Managing Spherical Aberration With Contact Lenses

Improving patients’ quality of vision with aspheric optics.

By Gregory DeNaeyer, OD; Alexis Vogt, PhD; and Marjorie J. Rah, OD, PhD

Manufacturers need to have access to new and improved contact lens materials to compete effectively in the contact lens market. Optics of contact lenses, however, play a crucial role in successfully fitting patients. Insights from patients are also imperative in designing lenses that meet their needs and expectations. During the past 5 or 6 years it seems as though much emphasis has been placed on enhancing the comfort of lenses, although dropout rates have remained stable throughout this time period.1,2 This leads to the question: What else may be causing patients to stop using their contact lenses?

The Needs, Symptoms, Incidence, Global Eye Health Trends Study

The NSIGHT (Needs, Symptoms, Incidence, Global Eye Health Trends) Study surveyed 3,800 vision-corrected patients to evaluate self-reported experiences, attitudes, and preferences.3 Patients included in the survey ranged in age from 15 to 65 years and were drawn from three global regions (United States, Europe, and Asia) that included seven countries (United States, United Kingdom, France, Italy, China, Korea, and Japan). The study reported two main outcomes: patients’ needs in relation to the selection of eye care products and the incidence of vision- and eyerelated symptoms.3,4

To assess their preferences when selecting eye care products, respondents reviewed 40 features of eye care products in multiple groupings of six. For each group, they chose one feature as the most compelling and one as the least compelling. These rankings were analyzed to produce a utility index score for each of the 40 features, which were then stratified into eight categories. Vision was the highest-ranked product feature category among respondents in all three of the global regions surveyed (Figure 1) for both contact lens wearers and spectacle wearers.3

In addition to patients’ preferences regarding products, the NSIGHT Study also evaluated the incidence, management strategy, and overall satisfaction with available treatments for symptoms. With respect to vision, two symptoms in the survey—glare and halo—stood out to researchers because these symptoms can be controlled with the design of contact lenses, specifically aspheric optics. In the NSIGHT Study, glare and halo were experienced by 46% and 39%, respectively, of all contact lens-corrected patients.4 When asked under what conditions the symptoms were experienced, driving, bright lights, sunlight, headlights, and being in the dark or nighttime viewing were commonly reported.

Spherical Aberration

The above-mentioned symptoms can be partially attributed to ocular aberration, and specifically, to spherical aberration. In a perfect ocular system without aberrations, a point source of light will come to a perfect point focused on the retina. In an eye with optical aberrations, the point source of light does not come to a perfect point of focus, and thus the result is a blurred retinal image. The general population has a mean positive value of spherical aberration, which is in contrast to an average close to zero with most other types of ocular aberrations such as coma and trefoil.

Spherical aberration refers to the inability of a lens or visual system to simultaneously focus the central and peripheral light rays to the same point. Instead, the peripheral light rays focus anterior to the ideal focal point resulting in degraded retinal image quality, reduced contrast sensitivity, and reduced low-contrast visual acuity. Spherical aberration can cause blurred vision, halos, and glare. In the eye, the magnitude of spherical aberration is directly affected by pupillary size, with higher amounts of spherical aberration associated with larger pupils. This may occur more frequently in low-light environments, such as driving in the dark, movie theaters, and dimly lit restaurants. Spherical aberration may degrade image quality even in photopic lighting conditions.

Aspheric Optics and Lens Design

One option for the correction of spherical aberration is to use advanced contact lenses that incorporate aspheric optics. Two components must be considered when designing spherical aberration-correcting aspheric contact lenses: inherent spherical aberration and induced spherical aberration. As mentioned previously, there is an inherent amount of spherical aberration in the average eye. In a study of 1,333 eyes (1,159 myopic and 174 hyperopic), a mean spherical aberration of +0.18 μm was reported for the myopic eyes and the age-matched hyperopic eyes.5

The optical properties of a soft contact lens that is soaking in solution differ from those of the same lens when placed on the eye. The wearers’ initial blinking upon insertion causes the lens to conform to the ocular surface (Figure 2). This conformation of the lens induces spherical aberration, the amount of which varies with the spherical power of the lens.6 To predict the behavior of a contact lens in this conformed state, a numerical-analysis technique called finite element modeling can be used.7 Finite element analysis creates a model of the structure under test conditions and subdivides the structure into small elements on which differential equations are solved to predict the behavior of each element as well as the whole structure. This type of modeling is used to predict the behavior of a contact lens in its conformed state and to subsequently design contact lenses in the state in which they are worn (Figure 2).

Clinical applications

Contact lenses that correct spherical aberration offer eye care practitioners a unique way to help improve visual quality for their patients. Unfortunately, it is often assumed by specialists and patients alike that halos and glare are an accepted side effect of contact lens wear. Prescribing a lens design that employs aspheric optics does not increase the difficulty of their fitting, because spherical aberration is symmetric and does not require a specific on-eye lens orientation like correcting for astigmatism. Providers should educate themselves on the way various lens designs attempt to incorporate aspheric optics across a range of powers, as there are significant differences in the efficacy of each aspheric design. The newest generation of aspheric lens designs, PureVision2 For Astigmatism with High Definition Optics (Bausch + Lomb), incorporates a population standard spherical aberration magnitude that is adjusted for specific lens powers, which improves the effectiveness of the aspheric design.

Summary

It is important to consider the complete lens-wearing experience when fitting patients with contact lenses (see Clinical Pearls). In addition to comfort and health, eye care practitioners should also take a closer look at patients’ quality of vision. The findings of the NSIGHT study have revealed that vision is what patients most desire.

Gregory DeNaeyer, OD, is the clinical director for Arena Eye Surgeons in Columbus, Ohio. He is a consultant or advisor to Aciont Inc., Bauch + Lomb, and Visionary Optics. Dr. DeNaeyer may be reached at gdenaeyer@arenaeyesurgeons.com.

Marjorie J. Rah, OD, PhD, is the manager of global medical affairs—vision care at Bausch + Lomb. Dr. Rah may be reached at marjorie.rah@bausch.com.

Alexis Vogt, PhD, is an optical design engineer in lens design at Bausch + Lomb. Dr. Vogt may be reached at alexis.vogt@bausch.com.

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