Soft Lens Correction of Higher-Order Aberration
This study compared seven popular soft lens designs to determine which are best at correcting HOA.
By Garold Edwards, OD, FAAO
Current soft contact lens designs typically do a good job of correcting lower-order aberrations (LOA) such as myopia, hyperopia and astigmatism.
The human eye has other optical imperfections known as higher-order aberrations (HOA). Until recently, HOA such as coma, trefoil and spherical aberration weren't easily detectable or correctable in the typical eye care office.
HOA are receiving much more attention today for a number of reasons. Refractive surgery has increased the awareness of induced HOA. In-office aberrometers have become available to help measure HOA. Most importantly, practical ways to correct HOA are becoming more readily available. Research and development within the optical industry will soon yield individually designed wavefront-guided soft lenses manufactured from in-office aberrometry readings. This may represent a significant advancement for a large number of patients. Recent research suggests that up to 50 percent of myopes and 15 percent of the general population have HOA that affect their visual performance.
Focusing on the Present
While aberration-altering contact lenses hold great promise for future correction, the goal of this study is to evaluate the effect of currently available soft lens designs on HOA. Of the current soft contact lens designs readily available on the market today, a number of them incorporate aspheric optics or other design features that could serve to correct or reduce some degree of HOA. The purpose of this study is to evaluate current soft contact lens designs to see if they increase or decrease HOA in patients.
We selected eight consecutive myopic patients who presented at the test locations, regardless of the degree or severity of either LOA or HOA within their eyes. We evaluated each eye initially utilizing a Z-View Aberrometer (Ophthonix, Inc., San Diego, California). For each eye the results were captured in a refractive map that yielded the amount of lower-order correction needed, as well as the amount of HOA that existed in the unaided eye. A sample map is available in the online version of the article at www.clspectrum.com. The spherical equivalent for each eye determined the power of the contact lens chosen for that eye.
We then gave patients the same power lens for each of the following currently available soft lenses: Acuvue Advance (Vistakon); Biomedics Premier, Frequency Aspheric (CooperVision); DefinitionAC (Optical Connection, Inc.); Night & Day, O2Optix (CIBA Vision); PureVision (Bausch & Lomb).
Patients placed one of the seven lenses on their eye. After the lens had stabilized, we evaluated the corrected eye with the Z-View Aberrometer. The investigator measured each eye without knowing which brand was worn at that time. We printed a refractive map showing the amount of HOA that existed on each patient's eye wearing each lens. We compiled the resulting HOA maps, which are available for comparison in Appendix 1 of the online version of this article at www.clspectrum.com. If total HOA had increased over the amount present in the unaided eye, then that lens induced HOA. If the amount of HOA had decreased from the amount present in the unaided eye, then that lens design reduced HOA.
We didn't control pupil size during the study; all testing took place in a darkened room. Pupil size (as measured by the aberrometer at the time of testing) ranged from a minimum of 4.0mm to a maximum of 7.2mm. Two pupils measured less than 4.5mm and one was 4.8mm. The pupils of the remaining 13 eyes measured greater than 5.0mm at the time of testing.
Understanding the Z-View Aberrometer
When the Z-View aberrometer measures HOA, the combined HOA is classified on the wavefront map as either significant (red), moderate (yellow) or insignificant (green). Ophthonix developed this "red, yellow, green" convention based on normative data from a fairly large in-house database. (The actual RMS figures are proprietary.) A patient who has HOA in the red zone will always benefit from HOA correction. A patient who has HOA in the yellow zone will usually appreciate the difference, and may see some benefit from HOA correction. HOA in the green zone is clinically insignificant.
The Z-View Aberrometer measures all HOA, up to and including the sixth order. The wavefront map that the instrument provides breaks out spherical aberration (SA), coma and trefoil. It also combines all HOA in a "total aberration" figure.
Ophthonix converts HOA data into diopters in the wavefront map. Although it's somewhat unusual to speak of HOA using diopters, the diopter convention provides a "language" for discussion with patients. For example, if SA in one eye is insignificant, you can place a +0.12D lens before that patient's eye to demonstrate the effect on vision. If SA is significantly higher in the fellow eye, you can use a +0.62D trial lens to show the SA effect on that eye.
Correcting Total Higher Order Aberration Of the 16 unaided eyes evaluated in this study, one had below average HOA, eight had average amounts of HOA, and seven eyes demonstrated above average amounts of HOA. The range of aberration for the 16 eyes was between 0.17D of total HOA to 0.45D of total HOA.
All of the lenses evaluated in this study did an acceptable job of correcting the spherical requirement on each study eye. Following is a breakdown of the spherical requirements of the 16 eyes in the study.
–1.00D 6 eyes
–2.00D 3 eyes
–3.00D 3 eyes
–5.00D 2 eyes
–6.00D 2 eyes
In evaluating how the seven study contact lenses performed in correcting HOA, we first evaluated each lens to see if it lowered total HOA for each of the 16 study eyes. Table 1 shows these results.
We also evaluated the data to determine the relative decrease or increase of total HOA with each study lens. We accomplished this by totaling the HOA of all study eyes, then calculating the average amount of HOA per study eye, which was 0.281D. We measured the total amount of HOA for each lens design on each study eye and calculated this relative decrease or increase for that lens design as a percentage change compared to the average HOA correction. Figure 1 demonstrates these findings.
Correcting Spherical Aberration A number of contact lenses in the study group have incorporated design features to reduce SA. Even though SA is only one component of total HOA, we evaluated the seven study lenses in the same way for SA alone.
Figure 2 illustrates the relative decrease or increase of total SA with each study lens. We totaled the SA of all study eyes, then calculated the average amount of SA per study eye. We measured the total amount of SA for each lens design on each study eye and calculated the relative decrease or increase for that lens design as a percentage change compared to the average SA correction.
Lowering Total HOA
|LENS BRAND||EYES WITH LOWERED HOA|
|Night & Day||5/16|
Varying levels of HOA exist in the general population, and no two eyes are alike. The variation is significant enough that some clinicians call it an "optical fingerprint." Researchers have analyzed HOA demographics and established average HOA levels for certain patient populations, but great variability can exist within any given patient population.
In addition to the natural HOA present in any individual's visual system, soft lenses induce their own aberrations when placed on the eye. SA and "lens wrap" aberration are both well documented. Aberration altering lenses incorporate aspheric optics in an attempt to reduce or eliminate the induced aberrations; some may also be designed to reduce the average HOA found in the general population.
Though many of the lenses included in this study were designed to reduce spherical aberration, a surprising difference exists in their relative ability to accomplish that task.
We would expect the design of any aspheric soft contact lens to have an effect on lowering SA, which is a rotationally symmetric aberration. Surprisingly, some designs in the study also decreased total HOA, including coma and trefoil in some individuals in the study group.
Uncorrected (or induced) HOA may result in patient discomfort, particularly in mesopic or scotopic conditions. Although many currently available lens designs incorporate advanced aspheric optics in an attempt to reduce HOA on the eye, it appears that not all lens designs are equal. Though you need to consider many factors when prescribing soft contact lenses for any individual, when crisp pristine vision is a top priority, you should consider lens designs that demonstrate the greatest amount of HOA correction.
To obtain references, visit www.clspectrum.com/references.asp and click on document #128.
Additional data tables are also available in the online version of this article at www.clspectrum.com.
Dr. Edwards has worked in optometric practice and in visioncare-related businesses and has contributed to bringing a number of ophthalmic medical technologies to the market. A national and international author and lecturer, he received his OD in 1982 from the Illinois College of Optometry. He serves as a consultant to Optical Connection, Inc. and other ophthalmic companies.