the average number of correctly identified high- and low-contrast letters after two weeks of lens wear, by subject, for each lens type. Table 3 shows approximated Snellen equivalent acuity based on the total number of correctly identified letters.

For the Night & Day lenses, the average high- and low-contrast Snellen equivalent acuity was 20/20⁺² and 20/25^-2 respectively. For the Focus Monthly lenses, the average high- and low-contrast Snellen equivalent acuity was 20/20⁺¹ and 20/25^-2 respectively. A paired t-test statistical comparison of the high-contrast Bailey-Lovie visual acuity scores between the Night & Day and the Focus Monthly lens types revealed no statistically significant difference (t = 1.037). Likewise, paired t-test comparison of the low-contrast Bailey-Lovie visual acuity scores between the Night & Day and the Focus Monthly lens types revealed no statistically significant difference (t = 0.094). Study patients also provided subjective vision scores following two weeks of wear for both the Night & Day and the Focus Monthly lens types (Table 4). Similarly, we found no statistically significant difference for subjective vision (t = 0.084).

Many different studies have evaluated visual acuity across various modes of refractive error correction, but results of these studies have been inconsistent. Some of the unpredictability of these results may stem from continual improvement in contact lens design, different methods of visual acuity measurement and/or small sample sizes. Interestingly, the combination of high- and low-contrast visual acuity is reportedly sensitive enough in normal subjects to provide the same information that you would gain by measuring contrast sensitivity. Because a contact lens rests directly on the eye, flexure, decentration and tear lens formation are all possible and each of these may impact final visual acuity. However, previous studies have demonstrated that flexure and tear-lens formation do not change the amount of on-eye aberration.

In addition to the differences in material characteristics, Night & Day, unlike Focus Monthly, is aspheric in design. This may contribute to a difference in either objective or subjective visual response. However, Vaz and Gundel (2003) failed to show any consistent difference in vision between aspheric vs. spherical design hydrogel lenses tested under similar conditions of normal room illumination. As most normal eyes show only small amounts of axial monochromatic aberration for photopic pupils up to around 3mm, the limits to retinal image quality would then be set by diffraction, uncorrected or imperfectly corrected refractive error, accommodative error and/or chromatic aberration.

Our results did not yield a statistically significant difference in either objective or subjective visual acuity between the two lens types in this study despite differences in lens material flexibility. Some of the possible reasons for this may include small study size, photopic pupil size and room illumination (as related to pupil size).

Although it's clear that silicone hydrogel lenses provide a superior environment for the human cornea, our study failed to reveal a difference in visual acuity and patient reported quality of vision between the Night & Day silicone hydrogel lens and the Focus Monthly hydrogel lens. Further studies limited to individuals who have large pupils and/or testing under scotopic conditions may yield different results.