The clear corneal incision (CCI) is now a standard technique for many types of cataract surgery.1–3 Previous studies4–8 found that various types of cataract surgery incisions induce different degrees of change in corneal astigmatism and the corneal shape after cataract surgery. In these studies, surgically induced changes in corneal astigmatism were assessed based on the anterior corneal surface only, and changes in the posterior corneal surface were assumed to be proportional to those in the anterior cornea. Astigmatism of the posterior cornea, however, is not necessarily proportional to that of the anterior cornea.9–12 Therefore, changes in the corneal astigmatism of the anterior and posterior cornea induced by cataract surgery should be determined.
It is reasonable to assume that the size of the cataract surgery incision affects the changes in corneal astigmatism and shape. Studies assessing the anterior corneal surface5,13–16 found that a wider CCI induced greater changes in corneal astigmatism. Our previous studies17,18 also found that the shape changes of the anterior cornea was significantly less affected in eyes that had a 2.0 mm wide CCI than in eyes that had a 2.65 mm or 3.0 mm wide CCI when the CCI length was the same. It remains unclear, however, whether CCI length affects the changes in corneal astigmatism and shape.
The present study assessed whether a longer length CCI induces greater changes in corneal shape and astigmatism. To precisely evaluate the surgically induced changes, the changes in astigmatism and the shape of the total, anterior, and posterior cornea were evaluated.
Patients and Methods
This was a prospective randomized clinical study and an exploratory study. A clinical research coordinator began screening all consecutive patients who were to have bilateral phacoemulsification with intraocular lens (IOL) implantation at Hayashi Eye Hospital, Fukuoka, Japan, on November 1, 2015. This research adhered to the tenets of the Declaration of Helsinki. The Institutional Review Board/Ethics committee of Hayashi Eye Hospital approved the study protocol, and all patients provided written informed consent to participate. This study was registered with the University Hospital Medical Information Network (UMIN000025345).
Exclusion criteria were pathology other than cataract, pseudoexfoliation syndrome, cases of planned extracapsular or intracapsular cataract extraction, zonular dehiscence, history of ocular surgery or inflammation, diabetes mellitus, patients who declined to be included in the study, and difficulties with examination or follow-up. In addition, patients who were enrolled in other studies were excluded from this study. The coordinator continued the patient screening until 120 patients were recruited (December 11, 2016).
The day before surgery, the study patients were randomly assigned to 1 of 2 groups. One group comprised left eyes that were to have a 2.4 mm wide CCI of 1.75 mm or longer and right eyes that were to have a 2.4 mm wide CCI shorter than 1.75 mm. The other group comprised left eyes that were to have a short-length CCI and right eyes that were to have a long-length CCI. The border of the long and short incisions was determined based on a finding that the mean CCI length is approximately 1.7 mm.8 The clinical research coordinator generated a randomization code with equal numbers using computer software and assigned each patient to 1 of the 2 groups according to the randomization code. A member of the operating room staff was informed of the group to which each patient was assigned. The surgeon was informed by the staff member about the length of the CCI just before surgery. The coordinator kept the assignment schedule concealed until all data were collected. Patients, examiners, and data analysts were unaware of the assignment schedule.
All surgeries were performed by the same surgeon (K.H.) using a previously described technique.17,18 For phacoemulsification, the surgeon created a 2.4 mm temporal CCI at the 3 o’clock meridian in left eyes and at the 9 o’clock meridian in right eyes. The meridian of the CCI was estimated by the surgeon. After 2 side-port incisions were made 90 degrees from the center of the CCI using a 0.6 mm steel slit knife, a continuous curvilinear capsulorhexis approximately 5.0 mm in diameter was created using a bent needle. A 2.4 mm single-plane CCI was made with a keratome from the posterior margin of the cornea. The surgeon intended to make a CCI of 1.75 mm or longer in eyes in the long CCI group and a CCI of less than 1.75 mm in eyes in the short CCI group. After hydrodissection, phacoemulsification of the nucleus and cortical aspiration were performed using a Constellation Vision System (Alcon Laboratories, Inc.). The capsular bag was then inflated with sodium hyaluronate 1.0% (Hyaguard). A single-piece aspheric hydrophobic acrylic IOL (Acrysof SN60WF, Alcon Laboratories, Inc.) was implanted in the lens capsule using a Monarch II injector (Alcon Laboratories, Inc.). After the IOL was inserted in the capsule, the ophthalmic viscosurgical device was removed. At the conclusion of surgery, the CCI and side ports were hydrated with a balanced salt solution to close the wounds. All IOLs were inserted in the capsular bag with no sutures. The preoperative and postoperative medication and anesthesia were identical between the long CCI and short CCI groups, and examiners could not distinguish the type of incision used.
Patients had videokeratography, visual acuity, objective refraction, and wavefront aberration evaluations 2 days and 2, 4, and 8 weeks postoperatively. The videokeratographic evaluations were performed using a Placido–Scheimpflug system (Topographic Modeling System, version 5, Tomey Corp.). Corrected distance visual acuity (CDVA) was measured on decimal charts, and the decimal visual acuity was converted to a logarithm of the minimum angle of resolution (logMAR) scale for statistical analysis. Objective refraction was measured using an autorefractor–keratometer (KR-7100, Topcon Corp.). The ocular wavefront higher-order aberrations (HOAs) were measured using a Placido-disk videokeratography and Hartmann-Shack wavefront aberrometer (KR-1W, Topcon Corp.). The nuclear palescence of the lens was graded using the Lens Opacities Classification System III.19
The videokeratographic data of the total, anterior, and posterior cornea were collected using the Placido–Scheimpflug system; details of this system are described elsewhere.20–22 Briefly, the Placido–Scheimpflug system includes a rotating Scheimpflug camera system and Placido-ring topographer. This device obtains data by merging Placido-ring topography with the Scheimpflug system. This system initially obtains 4 measurements using Placido-ring topography (ring topo mode), with each measurement lasting less than 0.5 second. The examiner then activates the Scheimpflug system (slit mode), and the Scheimpflug acquisition is performed.
The videokeratography data were used to determine regular corneal astigmatism and surgically induced changes in the corneal astigmatism and shape. Regular astigmatism was determined using the simulated keratometric values at the steeper and flatter meridians. Surgically induced astigmatism (SIA) was determined as the change vector from the preoperative astigmatism to postoperative astigmatism at each time interval. The SIA vector constitutes the magnitude and meridian of the change in corneal astigmatism and is not suitable for statistical analysis. To compare the SIA vector between groups, the SIA vector was decomposed to vertical–horizontal (Jackson cross-cylinder, axes at 180 degrees and 90 degrees [J0]) and oblique (Jackson cross-cylinder, axes at 45 degrees and 135 degrees [J45]) astigmatic change components using the power vector analysis described by Thibos and Horner.23 A positive J0 indicates a with-the-rule (WTR) astigmatic change, whereas a negative J0 indicates an against-the-rule (ATR) astigmatic change.
The data of corneal astigmatic change in left eyes and right eyes were analyzed together. The mean changes in corneal shape between preoperatively and each postoperative time interval were evaluated using the average of the difference map of the Placido–Scheimpflug system.8,17,18 The average of the difference map was created by subtracting the mean postoperative corneal topographic data from the preoperative data; this was expressed in 0.5 diopter (D) steps as a color-coded map. To produce the average of the difference map, videokeratographic data from the left eyes were converted to the right side and the maps of the left eyes and right eyes were expressed together.
Ocular and corneal wavefront aberrations were measured using the integrated Placido–disk videokeratography and Hartmann-Shack wavefront aberrometer system; the details of this instrument have been described.24–26 Ocular wavefront analysis is based on the Hartmann-Shack principle using a near-infrared laser beam. The corneal topographer uses conventional Placido-disk technology to measure the radius of the anterior corneal curvature; therefore, the data are computed to calculate only the anterior corneal HOAs. Ocular and corneal HOAs were calculated using standardized Zernike polynomials. After full mydriasis was achieved, the wavefront aberrations were measured in the central 4.0 optical zone. The wavefront aberration was expanded with normalized Zernike polynomials up to the 8th order. The HOAs were defined as the root mean square (RMS) of the 3rd- to 8th-order Zernike coefficients as follows:
where j is the Noll index, n is the radial degree, and m is the azimuthal degree. The RMS of Z(4,0) (spherical aberration) and Z(6,0) was defined as spherical-like aberrations, whereas the RMS of Z(3,−1) and Z(3,1) were defined as coma-like aberrations. Total HOAs were defined as the sums of the RMS of the 3rd- to 8th-order coefficients.
The wound length, width, and meridian of the long and short CCIs were evaluated 2 days after surgery using anterior segment optical coherence tomography (AS-OCT) (SS-1000, Tomey Corp.). The AS-OCT can scan across the anterior segment of the eye, traversing the main CCIs and side ports. Experienced ophthalmic technicians who were not aware of the purpose of the study performed all evaluations.
Data were tested for normality of distribution by inspecting histograms. The data for the SIA, J0 and J45, ocular HOAs, logMAR visual acuity, and other continuous variables followed a normal distribution. The magnitude of the SIA, HOAs, logMAR visual acuity, and other continuous variables were compared between groups using a paired t test. Two independent variables of the SIA vector (J0 and J45) were compared between groups using the multivariate analysis of variance (MANOVA). The magnitude of the SIA categorized by the type of astigmatism was compared between groups using the unpaired t test. The patient’s sex, ratio of the left eyes to right eyes, and other discrete variables were compared between groups using the chi-square test or Fisher exact test, where applicable. Differences with a P value less than 0.05 were considered statistically significant. Because this was an exploratory study and the primary outcome was the difference in the mean J0 and J45 of the total cornea 4 weeks and 8 weeks postoperatively, P values were not adjusted.27
Of the 120 patients, 10 (8.3%) were excluded from analysis. Two patients did not appear for the follow-up visit because of a scheduling conflict, 2 patients were referred to other hospitals, 2 patients refused examination, and 1 patient was unable to have an examination for psychological reasons. Three patients were excluded because of a complicated surgery. Accordingly, 110 patients (91.7%) remained for analysis. Because the surgical procedures and preoperative and postoperative medications were identical between groups, the patients were unaware of which eye had a long CCI or short CCI. Because the appearances of both eyes were similar, the examiners were unaware of which eye had long or short CCI. In addition, the data analysts did not know which eye had long or short CCI because the assignment schedule was not revealed until all data were collected.
Table 1 shows the patient characteristics at baseline and surgical factors in the long CCI and short CCI groups. The mean age of the 43 men and 67 women was 69.4 years ± 6.6 (SD). The preoperative corneal astigmatism, preoperative manifest spherical equivalent value, preoperative visual acuity, nuclear opalescence, surgery times, and other baseline characteristics and surgical factors did not differ significantly between groups. The distribution of WTR, ATR, and oblique astigmatism before surgery and the mean magnitude and meridian of the preoperative corneal astigmatism depending on the type of astigmatism were not significantly different between the long CCI group and short CCI group (Table 2).
Figure 1 shows the AS-OCT images of the incision of a representative eye in the long CCI group and short CCI group. In both eyes, the single-plane CCI began at the corneal limbus. The mean incision length was 2.34 ± 0.32 mm (range 1.74 to 3.28 mm) in the long CCI group and 1.37 ± 0.17 mm (range 1.06 to 1.75 mm) in the short CCI group; the CCI was significantly longer in the long CCI group than in the short CCI group (P < .0001). The mean postoperative width of CCI was 2.69 ± 0.17 mm in the long CCI group and 2.66 ± 0.20 mm in the short CCI group; there was no significant difference between groups (P = .3192). The mean meridian of CCI was 180.6 ± 17.4 degrees in the long CCI group and 178.0 ± 19.8 degrees in the short CCI group; the difference between groups was not significant (P = .0614).
Changes in Corneal Astigmatism
The mean magnitude of the SIA tended to be greater in the long CCI group than in the short CCI group; the difference was significant 2 weeks and 4 weeks postoperatively in the total cornea and at 2 days in the posterior cornea (P ≤ .0208) (Table 3). The mean meridian of the SIA did not differ significantly between groups at any postoperative interval (P ≥ .1238) (Table 3).
The mean J0 of the total and anterior cornea showed a WTR shift and that of the posterior cornea showed an ATR shift. The mean J45 was small, indicating only a slight oblique change. Based on the MANOVA, the mean J0 and J45 values of the total cornea were significantly greater in the long CCI group than in the short CCI group at all follow-ups (P ≤ .0290) (Figure 2). The mean J0 and J45 of the anterior cornea did not differ significantly between groups 2 days, 2 weeks, or 8 weeks postoperatively (P ≥ .2190) (Figure 2) and were significantly greater in the long CCI group than in the short CCI group at 4 weeks only (P = .0301). The mean J0 and J45 values of the posterior cornea were significantly greater in the long CCI group than in the short CCI group at 2 days, 2 weeks, and 4 weeks (P ≤ .0414) (Figure 2); there was no difference at 8 weeks (P = .7607). When a J0 of 0.20 D and a J45 of 0.20 D were assumed to be clinically meaningful differences between the 2 groups using the MANOVA, power analyses indicated that 110 patients provided a statistical power of over 85% in the total cornea, over 86% in the anterior cornea, and 100% in the posterior cornea.
Changes in Corneal Shape
Figures 3A, 3B, and 3C show changes in corneal shape in the long CCI group and short CCI group, which are shown by the average of the difference map, which is expressed in 0.5 D steps. In both groups, a wedge-shaped temporal flattening was observed around the CCI in the total and anterior cornea and a wedge-shaped steepening was observed in the posterior cornea 2 days postoperatively. These changes rapidly diminished thereafter; the wound-related flattening of the total and anterior cornea persisted for up to 8 weeks postoperatively, and the steepening of the posterior cornea was no longer detected at 4 weeks. The wedge-shaped flattening of the total and anterior cornea and the steepening of the posterior cornea extended closer to the central cornea in the long CCI group than in the short CCI group at all postoperative follow-ups.
Ocular and Corneal Higher Order Aberrations and Visual Acuity
Table 4 shows the mean ocular and corneal total, and coma-like and spherical-like aberrations, which did not differ significantly between the long CCI group and short CCI group at any follow-up (P ≥ .0521). The mean uncorrected and corrected distance visual acuities (logMAR) did not differ significantly between groups throughout the follow-up (P ≥ .0969).
The findings in the present study using videokeratography showed a wedge-shaped focal flattening in the total and anterior cornea and a focal steepening in the posterior cornea occurred around the CCI in the immediate postoperative period; however, these changes rapidly diminished. The focal steepening of the posterior cornea around the CCI might be attributable to corneal edema caused by wound hydration. These wound-related changes extended closer to the central cornea with a long CCI than with a short CCI. In addition, the focal steepening of the posterior cornea disappeared by 4 weeks postoperatively, whereas that of the total and anterior cornea persisted for up to 8 weeks. These findings suggest that the long-length CCI has a greater effect on the central cornea than the short-length CCI.
The surgically induced change in the corneal astigmatism was WTR in the total and anterior cornea and ATR in the posterior cornea after small-incision cataract surgery with a temporal CCI. On average, a slight oblique change was detected. Because the refractive power of the posterior cornea is a negative value, the meridian of the astigmatic change of the posterior cornea differs from that of the total and anterior cornea. The change in regular astigmatism was significantly greater with a long-length CCI than with a short-length CCI at all follow-ups in the total cornea, 4 weeks after surgery in the anterior cornea, and at 2 days, 2 weeks, and 4 weeks in the posterior cornea. Thus, the long CCI induced a greater astigmatic change in the total, anterior, and posterior cornea than the short CCI.
The mean incision length was approximately 1.0 mm longer in the long CCI group than in the short CCI group, although the CCIs were similarly started at the corneal limbus in both groups. Biologically, greater regular corneal astigmatism and more prominent wound-related shape changes after a long CCI than after a short CCI can be attributed to the fact that the entry site into the anterior chamber is closer to the central cornea in long CCIs than in short CCIs.
Ocular HOAs and visual acuity did not differ significantly between eyes that had a temporal 2.4 mm long-length CCI and those that had a 2.4 mm temporal short-length CCI. These findings suggest that a 2.4 mm temporal CCI is not wide enough to make a significant difference in visual acuity and HOAs between long CCIs and short CCIs. When the incision is wider or located superiorly, however, visual function will be impaired. We believe that the finding in this study that a longer incision induced greater changes in corneal shape and astigmatism than a shorter incision applies universally to any type of cataract surgery incision.
The changes in astigmatism and shape were greatest in the total cornea, followed by the anterior cornea and posterior cornea. These results are reasonable because a large portion of astigmatism in the total cornea is related to astigmatism in the anterior cornea.9–13 Indeed, most previous studies of corneal astigmatic changes caused by cataract surgery evaluated the changes in the anterior cornea only.4–9 In the present study, however, the changes in shape and astigmatism of the total cornea were not necessarily proportional to those of the anterior cornea and the changes of the posterior cornea were relatively large. These findings agree with those in most other studies,28–30 although a study by Klijin et al.31 found conflicting results. Based on these findings, we consider that examination of the astigmatism and shape of the total cornea is necessary to precisely assess the actual changes in corneal astigmatism and shape after cataract surgery.
Previous studies assessing the anterior corneal surface4,5 found that the SIA was greater in proportion to the incision width. Our previous studies17,18 showed significantly fewer changes in astigmatism and the shape of the anterior cornea in eyes that had a CCI with a 2.0 mm width than in eyes that had a CCI with a 2.65 mm or 3.0 mm width. However, to our knowledge no studies to date have compared the changes in corneal shape and astigmatism between long incisions and short incisions. Furthermore, although more recent studies examined SIA of the anterior and posterior cornea after temporal or superior incisions, these studies focused on the contribution of the posterior cornea to the astigmatic change in the total cornea in a single cohort.28–30 In the present study, the changes in the astigmatism and shape of the posterior cornea were quite large and not negligible, and the changes in astigmatism and shape were significantly greater with long-length CCIs than with short-length CCIs.
The present study has several limitations. First, the changes in corneal astigmatism and shape, HOAs, and visual acuity did not differ significantly between 4 weeks and 8 weeks after surgery. This is probably because the surgically induced changes in corneal astigmatism and shape rapidly diminished postoperatively because of the narrow incision width. Second, the sample size was not statistically determined before the study was performed. However, the statistical powers of the MANOVA for comparing the changes in the regular astigmatism of the total, anterior, and posterior cornea between groups were calculated to be more than 86% at all follow-ups. Thus, the statistical powers were sufficient to detect a clinically meaningful difference in the corneal astigmatic changes between groups.
In conclusion, SIA changes of the total cornea were significantly greater with a long-length CCI than with a short-length CCI at all follow-ups. Furthermore, wound-related flattening in the total and anterior cornea and wound-related steepening in the posterior cornea occurred around the CCI immediately after surgery, but rapidly diminished; these shape changes extended closer to the central cornea with a long CCI than with a short CCI. These findings suggest that a short incision is preferable to a long incision with regard to corneal astigmatic changes. Whether a short-length CCI is stable, similar to a long-length CCI, however, remains unclear. Further studies are required to compare wound stability and the risk for endophthalmitis between short incisions and long incisions.
What Was Known
- Previous studies assessing the anterior corneal surface showed that the surgically induced change in corneal astigmatism was greater in proportion to the width of CCIs in cataract surgery.
- Previous videokeratographic studies found that a wedge-shaped flattening occurred around the temporal CCI of the anterior cornea after cataract surgery; this wound-related flattening was greater and more persistent in eyes that had a CCI with a 2.0 mm width than in eyes that had a CCI with a 2.65 mm or 3.0 mm width.
What This Paper Adds
- The WTR astigmatic change that occurred in the total cornea and ATR change that occurred in the posterior cornea were significantly greater after long-length temporal CCIs (≥1.75 mm) than after short-length temporal CCIs (<1.75 mm).
- Videokeratography maps showed that a wedge-shaped focal flattening in the total and anterior cornea and a focal steepening in the posterior cornea occurred around the CCI postoperatively in eyes that had long-length and short-length CCIs, but these effects rapidly decreased. These wound-related changes in corneal shape extended closer to the central cornea with long-length CCIs than with short-length CCIs.
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None of the authors has a financial or proprietary interest in any material or method mentioned.