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Intraindividual comparison of changes in corneal biomechanical parameters after femtosecond lenticule extraction and small-incision lenticule extraction
ARTICLE
Intraindividual comparison of changes in corneal biomechanical parameters after femtosecond lenticule extraction and small-incision lenticule extraction Kazutaka Kamiya, MD, PhD, Kimiya Shimizu, MD, PhD, Akihito Igarashi, MD, PhD, Hidenaga Kobashi, MD, PhD, Nobuyuki Sato, MD, PhD, Rie Ishii, MD, PhD
PURPOSE: To compare the biomechanical changes after femtosecond lenticule extraction and small-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 eye and 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 a dynamic bidirectional applanation device (Ocular Response Analyzer) in relation to the amount of myopic 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 was statistically significant 1 week after both lenticule extraction procedures; however, the changes subsequently stabilized with no further deterioration (P<.001). There were no statistically significant differences between the biomechanical changes in the 2 procedures at any time; however, a significant correlation was found between the changes and the myopic correction 3 months after femtosecond lenticule extraction (r Z 0.41, PZ.046, CH; r Z 0.41, PZ.045, CRF) and after small-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 after femtosecond lenticule extraction and small-incision lenticule extraction; the changes were then nearly stable in relation to the amount of myopic correction. This suggests that the presence or absence of flap lifting does not significantly affect biomechanical parameters. Financial Disclosure: No author has a financial or proprietary interest in any material or method mentioned. J Cataract Refract Surg 2014; -:-–- Q 2014 ASCRS and ESCRS
The corneal resection refractive procedure was developed to remove a thin planar slice of corneal stroma for the correction of myopia.1 However, the results of this surgery were not necessarily satisfactory, possibly because the mechanical microkeratome was not precise enough to treat refractive errors. The femtosecond laser, a relatively new technology in medicine, has been mainly used in ophthalmology as an alternative to the mechanical microkeratome to create corneal flaps in laser in situ keratomileusis (LASIK). Recently, this technology has been used in Q 2014 ASCRS and ESCRS Published by Elsevier Inc.
a new refractive procedure, refractive lenticule extraction. The procedure does not require a microkeratome or an excimer laser; rather, the femtosecond laser system is used for flap and lenticule processing. The first clinical results of laser-induced extraction of a refractive lenticule were reported in highly myopic eyes2 and in blind or amblyopic eyes.3 Of the refractive lenticule extraction techniques in which the femtosecond laser is used are femtosecond lenticule extraction, in which the flap is lifted, and smallincision lenticule extraction, in which the flap is not
0886-3350/$ - see front matter http://dx.doi.org/10.1016/j.jcrs.2013.12.013
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lifted. Both techniques have been proposed as an alternative to conventional LASIK for the correction of refractive errors.4–11 We previously reported that femtosecond lenticule extraction and small-incision lenticule extraction performed well in all measures of safety, efficacy, predictability, and stability throughout a 6-month follow-up.11 However, it is possible that the differences between these 2 surgical procedures, with or without flap lifting, affect the biomechanical characteristics of the cornea, which may result in unpredictable refractive outcomes or the onset of iatrogenic keratectasia after surgery. A dynamic bidirectional applanation device (Ocular Response Analyzer, Reichert Technologies) assesses the biomechanical characteristics of the cornea,12 and it has been reported that these biomechanical parameters decrease significantly after other keratorefractive surgical procedures, such as LASIK and photorefractive keratectomy (PRK).12–16 However, to our knowledge, there have been no published studies of the biomechanical changes after femtosecond or small-incision lenticule extraction. Moreover, no comparative study of such changes after the 2 surgical procedures has been performed. The purpose of the current study was to compare the postoperative biomechanical variables of the cornea after femtosecond lenticule extraction and small-incision lenticule extraction for the equivalent correction of myopia in relation to the amount of achieved correction. PATIENTS AND METHODS This prospective intraindividual comparative study examined eyes of consecutive patients who had bilateral refractive lenticule extraction to correct myopia or myopic astigmatism 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 study of visual and refractive outcomes of femtosecond lenticule extraction and small-incision lenticule extraction.11 The study was approved by the Institutional Review Board, Kitasato University, and followed the tenets of the Declaration of Helsinki. After receiving an explanation of the nature and possible consequences of the study, all patients provided informed consent.
Submitted: April 12, 2013. Final revision submitted: December 9, 2013. Accepted: December 19, 2013. From the Department of Ophthalmology, University of Kitasato School of Medicine, Kanagawa, Japan. Corresponding author: Kazutaka Kamiya, MD, PhD, Department of Ophthalmology, University of Kitasato School of Medicine, 1-15-1 Kitasato, Sagamihara, Kanagawa 252-0374, Japan. E-mail: [email protected].
The inclusion criteria were a corrected distance visual acuity of 20/20 or better, unsatisfactory correction with spectacles or contact lenses, manifest spherical equivalent (SE) of 1.00 to 9.00 diopters (D), manifest cylinder of 0.00 to 4.00 D, sufficient corneal thickness (estimated total postoperative corneal thickness O400 mm; estimated residual stromal bed thickness O250 mm), intraocular pressure (IOP) of 21 mm Hg or less, no history of ocular surgery, severe dry eye, progressive corneal degeneration, cataract, or uveitis. Eyes with keratoconus were excluded from the study based on the keratoconus screening test of a Placido-disk videokeratographer (TMS-2, Tomey Corp.). Eligible patients were randomly allocated to have femtosecond lenticule extraction in 1 eye and small-incision lenticule extraction in the other eye. The sample size in this study offered 88% statistical power at the 5% level to detect a 1 mm Hg difference in corneal hysteresis (CH) between the 2 groups when the standard deviation (SD) of the mean difference was 1.5 mm Hg.
Surgical Technique For both procedures, the femtosecond laser was visually centered on the entrance pupil and a small, curved interface cone was used. The main femtosecond incisions were performed in the following automated sequence: posterior surface of the lenticule (spiral-in pattern), anterior surface of the lenticule (spiral-out pattern), and side cut of flap. The femtosecond laser parameters were as follows: 120 mm flap thickness, 7.5 mm flap diameter, 6.5 mm lenticule diameter, 140 nJ power for the lenticule and flap, 310-degree side cut with angles of 90 degrees for femtosecond lenticule extraction, and a 50-degree side cut for access to the lenticule with angles of 90 degrees for small-incision lenticule extraction. In all eyes, the preoperative manifest refraction was the target myopic correction. For femtosecond lenticule extraction, after completion of the laser sequence, a Seibel spatula (Rhein Medical, Inc.) was inserted under the flap near the hinge and the flap was lifted. The lenticule was then grasped with a forceps and extracted. Next, the flap was repositioned and the interface flushed. For small-incision lenticule extraction, the spatula was inserted through the side cut over the roof of the lenticule, dissecting this plane and then the bottom of the lenticule. The lenticule was subsequently grasped and removed. Next, the intrastromal space was flushed. After surgery, betamethasone 0.1% (Rinderon) and levofloxacin 0.3% (Cravit) were administered topically 4 times a day for 2 weeks, after which the frequency was steadily tapered.
Corneal Biomechanical Parameter Measurements The cornea's biomechanical parameters, characterized by CH and the corneal resistance factor (CRF), were measured using the dynamic bidirectional applanation device (software version 2.04). The measurements were taken before surgery and 1 week and 1 and 3 months after surgery. The details of the dynamic bidirectional applanation device function and the applanation pressures from which CH and the CRF are derived have been described.12 In brief, CH is calculated as the difference between the 2 pressure values at 2 applanation processes. The CRF is calculated as a linear function of the 2 pressures associated with the 2 applanation processes.A The measurements were performed at least 3 times with the patient sitting and with ocular fixation to ensure consistent signal quality and obtain consistent
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signal morphology and measurement values. The value with the highest waveform score was used for the statistical analysis according to the manufacturer's instructions. The waveform score is a composite index based on 5 mathematical aspects of the corneal deformation signal. The score ranges from 0 to 10, with a higher score indicating that the signal is closer to an ideal signal in a normal cornea. It was confirmed that the waveform scores were 6.5 or higher in all eyes. The central corneal thickness was also measured using an ultrasound pachymeter (DGH-500, DGH Technology, Inc.) before surgery and 1 week and 1 and 3 months after surgery.
Statistical Analysis All statistical analyses were performed using SPSS software (SPSS, Inc.). Repeated-measures analysis of variance (ANOVA) was used to assess the time course of changes with the Fisher least-significant-difference (LSD) test for multiple comparisons. The Wilcoxon signed-rank test was used to compare the data between the 2 groups. Unless otherwise indicated, the results are expressed as the mean G SD and a P value less than 0.05 was considered statistically significant.
RESULTS The study enrolled 48 eyes of 24 patients. The mean age of the 17 women and 7 men was 31.8 G 6.0 years. Table 1 shows the patients' preoperative demographics. All surgeries were uneventful, and no serious intraoperative complication was observed. Iatrogenic keratectasia did not occur in any case during the 3-month observation period. Table 2 shows the postoperative values of the corneal biomechanical parameters over time. Figure 1 shows representative examples of the preoperative and postoperative dynamic bidirectional applanation device waveforms by group. Figure 2 shows the differences in CH between femtosecond lenticule extraction and small-incision lenticule extraction over time. Figure 3 shows the differences in the CRF between the 2 groups over time. In the femtosecond lenticule extraction group, the variation in CH was statistically significant (P!.001, ANOVA). Multiple comparisons showed statistically significant differences in measurements between preoperatively and all postoperative times (P!.001, Fisher LSD test); there were no statistically significant differences in measurements between 1 week and 1 month postoperatively (PZ.19), between 1 week and 3 months postoperatively (PZ.79), or between 1 month and 3 months postoperatively (PZ.11). In the small-incision lenticule extraction group, the variation in CH was statistically significant (P!.001). Multiple comparisons showed statistically significant differences in measurements between preoperatively and all postoperative times (P!.001); there were no statistically significant differences in measurements
Table 1. Preoperative patient demographics by group. Lenticule Extraction Technique
Parameter Manifest spherical equivalent (D) Mean G SD Range Manifest cylinder (D) Mean G SD Range LogMAR UDVA Mean G SD Range LogMAR CDVA Mean G SD Range Goldmann-correlated IOP (mm Hg) Mean G SD Range Corneal-compensated IOP (mm Hg) Mean G SD Range Central corneal thickness (mm) Mean G SD Range
P Femtosecond Small Incision Value
4.1 G 1.7 1.50, 8.00
4.1 G 1.7 1.25, 8.25
.72
0.7 G 0.8 0.00, 2.75
0.5 G 0.8 0.00, 2.25
.16
1.07 G 0.30 0.30, 1.52
1.11 G 0.24 0.52, 1.52
.09
0.23 G 0.06 0.30, 0.18
0.23 G 0.06 0.30, 0.18
1.00
13.8 G 3.3 7.4, 20.2
13.3 G 3.2 6.9, 19.6
.26
14.2 G 2.9 8.6, 19.8
13.8 G 2.8 8.4, 19.2
.26
545.5 G 31.8 492, 626
543.1 G 32.4 483, 614
.10
CDVA Z corrected distance visual acuity; IOP Z intraocular pressure; UDVA Z uncorrected distance visual acuity
between 1 week and 1 month postoperatively (PZ.33), between 1 week and 3 months postoperatively (PZ.08), or between 1 month and 3 months postoperatively (PZ.22). Multiple comparisons found statistically significant differences in CH between the 2 groups 1 week postoperatively (P!.001) but not preoperatively (PZ.56), 1 month postoperatively (PZ.30), or 3 months postoperatively (PZ.18). In the femtosecond lenticule extraction group, the variation in the CRF was statistically significant (P!.001). Multiple comparisons showed statistically significant differences in CRF measurements between preoperatively and all postoperative times (P!.001); there were no statistically significant differences in measurements between 1 week and 1 month postoperatively (PZ.10), between 1 week and 3 months postoperatively (PZ.19), or between 1 month and 3 months postoperatively (PZ.24). In the small-incision lenticule extraction group, the variation in the CRF was statistically significant (P!.001). Multiple comparisons showed statistically significant differences in measurements between preoperatively and all postoperative
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Table 2. Corneal biomechanical parameters over time. Method/Parameter CH (mm Hg) Femtosecond Mean G SD 95% CI Small incision Mean G SD 95% CI CRF (mm Hg) Femtosecond Mean G SD 95% CI Small incision Mean G SD 95% CI CCT (mm) Femtosecond Mean G SD 95% CI Small incision Mean G SD 95% CI
Preop
1 Wk Postop
1 Mo Postop
3 Mo Postop
P Value
10.4 G 1.6 7.3, 13.6
7.9 G 1.4 5.2, 10.7
8.1 G 1.3 5.5, 10.7
8.3 G 1.1 6.2, 10.4
.001
10.5 G 1.3 8.0, 13.0
8.5 G 1.1 6.3, 10.6
8.3 G 1.0 6.3, 10.3
8.5 G 1.0 6.5, 10.5
.001
9.8 G 1.7 6.5, 13.1
6.8 G 1.5 4.0, 9.7
6.7 G 1.5 3.7, 9.7
6.7 G 1.4 4.0, 9.4
.001
10.0 G 1.7 6.5, 13.4
7.3 G 1.5 4.5, 10.6
6.9 G 1.4 4.2, 9.6
7.1 G 1.3 4.6, 9.6
.001
546 G 32 483, 608
471 G 37 398, 544
468 G 35 400, 536
474 G 32 410, 537
.001
543 G 32 480, 607
468 G 39 392, 544
472 G 34 405, 539
473 G 38 399, 547
.001
CCT Z central corneal thickness; CI Z confidence interval; CH Z corneal hysteresis; CRF Z corneal resistance factor
Figure 1. The preoperative and postoperative waveforms of eyes having femtosecond lenticule extraction and small-incision lenticule extraction (WS Z waveform score).
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Figure 2. Corneal hysteresis values over time. The bar represents the SD.
Figure 3. Corneal resistance factor values over time. The bar represents the SD.
times (P!.001); there were no statistically significant differences in measurements between 1 week and 1 month postoperatively (PZ.60), between 1 week and 3 months postoperatively (PZ.54), or between 1 month and 3 months postoperatively (PZ.93). Multiple comparisons found statistically significant differences in CRF between the 2 groups 1 week postoperatively (PZ.03) but no statistically significant differences between them preoperatively (PZ.48), 1 month postoperatively (PZ.55), or 3 months postoperatively (PZ.10). In both groups, there were no statistically significant differences in the changes in CH and the 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.53 and PZ.37, respectively). There was a statistically significant correlation between the amount of SE correction and the changes in biomechanical parameters 3 months after femtosecond lenticule extraction (Pearson correlation coefficient r Z 0.41, PZ.046 for CH; r Z 0.41, PZ.045 for CRF). A statistically significant correlation was also found between the amount of myopic correction and the changes in biomechanical parameters 3 months after small-incision lenticule extraction (r Z 0.62, PZ.001 for CH; r Z 0.67, P!.001 for CRF).
the energy-absorption capability of the cornea. The CRF is an indicator of the total corneal response, including the elastic resistance of the corneal tissue. Our findings indicate that femtosecond lenticule extraction and small-incision lenticule extraction may decrease not only the energy-absorption capability but also the elastic resistance of the corneal tissue in the early postoperative period, although no further changes occurred subsequently. We also found that the corneal biomechanical changes after both procedures were dependent on the amount of SE correction, indicating that the techniques can affect the biomechanical characteristics of the cornea, especially in highly myopic eyes that require a large amount of lenticule extraction. These findings are in line with previous biomechanical results of keratorefractive surgery, such as LASIK and PRK.15,16 This may also be supported the results in other studies,17–21 which found that iatrogenic keratectasia tended to occur in eyes with high myopia. We also found no significant differences in the changes in CH or CRF values in either group 1 month and 3 months postoperatively. Kirwan and O'Keefe22 report that the decrease in CH was not statistically significantly different after LASIK or laser-assisted subepithelial keratectomy, indicating that LASIK involving a 120 mm flap did not induce additional biomechanical change. On the other hand, Gatinel et al.23 report a single case in which CH and the CRF decreased immediately after a 159 mm flap cut without laser photoablation. Medeiros et al.24 found in a porcine model that CH and the CRF did not change significantly after flap creation in the thin-flap (100 mm) group but both decreased significantly after flap creation in the thick-flap (300 mm) group, suggesting that thicker flaps have a greater biomechanical impact on the cornea. We assume that the presence or absence of the standard 120 mm flap
DISCUSSION In the current study, CH and the CRF decreased significantly 1 week after femtosecond lenticule extraction and small-incision lenticule extraction. However, these changes subsequently stabilized with no further deterioration over the 3-month observation period. Corneal hysteresis is a dynamic measure of the viscous damping in corneal tissue, which represents
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lifting does not significantly affect these biomechanical parameters postoperatively. However, we found a larger variation in CH and the CRF in the femtosecond lenticule extraction group than in the small-incision lenticule extraction group, especially in highly myopic eyes that required greater amounts of correction. This suggests that corneal flap creation induces some postoperative biomechanical variations. There are concerns about the possible risk for keratectasia not only after LASIK but also after refractive lenticule extraction. Because we did not directly compare these biomechanical characteristics between LASIK and refractive lenticule extraction in this study, we cannot draw conclusions about this issue. Table 3 shows previous studies that evaluated CH and the CRF before and after myopic LASIK.13–16,22,25–30 In the current study, the deceases in CH and the CRF (per 1.00 D correction) were 0.48 mm Hg and 0.54 mm Hg, respectively, similar to those in the previous studies. Because the changes in the CH and CRF values were influenced by the amount of myopic correction, patient age, flap thickness, and the length of the postoperative period, it is difficult to directly compare the effect of LASIK and refractive lenticule extraction on these biomechanical parameters. The amount of lenticule thickness was slightly larger than that of excimer laser ablation for the equivalent myopic correction. For example, for a 4.0 D equivalent correction of myopia, the lenticule thickness for a 6.0 mm optical zone was 66 mm, whereas the ablation depth for a 6.0 mm optical zone was approximately
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 steroid eyedrops. We are performing a new study to compare the differences in corneal biomechanical variables after refractive lenticule extraction with those after LASIK. It is of clinical importance to assess the repeatability of measurements with the Ocular Response Analyzer to confirm the applicability of the data. We previously found that the mean difference between 2 consecutive measurements with the device was 0.1 G 0.5 mm Hg (95% limits of agreement [LOA], 1.1 to 1.0) for CH and 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 repeatability of the CH measurement was 0.8 mm Hg SD of the differences between 2 measurements. Accordingly, we believe that this device offers reasonable repeatability in the longitudinal evaluation of corneal biomechanical parameters. A limitation of this study is that the sample data were rather limited. However, the sample size in this study offered 88% statistical power at the 5% level. Moreover, this was a prospective intraindividual comparative study, which can provide more accurate information for a comparison of the biomechanical changes after the 2 lenticule extraction techniques because the patient age and sex were identical in the 2 groups and the amount of myopic correction and preoperative IOP were closely matched. Another limitation is that the follow-up was short. We evaluated the postoperative
Table 3. Summary of corneal hysteresis (CH) and corneal resistance factor (CRF) before and after myopic laser in situ keratomileusis (LASIK). Before LASIK Study*
Year Eyes (n) Mean Age (Y)
Pepose14 Oritz13 Chen25 Kamiya15 Kirwan22 Kamiya16 Qazi26 Shah27 Shah28 De Medeiros29 Uzbek30
2007 2007 2008 2008 2008 2009 2009 2009 2009 2010 2011
66 65 43 31 63 36 28 26 53 13 66
39.6 G 11.4 37 40.5 G 10.5 26.4 G 6.6 36.1 G 10.1 30.5 G 9.8 39 G 12 42.6 G 6.9 NA 32 G 10 30.3 G 6.1
MRSE (D) 5.1 G 2.8 4.3 G 3.0 4.0 G 2.0 4.1 G 1.4 3.8 G 1.8 4.4 G 1.4 5.3 G 2.7 NA NA 3.4 G 1.5 3.4 G 1.9/0.8 G 1.5†
Mean CH Mean CRF (mm Hg) (mm Hg) 9.7 G 1.8 10.4 G 1.7 11.5 G 1.3 10.8 G 1.4 10.8 G 1.4 10.6 G 1.7 10.0 G 1.8 11.9 G 2.3 11.9 G 2.0 11.4 G 1.5 9.6 G 2.0
After LASIK Mean CH (mm Hg)
Mean CRF (mm Hg) Postop Period
9.5 G 1.9 8.0 G 1.6 6.7 G 1.7 NA 10.1 G 2.0 9.3 G 1.9 8.1 G 1.9 1 mo 11.7 G 1.4 9.5 G 1.2 8.5 G 1.5 NA 10.3 G 1.5 8.6 G 0.9 7.7 G 1.3 3 mo NA 9.0 G 1.3 NA 3 mo 10.0 G 1.7 8.9 G 1.5 7.7 G 1.6 6 mo 9.9 G 2.0 8.6 G 2.3 7.4 G 2.5 6 mo 10.9 G 2.2 2.6 G 1.4z 2.7 G 1.4z 3 mo 10.4 G 1.8 9.8 G 1.9 8.0 G 1.9 NA 11.0 G 1.9 9.1 G 1.3 7.8 G 2.1 1 wk 10.9 G 2.8 8.0 G 1.7 7.9 G 1.7 Immediately
z Means G SD CH Z corneal hysteresis; CRF Z corneal resistance factor; LASIK Z laser in situ keratomileusis; MRSE Z manifest refraction spherical equivalent; NA Z not available *First author only † Sphere/cylinder z Change
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CH and CRF values up to 3 months after surgery, at which point, taking into account the wound-healing responses of the cornea, we considered the biomechanical properties of the cornea to have stabilized. We previously found no significant differences in biomechanical measurements between 3 months and 6 months after LASIK.16 However, in light of case reports of delayedonset keratectasia,32–34 we cannot refute the possibility that biomechanical changes may be ongoing over the long term. Further studies are needed to elucidate long-term biomechanical changes. In conclusion, we found that femtosecond lenticule extraction and small-incision lenticule extraction substantially decreased biomechanical parameters such as CH and the CRF depending on the amount of achieved myopic correction. In addition, no significant differences in the changes in CH or the CRF were observed 1 month and 3 months postoperatively in either group. These findings suggest that the presence or absence of flap lifting does not significantly affect these biomechanical characteristics. There were no significant differences in the CH or CRF values between the 2 lenticule extraction techniques. The Ocular Response Analyzer measures 37 additional biomechanical parameters that quantitatively describe several aspects of the applanation response during measurements using the manufacturer's latest generation software and other biomechanical parameters that were derived and reported by investigators to describe the waveform of response curve of the dynamic bidirectional applanation device.35,36 Further studies of the morphology of the Ocular Response Analyzer signals are necessary to clarify the biomechanical differences between the 2 lenticule extractions procedures. WHAT WAS KNOWN The refractive lenticule extraction technique for the correction of refractive errors can be used for femtosecond lenticule extraction by lifting the flap and for small-incision lenticule extraction without lifting the flap. The differences in these 2 procedures, with or without flap lifting, may affect the biomechanical characteristics of the cornea. However, the biomechanical changes after the procedures have not been reported. WHAT THIS PAPER ADDS Femtosecond lenticule extraction was essentially equivalent to small-incision lenticule extraction in terms of the CH and CRF values, indicating that the presence or absence of flap lifting does not significantly affect these biomechanical parameters.
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19. Spadea L, Palmieri G, Mosca L, Fasciani R, Balestrazzi E. Iatrogenic 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 LASEK. 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, HoangXuan T. Corneal hysteresis, resistance factor, topography, and pachymetry after corneal lamellar flap. J Refract Surg 2007; 23:76–84 24. Medeiros FW, Sinha-Roy A, Alves MR, Dupps WJ Jr. Biomechanical 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 biomechanical 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. Shah S, Laiquzzaman M. 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, Laiquzzaman M, 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
lu G, Mahmoud AM, Roberts CJ. Change 30. Uzbek AK, Kamburog in biomechanical parameters after flap creation using the Intralase 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 J Ophthalmol 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 keratectasia after LASIK in a low myopic patient. J Refract Surg 2005; 21:494–496 35. Kerautret J, Colin J, Touboul D, Roberts C. Biomechanical characteristics 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 MATERIAL A. 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?sidZ3427a143-09934537-a038-9896d5877b79. Accessed February 2, 2014
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First author: Kazutaka Kamiya, MD, PhD Department of Ophthalmology, University of Kitasato School of Medicine, Kanagawa, Japan
Intraindividual comparison of changes in corneal biomechanical parameters after femtosecond lenticule extraction and small-incision lenticule extraction Kazutaka Kamiya, MD, PhD, Kimiya Shimizu, MD, PhD, Akihito Igarashi, MD, PhD, Hidenaga Kobashi, MD, PhD, Nobuyuki Sato, MD, PhD, Rie Ishii, MD, PhD
PURPOSE: To compare the biomechanical changes after femtosecond lenticule extraction and small-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 eye and 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 a dynamic bidirectional applanation device (Ocular Response Analyzer) in relation to the amount of myopic 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 was statistically significant 1 week after both lenticule extraction procedures; however, the changes subsequently stabilized with no further deterioration (P<.001). There were no statistically significant differences between the biomechanical changes in the 2 procedures at any time; however, a significant correlation was found between the changes and the myopic correction 3 months after femtosecond lenticule extraction (r Z 0.41, PZ.046, CH; r Z 0.41, PZ.045, CRF) and after small-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 after femtosecond lenticule extraction and small-incision lenticule extraction; the changes were then nearly stable in relation to the amount of myopic correction. This suggests that the presence or absence of flap lifting does not significantly affect biomechanical parameters. Financial Disclosure: No author has a financial or proprietary interest in any material or method mentioned. J Cataract Refract Surg 2014; -:-–- Q 2014 ASCRS and ESCRS
The corneal resection refractive procedure was developed to remove a thin planar slice of corneal stroma for the correction of myopia.1 However, the results of this surgery were not necessarily satisfactory, possibly because the mechanical microkeratome was not precise enough to treat refractive errors. The femtosecond laser, a relatively new technology in medicine, has been mainly used in ophthalmology as an alternative to the mechanical microkeratome to create corneal flaps in laser in situ keratomileusis (LASIK). Recently, this technology has been used in Q 2014 ASCRS and ESCRS Published by Elsevier Inc.
a new refractive procedure, refractive lenticule extraction. The procedure does not require a microkeratome or an excimer laser; rather, the femtosecond laser system is used for flap and lenticule processing. The first clinical results of laser-induced extraction of a refractive lenticule were reported in highly myopic eyes2 and in blind or amblyopic eyes.3 Of the refractive lenticule extraction techniques in which the femtosecond laser is used are femtosecond lenticule extraction, in which the flap is lifted, and smallincision lenticule extraction, in which the flap is not
0886-3350/$ - see front matter http://dx.doi.org/10.1016/j.jcrs.2013.12.013
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lifted. Both techniques have been proposed as an alternative to conventional LASIK for the correction of refractive errors.4–11 We previously reported that femtosecond lenticule extraction and small-incision lenticule extraction performed well in all measures of safety, efficacy, predictability, and stability throughout a 6-month follow-up.11 However, it is possible that the differences between these 2 surgical procedures, with or without flap lifting, affect the biomechanical characteristics of the cornea, which may result in unpredictable refractive outcomes or the onset of iatrogenic keratectasia after surgery. A dynamic bidirectional applanation device (Ocular Response Analyzer, Reichert Technologies) assesses the biomechanical characteristics of the cornea,12 and it has been reported that these biomechanical parameters decrease significantly after other keratorefractive surgical procedures, such as LASIK and photorefractive keratectomy (PRK).12–16 However, to our knowledge, there have been no published studies of the biomechanical changes after femtosecond or small-incision lenticule extraction. Moreover, no comparative study of such changes after the 2 surgical procedures has been performed. The purpose of the current study was to compare the postoperative biomechanical variables of the cornea after femtosecond lenticule extraction and small-incision lenticule extraction for the equivalent correction of myopia in relation to the amount of achieved correction. PATIENTS AND METHODS This prospective intraindividual comparative study examined eyes of consecutive patients who had bilateral refractive lenticule extraction to correct myopia or myopic astigmatism 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 study of visual and refractive outcomes of femtosecond lenticule extraction and small-incision lenticule extraction.11 The study was approved by the Institutional Review Board, Kitasato University, and followed the tenets of the Declaration of Helsinki. After receiving an explanation of the nature and possible consequences of the study, all patients provided informed consent.
Submitted: April 12, 2013. Final revision submitted: December 9, 2013. Accepted: December 19, 2013. From the Department of Ophthalmology, University of Kitasato School of Medicine, Kanagawa, Japan. Corresponding author: Kazutaka Kamiya, MD, PhD, Department of Ophthalmology, University of Kitasato School of Medicine, 1-15-1 Kitasato, Sagamihara, Kanagawa 252-0374, Japan. E-mail: [email protected].
The inclusion criteria were a corrected distance visual acuity of 20/20 or better, unsatisfactory correction with spectacles or contact lenses, manifest spherical equivalent (SE) of 1.00 to 9.00 diopters (D), manifest cylinder of 0.00 to 4.00 D, sufficient corneal thickness (estimated total postoperative corneal thickness O400 mm; estimated residual stromal bed thickness O250 mm), intraocular pressure (IOP) of 21 mm Hg or less, no history of ocular surgery, severe dry eye, progressive corneal degeneration, cataract, or uveitis. Eyes with keratoconus were excluded from the study based on the keratoconus screening test of a Placido-disk videokeratographer (TMS-2, Tomey Corp.). Eligible patients were randomly allocated to have femtosecond lenticule extraction in 1 eye and small-incision lenticule extraction in the other eye. The sample size in this study offered 88% statistical power at the 5% level to detect a 1 mm Hg difference in corneal hysteresis (CH) between the 2 groups when the standard deviation (SD) of the mean difference was 1.5 mm Hg.
Surgical Technique For both procedures, the femtosecond laser was visually centered on the entrance pupil and a small, curved interface cone was used. The main femtosecond incisions were performed in the following automated sequence: posterior surface of the lenticule (spiral-in pattern), anterior surface of the lenticule (spiral-out pattern), and side cut of flap. The femtosecond laser parameters were as follows: 120 mm flap thickness, 7.5 mm flap diameter, 6.5 mm lenticule diameter, 140 nJ power for the lenticule and flap, 310-degree side cut with angles of 90 degrees for femtosecond lenticule extraction, and a 50-degree side cut for access to the lenticule with angles of 90 degrees for small-incision lenticule extraction. In all eyes, the preoperative manifest refraction was the target myopic correction. For femtosecond lenticule extraction, after completion of the laser sequence, a Seibel spatula (Rhein Medical, Inc.) was inserted under the flap near the hinge and the flap was lifted. The lenticule was then grasped with a forceps and extracted. Next, the flap was repositioned and the interface flushed. For small-incision lenticule extraction, the spatula was inserted through the side cut over the roof of the lenticule, dissecting this plane and then the bottom of the lenticule. The lenticule was subsequently grasped and removed. Next, the intrastromal space was flushed. After surgery, betamethasone 0.1% (Rinderon) and levofloxacin 0.3% (Cravit) were administered topically 4 times a day for 2 weeks, after which the frequency was steadily tapered.
Corneal Biomechanical Parameter Measurements The cornea's biomechanical parameters, characterized by CH and the corneal resistance factor (CRF), were measured using the dynamic bidirectional applanation device (software version 2.04). The measurements were taken before surgery and 1 week and 1 and 3 months after surgery. The details of the dynamic bidirectional applanation device function and the applanation pressures from which CH and the CRF are derived have been described.12 In brief, CH is calculated as the difference between the 2 pressure values at 2 applanation processes. The CRF is calculated as a linear function of the 2 pressures associated with the 2 applanation processes.A The measurements were performed at least 3 times with the patient sitting and with ocular fixation to ensure consistent signal quality and obtain consistent
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signal morphology and measurement values. The value with the highest waveform score was used for the statistical analysis according to the manufacturer's instructions. The waveform score is a composite index based on 5 mathematical aspects of the corneal deformation signal. The score ranges from 0 to 10, with a higher score indicating that the signal is closer to an ideal signal in a normal cornea. It was confirmed that the waveform scores were 6.5 or higher in all eyes. The central corneal thickness was also measured using an ultrasound pachymeter (DGH-500, DGH Technology, Inc.) before surgery and 1 week and 1 and 3 months after surgery.
Statistical Analysis All statistical analyses were performed using SPSS software (SPSS, Inc.). Repeated-measures analysis of variance (ANOVA) was used to assess the time course of changes with the Fisher least-significant-difference (LSD) test for multiple comparisons. The Wilcoxon signed-rank test was used to compare the data between the 2 groups. Unless otherwise indicated, the results are expressed as the mean G SD and a P value less than 0.05 was considered statistically significant.
RESULTS The study enrolled 48 eyes of 24 patients. The mean age of the 17 women and 7 men was 31.8 G 6.0 years. Table 1 shows the patients' preoperative demographics. All surgeries were uneventful, and no serious intraoperative complication was observed. Iatrogenic keratectasia did not occur in any case during the 3-month observation period. Table 2 shows the postoperative values of the corneal biomechanical parameters over time. Figure 1 shows representative examples of the preoperative and postoperative dynamic bidirectional applanation device waveforms by group. Figure 2 shows the differences in CH between femtosecond lenticule extraction and small-incision lenticule extraction over time. Figure 3 shows the differences in the CRF between the 2 groups over time. In the femtosecond lenticule extraction group, the variation in CH was statistically significant (P!.001, ANOVA). Multiple comparisons showed statistically significant differences in measurements between preoperatively and all postoperative times (P!.001, Fisher LSD test); there were no statistically significant differences in measurements between 1 week and 1 month postoperatively (PZ.19), between 1 week and 3 months postoperatively (PZ.79), or between 1 month and 3 months postoperatively (PZ.11). In the small-incision lenticule extraction group, the variation in CH was statistically significant (P!.001). Multiple comparisons showed statistically significant differences in measurements between preoperatively and all postoperative times (P!.001); there were no statistically significant differences in measurements
Table 1. Preoperative patient demographics by group. Lenticule Extraction Technique
Parameter Manifest spherical equivalent (D) Mean G SD Range Manifest cylinder (D) Mean G SD Range LogMAR UDVA Mean G SD Range LogMAR CDVA Mean G SD Range Goldmann-correlated IOP (mm Hg) Mean G SD Range Corneal-compensated IOP (mm Hg) Mean G SD Range Central corneal thickness (mm) Mean G SD Range
P Femtosecond Small Incision Value
4.1 G 1.7 1.50, 8.00
4.1 G 1.7 1.25, 8.25
.72
0.7 G 0.8 0.00, 2.75
0.5 G 0.8 0.00, 2.25
.16
1.07 G 0.30 0.30, 1.52
1.11 G 0.24 0.52, 1.52
.09
0.23 G 0.06 0.30, 0.18
0.23 G 0.06 0.30, 0.18
1.00
13.8 G 3.3 7.4, 20.2
13.3 G 3.2 6.9, 19.6
.26
14.2 G 2.9 8.6, 19.8
13.8 G 2.8 8.4, 19.2
.26
545.5 G 31.8 492, 626
543.1 G 32.4 483, 614
.10
CDVA Z corrected distance visual acuity; IOP Z intraocular pressure; UDVA Z uncorrected distance visual acuity
between 1 week and 1 month postoperatively (PZ.33), between 1 week and 3 months postoperatively (PZ.08), or between 1 month and 3 months postoperatively (PZ.22). Multiple comparisons found statistically significant differences in CH between the 2 groups 1 week postoperatively (P!.001) but not preoperatively (PZ.56), 1 month postoperatively (PZ.30), or 3 months postoperatively (PZ.18). In the femtosecond lenticule extraction group, the variation in the CRF was statistically significant (P!.001). Multiple comparisons showed statistically significant differences in CRF measurements between preoperatively and all postoperative times (P!.001); there were no statistically significant differences in measurements between 1 week and 1 month postoperatively (PZ.10), between 1 week and 3 months postoperatively (PZ.19), or between 1 month and 3 months postoperatively (PZ.24). In the small-incision lenticule extraction group, the variation in the CRF was statistically significant (P!.001). Multiple comparisons showed statistically significant differences in measurements between preoperatively and all postoperative
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Table 2. Corneal biomechanical parameters over time. Method/Parameter CH (mm Hg) Femtosecond Mean G SD 95% CI Small incision Mean G SD 95% CI CRF (mm Hg) Femtosecond Mean G SD 95% CI Small incision Mean G SD 95% CI CCT (mm) Femtosecond Mean G SD 95% CI Small incision Mean G SD 95% CI
Preop
1 Wk Postop
1 Mo Postop
3 Mo Postop
P Value
10.4 G 1.6 7.3, 13.6
7.9 G 1.4 5.2, 10.7
8.1 G 1.3 5.5, 10.7
8.3 G 1.1 6.2, 10.4
.001
10.5 G 1.3 8.0, 13.0
8.5 G 1.1 6.3, 10.6
8.3 G 1.0 6.3, 10.3
8.5 G 1.0 6.5, 10.5
.001
9.8 G 1.7 6.5, 13.1
6.8 G 1.5 4.0, 9.7
6.7 G 1.5 3.7, 9.7
6.7 G 1.4 4.0, 9.4
.001
10.0 G 1.7 6.5, 13.4
7.3 G 1.5 4.5, 10.6
6.9 G 1.4 4.2, 9.6
7.1 G 1.3 4.6, 9.6
.001
546 G 32 483, 608
471 G 37 398, 544
468 G 35 400, 536
474 G 32 410, 537
.001
543 G 32 480, 607
468 G 39 392, 544
472 G 34 405, 539
473 G 38 399, 547
.001
CCT Z central corneal thickness; CI Z confidence interval; CH Z corneal hysteresis; CRF Z corneal resistance factor
Figure 1. The preoperative and postoperative waveforms of eyes having femtosecond lenticule extraction and small-incision lenticule extraction (WS Z waveform score).
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Figure 2. Corneal hysteresis values over time. The bar represents the SD.
Figure 3. Corneal resistance factor values over time. The bar represents the SD.
times (P!.001); there were no statistically significant differences in measurements between 1 week and 1 month postoperatively (PZ.60), between 1 week and 3 months postoperatively (PZ.54), or between 1 month and 3 months postoperatively (PZ.93). Multiple comparisons found statistically significant differences in CRF between the 2 groups 1 week postoperatively (PZ.03) but no statistically significant differences between them preoperatively (PZ.48), 1 month postoperatively (PZ.55), or 3 months postoperatively (PZ.10). In both groups, there were no statistically significant differences in the changes in CH and the 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.53 and PZ.37, respectively). There was a statistically significant correlation between the amount of SE correction and the changes in biomechanical parameters 3 months after femtosecond lenticule extraction (Pearson correlation coefficient r Z 0.41, PZ.046 for CH; r Z 0.41, PZ.045 for CRF). A statistically significant correlation was also found between the amount of myopic correction and the changes in biomechanical parameters 3 months after small-incision lenticule extraction (r Z 0.62, PZ.001 for CH; r Z 0.67, P!.001 for CRF).
the energy-absorption capability of the cornea. The CRF is an indicator of the total corneal response, including the elastic resistance of the corneal tissue. Our findings indicate that femtosecond lenticule extraction and small-incision lenticule extraction may decrease not only the energy-absorption capability but also the elastic resistance of the corneal tissue in the early postoperative period, although no further changes occurred subsequently. We also found that the corneal biomechanical changes after both procedures were dependent on the amount of SE correction, indicating that the techniques can affect the biomechanical characteristics of the cornea, especially in highly myopic eyes that require a large amount of lenticule extraction. These findings are in line with previous biomechanical results of keratorefractive surgery, such as LASIK and PRK.15,16 This may also be supported the results in other studies,17–21 which found that iatrogenic keratectasia tended to occur in eyes with high myopia. We also found no significant differences in the changes in CH or CRF values in either group 1 month and 3 months postoperatively. Kirwan and O'Keefe22 report that the decrease in CH was not statistically significantly different after LASIK or laser-assisted subepithelial keratectomy, indicating that LASIK involving a 120 mm flap did not induce additional biomechanical change. On the other hand, Gatinel et al.23 report a single case in which CH and the CRF decreased immediately after a 159 mm flap cut without laser photoablation. Medeiros et al.24 found in a porcine model that CH and the CRF did not change significantly after flap creation in the thin-flap (100 mm) group but both decreased significantly after flap creation in the thick-flap (300 mm) group, suggesting that thicker flaps have a greater biomechanical impact on the cornea. We assume that the presence or absence of the standard 120 mm flap
DISCUSSION In the current study, CH and the CRF decreased significantly 1 week after femtosecond lenticule extraction and small-incision lenticule extraction. However, these changes subsequently stabilized with no further deterioration over the 3-month observation period. Corneal hysteresis is a dynamic measure of the viscous damping in corneal tissue, which represents
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lifting does not significantly affect these biomechanical parameters postoperatively. However, we found a larger variation in CH and the CRF in the femtosecond lenticule extraction group than in the small-incision lenticule extraction group, especially in highly myopic eyes that required greater amounts of correction. This suggests that corneal flap creation induces some postoperative biomechanical variations. There are concerns about the possible risk for keratectasia not only after LASIK but also after refractive lenticule extraction. Because we did not directly compare these biomechanical characteristics between LASIK and refractive lenticule extraction in this study, we cannot draw conclusions about this issue. Table 3 shows previous studies that evaluated CH and the CRF before and after myopic LASIK.13–16,22,25–30 In the current study, the deceases in CH and the CRF (per 1.00 D correction) were 0.48 mm Hg and 0.54 mm Hg, respectively, similar to those in the previous studies. Because the changes in the CH and CRF values were influenced by the amount of myopic correction, patient age, flap thickness, and the length of the postoperative period, it is difficult to directly compare the effect of LASIK and refractive lenticule extraction on these biomechanical parameters. The amount of lenticule thickness was slightly larger than that of excimer laser ablation for the equivalent myopic correction. For example, for a 4.0 D equivalent correction of myopia, the lenticule thickness for a 6.0 mm optical zone was 66 mm, whereas the ablation depth for a 6.0 mm optical zone was approximately
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 steroid eyedrops. We are performing a new study to compare the differences in corneal biomechanical variables after refractive lenticule extraction with those after LASIK. It is of clinical importance to assess the repeatability of measurements with the Ocular Response Analyzer to confirm the applicability of the data. We previously found that the mean difference between 2 consecutive measurements with the device was 0.1 G 0.5 mm Hg (95% limits of agreement [LOA], 1.1 to 1.0) for CH and 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 repeatability of the CH measurement was 0.8 mm Hg SD of the differences between 2 measurements. Accordingly, we believe that this device offers reasonable repeatability in the longitudinal evaluation of corneal biomechanical parameters. A limitation of this study is that the sample data were rather limited. However, the sample size in this study offered 88% statistical power at the 5% level. Moreover, this was a prospective intraindividual comparative study, which can provide more accurate information for a comparison of the biomechanical changes after the 2 lenticule extraction techniques because the patient age and sex were identical in the 2 groups and the amount of myopic correction and preoperative IOP were closely matched. Another limitation is that the follow-up was short. We evaluated the postoperative
Table 3. Summary of corneal hysteresis (CH) and corneal resistance factor (CRF) before and after myopic laser in situ keratomileusis (LASIK). Before LASIK Study*
Year Eyes (n) Mean Age (Y)
Pepose14 Oritz13 Chen25 Kamiya15 Kirwan22 Kamiya16 Qazi26 Shah27 Shah28 De Medeiros29 Uzbek30
2007 2007 2008 2008 2008 2009 2009 2009 2009 2010 2011
66 65 43 31 63 36 28 26 53 13 66
39.6 G 11.4 37 40.5 G 10.5 26.4 G 6.6 36.1 G 10.1 30.5 G 9.8 39 G 12 42.6 G 6.9 NA 32 G 10 30.3 G 6.1
MRSE (D) 5.1 G 2.8 4.3 G 3.0 4.0 G 2.0 4.1 G 1.4 3.8 G 1.8 4.4 G 1.4 5.3 G 2.7 NA NA 3.4 G 1.5 3.4 G 1.9/0.8 G 1.5†
Mean CH Mean CRF (mm Hg) (mm Hg) 9.7 G 1.8 10.4 G 1.7 11.5 G 1.3 10.8 G 1.4 10.8 G 1.4 10.6 G 1.7 10.0 G 1.8 11.9 G 2.3 11.9 G 2.0 11.4 G 1.5 9.6 G 2.0
After LASIK Mean CH (mm Hg)
Mean CRF (mm Hg) Postop Period
9.5 G 1.9 8.0 G 1.6 6.7 G 1.7 NA 10.1 G 2.0 9.3 G 1.9 8.1 G 1.9 1 mo 11.7 G 1.4 9.5 G 1.2 8.5 G 1.5 NA 10.3 G 1.5 8.6 G 0.9 7.7 G 1.3 3 mo NA 9.0 G 1.3 NA 3 mo 10.0 G 1.7 8.9 G 1.5 7.7 G 1.6 6 mo 9.9 G 2.0 8.6 G 2.3 7.4 G 2.5 6 mo 10.9 G 2.2 2.6 G 1.4z 2.7 G 1.4z 3 mo 10.4 G 1.8 9.8 G 1.9 8.0 G 1.9 NA 11.0 G 1.9 9.1 G 1.3 7.8 G 2.1 1 wk 10.9 G 2.8 8.0 G 1.7 7.9 G 1.7 Immediately
z Means G SD CH Z corneal hysteresis; CRF Z corneal resistance factor; LASIK Z laser in situ keratomileusis; MRSE Z manifest refraction spherical equivalent; NA Z not available *First author only † Sphere/cylinder z Change
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CH and CRF values up to 3 months after surgery, at which point, taking into account the wound-healing responses of the cornea, we considered the biomechanical properties of the cornea to have stabilized. We previously found no significant differences in biomechanical measurements between 3 months and 6 months after LASIK.16 However, in light of case reports of delayedonset keratectasia,32–34 we cannot refute the possibility that biomechanical changes may be ongoing over the long term. Further studies are needed to elucidate long-term biomechanical changes. In conclusion, we found that femtosecond lenticule extraction and small-incision lenticule extraction substantially decreased biomechanical parameters such as CH and the CRF depending on the amount of achieved myopic correction. In addition, no significant differences in the changes in CH or the CRF were observed 1 month and 3 months postoperatively in either group. These findings suggest that the presence or absence of flap lifting does not significantly affect these biomechanical characteristics. There were no significant differences in the CH or CRF values between the 2 lenticule extraction techniques. The Ocular Response Analyzer measures 37 additional biomechanical parameters that quantitatively describe several aspects of the applanation response during measurements using the manufacturer's latest generation software and other biomechanical parameters that were derived and reported by investigators to describe the waveform of response curve of the dynamic bidirectional applanation device.35,36 Further studies of the morphology of the Ocular Response Analyzer signals are necessary to clarify the biomechanical differences between the 2 lenticule extractions procedures. WHAT WAS KNOWN The refractive lenticule extraction technique for the correction of refractive errors can be used for femtosecond lenticule extraction by lifting the flap and for small-incision lenticule extraction without lifting the flap. The differences in these 2 procedures, with or without flap lifting, may affect the biomechanical characteristics of the cornea. However, the biomechanical changes after the procedures have not been reported. WHAT THIS PAPER ADDS Femtosecond lenticule extraction was essentially equivalent to small-incision lenticule extraction in terms of the CH and CRF values, indicating that the presence or absence of flap lifting does not significantly affect these biomechanical parameters.
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First author: Kazutaka Kamiya, MD, PhD Department of Ophthalmology, University of Kitasato School of Medicine, Kanagawa, Japan