From London Vision Clinic, London, United Kingdom (Reinstein, Archer, Gobbe); the Department of Ophthalmology, St Thomas’ Hospital - Kings College, London, United Kingdom (Reinstein); the Department of Ophthalmology, Weill Medical College of Cornell University, New York, NY (Reinstein); Centre Hospitalier National d’Ophtalmologie, Paris, France (Reinstein); and the Department of Physics, University of Miami, Miami, Fla (Johnson).
Dr Reinstein is a consultant for Carl Zeiss Meditec (Jena, Germany), has a proprietary interest in the Artemis technology (ArcScan Inc, Morrison, Colo), and is the author of patents related to VHF digital ultrasound administered by the Cornell Center for Technology Enterprise and Commercialization (CCTEC), Ithaca, NY. The remaining authors have no proprietary or financial interest in the materials presented herein.
Study concept and design (D.Z.R., T.J.A., M.G.); data collection (T.J.A., M.G.); analysis and interpretation of data (D.Z.R., T.J.A., M.G., N.J.); drafting of the manuscript (D.Z.R., T.J.A., M.G.); critical revision of the manuscript (D.Z.R., T.J.A., M.G., N.J.); statistical expertise (T.J.A., N.J.)
Correspondence: Dan Z. Reinstein, MD, MA(Cantab), FRCSC, FRCOphth, London Vision Clinic, 138 Harley St, London W1G 7LA, United Kingdom. Tel: 44 207 224 1005; Fax: 44 207 224 1055; E-mail email@example.com
Microkeratomes are used in LASIK to create a corneal flap of a desired thickness. The accuracy and reproducibility of flap thickness influences the ability to predict the residual stromal thickness, and as a consequence directly affects the safety and efficacy of the procedure.1,2 The cornea is thinnest centrally and the ablation of a myopic profile is deepest centrally; therefore, the central flap thickness is particularly important when considering residual stromal thickness. High variability in flap thickness can increase the risk of long-term plastic corneal changes, such as ectasia, in cases where a flap was significantly thicker than expected.1–3 On the other hand, if a flap is created much thinner than expected, there is the risk of producing a full thickness buttonhole4,5 or a cryptic buttonhole.6 Variability in flap thickness can also have a direct effect on refractive correction because the depth of keratectomy relates to the amount of induced bioelastic corneal change, which in turn can affect the accuracy of the desired curvature change.7
Central flap thickness standard deviation of certain mechanical microkeratomes has been reported to be in the range of 20 to 40 μm.8–14 More recent mechanical microkeratomes have improved flap thickness standard deviation to between 12 and 20 μm.2,15,16
Recently, femtosecond lasers have been used for flap creation with a view to further improving the accuracy and reproducibility of flap thickness. A number of femtosecond lasers are currently available: IntraLase femtosecond laser (IntraLase Corp, Irvine, Calif), FEMTO LDV (Ziemer Ophthalmic Systems AG, Port, Switzerland), Femtec femtosecond laser (20/10 Perfect Vision, Heidelberg, Germany) as well as the VisuMax femtosecond laser system (Carl Zeiss Meditec, Jena, Germany). The published flap thickness reproducibility (standard deviation) of femtosecond lasers reported varies between 5 μm17 and 21 μm,15 with an average of approximately 13 μm (Table 1).15–27
Table 1: Studies of LASIK Flap Thickness Using Femtosecond Laser Systems
The VisuMax femtosecond laser system obtained CE mark approval and was granted clearance to market by the US Food and Drug Administration (FDA) in January 2007. The purpose of this study was to measure the accuracy and reproducibility of central flap thickness for flaps created with the VisuMax femtosecond laser system.
Patients and Methods
This study was a prospective noncomparative case series of 12 consecutive volunteer patients with moderate myopia recruited from a population of patients seeking refractive surgery at the London Vision Clinic, London, United Kingdom. A complete ocular examination was performed to screen for corneal abnormalities and determine patient candidacy for refractive surgery. Patients included for study were myopic with a spherical equivalent refraction up to −6.50 diopters (D). Patients with ocular pathologies such as keratoconus, corneal scars, corneal dystrophies, and previous ocular surgery were excluded. Preoperative assessment included manifest refraction, logMAR best spectacle-corrected visual acuity (BSCVA) (CSV-1000 Vector Vision Inc, Greenville, Ohio), and cycloplegic refraction using one drop of tropicamide 1% (Alcon Laboratories UK Ltd, Hemel Hempstead, United Kingdom). Mesopic contrast sensitivity was measured with the CSV-1000. Topography and keratometry were assessed using the Orbscan II (Bausch & Lomb, Salt Lake City, Utah) and the Atlas (Carl Zeiss Meditec). Dynamic pupillometry was carried out using the Procyon P2000 pupillometer (Procyon Instruments, London, United Kingdom). Wavefront assessment was performed using the WASCA aberrometer (Carl Zeiss Meditec). Single-point handheld pachymetry was measured with the Corneo-Gage Plus (50 MHz) ultrasound pachymeter (Sonogage, Cleveland, Ohio). The white-to-white diameter was obtained from the Orbscan examination. Layered pachymetry of the cornea for the central 8- to 10-mm diameter was obtained using the Artemis 1 very high-frequency (VHF) digital ultrasound arc-scanner (ArcScan Inc, Morrison, Colo).
A written informed consent was obtained from all patients. The study adhered to the tenets of the Declaration of Helsinki and was performed in accordance with an institutional review board approved protocol.
VisuMax Femtosecond Laser System
The laser pulse repetition rate of the VisuMax femtosecond laser system is 200 kHz and the femtosecond laser pulse energy of the VisuMax system used for this study was between 0.10 and 0.15 mJ. The VisuMax femtosecond flap creation procedure starts with the application of a disposable contact glass to the laser aperture cone. The contact glass is similar to a gonio-scopic lens in that it possesses a curved surface designed to couple with the cornea with only a minimal contact glass application force. Before coupling, the VisuMax system self-calibrates the contact glass. One of the main differences between the VisuMax and other systems is that suction is applied directly to the cornea, not the bulbar conjunctiva and sclera as is the case with classical suction systems based on the design by Jose Ignacio Barraquer.3 The combination of a curved application and corneal suction means that the intraocular pressure (IOP) rise during the procedure is low enough that the patient is able to see the light during the entire procedure, which helps with corneal immobilization as the extraocular muscles stabilize the eye in the primary position. In a study performed to measure the IOP rise during flap creation with the VisuMax, it was found that the average IOP during flap creation in cadaver eyes was 84.9 mmHg.28 To further improve patient fixation and flap centration onto the visual axis, vergence of the fixation target within the system is adjusted using the manifest refraction of each individual eye. The eye’s keratometry data are also entered into the VisuMax to account for the difference between the relaxed cornea and the contact glass curvature. This allows the system to calculate the ratio between the intended clinical flap diameter on the relaxed eye and the technical incision diameter when cutting the eye attached to the contact glass. Flap parameters that can be adjusted include the flap thickness, flap diameter, hinge width, side-cut angle, and hinge location.
The contact glass is available in three sizes: small (S), medium (M), and large (L). The size of the contact glass is chosen depending on the white-to-white diameter. The S contact glass is recommended for white-to-white diameters ≤11.5 mm, the M contact glass is recommended for white-to white diameters between 11.6 and 12.6 mm, and the L contact glass is recommended for white-to-white diameters ≥12.7 mm. The maximum flap diameter using software release 2.3.0 was 7.6 mm for the S contact glass, 8.5 mm for the M contact glass, and 9.1 mm for the L contact glass.
All procedures were performed by the same surgeon (D.Z.R.) and consisted of the surgeon’s first consecutive 24 eyes using the VisuMax femtosecond laser system. Two drops of proxymetacaine 0.5% (Chauvin Pharmaceuticals Ltd, Surrey, United Kingdom) were instilled 5 minutes apart to anesthetize the eye. The VisuMax was pre-programmed for each procedure with a flap thickness of 110 μm, a side-cut angle of 110°, and a superior hinge. The manifest refraction and keratometry were entered into the VisuMax. The S size contact glass was used for one patient whose Orbscan white-to-white diameter was 11.4 mm; the intended flap diameter was 7.5 mm in both eyes. The M size contact glass was used in all other patients; the intended flap diameter was 8.0 mm in 14 eyes and 8.5 mm in 8 eyes. The intended flap hinge width was 3.8 mm in 4 eyes, 4.5 mm in 2 eyes, and 5.0 mm in 18 eyes.
The patient bed is moved by a joystick, which controls movement in the x-y and z directions so that the eye is brought up into contact with the contact glass while the patient is fixating on a flashing green light. This aligns the eye in the primary position, allowing the bed to be raised vertically while the surgeon observes the alignment of the contact glass application through the operating microscope as the contact glass is applied to the cornea in a self-centering way on the corneal vertex. Once contact is made between the cornea and the contact glass, the patient is able to see the flashing fixation target in clear focus. When full contact glass application is achieved, suction is applied, the eye is immobilized, and the laser is activated by the surgeon pressing on a foot pedal. Flap creation time in this study was approximately 40 seconds for each eye, using software release 2.3.0.
After the surgeon concluded creating the flaps for both eyes, the patient bed was rotated 180° to be positioned under the MEL 80 excimer laser (Carl Zeiss Meditec). Little to no residual bubbles were present by the time the patient was positioned under the MEL 80 to treat the first eye in which the flap had been created; therefore, the eye tracker was activated with no delay. The flap was lifted and laser ablation was carried out. The nomogram derived for treatments with the Hansatome zero compression microkeratome (Bausch & Lomb) and MEL 80 excimer laser at the London Vision Clinic was used for all eyes. After the flap was repositioned, the flap diameter and flap hinge width were measured using surgical calipers (Asico, Westmont, Ill).
Patients were instructed to wear plastic shields at night for 7 nights. Tobradex (Alcon Laboratories, Ft Worth, Tex) and Exocin (Allergan Ltd, Marlow, United Kingdom) were applied four times daily for the first week. Patients were reviewed at 1 day, 1 week, 1 month, and 3 months. All postoperative follow-up visits included measurement of manifest refraction, uncorrected visual acuity (UCVA), BSCVA, Orbscan II topography, WASCA aberrometry, mesopic contrast sensitivity, Visante optical coherence tomography (OCT, Carl Zeiss Meditec), and Artemis 1 VHF digital ultrasound.
Reinstein Flap Thickness Measurement
Calculation of the Reinstein Flap Thickness has been described previously.29 Briefly, central Reinstein Flap Thickness was measured by adding the thickness of the stromal component of the flap measured 3 months after surgery to the preoperative epithelial thickness. Measuring the flap at least 3 months after surgery ensures that postoperative edema has resolved, while using the preoperative epithelial thickness accounts for any postoperative epithelial changes known to occur after LASIK.7,30 The details of the Artemis VHF digital ultrasound arc-scanner and patient setup have been described previously.31–38 The repeatability of single point flap thickness measurements using the Artemis 1 VHF digital ultrasound arc-scanning system has been shown to be 1.14 μm.31
Descriptive statistics (mean, standard deviation, minimum, maximum, and range) were calculated for the central Reinstein Flap Thickness across eyes. Accuracy of central Reinstein Flap Thickness was calculated as the difference between the mean and the intended flap thickness (110 μm in this case series). Reproducibility of central Reinstein Flap Thickness was evaluated as the flap thickness standard deviation between eyes.
The accuracy of flap diameter was determined as the average difference between the obtained and intended flap diameter. The accuracy of flap hinge width was determined as the average difference between the obtained and intended flap hinge width.
Visual outcomes were recorded and plotted according to the standardized guidelines set out by Waring.39 The mean normalized mesopic contrast sensitivity ratio was calculated.40 Student paired t tests were used to assess the change in mesopic contrast sensitivity. The average pre- and postoperative higher order aberrations up to the sixth order were calculated for total higher order root-mean-square (RMS), coma, and spherical aberration. Wavefront examinations were analyzed in a 6.0-mm zone using Optical Society of America (OSA) notation.
Microsoft Excel 2003 (Microsoft Corp, Remond, Wash) was used for data entry and statistical analysis.
Twenty-four eyes of 12 patients were included in the study. Patients were examined at 1 day, 1 week, 1 month, and 3 months after surgery with 100% follow-up for all visits. Mean patient age was 31.6±7.5 years (median: 30.0 years, range: 23.9 to 52.1 years). Mean refraction was −2.98±1.66 D sphere (range: +0.25 to − 5.75 D) and −0.80±0.55 D cylinder (range: 0.00 to − 2.00 D). Mean spherical equivalent refraction was −3.40±1.63 D (range: −0.75 to −6.00 D). Keratometric power and central corneal thickness were within commonly found ranges for myopes. The mean keratometric power was 44.88±1.63 D (range: 41.80 to 48.80 D) in the steep meridian and 43.97±1.47 D (range: 41.40 to 47.20 D) in the flat meridian. The mean thinnest corneal thickness by Artemis was 528.4±27.4 μm (range: 483 to 572 μm). The mean thinnest corneal thickness by handheld ultrasound was 540.8±27.9 μm (range: 495 to 587 μm). Preoperative BSCVA was 20/20 in 37.5% of eyes, 20/16 in 50% of eyes, and 20/12.5 in 12.5% of eyes.
Accuracy and Reproducibility of Central Flap Thickness
Figure 1 shows a horizontal Artemis B-scan image of the cornea of one eye at 3 months. The mean central Reinstein Flap Thickness29 at 3 months for all eyes was 112.3±7.9 μm, giving an accuracy of +2.3 μm and reproducibility of 7.9 μm. The minimum central flap thickness was 102.6 μm and the maximum central flap thickness was 132.9 μm, giving a range of 30.3 μm. The distribution of central flap thickness for all eyes is presented in Figure 2. Twenty-five percent of eyes were within 2 μm of the intended flap thickness, 54.2% of eyes were within 5 μm of the intended flap thickness, and 87.5% were within 10 μm of the intended flap thickness.
Figure 1. Artemis Very-High Frequency Digital Ultrasound Horizontal B-Scan of a Cornea with a Flap Created Using the VisuMax Femtosecond Laser. Centrally, the Reinstein Flap Thickness Was Measured as 113 μm, Which Was 3 μm Thicker than the Predicted Flap Thickness of 110 μm. Digital Signal Processing Is Performed on the B-Scan Signal and Layer Thickness Measurements Are Obtained by a Computer Algorithm on the I-Scan, Resulting in the Red Line Image of the Interfaces.
Figure 2. Distribution of Flap Thickness in 24 Myopic Eyes Treated with the VisuMax Femtosecond Laser System and MEL 80 Excimer Laser. The y Axis Represents the Central Reinstein Flap Thickness in Microns. Each Blue Bar Represents the Flap Thickness for Each Individual Eye. The Red Line Represents the Intended Flap Thickness of 110 μm.
On average, the flap diameter was 0.47±0.21 mm greater than the intended flap diameter. The flap diameter obtained was equal to the intended flap diameter in 2 (8%) eyes, 0.25 mm greater than the intended flap diameter in 3 (13%) eyes, 0.50 mm greater than the intended flap diameter in 16 (67%) eyes, 0.75 mm greater than the intended flap diameter in 2 (8%) eyes, and 1.00 mm greater than the intended flap diameter in 1 (4%) eye. The flap diameter obtained was never less than the intended flap diameter.
On average, the flap hinge width was 0.08±0.30 mm less than the intended flap hinge width. The flap hinge width obtained was equal to the intended flap hinge width in 14 (58%) eyes, 0.25 mm less than the intended flap hinge width in 3 (13%) eyes, 0.50 mm less than the intended flap hinge width in 1 (4%) eye, 0.75 mm less than the intended flap hinge width in 1 (4%) eye, and 1.00 mm less than the intended flap hinge width in 1 (4%) eye. The flap hinge width obtained was 0.25 mm greater than the intended flap hinge width in 4 (17%) eyes.
Suction loss occurred in one eye half way through the flap creation. A new contact glass was connected to the VisuMax and the femtosecond flap creation procedure was carried out immediately using the same settings for flap thickness and all flap dimensions. The flap interface was smooth when lifted and no slivers of stromal tissue were present.
Figure 3 shows the Waring standard graphs summarizing the visual outcomes 3 months postoperatively. At the 3-month time point, 100% of eyes had UCVA of 20/20 or better. One eye was excluded from the efficacy analysis as the intended postoperative spherical equivalent refraction was −1.50 D for micro-monovision. There was a gain of 1 or more lines of BSCVA in 54% of eyes and a loss of 1 line of BSCVA in 4% of eyes. No eye lost 2 or more lines of BSCVA. Sixty-seven percent of eyes were within ±0.50 D and 92% were within ±1.00 D of the intended spherical equivalent refraction. A tendency for slight over-correction was noted on the postoperative spherical equivalent refraction histogram, whereas the R2 for the attempted versus achieved plot was found to be 0.98, demonstrating very low scatter.
Figure 3. Six Standard Graphs for Reporting Refractive Surgery Showing the Visual Outcomes for 24 Myopic Eyes Treated with the VisuMax Femtosecond Laser System and MEL 80 Excimer Laser.
The average preoperative normalized mesopic contrast sensitivity ratio was 0.99 at 3 cycles per degree (cpd), 1.00 at 6 cpd, 1.00 at 12 cpd, and 1.02 at 18 cpd. The average normalized contrast sensitivity ratio 3 months postoperatively was 1.02 at 3 cpd, 1.00 at 6 cpd, 1.01 at 12 cpd, and 0.99 at 18 cpd. No statistically significant difference was noted at any frequency level (P>.35).
Table 2 shows the preoperative and 3-month postoperative averages for total higher order RMS up to the sixth order, coma, and spherical aberration using OSA notation for a 6.0-mm diameter.
Table 2: Change in Higher Order Aberrations Analyzed at the 6.0-mm Zone
In this study, the VisuMax femtosecond laser system was found to produce accurate and reproducible flaps; for flaps with an intended thickness of 110 μm, the mean central Reinstein Flap Thickness was 112.3 μm and the reproducibility was 7.9 μm.
For reproducibility results of such a study to be valid, the precision of the measuring instrument needs to be smaller than the reproducibility of the data set being measured. For example, considering that 95% of measurements will be contained within a range of two standard deviations from the mean, if an instrument with a precision of 1 μm were to be used to repeatedly measure the same flap, known to be exactly 110 μm in thickness, 95% of the measurements would fall between 108 and 112 μm. On the other hand, if an instrument with a precision of 5 μm were to be used to repeatedly measure the same 110-μm flap, 95% of the measurements would fall between 100 and 120 μm. Therefore, the validity of a flap thickness study is compromised if the precision of the measuring instrument is too large to distinguish between two data points. As a general rule, given that 99% of measurements are within three standard deviations of the mean, the precision of the measuring tool should be at least one-third of the reproducibility of the data set to justify the validity of the reproducibility study. In the present study, the reproducibility of VisuMax flap thickness was measured to be 7.9 μm. Very-high frequency digital ultrasound, as used in the present study, has been shown to measure central flap thickness with a precision of 1.14 μm.31 Due to the high precision of the Artemis 1, we were able, in this case, to reliably characterize the accuracy and reproducibility of the VisuMax femtosecond laser system.
Alternative methods used to report flap thickness reproducibility include handheld ultrasound pachymetry, OCT, confocal microscopy through focusing, and online optical coherence pachymetry (OCP). With handheld ultrasound devices, the flap thickness is calculated as the difference between the preoperative corneal thickness and the intraoperative residual stromal bed thickness. Errors in establishing flap thickness reproducibility arise primarily from the instrument precision for measuring both the preoperative corneal thickness and the intraoperative residual stromal bed thickness. Instrument precision has been reported to be approximately 6 μm for corneal thickness measurement41 and is likely to be >6 μm for residual stromal bed measurement. Errors are also caused by changes in stromal bed hydration as well as the difficulty in finding the same location for the residual bed measurement that was used for the total corneal measurement. The combination of these errors would probably result in the instrument precision being degraded at a minimum of 10 to 12 μm. Given such a potential error of flap thickness measurement using intraoperative handheld ultrasound pachymetry, the reported flap thickness reproducibility of 5 μm with the FEMTO LDV26 is unreliable and unlikely because the reported reproducibility is half that of the potential error of the measurement method.
With OCT, flap thickness can be measured either by manual placement of the measuring tool on the OCT B-scan image17 or by using an automated computer algorithm,15 although this is not yet included for any of the commercially available OCT instruments. Neither method takes into account postoperative epithelial changes, and, in particular, the epithelial thickening occurring after myopic ablations.42–48 This will result in an overestimation of the flap thickness and introduce error due to the variable nature of epithelial response proportional to the level of myopia treated. Errors also come from the instrument flap thickness measurement precision, which has been reported to be 6.5 μm for the Visante OCT using a proprietary automated computer algorithm.15 Additional errors further reduce the instrument precision when using manual placement of the measuring tool on the OCT B-scan image. These include the intraobserver error in locating the flap interface and measurement error due to the discrete measurement increments when using the flap tool. For example, the flap tool with the Visante OCT only allows flap thickness measurements in 12-μm increments, meaning that there is a potential 6-μm measurement error. Therefore, manual measurements with the Visante OCT result in a 12-μm flap thickness measurement precision. As an example, one published study using the Visante OCT reported a flap thickness standard deviation of 5 μm for the IntraLase based on peripheral flap thickness measurements using manual location of the flap interface on a B-scan image without reference to the analytic signal.17 The authors concede that the subjective technique may be less precise than an automated measurement.17 As a comparison, the flap thickness standard deviation for the IntraLase was found to be 9 μm for an intended flap thickness of 110 μm and 11 μm for an intended flap thickness of 120 μm using an automated computer algorithm to analyze the flap thickness from the analytic signal for scans obtained with a prototype of the Visante OCT.15 However, the reported repeatability of flap thickness measurements with this automated system was still only 4.8 μm,15 meaning that the Visante OCT could be relied on to measure flap thickness reproducibility of 10 to 15 μm.
Online OCP has been used to measure flap thickness reproducibility.24,27 The repeatability of corneal thickness measurements using online OCP has been reported to be 4.74 μm,49 meaning that online OCP could be relied on to measure flap thickness reproducibility of 10 to 15 μm.
In this study, central flap thickness was measured with the Artemis 1 by adding the thickness of the stromal component of the flap measured 3 months postoperatively to the preoperative epithelial thickness.29 Using preoperative epithelial thickness rather than postoperative epithelial thickness eliminates the inaccuracy in central flap thickness measurement associated with epithelial changes induced by surgery.42–48 This method is likely to improve the precision with which central flap thickness is determined, and therefore enables the measurement of low microkeratome flap thickness reproducibility values.
A summary of recently published results of mean flap thickness obtained with femtosecond lasers and mechanical microkeratomes is presented in Table 1. Flap thickness reproducibility (standard deviation) reported for the IntraLase femtosecond laser ranged from 5 μm17 to 21 μm15 with an average of 12.9 μm,15–27 although this includes various intended flap thickness settings, IntraLase models, and pachymetric methods. Flap thickness reproducibility with the Femtec femtosecond laser in porcine eyes ranged from 9.1 μm for an intended flap thickness of 180 μm to 11.1 μm for an intended flap thickness of 120 μm.50 Flap thickness reproducibility with the FEMTO LDV was 5 μm.26 In studies comparing current mechanical microkeratomes and femtosecond lasers, the standard deviation of flap thickness tended to be lower with femtosecond lasers.15,16,20,22
The accuracy of flap thickness was calculated as the difference between the mean flap thickness and the intended flap thickness, which is also referred to as the bias. Positive values indicate a mean flap thickness thicker than the intended flap thickness, whereas negative values indicate a mean flap thickness thinner than the intended flap thickness. The bias varied between +40 μm15 and −1.3 μm19 for studies using the IntraLase femtosecond laser. The bias varied between +3.7 μm and −7.9 μm for the Femtec femtosecond laser in porcine eyes.50 For the FEMTO LDV, there was a bias of −20 μm.26 In the present study, the bias was −2.3 μm with the VisuMax femtosecond laser system.
Previously, we demonstrated that the choice of microkeratome had a marked impact on the risk of excessive keratectomy and hence the safety of LASIK.1,2 We showed that the safety of the procedure is improved by using a microkeratome with a low flap thickness standard deviation and with negative bias. To maximize the safety of the procedure, the flap thickness value used to predict the residual stromal thickness should represent the worst case scenario of obtaining the thickest flap possible for the microkeratome being used. Statistically, only 2.5% of values would be thicker than the mean plus two standard deviations, and 0.5% of values would be thicker than the mean plus three standard deviations. The mean plus two standard deviations would be a reasonable value to use. Therefore, we recommend using a flap thickness value of 128 μm (112.3+2×7.9) to calculate the predicted residual stromal thickness when creating flaps with an intended thickness of 110 μm with the VisuMax femtosecond laser system. The flap thickness to use for each microkeratome generated using this safety protocol is listed in the right hand column in Table 1. The Hansatome microkeratome appears to be labeled with sufficient negative bias,15,20 whereas many of the femtosecond laser systems appear to have positive bias. For example, in one study, the mean thickness of flaps with an intended thickness of 120 μm with the IntraLase Pulsion femtosecond laser was found to be 160±19 μm15—a positive bias of 40 μm, meaning that virtually all flaps would be thicker than intended. Our safety protocol would recommend using a flap thickness value of 198 μm (160±2×19) when creating flaps with an intended thickness of 120 μm with the IntraLase Pulsion femtosecond laser.
In the present study, we found that the flap diameter was on average 0.47 mm larger than the programmed diameter, with no flap ending up smaller than the programmed diameter. Femtosecond flap diameters tend to be smaller on average than those produced by modern microkeratomes. With the VisuMax, the minimum flap diameter coupled with centration of the flaps on the corneal vertex may help optimize the efficiency of stromal bed exposure for ablations centered on the corneal vertex. Since the date of this study, software release 2.4.0 allows the creation of larger flaps with the VisuMax; the maximum flap diameter with the new software is 8.0 mm for the S contact glass (translating to an achieved diameter of 8.47 mm), 8.8 mm for the M contact glass (translating to an achieved diameter of 9.27 mm), and 9.5 mm for the L contact glass (translating to an achieved diameter of 9.97 mm). Software release 2.4.0 also allowed flap creation time to be reduced to 20 seconds for an 8-mm diameter flap.
The VisuMax femtosecond laser system was found to create flaps with an intended thickness of 110 μm with an accuracy of +2.3 μm and a reproducibility of 7.9 μm. The low central flap thickness standard deviation achieved by the VisuMax should help improve the safety of LASIK, particularly if 18 μm (2.3+7.9×2=18.1) is added to the programmed flap thickness when calculating the predicted residual stromal thickness in LASIK.
- Reinstein DZ, Srivannaboon S, Archer TJ, Silverman RH, Sutton H, Coleman DJ. Probability model of the inaccuracy of residual stromal thickness prediction to reduce the risk of ectasia after LASIK part I: quantifying individual risk. J Refract Surg. 2006;22:851–860.
- Reinstein DZ, Srivannaboon S, Archer TJ, Silverman RH, Sutton H, Coleman DJ. Probability model of the inaccuracy of residual stromal thickness prediction to reduce the risk of ectasia after LASIK part II: quantifying population risk. J Refract Surg. 2006;22:861–870.
- Barraquer JI. Queratomileusis y Queratofakia. Bogota, Columbia: Instituto Barraquer de America; 1980.
- Pulaski JP. Etiology of buttonhole flaps. J Cataract Refract Surg. 2000;26:1270–1271. doi:10.1016/S0886-3350(00)00658-1 [CrossRef]
- Carrillo C, Chayet AS, Dougherty PJ, Montes M, Magallanes R, Najman J, Fleitman J, Morales A. Incidence of complications during flap creation in LASIK using the NIDEK MK-2000 microkeratome in 26,600 cases. J Refract Surg. 2005;21:S655–S657.
- Reinstein DZ. Consultation Section. J Cataract Refract Surg. 2001;27:1350–1352. doi:10.1016/S0886-3350(01)01083-5 [CrossRef]
- Reinstein DZ, Srivannaboon S, Silverman RH, Coleman DJ. The accuracy of routine LASIK; isolation of biomechanical and epithelial factors. Invest Ophthalmol Vis Sci. 2000;41:S318.
- Arbelaez MC. Nidek MK 2000 microkeratome clinical evaluation. J Refract Surg. 2002;18:S357–S360.
- Shemesh G, Dotan G, Lipshitz I. Predictability of corneal flap thickness in laser in situ keratomileusis using three different microkeratomes. J Refract Surg. 2002;18:S347–S351.
- Solomon KD, Donnenfeld E, Sandoval HP, Al Sarraf O, Kasper TJ, Holzer MP, Slate EH, Vroman DT. Flap thickness accuracy: comparison of 6 microkeratome models. J Cataract Refract Surg. 2004;30:964–977. doi:10.1016/j.jcrs.2004.01.023 [CrossRef]
- Miranda D, Smith SD, Krueger RR. Comparison of flap thickness reproducibility using microkeratomes with a second motor for advancement. Ophthalmology. 2003;110:1931–1934. doi:10.1016/S0161-6420(03)00786-3 [CrossRef]
- Modis L Jr, Langenbucher A, Behrens A, Seitz B. Flap quality in single versus multiple use of the same blade in the Flapmaker microkeratome. J Refract Surg. 2004;20:258–264.
- Jacobs BJ, Deutsch TA, Rubenstein JB. Reproducibility of corneal flap thickness in LASIK. Ophthalmic Surg Lasers. 1999;30:350–353.
- Ucakhan OO. Corneal flap thickness in laser in situ keratomileusis using the summit Krumeich-Barraquer microkeratome. J Cataract Refract Surg. 2002;28:798–804. doi:10.1016/S0886-3350(01)01304-9 [CrossRef]
- Li Y, Netto MV, Shekhar R, Krueger RR, Huang D. A longitudinal study of LASIK flap and stromal thickness with high-speed optical coherence tomography. Ophthalmology. 2007;114:1124–1132. doi:10.1016/j.ophtha.2006.09.031 [CrossRef]
- Talamo JH, Meltzer J, Gardner J. Reproducibility of flap thickness with IntraLase FS and Moria LSK-1 and M2 microkeratomes. J Refract Surg. 2006;22:556–561.
- Stahl JE, Durrie DS, Schwendeman FJ, Boghossian AJ. Anterior segment OCT analysis of thin IntraLase femtosecond flaps. J Refract Surg. 2007;23:555–558.
- Binder PS. One thousand consecutive IntraLase laser in situ keratomileusis flaps. J Cataract Refract Surg. 2006;32:962–969. doi:10.1016/j.jcrs.2006.02.043 [CrossRef]
- Binder PS. Flap dimensions created with the IntraLase FS laser. J Cataract Refract Surg. 2004;30:26–32. doi:10.1016/S0886-3350(03)00578-9 [CrossRef]
- Kezirian GM, Stonecipher KG. Comparison of the IntraLase femtosecond laser and mechanical keratomes for laser in situ keratomileusis. J Cataract Refract Surg. 2004;30:804–811. doi:10.1016/j.jcrs.2003.10.026 [CrossRef]
- Hu MY, McCulley JP, Cavanagh HD, Bowman RW, Verity SM, Mootha VV, Petroll WM. Comparison of the corneal response to laser in situ keratomileusis with flap creation using the FS15 and FS30 femtosecond lasers: clinical and confocal microscopy findings. J Cataract Refract Surg. 2007;33:673–681. doi:10.1016/j.jcrs.2006.12.021 [CrossRef]
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Studies of LASIK Flap Thickness Using Femtosecond Laser Systems
|Study (y)||Pachymetry Method||Pachymetry Instrument||Microkeratome||No. of Eyes||Manufacturer-labeled Flap Thickness (μm)||Mean Flap Thickness (μm)||Accuracy (μm)||Standard Deviation (μm)||Minimum (μm)||Maximum (μm)||Range (μm)||Recommended Flap Thickness for RST Prediction (Mean±2 SD)|
|Stahl et al17 (2007)||1 mo postop OCT||Visante||IntraLase FS60||25||100||112.0||+12.0||5.0||87||118||31||122|
|Pietilä et al26 (2009)||Intraop US||SP-3000||FEMTO LDV||787||110||90||−20.0||5.0||67||107||40||100|
|Alió & Piñero22 (2008)||1 mo postop VHFU||Artemis 2||IntraLase FS30||22||110||116.0||+6.0||6.2||101||126||25||128|
|Hu et al21 (2007)||3 mo postop CMTF||NR||IntraLase FS30||15||115 to 120||N/A||+13.9||7.1||N/A||N/A||N/A||N/A|
|Alió & Piñero22 (2008)||1 mo postop VHFU||Artemis 2||Moria M2||22||110||117.5||+7.5||7.8||101||131||30||133|
|Reinstein et al (2009)||Preop and 3 mo postop VHFU*||Artemis 1||VisuMax||24||110||112.3||+2.3||7.9||103||133||30||128|
|Alió & Piñero22 (2008)||1 mo postop VHFU||Artemis 2||Carriazo-Pendular†||22||110||118.1||+8.1||8.3||106||139||33||135|
|Li et al15 (2007)||1 wk postop OCT||CAS-OCT (Visante prototype)||IntraLase (Pulsion)||7||110||145.0||+35.0||9.0||132||154||22||163|
|Holzer et al50 (2006)||Micrometer||Digimatic||Femtec||10‡||180||179.6||−0.4||9.1||167||194||27||198|
|Sutton & Hodge25 (2008)||Intraop US||Corneo-Gage Plus||IntraLase FS30||141||115||114.0||−1.0||9.8||93||163||70||134|
|Li et al15 (2007)||Intraop US||Corneo-Gage 2 50 MHz||Hansatome†||12||160||116.0||−44.0||10.0||102||134||32||136|
|Binder18 (2006)||Intraop US||Cornea Scan II 50 MHz||IntraLase FS10||13||80||115.8||+35.8||10.1||96||136||40||136|
|Binder18 (2006)||Intraop US||Cornea Scan II 50 MHz||IntraLase FS15||21||110||131.1||+21.1||10.2||110||148||38||152|
|Holzer et al50 (2006)||Micrometer||Digimatic||Femtec||10‡||140||143.7||+3.7||10.7||131||159||28||165|
|Binder18 (2006)||Intraop US||Cornea Scan II 50 MHz||IntraLase FS15||249||90||115.8||+25.8||10.8||84||140||56||137|
|Sutton & Hodge25 (2008)||Intraop US||Corneo-Gage Plus||IntraLase FS15||119||105||116.8||+11.8||10.8||95||148||53||138|
|Li et al15 (2007)||Intraop US||Corneo-Gage 2 50 MHz||IntraLase (Pulsion)||7||110||140.0||+30.0||11.0||122||158||36||162|
|Li et al15 (2007)||1 wk postop OCT||CAS-OCT (Visante prototype)||IntraLase (Pulsion)||8||120||156.0||+36.0||11.0||136||167||31||178|
|Holzer et al50 (2006)||Micrometer||Digimatic||Femtec||10‡||120||112.1||−7.9||11.1||96||135||39||134|
|Hu et al21 (2007)||3 mo postop CMTF||NR||IntraLase FS15||15||115 to 125||N/A||+16.8||11.1||N/A||N/A||N/A||N/A|
|Binder18 (2006)||Intraop US||Cornea Scan II 50 MHz||IntraLase FS15||82||100||120.1||+20.1||11.8||94||150||56||144|
|Binder19 (2004)||Intraop US||NR||IntraLase FS||22||120||122.4||+2.4||11.9||103||148||45||146|
|Binder19 (2004)||Intraop US||NR||IntraLase FS||34||110||125.0||+15.0||12.0||94||154||60||149|
|Li et al15 (2007)||1 wk postop OCT||CAS-OCT (Visante prototype)||Hansatome†||12||160||125.0||−35.0||12.0||101||141||40||149|
|Talamo et al16 (2006)||Intraop US||Pachette II||IntraLase FS||99||110||119.0||+9.0||12.0||82||149||67||143|
|Binder18 (2006)||Intraop US||Cornea Scan II 50 MHz||IntraLase FS10||320||90||119.2||+29.2||12.4||78||152||74||144|
|Kim et al23 (2008)||3 mo postop OCT||Visante||IntraLase FS60||23||110||118.9||+8.9||13.6||98||142||44||146|
|Pfaeffl et al24 (2008)||Intraop OCP||Online OCP||IntraLase FS30||287||100||100.4||+0.4||13.6||57||138||81||128|
|Kezirian & Stonecipher20 (2004)||Intraop US||Pachette 50/60 KHz pachymeter||IntraLase FS||106||130||114.0||−16.0||14.0||78||155||77||142|
|Li et al15 (2007)||1 wk postop OCT||CAS-OCT (Visante prototype)||Hansatome†||24||180||143.0||−37.0||14.0||119||177||58||171|
|Binder18 (2006)||Intraop US||Cornea Scan II 50 MHz||IntraLase FS10||140||100||129.7||+29.7||14.3||89||165||76||158|
|Neuhann et al27 (2008)||Intraop OCP||Online OCP||IntraLase FS30||470||110||121.7||+11.7||14.7||84||164||80||151|
|Binder18 (2006)||Intraop US||Cornea Scan II 50 MHz||IntraLase FS10||49||110||127.4||+17.4||15.2||86||158||72||158|
|Binder19 (2004)||Intraop US||NR||IntraLase FS||21||130||128.7||−1.3||16.6||90||157||67||162|
|Li et al15 (2007)||Intraop US||Corneo-Gage 2 50 MHz||Hansatome†||24||180||131.0||−49.0||17.0||98||163||65||165|
|Binder18 (2006)||Intraop US||Cornea Scan II 50 MHz||IntraLase FS10||25||130||129.6||−0.4||17.1||90||156||66||164|
|Kim et al23 (2008)||3 mo postop OCT||Visante||IntraLase FS60||36||100||104.8||+4.8||17.6||75||128||53||140|
|Binder19 (2004)||Intraop US||NR||IntraLase FS||26||140||132.5||−7.5||18.5||80||158||78||170|
|Binder18 (2006)||Intraop US||Cornea Scan II 50 MHz||IntraLase FS10||38||140||130.6||−9.4||19.0||80||158||78||169|
|Li et al15 (2007)||Intraop US||Corneo-Gage 2 50 MHz||IntraLase (Pulsion)||8||120||160.0||+40.0||19.0||136||190||54||198|
|Talamo et al16 (2006)||Intraop US||Pachette II||Moria LSK-One†||100||160||130.0||−30.0||19.0||71||186||115||168|
|Binder18 (2006)||Intraop US||Cornea Scan II 50 MHz||IntraLase FS10||31||120||133.4||+13.4||22.1||103||187||84||178|
|Talamo et al16 (2006)||Intraop US||Pachette II||Moria M2†||135||130||142.0||+12.0||24.0||84||203||119||190|
|Kezirian & Stonecipher20 (2004)||Intraop US||Pachette 50/60 KHz pachymeter||Carriazo-Barraquer†||126||130||153.0||+23.0||26.0||59||210||151||205|
|Kezirian & Stonecipher20 (2004)||Intraop US||Pachette 50/60 KHz pachymeter||Hansatome†||143||180||156.0||−24.0||29.0||25||250||225||214|
Change in Higher Order Aberrations Analyzed at the 6.0-mm Zone
|Spherical Equivalent Refraction (D)||Higher Order RMS (μm)||Coma (μm)||Spherical Aberration (μm)|
|3 months postoperative||−0.09±0.52||0.48±0.12||0.25±0.11||0.27±0.15|