dr. vedat kaya (makale)

1Journal of Refractive Surgery • Vol. xx, No. x, 2011

O R I G I N A L A R T I C L E

ntrastromal corneal ring segments (ICRS) have been implanted to delay or prevent the need for penetrating keratoplasty and to achieve refractive correction with

improvement in visual acuity and quality in corneal ectatic diseases.1-14 Intacs (Addition Technology Inc, Sunnyvale, California) and Ferrara ICRS (Ferrara Ophthalmics Ltda, Belo Horizonte, Brazil) aim to improve geometry of the corneal sur-face and modulation of refractive effect with their unique de-signs, internal and external diameters, thicknesses, and arc lengths.2,12

Topographic and refractive results of ICRS implantation with different designs have been compared previously.15,16 Because the aim of ICRS implantation is visual rehabilitation, postop-erative visual quality is as important as topographic, refractive, and visual acuity outcomes. Improved lower and higher order aberrations and uncorrected and corrected distance visual acu-ities (UDVA and CDVA, respectively) after Intacs implantation have been reported.13 In our study, a comparative evaluation of Intacs and Ferrara ICRS implantations was performed in terms of visual acuity, refraction, topography, objective ocular opti-cal quality (ie, wavefront aberrations), and subjective visual quality (ie, contrast sensitivity function).

PATIENTS AND METHODSIn this retrospective, comparative study, eyes implanted

with Intacs ICRS (Intacs group) between January 2008 and January 2009 and Ferrara ICRS (Ferrara group) between May

IABSTRACT

PURPOSE: To evaluate the refractive, topographic, optical, visual acuity, and quality outcomes of two types of intrastromal corneal ring segment (ICRS) implanta-tions in keratoconus.

METHODS: In this retrospective study, 16 eyes that had been implanted with Intacs ICRS (Addition Technology Inc) (Intacs group) and 17 eyes implanted with Ferrara ICRS (Ferrara Ophthalmics Ltda) (Ferrara group) were evaluated. Pre- and postoperative examinations includ-ed uncorrected and corrected distance visual acuities (UDVA and CDVA, respectively), manifest refraction, slit-lamp examination, topography, and ocular wavefront analysis. Contrast sensitivity measurements under phot-opic, scotopic, and scotopic with glare conditions were performed 1 year postoperatively.

RESULTS: One year postoperatively, a signifi cant de-crease was noted in spherical equivalent refractive error of 3.76�0.39 diopters (D) and 3.42�0.88 D and keratometry of 3.43�0.24 D and 3.28�0.78 D in the Intacs and Ferrara groups, respectively; and increase in mean UDVA and CDVA in Snellen lines of 0.18�0.04 and 0.21�0.05, respectively, in the Intacs group and 0.21�0.09 and 0.26�0.08, respectively, in the Ferrara group (P�.01 for all). The postoperative increase in UDVA and CDVA and decrease in keratometry readings were not signifi cantly different between groups (P�.05 for all). Mean higher order aberrations decreased in the Intacs group and increased in the Ferrara group (P�.05 for both). Postoperatively, a signifi cant decrease was noted in scotopic contrast sensitivity when glare was in-troduced in the Ferrara group, which was positively cor-related with pupil diameter (r(15)=0.50, P=.04).

CONCLUSIONS: Both ICRS types provided comparable refractive, topographic, and optical quality outcomes. Eyes with Ferrara ICRS experienced greater decrease in scotopic contrast sensitivity under glare, which was sig-nifi cantly correlated with pupil diameter. [J Refract Surg. 2011;xx(x):xxx-xxx.] doi:10.3928/1081597X-2011

From Beyoglu Eye Research and Training Hospital (Kaya, Karakus, Kavadarli, Yilmaz); and Yeditepe University, Department of Ophthalmology (Utine), Istanbul, Turkey.

The authors have no financial or proprietary interest in the materials pre-sented herein.

This study was presented at the 44th Turkish Ophthalmology Society national meeting, September 29 - October 3, 2010, Antalya, Turkey.

Correspondence: Canan Asli Utine, MD, MSc, Yeditepe University, Dept of Ophthalmology, Gazi Umur Pasa sok. No: 28, Besiktas Balmumcu 34345 Istanbul Turkey. Tel: 90 533 5587635; Fax: 90 212 2112500; E-mail: [email protected]

Received: February 8, 2011; Accepted: July 18, 2011

Posted online:

Refractive and Visual Outcomes After Intacs vs Ferrara Intrastromal Corneal Ring Segment Implantation for Keratoconus: A Comparative StudyVedat Kaya, MD; Canan Asli Utine, MD, MSc; Sezen Harmanci Karakus, MD; Isilay Kavadarli, MD; Ömer Faruk Yilmaz, MD

2 Copyright © SLACK Incorporated

Intacs vs Ferrara Intrastromal Corneal Ring Segments/Kaya et al

2008 and December 2008 for visual rehabilitation of keratoconus were included. In all patients, diagnosis of keratoconus was based on corneal topography and slit-lamp observation (asymmetric bowtie pattern with or without skewed axes, keratometry �47.00 D, pres-ence of stromal thinning, conical protrusion of the cor-neal apex, Fleischer ring, Vogt striae, or anterior stro-mal scar). Nine eyes with grade 2 and 7 eyes with grade 3 keratoconus comprised the Intacs group, and 11 eyes with grade 2 and 6 eyes with grade 3 keratoconus com-prised the Ferrara group, which were classifi ed by cri-teria described by Krumeich et al.17

All surgeries were performed at Yeditepe Univer-sity, and 1-year postoperative examinations were per-formed at Beyoglu Eye Research and Training Hospi-tal, Istanbul, Turkey, by the same surgeon (V.K.). The study adhered to the tenets of the Declaration of Hel-sinki and written informed consent was obtained from all participants before examination.

Pre- and 1-year postoperative examinations included UDVA, CDVA, manifest refraction, slit-lamp examina-tion, corneal topography, and ocular wavefront analy-sis. At 1 year, contrast sensitivity measurements under photopic and scotopic with and without glare condi-tions were also performed.

All visual acuity measurements were completed using an Optec 6500P Vision Tester (Stereo Optical, Chicago, Illiniois), and results were recorded as total number of identifi ed letters and corresponding visual acuities in Snellen lines. Visual acuity testing was performed at a standardized luminance level of 85 and 3.0 cd/m2 (photopic and scotopic lighting, respec-tively). Corneal topographies were performed using the Orbscan II (Bausch & Lomb, Salt Lake City, Utah). Ocular wavefront analysis measurements were per-formed using the Ocular Wavefront Analyzer (COAS) (SCHWIND eye-tech-solutions GmbH & Co.KG, Kleinostheim, Germany) under scotopic conditions. Total and higher order ocular aberrations (μm) were recorded. Pupil diameters were measured under sco-topic conditions, using a handheld infrared Colvard pupillometer (Oasis Medical Inc, Glendora, Califor-nia), with the fellow eye occluded. Contrast sensitivity measurements with and without glare conditions were performed with the Functional Acuity Contrast Test (FACT) in both photopic and scotopic conditions us-ing the Optec 6500P. The average score in each spatial frequency test was recorded.

INTRACORNEAL RING SEGMENTSIntacs ICRS consist of a pair of semicircular ring

segments of hexagonal transverse shape and circum-ference arc length of 150°. Each segment has an external

diameter of 8.10 mm, an internal diameter of 6.77 mm, and variable thickness of 0.25 to 0.45 mm in 0.05-mm increments, which allows modulation of the refrac-tive effect.18

Ferrara ICRS have a triangular cross-section that theoretically induces a prismatic effect to reduce pho-tic phenomena.9 Circumference arc lengths of 90, 120, 160, and 210° are available. Each segment has an internal diameter of 4.40 mm, external diameter of 5.60 mm, and variable thickness of 0.15 to 0.35 mm in 0.05-mm increments.

SURGICAL TECHNIQUEAll surgical procedures were performed under top-

ical anesthesia, with the fi rst Purkinje refl ex marked as the corneal central point. A 60-kHz femtosecond laser (IntraLase; Abbott Medical Optics, Santa Ana, California) was used to create intrastromal channels for the intrastromal corneal ring with the following settings: entry cut thickness 1 μm, ring energy 1.50 to 2.50 μJ, and entry cut energy 1.50 μJ. In the Intacs group, a 10/0 monofi lament nylon suture was placed at the incision site, which was removed 1 month post-operatively. Postoperative treatment included topical ofl oxacin 0.3% (Exocin; Allergan, Mougins, France) and prednisolone acetate 1% (Predforte, Allergan) four times daily for 2 weeks; the latter was then tapered over 4 weeks. Preservative-free artifi cial tear substi-tute (Refresh Plus; Allergan, Irvine, California) was used as needed.

Thickness and degree of arc of ICRS were selected and their location was planned according to cone loca-tion on axial topography for the specifi c nomograms of each ICRS. In the Intacs group, implantation was performed according to the International Pre-Surgical Planning Guide and Comprehensive Nomogram forIntacs, May 2008 (http://www.lasermed.com.tr/nomogramlar/Nomogram_2008_intl_053008.pdf. Accessed September 2, 2011). In the Ferrara group, implantation was performed according to the nomo-gram described previously.11 Both types of ICRS were implanted in intracorneal channels from the incision placed on the steepest meridian of the cornea. For implantation of Intacs, if the cone was located sym-metrically, the incision site was placed on the posi-tive cylindrical axis of manifest refraction for CDVA. If the cone was located asymmetrically, the incision site was verifi ed by using the manifest refraction yield-ing CDVA, posterior fl oat in Orbscan topography, and peripheral fl attening and keratomety values. For im-plantation of the Ferrara ICRS, the steepest meridian of the anterior corneal surface was defi ned as the incision site per the nomogram.



Intracorneal channel depth was determined by the pachymetric map on Orbscan topography in both groups as 70% of the thinnest pachymetric reading in the 3.4-mm diameter zone in the Intacs group and the 2.4-mm diameter zone in the Ferrara group, with the origin as the fi xation point. Inner and outer diameters of the intracorneal channels were 6.8 and 7.8 mm, re-spectively, in the Intacs group and 4.8 and 5.6 mm, respectively, in the Ferrara group.

STATISTICAL ANALYSISStatistical analysis was performed using the Statis-

tical Package for Social Sciences (SPSS Inc, Chicago, Illinois), version 15.0. The Kolmogorov-Smirnov test was used for normality of distribution of each parameter. A Mann-Whitney U test was conducted to compare stages of keratoconus in both groups. If data were normally distributed, paired-samples t test was performed to compare pre- and postoperative fi nd-ings in each group. An independent-samples t test was performed to compare pre- and postoperative UDVA, CDVA, keratometry, spherical, cylindrical and spheri-cal equivalent manifest refractive errors, pupil diam-eters, total and higher order wavefront aberrations, and postoperative contrast sensitivity scores in both groups. Effi cacy index was defi ned as mean postop-erative UDVA divided by mean preoperative CDVA. Safety index was defi ned as mean postoperative CDVA

divided by mean preoperative CDVA. Correlations be-tween preoperative maximum keratometry and post-operative gain in UDVA and CDVA, and between pupil diameter and postoperative gain in contrast sensitivity function at photopic and scotopic conditions with or without glare were studied using Pearson’s correlation coeffi cient (r) if variables were normally distributed. A P value �.05 was considered statistically signifi cant.

RESULTSIn the Intacs group, 16 eyes of 11 patients (4 men and

7 women) were implanted and 17 eyes of 11 patients (7 men and 4 women) were implanted in the Ferrara group. Mean patient age was 23.0�2.7 and 24.2±3.9 years in the Intacs and Ferrara groups, respectively. Mean preoperative pupil diameter was 5.99�0.39 and 6.16�0.50 mm in the Intacs and Ferrara groups, respec-tively (P=.30). No signifi cant difference in stages of kera-toconus was detected between groups (U=78.50, P=.23).

All eyes in the Intacs group and all but three eyes

TABLE 1

Preoperative Characteristics of 22 Patients Implanted With Intacs

or Ferrara Intrastromal Corneal Ring SegmentsMean�Standard Deviation

Intacs ICRS Ferrara ICRS P Value

Spherical refractive error (D)

�4.16�0.89 �3.95�1.41 .63

Cylindrical refractive error (D)

�3.98�1.32 �3.54�1.26 .34

SEQ (D) �6.15�1.16 �5.39 �2.11 .21

Maximum K (D) 52.54�3.48 51.95�3.78 .64

UDVA (Snellen) 0.18�0.11 0.20�0.14 .72

CDVA (Snellen) 0.34�0.14 0.34�0.21 .97

Total aberrations (µm)

6.11�1.40 5.99�1.80 .84

HOA (µm) 1.88�0.45 1.77�0.51 .49

ICRS = intrastromal corneal ring segments, SEQ = spherical equivalent refraction, K = keratometry, UDVA = uncorrected distance visual acuity, CDVA = corrected distance visual acuity, HOA = higher order aberrations

TABLE 2

Postoperative Characteristics of 22 Patients Implanted With Intacs

or Ferrara Intrastromal Corneal Ring Segments

Mean�Standard Deviation

Intacs ICRS Ferrara ICRS P Value

Spherical refractive error (D)

�2.18�1.11 �1.81�2.11 .53

Cylindrical refractive error (D)

�2.61�0.93 �2.13�1.03 .17

SEQ (D) �2.39�0.77 �1.97�1.23 .24

Maximum K (D) 49.11�3.24 48.67�3.00 .70

UDVA (Snellen) 0.37�0.15 0.41�0.23 .53

CDVA (Snellen 0.55�0.19 0.60�0.29 .58

Total aberrations (µm)

4.33�1.42 4.46�2.04 .86

HOA (µm) 1.55�0.55 2.06�1.29 .25

Photopic CS 126.75�120.05 121.65�102.25 .90

Scotopic CS 103.44�92.94 72.94�53.46 .25

Scotopic CS with glare

90.25�74.39 40.06�49.28 .03*

ICRS = intrastromal corneal ring segments, SEQ = spherical equivalent refraction, K = keratometry, UDVA = uncorrected distance visual acuity, CDVA = corrected distance visual acuity, HOA = higher order aberrations, CS = contrast sensitivity*Statistically significant.Note. Luminance level was 85 cd/m2 and 3.0 cd/m2 for photopic and scotopic lighting, respectively.



in the Ferrara group were implanted with two ICRS; three eyes in the Ferrara group were implanted with a single segment. Patients were followed for 16.0�1.8 months in the Intacs group and 22.0�3.7 months in the Ferrara group.

Pre- and postoperative characteristics in both groups are shown in Tables 1 and 2. Preoperative refractive, keratometry, visual acuity, optical, and visual quality characteristics were not signifi cantly different in both groups (P�.05 for all).

REFRACTIVE AND KERATOMETRY OUTCOMESOne year postoperatively, spherical and cylindrical

refractive errors and spherical equivalent refraction decreased signifi cantly compared with preoperative values in each group (P�.01 for all). Postoperatively, one (6.3%) eye in the Intacs group and three (17.6%) eyes in the Ferrara group were within �1.00 D of em-metropia, in terms of spherical equivalent refraction. Postoperative change in spherical and cylindrical refractive errors and spherical equivalent refraction was not signifi cantly different between groups (P=.93, P=.81, and P=.39, respectively).

One year postoperatively, maximum keratometry decreased signifi cantly in each group compared with preoperative readings (P�.01 for both groups). Kera-tometry gains (ie, amount of decrease in keratometry readings) were not signifi cantly different between groups (P=.65).

VISUAL ACUITY OUTCOMESOne year postoperatively, mean UDVA and CDVA

improved signifi cantly in each group (P�.01 for all). Safety index was 1.62 in the Intacs group and 1.76 in the Ferrara group. Effi cacy index was 1.09 in the Intacs group and 1.21 in the Ferrara group.

In the Intacs group, UDVA increased at least 1 Snellen line in 14 (87.5%) eyes. In 10 (62.5%) eyes, this increase was �2 lines. In the Ferrara group, UDVA increased at least 1 Snellen line in 15 (88.2%) eyes. In 10 (58.8%) eyes, this increase was �2 lines. In 2 (12.5%) eyes of the Intacs group and 2 (11.8%) eyes in Ferrara group, no increase in UDVA was noted. The gain in UDVA was not signifi cantly different between groups (P=.48).

In the Intacs group, CDVA increased at least 1 Snel-len line in 14 (87.5%) eyes. In 8 (50%) eyes, this in-crease was �2 lines. In 2 (12.5%) eyes, no increase in CDVA was seen. In the Ferrara group, CDVA increased at least one Snellen line in all (100%) eyes. In 13 (76.5%) eyes, this increase was �2 lines. No eyes lost any lines of CDVA during the follow-up period. The gain in CDVA was not signifi cantly different between groups (P=.33).

In both groups, correlation between preoperative maximum keratometry and gain in UDVA and CDVA was not statistically signifi cant (r(14)=�0.25, P=.34 and r(14)=�0.14, P=.60, respectively, for the Intacs group and r(15)=0.43, P=.08 and r(15)=0.39, P=.12, re-spectively, for the Ferrara group).

OPTICAL QUALITY OUTCOMESPostoperative total aberration values decreased sig-

nifi cantly in each group (P=.03 for both). Mean higher order aberrations decreased in the Intacs group and in-creased in the Ferrara group; however, postoperative changes in higher order aberrations were not statisti-cally signifi cant in either group (P=.68 in the Intacs group and P=.29 in the Ferrara group).

VISUAL QUALITY OUTCOMESScotopic contrast sensitivity score decreased under

glare compared with no glare effect in both groups (P=.07 in the Intacs group; P�.01 in the Ferrara group) at 1 year. Mean decrease was 13.19�26.61 in the Intacs group and 32.88�28.56 in the Ferrara group (P=.05). A positive signifi cant correlation was noted between mean decrease and pupil diameter in the Ferrara group (r(15)=0.50, P=.04), but not in the Intacs group (r(14)=0.11, P=.69). A signifi cant negative correlation was seen between scotopic contrast sen-sitivity with glare and pupil diameter in the Ferr-ara group (r(15)=�0.52, P=.03), but not in the Intacs group (r(14)=�0.02, P=.95); however, no signifi cant correlations were present between pupil diameter and photopic and scotopic contrast sensitivities with-out glare in either group, (r(14)=�0.11, P=.68 and r(14)=0.02, P=.95, respectively, in the Intacs group, and r(15)=�0.15, P=.58 and r(15)=�0.21, P=.42, respectively, in the Ferrara group).

COMPLICATIONSNo intra- or postoperative complications that were

visually signifi cant or necessitated removal of ICRS occurred in any eye during follow-up. One eye in the Intacs group did not have the corneal incision suture removed postoperatively as the patient was lost to follow-up. At 1-year, neovascularization was noted at the stromal channel, which did not disturb integrity of corneal channel.

DISCUSSIONThe aim of ICRS implantation in patients with ectatic

corneal pathologies is to provide refractive correction and improvement in UDVA and CDVA, allow patients to wear contact lenses or spectacles, and delay or pre-vent the need for corneal transplantation.5,6 Because



corneal cross-linking surgery has been increasingly performed in eyes with keratoconus, the need for pen-etrating keratoplasty may decrease and subsequently, the importance of visual rehabilitation in these eyes has increased. Refractive predictability is particularly important when surgery is performed monocularly, to target a tolerable amount of anisometropia. To achieve high-quality vision postoperatively, preoperative low UDVA and CDVA should be improved, combined with a decrease in or minimal induction of wavefront ab-errations and avoidance of postoperative visual com-plaints (eg, halo and glare).

Signifi cant improvement in UDVA and CDVA after ICRS implantation has been reported, accompanied by decrease in spherical refraction in all studies1,2,7,8,19

and decrease in cylindrical refraction in some stud-ies,1,7,8,10,19 but not in others after Intacs implanta-tion.2,4,16 This has been explained by a greater segment diameter of Intacs ICRS, which induces only mini-mal central corneal fl attening.16 However, the closer the locations of segments from the pupil margins, the greater the likelihood of light scattering by ICRS, in-ducing blur and glare sensation and reduction in con-trast sensitivity.16

To date, few comparative studies exist between the two types of ICRS.15,16 Although good outcomes have been reported for Intacs, Ferrara, and KeraRing (Medi-phacos Ltda, Minas Gerais, Brazil) ICRS,1-14 in compar-ative studies, implantation of KeraRings, with virtu-ally the same characteristics as Ferrara ICRS, has been found to be superior to Intacs implantation in terms of refractive correction and visual outcome.15,16

In a comparative study, Kubaloglu et al15 found that patients implanted with KeraRing had greater im-provement in CDVA and greater decrease in maximum keratometry compared with Intacs at 6 months and 1 year postoperative. In that study, the nomogram used for Intacs implantation was not clearly indicated; but in the current study, nomograms recommended by manufacturers for each ICRS type were used. Piñero et al16 also compared the short-term refractive and ab-errometric performance of Intacs and Ferrara ICRS in ectatic corneas. In that study, although spherical error and spherical equivalent refraction were signifi cantly reduced in both groups, cylindrical error decreased signifi cantly in the Ferrara group but not in the Intacs group, indicating that Intacs have limited effect in cor-recting astigmatism.

In the current study, refractive and visual outcomes were better in the Ferrara group, but the difference between groups did not reach statistical signifi cance. Both types of ICRS rely on a similar mechanism of ac-tion, although with different optical diameters and de-

signs. Additionally, the small sample size may be the cause of the statistical insignifi cance. In both groups, refractive error was generally undercorrected. Postop-erative spherical equivalent refraction of �1.00 D was achieved in only 6.3% and 17.6% of eyes in the Intacs and Ferrara groups, respectively. All other eyes had myopic spherical equivalent refraction ��1.00 D. The relationship between uncorrected and corrected visual gain and preoperative maximum keratometry readings yielded a mild negative correlation in the Intacs group and moderate positive correlation in the Ferrara group, which was not statistically signifi cant.

To the best of our knowledge, Intacs and Ferrara ICRS have not been compared in terms of wavefront aberrations and contrast sensitivity in photopic and scotopic environments and under glare effect. Wave-front aberrations affect ocular optical quality that may not be accurately evaluated by visual acuity and conventional refractive error measurements alone.20 Contrast sensitivity is a measure of threshold contrast for seeing the target, impairment of which is closely linked to visual–task performance problems, including diffi culties in mobility, driving, reading, face recogni-tion, and an assortment of everyday tasks such as using tools and fi nding objects.21 In our study, visual qual-ity of patients implanted with these two types of ICRS was evaluated in terms of wavefront aberrations and contrast sensitivity function. Implanted ICRS were not selected according to patients’ preoperative character-istics or demands; but patients implanted with two dif-ferent types of ICRS at different times were examined 1 year postoperatively in this respect.

In the current study, total ocular aberrations were measured and compared, unlike previous studies,16,20 to outline the effect of ICRS implantation on ocular aberration profi les. Signifi cant improvement in total wavefront aberrations was noted in both groups post-operatively, which accompanied signifi cant improve-ments in spherical and cylindrical refractive errors. Mean higher order aberrations decreased in the Intacs group and increased in the Ferrara group postop-eratively, although not statistically signifi cantly. The smaller inner diameter of the Ferrara ring may cause greater effect on higher order aberrations, should any small amount of tilt or decentraton with respect to pupil occur in ICRS implantation.

In both groups, postoperative scotopic contrast sen-sitivity decreased signifi cantly when glare effect was introduced. Interestingly, this decrease in contrast sensitivity in eyes implanted with the Ferrara ICRS was greater compared with Intacs-implanted eyes and was signifi cantly negatively correlated with pupil diameter. These results suggest that smaller inner diameter, as



well as the design of the Ferrara ICRS may cause light scatter and loss of contrast sensitivity, particularly in conditions such as driving at night.

Our study has limitations in that the sample size was small and represents results of a single center only. Additionally, preoperative contrast sensitivity measurements were not available for further analysis.

This study showed that although slightly better refrac-tive and keratometry correction can be achieved with Ferrara ICRS compared to Intacs, implantation of Ferrara ICRS may lead to decreased scotopic contrast sensitivity when glare effect is introduced in eyes with a large pupil diameter. Implantation of ICRS with greater inner diam-eter that are placed farther away from the pupil margin may lead to better visual quality in these eyes.

AUTHOR CONTRIBUTIONSStudy concept and design (V.K., C.A.U.); data collection (V.K.,

S.H.K., I.K.); analysis and interpretation of data (V.K., C.A.U.,

O.F.Y.); drafting of the manuscript (C.A.U.); critical revision of the

manuscript (V.K., S.H.K., I.K., O.F.Y.); statistical expertise (C.A.U.);

supervision (V.K., O.F.Y.)

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3. Shabayek MH, Alió JL. Intrastromal corneal ring segment im-plantation by femtosecond laser for keratoconus correction. Ophthalmology. 2007;114(9):1643-1652.

4. Zare MA, Hashemi H, Salari MR. Intracorneal ring segment im-plantation for the management of keratoconus: safety and ef-fi cacy. J Cataract Refract Surg. 2007;33(11):1886-1891.

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7. Alió JL, Shabayek MH, Belda JI, Correas P, Diez 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.

8. Kanellopoulos AJ, Pe LH, Perry HD, Donnenfeld ED. Modifi ed intracorneal ring segment implantations (INTACS) for the man-agement of moderate to advanced keratoconus: effi cacy and complications. Cornea. 2006;25(1):29-33.

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12. Torquetti L, Ferrara P. Intrastromal corneal ring segment im-plantation for ectasia after refractive surgery. J Cataract Refract Surg. 2010;36(6):986-990.

13. Sansanayudh W, Bahar I, Kumar NL, et al. Intrastromal corneal ring segment SK implantation for moderate to severe keratoco-nus. J Cataract Refract Surg. 2010;36(1):110-113.

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15. 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.

16. Piñero DP, Alió JL, El Kady B, Pascual I. Corneal aberrometric and refractive performance of 2 intrastromal corneal ring seg-ment models in early and moderate ectatic disease. J Cataract Refract Surg. 2010;36(1):102-109.

17. Krumeich JH, Daniel J, Knulle A. Live-epikeratophakia for kera-toconus. J Cataract Refract Surg. 1998;24(4):456-463.

18. Ertan A, Colin J. Intracorneal rings for keratoconus and kera-tectectasia. J Cataract Refract Surg. 2007;33(7):1303-1314.

19. Kwitko S, Severo NS. Ferrara intracorneal ring segments for keratoconus. J Cataract Refract Surg. 2004;30(4):812-820.

20. Chalita MR, Krueger RR. Wavefront aberrations associated with the Ferrara intrastromal corneal ring in a keratoconic eye. J Refract Surg. 2004;20(6):823-830.

21. West SK, Rubin GS, Broman AT, Muñoz B, Bandeen-Roche K, Turano K. How does visual impairment affect performance on tasks of everyday life? The SEE Project. Salisbury Eye Evalua-tion. Arch Ophthalmol. 2002;120(6):774-780.

AUTHOR QUERIES

Page 4, right column: The format of correlation coeffi cient is not familiar. Please advise meaning of number in parentheses following the r.

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