Journal of Refractive Surgery

Original Article 

Change in Anterior and Posterior Curvature After Cataract Surgery

Yang Jae Kim, MD; Michael C. Knorz, MD; Gerd U. Auffarth, MD, PhD; Chul Young Choi, MD, PhD

Abstract

PURPOSE:

To analyze the change in anterior and posterior corneal curvature after cataract surgery using a Placido-dual rotating Scheimpflug device.

METHODS:

In a prospective cross-sectional study, corneal curvature was measured using the Galilei G4 device (Ziemer Ophthalmic Systems, Port, Switzerland) preoperatively and 1 week and 1, 3, and 6 months after cataract surgery with a temporal limbal self-sealing 2.2-mm incision. The surgically induced astigmatism (SIA) was determined on the anterior and posterior surfaces.

RESULTS:

Fifty-nine patients (68 eyes) were assessed. Based on the anterior corneal surface, 16 (23.5%) eyes had a vertically steep meridian (with-the-rule [WTR] astigmatism), 32 (47.1%) had a horizontally steep meridian (against-the-rule [ATR] astigmatism), and 20 (29.4%) had oblique astigmatism. Based on the posterior corneal surface, 2 (2.9%) eyes had a horizontally steep meridian (ATR astigmatism), 62 (91.2%) had a vertically steep meridian (WTR astigmatism), and 4 (5.9%) had oblique astigmatism. SIA of the anterior and posterior corneal surfaces was 0.61 ± 0.33 and 0.20 ± 0.17 diopters (D), respectively. However, there was no significant difference between the preoperative and the 6-month postoperative data in the Jackson coefficient orthogonal coordinate system for the anterior and posterior corneal surfaces. SIA of WTR astigmatism of the posterior cornea was 0.19 ± 0.16 D at 6 months. Sixty-one of 62 eyes with WTR astigmatism in the posterior corneal surface still showed WTR astigmatism after cataract surgery.

CONCLUSIONS:

The tendency of SIA of the posterior cornea may not be uniform, but type of posterior corneal astigmatism did not change in most cases after the 2.2-mm temporal limbal incision cataract surgery.

[J Refract Surg. 2016;32(11):754–759.]

Abstract

PURPOSE:

To analyze the change in anterior and posterior corneal curvature after cataract surgery using a Placido-dual rotating Scheimpflug device.

METHODS:

In a prospective cross-sectional study, corneal curvature was measured using the Galilei G4 device (Ziemer Ophthalmic Systems, Port, Switzerland) preoperatively and 1 week and 1, 3, and 6 months after cataract surgery with a temporal limbal self-sealing 2.2-mm incision. The surgically induced astigmatism (SIA) was determined on the anterior and posterior surfaces.

RESULTS:

Fifty-nine patients (68 eyes) were assessed. Based on the anterior corneal surface, 16 (23.5%) eyes had a vertically steep meridian (with-the-rule [WTR] astigmatism), 32 (47.1%) had a horizontally steep meridian (against-the-rule [ATR] astigmatism), and 20 (29.4%) had oblique astigmatism. Based on the posterior corneal surface, 2 (2.9%) eyes had a horizontally steep meridian (ATR astigmatism), 62 (91.2%) had a vertically steep meridian (WTR astigmatism), and 4 (5.9%) had oblique astigmatism. SIA of the anterior and posterior corneal surfaces was 0.61 ± 0.33 and 0.20 ± 0.17 diopters (D), respectively. However, there was no significant difference between the preoperative and the 6-month postoperative data in the Jackson coefficient orthogonal coordinate system for the anterior and posterior corneal surfaces. SIA of WTR astigmatism of the posterior cornea was 0.19 ± 0.16 D at 6 months. Sixty-one of 62 eyes with WTR astigmatism in the posterior corneal surface still showed WTR astigmatism after cataract surgery.

CONCLUSIONS:

The tendency of SIA of the posterior cornea may not be uniform, but type of posterior corneal astigmatism did not change in most cases after the 2.2-mm temporal limbal incision cataract surgery.

[J Refract Surg. 2016;32(11):754–759.]

The cornea accounts for the largest portion of the refractive power of the entire eye, and it is known that the degree or direction of corneal astigmatism changes with age.1,2 Therefore, ophthalmologists who perform cataract surgery have been continuously making efforts to evaluate patients' corneal astigmatism accurately and to minimize surgically induced astigmatism (SIA) or to correct corneal astigmatism. The size of the corneal incision is reduced during surgery to minimize induced corneal astigmatism; alternatively, corneal incisions at the steepest meridian, limbal relaxing incisions, or toric intraocular lenses are used to correct corneal astigmatism.3,4

Recently, rotating Scheimpflug imaging such as the Placidodual rotating Scheimpflug device (Galilei G4; Ziemer Ophthalmic Systems, Port, Switzerland) became available to allow measurements of the posterior corneal surface. Koch et al.1 reported that the steep corneal meridian was aligned vertically in 86.8% for the posterior surface. Owing to such equipment, the effects of posterior corneal astigmatism on total corneal astigmatism and changes in the posterior corneal surface before and after cataract surgery are attracting increasing interest.

Goggin et al.5 and Koch et al.6 reported that disregarding posterior corneal astigmatism resulted in an overestimation of against-the-rule (ATR) anterior corneal astigmatism and an underestimation of with-the-rule (WTR) anterior corneal astigmatism. Recent studies therefore focused on comparing results including measurements of posterior curvature to those not including posterior curvature. Nemeth et al.7 compared changes in the posterior corneal surface and SIA before and after cataract surgery, but the number of studies is still too small.

The purpose of the current study was to examine the effects of cataract surgery using a temporal 2.2-mm limbal incision on the anterior and posterior corneal surfaces; in particular, it focused on changes in the posterior corneal surface.

Patients and Methods

This is a prospective cross-sectional study that adhered to the tenets of the Declaration of Helsinki and was approved by the Kangbuk Samsung Hospital Institutional Review Board. Informed consent was obtained from all patients. Among adults aged at least 45 years who were scheduled for cataract surgery, a total of 68 eyes (59 patients; 16 men, 43 women; mean age: 67.2 ± 8.3 years [range: 49 to 84 years]) willing to report for at least 6 months of follow-up were included. Patients who had a history of eye surgery, retinal disease, glaucoma, or any corneal lesions that might affect any of the measured values of the cornea were excluded.

Measurements

The ophthalmologic examination included visual acuity, intraocular pressure, slit-lamp examination, axial length, and Scheimpflug imaging. These were performed preoperatively and 1 week and 1, 3, and 6 months after the cataract surgery. The Galilei G4 device uses two cameras opposite each other in combination with a Placido disk topographer to analyze the shape of the cornea. By detecting the edge provided by the dual Scheimpflug system images, the shape of the posterior cornea can be measured. The total scanning time was approximately 0.75 second, and more than 122,000 points were scanned. Every eye was measured three times by the Galilei G4 device and mean keratometric values (K1, K2, mean K, astigmatism, and axis) were used for data calculation. All measurements were performed continuously in individual patients between 10:00 am and 1:00 pm to avoid the effects of diurnal variation in corneal indices.8

Surgical Technique

Cataract surgery was performed by one experienced surgeon (CYC) using a three-plane, temporal limbo-corneal self-sealing 2.2-mm incision using a dual bevel slit knife (Intrepid ClearCut; Alcon Laboratories, Inc., Fort Worth, TX). In the right eye, the main incision was 2.2 mm at 190° with a 1-mm side incision at 110°. In the left eye, the main incision was 2.2 mm at 10° with a 1-mm side incision at 290°. We used the Gimbel Mendez Ring (Mastel Precision Surgical Instruments, Rapid City, SD) to obtain an exact incision site. A centered anterior 5.5-mm curvilinear continuous capsulorhexis was created using a curvilinear continuous capsulorhexis needle with CALLISTO eye (Carl Zeiss Meditec, Jena, Germany). Following a phaco-chop nucleofractis and irrigation/aspiration for cortex removal with a one-piece irrigation/aspiration handpiece, a hydrophobic foldable intraocular lens (enVista; Bausch & Lomb, Rochester, NY) was implanted using a Bausch & Lomb injector system (BLIS; Bausch & Lomb) without any enlargement of the incision.

Data Analysis

Patients were grouped based on the following criteria. Anterior corneal astigmatism was defined as WTR in case of a steep corneal curvature between 60° and 120°, oblique astigmatism between both 31° and 59° and 121° and 149°, and ATR astigmatism between both 0° and 30° and 150° and 180°. Posterior corneal astigmatism was defined as WTR astigmatism in case of a steep corneal curvature between 60° and 120°, oblique astigmatism between both 31° and 59° and 121° and 149°, and ATR astigmatism between both 0° and 30° and 150° and 180°.

Changes in the values of simulated keratometry and posterior keratometry (1- to 4-mm zone) were examined, and vector analyses considering the steep axis were also conducted.

The changes in corneal astigmatism were also analyzed using Thibos et al.'s method,9 in which the corneal power (K11; K22) is converted into rectangular forms of Fourier notation (Jackson coefficient M, J0, J45). Here, M is the mean corneal power and J0 and J45 are the horizontal and oblique components of the corneal astigmatism transposed in the Jackson coefficient orthogonal system, respectively.

SIA was calculated with the SIA Calculator version 3.1 using Holladay's approach based on the preoperative and postoperative keratometric data (ie, K1 and K2 as the flattest and steepest corneal curvatures [D], respectively, and the meridian of K2) obtained by the Galilei G4 device.10,11

Statistical Analysis

The results of continuous variables are presented as mean ± standard deviation, and those of categorical variables are presented as numbers. The measurements in both eyes of the same patient tend to be correlated, and a correction was made for this in the statistical analysis. The differences between preoperative and postoperative data at each time interval were examined by mixed-model analysis of variance for repeated measures for this purpose; the covariates were the fixed effect of time, random effects of patients, and errors. With this model, the preoperative and postoperative measurements could be compared while correcting for the correlation between the eyes of the same patient. We also analyzed the preoperative data and 6-month postoperative data by using a paired t test. The intraclass correlation coefficient (ICC) was calculated to assess the change in the anterior and posterior corneal surface. All statistical analysis was performed using SPSS software (version 18.0; SPSS, Inc., Chicago, IL).

Results

Astigmatism type and distribution before cataract surgery are shown in Table 1 and Figure 1. Preoperative posterior corneal astigmatism was greater than 0.50 D in 6 eyes (8.8%). The number of eyes with posterior WTR astigmatism (vertically steep meridian), which was 62 before surgery, changed slightly to 61 by 6 months after surgery.


Types of Preoperative Corneal Astigmatism (n = 68)

Table 1:

Types of Preoperative Corneal Astigmatism (n = 68)


Double-angle plot of the distribution of astigmatism in the anterior and posterior corneal surface before cataract surgery. The black dot indicates preoperative anterior corneal astigmatism and the white dot indicates preoperative posterior corneal astigmatism.

Figure 1.

Double-angle plot of the distribution of astigmatism in the anterior and posterior corneal surface before cataract surgery. The black dot indicates preoperative anterior corneal astigmatism and the white dot indicates preoperative posterior corneal astigmatism.

The keratometric values (K1, K2, mean K, and astigmatism) of the posterior cornea obtained through the Galilei G4 device before surgery and those of the anterior cornea showed no statistically significant difference at any follow-up visit. As can be seen in Tables 23, J0 and J45 did not show any statistically significant difference either (Bonferroni method; mixed-model analysis of variance for repeated measures). However, to review ICC values, whereas K1, K2, and mean K values showed high ICC values and the value changes were almost the same even after surgery, J0 and J45 showed low ICC values.


Change of Anterior and Posterior Curvature After Cataract Surgery

Table 2:

Change of Anterior and Posterior Curvature After Cataract Surgery


Change of Anterior and Posterior Curvature After Cataract Surgery by Paired t Test

Table 3:

Change of Anterior and Posterior Curvature After Cataract Surgery by Paired t Test

Table 4 shows that SIA occurred on the anterior and posterior cornea surfaces. At 6 months after cataract surgery, 39 eyes (57.4%) had anterior SIA greater than 0.50 D and 4 eyes (5.9%) had posterior SIA greater than 0.50 D.


Change of SIAa

Table 4:

Change of SIA

The results of the analysis of SIA conducted separately for anterior and posterior surfaces were expressed in the double-angle plots in Figures 23.


Double-angle plot of surgically induced astigmatism (SIA) in with-the-rule (WTR) and against-the-rule (ATR) astigmatism of the anterior cornea. The SIA values of the anterior cornea surfaces were shown to be 0.57 ± 0.33 @ 101 in the WTR group and 0.63 ± 0.34 @ 106 in the ATR group. Whereas the ATR group showed a tendency of flattening of the horizontal steep axis, the WTR group did not show such a tendency clearly. The black dot indicates SIA in WTR astigmatism of the anterior cornea and the white dot indicates SIA in ATR astigmatism of the anterior cornea.

Figure 2.

Double-angle plot of surgically induced astigmatism (SIA) in with-the-rule (WTR) and against-the-rule (ATR) astigmatism of the anterior cornea. The SIA values of the anterior cornea surfaces were shown to be 0.57 ± 0.33 @ 101 in the WTR group and 0.63 ± 0.34 @ 106 in the ATR group. Whereas the ATR group showed a tendency of flattening of the horizontal steep axis, the WTR group did not show such a tendency clearly. The black dot indicates SIA in WTR astigmatism of the anterior cornea and the white dot indicates SIA in ATR astigmatism of the anterior cornea.


Double-angle plot of surgically induced astigmatism (SIA) in with-the-rule (WTR) astigmatism of the posterior cornea. SIA in the WTR group of the posterior cornea was 0.19 ± 0.16 @ 75. Although the participants did not show a certain tendency, when their distribution according to the SIA axis was analyzed, 24 of 62 participants were shown in a range of 30° to 75° and 42 of 62 participants were shown in a range of 0° to 90°.

Figure 3.

Double-angle plot of surgically induced astigmatism (SIA) in with-the-rule (WTR) astigmatism of the posterior cornea. SIA in the WTR group of the posterior cornea was 0.19 ± 0.16 @ 75. Although the participants did not show a certain tendency, when their distribution according to the SIA axis was analyzed, 24 of 62 participants were shown in a range of 30° to 75° and 42 of 62 participants were shown in a range of 0° to 90°.

To review changes in the posterior curvature, preoperative and postoperative 6-month tomographic data did not show any statistically significant difference (LSD method, Bonferroni method; mixed-model analysis of variance for repeated measures). J0 and J45 did not show any statistically significant difference at any of the time points (Bonferroni method; mixed-model analysis of variance for repeated measures). There was no statistically significant difference when preoperative data and postoperative 6-month data were compared using paired t tests (Table 3). K1, K2, and mean K values showed ICC values not smaller than 0.9, thereby showing high reproducibility even after surgery; however, the astigmatism-related values changed greatly after surgery; in particular, J0 and J45 showed low ICC values. SIA in the WTR group of the posterior cornea (vertically steep meridian) was 0.19 ± 0.16 @ 75. Sixty-one of 62 eyes with WTR astigmatism, which is vertically steep in the posterior corneal surface, still showed WTR astigmatism after cataract surgery.

Discussion

Surgeons have not seriously considered the astigmatism of the posterior cornea because the exact measurement of the posterior corneal surface was not possible until recently. Another reason was the smaller difference in refractive indices between the posterior corneal surface and the aqueous humor.

As equipment capable of measuring the posterior cornea surface was developed, the change in the posterior cornea after cataract surgery attracted increasing interest. In a recent study,6 the authors reported that using devices that calculate the total corneal astigmatism based on the anterior corneal measurements only will overestimate WTR astigmatism by 0.50 to 0.60 D and underestimate ATR astigmatism by 0.20 to 0.30 D. Cheng et al.12 reported that the omission of the posterior corneal surface measurement when calculating the total corneal SIA can lead to significant inaccuracies in some eyes after cataract surgery. As such, many studies1,2,12 reported that not considering the posterior corneal surface will result in unexpected refractive errors.

The current study focused on analyzing the effects of a three-plane, temporal limbo-corneal self-sealing 2.2-mm incision on the WTR astigmatism of the posterior cornea, which is vertically steep. Astigmatism, J0, and J45 did not show any statistically significant differences at any of the time points. K1, K2, mean K, and astigmatism values have only the nature of magnitudes, and these values are not sufficient for drawing conclusions. Therefore, in the current study, J0 and J45 were also calculated; these are useful for statistical comparison because they express the magnitude and directional property of astigmatism. The ICC values and the results of the comparison of preoperative and postoperative 6-month data using paired t tests can be seen in Table 3. In our study, the magnitudes of K1, K2, and mean K values before and after surgery are not statistically significantly different and the reproducibility of the magnitudes after surgery is high. On the other hand, astigmatism, J0, and J45 did not show any statistically significant difference and showed low ICC values (Table 3). Given these results, although there were differences because changes occurred after surgery, the magnitudes were not large and the changes occurred without any certain directivity. These results appeared on the anterior corneal surface almost identically.

The changes of astigmatism type and axis in the posterior corneal surface were less than those of the anterior corneal surface. This might be due to the fact that, whereas the preoperative anterior astigmatism types of the patients were diverse, most preoperative posterior astigmatism was vertically steep, which is WTR astigmatism. However, given the changes in the steep axis and SIA found when the effects of temporal incisions were compared between the group with WTR astigmatism of the anterior cornea and the group with WTR astigmatism of the posterior cornea, the above results can be considered suggestive of the fact that the changes in the posterior cornea surface may be relatively smaller than those in the anterior cornea surface.

Anterior and posterior SIA did not change statistically significantly during the postoperative period, although the SIA for both tended to decrease (Table 4). On reviewing SIA of the anterior corneal surface in all 68 eyes at 6 months after surgery (Figure 2), the ATR group showed horizontal flattening, whereas the WTR group did not show any clear tendency. For posterior SIA, the WTR group showed an SIA of 0.19 ± 0.16 @ 75; however, this value could not be regarded as showing any certain tendency, as shown in Figure 3. In other words, because the steep axis was the vertical meridian for the WTR astigmatism of both the anterior and posterior cornea, the effects of temporal incisions (incision on the horizontal meridian) could not be uniformly predicted.

We considered why the posterior SIA showed a value of 0.20 ± 0.17 D despite the fact that keratometric values (including J0 and J45 values) did not show any statistically significant difference in the posterior cornea. SIA is influenced by the incision's size, location, corneal radius and thickness, and corneal rigidity.13–17 SIA is not consistent because eyes will always heal differently from each other, and this also affects SIA.18,19 The fact that the ICC values of J0 and J45 were low in the current study supports this.

Nemeth et al.7 reported that the difference between the preoperative and the postoperative posterior curvature was statistically significant, and posterior SIA was 0.31 ± 0.33 D. In addition, they found that 25% of eyes had a posterior SIA of 0.50 D or greater, compared to 5.9% in our study. This difference may be explained because the study by Nemeth et al. was limited to patients with WTR astigmatism, a 2.8-mm two-step corneal incision was used on the steepest superior corneal meridian, and follow-up was only 8.65 ± 2.4 weeks.

Whereas most previous studies analyzed data with 2 months of follow-up only, the current study looked at 6 months of follow-up.

Limitations of the current study include that patients with ATR and oblique posterior astigmatism could not be analyzed because there were too few such patients. Another limitation was that we could not measure exact internal incision size and inferred this from the size of the knife, even though the incision size tends to stretch perioperatively, particularly in small incisions. The failure of the ICC values to show similarity between the Jackson coefficient expressions could be due to axis variation. With 2.2-mm incision size, the predominant statistical effect can be test-to-test variations in keratometry and tomography manifesting as highly variable SIA axes compared to the small variations in incision axes. Further evaluation will be needed for analyzing the axis of effect of the surgical incision.

The authors of this study agree that using data obtained from the anterior cornea surface through only mathematical calculations without considering the preoperative posterior curvature may cause clinically significant refractive errors. The effects of SIA caused by cataract surgery on the posterior cornea should be evaluated. In particular, in the case of cataract surgery using toric or multifocal intraocular lenses, predicting more accurate and reliable refractive results is important to obtain optimal visual outcomes. In the current study, the tendencies of SIA were not uniform; however, the WTR astigmatism of the posterior cornea was maintained because it occurred after surgery in most cases when three-plane, temporal limbo-corneal self-sealing 2.2-mm incisions were performed.

References

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Types of Preoperative Corneal Astigmatism (n = 68)

AstigmatismPreoperative No. (%)
Anterior corneal
  With-the-rule16 (23.5%)
  Against-the-rule32 (47.1%)
  Oblique20 (29.4%)
Posterior corneal
  With-the-rule62 (91.2%)
  Against-the-rule2 (2.9%)
  Oblique4 (5.9%)

Change of Anterior and Posterior Curvature After Cataract Surgery

VariablePreoperativePostop 1 WeekPostop 1 MonthPostop 3 MonthsPostop 6 MonthsPa
Anterior curvature (mean ± SD)
  J00.061 ± 0.3350.009 ± 0.322−0.023 ± 0.334−0.025 ± 0.3530.036 ± 0.328.463
  J45−0.004 ± 0.309−0.029 ± 0.379−0.037 ± 0.3510.026 ± 0.351−0.011 ± 0.324.862
Posterior curvature (mean ± SD)
  J0−0.003 ± 0.136−0.015 ± 0.1540.032 ± 0.139−0.001 ± 0.127−0.013 ± 0.123.247
  J45−0.008 ± 0.1170.011 ± 0.134−0.044 ± 0.2010.005 ± 0.1660.023 ± 0.121.104

Change of Anterior and Posterior Curvature After Cataract Surgery by Paired t Test

VariablePreoperativePostop 6 MonthsPaICC
Anterior curvature (mean ± SD)
  J00.061 ± 0.3350.036 ± 0.328.6290.338
  J45−0.004 ± 0.309−0.011 ± 0.324.9010.036
Posterior curvature (mean ± SD)
  J0−0.003 ± 0.136−0.013 ± 0.123.6450.001
  J45−0.008 ± 0.1170.023 ± 0.121.4580.126

Change of SIAa

VariablePostop 1 WeekPostop 1 MonthPostop 3 MonthsPostop 6 MonthsPb
Anterior SIA (mean ± SD)0.76 ± 0.480.75 ± 0.510.76 ± 0.510.61 ± 0.33.091
Posterior SIA (mean ± SD)0.23 ± 0.230.26 ± 0.350.19 ± 0.160.20 ± 0.17.420
Authors

From the Department of Ophthalmology, Kangbuk Samsung Medical Center, Sungkyunkwan University School of Medicine, Seoul, Republic of Korea (YJK, CYC); FreeVis LASIK Zentrum, Universitätsmedizin Mannheim, Mannheim, Germany (MCK); and International Vision Correction Research Centre (IVCRC) and David J. Apple International Laboratory on Ocular Pathology, Department of Ophthalmology, University of Heidelberg, Heidelberg, Germany (GUA, CYC).

The authors have no financial or proprietary interest in the materials presented herein.

AUTHOR CONTRIBUTIONS

Study concept and design (YJK, CYC); data collection (YJK, CYC); analysis and interpretation of data (YJK, MCK, GUA, CYC); writing the manuscript (YJK, CYC); critical revision of the manuscript (YJK, MCK, GUA, CYC); administrative, technical, or material support (CYC); supervision (GUA, CYC)

Correspondence: Chul Young Choi, MD, PhD, Department of Ophthalmology, Kangbuk Samsung Medical Center, Sungkyunkwan University School of Medicine, Pyoung-Dong, Jongro-Ku, Seoul 110-746, Republic of Korea. E-mail: sashimi0@naver.com

Received: March 10, 2016
Accepted: August 08, 2016

10.3928/1081597X-20160816-01

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