Contemporary cataract surgery is performed with greater expectations by refractive surgery patients. In the past decade, a major milestone in cataract surgery has been the shift to smaller incisions to decrease the magnitude of surgically induced astigmatism (SIA) and corneal aberrations,1,2 and the correction of preexisting corneal astigmatism either during or after surgery.
The magnitude of SIA largely depends on the width and location of the corneal incision.3–9 A temporal incision induces less SIA than a superior or nasal incision10,11 and a posterior limbal or scleral incision induces less SIA than a clear corneal incision.8,12 The incision width and location are the surgeon’s preference and are usually relatively consistent for a given surgeon. SIA is greater in patients with greater preoperative astigmatism10 and older age,13 and in corneas with lower corneal hysteresis and corneal resistance factor.14 The associations between SIA and axial length, anterior chamber depth (ACD), the meridian of corneal astigmatism, and intraocular pressure (IOP) are less well known. Knowledge of the association between these factors and SIA would indicate whether the SIA used for the toric intraocular lens (IOL) power calculation should be adjusted for each patient according to various ocular factors. The purpose of this study was to identify ocular factors that are associated with the magnitude of SIA, including axial length, ACD, preoperative corneal astigmatism, the meridian of corneal astigmatism, and IOP.
Patients and Methods
This study was conducted by retrospective chart review and adhered to the tenets of the Declaration of Helsinki. Approval was obtained from the Institutional Review Board of Far Eastern Memorial Hospital. There were 605 eyes enrolled. Patients underwent smooth phacoemulsification by a single surgeon (S-WC) either through a 2.75-mm three-step superior limbal incision with the IOL inserted using a cartridge C injector (SA60AT; Alcon Laboratories, Inc., Fort Worth, TX) from October 2006 to September 2008, or a 2.2-mm incision with the IOL inserted using a cartridge D injector from October 2008 to January 2011. The incision tunnel was constructed to be a square when viewed through the surgical microscope. Patients with pterygium, corneal diseases, surgery with complications, or other prior eye surgeries were excluded.
Preoperative axial length and ACD were measured with an optical biometer (IOLMaster; Carl Zeiss Med-itec, Oberkochen, Germany), corneal curvature was measured with an autorefractor (KR-8900; Topcon, Tokyo, Japan), and IOP was measured with a pneumotonometer (CT80; Topcon). All patients underwent a preoperative examination and postoperative IOP and corneal curvature were examined at 1 day, 1 week, and 1, 2, and 3 months postoperatively. The magnitude of SIA, flattening effect, and torque were calculated with MATLAB software (R2009; MathWorks, Natick, MA) using the Alpins method (D1 = preoperative astigmatism magnitude [diopters], A1 = preoperative astigmatism meridian [degrees], D2 = postoperative astigmatism magnitude [diopters], A2 = postoperative astigmatism meridian [degrees], TIA = target induced astigmatism, AE = angle of error).15,16 Briefly,
All surgeries were performed by one experienced surgeon (S-WC). Topical anesthesia was applied with 0.05% proparacaine hydrochloride (ALCAINE; Alcon Laboratories, Inc.). With the surgeon sitting superior to the patient, incisions were made at the 12-o’clock position with paracentesis at the 2-o’clock position. A three-step incision was made 0.5-mm posterior to the limbus using a 2.2-mm (the 2.2-mm group) or 2.75-mm (the 2.75-mm group) clear corneal knife. A continuous curvilinear capsulorhexis measuring approximately 5.0 mm in diameter was created after the anterior chamber was inflated with viscoelastic (DuoVisc; Alcon Laboratories, Inc.). Nuclei were chopped after hydrodissection and nuclear fragments were removed by phacoemulsification. In-the-bag implantation of the IOL (SA60AT or SN6AD3; Alcon Laboratories, Inc.) was completed, and the viscoelastic was then completely removed from the anterior chamber. Incisions were sealed by standardized stromal hydration at the end of surgery. Only cases with no intraoperative complications were included.
Patients were grouped according to the width of the corneal incision. There were 357 eyes in the 2.75-mm group and 248 eyes in the 2.2-mm group. Differences in age, preoperative axial length, or ACD between the two groups were examined using independent t tests. Distribution of gender and astigmatism meridian in the two groups was examined using the chi-square test. Differences in SIA, flattening effect, torque, and IOP across time periods were analyzed separately in each group using repeated measures one-way analysis of variance (ANOVA). Differences between the two groups at each time period were examined using independent t tests. The associations among age, preoperative axial length, preoperative ACD, preoperative corneal curvature, and preoperative and postoperative IOP and SIA were examined separately in each group using Pearson correlation coefficients. The associations among SIAs and IOPs at different time points were also examined separately in each group using Pearson coefficients. Predictability of SIA was calculated using multiple regression analysis.
To assess whether the meridian of corneal astigmatism affected the magnitude of SIA, each group was further divided into three subgroups according to the location of the steepest corneal meridian: with-the-rule (WTR; steepest meridian from 60.1° to 120.0°), against-the-rule (ATR; steepest meridian from 0° to 30.0° or 150.1° to 180°), and oblique (steepest meridian from 30.1° to 60.0° or 120.1° to 150.0°). Differences in SIA, flattening effect, and torque among the WTR, ATR, and oblique astigmatism subgroups were analyzed separately in each group and at each time period using a one-way ANOVA. Differences between the 2.2- and 2.75-mm groups were examined for each subgroup at each time period using independent t tests.
For all tests, a P value less than .05 was considered statistically significant. Significant main effects in ANOVA were followed up with least significant difference post-hoc comparisons. All statistical analyses were performed using SPSS software version 17.0 (SPSS, Inc., Chicago, IL) for Windows.
Table 1 summarizes patient demographics and biometry parameters. Patients in the 2.2-mm group were older and had higher preoperative astigmatism than patients in the 2.75-mm group. There was no significant difference in the gender distribution and astigmatism meridian between groups. There was also no significant difference in preoperative axial length, ACD, or IOP between groups.
Preoperative Parameters in Both Groups
Preoperative Ocular Features
We found a significant correlation between patient age and axial length (r = −0.429, P < .001) and between patient age and ACD (r = −0.557, P < .001), whereby older patients had shorter axial length and shallower ACD. Eyes with longer axial length had deeper ACD (r = 0.569, P < .001) and a flatter cornea (r = 0.549, P < .001 for horizontal curvature; r = 0.386, P < .001 for vertical curvature). Eyes with deeper ACD had a flatter horizontal corneal curvature (r = 0.216, P < .001), but ACD was not related to vertical corneal curvature (P > .05).
Effect of Incision Width on SIA, Torque, and Flattening Effect
Eyes with greater SIA at 1 day postoperatively had greater SIA at all postoperative time periods up to 3 months postoperatively (P < .001) for all correlations among SIAs at different time periods in both groups. SIA was similar in the two groups at 1 day postoperatively (P = .922) (Figure 1). SIA was significantly greater in the 2.75-mm group than in the 2.2-mm group at 1 week postoperatively (P = .003) but was similar in the two groups thereafter (P = .101, .105, and .675 at 1, 2, and 3 months postoperatively, respectively) (Figure 1). In both groups, SIA was greatest at 1 day postoperatively (ANOVA P < .001 for both groups) (Figure 1). SIA stabilized at 1 week postoperatively in the 2.2-mm group (Figure 1) and at 1 month postoperatively in 2.75-mm group (Figure 1). Torque and flattening effect were significantly greater in the 2.75-mm group than in the 2.2-mm group at 1 week postoperatively (P = .014 and .006 for torque and flattening effect, respectively) but were similar in the two groups at other time periods (for torque, P = .896, = .982, = .392, and = .304 at 1 day, and 1, 2, and 3 months postoperatively, respectively; for flattening effect, P = .205, = .391, = .218, and = .713 at 1 day, and 1, 2, and 3 months postoperatively, respectively) (Figure 1). In both groups, the flattening effect was greatest at 1 day postoperatively, but the change over time was statistically significant only in the 2.2-mm group (ANOVA P = .004 and .207 for the 2.2- and 2.75-mm group, respectively) (Figure 1). There was also no significant change in torque over time in both groups (P = .212 and .949 for the 2.2- and 2.75-mm groups, respectively).
Magnitude of surgically induced astigmatism (SIA), torque (T), and flattening (F) after surgery with a 2.75- or 2.2-mm incision. Error bars represent standard deviations. * = P less than .05 between the two indicated time periods. # = P less than .05 between the 2.2- and 2.75-mm groups. 1D = 1 day postoperative; 1W = 1 week postoperative; 1M = 1 month postoperative; 2M = 2 months postoperative; 3M = 3 months postoperative
Association Between IOP and SIA
Eyes with higher preoperative IOP had a higher IOP at all postoperative time periods up to 3 months postoperatively (P < .001 for all correlations among IOPs at different time periods in both groups). In both the 2.75- and 2.2-mm group, IOP at 1 day postoperatively was higher than preoperative IOP (P = .001 for both groups), but IOP at all subsequent time periods was lower than preoperative IOP (for the 2.75-mm group, P = .006, = .001, < .001, and < .001 at 1 week and 1, 2, and 3 months, respectively; for the 2.2-mm group, P = .001, = .039, = .014, and = .217 at 1 week and 1, 2, and 3 months, respectively) (Figure A, available in the online version of this article). In the 2.75-mm group, preoperative IOP negatively correlated with SIA at all postoperative time periods, and IOP at 3 months postoperatively negatively correlated with SIA at 1, 2, and 3 months postoperatively, but not with SIA at 1 day or 1 week postoperatively (Table 2). By contrast, there were no associations between IOP, either preoperative or postoperative, and SIA in the 2.2-mm group.
Summary of Significant Correlations Between SIA and Other Dactors
Associations Between Age, Corneal Astigmatism, Ocular Biometry, and SIA
In both groups, eyes with older age, higher preoperative astigmatism, and shallower ACD had greater SIA up to 1 month postoperatively (Table 2). These associations were present up to 3 months postoperatively in the 2.75-mm group (Table 2). In the 2.2-mm group, age was associated with SIA up to 2 months postoperatively (Table 2). Shorter axial length and lower preoperative IOP were associated with greater SIA only in the 2.75-mm group, and both associations were present up to 3 months postoperatively (Table 2).
Effect of Preoperative Astigmatism Meridian on SIA, Flattening Effect, and Torque
In both groups, SIA tended to be greatest in the oblique astigmatism subgroup (Figure 2A) and stabilized later in the oblique astigmatism subgroup than in the WTR and ATR astigmatism subgroups. However, the difference between subgroups was statistically significant only in the 2.75-mm group at 1 month postoperatively (ANOVA, P = .140, = .313, = .007, = .206, and = .366 for 1 day, 1 week, and 1, 2, and 3 months, respectively). This difference in SIA among the WTR, ATR, and oblique astigmatism groups was significant up to 1 week postoperatively in the 2.2-mm group and up to 1 month postoperatively in the 2.75-mm group. The difference in the SIA between the 2.75- and 2.2-mm groups was greatest in the oblique astigmatism group and smallest in the WTR astigmatism group (Figure 2A). The flattening (Figure 2B) and torque (Figure 2C) components were also greatest in the oblique astigmatism subgroup in both groups. In the oblique subgroup, the summated vector means of the SIA was greater in the 2.75-mm group (Figure B, available in the online version of this article). The angle of error was larger in the oblique subgroup in both the 2.2- and 2.75-mm groups (P < .001 for both groups).
Magnitude of (A) surgically induced astigmatism, (B) flattening, and (C) torque after surgery with a 2.2- or 2.75-mm incision at each postoperative time period according to the preoperative corneal astigmatism meridian. Error bars represent standard deviations. * = P less than .05 between the two indicated time periiods. WTR = with the rule; ATR = against the rule; black bars = 2.75-mm group; gray bars = 2.2-mm group
Results of the multiple regression analysis of the association between SIA at 1 month postoperatively and other clinical variables are summarized in Table 3. In the 2.75-mm group, each diopter of preoperative astigmatism was associated with 0.177, 0.205, and 0.478 diopter of SIA in the WTR, ATR, and oblique astigmatism subgroups, respectively. By contrast, in the 2.2-mm group, each diopter of preoperative astigmatism was associated with 0.141 diopter of SIA in the WTR astigmatism subgroup, but there was no association between preoperative astigmatism and SIA in the ATR or oblique astigmatism subgroups. In both the 2.2- and the 2.75-mm groups, each year of age was associated with 0.017 and 0.016 diopter of SIA in the WTR astigmatism subgroup, but not in the ATR or oblique astigmatism subgroups.
Summary of Regression Model Analysis of the Association Between SIA at 1 Month and Other Clinical Variables
In this study, we reviewed the outcomes of a large number of patients operated on by a single surgeon using the same technique, allowing an in-depth analysis of the factors affecting the magnitude of SIA. We demonstrated for the first time that SIA was greater in eyes with shallower ACD, regardless of whether surgery was performed with a 2.75- or 2.2-mm superior limbal phacoemulsification incision. SIA was also greater in eyes with shorter axial length and lower preoperative IOP, but these associations were present only after surgery performed with a 2.75-mm incision. We also confirmed that SIA was greater in older patients13 and in eyes with higher preoperative corneal astigmatism,10 regardless of whether surgery was performed with a 2.75- or 2.2-mm incision.
Creation of a corneal incision allows opening and slippage of the corneal lamellae and results in SIA. Theoretically, the distance of the opening is larger with wider incisions3–7,14,17 and in stiffer corneas that absorb less stress.14 Our finding that SIA was greatest at 1 day postoperatively could be attributed to early wound gape,18–22 and our finding that SIA was greater in older corneas is compatible with the fact that corneal stiffness increases with age.23–25 Increased corneal stiffness could result in a larger opening in the early postoperative period and inhibit long-term recovery.
Lower preoperative and postoperative IOP was associated with greater SIA. We suggest that lower IOP resulted in less adaptation of the corneal wound, leading to posterior wound gape and slippage of the opened corneal lamellae. This is compatible with the early posterior wound gape reported in 86% to 89% of eyes19,21 and the correlation between low IOP and micro-leakage of the corneal wound in the early postoperative period,19 which led to a high SIA. The association between IOP and SIA was only significant after surgery performed with a 2.75-mm incision, suggesting a beneficial effect of a smaller corneal incision in reducing the effect of IOP on SIA.
We found that SIA was greatest in patients with an oblique astigmatism, and it stabilized later in those patients than in patients with a WTR or ATR astigmatism. This effect of astigmatism meridian was more striking after surgery with a 2.75-mm incision than with a 2.2-mm incision. The difference in SIA magnitude between the 2.2- and 2.75-mm groups was greatest in patients with an oblique astigmatism. Whether the progression of the corneal astigmatism toward ATR that occurs with aging26 contributes to an age-related laxity in the vertically oriented corneal fibers and the astigmatism meridian difference of SIA remains to be verified.
The magnitude of SIA was significantly greater in eyes with higher preoperative corneal astigmatism. If the magnitude of the preexisting astigmatism is regarded as partially representing the difference in tensile strength between the two major meridians, an incision in corneas with a greater preexisting astigmatism would be expected to result in greater SIA because it weakens the fiber strength at the incision meridian, especially a large incision. This is supported by our finding that higher preoperative astigmatism correlated with greater SIA.
The magnitude of SIA was greater in eyes with shorter axial length and shallower ACD. This effect was significant up to 1 month postoperatively in the 2.2-mm group and up to 3 months postoperatively in the 2.75-mm group. This is similar to the greater SIA induced by superior incision than by temporal incision10 because the incision is closer to the pupillary center in smaller eyes because it is in superior relative to temporal incision.
The magnitude of SIA in the 2.75-mm group in the current study is similar to that previously reported for a temporal clear cornea wound and slightly smaller than that reported for a superior clear corneal incision.17 This is reasonable because more numerous circular corneal fibers at the limbus make the cornea stronger at the limbus than at the clear cornea.23 It is also conceivable that a limbal incision smaller than 2.75 mm would minimize the ATR drift of SIA. Our result is also compatible with previous studies finding that the SIA is smaller in the 2.0/2.2-mm groups than in the 2.65/3.0-mm groups using corneal incision,3,8,9 but not significantly different using scleral incision.8
The summated vector mean of the SIA was less than the individual averaged SIA in each group. This could be attributed to the difference in calculation. The mean SIA was calculated by averaging the absolute magnitude of SIAs.27 In contrast, the summated vector mean of the SIA was calculated by summing all of the SIAs at their own orientations, and divided the net resultant vector’s total length by the number of SIA vectors.27 Different orientation of the vectors to be summed neutralized each other and resulted in a smaller vector. Because there were different orientations of each summated vector, the summated vector mean of the SIAs was thus smaller than the averaged magnitude of SIA.
A limitation of this study is that we included only cases that underwent surgery with superior corneal incision. Further studies on cases that underwent surgery with temporal incision will facilitate understanding of the factors that affect SIA following phacoemulsification. However, because the incidence of endophthalmitis for clear corneal incision is greater than that for limbal incisions,28 limbal incision is still preferred by some surgeons. Furthermore, because the temporal incision is more commonly adopted, our results pertaining to superior limbal incision are able to provide additional scientific merits and thus still an important reference for refractive cataract surgeons. Another limitation of this study is that we assumed the final incision width was the same as the initial incision width. Measuring the length of the wound with an inner gauge at the conclusion of surgery would potentially enhance the predictability of wound size on SIA. Further study incorporating the biomechanical properties of the cornea and anterior segment optical coherence tomography in analyzing the morphology of corneal incisions could also advance our understanding of ocular structure-related alterations in SIA. Finally, shallow ACD and shorter axial length is, in general, associated with smaller corneal diameter.29 It is possible that greater SIA would correlate with smaller corneas. However, correlation of axial length with corneal radius, ACD, and corneal diameter in normal eyes was not present in eyes with extreme myopia or hyperopia.29 In our study, we included eyes with axial length ranging from 18.5 to 30.36 mm, correlating SIA with corneal diameter and thus reducing the significance of the results. Further study to analyze whether corneal diameter correlates with SIA would be beneficial to cataract surgeons.
Traditional considerations for SIA focus on the location and width of the corneal incision.3–12 By analyzing a large number of cases operated on by a single surgeon, we found that patients with older age, greater preoperative corneal astigmatism, oblique corneal astigmatism, shallower ACD, shorter axial length, and lower IOP had greater SIA. Because ACD, axial length, corneal astigmatism, and patient age are all routinely measured before cataract surgery, and incision width, location, and structure are decided according to the surgeon’s preference, we advise surgeons to adjust their flattening component of SIA input for toric IOL calculation for each patient as needed, especially when using a corneal incision greater than 2.2 mm. This small but significant difference in ocular features associated with SIA, if taken into consideration by the major IOL calculation formulas, would potentially benefit the astigmatism control and surgical outcomes after premium refractive lens surgery.
- Denoyer A, Denoyer L, Marotte D, Georget M, Pisella PJ. Intra-individual comparative study of corneal and ocular wavefront aberrations after biaxial microincision versus coaxial small-incision cataract surgery. Br J Ophthalmol. 2008;92:1679–1684. doi:10.1136/bjo.2007.137067 [CrossRef]
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- Masket S, Wang L, Belani S. Induced astigmatism with 2.2- and 3.0-mm coaxial phacoemulsification incisions. J Refract Surg. 2009;25:21–24.
- Wilczynski M, Supady E, Piotr L, Synder A, Palenga-Pydyn D, Omulecki W. Comparison of surgically induced astigmatism after coaxial phacoemulsification through 1.8 mm microincision and bimanual phacoemulsification through 1.7 mm microincision. J Cataract Refract Surg. 2009;35:1563–1569. doi:10.1016/j.jcrs.2009.04.037 [CrossRef]
- Pakravan M, Nikkhah H, Yazdani S, et al. Astigmatic outcomes of temporal versus nasal clear corneal phacoemulsification. J Ophthalmic Vis Res. 2009;4:79–83.
- Kim EC, Byun YS, Kim MS. Microincision versus small-incision coaxial cataract surgery using different power modes for hard nuclear cataract. J Cataract Refract Surg. 2011;37:1799–1805. doi:10.1016/j.jcrs.2011.04.024 [CrossRef]
- Klamann MK, Gonnermann J, Maier AK, Torun N, Bertelmann E. Smaller incision size leads to higher predictability in microcoaxial cataract surgery. Eur J Ophthalmol. 2013;23:202–207. doi:10.5301/ejo.5000207 [CrossRef]
- Hayashi K, Yoshida M, Hayashi H. Corneal shape changes after 2.0-mm or 3.0-mm clear corneal versus scleral tunnel incision cataract surgery. Ophthalmology. 2010;117:1313–1323. doi:10.1016/j.ophtha.2009.11.041 [CrossRef]
- Hayashi K, Yoshida M, Hayashi H. Postoperative corneal shape changes: microincision versus small-incision coaxial cataract surgery. J Cataract Refract Surg. 2009;35:233–239. doi:10.1016/j.jcrs.2008.10.031 [CrossRef]
- Tejedor J, Perez-Rodriguez JA. Astigmatic change induced by 2.8-mm corneal incisions for cataract surgery. Invest Ophthalmol Vis Sci. 2009;50:989–994. doi:10.1167/iovs.08-2778 [CrossRef]
- Rho CR, Joo CK. Effects of steep meridian incision on corneal astigmatism in phacoemulsification cataract surgery. J Cataract Refract Surg. 2012;38:666–671. doi:10.1016/j.jcrs.2011.11.031 [CrossRef]
- Ernest P, Hill W, Potvin R. Minimizing surgically induced astigmatism at the time of cataract surgery using a square posterior limbal incision. J Ophthalmol. 2011:243170.
- Tadros A, Habib M, Tejwani D, Von Lany H, Thomas P. Opposite clear corneal incisions on the steep meridian in phacoemulsification: early effects on the cornea. J Cataract Refract Surg. 2004;30:414–417. doi:10.1016/S0886-3350(03)00649-7 [CrossRef]
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Figure A. Intraocular pressure (IOP) after surgery with a (A) 2.2- or (B) 2.75-mm incision. Error bars represent standard deviations. * = P less than .05 when compared with the preoperative time period; ** = P less than .05 when compared with the 1 day postoperative time period. preop = preoperative; 1D = 1 day postoperative; 1W = 1 week postoperative; 1M = 1 month postoperative; 2M = 2 months postoperative; 3M = 3 months postoperative.
Figure B. Summated vector means of surgically induced astigmatism of the 2.2- and 2.75-mm groups. WTR = with the rule; ATR = against the rule
Preoperative Parameters in Both Groups
|Group||No. of Eyes||Age (y)||Male/Female||Corneal Astigmatism (D)||Axial Length (mm)||ACD (mm)||IOP (mm Hg)|
|2.2 mm||248||70 ± 11||121/137||1.13 ± 0.9||23.51 ± 1.53||3.04 ± 0.47||14.3 ± 3.0|
|2.75 mm||357||66 ± 12||176/203||0.99 ± 0.77||23.96 ± 1.79||3.07 ± 0.50||13.7 ± 3.0|
Summary of Significant Correlations Between SIA and Other Dactors
|Time Postoperatively||Incision Size|
|2.75 mm||2.2 mm|
| Preoperative corneal astigmatism||0.244||.004||0.311||< .001|
| Age||0.348||< .001||0.255||< .001|
| Preoperative axial length||−0.215||.003||–||–|
| Preoperative IOP||−0.340||.006||–||–|
| Preoperative corneal astigmatism||0.315||.001||0.188||.006|
| Age||0.243||< .001||0.267||< .001|
| Preoperative axial length||−0.184||.006||–||–|
| Preoperative IOP||−0.355||.02||–||–|
| Preoperative corneal astigmatism||0.205||.021||0.236||.001|
| Age||0.344||< .001||0.22||.002|
| Preoperative axial length||0.184||.040||–||–|
| Preoperative IOP||−0.278||.005||–||–|
| IOP 3 months postoperatively||−0.271||.033||–||–|
| Preoperative corneal astigmatism||0.205||.001||0.251||.005|
| Age||0.293||< .001||–||–|
| Preoperative axial length||−0.146||.016||–||–|
| Preoperative IOP||−0.223||.030||–||–|
| IOP 3 months postoperatively||−0.317||.012||–||–|
| Preoperative corneal astigmatism||0.185||.006||–||–|
| Age||0.297||< .001||–||–|
| Preoperative axial length||0.203||.003||–||–|
| IOP 3 months postoperatively||−0.256||.044||–||–|
Summary of Regression Model Analysis of the Association Between SIA at 1 Month and Other Clinical Variables
|Preoperative Astigmatism Axis||Clinical Variable||Unstandardized Coefficient||P|
| WTR||Preoperative corneal astigmatism||0.177||.002|
| ATR||Preoperative corneal astigmatism||0.205||.002|
| Oblique||Preoperative corneal astigmatism||0.478||.001|
| WTR||Preoperative corneal astigmatism||0.141||< .001|