Journal of Cataract & Refractive Surgery
Volume 34, Issue 1 , Pages 40-45, January 2008

Corneal aberrations after microincision cataract surgery

From VISSUM, Instituto Oftalmologico de Alicante (Elkady, Alió, Ortiz, Montalbán), and Miguel Hernandez University (Alió), Alicante, Spain; and the Ain Shams University (Elkady), Cairo, Egypt

Accepted 10 August 2007.

Article Outline

Purpose

To study the effect of microincision cataract surgery (MICS) on the optical quality of the cornea, characterized in terms of Seidel aberrations.

Setting

Instituto Oftalmológico de Alicante, Vissum, Alicante, Spain.

Methods

This study comprised 25 eyes of 25 patients with nuclear or corticonuclear cataract of grade 2+ to 4+ (Lens Opacities Classification System III). Microincision cataract surgery was performed using low ultrasound power through a 1.6 to 1.8 mm clear corneal incision placed on the axis of the positive corneal meridian. An Acri.Smart 48S intraocular lens (Acri.Tec) was implanted in all eyes. Seidel aberration root-mean-square (RMS) values were obtained with a 6.0 mm aperture using the CSO topographer (Costruzione Strumenti Oftalmici) preoperatively and 1 and 3 months postoperatively.

Results

The total RMS after MICS decreased slightly from a mean of 2.15 μm ± 2.51 (SD) preoperatively to 1.96 ± 2.01 μm postoperatively; the decrease was not statistically significant (P = 1.00). The difference between the corneal astigmatism from preoperatively (−0.80 ± 0.76 diopter [D]) to postoperatively (−0.63 ± 0.62 D) was not statistically significant (P = 1.00) nor were the differences in Seidel aberrations, coma, or higher-order aberrations.

Conclusion

Microincision cataract surgery did not degrade the optical quality of the cornea or induce a modification in corneal astigmatism, including the axis.

 

Microincision cataract surgery (MICS) is performed through a sub-2.0 mm incision.1 A major advantage of MICS is that it causes less surgical trauma, resulting in a reduction in surgically induced astigmatism (SIA) and aberrations and improvement in the corneal optical quality; this leads to improved visual outcome and high patient satisfaction.2, 3

The optical quality of the cornea is essential to good visual and refractive outcomes as it plays an important role in recovery of visual function after cataract surgery. Visual function is determined by a combination of corneal and internal aberrations generated by the intraocular lens (IOL) and those induced by the surgery. These corneal refractive changes are attributed to the location and size of the corneal incision. The smaller the incision, the lower the aberrations and the better the optical quality.1, 4, 5, 6

The MICS technique induces considerably less corneal astigmatism than surgery using conventional corneal incisions.7, 8, 9 The amount of induced astigmatism is so small that most patients do not notice it.10 Alió et al.1 describe improved control of corneal SIA with MICS over conventional 3.0 mm phacoemulsification. By reducing the amount of SIA, MICS meets the increasingly common view of cataract surgery as a refractive procedure. However, controlling astigmatism is not enough as cataract surgery modifies the higher-order corneal aberrations and the pseudophakic eye has larger corneal aberrations than the average normal eye.11

Previous studies12, 13, 14 report a decrease in corneal optical performance in pseudophakic eyes after conventional coaxial phacoemulsification, with a significant increase in astigmatism and higher-order aberrations (HOAs) such as coma and trefoil. The increase in HOAs is related to the incision site and size. Today, aspherical monofocal and multifocal IOLs aim to reduce the spherical aberrations compared with those found in eyes with conventional spherical IOLs.2, 14 Considering that the optics of IOLs, combined with the eye's aberrations, produce the final retinal image, the optics of the cornea should remain relatively unchanged after surgery to ensure improved retinal image quality.11 This is supported by the finding that a corneal incision smaller than 2.0 mm has no impact on corneal curvature.6, 15, 16

Although several studies1, 6, 17, 18 report the outcomes of MICS, to our knowledge none has evaluated the evolution of HOAs after MICS using a direct comparison of higher-order corneal aberrations before and after surgery in the same patient. The aim of this study was to determine whether MICS using a sub-2.0 mm incision effectively decreases the induction of or changes in corneal HOAs during cataract surgery to evaluate the effect of MICS on corneal optical quality, characterized in terms of Seidel aberrations.

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Patients and methods 

This prospective cumulative interventional nonrandomized noncomparative case study comprised 25 eyes of 25 patients with nuclear or corticonuclear cataract of grade 2+ to 4+ (Lens Opacities Classification System III19). Inclusion criteria included age between 50 years and 85 years, no history of eye surgery or glaucoma, a transparent central cornea, pupil dilation at the preoperative examination of at least 7.0 mm, absence of biomicroscopic signs of pseudoexfoliation, a normal fundus examination, and an endothelial cell count of at least 1600 cells/mm2 at the central cornea. Cataracts other than nuclear or corticonuclear were excluded from the study. Eyes with more than 3.0 diopters (D) of astigmatism were also excluded as astigmatic incisions would have to be used in these eye.

All patients gave adequate informed consent. The study followed the tenets of the Declaration of Helsinki stated in 2004 (World Medical Association Declaration of Helsinki. Ethical Principles for Medical Research Involving Human Subjects. 52nd General Assembly, Edinburgh, Scotland, October 2000; amended 2004. Available at: http://www.wma.net/e/policy/b3.htm. Accessed September 23, 2004). The ethics committee approved the study.

Preoperative evaluation consisted of a full ophthalmic examination including topography (CSO topographer, Costruzione Strumenti Oftalmici), endothelial cell count (SP 2000 specular microscope, Topcon) using a previously described protocol,20 and corneal thickness (DGH 500 ultrasonic pachymeter, DGH Technology, Inc.). Flare and anterior chamber cells were measured with an LFC-1000 laser flare meter (Kowa) before pupil dilation on the day of the surgery. Visual data were obtained using Snellen charts and transformed into logMAR units for adequate statistical analysis.

All postoperative follow-ups were performed by an independent observer to increase the reliability of the study. All postoperative visits included slitlamp biomicroscopy, applanation tonometry, and measurement of uncorrected and best corrected visual acuities. Postoperatively, central corneal pachymetry was performed at 1 day, 1 week, and 1 and 3 months. Endothelial cell count and corneal topography were performed at 1 and 3 months.

Surgical Technique 

The same surgeon (J.L.A.) performed all surgeries. Topical anesthesia (preservative-free lidocaine 2%) and mild sedation with midazolam were used in all cases. Adequate dilation was obtained with intracameral mydriasis using 1 mL of a vial containing cyclopentolate 1% (1 mL), phenylephrine 10% (1.5 mL), lignocaine 2% (5 mL), and balanced salt solution (BSS) (10 mL), with each vial containing 2 mL.

The incision was placed on the axis of the positive corneal meridian, which was previously marked at the slitlamp to prevent cyclorotation. An Infiniti phacoemulsification platform (Alcon) was used for MICS; the settings are shown in Table 1. Surgery was performed using a previously described MICS protocol.1, 21

Table 1. Surgical parameters for the Infiniti phacoemulsification platform.
ParameterMeasure
Incision size (mm) (tentative)1.5
Capsulorhexis diameter (mm)5.5
Phacoemulsification tip (degrees)30 (microtip)
Phacoemulsification tip caliber0.9
PrechoppingYes
Power (%)20–30
Aspiration flow (cm3/min)25
Vacuum (mm Hg)550
Anterior chamber pressure (cm3 H2O)90
Mode (type/msec/%)Hyperpulse/80/40
Irrigation/aspiration of the cortical remnants (mm Hg)450
Irrigation/aspiration of OVD (mm Hg)40

OVD = ophthalmic viscosurgical device

An Acri.Smart 48S IOL (Acri.Tec) was loaded in a cartridge and inserted into a hydraulic injector (Acri.Glide, Acri.Tec). The incision was enlarged laterally at its internal side to approximately 1.5 mm with a diamond blade. The tip of the cartridge was introduced partially into the external part of the incision, after which the IOL was injected into the capsular bag (Figure 1). The proximal IOL was placed in the bag with a second instrument (Alió intraocular manipulator, Katena), which was inserted through the second incision. After the ophthalmic viscoelastic device was removed, the incisions were hydrated using a 30-gauge cannula (Alcon). Intraocular preservative-free cefuroxime 1% (0.1 cc) was injected into the anterior chamber. No sutures were used in any case. The length of the corneal incision was measured at the end of the procedure by applying the tips of a surgical caliper (Duckworth & Kent) to the inner lips of the incision.

  • View full-size image.
  • Figure 1 

    Injection of the IOL into the eye through the MICS incision. Note that the injector is not introduced into the eye but rather into the external lips of the corneal incision.

Postoperative topical therapy included a combination of topical antibiotics (dexamethasone 0.1% [Maxidex]) and a steroid (ofloxacin 0.3% [Exocin]).

Corneal Aberrometry 

Corneal aberrations were derived using data from the anterior surface of the cornea obtained with the CSO topographer. The topographer's software (EyeTop2005, Costruzione Strumenti Oftalmici) converts the corneal elevation profile into corneal wavefront data using Zernike polynomials, with expansion up to the 7th order. The topography system analyzes 6144 corneal points from a corneal zone between 0.33 mm and 10.00 mm with respect to corneal vertex. The CSO topographer has a panel in which the aberrations of the 3rd Seidel order (Figure 2, bottom), which correspond to the primary aberrations, are represented as follows: regular astigmatism, including the map, amount (diopters), axis, and root mean square (RMS); spherical aberration, including the map, amount (diopters) of longitudinal spherical aberration, and RMS; coma, including the RMS and direction; HOAs, with all components grouped with orders higher than the primary order; tilt, with the value (prism diopters), direction, and RMS; power, which is the sum of the dioptric power of the referenced surface and defocus component. The power is expressed in diopters and millimeters, with the conversion made using the refractive index of the stroma (1.376).

  • View full-size image.
  • Figure 2 

    Top: Corneal topography maps (axial maps) of 1 eye preoperatively (left) and 1 month (center) and 3 months (right) after MICS. Bottom: Corneal aberrometry maps (Seidel panels) of 1 eye preoperatively (left) and 1 month (center) and 3 months (right) after MICS.

To evaluate the changes in corneal aberrations, the RMS of the wave aberration (Seidel aberrations) for total, coma Z(3,±1), spherical Z(4,±0) (reported with its sign), astigmatism Z(2,±2), and HOA with a 6.0 mm aperture diameter were studied preoperatively and postoperatively at the 1-month and 3-month follow-up visits.

Statistical Analysis 

Statistical analysis was performed using SPSS for Windows (version 11.0.1, SPSS, Inc.). The paired t test and 1-way analysis of variance test with Bonferroni post hoc statistical analysis were used to compare the mean values of corneal aberrations at different follow-up times. Statistical tests were performed at the 95% confidence interval.

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Results 

Surgical caliper measurement showed all incisions were 1.8 mm or smaller (range 1.6 to 1.8 mm) at the end of surgery.

Figure 2 compares the preoperative corneal topography and aberrometry maps and the maps 1 and 3 months after MICS; the maps show a slight surgically induced change. The mean corneal power was 44.08 D ± 1.21 (SD) preoperatively, 43.97 ± 1.05 D 1 month postoperatively, and 44.17 ± 1.18 D at 3 months (Figure 3); there was no statistically significant difference in corneal power between 1 month and 3 months after surgery (P = 1.00) or between any follow-up visits (P>.05 Bonferroni). The mean corneal astigmatism (Figure 4) did not show statistically significant changes (−0.80 ± 0.76 D preoperatively, −0.61 ± 0.57 at 1 month, −0.63 ± 0.62 at 3 months); the difference between visits was not statistically significant (all P>.05, Bonferroni). The mean change in corneal astigmatism from preoperatively to 3 months postoperatively was −0.19 ± 0.40 D.

Figure 5 shows the evolution of the corneal aberrations after surgery. The RMS value of the total corneal aberrations decreased slightly after MICS (mean 2.15 ± 2.51 μm preoperatively, 1.87 ± 1.87 μm at 1 month, and 1.96 ± 2.01 μm at 3 months); there was no statistically significant difference between the 2 follow-up visits (both P = 1.00, Bonferroni). Analysis of individual Zernike terms showed a mean astigmatism of 0.85 ± 0.74 μm preoperatively, 0.65 ± 0.44 μm at 1 month, and 0.69 ± 0.46 μm at 3 months and a mean spherical aberration of −0.11 ± 0.25 μm, −0.09 ± 0.25 μm, and −0.19 ± 0.13 μm, respectively. Coma decreased (mean 0.45 ± 0.40 μm preoperatively, 0.39 ± 0.36 μm at 1 month, and 0.42 ± 0.44 μm at 3 months, respectively); there was no statistically significant difference between the 2 follow-up visits (both P = 1.00, Bonferroni). The mean HOA was 0.47 ± 0.26 μm preoperatively, 0.59 ± 0.32 μm at 1 month, and 0.54 ± 0.25 μm at 3 months; there was no statistically significant difference between the 2 follow-up visits (both P>.47, Bonferroni).

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Discussion 

Cataract surgery can degrade and change the optical quality of the cornea. It has been documented that cataract surgery with IOL implantation induces and increases HOAs that are not effectively corrected with spectacles, limiting the performance of the eye. Thus, measurement of corneal aberrations is important in cataract surgery.4, 6

Corneal incisions can alter the cornea's optical power, generating SIA and postoperative changes in aberration.17 The modern goal of cataract surgery is high patient satisfaction with vision, which is achieved through good retinal image quality and an eye that is as aberration free as possible. Thus, cataract surgery today is also a refractive procedure in many cases.

The production of a good retinal image depends on combining the eye's aberrations after surgery with those induced or modified by the IOL. Thus, IOL designs are being developed to decrease aberrations. In addition, new techniques and advances in cataract surgery have succeeded in achieving this goal by limiting the role of the corneal incision in inducing corneal aberrations.11

We believe the main tool to neutralize the effect of corneal incisions on induced aberrations and stabilize corneal optics postoperatively is to continue to decrease the size of the incision until successful cataract surgery with IOL implantation can be performed through a sub-2.0 mm incision, perfecting the MICS technique.1 This goes hand in hand with new advances in IOL technology (eg, IOLs with different aberration profiles) that compensate for and reduce ocular aberrations, thus improving retinal image quality. Aspherical IOLs12, 22 balance the positive corneal spherical aberration, and conventional spherical IOLs reduce aberration. Both improve contrast sensitivity compared with that with monofocal IOLs.13 Newer aberration- and wavefront-corrected IOLs, especially those with a prolate anterior surface, can eliminate spherical aberration in the average normal eye and reduce total optical spherical aberration in more than 90% of patients presenting for cataract surgery.22, 23, 24 Thus, it is important to neutralize the effect of the corneal incision on corneal aberrations.

In our study, we used corneal aberrometry to evaluate the optical quality of the cornea. Aberrometry provides the most important metrics for evaluating visual function25 as approximately 80% of all aberrations of the human eye occur in the corneal surface. The CSO topographer analyzes up to 6144 corneal points within a corneal zone between 0.33 mm and 10.00 mm with respect to corneal vertex, with more expressive analysis and assessment using the Zernike polynomials up to the 7th-order aberration. In addition, the topographer avoids superimposing the light spots associated with the different parts of the wavefront produced by highly aberrated eyes. Finally, some types of global aberrometry assume that the slope of the wavefront in each analyzed portion is locally flat, which could induce significant errors in the calculation of the final results.25

Although many authors1, 6, 17, 18 report changes in corneal aberrations after cataract surgery, the studies focused on the outcomes of small-incision (3.0 mm) surgery. These studies conclude that cataract surgery can degrade corneal optical quality by inducing changes in some aberrations. Previous studies11, 12 report decreased optical quality in pseudophakic eyes, with increased HOAs (eg, coma, trefoil, tetrafoil) generated on the cornea. Guirao et al.11 found changes in the magnitude and orientation of aberrations, with a mean induced astigmatism at the surgical meridian of −1.0 ± 0.9 D. In a study of 2 aspherical IOLs (Tecnis Z9000, Advanced Medical Optics, Inc.; AcrySof IQ SN60WF, Alcon), Marcos et al.12 found significant changes in vertical astigmatism (2.47 μm and 1.74 μm, respectively), vertical trefoil (1.81 μm and 1.20 μm, respectively), and tetrafoil (−1.10 μm and −0.89 μm, respectively). Our study focused on MICS (ie, sub-2.0 mm incisions), evaluating corneal astigmatism and HOA as a measure of postoperative corneal changes. We found that on average, the RMS value of the total corneal aberrations decreased slightly after MICS, although the differences were not statistically significant. Our finding agrees with the results in a study by Yao et al.17

Although the changes in corneal astigmatism were not statistically significant in our study, there were slight differences between the preoperative and 1- and 3-month postoperative values (all P>.05, Bonferroni). The mean change in corneal astigmatism preoperatively to 3 months was −0.19 ± 0.40 D, with little difference between 1-month and 3-month values, indicating that MICS provides a stable incision for up to 3 months postoperatively. This supports the findings in previous studies,1, 6, 17, 26 reinforcing that MICS has advantages over traditional small-incision cataract surgery, which induces low levels of astigmatism.17 Thus, we can conclude that MICS provides a neutral incision in terms of astigmatism.

When the aberrations were evaluated by analysis of 4 Zernike terms (astigmatism, spherical, coma, HOA), no statistically significant changes were found in any aberration postoperatively. Similar results have been reported.6, 17 All aberration values except HOA decreased slightly, with no statistically significant differences between the follow-up visits. All aberration values were stable for 3 months after surgery, indicating that successful MICS depends on preventing induction of HOAs as well as a surgically neutral and stable procedure. Successful MICS gives visual quality equal to that in persons of the same age without pathology and leads to good patient satisfaction.

We believe the reliability of our study is the result of 3 main factors: similar incision size (range 1.6 to 1.8 mm) in all the eyes, no incision larger than 1.8 mm, and all incisions placed on the axis of the positive corneal meridian. Also, only 1 type of IOL was used, making our results more reliable than those in previous studies that used different incision sizes, sites, and/or IOL types.6, 11, 12, 17

Measurements of corneal quality were performed in the same eyes preoperatively and postoperatively, making our conclusion stronger than that of Guirao et al.11 and in agreement with the results of Marcos et al.12 The smallest changes were in spherical and coma aberrations, supporting the value of aspherical IOL designs in compensating for spherical aberrations of the cornea, which remains almost unchanged after surgery. Similar results have been reported.11, 12

In conclusion, the MICS sub-2.0 mm incision was effective in stabilizing the cornea after surgery, effectively decreasing the induction of corneal HOAs. It did not degrade the corneal optical quality, nor did it modify the corneal astigmatism, even in the axis. These findings confirm that using the smallest incision in sub-2.0 mm surgery improves control of the optical performance of the human eye.

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References 

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 No author has a financial or proprietary interest in any material or method mentioned.

 Supported in part by a grant from the Spanish Ministry of Health, Instituto Carlos III, Red Temática de Investigación en Oftalmologia, Subproyecto de Cirugia Refractiva y Calidad Visual (C03/13), Spain.

PII: S0886-3350(07)01659-8

doi:10.1016/j.jcrs.2007.08.022

Journal of Cataract & Refractive Surgery
Volume 34, Issue 1 , Pages 40-45, January 2008

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