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Changes in corneal wavefront aberrations in microincision and small-incision cataract surgery
ARTICLE
Changes in corneal wavefront aberrations in microincision and small-incision cataract surgery NuXia Tong, OD, Ji C. He, PhD, Fan Lu, MD, OD, QinMei Wang, MD, Jia Qu, MD, MSc, Yun-E. Zhao, MD
PURPOSE: To study the effect of incision size on the optical quality of the anterior cornea by comparing the changes in corneal wavefront aberrations between microincision cataract surgery (MICS) and small-incision cataract surgery (SICS). SETTING: Eye Hospital of Wenzhou Medical College, Wenzhou, Zhejiang, China. METHODS: This prospective randomized clinical study included 36 eyes having MICS (1.5 mm) and 38 eyes having SICS (3.0 mm). Anterior corneal topography was measured preoperatively and 3 to 6 months postoperatively. The data were used to calculate anterior corneal Zernike aberrations (through the 6th order) for a 6.0 mm central area. RESULTS: In the MICS group, 2 corneal Zernike aberrations (trefoil and tetrafoil) changed significantly from preoperatively to postoperatively (both P<.0001). In the SICS group, in addition to trefoil and tetrafoil, oblique astigmatism (P<.0001), secondary oblique astigmatism (P Z .001), and vertical tetrafoil (P Z .001) changed significantly. The SICS group had greater changes than the MICS group in oblique astigmatism (P Z .0001), oblique trefoil (P Z .0035), and vertical tetrafoil (P Z .0023). The changes in the SICS group were significantly greater than in the MICS group in the total root mean square (RMS) (P Z .007) and higher-order RMS (P Z .023) of corneal wavefront aberrations. CONCLUSIONS: Cataract surgery-related changes in corneal wavefront aberrations were dependent on incision size. The MICS technique had advantages over the SICS technique in minimizing the effect of the incision size on the optical quality of the cornea. J Cataract Refract Surg 2008; 34:2085–2090 Q 2008 ASCRS and ESCRS
In microincision cataract surgery (MICS), the incision is approximately 1.5 mm, which is about half the size of the incision in small-incision cataract surgery (SICS). Because of the smaller incision, MICS is expected to have less effect than SICS on the optical quality of the cornea. This has been shown in studies1–4 of corneal astigmatism, which found less induced astigmatism with MICS than with SICS. At present, the most widely used technique for evaluating corneal optical quality is corneal topography, which produces color maps that show, among other things, corneal astigmatism. Another technique, wavefront analysis, allows quantitative characterization of localized changes in the corneal shape with Zernike polynomial functions from the same set of the corneal topography data, given that the system is carefully calibrated.5–7 Guirao et al.8 applied the wavefront technique to assess wavefront aberrations in the anterior corneal surface of patients having SICS. In addition to astigmatism, the authors found that the surgery induced significant trefoil, a higherQ 2008 ASCRS and ESCRS Published by Elsevier Inc.
order Zernike aberration. In a later study, Marcos et al.9 found that SICS caused significant changes in vertical tetrafoil, a 4th-order higher-order aberration (HOA). Given the high accuracy of wavefront analysis, studies have used it to compare the influence of incision size (ie, MICS versus SICS) on corneal optical quality. Yao et al.10 compared postoperative corneal astigmatism and HOAs with a 5.0 mm pupil in 30 eyes with SICS and 30 eyes with MICS; they found no significant difference between the 2 groups in HOAs, including coma, trefoil, spherical aberration, and total tetrafoil. They concluded that MICS had no significant advantage over SICS in reducing corneal HOAs even though it induced less astigmatism and produced better modulation transfer function (MTF). Thus, their study implies that MICS induces the same amounts of HOA as SICS. However, the authors did not measure preoperative aberrations and the exact changes in corneal aberrations postoperatively were not detailed. In a recent study, Elkady et al.11 found no significant 0886-3350/08/$dsee front matter doi:10.1016/j.jcrs.2008.08.020
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induction of HOAs in 25 eyes after MICS compared with preoperative aberration levels. Based on the varying results in previous studies, there is controversy over whether MICS induces the same amounts of HOAs as SICS or induces no significant HOA. To explore this issue, we measured the preoperative and postoperative corneal wavefront aberrations in 2 groups of patients, 1 having MICS and 1 having SICS. This allowed us to directly test the surgically induced incision-related changes in corneal HOAs within each group and compare the difference in induced HOAs between the 2 groups. PATIENTS AND METHODS This prospective randomized clinical study comprised 53 cataract patients (74 eyes). Practices and research adhered to the tenets of the Declaration of Helsinki. Informed consent was obtained from all patients after they received an explanation of the nature and possible consequences of the procedures. Inclusion criteria included good general health, no other ocular pathology, corneal astigmatism less than 1.5 diopters, and no complications during or after surgery. The 74 eyes were randomly assigned to have clear corneal MICS or clear corneal SICS. Because of the randomization, some patients had MICS in 1 eye and SICS in the fellow eye. All clinical examinations and surgeries were conducted at the Affiliated Eye Hospital of Wenzhou Medical College, Zhejiang Wenzhou, China. Clinical examination data recorded included uncorrected visual acuity, intraocular pressure, and biomicroscopic anterior and posterior segment evaluations. All tests were performed within 3 days before surgery and 1 week, and 3 to 6 months after surgery by an independent ophthalmologist (ie, not the surgeon who performed the MICS and SICS). Videokeratoscopy was performed preoperatively and 3 to 6 months postoperatively.
Surgical Technique The same surgeon (Z.Y.E.) performed all surgeries on an inpatient basis using topical anesthesia. Phacoemulsification
Accepted for publication August 7, 2008. From the School of Optometry and Ophthalmology (Tong, He, Lu, Wang, Qu, Zhao), Eye Hospital, Wenzhou Medical College, Wenzhou, Zhejiang, China; New England College of Optometry (He), Boston, Massachusetts, USA. No author has a financial or proprietary interest in any material or method mentioned. Supported by research grants from Chinese National Key Technologies R&D Program, Beijing, China (2007BAI18B09, Dr. Lu), Science and Technology Department of Zhejiang Province (2006C3021, Dr. Zhao), and Zhejiang Provincial Program for the Cultivation of High-level Innovative Health Talents (Dr. Lu). Corresponding author: Yun-E Zhao, MD, School of Ophthalmology and Optometry, Wenzhou Medical College, 82 Xueyuan Road, Wenzhou, Zhejiang, 325027, China. E-mail: [email protected].
was performed with the Sovereign System (Advanced Medical Optics). In the MICS group, a 1.2 mm superolateral clear corneal incision (at the 10:30 o’clock position) and a 1.2 mm side puncture (at 2 o’clock) were created with a steel blade. A capsulorhexis was created after the anterior chamber was filled with an ophthalmic viscosurgical device (OVD). After the nuclear and cortical material was removed, the capsule was cleaned with the aspiration tip under low vacuum (15 mm Hg). The superolateral clear corneal incision was enlarged to 1.5 mm, and an Acri.Smart intraocular lens (IOL) (Acri.Tec) was implanted using an acri.Smart IOL unfolder. Once the OVD was removed, the incision was closed by stromal hydration without sutures. In the SICS group, a 3.0 mm clear corneal incision was created superolaterally at the 10:30 o’clock position and a 1.0 mm side puncture was made at the 2 o’clock position. After OVD was introduced into the anterior chamber, a 5.5 mm capsulorhexis was made. This was followed by hydrodissection, phacoemulsification, aspiration of cortical masses, and introduction of a foldable IOL through the 3.0 mm incision. Once the OVD was removed, the incision was closed by stromal hydration without sutures.
Anterior Corneal Aberrations Corneal topography was performed preoperatively and postoperatively using a Humphrey Atlas corneal topography system (Carl Zeiss Meditec). The system directly provided estimates of the pupil center location, vertex location, curvatures of the anterior corneal surface, and keratometric measurements. The corneal curvatures and corneal heights were exported from the topography system and used to calculate corneal aberrations. A customized MatLab ray-tracing program (The MathWorks, Inc.)7,12–14 was used to derive Zernike aberrations up to the 6th order (28 terms). For each eye, the mean of 3 measurements at a 6.0 mm diameter central area with respect to the pupil center was calculated and used as the final estimate of the aberration measurements. The single-index conversion (ie, j)15 was used to name the Zernike aberrations.
Statistical Analysis An unpaired t test was used to compare the preoperative and postoperative mean corneal Zernike aberrations and the root mean square (RMS) of the corneal wavefront aberrations in the 2 groups. Mean changes in Zernike aberrations and RMS values in each group from preoperatively to postoperatively were calculated and tested with a paired t test. The difference in mean changes between the MICS group and the SICS group was tested with an unpaired t test. All analyses were performed using SPSS software (version 12.0, SPSS, Inc.). A P value less than 0.05 after Bonferroni correction for multiple comparison (significance level divided by comparison number) was considered statistically significant for individual Zernike aberrations.
RESULTS Microincision cataract surgery was performed in 36 eyes and SICS, in 38 eyes. The mean age of the patients was 66.38 years G 10.05 (SD) (range 41 to 80 years). Table 1 shows the patients’ baseline data. There was no significant difference between the 2 groups in preoperative
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Table 1. Preoperative patient data. Mean G SD MICS Group (n Z 36)
Parameter Age (y) Corneal astigmatism (D) Corneal power (D) Time between surgery and postop measurements (d)
SICS Group (n Z 38)
P Value
68.86 G 7.85 64.97 G 11.73 0.65 G 0.39 0.72 G 0.42
.097 .462
44.36 G 1.22 43.99 G 1.78 112 G 29 113 G 24
.299 .819
MICS Z microincision cataract surgery; SICS Z small-incision cataract surgery
age, length of follow-up, corneal astigmatism, or corneal power. No eye had surgical complications. Root Mean Square Values and Zernike Aberrations Table 2 shows the mean preoperative and postoperative corneal aberrations over a 6.0 mm area centered
on the pupil center for Zernike terms from the 2nd to 4th order (except defocus, j Z 4). The Zernike terms of 5th order up are not shown because they were small in magnitude. Table 2 also shows the RMS values for total corneal wavefront aberrations from the 2nd to 6th order (with j Z 4 excluded) and HOA RMS values from the 3rd to 6th order. In the SICS group, the mean values for oblique astigmatism (j Z 3), oblique trefoil (j Z 9), and oblique tetrafoil (j Z 10) were statistically significantly different between preoperatively and postoperatively; the differences in secondary oblique astigmatism (j Z 11) and tetrafoil (j Z 14) approached significance. Although the total RMS was not significantly different between preoperatively and postoperatively, the change was borderline significant. The mean postoperative trefoil was statistically significantly greater than preoperatively. In the MICS group, the mean values for oblique trefoil and oblique tetrafoil were statistically significantly different between preoperatively and postoperatively. The mean postoperative trefoil was statistically significantly greater than the mean preoperative level.
Table 2. Mean preoperative and postoperative RMS values and Zernike aberrations (mm) in the anterior cornea over a 6.0 mm diameter central area. SICS Group Parameter tRMS hoRMS j index/Z term 3/Z(2,2) 5/Z(2,2) 6/Z(3,3) 7/Z(3,1) 8/Z(3,1) 9/Z(3,3) 10/Z(4,4) 11/Z(4,2) 12/Z(4,0) 13/Z(4,2) 14/Z(4,4) Astigk Comak Trefoilk
Preop
Postop
0.941 G 0.375 0.612 G 0.196
1.124 G 0.472 0.698 G 0.243
0.064 G 0.447 0.134 G 0.636 0.184 G 0.186 0.030 G 0.285 0.005 G 0.312 0.004 G 0.145 0.014 G 0.066 0.006 G 0.046 0.312 G 0.088 0.007 G 0.090 0.024 G 0.100 0.662 G 0.421 0.364 G 0.208 0.261 G 0.142
0.460 G 0.532 0.187 G 0.652 0.193 G 0.219 0.008 G 0.309 0.028 G 0.269 0.263 G 0.209 0.105 G 0.122 0.035 G 0.050 0.296 G 0.095 0.025 G 0.095 0.044 G 0.123 0.830 G 0.504 0.339 G 0.225 0.396 G 0.200
MICS Group P Value* .069 .099 .001† .722 .833 .584 .733 !.0001x !.0001x .014 .443 .416 .012 .123 .615 .0016z
Preop
Postop
0.884 G 0.295 0.602 G 0.203
0.977 G 0.395 0.652 G 0.185
0.025 G 0.271 0.042 G 0.634 0.173 G 0.194 0.073 G 0.323 0.084 G 0.267 0.010 G 0.118 0.030 G 0.063 0.002 G 0.053 0.300 G 0.118 0.009 G 0.080 0.018 G 0.076 0.589 G 0.346 0.371 G 0.218 0.258 G 0.120
0.144 G 0.364 0.105 G 0.708 0.191 G 0.280 0.108 G 0.297 0.045 G 0.249 0.143 G 0.146 0.057 G 0.098 0.020 G 0.064 0.279 G 0.110 0.014 G 0.073 0.025 G 0.084 0.663 G 0.463 0.351 G 0.194 0.369 G 0.134
P Value* .262 .279 .123 .694 .765 .636 .538 !.0001x !.0001x .191 .447 .805 .692 .448 .695 .0007x
Means G SD Astig Z astigmatism; hoRMS Z higher-order root mean square; MICS Z microincision cataract surgery; SICS Z small-incision cataract surgery; tRMS Z total corneal wavefront aberration root mean square; Z Z Zernike *Significance corrected for multiple comparison for Zernike aberrations of j Z 3–14 (11 terms) † P!.05 z P!.01 x P!.001 k The terms of astigmatism, coma, and trefoil indicate the RMS values of the 2 Zernike astigmatisms (j Z 3 and j Z 5), comas (j Z 7 and j Z 8), and trefoils (j Z 6 and j Z 9), respectively.
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There were no statistically significant differences between the MICS group and the SICS group in mean RMS values or Zernike aberrations preoperatively. Postoperatively, the differences between groups in oblique astigmatism, oblique trefoil, and tetrafoil were borderline significant (P Z .0059, P Z .0067, and P Z .0073, respectively). Changes in Root-Mean-Square Values and Zernike Aberrations Table 3 shows the mean changes in Zernike aberrations and RMS values from preoperatively to postoperatively in the MICS and SICS groups. Five corneal aberration terms (trefoil, tetrafoil, oblique astigmatism, secondary oblique astigmatism, and vertical tetrafoil) in the SICS group and 2 terms (trefoil and tetrafoil) in the MICS group were statistically significantly different after surgery. All changes in RMS values were statistically significantly different from zero in both groups. The changes in total RMS and HOA RMS were statistically significantly greater in the SICS group
than in the MICS group (Table 3). Oblique astigmatism, oblique trefoil, and vertical tetrafoil showed change most in the SICS group compared with the MICS group. The mean induced astigmatism was 0.324 G 0.198 mm in the MICS group and 0.478 G 0.299 mm in the SICS group; the difference between groups was statistically significant (P Z .0063). The mean induced trefoil was 0.233 G 0.134 mm in the MICS group and 0.319 G 0.164 mm in the SICS group; the difference between groups was statistically significant (P Z .009). There was no statistically significant difference between groups in induced spherical aberration or induced coma aberration. DISCUSSION We measured wavefront aberrations in the anterior corneal surface preoperatively and postoperatively in 36 eyes having MICS and 38 eyes having SICS to evaluate the influence of incision size on surgery-related change in corneal aberrations postoperatively. The mean level of total corneal wavefront aberrations (total RMS) did not change significantly in the MICS
Table 3. Mean changes in RMS values and Zernike aberrations (mm) in the anterior cornea over a 6.0 mm diameter central area between preoperatively and postoperatively.
SICS Group
Small-Incision Versus Microincision
MICS Group
Parameter
Change (mm)
P Value*
Change (mm)
P Value
P Value*
tRMS hoRMS j index/Z term 3/Z(2,2) 5/Z(2,2) 6/Z(3,3) 7/Z(3,1) 8/Z(3,1) 9/Z(3,3) 10/Z(4,4) 11/Z(4,2) 12/Z(4,0) 13/Z(4,2) 14/Z(4,4) Astigk Comak Trefoilk
0.697 G 0.316 0.480 G 0.195
!.0001x !.0001x
0.523 G 0.209 0.390 G 0.143
!.0001x !.0001x
.007z .023†
0.396 G 0.316 0.053 G 0.244 0.010 G 0.181 0.038 G 0.138 0.023 G 0.139 0.259 G 0.172 0.118 G 0.119 0.028 G 0.050 0.016 G 0.065 0.018 G 0.078 0.068 G 0.120 0.478 G 0.299 0.168 G 0.105 0.319 G 0.164
!.0001x .190 .737 .100 .315 !.0001x !.0001x .001† .128 .173 .001† !.0001x !.0001x !.0001x
0.120 G 0.260 0.063 G 0.247 0.017 G 0.167 0.035 G 0.107 0.039 G 0.145 0.153 G 0.147 0.088 G 0.097 0.018 G 0.045 0.020 G 0.068 0.004 G 0.062 0.008 G 0.096 0.324 G 0.198 0.154 G 0.105 0.233 G 0.134
.009 .135 .543 .059 .117 !.0001x !.0001x .019 .078 .667 .639 !.0001x !.0001x !.0001x
.0001z .431 .430 .462 .317 .0035† .117 .187 .391 .215 .0023† .0063z .278 .009z
Means G SD Astig Z astigmatism; hoRMS Z higher-order root mean square; MICS Z microincision cataract surgery; SICS Z small-incision cataract surgery; tRMS Z total corneal wavefront aberration root mean square; Z Z Zernike *Significance corrected for multiple comparison for Zernike aberrations of j Z 3–14 (11 terms) † P!.05 z P!.01 x P!.001 k The terms of astigmatism, coma, and trefoil indicate the RMS values of the 2 Zernike astigmatisms (j Z 3 and j Z 5), comas (j Z 7 and j Z 8), and trefoils (j Z 6 and j Z 9), respectively.
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group; the mean level in the SICS group was higher, although the difference was only borderline significant. Because the HOA RMS did not increase after surgery in either group, the higher total RMS in the SICS group can be attributed to the increase in oblique astigmatism. The results, therefore, indicate that surgically induced astigmatism depends on incision size. That the mean levels of HOA RMS were not increased in the MICS group or the SICS group does not imply that the levels of individual Zernike aberrations were not altered. In fact, the mean oblique trefoil and oblique tetrafoil values were higher in both groups. In addition to oblique trefoil and oblique tetrafoil, secondary astigmatism and vertical tetrafoil changed in the SICS group, although the changes were borderline significant. Moreover, 2 HOA terms changed in the MICS group and 4 in the SICS group and the changes in the SICS group were greater than in the MICS group in 2 higher-order terms. These results suggest that both types of surgery cause changes in HOA but that the changes are greater with SICS than with MICS. The surgery-related changes in Zernike astigmatism, trefoil, and tetrafoil in our SICS group agree with results in previous studies.1–4,8,9 However, the changed terms were different than in previous studies because the incision site was superolateral in our study instead of dominantly temporal8 or superior.9 In the MICS group, there were also changes in trefoil and tetrafoil, although the magnitudes were smaller than in the SICS group. No patient in our study had a significant change in coma or spherical aberration. This finding is consistent with that in the study by Elkady et al.,11 in which Seidel coma and spherical aberration were not changed in 25 patients after MICS. Elkady et al. found no significant change in RMS HOA in the MICS group when only Seidel aberrations were analyzed, perhaps because Seidel aberrations include few HOAs and no Seidel term has an order as high as Zernike trefoil and tetrafoil. In a comparative study of postoperative corneal HOAs between MICS and SICS, Yao et al.10 found no significant differences in mean levels of total coma, total trefoil, spherical aberration, or total tetrafoil between the 2 groups. The total aberration in their study might be equivalent to the RMS of aberrations in our study. For example, the total coma might be the RMS of the horizontal and the vertical comas. The same results were observed in postoperative RMS values between the 2 groups in this study. However, 3 postoperative Zernike terms (oblique astigmatism, oblique trefoil, and tetrafoil [j Z 3, j Z 9, and j Z 14, respectively]) were on the border of significance after correction for multiple comparison. In our study, the
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changes in total RMS and HOA RMS were greater in the SICS group than in the MICS group. The total trefoil changed more after SICS than after MICS, and 3 Zernike terms (oblique astigmatism, oblique trefoil, tetrafoil) changed more in the SICS group than in the MICS group. On corneal topography, we measured the corneal heights and calculated the change in wavefront aberrations in the anterior corneal surface only. The incision should alter the posterior corneal surface in a similar way and thus produce similar changes in Zernike aberrations. With smaller amplitude and opposite signs than those in the anterior cornea because the difference in the refractive index across the posterior surface is smaller than that across the anterior corneal surface and in the opposite direction. Therefore, the posterior corneal aberrations might be able to compensate for part of the anterior corneal aberrations and increase the optical quality of the whole cornea. However, the posterior cornea could also induce more aberrations to the whole cornea for some Zernike terms due to the difference in surface shape between the anterior and posterior cornea (He JC, et al. IOVS 2008; 49:ARVO E-Abstract 986). Further study is required to clarify this point. From the HOA standard deviation data in Tables 2 and 3, it is clear that there was substantial individual variation in induced higher-order wavefront aberrations. Some inconsistencies between the results in our study and those in previous studies might be attributed to the individual variation in corneal aberrations. In our study, we did not analyze the surgically induced change in corneal aberrations to calculate its effect on image quality using an image plane metric, such as the point-spread function or MTF. Further study of this aspect will lead to a better understanding of the effect of the change in corneal aberrations on the visual performance of the eye. Pupil size in cataract patients could be smaller than the size we used to calculate the corneal aberrations (6.0 mm), and it varies from individual to individual. Therefore, future studies should evaluate the effect of pupil size on image quality. In conclusion, both MICS and SICS caused changes in astigmatism and higher-order wavefront aberrations in the anterior corneal surface, with the extent of the changes depending on the incision size. Our results show that MICS has an advantage over SICS in minimizing the effect of incision size on corneal optical quality. Incision size plays a role in determining corneal wavefront aberrations and should be considered when evaluating the origin of postoperative total aberration in the whole eye and in assessing the optical outcomes of cataract surgery.
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REFERENCES 1. Tsuneoka H, Shiba T, Takahashi Y. Feasibility of ultrasound cataract surgery with a 1.4 mm incision. J Cataract Refract Surg 2001; 27:934–940 2. Agarwal A, Agarwal A, Agarwal S, Narang P, Narang S. Phakonit: phacoemulsification through a 0.9 mm corneal incision. J Cataract Refract Surg 2001; 27:1548–1552 3. Tsuneoka H, Shiba T, Takahashi Y. Ultrasonic phacoemulsification using a 1.4 mm incision: clinical results. J Cataract Refract Surg 2002; 28:81–86 4. Alio´ J, Rodriguez-Prats JL, Galal A, Ramzy M. Outcomes of microincision cataract surgery versus coaxial phacoemulsification. Ophthalmology 2005; 112:1997–2003 5. Artal P, Guirao A, Berrio E, Williams DR. Compensation of corneal aberrations by the internal optics in the human eye. J Vision 2001; 1:1–8. Available at: http://journalofvision.org/1/1/1/. Accessed August 17, 2008 6. Salmon TO, Thibos LN. Videokeratoscopedline-of-sight misalignment and its effect on measurements of corneal and internal ocular aberrations. J Opt Soc Am A Opt Image Sci Vis 2002; 19:657–669 7. He JC, Gwiazda J, Thorn F, Held R. Wave-front aberrations in the anterior corneal surface and the whole eye. J Opt Soc Am A Opt Image Sci Vis 2003; 20:1155–1163 8. Guirao A, Tejedor J, Artal P. Corneal aberrations before and after small-incision cataract surgery. Invest Ophthalmol Vis Sci 2004; 45:4312–4319. Available at: http://www.iovs.org/cgi/reprint/45/12/4312. Accessed August 17, 2008 9. Marcos S, Rosales P, Llorente L, Jime´nez-Alfaro I. Change in corneal aberrations after cataract surgery with 2 types of aspherical intraocular lenses. J Cataract Refract Surg 2007; 33:217–226
10. Yao K, Tang X, Ye P. Corneal astigmatism, high order aberrations, and optical quality after cataract surgery: microincision versus small incision. J Refract Surg 2006; 22:S1079–S1082 11. Elkady B, Alio´ JL, Ortiz D, Montalba´n R. Corneal aberrations after microincision cataract surgery. J Cataract Refract Surg 2008; 34:40–45 12. Qu J, Lu F, Wu JX, Wang QM, Xu CC, Zhou XT, He JC. Wavefront aberration and its association with intraocular pressure and central corneal thickness for the myopic eyes. J Cataract Refract Surg 2007; 33:1447–1454 13. Lu F, Wu J, Shen Y, Qu J, Wang Q, Xu C, Chen S, Zhou X, He JC. On the compensation of horizontal coma aberrations in young human eyes. Ophthalmic Physiol Opt 2008; 28:277–282 14. Lu F, Wu JX, Qu J, Wang QM, Xu CC, Zhou XT, Shen YY, He JC. Association between offset of the pupil center from the corneal vertex and wavefront aberrations. J Optom 2008; 1:1–6. Available at: http://www.journalofoptometry.org/Archive/vol1/ pdf/04%20Vol1-n1%20Original%20Article.pdf. Accessed August 17, 2008 15. Thibos LN, Applegate RA, Schwiegerling JT, Webb R. Standards for reporting the optical aberrations of eyes; VSIA Standards Taskforce Members. J Refract Surg 2002; 18:S652–S660
First author: NuXia Tong, OD School of Optometry and Ophthalmology, Eye Hospital, Wenzhou Medical College, Wenzhou, Zhejiang, China
J CATARACT REFRACT SURG - VOL 34, DECEMBER 2008
Changes in corneal wavefront aberrations in microincision and small-incision cataract surgery NuXia Tong, OD, Ji C. He, PhD, Fan Lu, MD, OD, QinMei Wang, MD, Jia Qu, MD, MSc, Yun-E. Zhao, MD
PURPOSE: To study the effect of incision size on the optical quality of the anterior cornea by comparing the changes in corneal wavefront aberrations between microincision cataract surgery (MICS) and small-incision cataract surgery (SICS). SETTING: Eye Hospital of Wenzhou Medical College, Wenzhou, Zhejiang, China. METHODS: This prospective randomized clinical study included 36 eyes having MICS (1.5 mm) and 38 eyes having SICS (3.0 mm). Anterior corneal topography was measured preoperatively and 3 to 6 months postoperatively. The data were used to calculate anterior corneal Zernike aberrations (through the 6th order) for a 6.0 mm central area. RESULTS: In the MICS group, 2 corneal Zernike aberrations (trefoil and tetrafoil) changed significantly from preoperatively to postoperatively (both P<.0001). In the SICS group, in addition to trefoil and tetrafoil, oblique astigmatism (P<.0001), secondary oblique astigmatism (P Z .001), and vertical tetrafoil (P Z .001) changed significantly. The SICS group had greater changes than the MICS group in oblique astigmatism (P Z .0001), oblique trefoil (P Z .0035), and vertical tetrafoil (P Z .0023). The changes in the SICS group were significantly greater than in the MICS group in the total root mean square (RMS) (P Z .007) and higher-order RMS (P Z .023) of corneal wavefront aberrations. CONCLUSIONS: Cataract surgery-related changes in corneal wavefront aberrations were dependent on incision size. The MICS technique had advantages over the SICS technique in minimizing the effect of the incision size on the optical quality of the cornea. J Cataract Refract Surg 2008; 34:2085–2090 Q 2008 ASCRS and ESCRS
In microincision cataract surgery (MICS), the incision is approximately 1.5 mm, which is about half the size of the incision in small-incision cataract surgery (SICS). Because of the smaller incision, MICS is expected to have less effect than SICS on the optical quality of the cornea. This has been shown in studies1–4 of corneal astigmatism, which found less induced astigmatism with MICS than with SICS. At present, the most widely used technique for evaluating corneal optical quality is corneal topography, which produces color maps that show, among other things, corneal astigmatism. Another technique, wavefront analysis, allows quantitative characterization of localized changes in the corneal shape with Zernike polynomial functions from the same set of the corneal topography data, given that the system is carefully calibrated.5–7 Guirao et al.8 applied the wavefront technique to assess wavefront aberrations in the anterior corneal surface of patients having SICS. In addition to astigmatism, the authors found that the surgery induced significant trefoil, a higherQ 2008 ASCRS and ESCRS Published by Elsevier Inc.
order Zernike aberration. In a later study, Marcos et al.9 found that SICS caused significant changes in vertical tetrafoil, a 4th-order higher-order aberration (HOA). Given the high accuracy of wavefront analysis, studies have used it to compare the influence of incision size (ie, MICS versus SICS) on corneal optical quality. Yao et al.10 compared postoperative corneal astigmatism and HOAs with a 5.0 mm pupil in 30 eyes with SICS and 30 eyes with MICS; they found no significant difference between the 2 groups in HOAs, including coma, trefoil, spherical aberration, and total tetrafoil. They concluded that MICS had no significant advantage over SICS in reducing corneal HOAs even though it induced less astigmatism and produced better modulation transfer function (MTF). Thus, their study implies that MICS induces the same amounts of HOA as SICS. However, the authors did not measure preoperative aberrations and the exact changes in corneal aberrations postoperatively were not detailed. In a recent study, Elkady et al.11 found no significant 0886-3350/08/$dsee front matter doi:10.1016/j.jcrs.2008.08.020
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induction of HOAs in 25 eyes after MICS compared with preoperative aberration levels. Based on the varying results in previous studies, there is controversy over whether MICS induces the same amounts of HOAs as SICS or induces no significant HOA. To explore this issue, we measured the preoperative and postoperative corneal wavefront aberrations in 2 groups of patients, 1 having MICS and 1 having SICS. This allowed us to directly test the surgically induced incision-related changes in corneal HOAs within each group and compare the difference in induced HOAs between the 2 groups. PATIENTS AND METHODS This prospective randomized clinical study comprised 53 cataract patients (74 eyes). Practices and research adhered to the tenets of the Declaration of Helsinki. Informed consent was obtained from all patients after they received an explanation of the nature and possible consequences of the procedures. Inclusion criteria included good general health, no other ocular pathology, corneal astigmatism less than 1.5 diopters, and no complications during or after surgery. The 74 eyes were randomly assigned to have clear corneal MICS or clear corneal SICS. Because of the randomization, some patients had MICS in 1 eye and SICS in the fellow eye. All clinical examinations and surgeries were conducted at the Affiliated Eye Hospital of Wenzhou Medical College, Zhejiang Wenzhou, China. Clinical examination data recorded included uncorrected visual acuity, intraocular pressure, and biomicroscopic anterior and posterior segment evaluations. All tests were performed within 3 days before surgery and 1 week, and 3 to 6 months after surgery by an independent ophthalmologist (ie, not the surgeon who performed the MICS and SICS). Videokeratoscopy was performed preoperatively and 3 to 6 months postoperatively.
Surgical Technique The same surgeon (Z.Y.E.) performed all surgeries on an inpatient basis using topical anesthesia. Phacoemulsification
Accepted for publication August 7, 2008. From the School of Optometry and Ophthalmology (Tong, He, Lu, Wang, Qu, Zhao), Eye Hospital, Wenzhou Medical College, Wenzhou, Zhejiang, China; New England College of Optometry (He), Boston, Massachusetts, USA. No author has a financial or proprietary interest in any material or method mentioned. Supported by research grants from Chinese National Key Technologies R&D Program, Beijing, China (2007BAI18B09, Dr. Lu), Science and Technology Department of Zhejiang Province (2006C3021, Dr. Zhao), and Zhejiang Provincial Program for the Cultivation of High-level Innovative Health Talents (Dr. Lu). Corresponding author: Yun-E Zhao, MD, School of Ophthalmology and Optometry, Wenzhou Medical College, 82 Xueyuan Road, Wenzhou, Zhejiang, 325027, China. E-mail: [email protected].
was performed with the Sovereign System (Advanced Medical Optics). In the MICS group, a 1.2 mm superolateral clear corneal incision (at the 10:30 o’clock position) and a 1.2 mm side puncture (at 2 o’clock) were created with a steel blade. A capsulorhexis was created after the anterior chamber was filled with an ophthalmic viscosurgical device (OVD). After the nuclear and cortical material was removed, the capsule was cleaned with the aspiration tip under low vacuum (15 mm Hg). The superolateral clear corneal incision was enlarged to 1.5 mm, and an Acri.Smart intraocular lens (IOL) (Acri.Tec) was implanted using an acri.Smart IOL unfolder. Once the OVD was removed, the incision was closed by stromal hydration without sutures. In the SICS group, a 3.0 mm clear corneal incision was created superolaterally at the 10:30 o’clock position and a 1.0 mm side puncture was made at the 2 o’clock position. After OVD was introduced into the anterior chamber, a 5.5 mm capsulorhexis was made. This was followed by hydrodissection, phacoemulsification, aspiration of cortical masses, and introduction of a foldable IOL through the 3.0 mm incision. Once the OVD was removed, the incision was closed by stromal hydration without sutures.
Anterior Corneal Aberrations Corneal topography was performed preoperatively and postoperatively using a Humphrey Atlas corneal topography system (Carl Zeiss Meditec). The system directly provided estimates of the pupil center location, vertex location, curvatures of the anterior corneal surface, and keratometric measurements. The corneal curvatures and corneal heights were exported from the topography system and used to calculate corneal aberrations. A customized MatLab ray-tracing program (The MathWorks, Inc.)7,12–14 was used to derive Zernike aberrations up to the 6th order (28 terms). For each eye, the mean of 3 measurements at a 6.0 mm diameter central area with respect to the pupil center was calculated and used as the final estimate of the aberration measurements. The single-index conversion (ie, j)15 was used to name the Zernike aberrations.
Statistical Analysis An unpaired t test was used to compare the preoperative and postoperative mean corneal Zernike aberrations and the root mean square (RMS) of the corneal wavefront aberrations in the 2 groups. Mean changes in Zernike aberrations and RMS values in each group from preoperatively to postoperatively were calculated and tested with a paired t test. The difference in mean changes between the MICS group and the SICS group was tested with an unpaired t test. All analyses were performed using SPSS software (version 12.0, SPSS, Inc.). A P value less than 0.05 after Bonferroni correction for multiple comparison (significance level divided by comparison number) was considered statistically significant for individual Zernike aberrations.
RESULTS Microincision cataract surgery was performed in 36 eyes and SICS, in 38 eyes. The mean age of the patients was 66.38 years G 10.05 (SD) (range 41 to 80 years). Table 1 shows the patients’ baseline data. There was no significant difference between the 2 groups in preoperative
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Table 1. Preoperative patient data. Mean G SD MICS Group (n Z 36)
Parameter Age (y) Corneal astigmatism (D) Corneal power (D) Time between surgery and postop measurements (d)
SICS Group (n Z 38)
P Value
68.86 G 7.85 64.97 G 11.73 0.65 G 0.39 0.72 G 0.42
.097 .462
44.36 G 1.22 43.99 G 1.78 112 G 29 113 G 24
.299 .819
MICS Z microincision cataract surgery; SICS Z small-incision cataract surgery
age, length of follow-up, corneal astigmatism, or corneal power. No eye had surgical complications. Root Mean Square Values and Zernike Aberrations Table 2 shows the mean preoperative and postoperative corneal aberrations over a 6.0 mm area centered
on the pupil center for Zernike terms from the 2nd to 4th order (except defocus, j Z 4). The Zernike terms of 5th order up are not shown because they were small in magnitude. Table 2 also shows the RMS values for total corneal wavefront aberrations from the 2nd to 6th order (with j Z 4 excluded) and HOA RMS values from the 3rd to 6th order. In the SICS group, the mean values for oblique astigmatism (j Z 3), oblique trefoil (j Z 9), and oblique tetrafoil (j Z 10) were statistically significantly different between preoperatively and postoperatively; the differences in secondary oblique astigmatism (j Z 11) and tetrafoil (j Z 14) approached significance. Although the total RMS was not significantly different between preoperatively and postoperatively, the change was borderline significant. The mean postoperative trefoil was statistically significantly greater than preoperatively. In the MICS group, the mean values for oblique trefoil and oblique tetrafoil were statistically significantly different between preoperatively and postoperatively. The mean postoperative trefoil was statistically significantly greater than the mean preoperative level.
Table 2. Mean preoperative and postoperative RMS values and Zernike aberrations (mm) in the anterior cornea over a 6.0 mm diameter central area. SICS Group Parameter tRMS hoRMS j index/Z term 3/Z(2,2) 5/Z(2,2) 6/Z(3,3) 7/Z(3,1) 8/Z(3,1) 9/Z(3,3) 10/Z(4,4) 11/Z(4,2) 12/Z(4,0) 13/Z(4,2) 14/Z(4,4) Astigk Comak Trefoilk
Preop
Postop
0.941 G 0.375 0.612 G 0.196
1.124 G 0.472 0.698 G 0.243
0.064 G 0.447 0.134 G 0.636 0.184 G 0.186 0.030 G 0.285 0.005 G 0.312 0.004 G 0.145 0.014 G 0.066 0.006 G 0.046 0.312 G 0.088 0.007 G 0.090 0.024 G 0.100 0.662 G 0.421 0.364 G 0.208 0.261 G 0.142
0.460 G 0.532 0.187 G 0.652 0.193 G 0.219 0.008 G 0.309 0.028 G 0.269 0.263 G 0.209 0.105 G 0.122 0.035 G 0.050 0.296 G 0.095 0.025 G 0.095 0.044 G 0.123 0.830 G 0.504 0.339 G 0.225 0.396 G 0.200
MICS Group P Value* .069 .099 .001† .722 .833 .584 .733 !.0001x !.0001x .014 .443 .416 .012 .123 .615 .0016z
Preop
Postop
0.884 G 0.295 0.602 G 0.203
0.977 G 0.395 0.652 G 0.185
0.025 G 0.271 0.042 G 0.634 0.173 G 0.194 0.073 G 0.323 0.084 G 0.267 0.010 G 0.118 0.030 G 0.063 0.002 G 0.053 0.300 G 0.118 0.009 G 0.080 0.018 G 0.076 0.589 G 0.346 0.371 G 0.218 0.258 G 0.120
0.144 G 0.364 0.105 G 0.708 0.191 G 0.280 0.108 G 0.297 0.045 G 0.249 0.143 G 0.146 0.057 G 0.098 0.020 G 0.064 0.279 G 0.110 0.014 G 0.073 0.025 G 0.084 0.663 G 0.463 0.351 G 0.194 0.369 G 0.134
P Value* .262 .279 .123 .694 .765 .636 .538 !.0001x !.0001x .191 .447 .805 .692 .448 .695 .0007x
Means G SD Astig Z astigmatism; hoRMS Z higher-order root mean square; MICS Z microincision cataract surgery; SICS Z small-incision cataract surgery; tRMS Z total corneal wavefront aberration root mean square; Z Z Zernike *Significance corrected for multiple comparison for Zernike aberrations of j Z 3–14 (11 terms) † P!.05 z P!.01 x P!.001 k The terms of astigmatism, coma, and trefoil indicate the RMS values of the 2 Zernike astigmatisms (j Z 3 and j Z 5), comas (j Z 7 and j Z 8), and trefoils (j Z 6 and j Z 9), respectively.
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There were no statistically significant differences between the MICS group and the SICS group in mean RMS values or Zernike aberrations preoperatively. Postoperatively, the differences between groups in oblique astigmatism, oblique trefoil, and tetrafoil were borderline significant (P Z .0059, P Z .0067, and P Z .0073, respectively). Changes in Root-Mean-Square Values and Zernike Aberrations Table 3 shows the mean changes in Zernike aberrations and RMS values from preoperatively to postoperatively in the MICS and SICS groups. Five corneal aberration terms (trefoil, tetrafoil, oblique astigmatism, secondary oblique astigmatism, and vertical tetrafoil) in the SICS group and 2 terms (trefoil and tetrafoil) in the MICS group were statistically significantly different after surgery. All changes in RMS values were statistically significantly different from zero in both groups. The changes in total RMS and HOA RMS were statistically significantly greater in the SICS group
than in the MICS group (Table 3). Oblique astigmatism, oblique trefoil, and vertical tetrafoil showed change most in the SICS group compared with the MICS group. The mean induced astigmatism was 0.324 G 0.198 mm in the MICS group and 0.478 G 0.299 mm in the SICS group; the difference between groups was statistically significant (P Z .0063). The mean induced trefoil was 0.233 G 0.134 mm in the MICS group and 0.319 G 0.164 mm in the SICS group; the difference between groups was statistically significant (P Z .009). There was no statistically significant difference between groups in induced spherical aberration or induced coma aberration. DISCUSSION We measured wavefront aberrations in the anterior corneal surface preoperatively and postoperatively in 36 eyes having MICS and 38 eyes having SICS to evaluate the influence of incision size on surgery-related change in corneal aberrations postoperatively. The mean level of total corneal wavefront aberrations (total RMS) did not change significantly in the MICS
Table 3. Mean changes in RMS values and Zernike aberrations (mm) in the anterior cornea over a 6.0 mm diameter central area between preoperatively and postoperatively.
SICS Group
Small-Incision Versus Microincision
MICS Group
Parameter
Change (mm)
P Value*
Change (mm)
P Value
P Value*
tRMS hoRMS j index/Z term 3/Z(2,2) 5/Z(2,2) 6/Z(3,3) 7/Z(3,1) 8/Z(3,1) 9/Z(3,3) 10/Z(4,4) 11/Z(4,2) 12/Z(4,0) 13/Z(4,2) 14/Z(4,4) Astigk Comak Trefoilk
0.697 G 0.316 0.480 G 0.195
!.0001x !.0001x
0.523 G 0.209 0.390 G 0.143
!.0001x !.0001x
.007z .023†
0.396 G 0.316 0.053 G 0.244 0.010 G 0.181 0.038 G 0.138 0.023 G 0.139 0.259 G 0.172 0.118 G 0.119 0.028 G 0.050 0.016 G 0.065 0.018 G 0.078 0.068 G 0.120 0.478 G 0.299 0.168 G 0.105 0.319 G 0.164
!.0001x .190 .737 .100 .315 !.0001x !.0001x .001† .128 .173 .001† !.0001x !.0001x !.0001x
0.120 G 0.260 0.063 G 0.247 0.017 G 0.167 0.035 G 0.107 0.039 G 0.145 0.153 G 0.147 0.088 G 0.097 0.018 G 0.045 0.020 G 0.068 0.004 G 0.062 0.008 G 0.096 0.324 G 0.198 0.154 G 0.105 0.233 G 0.134
.009 .135 .543 .059 .117 !.0001x !.0001x .019 .078 .667 .639 !.0001x !.0001x !.0001x
.0001z .431 .430 .462 .317 .0035† .117 .187 .391 .215 .0023† .0063z .278 .009z
Means G SD Astig Z astigmatism; hoRMS Z higher-order root mean square; MICS Z microincision cataract surgery; SICS Z small-incision cataract surgery; tRMS Z total corneal wavefront aberration root mean square; Z Z Zernike *Significance corrected for multiple comparison for Zernike aberrations of j Z 3–14 (11 terms) † P!.05 z P!.01 x P!.001 k The terms of astigmatism, coma, and trefoil indicate the RMS values of the 2 Zernike astigmatisms (j Z 3 and j Z 5), comas (j Z 7 and j Z 8), and trefoils (j Z 6 and j Z 9), respectively.
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group; the mean level in the SICS group was higher, although the difference was only borderline significant. Because the HOA RMS did not increase after surgery in either group, the higher total RMS in the SICS group can be attributed to the increase in oblique astigmatism. The results, therefore, indicate that surgically induced astigmatism depends on incision size. That the mean levels of HOA RMS were not increased in the MICS group or the SICS group does not imply that the levels of individual Zernike aberrations were not altered. In fact, the mean oblique trefoil and oblique tetrafoil values were higher in both groups. In addition to oblique trefoil and oblique tetrafoil, secondary astigmatism and vertical tetrafoil changed in the SICS group, although the changes were borderline significant. Moreover, 2 HOA terms changed in the MICS group and 4 in the SICS group and the changes in the SICS group were greater than in the MICS group in 2 higher-order terms. These results suggest that both types of surgery cause changes in HOA but that the changes are greater with SICS than with MICS. The surgery-related changes in Zernike astigmatism, trefoil, and tetrafoil in our SICS group agree with results in previous studies.1–4,8,9 However, the changed terms were different than in previous studies because the incision site was superolateral in our study instead of dominantly temporal8 or superior.9 In the MICS group, there were also changes in trefoil and tetrafoil, although the magnitudes were smaller than in the SICS group. No patient in our study had a significant change in coma or spherical aberration. This finding is consistent with that in the study by Elkady et al.,11 in which Seidel coma and spherical aberration were not changed in 25 patients after MICS. Elkady et al. found no significant change in RMS HOA in the MICS group when only Seidel aberrations were analyzed, perhaps because Seidel aberrations include few HOAs and no Seidel term has an order as high as Zernike trefoil and tetrafoil. In a comparative study of postoperative corneal HOAs between MICS and SICS, Yao et al.10 found no significant differences in mean levels of total coma, total trefoil, spherical aberration, or total tetrafoil between the 2 groups. The total aberration in their study might be equivalent to the RMS of aberrations in our study. For example, the total coma might be the RMS of the horizontal and the vertical comas. The same results were observed in postoperative RMS values between the 2 groups in this study. However, 3 postoperative Zernike terms (oblique astigmatism, oblique trefoil, and tetrafoil [j Z 3, j Z 9, and j Z 14, respectively]) were on the border of significance after correction for multiple comparison. In our study, the
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changes in total RMS and HOA RMS were greater in the SICS group than in the MICS group. The total trefoil changed more after SICS than after MICS, and 3 Zernike terms (oblique astigmatism, oblique trefoil, tetrafoil) changed more in the SICS group than in the MICS group. On corneal topography, we measured the corneal heights and calculated the change in wavefront aberrations in the anterior corneal surface only. The incision should alter the posterior corneal surface in a similar way and thus produce similar changes in Zernike aberrations. With smaller amplitude and opposite signs than those in the anterior cornea because the difference in the refractive index across the posterior surface is smaller than that across the anterior corneal surface and in the opposite direction. Therefore, the posterior corneal aberrations might be able to compensate for part of the anterior corneal aberrations and increase the optical quality of the whole cornea. However, the posterior cornea could also induce more aberrations to the whole cornea for some Zernike terms due to the difference in surface shape between the anterior and posterior cornea (He JC, et al. IOVS 2008; 49:ARVO E-Abstract 986). Further study is required to clarify this point. From the HOA standard deviation data in Tables 2 and 3, it is clear that there was substantial individual variation in induced higher-order wavefront aberrations. Some inconsistencies between the results in our study and those in previous studies might be attributed to the individual variation in corneal aberrations. In our study, we did not analyze the surgically induced change in corneal aberrations to calculate its effect on image quality using an image plane metric, such as the point-spread function or MTF. Further study of this aspect will lead to a better understanding of the effect of the change in corneal aberrations on the visual performance of the eye. Pupil size in cataract patients could be smaller than the size we used to calculate the corneal aberrations (6.0 mm), and it varies from individual to individual. Therefore, future studies should evaluate the effect of pupil size on image quality. In conclusion, both MICS and SICS caused changes in astigmatism and higher-order wavefront aberrations in the anterior corneal surface, with the extent of the changes depending on the incision size. Our results show that MICS has an advantage over SICS in minimizing the effect of incision size on corneal optical quality. Incision size plays a role in determining corneal wavefront aberrations and should be considered when evaluating the origin of postoperative total aberration in the whole eye and in assessing the optical outcomes of cataract surgery.
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First author: NuXia Tong, OD School of Optometry and Ophthalmology, Eye Hospital, Wenzhou Medical College, Wenzhou, Zhejiang, China
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