Reader and Industry Forum
Aspheric Optics for Aberration Control vs. Conventional Optics
By Gerard Cairns, PhD, MCOptom, FAAO
Getting the most out of an individual's vision is imperative for every eyecare professional. In fact, in a recent survey of 3,800 vision-corrected individuals from seven countries, when participants were asked to rank 40 different features associated with their eye care (e.g., health, comfort, vision, etc.), six of the top seven features were related to vision. For many people, contact lenses can provide the best quality of vision because of improved peripheral vision, absence of spectacle glare and potential accommodation benefits.
In addition to the overall advantages of contact lenses, specific lens designs may offer other potential benefits. For example, the average spherical aberration population-wide has been reported to be +0.18μm (Cox et al, 2011), an amount capable of reducing retinal image quality. In addition, conventional spherical surface (nonaspheric) contact lenses themselves induce spherical aberration, which can vary by power and design. Aspheric optics can be incorporated into soft lens designs to reduce spherical aberration. An ideal contact lens with aspheric optics would correct both the spherical aberration of the eye and the spherical aberration induced by the lens across all lens powers, even those that include astigmatism correction.
The purpose of this series of studies was to determine if introducing spherical aberration control into contact lenses provides benefits to vision.
To compare high-contrast visual acuity (VA) and spherical aberration measurements with spherical and aspheric soft contact lenses, we conducted four studies using balafilcon A lenses with aspheric optics (Test; PureVision2 HD, Bausch + Lomb [B+L]) and senofilcon A lenses with conventional spherical optics (Control; Acuvue Oasys, Vistakon). Ethics approval was obtained for each study prior to enrollment.
A total of 94 adapted contact lens wearers participated in these studies. Between 22 and 25 subjects completed each of the four studies, as follows:
• Study A compared +3.00D Test and Control lenses on a group of hyperopes
• Study B compared −1.00D Test and Control lenses on a group of low myopes
• Study C compared −5.00D Test and Control lenses on a group of moderate myopes
• Study D compared −9.00D Test and Control lenses on a group of high myopes.
Following pupil dilation to 6mm with 1% tropicamide, baseline spherical aberration readings were recorded using a Zywave aberrometer (B+L) followed by a second reading through the contact lenses. Lenses were allowed to settle for 3 minutes before VA was assessed. High contrast logMAR visual acuities were recorded through the contact lenses using a 6mm artificial aperture. Within each study, paired comparisons were conducted between Test and Control lenses. A value of P<.05 was considered statistically significant.
We found statistically significant differences in logMAR VA between subjects wearing the Test lenses and those wearing the Control lenses. For the −9.00D, −5.00D and +3.00D powers, the Test lenses provided significantly better VA (4.5 letters, 3.5 letters and 6.5 letters, respectively; P<.05 in each group). The Test lenses provided better logMAR VA for the −1.00D lenses (1.0 letters); however, this result was not statistically significant (P>.05) (Table 1; Figures 1-4).
|Table 1 LogMAR VA at Distance, Normalized to Spectacle VA
(Positive values represent results that better spectacles.)
|Study||Balafilcon A||Senofilcon A||Difference|
|A (+3.00DS)||−0.05||−0.18||PV + 6.5 letters*|
|B (−1.00DS)||−0.02||−0.04||PV + 1.0 letter|
|C (−5.00DS)||0.05||−0.02||PV + 3.5 letters*|
|D (−9.00DS)||0.00||−0.09||PV + 4.5 letters*|
|* Statistically significant (P< 0.05)|
Figures 1-4. Schematic representations of differences in high contrast, high illumination visual acuity with spherical and aspheric lens optics.
The data from all four studies were stacked and used to fit a linear mixed model. The model included the fixed effect of lens type and the random effect of subject nested within lens power. Model adequacy was verified by examining residual plots. Least squares means were estimated. Overall, the mean difference between the balafilcon A (Test) lenses and the senofilcon A (Control) lenses was 0.073 logMAR or 3.5 letters of logMAR acuity.
Furthermore, over the four studies, there was a statistically significant difference in spherical aberration reduction between the lenses. The balafilcon A (Test) lenses with aspheric optics reduced the mean spherical aberration by 0.136μm, whereas the senofilcon A (Control) lenses with conventional spherical optics reduced the spherical aberration by 0.054μm.
Contact lens practitioners are routinely challenged to provide the best-in-class product to meet the varied demands of their patients. Incorporating aspheric optics into contact lens design to reduce the estimated population-wide spherical aberration advances the science of contact lens care and vision correction. Furthermore, contact lenses with aspheric optics not only reduced spherical aberration but provided real benefits in VA.
This series of studies has shown that balafilcon A lenses with aspheric optics significantly reduced spherical aberration and provided better VA over conventional optics: true points of differentiation when recommending contact lens products. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #193.
Dr. Cairns is manager, Global Medical Affairs for Bausch + Lomb.