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Comparison of the Femtosecond Laser and Mechanical Keratome for Laser In Situ Keratomileusis
Annie Chan, MD;
Judy Ou, MD;
Edward E. Manche, MD
Arch Ophthalmol. 2008;126(11):1484-1490.
ABSTRACT
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Objective To compare clinical outcomes between fellow eyes randomized to femtosecond laser–created flaps (femtosecond group) or mechanical keratome–created flaps (mechanical group) during wavefront-guided laser in situ keratomileusis.
Design Prospective, randomized, comparative clinical study.
Main Outcome Measures Efficacy, safety, predictability, stability, changes in corneal optical aberrations, and low-contrast visual acuity before and 1 week and 1, 3, 6, and 12 months after surgery.
Results Forty-three patients underwent evaluation in this study. One month after surgery, the mean (SD) spherical equivalent was –0.15 (0.30) diopters (D) for the femtosecond group and –0.12 (0.29) D for the mechanical group (differences were not statistically significant). Twelve months after surgery, 39 eyes (98%) in the femtosecond group had uncorrected visual acuity of 20/20 or better compared with 37 (95%) in the mechanical group. The femtosecond group had fewer high-order, spherical, and coma aberrations and more trefoil aberrations than the mechanical group at 1 month (P = .55), 3 months (P = .05), 6 months (P = .33), and 12 months (P = .48) after surgery. At 25% contrast, the femtosecond group had gains at 1 month (P = .01) and 6 months (P = .008) after surgery.
Conclusion Twelve months after keratomileusis, clinical outcomes were similar for both groups.
INTRODUCTION
Laser in situ keratomileusis (LASIK) is the most common corneal refractive surgery performed for the correction of myopia.1 Recently, this technique has been refined with the introduction of femtosecond lasers used in the creation of precise flaps, a critical step in the surgical procedure.2-4 Femtosecond lasers use ultrafast pulses of energy to induce photodisruption of tissue with minimal collateral tissue damage and inflammation at a preset depth.5 Several studies describe the early clinical outcomes of flaps created by femtosecond lasers vs mechanical keratomes in patients who underwent LASIK.6-11 At present, however, no published reports of long-term results have compared the 2 methods of flap creation during LASIK.
The purpose of this study was to evaluate and compare the efficacy, safety, predictability, stability, changes in higher-order aberrations (HOAs), and low-contrast visual acuity between fellow eyes randomized to flap creation with a femtosecond laser or a mechanical keratome during LASIK.
METHODS
This study was a prospective, randomized comparison of fellow eyes undergoing wavefront-guided LASIK using a femtosecond laser keratome (IntraLase; IntraLase Corp/Advanced Medical Optics, Inc, Santa Anna, California) (femtosecond group) or a mechanical keratome (Hansatome; Bausch & Lomb, Rochester, New York) (mechanical group). The study was conducted and approved by the institutional review board at Stanford University. Informed consent was obtained after a detailed review of the procedure and study was discussed with all patients. Excimer laser ablation was performed with a commercially available excimer laser system, the Star S4 Excimer Laser System with WaveScan (VISX, Inc/Advanced Medical Optics, Inc, Santa Clara, California), by one surgeon (E.E.M.) with a target of full correction in all eyes. Randomization was performed by assigning the dominant eye to one keratotomy method and the fellow eye to the other method according to a prepared randomization schedule.
A drop of anesthetic was instilled in the inferior cul-de-sac after the patients underwent sterile draping and preparation. The reticle of the excimer laser system was positioned over the pupil. Autocentration was engaged in all cases. Pachymetry was measured by placing the sterile pachymeter tip directly over the pupil center. A flap was created with either the femtosecond laser or the keratome. Each subject had 2 flaps created, 1 with each keratome. All procedures were performed with superior hinged flaps. The flap was lifted and pachymetry of the bed was performed. Centration of the pachymeter was ensured using the excimer laser reticle. The first 8 subjects were treated before the laser received a software upgrade; in these patients, we used Fourier calculations for the wavefront treatment. The remaining 43 subjects underwent treatment after the upgrade was installed. Patients underwent postoperative examinations 1 day, 1 week, and 1, 3, 6, and 12 months after LASIK. Data were collected on case report forms that were completed for each of these visits and for preoperative and operative visits.
Femtosecond laser flaps were programmed with 120-µm thickness and 9.0-µm diameter, operating at 15 kHz. The mechanical keratome used a 160-µm head and a 9-mm suction ring. Both instruments were used to create superior hinged flaps.
Fifty-one patients were enrolled from May 27, 2004, to November 17, 2005, and underwent evaluation at the Refractive Surgery Center of Stanford University. Inclusion criteria were no more than 6.00 diopters (D) of spherical myopia, no more than 3.00 D of refractive astigmatism, stable refraction (<0.50 D of sphere or cylinder), corneal diameter of less than 11.0 mm to allow for suction ring fixation, discontinuation of soft contact wear at least 7 days before the preoperative evaluation, visual acuity correctable to at least 20/20 in both eyes, age older than 21 years, and ability to participate in follow-up examinations for 12 months after LASIK. To compare the visual outcomes of the 2 keratomes fairly, patients were required to have closely matched eyes in terms of refractive error. Exclusion criteria consisted of the following: rigid gas-permeable contact lens use; severe dry eye; severe blepharitis; anterior segment abnormalities (ie, cataracts, corneal scarring, or neovascularization within 1 mm of the intended ablation zone); recurrent corneal erosion; severe basement membrane disease; progressive or unstable myopia or keratoconus; unstable corneal mires on central keratometry results; corneal thickness in which the LASIK procedure could result in less than 250 µm of remaining posterior corneal thickness below the flap postoperatively; baseline standard manifest refraction exhibiting greater than 0.75 D more minus in sphere power or a difference of greater than 0.50 D in cylinder power, or a different type of astigmatism, ie, "with-the-rule," "against-the-rule," or "oblique" when the cylinder is greater than 0.50 D compared with the baseline standard cycloplegic refraction; preoperative assessment of ocular topography and/or aberrations indicating that either eye is not a suitable candidate for the LASIK vision correction procedure (ie, forme fruste keratoconus, corneal warpage, or pellucid marginal degeneration); previous intraocular or corneal surgery; history of herpes zoster or herpes simplex virus keratitis; current use of systemic corticosteroid or immunosuppressive therapy; immunocompromise or clinically significant atopic disease; connective tissue disease; diabetes mellitus; steroid response; macular abnormality; pregnancy or lactation; sensitivity to the planned study concomitant medications; or participation in another clinical trial of an ophthalmic drug or device.
A comprehensive eye examination was performed before and after LASIK, including manifest refraction, uncorrected visual acuity (UCVA), best-corrected visual acuity (BCVA), and HOA analysis with the study aberrometer. Low-contrast visual acuity was measured and recorded using Early Treatment of Diabetic Retinopathy Study contrast charts at 5% and 25%. Patients were instructed to use 1 drop each of prednisolone acetate and moxifloxacin hydrochloride 4 times daily for 1 week.
Data analysis was performed using commercially available software (SPSS for Windows, version 11.0; SPSS Inc, Chicago, Illinois). We performed the unpaired t test to compare both groups. A 1-way analysis of variance was used to compare keratomes in terms of the difference between the actual and expected corneal flap thicknesses. Differences were considered to be statistically significant when P < .05. Unless otherwise indicated, data are expressed as mean (SD).
RESULTS
Patient baseline characteristics are shown in Table 1. Fifty-one patients were enrolled in the study. The mean age was 39.7 (7.8) (range, 25-59) years. Thirty-two (63%) were women, 35 (69%) white, 9 (18%) Asian, 5 (10%) Hispanic, and 2 (4%) Indian. Mean preoperative spherical equivalent was –3.76 (1.41) D in the femtosecond group and –3.77 (1.40) D in the mechanical group. Four patients (5 eyes) discontinued the study 6 months after surgery or later and required retreatment with good outcomes. Two patients (4 eyes) were lost to follow-up.
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Table 1. Baseline Patient Characteristicsa
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Because of the excimer laser software upgrade after the first 8 patients underwent LASIK, only the remaining 43 patients are included in the refractive outcomes and patient preference analyses to rule out any issues related to the initial version of excimer laser software.
FLAP THICKNESS
In the femtosecond group, the mean flap thickness when we used a 110-µm depth setting was 118.7 (18.2) µm (n = 51); in the mechanical group, the mean flap thickness when we used a 160-µm head was 137.0 (21.1) µm (n = 51). Actual flap thickness in each eye differed significantly from expected flap thickness according to the manufacturer's specifications (P < .001 for the femtosecond group and P = .002 for the mechanical group). In addition, there was no difference in the reproducibility (variance) of flap thickness between methods (P = .17).
EFFICACY
Preoperative BCVA was 20/20 or better in 43 eyes (100%). One month after surgery, UCVA was 20/20 or better in 43 eyes (100%) of the femtosecond group and 42 eyes (98%) of the mechanical group (Table 2). Three months after surgery, UCVA was 20/20 or better in 41 eyes (98%) of the femtosecond group and 41 eyes (98%) of the mechanical group. By 6 months, UCVA was 20/20 or better in 42 eyes (100%) of the femtosecond group and 39 eyes (93%) of the mechanical group. At the final 12-month postoperative visit, UCVA was 20/20 or better in 39 eyes (98%) of the femtosecond group and 37 eyes (95%) of the mechanical group.
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Table 2. Postoperative Uncorrected Visual Acuity (UCVA) in the Femtosecond and Mechanical Groupsa
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SAFETY
The safety of the 2 keratomes was illustrated by the number of lines gained or lost after surgery (Figure 1). To calculate the safety index, BCVA is converted to Snellen acuities in logMAR values, and the mean postoperative BCVA is then divided by the mean preoperative BCVA. Six months after LASIK, the safety index was 1.19 for the femtosecond group and 1.15 for the mechanical keratome group.
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Figure 1. Changes in best-corrected visual acuity (BCVA) (safety) 12 months after laser in situ keratomileusis in both groups. Groups are described in the "Methods" section.
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PREDICTABILITY
For the femtosecond group, the mean postoperative spherical equivalent was –0.30 (0.26) D 12 months after surgery, whereas the mean postoperative spherical equivalent for the mechanical group was –0.20 (0.31) D (Figure 2). After 12 months, all eyes in both groups were within 1 D of the intended refractive change. Thirty-seven eyes (93%) in the femtosecond group and 35 eyes (90%) in the mechanical group were within 0.5 D of the target refractive change.
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Figure 2. Intended vs achieved correction of spherical equivalent (SphEq) (predictability) 12 months after laser in situ keratomileusis in both groups. Groups are described in the "Methods" section. D indicates diopter; MRSphEq, manifest SphEq.
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STABILITY
For the femtosecond group, the change in mean spherical equivalent between 1 and 3 months was –0.05 D, that between 3 and 6 months was –0.06 D, that between 6 and 12 months was –0.04 D, and the overall regression was –0.30 D. For the mechanical group, the change in mean spherical equivalent between 1 and 3 months was 0.04 D, that between 3 and 6 months was –0.02 D, that between 6 and 12 months was –0.02 D, and the overall regression was –0.20 D (Figure 3). Both keratomes had similar results by the final 12-month visit.
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Figure 3. Time course of the spherical equivalent and standard deviation after laser in situ keratomileusis for both groups. Groups are described in the "Methods" section. D indicates diopter.
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HIGHER-ORDER ABERRATIONS
There was a trend for flaps in the femtosecond group to induce fewer HOAs (Figure 4), spherical aberrations, and coma aberrations and to reduce more trefoil aberrations than flaps in the mechanical group 1 month (P = .55), 3 months (P = .05), 6 months (P = .33), and 12 months (P = .48) after surgery. This finding was statistically significant only at the 3-month visit (P = .05).
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Figure 4. Change in higher order aberrations (HOAs) 12 months from baseline. Bars and lines represent means and SDs, respectively. Groups are described in the "Methods" section.
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LOW-CONTRAST VISUAL ACUITY
Contrast visual acuity testing was added after the study had started to determine whether there was a difference between the 2 keratomes in visual acuity under different contrast conditions (5% and 25%). Therefore, the number of patients with baseline and postoperative data is limited. The values at 5% contrast are similar (Figure 5). At 25% contrast, the femtosecond group, compared with the mechanical group, had a statistically significant gain from baseline 1 month (P = .01) and 6 months (P = .008) after surgery (Table 3). However, this finding lost statistical significance at the final 12-month visit (P = .29).
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Figure 5. Contrast sensitivity visual acuity changes from baseline at 5% contrast and 25% contrast 12 months after laser in situ keratomileusis for both groups. The 12-month postoperative examination included 23 eyes in the femtosecond group (F) and 22 in the mechanical group (M). Groups are described in the "Methods" section.
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Table 3. Change in 25% Contrast Visual Acuity From Baselinea
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In addition to refractive outcomes, patients were given self-evaluation questionnaires concerning which eye had better vision (Figure 6). Initially, a greater proportion of patients preferred the eye that had undergone femtosecond flap creation. At the final postoperative visit, fewer patients expressed a preference for one eye over the other.
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Figure 6. Postoperative patient eye preference. Groups are described in the "Methods" section.
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COMPLICATIONS
Complications from this study are listed in Table 4. One eye reportedly had fiber debris in the interface at all postoperative visits. The fiber was outside the visual axis and the eye had excellent refractive outcomes (UCVA, 20/12.5). Ten patients had diffuse lamellar keratitis (DLK) at the 1-day postoperative visit. Three patients had DLK in both eyes. In addition, 7 patients had DLK only in the eye in the femtosecond group. All cases of DLK were graded as trace and resolved by the 1-week visit without surgical intervention or loss of vision. One eye in the femtosecond group had interface haze at the 1-month visit, and about 2 weeks later the eye had interface fibrosis. At the 3-month postoperative visit, no haze, fibrosis, or further complications were reported. At the 6-month visit, episcleritis that was unrelated to treatment developed in 1 eye despite its having 20/10 UCVA. At the 12-month visit, no complications were reported, and UCVA and BCVA were both 20/10 for that eye.
PATIENT PREFERENCE
One month after LASIK, 22 of 50 patients who responded preferred vision in the eye that received the femtosecond laser–created flap, 13 patients preferred vision in the eye that received the mechanical keratome–created flap, 14 patients expressed no preference, and 1 patient was unsure. Similarly, 3 months after LASIK, 22 of 48 patients preferred vision in the eye that received the femtosecond laser–created flap, 11 patients preferred vision in the eye that received the mechanical keratome–created flap, 12 patients expressed no preference, and 3 patients were unsure. Six months after LASIK, 13 of 48 patients preferred vision in the eye that received the femtosecond laser–created flap, 16 patients preferred vision in the eye that received the mechanical keratome–created flap, 16 patients expressed no preference, and 3 patient were unsure. Twelve months after LASIK, 11 of 45 patients preferred vision in the eye that received the femtosecond laser–created flap, 12 patients preferred vision in the eye that received the mechanical keratome–created flap, 17 patients expressed no preference, and 5 patients were unsure.
COMMENT
This prospective study compared fellow eyes that were randomized to receive flaps created by a femtosecond laser or by a mechanical keratome during wavefront-guided LASIK in the treatment of myopia. We evaluated efficacy, safety, predictability, stability, and HOAs. In the early postoperative period, specifically postoperative day 1 and week 1, eyes in the femtosecond group enjoyed faster visual rehabilitation compared with those in the mechanical group. One month after surgery, all 43 eyes in the femtosecond group had achieved UCVA of 20/20 or better, whereas 41 eyes in the mechanical group (98%) achieved UCVA of 20/20 or better. By 3 months, both groups experienced similar results, with 41 patients in both groups (98%) achieving UCVA of 20/20 or better. Initially, the femtosecond group had less residual astigmatism and less spherical aberration when compared with the mechanical group, a difference that was statistically significant at the 3-month visit (P = .05). However, this significance was lost by 12 months. The overall regression was similar in both groups, at –0.30 D in the femtosecond group and –0.20 D in the mechanical group. Twelve months after LASIK, all eyes in both groups were within 1 D of the intended refractive change.
Although the femtosecond group in this study had better short-term results compared with the mechanical group, 12 months after surgery there was no clinical or statistical difference in UCVA, manifest refractive error, contrast visual acuity, or patient eye preference between groups. These results support recently published 6-month data by Patel et al6 but are in contrast to 6-month data by Montés-Micó et al.7
In terms of flap thickness, the standard deviation was 18.2 µm in the femtosecond group and 21.1 µm in the mechanical group. There was no significant difference in the standard deviation between groups (P = .17). These standard deviations are consistent with those of other published reports. Flanagan and Binder12 reported 22.9 and 24.1 µm for 2 different microkeratomes. In a different study, Binder13 found standard deviations of 12 to 18 µm, depending on the attempted flap thickness, with use of a femtosecond laser. More recently, Patel et al6 reported standard deviations of 16 µm with femtosecond lasers and 22 µm with the mechanical microkeratome.
Total HOAs increased in both groups, with spherical aberrations as the major component. These findings are consistent with other studies.14-17 Femtosecond laser–created flaps induced fewer spherical aberrations and coma aberrations than did mechanical keratome–created flaps at all postoperative visits. Femtosecond flaps also reduced more trefoil aberrations than did mechanical keratome flaps at all postoperative visits. These values were not statistically significant, except for the difference in spherical aberrations at the 3-month visit (P = .05). The CustomVue laser system (VISX, Inc/Advanced Medical Optics, Inc) has been shown to reduce trefoil while increasing other types of HOAs.18 It is not entirely clear why trefoil aberrations were reduced in both groups. Porter et al19 hypothesized that the inherent properties of a superior hinge may influence the negative shift in trefoil. Furthermore, based on those studies, they found that most of the spherical aberration induced by the LASIK procedure seemed to result from the laser ablation and not the creation of the flap.
There was no significant difference in low-contrast visual acuity between the 2 groups except at the 12-month postoperative visit. At 25% contrast visual acuity, the femtosecond group had a slight gain in contrast compared with the mechanical group, which reached statistical significance 6 months after surgery (P = .008).
In terms of complications, the femtosecond group experienced a higher rate of DLK compared with the mechanical group (10 eyes [20%] vs 3 [6%]). Our rate of 20% is consistent with the rates of 19.4% by Binder13 and 17% by Javaloy et al.20 Gil-Cazorla et al21 also found a higher rate of DLK in their femtosecond group when compared with the mechanical group but with a lower overall rate in both groups (0.5% in the femtosecond group and 0.1% in the microkeratome group). Gil-Cazorla et al attributed the lower rates in their study to the more intense anti-inflammatory therapy (8 times daily in the femtosecond group instead of 4 times daily in our study) during the first postoperative week. It is possible that the higher incidence of DLK associated with the femtosecond laser is a result of higher energy at the interface. Binder13 was able to decrease the amount of interface inflammation by decreasing the side-cut energy and angle.
CONCLUSIONS
This report contains, to our knowledge, the longest follow-up data available comparing the clinical outcomes of eyes after LASIK surgery with either a femtosecond laser– or a mechanical keratome–created flap. Although our study demonstrated superior outcomes in many clinical measures in the early postoperative period and a more accurate and predictable flap thickness in the femtosecond group, we found no clinical or statistical differences between fellow eyes randomized to the femtosecond group or to the mechanical group 1 year after surgery. This finding supports other reports that either method of flap creation is acceptable. Recent studies using anterior segment optical coherence tomography have demonstrated differences between mechanical keratome–created flaps and femtosecond laser–created flaps. These studies demonstrate that the femtosecond laser–created flaps are uniform throughout with a planar flap configuration and that mechanical keratome–created flaps are less uniform with a meniscus flap configuration. This difference in flap architecture may be important from a corneal biomechanical standpoint. The planar configuration of the femtosecond laser–created flaps may induce fewer biomechanical changes to the cornea than mechanical keratome–created flaps. This difference may be important in preventing or reducing the incidence of post-LASIK corneal ectasia.22
Further long-term studies with large patient populations are needed to evaluate the advantages and disadvantages, including corneal biomechanical stability and safety of newer-generation femtosecond laser–created flaps vs mechanical keratome–created flaps.
AUTHOR INFORMATION
Correspondence: Edward E. Manche, MD, Department of Ophthalmology, Stanford University School of Medicine, 900 Blake Wilbur Dr, Third Floor, Stanford, CA 94305 (edward.manche{at}stanford.edu).
Submitted for Publication: March 4, 2008; final revision received June 25, 2008; accepted July 2, 2008.
Financial Disclosure: None reported.
Previous Presentation: This paper was presented in part at the American Society of Cataract and Refractive Surgery meeting; March 20, 2006; San Francisco, California; and at the European Society of Cataract and Refractive Surgery meeting; September 10, 2006; London, England.
Author Affiliations: Department of Ophthalmology, Stanford University School of Medicine, Stanford, California.
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12. Flanagan GW, Binder PS. Precision of flap measurements for laser in situ keratomileusis in 4428 eyes. J Refract Surg. 2003;19(2):113-123.
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13. Binder PS. Flap dimensions created with the IntraLase FS laser. J Cataract Refract Surg. 2004;30(1):26-32.
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14. Mrochen M, Kaemmerer M, Mierdel P, Seiler T. Increased higher-order optical aberrations after laser refractive surgery: a problem of subclinical decentration. J Cataract Refract Surg. 2001;27(3):362-369.
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16. Marcos S, Barbero S, Llorente L, Merayo-Lloves J. Optical response to LASIK surgery for myopia from total and corneal aberration measurements. Invest Ophthalmol Vis Sci. 2001;42(13):3349-3356.
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20. Javaloy J, Vidal MT, Abdelrahman AM, Artola A, Alió JL. Confocal microscopy comparison of IntraLase femtosecond laser and Moria M2 microkeratome in LASIK. J Refract Surg. 2007;23(2):178-187.
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21. Gil-Cazorla R, Teus MA, de Benito-Llopis L, Fuentes I. Incidence of diffuse lamellar keratitis after laser in situ keratomileusis associated with the IntraLase 15 kHz femtosecond laser and Moria M2 microkeratome. J Cataract Refract Surg. 2008;34(1):28-31.
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SECTION EDITOR: ROY W. BECK, MD, PhD
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