Varying lens power profiles appear to be visually inconsequential

September 15, 2005

Sydney, Australia—Currently available soft contact lenses vary substantially in their optic zone refractive power distribution, and labeled lens power may not correspond exactly to the effective power of the lens on the eye. However, with attention to proper lens fitting, the power profile differences do not appear to result in measurable differences on short-term visual performance, according to the results of a study undertaken by Australian researchers.

"Presumably, variations in power profiles of soft contact lenses reflect attempts by lens designers to control spherical aberration. The designs represent a compromise approach based on population average data and vary in nature between manufacturers," said Eric Papas, PhD, McOptom, executive director, Research & Development, Vision Co-operative Research Centre. "Our findings suggest that on average, these variations do not appear to be visually influential among individuals with low to moderate myopia, although they do not rule out the possibility that for some individuals one lens type or another may perform better or worse.

"Of course, achieving optimum vision depends on proper power selection, and practitioners need to be aware that there can be differences between effective in-eye lens power and what is written on the box. Therefore, accurate over-refraction is requisite for all fittings and especially when switching wearers between lens types," said Dr. Papas, adjunct senior lecturer, School of Optometry & Vision Science, University of New South Wales, Sydney, Australia.

Five brands tested

The lenses tested were the Acuvue 2 and Acuvue Advance with Hydraclear (Johnson & Johnson Vision Care), the O2Optix and Night & Day (CIBA Vision), and the PureVision (Bausch & Lomb). The subjects were fitted in their right eye only with each of the lenses in randomized order. In-eye lens power was adjusted to give plano over-refraction, and vision testing was performed after allowing for a 20-minute wear period, with the left eye occluded, and with the subject and examiner masked to lens identity.

The assessments included high-contrast visual acuity (HCVA) and low-contrast visual acuity (LCVA) measured under normal lighting and HCVA measured in dim illumination.

In addition, the participants were asked to evaluate their vision subjectively, comparing visual quality with their best-corrected spectacle vision, and to rate their visual satisfaction on a 5-point Likert scale (very dissatisfied to very satisfied).

Statistical analyses based on group mean data showed no significant differences between lenses for contrast acuity under normal or reduced illumination.

"Pupil size is an important factor in determining visual performance, and therefore, differences in vision performance between lenses might well become evident at certain pupil sizes and under some conditions. We measured performance under low light conditions in an attempt to capture something of this behavior," Dr. Papas explained.

The subjective evaluations also showed no significant differences between lenses. Ratings of mean visual quality for all lenses were high and considered generally better than that achieved with spectacle wear, and about 80% of subjects indicated that they were satisfied or very satisfied with their vision while wearing each of the different lenses.

After the lenses were removed, back vertex power (BVP) was measured using the Nikon projection focimeter, and then BVP error was calculated as the difference between the labeled power and focimeter measured power. In addition, lens power profile was determined (Visionix VC 2001) for three nominal BVPs (–1, –3, and –6 D) of each lens type using several lenses of each BVP.