ASPHERIC LENS PERFORMANCE
Evaluating The Visual Performance of an Aspheric Contact Lens
This study set out to determine if a particular aspheric contact lens could improve subjective vision.
By Linda Tran, OD, Renee Wyckoff, OD, Vandi Wharton, OD, Jennifer Smythe, OD, MS, FAAO, Peter Bergenske, OD, FAAO, and Patrick Caroline, FAAO
Some research indicates that certain individuals, particularly those who have low amounts of uncorrected refractive astigmatism, may appreciate better visual performance with front-surface aspheric lenses than they do with conventional bi-spherical soft contact lenses. Practitioners commonly fit patients who have low amounts of refractive astigmatism with spherical equivalent correction, which often results in vision with the soft contact lenses that's inferior to that with their spectacles.
Although aspheric lenses don't purport to correct for astigmatism, their ability to reduce spherical aberration may counterbalance the blur induced by uncorrected astigmatism and thus result in better visual performance than patients might experience with spherical lenses.
The Aspheric Lens Concept
A lens with spherical surfaces will suffer from errors of focus that occur because a spherical lens focuses parallel rays along a line, rather than to a point. In other words, rays that are parallel to the optic axis but at different distances from the optic axis fail to converge to the same point. As a result, off-axis rays come to a focus closer to the lens than do on-axis rays. The degree of induced error is proportional to the distance from the optical axis of the lens, resulting in a lack of clarity of the created image. We call this effect spherical aberration. Although a single lens can't eliminate spherical aberration, it can minimize it through appropriate alteration of the lens shape. Front-surface aspheric lens designs are used to reduce aberration caused by spherical front surface curves. The radius changes from center to periphery, bringing the focus back to an improved position and theoretically providing a sharper image.
Recent reports indicate that aspheric soft contact lenses may mask residual astigmatism and may provide better overall visual acuity. Practitioners are aware, however, that examination room performance doesn't always correlate with patients' subjective evaluations of vision in their normal environments.
We undertook to investigate whether patient subjective evaluation of visual quality with an aspheric lens was significantly different from their visual quality with their habitual, spherical soft contact lenses.
Our two aims of this study were to determine:
1. Whether aspheric soft contact lenses would offer better visual performance than habitual spherical lens correction.
2. Whether patient satisfaction with aspheric lenses would differ from that with standard spherical lenses.
We also evaluated the effect of cylinder magnitude or the type of astigmatism (with-the-rule [WTR] vs. against-the-rule [ATR] or oblique) on both performance and satisfaction.
Materials and Methods
This study was a randomized, single-blind (masked examiner, open-label for subjects), cross-over design in which subjects wore a pair of their habitual spherical soft contact lenses and a pair of aspheric soft contact lenses (CooperVision's Frequency 55 Aspheric) for one week each.
We screened candidates from the outpatient clinic at Pacific University College of Optometry. We diagnostically fit all eligible subjects with the aspheric lenses from the clinic lens inventory. We selected initial lens powers according to the spherical component of the manifest minus-cylinder refraction. We performed a spherical over-refraction to achieve binocular balance and best visual acuity. We then followed the same procedure for the habitual lens type to ensure optimal correction with that lens type.
Once we established the optimum lens fit and powers, we randomly dispensed one pair of lenses to each subject. Fifteen subjects received a pair of aspheric soft lenses and 15 subjects received a new pair of their habitual spherical soft lenses (corrected for optimal power, if indicated). Although less than optimal, we deemed it impossible to mask the subjects as to lens type because we felt they would readily recognize their habitual lens type from the test lens, particularly as they wore the lenses on a daily wear basis. We instructed the subjects to wear the lenses for a minimum of 10 hours each day and to use their habitual lens care system as outlined in the respective package insert. At the end of the first week, the subjects crossed over to the other lens design.
At each one-week follow-up visit, we asked the subjects to complete subjective questionnaires assessing overall vision, distance vision, near vision and frequency of symptoms with each pair of contact lenses. We masked the examiner who administered the questionnaires as to what lens type the subjects had worn. We further broke down subjective assessment of vision into specific tasks such as computer use, reading, daytime and nighttime driving, etc. The symptoms we evaluated included headache, nausea, eye fatigue and strain, difficulty judging distances, ghost images, shadows, halos, glare and fluctuating vision.
A masked examiner then performed acuity assessment that included distance visual acuity (high- and low-contrast Bailey-Lovie), near visual acuity (Bailey-Lovie word card) and sphero-cylindrical over-refraction in the phoropter.
At the end of the study, we asked subjects whether they preferred the aspheric lenses, their habitual lenses or whether they had no preference based on vision, comfort, handling and overall preference.
Thirty subjects completed the study (16 female and 14 male). Refractive error of the subjects ranged from 2.50D to 8.50D with an average of 4.67D. All subjects manifested between 0.25D to 0.75D of astigmatism (31 eyes WTR, 18 oblique and 11 ATR). Regarding cylinder magnitude, 25 eyes manifested 0.25D, 26 eyes manifested 0.50D and nine eyes manifested 0.75D. Figure 1 shows the broad range of habitual lens designs.
Figure 1. Habitual lens designs.
Objective: Comparison of Vision Performance We evaluated objective vision performance by comparing high-contrast distance visual acuity, low-contrast distance visual acuity and near visual acuity with the two lens designs. Overall, we found no difference between the aspheric and the habitual lens designs for any of the three tests. Likewise, we observed no significant differences with respect to astigmatism magnitude or type. Therefore, the two lens designs objectively performed the same.
Subjective: Comparison of Patient Satisfaction Satisfaction ratings of overall visual demands were better with the aspheric design. This was especially true for distance tasks including daytime driving and low-light, nighttime driving. Tables 1 through 3 show a summary of satisfaction ratings and p-values for differences.
Cylinder Magnitude, Astigmatism Type and Symptoms We found no significant relationship between satisfaction difference and cylinder magnitude for the two lens types. Likewise, we observed no significant difference between WTR and ATR astigmatism with regard to satisfaction score differences. However, as cylinder magnitude increased, we noted more symptoms of "strain" and "glare" with habitual lenses. No other symptom differences were significant. Astigmatism type didn't impact symptom rate differences.
Subject Preference At the end of the study, subjects preferred the aspheric lens with regard to vision (p=0.0026) and overall performance (p=0.05). No matter what type of astigmatism, the final preference for the aspheric lens was significant and this was especially true for subjects manifesting oblique and ATR astigmatism.
It's interesting to note that subjects didn't prefer their habitual lenses over the aspheric design for any of the evaluated conditions, although the subjects were all successful habitual spherical lens wearers before study enrollment. Unlike previous reports, we couldn't demonstrate a difference in measured visual acuity between the aspheric and the habitual spherical lens type. Clinically, however, it's most often a patient's perception of the quality of his vision that ultimately determines the final lens choice and patient satisfaction.
These results indicate that even though we may not observe differences in recorded visual acuity during an eye examination, low astigmatic patients may perceive better vision with an aspheric lens design in comparison to standard spherical soft lenses. This knowledge may offer patients and practitioners an advantage in that practitioners can enhance perceived visual performance for low astigmatic patients at a lower cost and use less chair time than with toric and GP lenses.
We must acknowledge a possibility of subject bias toward the test lens, which we couldn't practically mask from the subjects with this test design. Further evaluation of our study findings should include testing that completely masks subjects as well as examiners from lens type to confirm that our findings are the result of real effect, rather than the possible influence of subject bias.
CooperVision sponsored this study.
TABLE 1 Satisfaction Rating of Overall Vision
TABLE 2 Satisfaction Rating of Distance Vision
|Distance for work||Aspheric||0.016|
TABLE 3 Satisfaction Rating of Near Vision
|Close range (work)||None||N/A|
|Close range (leisure||Aspheric||0.027|
Table 4 Test Used to Determine Statistical Significance
|Cylinder magnitude||Pearson's correlation|
|Astigmatism type||One way ANOVA|
|Cylinder magnitude and satisfaction||Pearson's correlation|
|Cylinder type and satisfaction||One way ANOVA|
|Forced choice question||Chi-square analysis|
Dr. Tran is a 2003 graduate of Pacific University College of Optometry.
Dr. Wyckoff is a 2003 graduate of Pacific University College of Optometry.
Dr. Wharton is a 2003 graduate of Pacific University College of Optometry.
Dr. Smythe is an associate professor of optometry at Pacific University and is in private group practice in Beaverton, Oregon.
Dr. Bergenske, a past chair of the American Academy of Optometry's Section on Cornea and Contact Lenses, has practiced for over 20 years in Wisconsin and now is on the faculty at Pacific University College of Optometry. E-mail him at: firstname.lastname@example.org.
Patrick Caroline is an associate professor of optometry at Pacific University and is an assistant professor of ophthalmology at the Oregon Health Sciences University.