Ⅰ. Introduction

Nowadays, wearing contact lens remains one of the most popular methods to correct refractive error. It is well known that contact lenses are designed to improve vision, most commonly by correcting spherical and astigmatic refractive errors as lower-order aberrations for clear vision. Recently, there has been renewed interest in aspheric contact lens designs, which could be achieved the correction of higher-order aberrations (HOAs) by spherical aberration to improve visual quality. With the advances in the optical design of soft contact lenses, contact manufacturers have incorporated aspheric optics into soft contact lens designs in an attempt to reduce HOAs, mainly spherical aberration (SA).^1,2)

It has been feasible to manipulate the magnitude of SA for each lens power with the anterior surface of aspheric soft contact lens designs and possible to postulate that aspheric soft contact lenses designed to control SA would be better than conventional spherical lenses for improving visual quality.

With aberrometers that are able to quantify the on-eye aberrations induced by various contact lens designs, many studies have been trying to find out the effect of aspheric hydrogel soft lenses on improving visual quality.^1,3-6) Some studies reported visual performance to be better with aspheric soft contact lenses,^4-5) whereas, other studies showed that they were not able to find out any difference in visual performance between aspheric and spherical soft lens designs.^1,6) For this reason, recent studies examining commercially available soft contact lenses have been skeptical about the visual benefit of wearing aspheric over spherical soft contact lenses.^7-9)

In the meantime, several studies have reported that rigid contact lenses decreased more aberrations on average than soft contact lens designs, so that rigid contact lenses provided better aberration profiles compared to soft contact lenses.^10-12) With relate to their finding, aspheric silicone hydrogel contact lenses may suggest as an alternative to aspheric hydrogel soft contact lenses for optical effect as to correcting ocular SA. In addition, we put the case that modulus as the inherent ‘stiffness’, which is one of silicone hydrogel material characteristics, would have an effect on ocular HOAs differences between aspheric and spherical silicone hydrogel contact lenses.

There is little published work on the effect of aspheric silicone hydrogel contact lenses on SA and other HOAs. Thus, this study was to examine ocular HOAs and visual quality of aspheric silicone hydrogel contact lenses designed to reduce SA compared to spherical silicone hydrogel contact lenses.

Ⅱ. Methods

1. Subjects

Subjects were recruited from contact lens wearers and non-contact lens wearers in the Department of Optometry of Eulji University. Thirty myopic subjects were enrolled in the study. To be included in the study, subjects had to have between -1.00 to -8.00 D of spherical refractive errors and no more than 0.75 D of astigmatic refractive errors. Best-corrected monocular visual acuity of 20/20 or better with both contact lenses and spectacles was observed in each eye for all subjects. None of the subjects had any ocular diseases, or any contraindication for soft contact lens wear. Verbal informed consent was obtained following a detailed explanation of the study procedure.

2. Contact Lenses

The soft contact lenses evaluated were aspheric silicone hydrogel contact lenses (Biofinity, CooperVsion, Fareham, UK) and spherical silicone hydrogel contact lenses (Acuvue Oasys, Vistakon, USA). The study lenses are outlined in Table 1.

3. Visual Acuity (VA)

Measurements of VA for distance were made under photopic (340 lx) and mesopic (1 lx) conditions after 2 hours of the new soft contact lens wear, using a high (100%) and low (10%) - contrast ETDRS acuity charts. VA was recorded by logMAR units. The testing was carried out monocularly at a distance of 4m.

4. Contrast Sensitivity (CS)

CS was measured using the CSV-1000 instrument (Vector Vision, Inc, USA), which presents a translucent chart divided into four cycles with spatial frequencies of 3, 6, 12, and 18 cycles per degree (cpd). It is provided by a fluorescent luminance source and is automatically calibrated to 85 cd/m². After 2 hours of wearing the new soft contact lens, CS test was started with the lowest spatial frequency (3 cpd) first under photopic (280 lx) and then under mesopic (1 lx) conditions. In order to test mesopic conditions, mesopic filters (dark neutral density filters) with with a trial frame were employed. The recorded CS values were transformed into log values as described by Vector Vision. The testing was performed mono- cularly at a distance of 2.5 m.

5. Wavefront higher-order aberrations (HOAs)

Monochromatic wavefront analysis was performed by Wavefront Analyzer (KR-1W, Topcon, Japan) with the Hartmann-shack technique. In this study, ocular HOAs were total HOAs, third-order aberrations that include coma, and fourth-order aberrations that include SA. Ocular HOAs were measured for 4 mm and 6 mm pupils after 2 hours of the new soft contact lens wear. The root mean square (RMS) of the third-order and fourth-order Zernike coefficients was used to represent third-order aberrations and fourthorder aberrations, respectively. Total HOAs were calculated as the RMS of the third-order and fourth-order coefficients. In order to maximize the influence of pupil size, wavefront aberrations were measured in mesopic lighting conditions. Also, to minimize the effects of any local tear film changes and blink-induced lens movements, each single measurement was taken immediately after observing alignment of mire image and stabilization of the lens on the eye after a blink through the instrument. Three consecutive measurements were taken in each eye.

6. Pupillometry

The natural pupil diameter was taken under the mesopic conditions using Digital Variable Pupillometer(VIP^TM-200, Neuroptics, USA). The pupil diameter can measure variable light levels (scotopic, low mesopic and high mesopic) in one sequence. Measurement was performed three times alternately between the right and left eye.

7. Experimental procedure

This study involved three visits with the following procedures conducted at each visit. At the first visit, all subjects were undergone an initial optometric examination to determine optimal lens power and checked according to the inclusion criteria. Also, ocular aberrations were measured without contact lens wear. Subjects attended two visits over two days. After ordering the lenses, each subject received one of the two types of contact lenses used in this study.

At the second visit, all subjects after 2 hours of the new contact lens wear were examined for contact lens-fitting assessment (centration, coverage, movement) using a slit lamp and followed by high and low contrast VA, CS, and ocular HOAs. Another new contact lenses used in this study were then dispensed to each subject with instructions to wear for another time.

After that, each subject returned in the morning for the final examination of wearing the second new contact lenses, and all measurements were repeated in the same way as in the second visiting procedure.

At each visit, all measurements were carried out monocularly with a natural pupil and completed by one examiner. Measurements with contact lenses were conducted by having each subject to wear one lens design per one day during two days.

8. Statistical analysis

VA, CS, and ocular HOAs were compared between aspheric and spherical soft contact lenses by performing the paired Student t-test. SPSS/window programs version 18.0 (SPSS, Chicago, IL) was used for statistical analysis throughout this study and graphs were produced using Origin 6.0 program (OriginLab Co., Northampton, USA) and Microsoft Excel (Microsoft, USA). Difference was considered statistically significant when p-value was < 0.05.

Ⅲ. Results

1. Assessment of contact lens fit

Of the 60 eyes fitted with aspheric lenses (BOZR:8.6 mm) and spherical lenses (BOZR:8.8 mm), fifty eyes (83%) were considered acceptable fit in both two lens designs, but ten eyes (17%) could not be acceptably fitted with either aspheric lenses or spherical lenses. In the 50 eyes fitted with two type lenses, 38 eyes (63%) with aspheric lenses and 28 eyes (47%) with spherical lenses were considered to be optimum fit.

2. Subjects

Thirty subjects completed the work, but data for this study were captured for 25 subjects and data of 5 subjects were discarded taking into account poor centration and excessive movement. For the subjects, contact lens wearers were 21 subjects and non-contact lens wearers were 4 subjects. The average age of subjects was 24.32±2.87 years (range 20 to 33). Pupil size was 6.64±0.63 mm under mesopic conditions. The demographics and biometric data of 25 subjects (50 eyes) are listed in Table 2.

3. Visual Acuity

Results of logMAR VA under photopic and mesopic lighting conditions are summarized in Table 3. VA was worse in mesopic conditions than that of photopic conditions as expected. HCVA and LCVA under photopic or mesopic conditions were no statistically significant difference between the aspheric and spherical lenses.

4. Contrast Sensitivity

The limits of CS values in log units according to spatial frequencies provided by the Vector Vision runs as follows: 0.70~2.08 log units at 3 cpd, 0.91~2.29 log units at 6 cpd, 0.61~1.99 log units at 12 cpd, and 0.17~1.55 log units at 18 cpd.

For CS under photopic conditions, the mean CS log values of the aspheric lens on the eye were 1.70±0.109(3 cpd), 2.00±0.117(6 cpd), 1.67± 0.137 (12 cpd), and 1.28±0.129(18 cpd). The mean CS log values of the spherical lens on the eye were 1.73±0.106(3 cpd), 1.99±0.130(6 cpd), 1.69± 0.165 (12 cpd), and 1.25±0.133(18 cpd). No statistically significant differences in CS between the two lens designs were observed for given all spatial frequencies under photopic conditions.

For CS under mesopic conditions, the mean CS log values of the aspheric lens on the eye were 1.54±0.117(3 cpd), 1.68±0.134(6 cpd), 1.25± 0.178 (12 cpd), and 0.80±0.175(18 cpd). The mean CS log values of the spherical lens on the eye were 1.56±0.125(3 cpd), 1.70±0.124(6 cpd), 1.16± 0.223 (12 cpd), and 0.75±0.168(18 cpd). There was a statistically significant difference in spatial frequency of 12 cpd between the two lens designs (p=0.012) under mesopic conditions, but there were no statistically significant differences in spatial frequencies of 3 cpd, 6 cpd, and 18 cpd between the two lens designs under mesopic conditions.

The CS log values for the two lens designs on the eyes under photopic and mesopic conditions are presented in Fig. 1. The mean value at 6 cpd was higher than that of any other spatial frequencies.

5. Ocular higher-order aberrations

The RMS values of total HOAs, third-order and fourth-order aberrations and the amount of coma and SA with baseline (no lens) and with the aspheric and spherical contact lenses on the eyes are given in Table 4 and 5.

For a 4-mm pupil size, the RMS values of total HOAs, third-order and fourth-order aberrations in the aspheric lenses on the eyes were 0.104±0.040 μm, 0.090±0.042 μm and 0.047± 0.021 μm respectively, and coma and SA were 0.056±0.042 μm and -0.010±0.027 μm. In the spherical lenses on the eyes, the RMS values of total HOAs, third-order and fourth-order aberrations were 0.108±0.040 μm, 0.096±0.041 μm and 0.044±0.019 μm respectively, and coma and SA were 0.065±0.039 μm and 0.000±0.028 μm. There was statistically significant difference in SA with a 4-mm pupil between the aspheric and spherical lenses(p=0.002), however, there was no significant difference in any other ocular HOAs with a 4-mm pupil between the two lens types.

For a 6-mm pupil size, the RMS values of total HOAs, third-order and fourth-order aberrations in the aspheric lenses on the eyes were 0.338± 0.089 μm, 0.254±0.093 μm and 0.155±0.088 μm respectively, and coma and SA were 0.170±0.090 μm and -0.048±0.080 μm. In the spherical lenses on the eyes, the RMS values of total HOAs, third-order and fourth-order aberrations were 0.377±0.103 μm, 0.285±0.105 μm and 0.172± 0.075 μm respectively, and coma and SA were 0.201±0.103 μm and -0.005±0.088 μm. The difference in SA between the aspheric and spherical lenses on the eyes was statistically significant with a 6-mm pupil (p=0.001), whereas there were no significant differences in total HOAs, thirdorder aberrations, fourth-order aber-rations, and coma between the two lens types for a 6-mm pupil.

Ocular aberrations of total HOAs, third-order aberrations, fourth-order aberrations, coma and SA for the two lens designs on the eyes as well as for the baseline (no lens) with 4-mm and 6-mm pupils are shown in Fig. 2. The values of total HOAs, third-order aberrations, fourth-order aberrations, coma and SA were reduced by the aspheric and spherical contact lenses compared to wearing no lens. Especially, the changes in SA between the baseline and the aspheric or spherical lenses on the eyes with both 4-mm and 6-mm pupils were greater than those of any other HOAs.

Ⅳ. Discussion

The purpose of this study was to investigate VA and CS with respect to visual quality under different lighting conditions, and ocular HOAs with different pupil sizes between the aspheric and the spherical silicone hydrogel contact lenses that are commonly fitted in clinical practice.

With the increase in the number of soft contact lenses available, it is crucial for contact lens wearers and eye care professionals to be aware of the influence that contact lens design and material have on ocular aberrations and visual quality.

All minus powered contact lenses used in this study considerably reduced the inherent positive SA of the eye with both aspheric and spherical silicone hydrogel lenses, although the amount of change in SA was rely on their lens design. The SA observed in this study was on average close to zero or negative values in both aspheric and spherical silicone hydrogel lenses for 4-mm and 6-mm pupils, inducing more negative SA with a 6-mm pupil than with a 4-mm pupil. The aspheric silicone hydrogel lenses on the eyes resulted in an on average negative SA with 4-mm and 6-mm pupils. This study also noted that there was significantly change in the SA of the spherical silicone hydrogel lenses on the eyes that the manufacturer made no attempt to control SA. The reduction in positive SA was confirmed by Dietze and Cox⁸⁾ who used ray tracing to evaluate the expected negative SA that is related to the amount of minus power in the lens. Generally, spherically surfaced negative lenses occur negative SA, whereas spherically surfaced positive lenses occur positive SA in direct proportion to their sphere power.¹³⁾ The trends toward negative SA in our study agree with the findings of Lindskoog Pettersson et al.⁶⁾ who investigated SA in relation to visual performance with various lens materials and designs. Moreover, these results are similar to those of our previous publication¹⁴⁾ which reported change of SA with aspheric hydrogel contact lenses (Soflens Daily Disposable) that include controlled levels of SA and finding of Rae et al.⁵⁾ who evaluated HCVA and LCVA with custom-made contact lenses making SA control. In our results, aspheric silicone hydrogel lenses on the eyes made SA more negative for 4-mm and 6-mm pupils than spherical silicone hydrogel lenses, with statistically significant difference in the amount of change between the two lens designs.

In addition, as confirmed by the result of this study, all RMS values of ocular HOAs including total HOAs, third-order aberrations, fourthorder aberrations, coma were reduced by both the two lens designs on the eyes compared to wearing no lens with both 4-mm and 6-mm pupils. But when examining the ocular HOAs results, there was no statistically significant difference in the amount of total HOAs between the two designs on the eyes for 4-mm and 6-mm pupils, regardless of the amount of considerably reduced SA in both the two lens designs on the eyes. These results do not agree with previous findings^12,15) that soft contact lenses increase HOAs when compared to wearing no lens. McAlinden and colleagues¹⁶⁾ reported that no statistically significant changes induced in SA with the PureVision and Biofinity. They also found that these aspheric silicone hydrogel lenses did not correct or reduce any HOAs, even caused increases in some aberrations compared to wearing no lens. Of course, these differences may reflect that the effect of correcting SA with aspheric contact lenses on the eyes is contingent upon the baseline aberrations of subjects that have considerable individual variations^12,17,18) and pupil size.^19,20)

Previous studies^11,21) reported that the amount of SA induced by contact lens on the eyes, which account substantially for HOAs,^21,22) was correlated with visual quality. Recently, CS or LCVA testing has been used to assess the visual quality^23,24) and CS under mesopic conditions has been considered as a cardinal clinical test to evaluate detailed changes and difference in quality of visual.^25,26) In the current study, we expected to some extent that incorporating negative SA into contact lenses would lead to better visual performance compared to spherical contact lenses. However, our results showed that there were no clinically meaningful differences in visual quality analyzed through the HCVA and LCVA as well as CS under photopic and mesopic conditions between the two lens designs on the eyes, resulting in no significant difference in the amount of total HOAs between the two designs. These findings are in accord with previous studies^6,7) investigating visual quality under photopic conditions with aspheric and spherical hydrogel soft lenses. Efron et al.⁷⁾ found that there was no any difference in visual quality under mesopic conditions between the two lens designs. On the other hand, Rea et al.⁵⁾ showed that custom-made contact lenses induced negative SA of 0.1 μm made statistically significant improvements in HCVA and LCVA, but they deemed that it was not likely to be of clinical meaningful difference which considered to being a half-line or more.²⁷⁾ Chateau et al.²⁸⁾ also reported that there was a slightly better CS with the lenses that did not correct for SA. De Brabander et al.²⁹⁾ suggested that selective manipulation of SA in individuals with high power soft contact lenses could be significantly improvement in CS.

One reason why there are the differences found by various soft contact lenses among the studies is that while the lenses used in this study made of silicone hydrogel materials, the lenses used in previous studies described above largely made of hydrogel materials. Lens flexure on the eye may result in different on-eye aberration profiles.³⁰⁾ Further study is necessary to investigate in this area. In fact, it is not possible to determine conclusively whether no differences in VA and CS between the two lens designs on the eyes are due to the silicone hydrogel material, interaction of other aberrations, eye-lens interactions, or lens flexure on the eye and so on.

In summary, it could be possible to correct ocular SA with soft contact lenses and our study found that baseline SA shifted toward close to zero or negative values with aspheric and spherical silicone hydrogel lenses used in this study. With our findings, we support the assumption of Lindskoog Pettersson et al.⁶⁾ that the amount of SA in the average population is small and that low level of corrected SA of soft contact lenses on the eye, even aspheric soft lens designs(not custom-made contact lenses), does not encourage the contact lens wearers to have meaningful effect on visual quality. Further study is needed to explore the speculations on achieving the best visual quality with advanced soft contact lens designs and materials.

Ⅴ. Conclusions

In this study, no statistically significant and clinically meaningful differences in ocular HOAs were found between the aspheric and spherical silicone hydrogel contact lenses, indicating no changes in HCVA and LCVA as well as CS under both photopic and mesopic conditions between the two lens designs.

Based on our findings, we conclude that aspheric silicone hydrogel lens design used in this study did not provide better visual quality for the contact lens wearer compared to spherical silicone contact lens design, although there was statistically significant difference in the amount of SA between the two lens designs.