ARTICLE

Intraindividual compari

son of changes in cornealbiomechanical parameters after femtosecond

lenticule extraction and small-incision lenticuleextraction

Kazutaka Kamiya, MD, PhD, Kimiya Shimizu, MD, PhD, Akihito Igarashi, MD, PhD,Hidenaga Kobashi, MD, PhD, Nobuyuki Sato, MD, PhD, Rie Ishii, MD, PhD

Q 2014 A

Published

SCRS an

by Elsev

PURPOSE: To compare the biomechanical changes after femtosecond lenticule extraction andsmall-incision lenticule extraction for myopia.

SETTING: Department of Ophthalmology, Kitasato University, Kanagawa, Japan.

DESIGN: Comparative case series.

METHODS: In eyes of consecutive patients, femtosecond lenticule extraction was performed in 1 eyeand small-incision lenticule extraction in the other eye (both Visumax laser) by random assignment.Corneal hysteresis (CH) and the corneal resistance factor (CRF) were quantitatively assessed using adynamic bidirectional applanation device (Ocular Response Analyzer) in relation to the amount ofmyopic correction preoperatively and 1 week and 1 and 3 months postoperatively.

RESULTS: This study comprised 48 eyes (24 patients). The decrease in CH and the CRF wasstatistically significant 1 week after both lenticule extraction procedures; however, the changes sub-sequently stabilized with no further deterioration (P<.001). There were no statistically significantdifferences between the biomechanical changes in the 2 procedures at any time; however, asignificant correlation was found between the changes and the myopic correction 3 months afterfemtosecond lenticule extraction (r Z 0.41, PZ.046, CH; r Z 0.41, PZ.045, CRF) and aftersmall-incision lenticule extraction (r Z 0.62, PZ.001, CH; r Z 0.67, P<.001, CRF).

CONCLUSIONS: The greatest changes in biomechanical parameters occurred within 1 week afterfemtosecond lenticule extraction and small-incision lenticule extraction; the changes were thennearly stable in relation to the amount of myopic correction. This suggests that the presence orabsence of flap lifting does not significantly affect biomechanical parameters.

Financial Disclosure: No author has a financial or proprietary interest in any material or methodmentioned.

J Cataract Refract Surg 2014; -:-–- Q 2014 ASCRS and ESCRS

The corneal resection refractive procedure was devel-oped to remove a thin planar slice of corneal stromafor the correction of myopia.1 However, the resultsof this surgery were not necessarily satisfactory,possibly because the mechanical microkeratome wasnot precise enough to treat refractive errors.

The femtosecond laser, a relatively new technologyin medicine, has been mainly used in ophthalmologyas an alternative to the mechanical microkeratome tocreate corneal flaps in laser in situ keratomileusis(LASIK). Recently, this technology has been used in

d ESCRS

ier Inc.

a new refractive procedure, refractive lenticule extrac-tion. The procedure does not require a microkeratomeor an excimer laser; rather, the femtosecond lasersystem is used for flap and lenticule processing.

The first clinical results of laser-induced extractionof a refractive lenticule were reported in highlymyopic eyes2 and in blind or amblyopic eyes.3 Of therefractive lenticule extraction techniques in which thefemtosecond laser is used are femtosecond lenticuleextraction, in which the flap is lifted, and small-incision lenticule extraction, in which the flap is not

0886-3350/$ - see front matter 1http://dx.doi.org/10.1016/j.jcrs.2013.12.013

2 CORNEAL BIOMECHANICS: FEMTOSECOND AND SMALL-INCISION LENTICULE EXTRACTION OUTCOMES

lifted. Both techniques have been proposed as an alter-native to conventional LASIK for the correction ofrefractive errors.4–11 We previously reported thatfemtosecond lenticule extraction and small-incisionlenticule extraction performed well in all measures ofsafety, efficacy, predictability, and stability throughouta 6-month follow-up.11 However, it is possible that thedifferences between these 2 surgical procedures,with orwithout flap lifting, affect the biomechanical character-istics of the cornea, which may result in unpredictablerefractive outcomes or the onset of iatrogenic keratecta-sia after surgery.

A dynamic bidirectional applanation device (OcularResponse Analyzer, Reichert Technologies) assessesthe biomechanical characteristics of the cornea,12 andit has been reported that these biomechanical parame-ters decrease significantly after other keratorefractivesurgical procedures, such as LASIK andphotorefractivekeratectomy (PRK).12–16 However, to our knowledge,there have been no published studies of the biomechan-ical changes after femtosecond or small-incision lenti-cule extraction. Moreover, no comparative study ofsuch changes after the 2 surgical procedures has beenperformed.

The purpose of the current study was to comparethe postoperative biomechanical variables of thecornea after femtosecond lenticule extraction andsmall-incision lenticule extraction for the equivalentcorrection of myopia in relation to the amount ofachieved correction.

PATIENTS AND METHODS

This prospective intraindividual comparative studyexamined eyes of consecutive patients who had bilateralrefractive lenticule extraction to correct myopia or myopicastigmatism using the Visumax femtosecond laser system(Carl Zeiss Meditec AG) with a 500 kHz repetition rate.The patients comprised in part those in a preceding studyof visual and refractive outcomes of femtosecond lenticuleextraction and small-incision lenticule extraction.11 Thestudywas approved by the Institutional Review Board, Kita-sato University, and followed the tenets of the Declaration ofHelsinki. After receiving an explanation of the nature andpossible consequences of the study, all patients providedinformed consent.

Submitted: April 12, 2013.Final revision submitted: December 9, 2013.Accepted: December 19, 2013.

From the Department of Ophthalmology, University of KitasatoSchool of Medicine, Kanagawa, Japan.

Corresponding author: Kazutaka Kamiya, MD, PhD, Department ofOphthalmology, University of Kitasato School of Medicine, 1-15-1Kitasato, Sagamihara, Kanagawa 252-0374, Japan. E-mail:kamiyak-tky@umin.ac.jp.

J CATARACT REFRACT SURG

The inclusion criteria were a corrected distance visualacuity of 20/20 or better, unsatisfactory correction withspectacles or contact lenses, manifest spherical equivalent(SE) of �1.00 to �9.00 diopters (D), manifest cylinder of0.00 to 4.00 D, sufficient corneal thickness (estimated totalpostoperative corneal thickness O400 mm; estimated resid-ual stromal bed thickness O250 mm), intraocular pressure(IOP) of 21 mm Hg or less, no history of ocular surgery, se-vere dry eye, progressive corneal degeneration, cataract, oruveitis. Eyeswith keratoconuswere excluded from the studybased on the keratoconus screening test of a Placido-diskvideokeratographer (TMS-2, Tomey Corp.). Eligible patientswere randomly allocated to have femtosecond lenticuleextraction in 1 eye and small-incision lenticule extraction inthe other eye. The sample size in this study offered 88%statistical power at the 5% level to detect a 1 mm Hgdifference in corneal hysteresis (CH) between the 2 groupswhen the standard deviation (SD) of the mean differencewas 1.5 mm Hg.

Surgical Technique

For both procedures, the femtosecond laser was visuallycentered on the entrance pupil and a small, curved interfacecone was used. The main femtosecond incisions wereperformed in the following automated sequence: posteriorsurface of the lenticule (spiral-in pattern), anterior surface ofthe lenticule (spiral-out pattern), and side cut of flap. Thefemtosecond laser parameters were as follows: 120 mm flapthickness, 7.5 mm flap diameter, 6.5 mm lenticule diameter,140 nJ power for the lenticule and flap, 310-degree side cutwith angles of 90 degrees for femtosecond lenticule extraction,and a 50-degree side cut for access to the lenticule with anglesof 90 degrees for small-incision lenticule extraction. In all eyes,the preoperative manifest refraction was the target myopiccorrection. For femtosecond lenticule extraction, after comple-tion of the laser sequence, a Seibel spatula (Rhein Medical,Inc.) was inserted under the flap near the hinge and the flapwas lifted. The lenticule was then grasped with a forcepsand extracted. Next, the flap was repositioned and the inter-face flushed. For small-incision lenticule extraction, thespatula was inserted through the side cut over the roof ofthe lenticule, dissecting this plane and then the bottom ofthe lenticule. The lenticule was subsequently grasped andremoved. Next, the intrastromal space was flushed.

After surgery, betamethasone 0.1% (Rinderon) and levo-floxacin 0.3% (Cravit) were administered topically 4 timesa day for 2 weeks, after which the frequency was steadilytapered.

Corneal Biomechanical Parameter Measurements

The cornea's biomechanical parameters, characterized byCH and the corneal resistance factor (CRF), were measuredusing the dynamic bidirectional applanation device(software version 2.04). The measurements were takenbefore surgery and 1 week and 1 and 3 months after surgery.The details of the dynamic bidirectional applanation devicefunction and the applanation pressures from which CHand the CRF are derived have been described.12 In brief,CH is calculated as the difference between the 2 pressurevalues at 2 applanation processes. The CRF is calculated asa linear function of the 2 pressures associated with the 2 ap-planation processes.A The measurements were performed atleast 3 times with the patient sitting and with ocular fixationto ensure consistent signal quality and obtain consistent

- VOL -, - 2014

Table 1. Preoperative patient demographics by group.

Parameter

Lenticule ExtractionTechnique

PValueFemtosecond Small Incision

Manifest sphericalequivalent (D)Mean G SD �4.1 G 1.7 �4.1 G 1.7 .72Range �1.50, �8.00 �1.25, �8.25

Manifest cylinder (D)

3CORNEAL BIOMECHANICS: FEMTOSECOND AND SMALL-INCISION LENTICULE EXTRACTION OUTCOMES

signal morphology andmeasurement values. The value withthe highest waveform score was used for the statistical anal-ysis according to the manufacturer's instructions. The wave-form score is a composite index based on 5 mathematicalaspects of the corneal deformation signal. The score rangesfrom 0 to 10, with a higher score indicating that the signalis closer to an ideal signal in a normal cornea. It wasconfirmed that the waveform scores were 6.5 or higher inall eyes. The central corneal thickness was also measuredusing an ultrasound pachymeter (DGH-500, DGH Techno-logy, Inc.) before surgery and 1 week and 1 and 3 monthsafter surgery.

Mean G SD �0.7 G 0.8 �0.5 G 0.8 .16Range �0.00, �2.75 �0.00, �2.25

Statistical Analysis

LogMAR UDVAMean G SD 1.07 G 0.30 1.11 G 0.24 .09Range 0.30, 1.52 0.52, 1.52

LogMAR CDVAMean G SD �0.23 G 0.06 �0.23 G 0.06 1.00Range �0.30, �0.18 �0.30, �0.18

Goldmann-correlatedIOP (mm Hg)Mean G SD 13.8 G 3.3 13.3 G 3.2 .26Range 7.4, 20.2 6.9, 19.6

Corneal-compensatedIOP (mm Hg)Mean G SD 14.2 G 2.9 13.8 G 2.8 .26Range 8.6, 19.8 8.4, 19.2

Central cornealthickness (mm)Mean G SD 545.5 G 31.8 543.1 G 32.4 .10Range 492, 626 483, 614

CDVA Z corrected distance visual acuity; IOP Z intraocular pressure;UDVA Z uncorrected distance visual acuity

All statistical analyses were performed using SPSS soft-ware (SPSS, Inc.). Repeated-measures analysis of variance(ANOVA) was used to assess the time course of changeswith the Fisher least-significant-difference (LSD) test formultiple comparisons. The Wilcoxon signed-rank test wasused to compare the data between the 2 groups. Unlessotherwise indicated, the results are expressed as the meanG SD and a P value less than 0.05 was considered statisti-cally significant.

RESULTS

The study enrolled 48 eyes of 24 patients. The meanage of the 17 women and 7 men was 31.8 G 6.0 years.Table 1 shows the patients' preoperative demo-graphics. All surgeries were uneventful, and noserious intraoperative complication was observed. Iat-rogenic keratectasia did not occur in any case duringthe 3-month observation period.

Table 2 shows the postoperative values of thecorneal biomechanical parameters over time.Figure 1 shows representative examples of the preop-erative and postoperative dynamic bidirectional ap-planation device waveforms by group.

Figure 2 shows the differences in CH betweenfemtosecond lenticule extraction and small-incisionlenticule extraction over time. Figure 3 shows the dif-ferences in the CRF between the 2 groups over time.In the femtosecond lenticule extraction group, the vari-ation in CH was statistically significant (P!.001,ANOVA). Multiple comparisons showed statisticallysignificant differences in measurements between pre-operatively and all postoperative times (P!.001,Fisher LSD test); there were no statistically significantdifferences in measurements between 1 week and1 month postoperatively (PZ.19), between 1 weekand 3 months postoperatively (PZ.79), or between1 month and 3 months postoperatively (PZ.11). Inthe small-incision lenticule extraction group, the vari-ation in CH was statistically significant (P!.001).Multiple comparisons showed statistically significantdifferences in measurements between preoperativelyand all postoperative times (P!.001); there were nostatistically significant differences in measurements

J CATARACT REFRACT SURG

between 1 week and 1 month postoperatively(PZ.33), between 1 week and 3 months postopera-tively (PZ.08), or between 1 month and 3 monthspostoperatively (PZ.22). Multiple comparisons foundstatistically significant differences in CH between the 2groups 1 week postoperatively (P!.001) but not pre-operatively (PZ.56), 1 month postoperatively(PZ.30), or 3 months postoperatively (PZ.18).

In the femtosecond lenticule extraction group, thevariation in the CRF was statistically significant(P!.001). Multiple comparisons showed statisticallysignificant differences in CRF measurements betweenpreoperatively and all postoperative times (P!.001);there were no statistically significant differences inmeasurements between 1week and 1month postoper-atively (PZ.10), between 1week and 3months postop-eratively (PZ.19), or between 1 month and 3 monthspostoperatively (PZ.24). In the small-incision lenti-cule extraction group, the variation in the CRFwas sta-tistically significant (P!.001). Multiple comparisonsshowed statistically significant differences inmeasure-ments between preoperatively and all postoperative

- VOL -, - 2014

Table 2. Corneal biomechanical parameters over time.

Method/Parameter Preop 1 Wk Postop 1 Mo Postop 3 Mo Postop P Value

CH (mm Hg)Femtosecond

Mean G SD 10.4 G 1.6 7.9 G 1.4 8.1 G 1.3 8.3 G 1.1 .00195% CI 7.3, 13.6 5.2, 10.7 5.5, 10.7 6.2, 10.4

Small incisionMean G SD 10.5 G 1.3 8.5 G 1.1 8.3 G 1.0 8.5 G 1.0 .00195% CI 8.0, 13.0 6.3, 10.6 6.3, 10.3 6.5, 10.5

CRF (mm Hg)Femtosecond

Mean G SD 9.8 G 1.7 6.8 G 1.5 6.7 G 1.5 6.7 G 1.4 .00195% CI 6.5, 13.1 4.0, 9.7 3.7, 9.7 4.0, 9.4

Small incisionMean G SD 10.0 G 1.7 7.3 G 1.5 6.9 G 1.4 7.1 G 1.3 .00195% CI 6.5, 13.4 4.5, 10.6 4.2, 9.6 4.6, 9.6

CCT (mm)Femtosecond

Mean G SD 546 G 32 471 G 37 468 G 35 474 G 32 .00195% CI 483, 608 398, 544 400, 536 410, 537

Small incisionMean G SD 543 G 32 468 G 39 472 G 34 473 G 38 .00195% CI 480, 607 392, 544 405, 539 399, 547

CCT Z central corneal thickness; CI Z confidence interval; CH Z corneal hysteresis; CRF Z corneal resistance factor

Figure 1. The preoperative andpostoperative waveforms of eyeshaving femtosecond lenticuleextraction and small-incision lenti-cule extraction (WS Z waveformscore).

4 CORNEAL BIOMECHANICS: FEMTOSECOND AND SMALL-INCISION LENTICULE EXTRACTION OUTCOMES

J CATARACT REFRACT SURG - VOL -, - 2014

Figure 2. Corneal hysteresis values over time. The bar representsthe SD.

Figure 3. Corneal resistance factor values over time. The bar repre-sents the SD.

5CORNEAL BIOMECHANICS: FEMTOSECOND AND SMALL-INCISION LENTICULE EXTRACTION OUTCOMES

times (P!.001); there were no statistically significantdifferences in measurements between 1 week and1 month postoperatively (PZ.60), between 1 weekand 3 months postoperatively (PZ.54), or between1 month and 3 months postoperatively (PZ.93).

Multiple comparisons found statistically significantdifferences in CRF between the 2 groups 1 week post-operatively (PZ.03) but no statistically significant dif-ferences between them preoperatively (PZ.48), 1month postoperatively (PZ.55), or 3months postoper-atively (PZ.10). In both groups, there were no statisti-cally significant differences in the changes in CH andthe CRF between any postoperative timepoint(1 week: PZ.17 for CH and PZ.20 for CRF; 1 month:PZ.83 and PZ.94, respectively; 3 months: PZ.53and PZ.37, respectively).

There was a statistically significant correlationbetween the amount of SE correction and the changesin biomechanical parameters 3 months after femto-second lenticule extraction (Pearson correlation coeffi-cient r Z 0.41, PZ.046 for CH; r Z 0.41, PZ.045 forCRF). A statistically significant correlation was alsofound between the amount of myopic correction andthe changes in biomechanical parameters 3 monthsafter small-incision lenticule extraction (r Z 0.62,PZ.001 for CH; r Z 0.67, P!.001 for CRF).

DISCUSSION

In the current study, CH and the CRF decreased signif-icantly 1 week after femtosecond lenticule extractionand small-incision lenticule extraction. However,these changes subsequently stabilized with no furtherdeterioration over the 3-month observation period.

Corneal hysteresis is a dynamic measure of theviscous damping in corneal tissue, which represents

J CATARACT REFRACT SURG

the energy-absorption capability of the cornea. TheCRF is an indicator of the total corneal response,including the elastic resistance of the corneal tissue.Our findings indicate that femtosecond lenticule extrac-tion and small-incision lenticule extraction maydecrease not only the energy-absorption capability butalso the elastic resistanceof the corneal tissue in the earlypostoperative period, although no further changesoccurred subsequently. We also found that the cornealbiomechanical changes after both procedures weredependent on the amount of SE correction, indicatingthat the techniques can affect the biomechanical charac-teristics of the cornea, especially in highly myopic eyesthat require a large amount of lenticule extraction. Thesefindings are in line with previous biomechanical resultsof keratorefractive surgery, such asLASIKandPRK.15,16

This may also be supported the results in otherstudies,17–21 which found that iatrogenic keratectasiatended to occur in eyes with high myopia.

We also found no significant differences in thechanges in CH or CRF values in either group 1 monthand 3 months postoperatively. Kirwan and O'Keefe22

report that the decrease in CH was not statisticallysignificantly different after LASIK or laser-assisted sub-epithelial keratectomy, indicating that LASIK involvinga 120 mm flap did not induce additional biomechanicalchange. On the other hand, Gatinel et al.23 report asingle case in which CH and the CRF decreased imme-diately after a 159 mm flap cut without laser photoabla-tion.Medeiros et al.24 found in a porcinemodel that CHand the CRF did not change significantly after flap cre-ation in the thin-flap (100 mm)group but both decreasedsignificantly after flap creation in the thick-flap (300mm)group, suggesting that thicker flaps have a greaterbiomechanical impact on the cornea. We assume thatthe presence or absence of the standard 120 mm flap

- VOL -, - 2014

6 CORNEAL BIOMECHANICS: FEMTOSECOND AND SMALL-INCISION LENTICULE EXTRACTION OUTCOMES

lifting does not significantly affect these biomechanicalparameters postoperatively. However, we found alarger variation in CH and the CRF in the femtosecondlenticule extraction group than in the small-incisionlenticule extraction group, especially in highly myopiceyes that required greater amounts of correction. Thissuggests that corneal flap creation induces some post-operative biomechanical variations.

There are concerns about the possible risk for kera-tectasia not only after LASIK but also after refractivelenticule extraction. Because we did not directlycompare these biomechanical characteristics betweenLASIK and refractive lenticule extraction in this study,we cannot draw conclusions about this issue. Table 3shows previous studies that evaluated CH and theCRF before and after myopic LASIK.13–16,22,25–30 Inthe current study, the deceases in CH and the CRF(per 1.00 D correction) were 0.48 mm Hg and0.54 mmHg, respectively, similar to those in the previ-ous studies. Because the changes in the CH and CRFvalues were influenced by the amount of myopiccorrection, patient age, flap thickness, and the lengthof the postoperative period, it is difficult to directlycompare the effect of LASIK and refractive lenticuleextraction on these biomechanical parameters. Theamount of lenticule thickness was slightly largerthan that of excimer laser ablation for the equivalentmyopic correction. For example, for a 4.0 D equivalentcorrection of myopia, the lenticule thickness for a6.0 mm optical zone was 66 mm, whereas the ablationdepth for a 6.0 mm optical zone was approximately

Table 3. Summary of corneal hysteresis (CH) and corneal resistance f(LASIK).

Study* Year Eyes (n) Mean Age (Y) MRSE (D)

Pepose14 2007 66 39.6 G 11.4 �5.1 G 2.8Oritz13 2007 65 37 �4.3 G 3.0Chen25 2008 43 40.5 G 10.5 �4.0 G 2.0Kamiya15 2008 31 26.4 G 6.6 �4.1 G 1.4Kirwan22 2008 63 36.1 G 10.1 �3.8 G 1.8Kamiya16 2009 36 30.5 G 9.8 �4.4 G 1.4Qazi26 2009 28 39 G 12 �5.3 G 2.7Shah27 2009 26 42.6 G 6.9 NAShah28 2009 53 NA NADe Medeiros29 2010 13 32 G 10 �3.4 G 1.5Uzbek30 2011 66 30.3 G 6.1 �3.4 G 1.9/�0.8 G 1.5†

zMeans G SDCH Z corneal hysteresis; CRF Z corneal resistance factor; LASIK Z laser in situ kavailable*First author only†Sphere/cylinderzChange

J CATARACT REFRACT SURG

60 mm. Other biomechanical factors may play a role.These include differences in the surgical technique(excimer laser ablation versus lenticule extraction),wound-healing process, or long-term use of steroideyedrops. We are performing a new study to comparethe differences in corneal biomechanical variables afterrefractive lenticule extraction with those after LASIK.

It is of clinical importance to assess the repeatabilityof measurements with the Ocular Response Analyzerto confirm the applicability of the data. We previouslyfound that the mean difference between 2 consecutivemeasurements with the device was�0.1G 0.5 mmHg(95% limits of agreement [LOA], �1.1 to 1.0) for CHand 0.0 G 0.5 mm Hg (95% LOA, �0.9 to 1.0 mm Hg)for CRF.14 Moreover, Lu et al.31 found that the repeat-ability of the CH measurement was 0.8 mm Hg SDof the differences between 2 measurements. Accord-ingly, we believe that this device offers reasonablerepeatability in the longitudinal evaluation of cornealbiomechanical parameters.

A limitation of this study is that the sample datawererather limited. However, the sample size in this studyoffered 88% statistical power at the 5% level. Moreover,this was a prospective intraindividual comparativestudy, which can provide more accurate informationfor a comparison of the biomechanical changes afterthe 2 lenticule extraction techniques because the patientage and sex were identical in the 2 groups and theamount of myopic correction and preoperative IOPwere closely matched. Another limitation is that thefollow-up was short. We evaluated the postoperative

actor (CRF) before and after myopic laser in situ keratomileusis

Before LASIK After LASIK

Postop PeriodMean CH(mm Hg)

Mean CRF(mm Hg)

Mean CH(mm Hg)

Mean CRF(mm Hg)

9.7 G 1.8 9.5 G 1.9 8.0 G 1.6 6.7 G 1.7 NA10.4 G 1.7 10.1 G 2.0 9.3 G 1.9 8.1 G 1.9 1 mo11.5 G 1.3 11.7 G 1.4 9.5 G 1.2 8.5 G 1.5 NA10.8 G 1.4 10.3 G 1.5 8.6 G 0.9 7.7 G 1.3 3 mo10.8 G 1.4 NA 9.0 G 1.3 NA 3 mo10.6 G 1.7 10.0 G 1.7 8.9 G 1.5 7.7 G 1.6 6 mo10.0 G 1.8 9.9 G 2.0 8.6 G 2.3 7.4 G 2.5 6 mo11.9 G 2.3 10.9 G 2.2 �2.6 G 1.4z �2.7 G 1.4z 3 mo11.9 G 2.0 10.4 G 1.8 9.8 G 1.9 8.0 G 1.9 NA11.4 G 1.5 11.0 G 1.9 9.1 G 1.3 7.8 G 2.1 1 wk9.6 G 2.0 10.9 G 2.8 8.0 G 1.7 7.9 G 1.7 Immediately

eratomileusis; MRSE Z manifest refraction spherical equivalent; NA Z not

- VOL -, - 2014

7CORNEAL BIOMECHANICS: FEMTOSECOND AND SMALL-INCISION LENTICULE EXTRACTION OUTCOMES

CH and CRF values up to 3 months after surgery, atwhich point, taking into account the wound-healing re-sponses of the cornea, we considered the biomechanicalproperties of the cornea to have stabilized. We previ-ously found no significant differences in biomechanicalmeasurements between 3 months and 6 months afterLASIK.16 However, in light of case reports of delayed-onset keratectasia,32–34 we cannot refute the possibilitythat biomechanical changes may be ongoing over thelong term. Further studies are needed to elucidatelong-term biomechanical changes.

In conclusion, we found that femtosecond lenticuleextraction and small-incision lenticule extraction sub-stantially decreased biomechanical parameters suchas CH and the CRF depending on the amount ofachievedmyopic correction. In addition, no significantdifferences in the changes in CH or the CRF wereobserved 1 month and 3 months postoperatively ineither group. These findings suggest that the presenceor absence of flap lifting does not significantly affectthese biomechanical characteristics. There were no sig-nificant differences in the CH or CRF values betweenthe 2 lenticule extraction techniques. The OcularResponse Analyzer measures 37 additional biome-chanical parameters that quantitatively describeseveral aspects of the applanation response duringmeasurements using the manufacturer's latest genera-tion software and other biomechanical parameters thatwere derived and reported by investigators to describethe waveform of response curve of the dynamic bidi-rectional applanation device.35,36 Further studies ofthe morphology of the Ocular Response Analyzer sig-nals are necessary to clarify the biomechanical differ-ences between the 2 lenticule extractions procedures.

WHAT WAS KNOWN

� The refractive lenticule extraction technique for thecorrection of refractive errors can be used for femto-second lenticule extraction by lifting the flap and forsmall-incision lenticule extraction without lifting the flap.

� The differences in these 2 procedures, with or without flaplifting, may affect the biomechanical characteristics of thecornea. However, the biomechanical changes after theprocedures have not been reported.

WHAT THIS PAPER ADDS

� Femtosecond lenticule extraction was essentially equiva-lent to small-incision lenticule extraction in terms of theCH and CRF values, indicating that the presence orabsence of flap lifting does not significantly affect thesebiomechanical parameters.

J CATARACT REFRACT SURG

REFERENCES1. Barraquer JI. The history and evolution of keratomileusis. Int

Ophthalmol Clin 1996; 36(4):1–7

2. Krueger RR, Juhasz T, Gualano A, Marchi V. The picosecond

laser for nonmechanical laser in situ keratomileusis. J Refract

Surg 1998; 14:467–469

3. Ratkay-Traub I, Ferincz IE, Juhasz T, Kurtz RM, Krueger RR.

First clinical resultswith the femtosecond neodynium-glass laser

in refractive surgery. J Refract Surg 2003; 19:94–103

4. Sekundo W, Kunert K, Russmann C, Gille A, Bissmann W,

Stobrawa G, Sticker M, Bischoff M, Blum M. First efficacy and

safety study of femtosecond lenticule extraction for the correc-

tion of myopia: six-month results. J Cataract Refract Surg

2008; 34:1513–1520; erratum, 1819

5. Blum M, Kunert K, Schr€oder M, Sekundo W. Femtosecond

lenticule extraction for the correction of myopia: preliminary

6-month results. Graefes Arch Clin Exp Ophthalmol 2010;

248:1019–1027

6. Blum M, Kunert KS, Engelbrecht C, Dawczynski J, Sekundo W.

Femtosekunden-Lentikel-Extraktion (FLEx) – Ergebnisse nach

12 Monaten bei myopen Astigmatismus [Femtosecond lenticule

extraction (FLEx) – results after 12 months in myopic astigma-

tism]. Klin Monatsbl Augenheilkd 2010; 227:961–965

7. Sekundo W, Kunert KS, Blum M. Small incision corneal refrac-

tive surgery using the small incision lenticule extraction (SMILE)

procedure for the correction of myopia and myopic astigmatism:

results of a 6 month prospective study. Br J Ophthalmol 2011;

95:335–339

8. Shah R, Shah S, Sengupta S. Results of small incision lenticule

extraction: all-in-one femtosecond laser refractive surgery.

J Cataract Refract Surg 2011; 37:127–137

9. Shah R, Shah S. Effect of scanning patterns on the results of

femtosecond laser lenticule extraction refractive surgery.

J Cataract Refract Surg 2011; 37:1636–1647

10. Kamiya K, Igarashi A, Ishii R, Sato N, Nishimoto H, Shimizu K.

Early clinical outcomes, including efficacy and endothelial cell

loss, of refractive lenticule extraction using a 500 kHz femto-

second laser to correct myopia. J Cataract Refract Surg 2012;

38:1996–2002

11. Kamiya K, Shimizu K, Igarashi A, Kobashi H. Visual and refrac-

tive outcomes of femtosecond lenticule extraction and small inci-

sion lenticule extraction for myopia. Am J Ophthalmol 2014;

157:128–134.e2

12. Luce DA. Determining in vivo biomechanical properties of the

cornea with an ocular response analyzer. J Cataract Refract

Surg 2005; 31:156–162

13. Ortiz D, Pi~nero D, Shabayek MH, Arnalich-Montiel F, Ali�o JL.

Corneal biomechanical properties in normal, post-laser in situ

keratomileusis, and keratoconic eyes. J Cataract Refract Surg

2007; 33:1371–1375

14. Pepose JS, Feigenbaum SK, Qazi MA, Sanderson JP,

Roberts CJ. Changes in corneal biomechanics and intraocular

pressure following LASIK using static, dynamic, and noncontact

tonometry. Am J Ophthalmol 2007; 143:39–47

15. Kamiya K, Shimizu K, Ohmoto F. Comparison of the changes in

corneal biomechanical properties after photorefractive keratec-

tomy and laser in situ keratomileusis. Cornea 2009; 28:765–769

16. Kamiya K, Shimizu K, Ohmoto F. Time course of corneal biome-

chanical parameters after laser in situ keratomileusis.

Ophthalmic Res 2009; 42:167–171

17. Seiler T, Koufala K, Richter G. Iatrogenic keratectasia after laser

in situ keratomileusis. J Refract Surg 1998; 14:312–317

18. Rao SN, Epstein RJ. Early onset ectasia following laser in situ

keratomileusus: case report and literature review. J Refract

Surg 2002; 18:177–184

- VOL -, - 2014

8 CORNEAL BIOMECHANICS: FEMTOSECOND AND SMALL-INCISION LENTICULE EXTRACTION OUTCOMES

19. Spadea L, Palmieri G, Mosca L, Fasciani R, Balestrazzi E. Iatro-

genic keratectasia following laser in situ keratomileusis.

J Refract Surg 2002; 18:475–480

20. Rad AS, Jabbarvand M, Saifi N. Progressive keratectasia after

laser in situ keratomileusis. J Refract Surg 2004; 20:S718–S722

21. Randleman JB. Post-laser in-situ keratomileusis ectasia: current

understanding and future directions. Curr Opin Ophthalmol

2006; 17:406–412

22. Kirwan C, O’Keefe M. Corneal hysteresis using the Reichert

ocular response analyser: findings pre- and post-LASIK and LA-

SEK. Acta Ophthalmol 2008; 86:215–218. Available at: http://

onlinelibrary.wiley.com/doi/10.1111/j.1600-0420.2007.01023.x/

pdf. Accessed February 2, 2014

23. Gatinel D, Chaabouni S, Adam P-A, Munck J, Puech M, Hoang-

XuanT. Corneal hysteresis, resistance factor, topography, and pa-

chymetry after corneal lamellar flap. JRefract Surg2007; 23:76–84

24. Medeiros FW, Sinha-Roy A, Alves MR, Dupps WJ Jr. Biome-

chanical corneal changes induced by different flap thickness

created by femtosecond laser. Clinics (Sao Paulo) 2011;

66:1067–1071. Available at: http://www.ncbi.nlm.nih.gov/

pmc/articles/PMC3129968/pdf/cln-66-06-1067.pdf. Accessed

February 2, 2014

25. Chen MC, Lee N, Bourla N, Hamilton DR. Corneal biomechan-

ical measurements before and after laser in situ keratomileusis.

J Cataract Refract Surg 2008; 34:1886–1891

26. Qazi MA, Sanderson JP, Mahmoud AM, Yoon EY, Roberts CJ,

Pepose JS. Postoperative changes in intraocular pressure and

corneal biomechanical metrics; laser in situ keratomileusis

versus laser-assisted subepithelial keratectomy. J Cataract

Refract Surg 2009; 35:1774–1788

27. ShahS, LaiquzzamanM.Comparison of corneal biomechanics in

pre and post-refractive surgery and keratoconic eyes by Ocular

Response Analyser. Cont Lens Anterior Eye 2009; 32:129–132

28. Shah S, LaiquzzamanM, Yeung I, Pan X, Roberts C. The use of

the Ocular Response Analyser to determine corneal hysteresis

in eyes before and after excimer laser refractive surgery. Cont

Lens Anterior Eye 2009; 32:123–128

29. de Medeiros FW, Sinha-Roy A, Alves MR, Wilson SE,

Dupps WJ Jr. Differences in the early biomechanical effects of

hyperopic and myopic laser in situ keratomileusis. J Cataract

Refract Surg 2010; 36:947–953

J CATARACT REFRACT SURG

30. Uzbek AK, Kamburo�glu G, Mahmoud AM, Roberts CJ. Change

in biomechanical parameters after flap creation using the Intra-

lase femtosecond laser and subsequent excimer laser ablation.

Curr Eye Res 2011; 36:614–619

31. Lu F, Xu S, Qu J, Shen M, Wang X, Fang H, Wang J. Central

corneal thickness and corneal hysteresis during corneal swelling

induced by contact lens wear with eye closure. Am JOphthalmol

2007; 143:616–622

32. Geggel HS, Talley AR. Delayed onset keratectasia following

laser in situ keratomileusis. J Cataract Refract Surg 1999;

25:582–586

33. Leung AT, Rao SK, Lam DS. Delayed onset keratectasia after

LASIK [letter]. J Cataract Refract Surg 1999; 25:1036–1037

34. Lifshitz T, Levy J, Klemperer I, Levinger S. Late bilateral keratec-

tasia after LASIK in a low myopic patient. J Refract Surg 2005;

21:494–496

35. Kerautret J, Colin J, Touboul D, Roberts C. Biomechanical char-

acteristics of the ectatic cornea. J Cataract Refract Surg 2008;

34:510–513

36. Hallahan KM, Roy AS, Ambrosio R Jr, Salomao M,

Dupps WJ Jr. Discriminant value of custom Ocular Response

Analyzer waveform derivatives in keratoconus. Ophthalmology

2014; 121:459–468

OTHER CITED MATERIALA. Luce D. Methodology for cornea compensated IOP and corneal

resistance factor for Reichert Ocular Response Analyzer. IOVS

2006; 47:ARVO E-Abstract 2266. Available at: http://abstracts.

iovs.org//cgi/content/abstract/47/5/2266?sidZ3427a143-0993-

4537-a038-9896d5877b79. Accessed February 2, 2014

- V

OL -, - 2014

First author:Kazutaka Kamiya, MD, PhD

Department of Ophthalmology,University of Kitasato Schoolof Medicine, Kanagawa, Japan