Visual performance of Acrysof ReSTOR compared with a monofocal intraocular lens following implantation in cataract surgery

The aim of this study was to compare the visual performance of Acrysof ReSTOR and Acrysof Natural intraocular lenses (IOLs) following cataract surgery. A randomized prospective study was performed in which 64 eyes (51 patients) were divided randomly into two groups. Monofocal IOLs (Acrysof Natural) were implanted into 34 eyes (27 patients) and multifocal IOLs (Acrysof ReSTOR) were implanted into 30 eyes (24 patients) using phacoemulsification surgery. The corrected distance visual acuity, near visual acuity, pseudoaccommodation, contrast sensitivity (CS) and wavefront analysis were measured at 1 week, 1 month and 3 months after surgery. The distance vision of the monofocal and ReSTOR patients improved equally with glasses (P<0.05). A greater improvement in near vision without glasses was observed in the ReSTOR-implanted patients (P<0.01). The CS values of the multifocal IOL group were significantly lower than those of the monofocal IOL group for all spatial frequencies tested (P<0.05). The spherical aberration was significantly higher in the multifocal IOL group compared with the monofocal IOL group (P<0.05). We observed no differences in coma between the two groups. The difference in the amplitude of pseudoaccommodation between the two groups was statistically significant (−3.14±0.91 D in the ReSTOR group vs. −1.03±0.33 D in the Natural group, P<0.01). The improvement in near vision was significantly more evident in the ReSTOR patients. Compared with the monofocal IOL lens, the multifocal lens is able to increase the amplitude of pseudoaccommodation. However, increased spherical aberration may contribute to lower CS values in the multifocal IOL group.


Introduction
Intraocular lenses (IOLs) are designed to provide the best quality of vision following cataract surgery. Monofocal IOLs are capable of providing excellent distance vision quality. However, patients with monofocal IOLs require glasses for near vision. Multifocal IOLs have been developed to reduce the patients' dependence on glasses. Clinically, multifocal IOLs have been reported to provide functional near distance vision with an acceptable level of satisfaction (1). Snellen visual acuity insufficiently describes the quality of eye optics before and after surgery (2). The deficiencies in the optical quality of vision may be effectively evaluated using a contrast sensitivity (CS) test and wavefront analysis. Improvements in uncorrected near visual acuity have been achieved with multifocal IOLs but a loss of clarity, low CS and complaints of halos and glare have also been reported (3). Yoon et al (4) reported spherical aberration to be one of the most significant higher-order aberrations (HOAs) that reduce retinal image quality. The present study compared the improvement in near vision and distance vision-associated limitations between ReSTOR and monofocal IOLs following their implantation using phacoemulsification. In addition, the pseudoaccommodation, CS and HOA values of the patients were measured.

Patients and methods
Patients. This randomized study was conducted in the Ophthalmology Department of the Shanghai Ninth People's Hospital (Shanghai, China). Patients between 50 and 75 years old with age-associated cataracts were enrolled. Patients with ocular diseases, including corneal astigmatism of >1. 5  ) with 4.0 and 6.0 mm pupils. All the measurements for CS and HOAs were obtained using the best spectacle-corrected acuity.
Statistical analysis. Statistical analysis was performed using the mean and standard deviation for quantitative variables. The comparison of quantitative variables was performed using analysis of variance (ANOVA) and the differences were calculated using a multiple comparison Tukey's test. For multiple measurements, Bonferroni correction was applied when necessary. All results were presented with 95% confi-dence limits. P<0.05 was considered to indicate statistically significant differences.
The mean UNVA values in the ReSTOR and Natural groups were: 0.29±0.07 and 0.18±0.08, respectively, on postoperative day 7; 0.48±0.09 and 0.24±0.06, respectively, on postoperative day 30 (P=0.15); and 0.58±0.09 and 0.21±0.16, respectively, on postoperative day 90 (P=0.008). No significant difference was observed between the two groups on postoperative days 7 and 30 (P>0.05). A significant difference was observed in UNVAs between the two groups on postoperative day 90 (P<0.05). All the average BCDVAs were obtained from Snellen visual chart values and the UNVAs were obtained using Jaeger visual charts (Table III). Acrysof Natural IOLs under mesopic and photopic conditions. Statistically significant differences were observed in all spatial frequencies between the groups at all follow-up times (P<0.05).

Discussion
One of the major defects of monofocal IOLs as replacements for human crystalline lenses is the fixed focus of the IOLs (5). Although patients may see well at a distance following cataract surgery, reading spectacles are generally required for near vision. To address this issue, multifocal IOLs that provide refractive correction for near and distance vision are now available (6). Brydon et al (7) compared the BCDVA, UBVA, UNVA and BCNVA values of patients with multifocal or monofocal IOLs. The results demonstrated that there was no difference between the two groups in terms of BCDVA and UNVA values. However, patients with multifocal IOLs may have improved near visual acuity without glasses. In the present study, the number of patients in the ReSTOR group with a UNVA of 0.3 or more was larger than that of the Natural group during all the follow-up periods. However, there was no difference between the two groups in terms of the number of patients with a BCDVA of 0.5 or more. These results indicate that the multifocal IOLs may decrease the dependence on glasses of patients who attained satisfactory distance visual acuities.
Accommodation is defined as the eye's ability to focus on near objects by changing its refractive power (8). As the human lens ages, its accommodative amplitude decreases. People cannot obtain clear visual acuity when they see things nearby. This phenomenon is called presbyopia. Patients with an implanted pseudophakic eye are similar to an absolute presbyope following cataract surgery. A ReSTOR IOL employs a central apodized diffractive zone surrounded by a purely refractive outer zone. It has a central 3.6-mm diffractive optic region where 12 concentric diffractive zones on the anterior surface of the lens divide light into two diffraction orders to create two lens powers. The central 3.6-mm zone is surrounded by a region that has no diffractive structure over the remainder of the 6-mm diameter lens. The near correction is calculated at +4.0 D at the lens plane, resulting in approximately +3.2 D at the spectacle plane. This provides 6 D of pseudoaccommodation. Sugitani, Hardman et al and Nakazawa and Ohtsuki (9)(10)(11) reported that the pseudophakic eye also reserves 2 D of amplitude for pseudoaccommodation. In the present study, the pseudoaccommodation in the ReSTOR group approaches 4 D, which allows the patient to achieve complete distance vision. Additionally, patients in the Natural group exhibited some pseudoaccommodation. However, the difference between the two groups was not significant.
Although the multifocal IOLs may afford cataract patients complete distance visual acuity, they may increase the depth of focus in exchange for a loss of CS. It has been reported that AMO Array multifocal IOLs [Tecnis ZM900,Abbott Medical Optics, Inc. (AMO)] may cause more glare and lower CS   than monofocal IOLs (12). Bellucci (13) reported that patient satisfaction was no higher for multifocal IOLs than monofocal IOLs and the poor visual performance was attributed to the reduction of CS and the presence of halos. The present study demonstrated that CS values were decreased and HOAs were increased in the multifocal IOL group. CS values were significantly lower in the ReSTOR group compared with the Natural group for all spatial frequencies under all conditions. HOAs, particularly spherical aberrations, were increased significantly in the ReSTOR group. The present study revealed that there may be an association between decreased CSs and increased spherical aberrations in the ReSTOR group. A possible cause of this issue is the division of light energy through the two focal points produced by the multifocal IOL, suggesting that spherical aberration is involved in decreasing the CS of multifocal IOL-implanted eyes. Residual refractive errors and delicate decentration of the IOLs associated with the pupil size may affect the visual performance of the patients. Thus, spherical aberration increases in multifocal IOLs with large pupil sizes. The present study revealed that the value of the wavefront increased with the 6-mm pupil size for RMS, Z 3 -1 and Z 4 0 at all follow-up times. This suggests that visual acuity decreases when patients with multifocal IOLs drive at night.
In conclusion, ReSTOR IOLs provide the additional benefit of uncorrected near vision. ReSTOR also provides patients with a comfortable distance vision that is comparable to that of monofocal IOLs and a comfortable near vision without glasses that is significantly superior to that of monofocal IOLs. However, decreased CSs are associated with low visual performance satisfaction in multifocal IOL-implanted patients and spherical aberration appears to be a key contributor to reduced CS in these patients. Thus, when doctors select the type of IOL for cataract patients, the patients' personal requirements should be taken into account in order to improve the quality of their lives.