Reproducibility of LASIK flap thickness using the Hansatome microkeratome

The Accuracy of Flap Thickness and Diameter in LASIK Using a Femtosecond Laser

Purpose

To evaluate the resultant thickness of the WaveLight FS200 (Alcon Vision, LLC) created LASIK flap compared to intended thickness at the surgical visit, using the WaveLight EX500 (Alcon Vision, LLC) pachymeter and the resultant flap diameter compared to intended diameter.

Methods

This single arm, prospective, single surgeon study assessed the accuracy of the intended flap thickness and diameter, after successful bilateral LASIK surgery. The WaveLight FS200 femtosecond laser was used to create all flaps with an intended thickness of 120 μm. Flap thickness was calculated by subtracting the stromal bed thickness after flap lift from the preoperative corneal thickness using the WaveLight EX500 on-board optical pachymeter. Flap diameter was determined using digital analysis.

Results

A total of 58 subjects (116 eyes) completed the study. The calculated mean flap thickness was 120.6 ± 9.0 μm (range 102 to 143 μm) using the EX500 pre- and post-flap pachymetry measurements. There was no statistically significant difference between the planned and achieved flap thickness (p > 0.05). The mean difference in flap diameter between planned and actual was 0.02 ± 0.05 mm. Corneal thickness measured by Pentacam at up to 2 months preoperatively versus EX500 just prior to surgery was similar, with EX500 measuring 2 μm less on average than the Pentacam.

Conclusion

The results suggest that the WaveLight FS200 laser is reliable for LASIK flap thickness and diameter and accurately created flaps at the intended thickness and the intended diameter.

Reproducibility of flap thickness in sub-Bowman keratomileusis using a mechanical microkeratome

PURPOSE

To examine the predictability of flap thickness using a mechanical microkeratome (One Use-Plus) and identify factors related to variations in flap thickness in sub-Bowman keratomileusis (SBK).

SETTING

Tadawi Surgical Center, Taif, Saudi Arabia.

DESIGN

Retrospective case series.

METHODS

Patients with a stable refraction for 1 year, corrected distance visual acuity (CDVA) of at least 20/20 in each eye, and minimum central corneal thickness of 480 μm in each eye had microkeratome-assisted SBK. Parameters included manifest refraction, uncorrected distance visual acuity (UDVA), CDVA, pachymetry, and higher-order aberrations (HOAs).

RESULTS

Seventy eyes (36 patients) were enrolled. The mean flap thickness was 88.74 μm ± 15.40 (SD) and the mean change in HOAs, 0.11 ± 0.30 μm(2). There was no correlation between flap thickness and age (r = -0.10), preoperative manifest refraction spherical equivalent (MRSE) (r = -0.08), preoperative cylinder (r = 0.13), postoperative CDVA (r = -0.17), or postoperative change in HOAs (r = -0.07). A strong positive correlation with preoperative pachymetry (r = 0.41) and a weak negative correlation with preoperative sphere (r = -0.21) were observed. There was no or a moderate negative correlation of pachymetry with age (r = -0.14), preoperative sphere (r = -0.36), cylinder (r = -0.04), or MRSE (r = -0.36).

CONCLUSIONS

The microkeratome was reliable with reasonable predictability for SBK flap creation. Extra caution in handling the flaps to avoid flap striae or tears is advised. Flap thickness correlated positively with preoperative pachymetry; however, the variation in flap thickness did not affect visual outcomes.

FINANCIAL DISCLOSURE

The author has no financial or proprietary interest in any material or method mentioned.

Post-LASIK keratectasia triggered by eye rubbing and treated with topography-guided ablation and collagen cross-linking--a case report

PURPOSE

To report a case of unilateral post-laser-assisted in situ keratomileusis (LASIK) keratectasia in a 35-year-old woman who had no known predisposing risk factors but who rubbed her affected eye frequently and vigorously in response to allergic conjunctivitis.

METHODS

Case report with relevant literature review.

RESULTS

A 35-year-old woman, with a cumulative risk scale score of 0 (according to the Randleman criteria), who underwent bilateral LASIK developed unilateral post-LASIK keratectasia 32 months later. She presented with a history of vigorous eye rubbing of the affected eye since about a year after allergic conjunctivitis. The fellow eye, which was not rubbed, remained normal. She complained of glare, halos, and ghost images in her affected eye. She underwent transepithelial topography-guided customized ablation with simultaneous UV-A corneal collagen cross-linking, after which she improved symptomatically and topographically.

CONCLUSIONS

Eye rubbing could contribute to the development of keratectasia, even in an eye that has no subclinical features of the disease. When detected early, a simultaneous combined topography-guided customized ablation treatment and collagen cross-linking is effective in improving the irregular corneal contour and restoring biomechanical stability.

Factors influencing corneal flap thickness in laser in situ keratomileusis with a femtosecond laser

Purpose

To evaluate factors responsible for the variability between intended and achieved corneal-flap thickness during femtosecond laser-assisted laser in situ keratomileusis (LASIK).

Methods

A prospective, nonrandomized, case study was performed on 35 eyes of 18 consecutive patients who underwent LASIK surgery using the 60 kHz femtosecond laser microkeratome. Eyes were assigned to three different thickness groups, with 110-, 120-, or 130-µm cut depths. Anterior segment optical coherence tomography was used to assess the morphology of 35 LASIK flaps at postoperative one week postoperatively. The flap thickness was assessed at seven measuring points across each flap. Patient age, preoperative spherical equivalent, manual keratometry, preoperative central pachymetry, and regional variability of the cornea were evaluated to determine where they influenced the achieved corneal flap thickness.

Results

Cuttings of all flaps were easily performed without any intraoperative complications. Flap-thickness measurements had a mean of 115.21 ± 4.98 µm (intended thickness, 110 µm), 121.90 ± 5.79 µm (intended, 120 µm), and 134.38 ± 5.04 µm (intended, 130 µm), respectively. There was no significant difference between the 110-µm and 120-µm groups when compared with the 130-µm group (one-way analysis of variance test, p > 0.05). Patients' age, preoperative spherical equivalent, manual keratometry, and preoperative central pachymetry did not affect the achieved flap thickness (Pearson correlations test, p > 0.05). The reproducibility of flap thickness in the central 1.5-mm radius area was more accurate than that in the peripheral 3.0 to 4.0-mm radius area (paired samples t-test, p < 0.05).

Conclusions

Femtosecond laser-assisted LASIK is likely to reproduce a reliable thickness of the corneal flap, which is independent of corneal shape factors or refractive status. Future studies should focus on variations in corneal biomechanical factors, which may also play an important role in determining flap thickness.

Safety and effectiveness of thin-flap LASIK using a femtosecond laser and microkeratome in the correction of high myopia in Chinese patients

PURPOSE

To establish safety and effectiveness of thin-flap LASIK using a femtosecond laser and microkeratome in correcting high myopia in Chinese patients.

METHODS

Two hundred seventy-four eyes of 148 Chinese patients with high myopia whose spherical equivalent refraction (SE) ranged from -6.12 to -15.75 diopters (D) received thin-flap LASIK with the VISX S4 IR excimer laser system. Corneal flaps were created with a femtosecond laser (15-kHz IntraLase, 134 eyes of 76 patients, target flap thickness 100 microm) and Moria M2 microkeratome (90-microm head, 140 eyes of 72 patients, target flap thickness 110 microm). Clinical outcomes were assessed with uncorrected (UCVA) and best spectacle-corrected visual acuity (BSCVA), manifest refraction, wavefront aberrometry, Schirmer tests, and tear break-up time (TBUT) at 1 day, 1 week, and 1 and 3 months postoperatively.

RESULTS

At 3 months, both groups showed comparable clinical outcomes in most parameters assessed, including the percent of postoperative UCVA better than or equal to preoperative BSCVA (P=.642), mean residual spherical equivalent refraction (P=.448), mean Schirmer test (P=.950), and mean TBUT (P=.867). Postoperative coma, trefoil, and spherical aberration were similar in both groups (P=.202, P=.898, and P=.890, respectively). Both groups had a similar percent of eyes with a change of SE of <1.00 D (P=.284).

CONCLUSIONS

Thin-flap LASIK with a femtosecond laser and microkeratome are both safe and effective for the correction of high myopia in Chinese patients. Femtosecond laser shows similar predictability, stability, and induction of higher order aberrations to the microkeratome.

Accuracy of corneal flap thickness achieved by two different age MK-2000 microkeratomes

AIMS

To investigate the accuracy of corneal flap thickness (FT) using two different age MK-2000 microkeratomes.

METHODS

The prospective cohort study enroled 260 patients with refractive error. Flaps were created using two microkeratomes A and B (new and aged, respectively) with 130-mum heads in two patient groups and two times the same blade in both treated eyes of each patient. The variations in FTs were compared between two groups and between both operated eyes of each patient. The correlations were analysed between FT and CCT or keratometric power.

RESULTS

In the A and B groups, the average FTs were 123.3+/-18.7 and 147.5+/-19.1 mum respectively. Difference in measurements between the actual FTs of first eye operations in the A group and intended 130 mum of FTs was not significant (P=0.462), but those of second operated eyes in the A group and both treated eyes in the B group were significant (P<0.001). Second cut achieved a thinner flap and increased the variability in FT, and an aged microkeratome achieved a thicker flap than a new microkeratome and than that claimed by the manufacturer. Positive correlations were observed between preoperative CCT and FT (P<0.05).

CONCLUSIONS

The first eye operation by a new MK-2000 microkeratome achieves the accuracy of the intended FT. FTs varied between first and second cuts of each patient and between two different age MK-2000 microkeratomes. LASIK surgeons should compare FT when using an aged MK-2000 microkeratome, and frequent and periodic comparison of FT achieved by all microkeratomes may be also recommended.

Thin-flap (sub-Bowman keratomileusis) versus thick-flap laser in situ keratomileusis for moderate to high myopia: case-control analysis

PURPOSE

To compare the refractive and visual outcomes of sub-Bowman keratomileusis (SBK) and thick-flap laser in situ keratomileusis (LASIK) for moderate to high myopia and evaluate the effect of corneal flap thickness on outcomes.

SETTING

Massachusetts Eye and Ear Infirmary, Boston, Massachusetts, USA.

METHODS

Two studies were performed. In the first study, the refractive and visual outcomes in 33 eyes that had SBK (flap thickness 82 to 120 microm) and 62 eyes that had thick-flap LASIK (flap thickness >or=160) were retrospectively analyzed. Inclusion criteria were spherical equivalent -4.0 to -10.0 diopters (D), astigmatism 3.0 D or less, and follow-up 3 months or more. In the second study, the influence of flap thickness was evaluated. A case-control matched study (21 pairs) that controlled for residual stromal bed (RSB) thickness was performed.

RESULTS

The mean flap thickness was 110.2 microm+/-9.2 (SD) in the SBK group and 179.2+/-19.5 microm in the thick-flap LASIK group. There were no significant differences in visual outcomes. In the second study with equivalent RSB thickness, case-control matched comparisons between SBK (mean 108.6+/-8.0 microm) and thick-flap LASIK (mean 165.7+/-12.6 microm) showed no differences in preoperative and postoperative refractive and visual outcomes. Comparison of the intended versus achieved correction showed no significant differences between the 2 groups.

CONCLUSION

Retrospective analyses showed that the safety, efficacy, and predictability of SBK were similar to those of conventional thick-flap LASIK in corneas with equivalent RSB thickness.

Blade source effect on laser in situ keratomileusis flap thickness with the Amadeus I microkeratome

PURPOSE

To determine the effect of different blades on laser in situ keratomileusis (LASIK) flap thickness created with the Amadeus I microkeratome (Ziemer Ophthalmic Systems).

SETTING

Emory University Department of Ophthalmology and Emory Vision, Atlanta, Georgia, USA.

METHODS

This retrospective nonrandomized comparative case study from January 2005 through June 2006 compared LASIK flap thickness created with blades from 2 manufacturers: the Surepass from Surgical Instrument Systems and distributed by AMO and the ML7090 CLB distributed by Med-Logics, Inc. Sex, preoperative corneal thickness, surgical-eye sequence, flap thickness and variance, and residual stromal bed were evaluated in each group.

RESULTS

This study evaluated 424 eyes of 226 patients. Surepass blades were used in 238 eyes and ML7090 CLB blades in 186 eyes. There were no significant differences between the 2 blade groups in preoperative corneal thickness, sex, or cases with corneal thickness greater than 550 microm. Mean flap thickness and variance were significantly lower in the ML7090 CLB group than in the Surepass group (P<.0001). There were no significant differences in flap thickness in either group based on sex; however, in both groups, flap thickness was significantly lower in second eyes and in eyes with a preoperative thickness less than 550 microm (P<.001).

CONCLUSIONS

The Amadeus I microkeratome created thinner, more consistent LASIK flaps with the ML7090 CLB blade than with the Surepass blade. Preoperative corneal thickness and eye sequence affected flap thickness, while sex did not.

Comparison of corneal flap thickness between primary eyes and fellow eyes using the Zyoptix XP microkeratome

PURPOSE

To evaluate corneal flap thickness created in myopic laser in situ keratomileusis (LASIK) using the Zyoptix XP 120 microkeratome (Bausch & Lomb).

SETTING

University-based eye clinic.

METHODS

Corneal thickness was measured preoperatively and intraoperatively after flap creation in 62 consecutive patients (124 eyes) who had LASIK for the correction of myopia. Corneal flap thickness was calculated by subtracting stromal bed thickness from total corneal thickness. In each patient, both corneas (right followed by left) were cut by 1 the same Zyoptix XP 120 microkeratome blade at the same session.

RESULTS

The mean actual flap thickness was 115.34 microm +/- 16.34 (SD) in right eyes and 104.55 +/- 14.34 mum in left eyes. The mean actual flap thickness in right eyes was not statistically significantly different from the 120 microm proposed by the manufacturer (P = .142); however, the mean actual flap thickness in left eyes was statistically significantly different from the 120 microm (P<.001). The mean flap thickness in the second eye was also statistically significantly thinner than in the first eye (P<.001).

CONCLUSIONS

The corneal flaps were thinner than expected when a Zyoptix XP 120 microkeratome was used in LASIK. Although the first cuts produced thinner flaps, this difference was not statistically significant. However, the second cuts produced significantly thinner flaps.

Comparative performance of the Zyoptix XP and Hansatome zero-compression microkeratomes

PURPOSE

To compare flap parameters produced by the Zyoptix XP and Hansatome microkeratomes (both Bausch & Lomb) and to evaluate preoperative variables contributing to flap thickness variation.

SETTING

Private practice, St. Louis, Missouri, USA.

METHODS

Flap dimensions in 75 eyes that prospectively had laser in situ keratomileusis using the Zyoptix XP microkeratome were compared with a historical control sample of 75 eyes treated with the Hansatome microkeratome. The 2 groups were matched for mean keratometry, central corneal pachymetry, spherical equivalent (SE), age, and microkeratome head size and suction ring diameter.

RESULTS

The mean ultrasound-measured flap thickness was 126.54 microm +/- 14.6 (SD) and 143.74 +/- 15.0 microm for the 120 microm and 140 microm Zyoptix XP heads, respectively, and 128.90 +/- 20.4 microm and 143.32 +/- 21.0 microm for the 160 microm and 180 microm Hansatome heads, respectively. The standard deviation in flap thickness was smaller and statistically significant for the Zyoptix XP (+/-14.8 microm) versus the Hansatome (+/-20.7 microm) (P = .0039, F test). Preoperative pachymetry and SE (P<.001) accounted for 20% of the variability in measured flap thickness using the Hansatome. For the Zyoptix XP, preoperative pachymetry was statistically significant in explaining the variation in flap thickness with the 120 microm head (P = .02) but not with the 140 microm head. Variation in flap thickness from either Zyoptix XP head was not statistically related to the preoperative SE.

CONCLUSION

Although the 2 microkeratomes produced flaps of similar mean thickness, the Zyoptix XP showed significantly less variation in flap thickness than the Hansatome, was less affected by measurable preoperative variables such as SE, and was closer to nominal labeling.

LASIK Flap Characteristics Using the Moria M2 Microkeratome With the 90-µm Single Use Head

PURPOSE

To evaluate the accuracy and consistency of corneal flap thickness, horizontal diameter, and hinge size with the Moria M2 90-microm single use head.

METHODS

Fifty-two myopic patients (104 eyes), mean age 32.6 years, underwent bilateral LASIK with a superior hinged flap using the Moria M2 microkeratome (90-microm single use head). Prospective evaluation included flap thickness (subtraction method), diameter, hinge size, interface particles, intraoperative complications, and visual recovery.

RESULTS

The mean preoperative spherical equivalent refraction was -5.72 +/- 2.59 diopters (D) (range: -2.88 to -10.75 D) and -5.84 +/- 2.73 D (range: -3.13 to -9.38 D) for right and left eyes, respectively. The mean preoperative central corneal thickness was 548 +/- 24 microm and 547 +/- 25 microm for right and left eyes, respectively. The mean preoperative steepest K was 44.12 +/- 1.28 D and 44.41 +/- 1.27 D for right and left eyes, respectively. Corneal diameter (white-to-white) was 12 +/- 0.4 mm and 11.9 +/- 0.4 mm for right and left eyes, respectively. The mean postoperative flap thickness was 109 +/- 18 microm (range: 67 to 152 microm) and 103 +/- 15 microm (range: 65 to 151 microm) for right and left eyes, respectively. The mean postoperative flap diameter was 9.4 +/- 0.3 mm (expected mean according to the nomogram given by the company was 9.5 mm). The mean postoperative hinge chord was 4.4 +/- 0.4 mm (expected mean 4.2 mm). No interface particles were detected on slit-lamp examination.

CONCLUSIONS

The Moria M2 90-microm single use head is safe with reasonable predictability for LASIK flap creation.

Femtosekundenlaser für die refraktive Hornhautchirurgie

Modern corneal laser surgery for the correction of optical errors of the eye requires a precise, reliable and reproducible creation of corneal lenticels (flaps). The use of ultra-short laser pulses with pulse durations of a few 100 femtoseconds (10(-13) s) allows for non-thermal cuts of ocular tissue. Mean flap thicknesses as small as 100 microm with a reproducibility of 10 microm (standard deviation) can be created by using mechano-optical adaptations through the eye. Thus, the femtosecond laser can be considered a good alternative approach with a safety in clinical use that is comparable with that of mechanical microkeratomes.

Probability Model of the Inaccuracy of Residual Stromal Thickness Prediction to Reduce the Risk of Ectasia After LASIK Part II: Quantifying Population Risk

PURPOSE

To derive a statistical model to estimate the rate of excessive keratectomy depth below a selected cut-off residual stromal thickness (RST) given a minimum target RST and specific Clinical Protocol; apply the model to estimate the RST below which ectasia appears likely to occur and back-calculate the safe minimum target RST that should be used given a specific Clinical Protocol.

METHODS

Myopia and corneal thickness distribution were modeled for a population of 5212 eyes that underwent LASIK. The probability distribution of predicted target RST error (Part I) was used to calculate the rate of excessive keratectomy depth for this series. All treatments were performed using the same Clinical Protocol; one surgeon, Moria LSK-One microkeratome, NIDEK EC-5000 excimer laser, Orbscan pachymetry, and a minimum target RST of 250 microm--the Vancouver Clinical Protocol. The model estimated the RST below which ectasia appears likely to occur and back-calculated the safe minimum target RST. These values were recalculated for a series of microkeratomes using published flap thickness statistics as well as for the Clinical Protocol of one of the authors-the London Clinical Protocol.

RESULTS

In the series of 5212 eyes, 6 (0.12%) cases of ectasia occurred. The model predicted an RST of 191 microm for ectasia to occur and that a minimum target RST of 329 microm would have reduced the -rate of ectasia to 1: 1,000,000 for the Vancouver Clinical Protocol. The model predicted that the choice of microkeratome varied the rate of ectasia between 0.01 and 11,623 eyes per million and the safe minimum target RST between 220 and 361 microm. The model predicted the rate of ectasia would have been 0.000003: 1,000,000 had the London Clinical Protocol been used for the Vancouver case series.

CONCLUSIONS

There appears to be no universally safe minimum target RST to assess suitability for LASIK largely due to the disparity in accuracy and reproducibility of microkeratome flap thickness. This model may be used as a tool to evaluate the risk of ectasia due to excessive keratectomy depth and help determine the minimum target RST given a particular Clinical Protocol.

Laser in situ keratomileusis flap thickness using the Hansatome microkeratome with zero compression heads

PURPOSE

To evaluate predictability and possible factors affecting flap thickness in laser in situ keratomileusis (LASIK) using the Hansatome microkeratome (Bausch & Lomb Surgical) with zero compression heads.

SETTING

Zentrum für Refraktive Chirurgie Münster, Münster, Germany.

METHODS

A prospective nonrandomized comparative (self-controlled) trial analyzing flap thickness in 153 consecutive patients having LASIK using the Hansatome microkeratome with zero compression was conducted. Two hundred thirty-eight eyes that had uncomplicated primary LASIK (n = 237) or secondary LASIK (n = 1) by the same surgeon and same technique using 4 different microkeratomes of the same model were evaluated. Each keratome cut was performed with a new Accuglide (Bausch & Lomb) blade in a 160 microm (n = 89), 180 microm (n = 128), or 200 microm (n = 21) Hansatome zero compression head coupled to a 8.5 mm (n = 106) or 9.5 mm (n = 131) suction ring. Only Hansatome elements with the same serial numbers were combined. Ultrasound subtraction pachymetry was routinely used to determine intraoperative flap thickness. Flap thickness was correlated with microkeratome head dimension, suction ring size, preoperative keratometry obtained by Orbscan IIz (Bausch & Lomb), preoperative corneal thickness as obtained by ultrasound pachymetry, refractive error, and age.

RESULTS

Measured intraoperative flap thickness was significantly different (P<.01) from predicted flap thickness. The mean flap thickness was 97 microm +/- 18 (SD) (range 65 to 163 microm), 111 +/- 20 microm (range 61 to 177 microm), and 131 +/- 20 microm (range 89 to 162 microm) for the 160 microm, the 180 microm, and 200 microm heads, respectively. There was a good correlation between microkeratome head and corneal flap thickness. However, there was a variability between devices. There was a low correlation between baseline ultrasound pachymetry at the time of surgery and corneal flap thickness (r = .26) and a small effect of ring size. There was no correlation with keratometry, refractive error, or age.

CONCLUSIONS

There was a remarkable difference in the flap thickness of microkeratomes of the same make and model. This emphasizes the need to measure intraoperative flap thickness and to evaluate every microkeratome separately. Factors affecting flap thickness seem to be more device dependent than patient related; obtaining flap thickness in the first eye did not enable predictions of the flap thickness in the fellow eye.

Factors Predictive of LASIK Flap Thickness With the Hansatome Zero Compression Microkeratome

PURPOSE

To determine the explanatory power of preoperative variables and comeal flap thickness in laser in situ keratomileusis (LASIK) using the Hansatome zero compression microkeratome (Bausch & Lomb, Rochester, NY).

METHODS

A prospective, nonrandomized, comparative interventional case study was performed on 250 eyes of 129 consecutive patients who underwent LASIK surgery using the Hansatome zero compression microkeratome. A 160-microm or 180-microm microkeratome head and an 8.5- or 9.5-mm suction ring were used in the procedures. Preoperative measurements included refraction, spherical equivalent, keratometry, intraocular pressure, corneal white-to-white, anterior chamber depth, and corneal eccentricity. Corneal thickness was measured intraoperatively using ultrasonic pachymetry before and after flap creation, and the difference was taken as flap thickness. Flap diameter was measured with a corneal gauge. Data were analyzed using simple, multiple, stepwise linear and non-linear regression analyses and two-tailed t tests.

RESULTS

The mean flap thickness was 124 +/- 17 microm with the nominal 160-microm head and 142 +/- 20 microm with the nominal 180-microm head. One third (33%) of the total variation in flap thickness could be accounted for by three preoperative variables: average corneal thickness, spherical equivalent refraction, and choice of 160- or 180-microm microkeratome head. A simple correlation of 0.114 was noted between corneal eccentricity and flap thickness, but this variable did not add significant explanatory power on multiple regression analysis. Linear regression analysis allowed determination of a flap thickness nomogram with a standard error of the estimate of 16.9 microm and a 95% confidence interval of +/- 33.1.

CONCLUSIONS

Comeal thickness is the most systematic predictor of corneal flap thickness using the Hansatome microkeratome. Because three preoperative variables account for only 33% of the range in flap thickness, future studies should focus on variations in blade extension and corneal biomechanical factors, which may also play an important role in determining flap thickness.

Randomized prospective clinical study comparing induced aberrations with IntraLase and Hansatome flap creation in fellow eyes

PURPOSE

To measure and compare the changes in objective wavefront aberration and subjective manifest refraction after laser in situ keratomileusis (LASIK) flap creation with a mechanical microkeratome and a femtosecond laser.

SETTING

Private practice refractive surgery center, Irvine, California, USA.

METHODS

This randomized prospective study comprised 9 patients (18 eyes) treated with a 2-step LASIK procedure: lamellar keratectomy with a Hansatome microkeratome (Bausch & Lomb) or the IntraLase femtosecond laser in fellow eyes followed by non-wavefront-guided (standard) excimer laser treatment with the Technolas 217A (Bausch & Lomb) excimer laser 10 weeks later. Fellow eyes were matched to within 0.75 diopter (D) sphere and 0.50 D cylinder. Patients were followed for 3 months after excimer laser treatment. Preoperative and post-flap creation wavefront aberrometry using a Hartmann-Shack aberrometer and manifest refraction were compared between the 2 groups. The same tests were performed 3 months after excimer laser ablation.

RESULTS

Statistically significant changes were seen in defocus wavefront aberrations after Hansatome (P=.004) and IntraLase (P=.008) flap creation. A hyperopic shift in manifest refraction was noted in the Hansatome group after the creation of the corneal flap (P=.04); no statistically significant changes in manifest refraction were seen in the IntraLase group. Statistically significant changes in total higher-order aberrations (HOAs) (trefoil and quadrafoil Zernike terms) were seen after flap creation in the Hansatome group (P=.02). No significant changes in HOAs were noted after flap creation in the IntraLase group. After the flap was relifted and standard excimer laser ablation was performed, a statistically significant increase in coma occurred in the Hansatome group (P=.008). Standard refractive outcomes in the 2 groups were similar.

CONCLUSIONS

The creation of the LASIK flap alone can modify the eye's optical characteristics in low-order aberrations and HOAs. A significant increase in HOAs was seen in the Hansatome group but not in the IntraLase group. This may have significant clinical implications in wavefront-guided LASIK treatments, which are based on measurements made before flap creation.