Higher-Order Aberrations and Contact Lenses
BY S. BARRY EIDEN, OD, FAAO, & PAUL VELTING, OD
The eye has several optical elements that focus light rays on the retina. Anomalies of these elements may cause the light rays to deviate from the optimal path. These deviations, referred to as optical or wavefront aberrations, result in blurred images and diminished visual performance (Schwiegerling, 2000).
Low-order aberrations (LOAs) typically refer to traditional refractive errors of myopia, hyperopia, and astigmatism. High-order aberrations (HOAs) describe a series of optical imperfections that cannot be corrected by traditional refractive means. Such HOAs in the normal eye can contribute as much as 15% of the total ocular aberrations (Resan et al, 2012). Abnormalities and ocular diseases that negatively impact the optical elements of the eye—irregular corneal diseases and lenticular anomalies, such as cataracts or posterior capsular opacities following cataract surgery, among others—will result in significantly higher degrees of HOAs. The most common and visually significant HOAs in the human visual system are coma, trefoil, and spherical aberrations (although other HOAs exist as well).
Today, clinically available technologies are able to identify and quantify HOAs. Aberrometry systems allow clinicians to not only measure LOAs and HOAs, but through integrated technologies they can identify the source of the HOAs (cornea versus internal). Contact lens wear can significantly impact HOAs. What is some of the current research telling us about HOAs and their relationship with contact lenses?
Aberrations in Normal Eyes
Levy et al (2005) evaluated HOAs in eyes that had supernormal vision (natural uncorrected visual acuity [UCVA] ≥ 20/15). Ocular HOAs were examined across a naturally dilated pupil with a diameter ≥ 6.0mm in 70 eyes of 35 subjects who had ≥ 20/15 UCVA (mean age 24.3 ± 7.7 years [SD]) using the Nidek OPD scan wavefront aberrometer. Root-mean-square (RMS) values of HOA, total spherical aberration (TSA), total coma (TC), and total trefoil (TT) were analyzed. Correlation analysis was performed to assess the association between ocular HOAs and age as well as the correlation of HOAs between right and left eyes.
The mean RMS values were 0.334μm ± 0.192μm for total HOAs, 0.110μm ± 0.077μm for TSA, 0.136μm ± 0.081μm for TC, and 0.268μm ± 0.220μm for TT. There were no significant differences in the mean values of HOA, TSA, TC, and TT between right and left eyes. No significant correlation was found between each of the ocular aberrations and age. They concluded that the amount of ocular HOAs in eyes with natural supernormal vision is not negligible; it is comparable to the reported amount of HOAs in myopic eyes.
Zhang et al (2013) evaluated the relationship between HOAs and myopia progression in school-aged children. They defined myopia progression rate as the change in spherical equivalent refractive error divided by the time span in years. Significant correlations were found between the RMS values of total HOAs (r=0.193, P=0.019), RMS values of HOAs without spherical aberration (r=0.23, P=0.005), RMS values of coma (r=0.235, P=0.004), RMS values of third-order aberrations (r=0.243, P=0.003), and the myopia progression rate. The authors concluded that these results indicate a relationship between HOAs and myopia progression.
Aberrometry to Evaluate Contact Lens Optical Quality
Montes-Mico et al (2013) attempted to quantify the optical quality of various daily disposable contact lenses and to ascertain variability in terms of wearing time by utilizing measurements of wavefront patterns. Wavefront readings were taken in vivo for 15 myopic eyes prior to and at two-hour intervals following lens application. Seven types of daily disposable designs were evaluated for each subject. The optical quality was described by RMS values, modulation transfer function (MTF), and point spread function (PSF) data from aberrometry.
The results indicated that RMS increased both after lens application and over time. Each type of lens induced a different amount of wavefront aberration and also varied differently over time. One of the seven designs demonstrated the lowest RMS and MTF both at application and following 12 hours of wear. The researchers concluded that aberrometry makes it possible to analyze the optical quality of contact lenses in vivo. They suggested that variations across lens materials and designs may result in differences in visual performance.
Multifocal Contact Lenses and Aberrometry Outcomes
Sivardeen et al (2016) conducted a double-masked, randomized crossover trial to assess the performance of Air Optix Multifocal (Alcon), PureVision2 for Presbyopia (Bausch + Lomb), Acuvue Oasys for Presbyopia (Johnson & Johnson Vision Care, Inc.), and Biofinity Multifocal (CooperVision) monthly replacement silicone hydrogel lenses as compared to monovision with Biofinity sphere contact lenses. Thirty-five presbyopes (mean age 54.3 ± 6.2 years) were fit in lenses, and a multitude of visual performance measures were taken after four weeks of wear.
The aberrometry measurements showed that there were no significant differences among the presbyopic contact lens options, but the visual performance outcomes suggested that the multifocals outperformed monovision. The authors concluded that although ocular aberration variations among individuals largely masked the optical differences among multifocal lens designs, certain multifocal designs can outperform monovision even in early presbyopes.
Piñero et al (2015) compared the performance of Duette Multifocal (SynergEyes) hybrid lenses to two monthly replacement soft multifocal lenses (Air Optix multifocal and Biofinity multifocal). Aberrometry showed that trefoil was significantly higher with the soft multifocals as compared to the hybrid multifocal (p<0.01). The conclusion overall was that the hybrid multifocal provided similar visual quality outcomes, but aberrometry differentiated performance.
Keratoconus, Aberrometry, and Contact Lenses
Keratoconus is well known to result in significant elevations of HOAs. Contact lenses reduce HOAs by masking the anterior corneal surface irregularity and by creating a regular anterior optical surface.
Jinabhai et al (2011) evaluated ocular aberration measurements in keratoconus patients in an effort to evaluate the repeatability of aberrometry outcomes in these subjects. Aberrometry was conducted on 31 eyes of 31 patients who had keratoconus. Measurements were taken four times consecutively, and the data was analyzed up to the fifth Zernike order for a 4mm pupil diameter.
As expected, both low-order and high-order measurements were elevated in the keratoconus subjects; however, variability of individual HOAs with repeated measures was increased in these subjects. Interestingly, RMS values (which average out HOAs similarly to what the spherical equivalent does for sphere and cylinder) were comparatively less variable than were the individual HOAs. The researchers concluded that the LOAs and HOAs measured in keratoconus subjects were more variable compared to previous studies of visually normal subjects. They suggested that this should be taken into account when evaluating aberration-controlling contact lenses for patients who have keratoconus.
The optical system of the eye is extremely complex. For centuries, we have limited our analysis to low-order spherical and cylindrical corrections. Our understanding, measuring ability, and initial attempts at visual correction of HOAs are advancing at exponential rates. Research-based outcomes will enable us to most effectively advance this important area of vision science. CLS
For references, please visit www.clspectrum.com/references and click on document #246.
Dr. Eiden is president and medical director of North Suburban Vision Consultants, president and founder of the International Keratoconus Academy of Eye Care Professionals, and co-founder of EyeVis Eye and Vision Research Institute. He is an adjunct faculty member at The University of Illinois Medical Center as well as at the Indiana and Illinois Colleges of Optometry and Pennsylvania College of Optometry at Salus University. Dr. Velting is co-director of contact lens services at North Suburban Vision Consultants and is an adjunct faculty member at the Indiana, Illinois, UMSL, and Salus University Colleges of Optometry.