Journal of Refractive Surgery

Therapeutic Refractive Surgery Supplemental Data

Intracorneal Ring Segments Implantation Outcomes Using Two Different Manufacturers' Nomograms for Keratoconus Surgery

Guilherme Rocha, MD; Ludmila Nascimento Pinto Silva, MD; Luís Fernando Oliveira Borges Chaves, MD; Pedro Bertino, MD; Leonardo Torquetti, MD, PhD; Luciene Barbosa de Sousa, MD, PhD

Abstract

PURPOSE:

To compare the outcomes of intracorneal ring segments (ICRS) implantation in keratoconic eyes with a similar tomographic pattern, using two different manufacturers' nomograms for surgical planning.

METHODS:

ICRS were implanted alternately in patients with the same tomographic pattern of keratoconus divided into two groups according to the surgical planning proposed by the ICRS manufacturers (Keraring, Mediphacos, Belo Horizonte, Brazil, and Ferrara Ring, AJL Ophthalmics, Vitoria, Spain). Visual, refractive, keratometric, corneal aberrometry, and optical quality changes were evaluated during a 6-month follow-up. Corneal and manifest refractive astigmatic changes were also analyzed using the double-angle polar plot and the Alpins vectorial method through the following components: target induced astigmatism, surgically induced astigmatism, difference vector, correction index, angle of error, index of success, flattening effect, and torque.

RESULTS:

After ICRS implantation, both groups showed significant improvement (P < .05) in visual and keratometric data. Corneal aberrometric changes and optical quality improvement were also statistically significant in both groups, except for trefoil (P > .05 in all intervals). The Alpins method analysis showed a better performance in the Keraring group, but with no statistically significant difference between groups (P > .05). Comparison between groups showed a statistically significant difference only in tomographic astigmatism in double-angle polar plot analysis (P = .03), with more significant improvement in the Keraring group.

CONCLUSIONS:

Both manufacturers' nomograms resulted in statistically significant improvement in most of the parameters analyzed, with greater correction of corneal tomographic astigmatism in the group operated on according to the spherical equivalent/tomographic astigmatism nomogram.

[J Refract Surg. 2019;35(10):673–683.]

Abstract

PURPOSE:

To compare the outcomes of intracorneal ring segments (ICRS) implantation in keratoconic eyes with a similar tomographic pattern, using two different manufacturers' nomograms for surgical planning.

METHODS:

ICRS were implanted alternately in patients with the same tomographic pattern of keratoconus divided into two groups according to the surgical planning proposed by the ICRS manufacturers (Keraring, Mediphacos, Belo Horizonte, Brazil, and Ferrara Ring, AJL Ophthalmics, Vitoria, Spain). Visual, refractive, keratometric, corneal aberrometry, and optical quality changes were evaluated during a 6-month follow-up. Corneal and manifest refractive astigmatic changes were also analyzed using the double-angle polar plot and the Alpins vectorial method through the following components: target induced astigmatism, surgically induced astigmatism, difference vector, correction index, angle of error, index of success, flattening effect, and torque.

RESULTS:

After ICRS implantation, both groups showed significant improvement (P < .05) in visual and keratometric data. Corneal aberrometric changes and optical quality improvement were also statistically significant in both groups, except for trefoil (P > .05 in all intervals). The Alpins method analysis showed a better performance in the Keraring group, but with no statistically significant difference between groups (P > .05). Comparison between groups showed a statistically significant difference only in tomographic astigmatism in double-angle polar plot analysis (P = .03), with more significant improvement in the Keraring group.

CONCLUSIONS:

Both manufacturers' nomograms resulted in statistically significant improvement in most of the parameters analyzed, with greater correction of corneal tomographic astigmatism in the group operated on according to the spherical equivalent/tomographic astigmatism nomogram.

[J Refract Surg. 2019;35(10):673–683.]

Keratoconus is a non-inflammatory condition in which the cornea assumes a conical shape because of thinning and protrusion.1 It is bilateral, asymmetric, and progressive. The cornea thins, steepens, and protrudes, and its outer surface becomes irregular, distorted, and sometimes even scarred, resulting in blurry vision, even with visual correction.2

Keratoconus treatment has evolved significantly in the past few years. The benefits of intracorneal ring segments (ICRS) have been established in the current literature regarding visual acuity gain, reduction in spherical equivalent (SE), and improvement in corneal tomographic parameters.3–13

Although many studies have shown the benefits of ICRS implantation, few studies compared the results among different ICRS types or analyzed different surgical planning.14–17 Most corneal surgeons, especially those in training, base their ICRS implantation planning on two nomograms provided by the manufacturers: (1) one based on SE and tomographic astigmatism (SE/dK-based nomogram)18 (2) and one based on average corneal asphericity (Q-based nomogram), measured in 30° of arc (Q 30°).19

This study compared the visual acuity improvement and corneal tomographic, corneal aberrometric, and vectorial astigmatic changes in patients implanted with ICRS, using two different manufacturers' nomograms for surgical planning.

Patients and Methods

This prospective, comparative, and interventional case series study comprised a total of 64 keratoconic eyes of 49 patients treated with ICRS implantation between December 2013 and January 2016. Preoperatively, all patients had corrected distance visual acuity (CDVA) of 0.3 logMAR or worse (Snellen visual acuity of 20/40 or worse) and inability to wear corneal or scleral contact lenses.

Patients were alternately divided into two different groups: Keraring ICRS (Mediphacos; Belo Horizonte, Brazil) with three different arcs (160°, 120°, and 90°) according to the SE/dK-based nomogram18 and Ferrara Ring ICRS (AJL Ophthalmics, Vitoria, Spain) with two different arcs (160° and 210°) according to the Q-based nomogram.19 To ensure comparative effectiveness, only keratoconic eyes with a similar tomographic pattern were selected. All patients presented with type 2 keratoconus according to the nomogram instruction based on keratoconus distribution18: approximately one-third of the ectatic area located on one side of the steepest corneal meridian and two-thirds located on the opposite side. Also, ICRS thicknesses were selected based on each nomogram indication. Finally, no personal adjustment to the manufacturers' nomograms was performed.

Exclusion criteria were patients who did not complete minimum follow-up intervals, history of previous ocular surgeries, and active ocular diseases other than keratoconus. After thoroughly explaining the purpose and procedures of the study, all patients were asked to sign an informed consent form before treatment. This study was approved by the São Paulo Federal University Review Board at the Department of Ophthalmology and followed the tenets of the Declaration of Helsinki.

Examination Protocol

Preoperative and postoperative evaluation included a comprehensive ophthalmologic examination with uncorrected distance visual acuity (UDVA) and CDVA, SE refraction, and manifest refractive astigmatism (MRA), slit-lamp examination, Goldmann tonometry, fundus examination, corneal tomography (Pentacam; Oculus Optikgeräte, Wetzlar, Germany); and corneal aberrometry (OPD Scan III; Nidek Inc., Tokyo, Japan).

The following corneal tomographic data between preoperatively and 3 and 6 months postoperatively were evaluated: flat and steep corneal keratometry, dK and respective axis, mean keratometry, maximum keratometry, and average corneal asphericity at 30° angle (Q 30°).

From corneal aberrometry records, we collected data from wavefront summary maps and optical quality maps. To avoid pupil size bias, we manually set the pupil size to 5 mm (presumed optical zone for ICRS location) and 6 mm (chosen for analysis inclusion). The corresponding aberration coefficients and root mean square values were calculated for the following types of aberration: total aberrations, higher order aberrations, total coma, total trefoil, and total spherical aberrations. From optical quality maps, we collected the Strehl ratio and wavefront error from the point spread function map, and total aberrations and higher order aberrations area ratio (%) measured from the modulation transfer function graphic.

To ensure comparable data, both groups had similar preoperative data (Table 1).

Preoperative Data

Table 1:

Preoperative Data

Surgical Technique

The first nomogram (SE/dK-based)18 takes into consideration four parameters for choosing ICRS: CDVA, the spherical magnitude in refraction, MRA or dK depending on CDVA, and SE. This nomogram uses the principle that shorter ICRS arcs (120° and 90°) promote a higher astigmatic correction and consequently better correction of MRA and dK when used in combination with a 160° ICRS in keratoconus presenting the tomographic pattern enrolled in this study.

The second nomogram (Q-based)19 takes into consideration three parameters for choosing ICRS: the keratoconus distribution in the axial tomographic map, the dK magnitude, and Q 30°. Patients with keratoconus presenting the tomographic pattern enrolled in this study will have a single 160° arc, a pair of 160° arcs, or a 210° arc chosen based on Q 30°. This nomogram uses the principle of trying to reshape the cornea better, using longer arcs to reduce corneal asphericity.

All surgical procedures were performed under topical anesthesia with proxymetacaine hydrochloride 0.5% drops (Anestalcon; Alcon Laboratories, Inc., Fort Worth, TX) by the same surgeon (GR) at Brasília Ophthalmological Hospital (Brasília, Brazil). Incision and stromal tunnel were both created by using a femtosecond laser (60-kHz IntraLase; Abbott Medical Optics, Inc., Santa Ana, CA). Laser parameters included: (1) incision placed at steepest tomography axis; (2) incision and tunnel depth of 70% of corneal thickness at the thinnest point in the ring track; (3) inner diameter of 5 mm; (4) outer diameter of 5.9 mm; (5) entry cut length of 1.2 mm; (6) entry cut thickness of 1 µm; (7) ring energy of 1.3 mJ; and (8) entry cut energy of 1.3 mJ.

After incision and tunnel creation, the ICRS was inserted into the circular tunnel using an implantation forceps. Subsequently, a silicone-hydrogel bandage contact lens (Air Optix; Alcon Laboratories, Inc.) was placed onto the cornea. The postoperative regimen included moxifloxacin 0.5% and dexamethasone 0.1% eye drops (Vigadexa; Alcon Laboratories, Inc.) four times a day for 10 days and topical lubricant (Systane UL; Alcon Laboratories, Inc.). No intraoperative complications occurred.

Follow-up Evaluation

Postoperative visits were scheduled for 1 day and 3 and 6 months. On the first postoperative day, UDVA measurement, slit-lamp examination, and bandage contact lens removal were performed. The subsequent examinations included UDVA, CDVA, manifest refraction, slit-lamp evaluation, corneal tomography, and corneal aberrometry. No postoperative complications occurred.

Vector Analysis of Refraction and Corneal Astigmatism Changes

Vector analysis of MRA and dK changes between preoperatively and the final follow-up visit at 6 months were performed using Alpins vector analysis20,21 and the double-angle polar plot.22,23

All vector calculations for the Alpins method were performed using VECTrAK software (version 2.4.6; ASSORT Pty. Ltd., Cheltenham, Australia). The following vectors were determined and evaluated: target induced astigmatism (TIA), which is the vector of the proposed change in the cylinder for each treatment; surgically induced astigmatism (SIA), which is the vector of the actual change achieved; and difference vector, which is the additional astigmatic change required to achieve the intended target of the initial surgery. In addition, five parameters derived from the relationship between the vectors were calculated and analyzed. First, the correction index is the ratio of the SIA to the TIA. The correction index is preferably 1.00; it is higher than 1.00 if an overcorrection occurs and less than 1.00 if there is an undercorrection. Second, the angle of error is the angle described by the vectors of the achieved (SIA) and intended (TIA) correction. Third, the index of success is calculated by dividing the difference vector by the TIA, with an intended value of zero. Fourth, the flattening effect is the amount of astigmatism reduction achieved by the effective proportion of the SIA at the intended meridian. Fifth, torque is the amount of astigmatic change induced by the SIA that, due to treatment non-alignment, was ineffective in reducing astigmatism at the intended meridian and that caused rotation and a small increase in existing astigmatism.

The calculation concerning the double-angle polar plot was assessed using a Microsoft Excel for Mac 2011 (version 14.7.2; Microsoft Corporation, Redmond, WA) spreadsheet available as an online tool.23 In this method, the centroid is the mean of a set of x and y values, the vectorial center of the data. An area surrounding the centroid represents the standard deviation. The shape of this area will vary depending on the length of the major and minor axis. The smaller ellipse is called the 95% confidence ellipse of the centroid and is analogous to the 95% confidence interval of the mean in a univariate analysis. The larger ellipse is the 95% confidence ellipse of the dataset and is similar to the 95% confidence interval of the observations in a univariate analysis. Thus, after ICRS implantation, the centroid tends to approximate the polar plot origin, as an astigmatism improvement result.

Statistical Analysis

Statistical analysis was performed using SPSS for Mac software (version 24; SPSS, Inc., Chicago, IL). The data were checked for normality using the Kolmogorov– Smirnov test before statistical evaluation. For paired tests, normally distributed data were evaluated using the paired Student's t test, and non-normally distributed data were analyzed with the Wilcoxon signed-rank test. For unpaired tests (comparison between groups), normally distributed data were evaluated using the un-paired t test, and non-normally distributed data were analyzed with the Mann–Whitney U test. A P value of less than .05 was considered statistically significant.

Results

A total of 64 eyes of 49 patients were included. The mean age was 28.1 ± 7.6 years (range: 12 to 43 years) in the Keraring group and 27.6 ± 6.5 years (range: 17 to 43 years) in Ferrara group (P = .79). The mean follow-up interval was 6.18 ± 0.76 months (range: 5.97 to 8.60 months) in the Keraring group and 6.46 ± 1.20 months (range: 5.73 to 11.00 months) in the Ferrara group (P = .32).

The mean incision axis was 89.3° ± 28.7° (range: 51° to 161°) in the Keraring group and 93.3° ± 31.4° (range: 40° to 150°) in the Ferrara group (P = .60). The mean incision depth was 354.1 ± 34.4 µm (range: 291 to 411 µm) in the Keraring group and 359.2 ± 36.5 µm (range: 291 to 419 µm) in the Ferrara group (P = .55).

Implanted ICRS

According to each manufacturer's nomogram, we found some similarities and differences between the groups. The main similarity was the number of patients who received a single ICRS in the inferior temporal quadrant: 12 eyes (37.50%) in the Keraring group and 13 eyes (40.62%) in the Ferrara group. All of these were 160° ICRS.

Regarding the differences between the groups, we found two that deserve to be mentioned. First, the Q-based nomogram19 uses thinner segments than the SE/dK nomogram18 for the inferior temporal quadrant. In the Keraring group, 11 (34.37%) and 13 (40.62%) eyes received a 300- or 250-µm thick ICRS, respectively. In the Ferrara group, no eyes received a 300-µm ICRS, 4 eyes (12.50%) received a 250-µm ICRS, and 19 eyes (59.37%) received a 200-µm ICRS. Second, following these nomograms, 20 (62.50%) and 19 (59.37%) eyes in the Keraring and the Ferrara groups, respectively, received the second segment in the superior nasal quadrant. In the Keraring group, 5 eyes (25%) received a 90° ICRS, 11 eyes (55%) received a 120° ICRS, and 4 eyes (20%) received a 160° ICRS. In the Ferrara group, all 19 eyes received a 160° ICRS in the superior nasal quadrant.

Visual Acuity and Refraction

Table 2 shows the visual and refractive outcomes. Both groups showed significant improvement in UDVA, CDVA, SE, and MRA between preoperative and postoperative intervals. Comparison of between-group performance showed no statistically significant differences (P = .39 and .79 for UDVA; P = .14 and .12 for CDVA; P = .28 and .36 for SE; and P = .08 and .08 for MRA, respectively, after 3 and 6 months). Figure 1 shows the efficacy of ICRS surgery by comparing postoperative UDVA and preoperative CDVA in both groups. In these respective graphics, 40% of patients in the Keraring group and 34% of patients in the Ferrara group showed a postoperative UDVA of 20/40 or better. Figure 2 shows the safety of ICRS surgery as the percentage of gain or loss of Snellen lines of CDVA. In these respective graphics, 90% of patients in the Keraring group and 72% of patients in the Ferrara group gained two or more lines of CDVA after 6 months. Figure 3 shows the stability of SE refraction between preoperative and postoperative intervals in both groups. The Keraring group showed less variation between postoperative intervals. Figure 4 shows the MRA before and after treatment. The preoperative MRA ranged from 4.01 to 5.00 D in both groups (28% in the Keraring group and 25% in the Ferrara group). An MRA of 2.00 D or less after treatment was present in 12% of patients preoperatively and 66% of patients postoperatively in the Keraring group, and 16% of patients preoperatively and 47% of patients postoperatively in the Ferrara group. An MRA of 1.00 D or less was present in 3% of patients preoperatively and 28% of patients postoperatively in the Keraring group and 0% of patients preoperatively and 13% of patients postoperatively in the Ferrara group. Both groups had 91% of patients presenting a postoperative MRA of 4.00 D or less, compared to 44% and 57% in the Keraring and Ferrara groups, respectively, before ICRS insertion.

Visual Acuity and Refraction Outcomes

Table 2:

Visual Acuity and Refraction Outcomes

Efficacy. Postoperative uncorrected distance visual acuity (UDVA) versus preoperative corrected distance visual acuity (CDVA) in the Keraring (Mediphacos, Belo Horizonte, Brazil) group (left) and in the Ferrara (AJL Ophthalmics, Vitoria, Spain) group (right).

Figure 1.

Efficacy. Postoperative uncorrected distance visual acuity (UDVA) versus preoperative corrected distance visual acuity (CDVA) in the Keraring (Mediphacos, Belo Horizonte, Brazil) group (left) and in the Ferrara (AJL Ophthalmics, Vitoria, Spain) group (right).

Safety. Change in Snellen lines of corrected distance visual acuity (CDVA) in the Keraring (Mediphacos, Belo Horizonte, Brazil) group (left) and in the Ferrara (AJL Ophthalmics, Vitoria, Spain) group (right).

Figure 2.

Safety. Change in Snellen lines of corrected distance visual acuity (CDVA) in the Keraring (Mediphacos, Belo Horizonte, Brazil) group (left) and in the Ferrara (AJL Ophthalmics, Vitoria, Spain) group (right).

Stability. Spherical equivalent refraction stability during postoperative intervals in the Keraring (Mediphacos, Belo Horizonte, Brazil) group (left) and in the Ferrara (AJL Ophthalmics, Vitoria, Spain) group (right).

Figure 3.

Stability. Spherical equivalent refraction stability during postoperative intervals in the Keraring (Mediphacos, Belo Horizonte, Brazil) group (left) and in the Ferrara (AJL Ophthalmics, Vitoria, Spain) group (right).

Manifest refractive astigmatism (MRA) distribution before and after treatment in the Keraring (Mediphacos, Belo Horizonte, Brazil) group (left) and in the Ferrara (AJL Ophthalmics, Vitoria, Spain) group (right). D = diopters

Figure 4.

Manifest refractive astigmatism (MRA) distribution before and after treatment in the Keraring (Mediphacos, Belo Horizonte, Brazil) group (left) and in the Ferrara (AJL Ophthalmics, Vitoria, Spain) group (right). D = diopters

Corneal Tomography

Table 3 shows the corneal tomography outcomes. Our results showed significant improvement in keratometry readings for both groups at both postoperative evaluations. Comparison between groups showed no statistically significant differences (P = .18 and .28 for flat keratometry; P = .22 and .29 for steep keratometry; P = .94 and .96 for mean keratometry; P = .33 and .71 for maximum keratometry; and P = .45 and .46 for Q 30º; respectively, after 3 and 6 months).

Corneal Tomography Outcomes

Table 3:

Corneal Tomography Outcomes

Corneal Aberrations and Optical Quality

Table A (available in the online version of this article) shows the corneal aberrometry outcomes. With a 5-mm pupil size, both groups showed the same performance. They presented a significant decrease in total aberrations, higher order aberrations, and total coma (P < .05 after 3 and 6 months); a significant increase in total spherical aberrations (P < .05 after 3 and 6 months); and showed no significant difference in total trefoil. Comparison between groups showed no significant differences in performance between preoperative and postoperative intervals (P = .64 and .88 for total aberrations; P = .91 and .88 for higher order aberrations; P = .83 and .41 for coma; P = .59 and .53 for trefoil; and P = .46 and .48 for spherical aberrations, respectively after 3 and 6 months).

Corneal Aberrometry Outcomes at 5- and 6-mm Pupil Diameters

Table A:

Corneal Aberrometry Outcomes at 5- and 6-mm Pupil Diameters

With a 6-mm pupil size, both groups showed a significant reduction in total aberrations and coma (P < .05) after 3 and 6 months. For higher order aberrations, the Keraring group showed a significant reduction after both postoperative intervals, whereas the Ferrara group had this performance only after 6 months (P = .12 after 3 months in the Ferrara group). For spherical aberrations, both groups showed a significant increase after 6 months, but only the Ferrara group had this result after 3 months (P = .13 after 3 months in the Keraring group). Comparison between groups showed no significant differences in performance between preoperative and postoperative intervals (P = .62 and .81 for total aberrations; P = .89 and .67 for higher order aberrations; P = .80 and .89 for coma; P = .91 and .93 for trefoil; and P = .47 and .66 for spherical aberrations, respectively, after 3 and 6 months).

We collected data from measured Strehl ratio and wavefront error in mesopic conditions using the optical quality maps. In both groups, both Strehl ratio and wavefront error significantly improved (P < .05) between preoperative and postoperative intervals, with no difference between groups (P = .31 and .85 for Strehl ratio; P = .28 and .80 for wavefront error, respectively, after 3 and 6 months).

Regarding total aberrations in the modulation transfer function graph, both groups showed a significant increase (P < .05) in area ratio, with no difference in between-group performance (P = .77 and .85, respectively, after 3 and 6 months). Higher order aberrations area ratio in the modulation transfer function graph significantly decreased (P < .05) in both groups after 3 and 6 months. Again, no difference occurred after between-group comparison (P = .88 and .97, respectively, after 3 and 6 months).

Vector Analysis of Refraction and Corneal Astigmatism Changes

Table B (available in the online version of this article) shows the changes in the vectors analyzed by the Alpins method. For dK, mean magnitude in TIA was 4.36 ± 1.38 and 3.92 ± 1.78, respectively, in the Keraring and Ferrara groups (P = .27). For MRA, the mean magnitude in TIA was 4.34 ± 1.88 and 3.95 ± 1.88, respectively, in the Keraring and Ferrara groups (P = .42).

Results of Vectors Analyzed With the Alpins Method

Table B:

Results of Vectors Analyzed With the Alpins Method

Regarding the mean magnitude in SIA, we found 6 overcorrections (18.75%) regarding dK in each group, and 11 (34.38%) and 10 (31.25%) overcorrections regarding MRA, respectively, in the Keraring and Ferrara groups. Comparison between groups showed a higher mean magnitude in SIA in the Keraring group for both dK and MRA, but there was no statistical significance (P = .20 for dK and .16 for MRA). Figure 5 shows the TIA versus SIA for dK in both groups, and Figure 6 shows the TIA versus SIA for MRA in both groups.

Target induced (TIA) versus surgically induced (SIA) astigmatism for corneal tomographic astigmatism (dK) in the Keraring (Mediphacos, Belo Horizonte, Brazil) group (left) and in the Ferrara (AJL Ophthalmics, Vitoria, Spain) group (right).

Figure 5.

Target induced (TIA) versus surgically induced (SIA) astigmatism for corneal tomographic astigmatism (dK) in the Keraring (Mediphacos, Belo Horizonte, Brazil) group (left) and in the Ferrara (AJL Ophthalmics, Vitoria, Spain) group (right).

Target induced (TIA) versus surgically induced (SIA) astigmatism for manifest refractive astigmatism (MRA) in the Keraring (Mediphacos, Belo Horizonte, Brazil) group (left) and in the Ferrara (AJL Ophthalmics, Vitoria, Spain) group (right). D = diopters

Figure 6.

Target induced (TIA) versus surgically induced (SIA) astigmatism for manifest refractive astigmatism (MRA) in the Keraring (Mediphacos, Belo Horizonte, Brazil) group (left) and in the Ferrara (AJL Ophthalmics, Vitoria, Spain) group (right). D = diopters

The mean magnitude of the difference vector was lower in the Keraring group for dK and MRA, but with no statistically significant difference between groups (P = .42 for dK and .39 for MRA).

From the relationship between studied vectors, five parameters were analyzed. First, the mean magnitude of correction index was lower than 1.00 in both groups as a result of the higher percentage of undercorrection, with no statistical difference between groups (P = .86 for dK and P = .39 for MRA). Second, the mean magnitude of the angle of error regarding dK was negative (achieved correction clockwise to the intended axis) in the Keraring group, and positive (achieved correction counterclockwise to the intended axis) in the Ferrara group, with no statistically significant difference between groups (P = .49). For MRA, the mean magnitude of the angle of error was negative in both groups, with no statistically significant difference between groups (P = .75). Third, the mean value in the index of success was closer to zero in both dK and MRA in the Keraring group, with no statistically significant difference between groups (P = .08 for dK and .09 for MRA). Fourth, the flattening effect component was higher in the Keraring group, with no statistically significant difference between groups (P = .10 for dK and .11 for MRA). Finally, the magnitude of the torque was always positive in both groups, both for dK and MRA, with no statistically significant difference between groups (P = .51 for dK and .47 for MRA).

Figure A (available in the online version of this article) shows the double-angle plots of the individual astigmatism types, providing an overview of the astigmatism magnitude (diopters – D) and axis (degrees) of each data point. The radius from the center of the plot to each point represents the magnitude of astigmatism. The preoperative centroid of dK in the Keraring group was 2.80 @ 83º ± 3.64 D, and the postoperative centroid was 1.30 @ 87º ± 2.28 D. In the Ferrara group, the preoperative centroid dK was 2.27 @ 98º ± 3.71 D, and the postoperative centroid was 1.46 @ 94º ± 2.54 D. As a result of dK improvement, the keratometric astigmatism centroid was 1.50 ± 1.36 D and 0.81 ± 1.17 D closer to zero after ICRS implantation, respectively, in the Keraring and Ferrara groups, with a statistically significant greater correction in the Keraring group (P = .03).

Preoperative (left) and 6-month postoperative (right) double-angle polar plot changes for corneal tomographic astigmatism (dK) in the Keraring (KR) (Mediphacos, Belo Horizonte, Brazil) group (superior) and in the Ferrara (FR) (AJL Ophthalmics, Vitoria, Spain) group (inferior). D = diopters

Figure A.

Preoperative (left) and 6-month postoperative (right) double-angle polar plot changes for corneal tomographic astigmatism (dK) in the Keraring (KR) (Mediphacos, Belo Horizonte, Brazil) group (superior) and in the Ferrara (FR) (AJL Ophthalmics, Vitoria, Spain) group (inferior). D = diopters

Discussion

This study found that ICRS implantation as a keratoconus surgical treatment not only effectively improves visual acuity and keratometry readings, as shown in previous studies,3–13 but also improves corneal aberrometry and optical quality, as wavefront summary and optical quality maps showed. Most studies on corneal aberrations after ICRS have shown no statistically significant difference between the preoperative and postoperative data. Vega-Estrada et al.24 published one study that evaluated the behavior of anterior corneal higher order aberrations according to the degree of visual impairment after ICRS implantation. They found that there was no statistically significant difference between the preoperative and postoperative period regarding corneal aberrometric data. However, the group of patients with the most advanced keratoconus showed the most substantial decrease in those variables. They concluded that the higher the preoperative root mean square value of the cornea, the higher the amount of reduction that can be achieved after ICRS implantation. Piñero et al.25 showed a non-significant decrease in corneal coma-like and higher order aberrations and reported coma reduction only in preoperative values greater than 3 µm. On the other hand, our results revealed that most of the aberrometric studied variables showed significant improvement between preoperative and postoperative intervals, except for trefoil, in both groups and in both pupil diameters. Moreover, both surgical planning strategies showed mostly the same pattern of corneal aberrometric data improvement, with little difference between them, and none of them was superior regarding aberrometric changes (P > .05 for all comparisons).

ICRS implantation aims to effectively reduce the excess of asphericity and keratometry readings in eyes with keratoconus by modifying the cornea to a more physiologic aspheric shape. Torquetti and Ferrara26 showed this relationship between average corneal asphericity decrease and visual improvement, and this led to a Q-based nomogram proposal.19 In their study, ICRS were placed in 135 eyes of 123 patients with keratoconus for a mean follow-up period of 16.46 months. They found that ICRS implantation significantly increased Q 30° (from −0.85 to −0.32) in all grades of keratoconus. We also found significant asphericity reduction after ICRS implantation, independent from the manufacturer's nomogram used. Thus, it seems that considering asphericity for surgical planning does not imply better corneal reshaping. This is supported by our findings in the Keraring group because Q 30° also showed significant improvement, with no statistical difference between groups. Therefore, the SE/dK-based nomogram also provides statistically significant corneal reshaping after ICRS insertion. Another theoretical advantage of the Q-based nomogram is the implantation of less thick ICRS to achieve the expected outcome with fewer complications such as ICRS extrusion.19 Based on this nomogram, no 300-µm ICRS are used and, in few cases, 250-µm ICRS are temporally implanted, whereas in the SE/dK-based nomogram, the higher the magnitude of spherical component and MRA or dK, the higher the indicated thickness of the implant. This was showed in our results when 24 eyes (75%) received a 300- or 250-µm ICRS temporally in the Keraring group and 4 eyes (12.50%) received 250-µm and 19 eyes (59.37%) received a 200-µm ICRS temporally in the Ferrara group. With the use of a pair of 160° ICRS, a more significant flattening effect on keratometry was expected when compared to a single 160° ICRS associated with a 90° or 120° ICRS, but the difference in the choice of ICRS thicknesses probably offset this difference, because both groups showed no statistically significant difference between their performance on keratometry readings improvement.

Another essential analysis should be considered for dK and MRA improvement after ICRS implantation. The astigmatic improvement between two intervals must be calculated by vector analysis.20–23 When the cylinder axis is treated as being independent of cylinder power or is ignored, the real difference in power and direction of astigmatism is masked, even for surgical outcomes,20 and errors in the correction of the astigmatic axis could induce aberrations and lead to poor predictability of the spherocylindrical correction.25 We decided to analyze dK and MRA changes because these are important findings associated with the use of ICRS. However, MRA can be biased by the variability in focus caused by commonly increased values of higher order aberrations in keratoconic eyes.25 In the other hand, vectorial changes in dK are an objective variable, and, differently from MRA, cannot be biased by this variability. Ruckhofer et al.27 reported preliminary results of ICRS implantation using 130° ICRS for astigmatism correction in patients with healthy corneas. Sandes et al.28 reported results concerning the implantation of pairs of ICRS with 140° of arc in patients with keratoconus. Both studies showed that shorter ICRS arc lengths produce less central corneal flattening and increase corneal toricity, with good improvement in astigmatism correction. In our research, the Keraring group involved implantation of 160° ICRS alone or combined with 120° or 90° ICRS, whereas the Ferrara group involved implantation of paired 160° ICRS, 160° ICRS alone, or 210° ICRS alone.

The purpose of using shorter ICRS arc lengths (120° or 90° ICRS) is to increase astigmatic correction. Using the double-angle plot,23 the centroid of dK was 1.50 ± 1.36 D closer to zero (53.57% of decrease) in the Keraring group and 0.81 ± 1.17 D closer to zero (35.68% of reduction) in the Ferrara group (P = .03). Therefore, the use of shorter arc segments combined with 160° ICRS promoted a superior result in terms of correction of dK when compared to single 160° or 210° ICRS, or a pair of 160° ICRS, following the literature published on shorter arc ICRS.27,28 Nevertheless, this superior improvement in astigmatic correction did not result in a better visual outcome, as both groups had no statistically significant differences in UDVA and CDVA after 3 or 6 months postoperatively (P > .05 in all comparisons). However, the Keraring group showed higher efficacy, safety, and stability, as previously demonstrated.

Using the Alpins method20,21 for vectorial analysis of astigmatic changes brings other significant results for manufacturers' nomograms comparison. As previously shown in another study,25 TIA and SIA are not coincident after ICRS insertion, with lower values for SIA representing an undercorrection for dK and MRA. This is an expected result because ICRS surgery for keratoconus is not a refractive surgery, but a procedure to reduce corneal irregularity and improve visual acuity. Moreover, Piñero et al.25 discussed the higher values of difference vector as a poor predictor of nomogram effectiveness. Using their mean values of TIA, SIA, and difference vector, we are able to calculate two essential indexes: correction index, the ratio of the SIA to the TIA, resulting in 0.73, and index of success, the rate of the difference vector to the TIA, resulting in 0.55. In our study, we found similar mean values regarding dK change, with a mean value of 0.65 and 0.73 for correction index and 0.54 and 0.74 for index of success, respectively, in the Keraring and Ferrara groups. As shown, this low predictability of nomograms is related to the use of refractive and topographic parameters only, in ICRS nomograms.25

The flattening effect and torque vectors were also studied. The flattening effect vector represents the amount of astigmatism reduction achieved by the effective proportion of the SIA at the intended meridian, and the torque vector represents the amount of astigmatic change induced by the SIA because of non-alignment of the treatment that is ineffective in reducing astigmatism at the intended meridian.20 Regarding the flattening effect, in a situation of perfect treatment, SIA and flattening effect would be equal. Although this did not occur, the mean magnitude in the Keraring group was closer to SIA than in the Ferrara group for both dK and MRA, but this difference in performance showed no statistical significance. Regarding the torque vector, if the treatment were valid, it would have been zero. However, the mean value of the torque vector was closer to 1.00 in both groups, and in both studied astigmatism types, with no statistically significant difference between them.

Both manufacturers' nomograms resulted in statistically significant improvement in all visual and corneal tomographic analyzed parameters, with more substantial correction of vectorial dK in the SE/dK-based nomogram group, according to the double-angle plot analysis. Also, we found better performance in the Keraring group using the Alpins method for MRA and dK vectorial changes, but these were not statistically significant.

References

  1. Rabinowitz YS. Keratoconus. Surv Ophthalmol. 1998;42(4):297–319. https://doi.org/10.1016/S0039-6257(97)00119-7 PMID: doi:10.1016/S0039-6257(97)00119-7 [CrossRef]9493273
  2. Zadnik K, Barr JT, Edrington TB, et al. Baseline findings in the Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study. Invest Ophthalmol Vis Sci. 1998;39(13):2537–2546. PMID:9856763
  3. Coskunseven E, Kymionis GD, Tsiklis NS, et al. One-year results of intrastromal corneal ring segment implantation (KeraRing) using femtosecond laser in patients with keratoconus. Am J Ophthalmol. 2008;145(5):775–779. https://doi.org/10.1016/j.ajo.2007.12.022 PMID: doi:10.1016/j.ajo.2007.12.022 [CrossRef]18291344
  4. Ertan A, Kamburoglu G. Intacs implantation using a femtosecond laser for management of keratoconus: comparison of 306 cases in different stages. J Cataract Refract Surg. 2008;34(9):1521–1526. https://doi.org/10.1016/j.jcrs.2008.05.028 PMID: doi:10.1016/j.jcrs.2008.05.028 [CrossRef]18721713
  5. Shabayek MH, Alió JL. Intrastromal corneal ring segment implantation by femtosecond laser for keratoconus correction. Ophthalmology. 2007;114(9):1643–1652. https://doi.org/10.1016/j.ophtha.2006.11.033 PMID: doi:10.1016/j.ophtha.2006.11.033 [CrossRef]17400293
  6. Kymionis GD, Siganos CS, Tsiklis NS, et al. Long-term follow-up of Intacs in keratoconus. Am J Ophthalmol. 2007;143(2):236–244. https://doi.org/10.1016/j.ajo.2006.10.041 PMID: doi:10.1016/j.ajo.2006.10.041 [CrossRef]
  7. Alió JL, Shabayek MH, Artola A. Intracorneal ring segments for keratoconus correction: long-term follow-up. J Cataract Refract Surg. 2006;32(6):978–985. https://doi.org/10.1016/j.jcrs.2006.02.044 PMID: doi:10.1016/j.jcrs.2006.02.044 [CrossRef]16814056
  8. Colin J. European clinical evaluation: use of Intacs for the treatment of keratoconus. J Cataract Refract Surg. 2006;32(5):747–755. https://doi.org/10.1016/j.jcrs.2006.01.064 PMID: doi:10.1016/j.jcrs.2006.01.064 [CrossRef]16765790
  9. Kanellopoulos AJ, Pe LH, Perry HD, Donnenfeld ED. Modified intracorneal ring segment implantations (INTACS) for the management of moderate to advanced keratoconus: efficacy and complications. Cornea. 2006;25(1):29–33. https://doi.org/10.1097/01.ico.0000167883.63266.60 PMID: doi:10.1097/01.ico.0000167883.63266.60 [CrossRef]
  10. Colin J, Malet FJ. Intacs for the correction of keratoconus: two-year follow-up. J Cataract Refract Surg. 2007;33(1):69–74. https://doi.org/10.1016/j.jcrs.2006.08.057 PMID: doi:10.1016/j.jcrs.2006.08.057 [CrossRef]
  11. Bourcier T, Borderie V, Laroche L. Late bacterial keratitis after implantation of intrastromal corneal ring segments. J Cataract Refract Surg. 2003;29(2):407–409. https://doi.org/10.1016/S0886-3350(02)01484-0 PMID: doi:10.1016/S0886-3350(02)01484-0 [CrossRef]12648660
  12. Alió JL, Shabayek MH, Belda JI, Correas P, Feijoo ED. Analysis of results related to good and bad outcomes of Intacs implantation for keratoconus correction. J Cataract Refract Surg. 2006;32(5):756–761. https://doi.org/10.1016/j.jcrs.2006.02.012 PMID: doi:10.1016/j.jcrs.2006.02.012 [CrossRef]16765791
  13. Rabinowitz YS. INTACS for Keratoconus. Int Ophthalmol Clin. 2010;50(3):63–76. https://doi.org/10.1097/IIO.0b013e3181e21b76 PMID: doi:10.1097/IIO.0b013e3181e21b76 [CrossRef]20611018
  14. Yousif OM, Said AMA. Comparative study of 3 intracorneal implant types to manage central keratoconus. J Cataract Refract Surg. 2018;44(3):295–305 https://doi.org/10.1016/j.jcrs.2017.12.020 doi:10.1016/j.jcrs.2017.12.020 [CrossRef]29610025
  15. Hashemian MN, Zare MA, Mohammadpour M, Rahimi F, Fallah MR, Panah FK. Outcomes of single segment implantation of conventional Intacs versus Intacs SK for keratoconus. J Ophthalmic Vis Res. 2014;9(3):305–309. PMID:25667729
  16. Haddad W, Fadlallah A, Dirani A, et al. Comparison of 2 types of intrastromal corneal ring segments for keratoconus. J Cataract Refract Surg. 2012;38(7):1214–1221. https://doi.org/10.1016/j.jcrs.2012.02.039 PMID: doi:10.1016/j.jcrs.2012.02.039 [CrossRef]22727290
  17. Kubaloglu A, Cinar Y, Sari ES, Koytak A, Ozdemir B, Ozertürk Y. Comparison of 2 intrastromal corneal ring segment models in the management of keratoconus. J Cataract Refract Surg. 2010;36(6):978–985. https://doi.org/10.1016/j.jcrs.2009.12.031 PMID: doi:10.1016/j.jcrs.2009.12.031 [CrossRef]20494770
  18. Kiliç A, Alió J, Vega Estrada A. Intracorneal ring segments: types, indications and outcomes. In: Alió JL, ed. Keratoconus: Recent Advances in Diagnosis and Treatment. New York: Springer International Publishing; 195–208.
  19. Ferrara P, Torquetti L. The new Ferrara ring nomogram: the importance of corneal asphericity in ring selection. Vision Pan-America. 2010:92–95.
  20. Alpins N. Astigmatism analysis by the Alpins method. J Cataract Refract Surg. 2001;27(1):31–49. https://doi.org/10.1016/S0886-3350(00)00798-7 PMID: doi:10.1016/S0886-3350(00)00798-7 [CrossRef]11165856
  21. Alpins NA. A new method of analyzing vectors for changes in astigmatism. J Cataract Refract Surg. 1993;19(4):524–533. https://doi.org/10.1016/S0886-3350(13)80617-7 PMID: doi:10.1016/S0886-3350(13)80617-7 [CrossRef]8355160
  22. Holladay JT, Dudeja DR, Koch DD. Evaluating and reporting astigmatism for individual and aggregate data. J Cataract Refract Surg. 1998;24(1):57–65. https://doi.org/10.1016/S0886-3350(98)80075-8 PMID: doi:10.1016/S0886-3350(98)80075-8 [CrossRef]9494900
  23. Abulafia A, Koch DD, Holladay JT, Wang L, Hill WE. Pursuing perfection in intraocular lens calculations: IV. Rethinking astigmatism analysis for intraocular lens-based surgery: Suggested terminology, analysis, and standards for outcome reports. J Cataract Refract Surg. 2018;44(10):1169–1174. https://doi.org/10.1016/j.jcrs.2018.07.027 PMID: doi:10.1016/j.jcrs.2018.07.027 [CrossRef]30243391
  24. Vega-Estrada A, Alio JL, Brenner LF, et al. Outcome analysis of intracorneal ring segments for the treatment of keratoconus based on visual, refractive, and aberrometric impairment. Am J Ophthalmol. 2013;155(3):575–584.e1. https://doi.org/10.1016/j.ajo.2012.08.020 PMID: doi:10.1016/j.ajo.2012.08.020 [CrossRef]
  25. Piñero DP, Alió JL, Teus MA, Barraquer RI, Michael R, Jiménez R. Modification and refinement of astigmatism in keratoconic eyes with intrastromal corneal ring segments. J Cataract Refract Surg. 2010;36(9):1562–1572. https://doi.org/10.1016/j.jcrs.2010.04.029 PMID: doi:10.1016/j.jcrs.2010.04.029 [CrossRef]20692571
  26. Torquetti L, Ferrara P. Corneal asphericity changes after implantation of intrastromal corneal ring segments in keratoconus. J Emmetropia. 2010;1:178–181.
  27. Ruckhofer J, Stoiber J, Twa MD, Grabner G. Correction of astigmatism with short arc-length intrastromal corneal ring segments: preliminary results. Ophthalmology. 2003;110(3):516–524. https://doi.org/10.1016/S0161-6420(02)01773-6 PMID: doi:10.1016/S0161-6420(02)01773-6 [CrossRef]12623814
  28. Sandes J, Stival LRS, de Ávila MP, et al. Clinical outcomes after implantation of a new intrastromal corneal ring with 140-degree of arc in patients with corneal ectasia. Int J Ophthalmol. 2018;11(5):802–806. https://doi.org/10.18240/ijo.2018.05.14 PMID:29862179

Preoperative Data

VariableKR GroupFR GroupP
Ophthalmological examination
  UDVA (logMAR)1.14 ± 0.491.13 ± 0.50.91
  CDVA (logMAR)0.45 ± 0.220.40 ± 0.20.46
  SE (D)−6.45 ± 4.42−6.09 ± 3.99.56
Corneal tomography
  K1 (D)47.29 ± 3.1147.54 ± 3.06.62
  K2 (D)51.63 ± 3.5251.46 ± 3.31.80
  Km (D)49.36 ± 3.2249.41 ± 3.06.83
  dK (D)4.36 ± 1.383.92 ± 1.78.25
  Kmax (D)57.18 ± 4.7356.30 ± 4.30.41
  Q 30º−0.87 ± 0.35−0.81 ± 0.28.41
5-mm corneal aberrometry
  Total (µm)8.13 ± 3.497.20 ± 3.04.27
  HOA (µm)2.47 ± 1.032.21 ± 1.02.25
  Coma (µm)2.23 ± 0.972.02 ± 0.97.34
  Trefoil (µm)0.74 ± 0.360.65 ± 0.42.27
  Spherical (µm)0.46 ± 0.420.30 ± 0.29.10
6-mm corneal aberrometry
  Total (µm)13.10 ± 5.7211.53 ± 4.92.21
  HOA (µm)3.82 ± 1.553.59 ± 1.55.48
  Coma (µm)3.45 ± 1.453.31 ± 1.45.67
  Trefoil (µm)1.06 ± 0.530.96 ± 0.54.32
  Spherical (µm)0.77 ± 0.740.49 ± 0.51.08

Visual Acuity and Refraction Outcomes

VariablePreoperative3 Months PostoperativePaPb6 Months PostoperativePaPc
UDVA (logMAR)
  KR1.14 ± 0.490.75 ± 0.38< .05.390.70 ± 0.39< .05.79
  FR1.13 ± 0.500.82 ± 0.52< .050.71 ± 0.41< .05
CDVA (logMAR)
  KR0.45 ± 0.220.13 ± 0.13< .05.140.12 ± 0.11< .05.12
  FR0.40 ± 0.200.15 ± 0.12< .050.14 ± 0.12< .05
SE (D)
  KR−6.45 ± 4.42−4.09 ± 3.05< .05.28−3.94 ± 2.98< .05.36
  FR−6.09 ± 3.99−4.61 ± 3.73< .05−4.25 ± 3.25< .05
MRA (D)
  KR4.34 ± 1.882.01 ± 1.41< .05.081.99 ± 1.38< .05.08
  FR3.95 ± 1.882.42 ± 1.59< .052.27 ± 1.23< .05

Corneal Tomography Outcomes

VariablePreoperative3 Months PostoperativePaPb6 Months PostoperativePaPc
K1 (D)
  KR47.29 ± 3.1146.44 ± 2.62< .05.1846.49 ± 2.61< .05.28
  FR47.54 ± 3.0646.35 ± 2.74< .0546.44 ± 2.69< .05
K2 (D)
  KR51.63 ± 3.5248.63 ± 3.14< .05.2248.79 ± 2.90< .05.29
  FR51.46 ± 3.3148.88 ± 3.16< .0549.00 ± 3.21< .05
Km (D)
  KR49.36 ± 3.2247.50 ± 2.80< .05.9447.61 ± 2.68< .05.96
  FR49.41 ± 3.0647.57 ± 2.86< .0547.68 ± 2.86< .05
Kmax (D)
  KR57.18 ± 4.7354.87 ± 4.67< .05.3355.15 ± 4.61< .05.71
  FR56.30 ± 4.3054.68 ± 5.09< .0554.58 ± 5.18< .05
Q 30º
  KR−0.87 ± 0.35−0.47 ± 0.33< .05.45−0.51 ± 0.28< .05.46
  FR−0.81 ± 0.28−0.38 ± 0.28< .05−0.41 ± 0.23< .05

Corneal Aberrometry Outcomes at 5- and 6-mm Pupil Diameters

VariablePreoperative3 Months PostoperativePaPb6 Months PostoperativePaPc
5-mm
  RMS TOT (µm)
    KR8.13 ± 3.496.76 ± 3.09< .05.646.97 ± 2.82< .05.88
    FR7.20 ± 3.045.99 ± 2.28< .055.97 ± 2.19< .05
  RMS HOA (µm)
    KR2.47 ± 1.032.20 ± 0.75< .05.912.15 ± 0.66< .05.88
    FR2.21 ± 1.021.96 ± 0.77< .051.87 ± 0.68< .05
  RMS coma (µm)
    KR2.23 ± 0.971.81 ± 0.70< .05.831.69 ± 0.69< .05.41
    FR2.02 ± 0.971.63 ± 0.67< .051.62 ± 0.64< .05
  RMS trefoil (µm)
    KR0.74 ± 0.360.71 ± 0.37.70.590.68 ± 0.32.35.53
    FR0.65 ± 0.420.58 ± 0.41.200.54 ± 0.31.13
  RMS sphere (µm)
    KR0.46 ± 0.420.62 ± 0.46< .05.460.69 ± 0.51< .05.48
    FR0.30 ± 0.290.51 ± 0.37< .050.49 ± 0.37< .05
6-mm
  RMS TOT (µm)
    KR13.10 ± 5.7211.29 ± 5.08< .05.6211.57 ± 4.69< .05.81
    FR11.53 ± 4.9210.01 ± 3.80< .059.83 ± 3.61< .05
  RMS HOA (µm)
    KR3.82 ± 1.553.46 ± 1.24< .05.893.39 ± 1.14< .05.67
    FR3.59 ± 1.553.27 ± 1.49.123.05 ± 1.10< .05
  RMS coma (µm)
    KR3.45 ± 1.452.88 ± 1.16< .05.802.74 ± 1.18< .05.89
    FR3.31 ± 1.452.79 ± 1.10< .052.64 ± 0.99< .05
  RMS trefoil (µm)
    KR1.06 ± 0.531.00 ± 0.54.54.910.94 ± 0.49.20.93
    FR0.96 ± 0.540.92 ± 0.89.730.86 ± 0.56.35
  RMS sphere (µm)
    KR0.77 ± 0.740.88 ± 0.74.13.470.95 ± 0.79< .05.66
    FR0.49 ± 0.510.67 ± 0.53< .050.72 ± 0.62< .05

Results of Vectors Analyzed With the Alpins Method

VariableKRRangeFRRangeP
Corneal astigmatism
  TIA (D)4.36 ± 1.381.30 to 7.803.92 ± 1.780.60 to 8.80.27
  SIA (D)2.73 ± 1.320.54 to 5.602.33 ± 1.100.23 to 4.52.20
  DV (D)2.30 ± 1.210.10 to 4.802.56 ± 1.340.70 to 6.50.42
  CIa0.65 ± 0.310.15 to 1.290.73 ± 0.560.08 to 2.69.86
  AE (degrees)−1.78 ± 13.20−28.00 to 32.001.88 ± 26.64−75.00 to 81.00.49
  IOSb0.54 ± 0.230.02 to 0.920.74 ± 0.410.23 to 2.00.08
  Flattening effect (D)2.49 ± 1.320.45 to 5.601.89 ± 1.54−2.74 to 4.38.10
  Torque (D)0.82 ± 0.750.00 to 2.950.72 ± 0.510.07 to 1.94.51
Manifest refractive astigmatism
  TIA (D)4.34 ± 1.881.00 to 10.003.95 ± 1.881.25 to 9.50.42
  SIA (D)3.49 ± 1.910.98 to 10.022.89 ± 1.400.82 to 5.79.16
  DV (D)1.99 ± 1.380.50 to 6.252.27 ± 1.230.00 to 5.50.39
  CIa0.86 ± 0.420.30 to 2.180.88 ± 0.720.27 to 3.95.39
  AE (degrees)−1.66 ± 11.49−22.00 to 29.00−0.63 ± 13.81−26.00 to 30.00.75
  IOSb0.49 ± 0.260.11 to 1.250.66 ± 0.510.00 to 3.00.09
  Flattening effect (D)3.27 ± 1.780.52 to 8.112.63 ± 1.400.60 to 5.26.11
  Torque (D)0.87 ± 1.120.07 to 5.890.92 ± 0.790.00 to 2.95.47
Authors

From the Department of Refractive Surgery, Cornea and Contact Lens Sector, Brasilia Ophthalmological Hospital; Brasilia, Brazil (GR, LNPS); Private Clinic, Goiânia, Brazil (LFOBC); Private Clinic, Brasilia, Brazil (PB); Private Clinic, Pará de Minas, Brazil (LT); and Federal University of São Paulo (UNIFESP/EPM); São Paulo; Brazil (LBD).

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

AUTHOR CONTRIBUTIONS

Study concept and design (GR, LT, LBS); data collection (GR, LNPS, LFOBC, PB, LT); analysis and interpretation of data (GR, PB, LT, LBS); writing the manuscript (GR, LNPS, LFOBC, PB, LT); critical revision of the manuscript (PB, LT, LBS); statistical expertise (GR, PB, LT); supervision (LBS)

Correspondence: Guilherme Rocha, MD, Brasilia Ophthalmological Hospital; Av. L2 Sul SGAS 607 Módulo G, Brasilia 70.200-670, Brazil. E-mail: drguilhermerocha@hotmail.com

Received: March 28, 2019
Accepted: September 16, 2019

10.3928/1081597X-20190916-01

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