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Optical quality and visual performance after cataract surgery with biaxial microincision intraocular lens implantation

Jiménez, Raimundo PhD*; Valero, Almudena MD; Fernández, Joaquín MD; Anera, Rosario G. PhD; Jiménez, José R. PhD

Author Information
Journal of Cataract & Refractive Surgery: July 2016 - Volume 42 - Issue 7 - p 1022-1028
doi: 10.1016/j.jcrs.2016.03.039
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Abstract

The elimination of spherical refractive error and the minimization of corneal astigmatism induced by cataract surgery are the main goals that lead to patient visual satisfaction. Advanced ocular biometry and formulas have largely solved the first goal, and the minimization or elimination of surgically induced astigmatism (SIA) has been achieved with the advent of microincision cataract surgery (MICS).1,2

Microincision cataract surgery is defined as surgery performed through incisions smaller than 2.0 mm. This technique reduces surgical trauma, leads to quicker visual recovery, and reduces ocular complication rates with respect to other surgical techniques.3,4 When the incision decreases from 2.2 mm to 1.8 mm, or even 1.6 mm, no significant beneficial clinical effect on SIA has been noted.1,5,6 However, as indicated in a study by Luo et al.,7 smaller incisions are not always better if this type of surgery is not accompanied by new developments in the instruments used, the phacoemulsification system used, and new intraocular lens (IOL) designs that provide good optical quality for satisfactory health and visual outcomes.

The repeatability of objective measurements of optical quality of the human eye made with devices based on the double-pass technique has steadily enabled more widespread use in clinical practice,8 in particular in the evaluation of ocular optical performance in patients who have refractive surgery procedures.1,9,10

This study evaluated the visual and refractive outcomes in patients having 1.6 mm biaxial MICS and implantation of a monofocal MICS IOL (Incise, Bausch & Lomb). We assessed the effect of this procedure on the ocular optical quality achieved with a device based on the double-pass technique, providing data on the modulation transfer function (MTF) cutoff frequency, Strehl ratio, and objective scatter index (OSI). In addition, we analyzed the visual performance by assessing the contrast sensitivity function (CSF) and the discrimination capacity to peripheral stimuli under low illumination conditions (visual disturbance index).

Patients and methods

Eyes with cataract grade II to IV (Lens Opacities Classification System III)11 were enrolled in this prospective nonrandomized controlled study performed at Torrecárdenas Public Hospital, Almería, Spain. The study was performed according to the Declaration of Helsinki. Written informed consent from all patients and clearance from the hospital’s Ethics Committee were obtained for the study.

Inclusion criteria were age between 40 years and 80 years and patients who had cataract extraction and IOL implantation. Exclusion criteria included previous ocular or intraocular surgery (including laser in situ keratomileusis), presence of corneal disease, fundus abnormalities, glaucoma, uveitis, amblyopia, posterior capsule rupture, IOL decentration greater than 0.5 mm, and systemic disease such as diabetes mellitus.

Preoperative Evaluation

Clinical preoperative examination data included measurement of corrected distance visual acuity (CDVA) using Snellen distance charts, refraction, keratometry (Humphrey autorefractor/keratometer model 599, Carl Zeiss Meditec AG), biomicroscopic anterior and posterior segment evaluation, biometry, and IOL calculation (Ocuscan RxP Ophthalmic Ultrasound System, Alcon Surgical, Inc.).

Intraocular Lens

An Incise monofocal MICS IOL was implanted in all cases. It has a 22% water content, ultraviolet protection of 10% transmittance at 371 nm for +20.0 D, and aberration-free aspheric anterior and posterior surfaces. The IOL spherical equivalent power was calculated using the SRK II formula.12

Surgical Technique

Biaxial MICS phacoemulsification was performed between January 2014 and January 2015 by the same surgeon (J.F.) using topical anesthesia (proparacaine hydrochloride 0.5%). The biaxial procedure began with a 1.6 mm incision using a corneal 1-step tunnel that was 1.6 mm wide and 2.0 mm long at the 10 o’clock position. A second incision 1.5 mm wide was created at the 2 o’clock position. After surgery, the microincision was measured with a gauge and in no case surpassed 1.7 mm. The microincision monofocal IOL was implanted using a single-use injector (Viscoject 1.5, Medicel AG), which allows insertion in the bag through a 1.8 mm incision and 1.4 mm wound-assisted implantation (Stellaris Vision Enhancement System 2008, Bausch & Lomb).

Induced Astigmatism

The SIA was calculated by vector analyses using the Holladay-Cravy-Koch method13 from keratometry values obtained with the autorefractor/keratometer.

Measurement of Optical Quality

Optical quality was measured preoperatively and 1, 3, and 6 months postoperatively using the Optical Quality Analysis System (Visiometrics SL). This device is based on a double-pass technique.8–10 and provides data on diffraction, ocular aberrations, and scattering that diminish image clarity, reducing the patient’s visual quality. For a quantitative measure of the retinal image quality, the MTF cutoff and Strehl ratio were used. The MTF cutoff is the frequency at which the MTF reaches a value of 0.01, the threshold at which the eye can image an object in the retina with a significant 1.0% contrast. A higher MTF cutoff value means better contrast sensitivity. The Strehl ratio is defined as the ratio between the MTF area of the eye and the diffraction-limited MTF area. The Strehl ratio ranges from 0 to 1. A lower value of this parameter indicates a greater contribution of the aberrations and ocular scattering and therefore poorer optical quality.14 Finally, the OSI, a parameter that offers objective quantification of intraocular scattering, was calculated. In healthy younger eyes, the OSI value is lower than 0.5. A higher OSI indicates a higher influence of intraocular scattering.14 All these results refer to a pupil size of 4.0 mm for all patients.

Visual Performance

The CSF and the discrimination capacity to peripheral stimuli under low illumination conditions were measured preoperatively and 1, 3, and 6 months postoperatively.

Contrast Sensitivity Function

The CSF was measured at 4 spatial frequencies (3, 6, 12, and 18 cycles per degree [cpd]) and 8 levels of contrast using the CSV-1000E contrast chart (VectorVision).15 This test has been used studies to evaluate the CSF in patients having different types of eye surgery.16

The CSV-1000 system contains a translucent chart that is retroilluminated and calibrates luminance to maintain a constant 85 candelas (cd) per square meter (cd/m2). Contrast sensitivity was graded as 1 of 9 levels at the 4 spatial frequencies tested. Each spatial frequency is presented on a separate row of the test, and in each row the patches that present gratings decrease in contrast, moving from left to right across the row. The contrast level diminishes in a uniform logarithmic fashion in steps of 0.17 log units for contrast levels 1 through 3 and 0.15 log units for steps 3 through 8. The contrast change between the sample patch and level 1 is 0.3 log units. For each spatial frequency, patients were asked to indicate the orientation of the last grating (right, left, or up), and the last grating seen was plotted on a contrast sensitivity curve. The CSF was computed as the reciprocal of the contrast threshold. These raw grades were then converted into their respective log contrast sensitivity scores according to definitions provided by the manufacturer of the contrast chart.

Pupils were not dilated during test procedures, and testing was performed at the recommended distance of 2.5 m. Each eye was tested using spectacle correction.

Visual Discrimination Capacity

The discrimination capacity of peripheral stimuli was quantified in the presence of night-vision disturbances (eg, glare, halos, and starbursts) using the visual disturbance index. Halo freewareA (version 1.0, Laboratory of Vision Sciences and Applications, University of Granada, Spain), a visual test based on an experimental device called a halometer, was used.17 The patient’s task consisted of discriminating luminous peripheral stimuli around a central high-luminance stimulus against a dark background. On detection of peripheral spots, the patient pressed a button, storing this information for subsequent treatment and calculation of the visual disturbance index.18 This test has been used in different groups of subjects and in many studies.19–21

The discrimination capacity was assessed using a liquid-crystal display monitor with the resolution set at 1024 pixels × 768 pixels, and the distance from the observer to the monitor was 2.5 m. The size of the stimuli was 30 pixels (subtending 0.46 degrees) for the radius of the central stimulus and 1 pixel (0.02 degrees) for the radius of the peripheral stimuli. The luminance of the stimuli was measured with a spectroradiometer (Spectrascan PR-650, Photo Research, Inc.); it was set at 176.1 cd/m2 for the main stimulus and 61.1 cd/m2 for the peripheral stimulus, with the luminance for the background monitor of 0.71 cd/m2. The monitor shows 144 peripheral stimuli around the central one; the stimuli are distributed along 18 semiaxes (4 stimuli per semiaxis). Each peripheral stimulus was shown 2 times. This configuration agreed with spatial parameters used in other studies.18–21

The formal monocular testing began after the observer’s position was fixed in front of the monitor with chinrests and forehead rests. After a 3-minute adaptation period to darkness of the monitor background, there was a 1-minute adaptation to the central stimulus. Then, the observer was randomly presented with peripheral stimuli around the main stimulus to avoid learning effects. The visual disturbance index has values from 0 to 1. The greater the index, the lower the discrimination capacity and therefore the greater the influence of visual disturbances such as glare, starbursts, and halos around luminous stimuli.

Statistical Analysis

Data were analyzed using the SPSS software (version 15.0, SPSS Inc.). Repeated-measures analysis of variance was performed during the follow-up. For visual parameters that presented statistically significant differences, a multiple comparison was performed using the post hoc Bonferroni test. A P value less than 0.05 was considered statistically significant.

Results

The study comprised 32 eyes of 32 patients. The mean age of the patients was 70.7 years ± 5.3 (SD) (range 57 to 80 years). The mean power of the implanted IOLs was 20.5 ± 1.5 diopter (D) (range 17.5 to 25.0 D).

Table 1 shows preoperative and postoperative refractive error and CDVA data. One month postoperatively, the spherical refractive error was statistically significantly lower than preoperatively (P < .001); there were no statistically significant variations between postoperative visits (P > .05). However, astigmatism did not change significantly in power or on the axis between preoperatively and any postoperative timepoint (P > .05). The SIA at 1 month was not statistically significant different than at any of the other follow-up visits (P > .05). The mean CDVA was statistically significantly better preoperatively than 6 months postoperatively (P < .001).

Table 1
Table 1:
Preoperative and postoperative refractive data.

Optical Quality

Table 2 shows the preoperative and postoperative mean MTF cutoff frequency, Strehl ratio, and OSI. For all 3 parameters, there were statistically significant differences between the preoperative visit and all postoperative visits (P < .05). No significant difference was found between any of the postoperative visits (P > .05).

Table 2
Table 2:
Preoperative and postoperative measurement of optical and visual disturbance index.

Contrast Sensitivity Function and Visual Disturbance Index

Figure 1 shows the contrast sensitivity values over time plotted for each spatial frequency. At each spatial frequency, significant differences were found between preoperative values and each follow-up value. No significant difference was found between any of the postoperative visits (P > .05).

Figure 1
Figure 1:
Evolution of log contrast sensitivity at each spatial frequency during the study period (cpd = cycle per degree).

Finally, the mean visual disturbance index was significantly higher preoperatively than 6 months postoperatively (P < .05). No significant difference was found between any of the postoperative visits (P > .05).

Discussion

This study was performed to assess the visual performance and optical quality differences of patients who had cataract surgery and implantation of the Incise monofocal IOL during biaxial MICS (1.6 mm) phacoemulsification.

It has been established that with smaller incisions, SIA can be minimized,1,2,22 but no substantial differences in results were found between the microincision technique and small-incision surgery (≥2.2 mm and ≤2.75 mm).1,5,6,23 Results in a study by Alió et al.23 indicate that incisions of approximately 2.0 mm constitute the limit for the induction of nonsignificant optical changes in the human cornea after cataract surgery. On the other hand, other studies advise the use of biaxial MICS rather than coaxial MICS to minimize SIA.22–24

In the present work, we analyzed the variation in the total preoperative and postoperative astigmatic error, including a follow-up of 1, 3, and 6 months and found no significant differences in the refractive astigmatism value or in its axis. The SIA in these eyes was similar to values found in other studies,5,22,24 remaining fairly steady from the first month on. Most patients included in this study had good visual results in terms of logMAR CDVA, indicating that visual function was restored quickly. This agrees with studies reporting earlier improvement in CDVA using the biaxial technique.24 Subjective visual acuity would be reduced in the event of blurring of the retinal image, and this is directly related to the MTF cutoff value, which is highest when optical quality is better. It is normally assumed that a cutoff frequency of 30 cpd in the CSF, which includes the contrast degradation imposed by the optics and subsequent visual processing, corresponds to a logMAR acuity of 0.0.25 In the present work, as might be expected, the MTF cutoff frequency improved appreciably in the postoperative period (30.05 ± 13.86 cpd in the first month). In a population with aspheric IOLs implanted, Lee et al.10 found a mean MTF cutoff value of 26.49 ± 12.17 cpd 3 months after cataract surgery, a value close to ours (25.95 ± 10.49 cpd) for the same timepoint. Debois et al.26 and Xiao et al.27 implanted toric IOLs and found results comparable to ours (27.28 ± 8.45 cpd and 22.86 ± 5.58 cpd, respectively). Ortiz et al.20 reported a value of 29.67 ± 10.73 cpd for an age range that was similar to ours but in healthy eyes. With respect to the time course of the mean MTF cutoff value in the postoperative period, our results confirm that there were no significant differences between the 3 postoperative assessments. These results confirm the early reestablishment and stability of ocular optical quality in the first month after surgery.

We found an increase in the mean value of the Strehl ratio in the first month of follow-up, with the value remaining roughly constant over the 6-month follow-up. At 6 months, the mean value was 0.15 ± 0.07, similar to findings of Lee et al.10 with aspheric IOLs (0.15 ± 0.05) and Xiao et al.27 with toric IOLs (0.15 ± 0.04). Guirao et al.9 also found comparable values for the Strehl ratio (0.12 ± 0.03) in patients in whom IOLs had been implanted. Comparable values were found even in control groups of healthy older subjects (0.14 ± 0.03 in the study by Guirao et al.9 and 0.17 ± 0.04 in the study by Ortiz et al.20).

The mean preoperative OSI value in our work (7.44 ± 3.25) was greater than 5, indicating that the patients had mature cataract according to the objective classification of cataract patients by Pujol et al.B Postoperative OSI values were below 2, corresponding to eyes without cataract according to this classification. Others, such as Debois et al.26 and Xiao et al.27 with toric IOLs and Lee et al.10 with aspheric IOLs reported results similar to ours after surgery (1.76 ± 0.64, 1.80 ± 0.84, and 1.38 ± 0.73, respectively). These values are slightly higher than those in a study of a population of the same age but with healthy eyes (1.11 ± 0.50),20 indicating that once the cataract is eliminated and the refractive problem is resolved, these eyes continue to present more scattering than a healthy eye of a subject of the same age. As occurred with the other parameters, we found no significant differences between the values at the 3 follow-up assessments, corroborating that the reestablishment of ocular optical quality was reached during the first month.

With respect to contrast sensitivity, no statistical differences were found at any spatial frequency (3, 6, 12, and 18 cpd) between 1 month, 3 month, and 6 months postoperatively. Our results agree with those in other studies,6,28 although as pointed out by Schuster et al.29 in a systematic review with metaanalysis, an aspheric IOL achieves better contrast sensitivity than a spherical IOL, especially under photopic and mesopic conditions. However, almost none of the studies in their review analyzed the time course of CSF in 3 postoperative assessments (1, 3, and 6 months). In the available literature, only 1 study measured the CSF to the third postoperative month. The results agree with ours; that is, that there were no statistical differences at any spatial frequency between the assessments at 1 month and 3 months. Data from the Halo freewareA indicated impairment of the discrimination capacity in the preoperative session. We found that a higher number of stimuli were undetected when the cataract was present, giving a higher disturbance index (mean 0.70 ± 0.28). In these eyes, a higher level of ocular scattering contributed to the sensation of glare as well as to the perception of larger halos around central lights, severely diminishing the capacity to detect peripheral lights surrounding the central source and deteriorating the visual performance.30 After cataract surgery, the mean visual disturbance index value decreased almost by one half during the first month (0.36 ± 0.20) but did not vary significantly with respect to the sixth month (0.31 ± 0.17), approaching the mean values measured in similar age-control populations (without cataract).20 Our results show that from the first month after surgery, stabilization occurred in the CDVA, CSF, and visual disturbance index, indicating the rapid recuperation and temporal stability of visual function in patients who had microincision monofocal IOL implantation during biaxial MICS.

In this study, implantation of the Incise microincision monofocal IOL by biaxial MICS was safe, effective, and stable, providing excellent clinical outcomes. The optical quality and the visual performance significantly improved 1 month after surgery, and these results remained stable at 6 months. This shows that visual recuperation after the application of this technique is rapid, reaching visual stability 1 month after surgery. The postoperative visual function of the patients with this IOL was similar to that found in subjects of the same age with healthy eyes.

What Was Known

  • Intraocular lens implantation with the biaxial microincision technique provides rapid recuperation and good visual quality after surgery. The optical and visual qualities of patients with the microincision monofocal IOL have not been previously studied.

What This Paper Adds

  • The microincision monofocal IOL, from the first month after surgery, provided good optical and visual quality, similar to that of subjects in the same age range with healthy eyes.

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Other Cited Material

A. Laboratory of Vision Sciences and Applications, University of Granada, Spain. New Halo v1.0 free download. Available at: http://www.ugr.es/˜labvisgr/. Accessed April 30, 2016
B. Pujol J, Vilaseca M, Salvadó A, Romero MJ, Pérez GM, Issolio L, Artal P, “Cataract Evaluation With an Objective Scattering Index Based on Double-pass Image Analysis” presented at the annual meeting of the Association for Ophthalmology and Vision, Fort Lauderdale, Florida, USA, May 2009. ARVO E-Abstract 6127. Available at: http://iovs.arvojournals.org/article.aspx?articleid=2368484. Accessed April 30, 2016
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