Efficacy, safety, and flap dimensions of a new femtosecond laser for laser in situ keratomileusis

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1 ARTICLE Efficacy, safety, and flap dimensions of a new femtosecond laser for laser in situ keratomileusis Jérôme C. Vryghem, MD, Thibaut Devogelaere, MD, FEBO, Pavel Stodulka, MD, PhD PURPOSE: To evaluate the clinical results of a preproduction femtosecond laser for flap creation in laser in situ keratomileusis (LASIK). SETTING: Private practice, Brussels, Belgium. METHODS: This study comprised myopic eyes with a plano target refraction and a target flap thickness of 110 mm. The LASIK flap was created with a Ziemer LDV femtosecond laser. Prospective evaluation included flap dimensions, intraoperative and postoperative complications, and visual outcomes. RESULTS: Sixty-three patients (111 eyes; mean age 37.2 years) were evaluated. Preoperatively, the mean corrected distance visual acuity (CDVA) was 1.34 (Snellen) and the mean manifest refraction spherical equivalent (MRSE), 4.91 diopters (D) G 2.45 (SD). Six months postoperatively, the mean CDVA was 1.33; the mean MRSE, 0.05 G 0.3 D; and the mean uncorrected distance visual acuity (UDVA), The UDVA was 20/25 or better in 98.2% of eyes and 20/20 or better in 94.6% of eyes. The MRSE was within G0.50 D in 95.5% of eyes and within G1.00 D in 99.1% of eyes. The cylinder was 0.50 D or less in 99.1% of eyes. The mean flap thickness was G 12.6 mm. The most frequent complications were epithelial sloughing (10.8%), a decentered cut (4.5%), flap adhesions (5.4%), a slightly irregular flap border (5.4%), and microstriae (5.4%); all were mild. CONCLUSIONS: Overall, the flap dimensions and refractive results were predictable and the complication rate was acceptable after LASIK using the new femtosecond laser for flap creation. Financial Disclosure: No author has a financial or proprietary interest in any material or method mentioned. J Cataract Refract Surg 2010; 36: Q 2010 ASCRS and ESCRS Flap creation is probably the most important step during laser in situ keratomileusis (LASIK), and complications during it can affect the rest of the procedure and Submitted: November 1, Final revision submitted: September 18, Accepted: September 22, From private practices, Brussels, Belgium (Vryghem, Devogelaere), and Zlin, Czecho Republic (Stodulka). Presented in part at the ASCRS Symposium on Cataract, IOL and Refractive Surgery, San Diego, California, USA, April 2007; the XXV Congress of the European Society of Cataract & Refractive Surgeons, Stockholm, Sweden, September 2007; and the XXVI Congress of the European Society of Cataract & Refractive Surgeons, Berlin, Germany, September Corresponding author: Jérôme C. Vryghem, Boulevard Saint-Michel, 12-16, B-1150 Brussels, Belgium. info@vryghem.be. cause permanent visual loss. 1 3 Variability in flap thickness limits accurate calculation of the residual stromal bed (RSB), which can be critical when LASIK is performed in eyes with high myopia or a thin cornea. 4 7 Creating the LASIK corneal flap with a femtosecond laser rather than with a standard mechanical microkeratome may yield better safety and reproducibility. The working principle of the femtosecond laser has been described. 8 This clinical study evaluated corneal flap creation with a preproduction femtosecond laser in patients having LASIK for myopia. The laser is nonamplified and solid state and is not sensitive to environmental influences (eg, temperature, shocks). 9 PATIENTS AND METHODS Study Design This prospective consecutive-enrollment study comprised patients treated at a private practice in 442 Q 2010 ASCRS and ESCRS Published by Elsevier Inc /10/$dsee front matter doi: /j.jcrs

2 CLINICAL RESULTS OF FEMTOSECOND LASER LASIK 443 Brussels, Belgium, between October 2006 and December No ethics committee approval was required because the femtosecond laser used in the study has the Communauté Européenne mark of approval. Enrollment Criteria All patients who qualified for conventional LASIK were eligible for enrollment in the study. Only myopic eyes with a plano target refraction and preoperative manifest astigmatism of 4.50 diopters (D) or less at the spectacle plane were included to allow comparison with published LASIK results Eyes with a history of corneal procedures (penetrating keratoplasty, aborted mechanical microkeratome cuts) or followup of fewer than 6 months were excluded. Clinical Outcomes Measures Preoperative assessment included uncorrected (UDVA) and corrected (CDVA) distance visual acuities, manifest refraction, slitlamp and fundus evaluation, scotopic pupil measurement (Procyon P2000, Procyon Instruments), pachymetry by optical low-coherence reflectometry (OLCR, Haag-Streit AG), and corneal topography (WaveLight, WaveLight AG). All visual acuity measurements were performed using Snellen charts. Intraoperative analysis included flap diameter (horizontal and vertical), hinge size, flap thickness (Corneo-Gage, Sonogage), intraoperative complications, and stromal bed evaluation. Flap dimensions were measured with a Moria caliper. Postoperative assessment included CDVA, UDVA, refractive outcomes, slitlamp examination, and complications. Femtosecond Laser All corneal flaps were created with a preproduction LDV femtosecond laser (Ziemer Group). The laser delivers single femtosecond laser pulses with a repetition rate greater than 1 MHz. The laser beam is deviated by a fast-spinning polygon mirror. The frequency of the incoming pulses of the laser source is higher than the spinning speed of the mirror; therefore, the subsequent pulse partially overlaps the previous pulse. The pulse energy is low (!100 nj) and the pulse duration short (typically 250 femtoseconds). The corneal cut is made by discrete microcavitation bubbles within the cutting plane. The bubbles disappear when the flap is lifted. Therefore, there is no pocket for bubble accumulation. The spot diameter is in the range of 2 mm, and the overlap rate between pulses is up to 80%; thus, no tissue bridges are left between stops (Figure 1). The high numerical aperture and small focal point focus the laser energy at the depth at which optical breakdown Figure 1. Overlap of laser pulses. occurs. 9 The focal point is at a constant distance of 250 mm below the applanation window of the laserhead. Flap thickness is altered by a plastic foil spacer (Intershield Spacer, Ziemer Group) interposed between the laser head and the cornea (Figure 2). Because the focal distance of the laser head is constant, the interposition of the spacer alters the cutting plane in the stroma. The thicker the spacer, the thinner the flap. The spacer also functions as a sterile barrier. One spacer is used for each bilateral procedure. Vacuum is controlled by a computer. Flap creation can be seen only virtually on the computer screen because the cutting optic is integrated into the laser head and thus obstructs the view of the cutting process. This is necessary to obtain the high numerical aperture. The eye is fixated with a suction ring and the cornea applanated before cutting begins. The cutting distance is parallel to the applanation surface. The cutting takes place in the anterior stroma of the cornea at a depth corresponding to: 250 mm minus the thickness of the spacer. The flaps are created Figure 2. Plastic foil (spacer) interpositioned between the laser head and the cornea. print & web 4C=FPO

3 444 CLINICAL RESULTS OF FEMTOSECOND LASER LASIK parallel to the anterior surface and are not influenced by corneal thickness or biomechanical parameters (eg, corneal stiffness). During cutting, no moving parts exert pressure on the cornea; therefore, flap thickness should not be dependent on corneal rigidity. The flap diameter is slightly dependent on corneal curvature because a given suction ring dimension applanates less with a low corneal curvature and slightly more with a steeper cornea. The variance is significantly less than with a standard mechanical microkeratome. The expected flap diameter is directly related to the visible applanation area, which can be seen before the cutting process starts. Thus, the surgeon can adapt the size of the suction ring. After the corneal cut is made, the flap is lifted and excimer ablation performed. The patient is not repositioned between the corneal cut and the excimer ablation. Technically, the workflow is similar to that with a mechanical microkeratome. Surgical Technique The cornea was hydrated with sodium hyaluronate 0.25% (LaserVisc) before the femtosecond laser head was applied to ensure smooth contact between the plastic spacer and the corneal epithelium. At the edge of the applanation, the cornea was marked with gentian violet after the laser cut was made, so the ink did not interfere with the laser beam. The microbubbles at the interface disappeared when the flap was lifted. The flap was elevated with a manipulator (Storz E 9071, Bausch & Lomb) or a spatula (Vryghem 19087, Moria). If moisture was present on the stromal bed when the flap was elevated, it was removed with a sponge and the ablation performed. The ablation was performed with an Allegretto EYE-Q 400 excimer laser (WaveLight AG). The optical zone of the ablation was 6.5 mm in all cases. The standard LASIK nomogram was used. The laser room temperature was maintained at 22 C and the relative humidity at 50%. An RSB after ablation of at least 250 mm was planned. After the excimer laser ablation, the stromal bed was irrigated and the flap was floated back to its original position. Flap alignment was checked using the corneal marks, and the flap was examined for proper adherence. Postoperative Management Bandage contact lenses were used in cases of epithelial defect or free flap. All patients were examined 30 minutes after surgery to check for proper flap position and interface debris. Postoperative treatment consisted of neomycin sulfate 0.5%, polymyxin B 5000 UI/mL, and prednisolone acetate 0.5% 4 times a day for 1 week, ketorolac tromethamine 0.5% eyedrops as needed, and frequent instillation of artificial tears. Postoperative examinations were performed at 1 day, 10 days (optional), 6 weeks, and 6 months. The evaluations included UDVA, CDVA, manifest refraction, and slitlamp microscopy. In cases of pseudokeratitis sicca, topical cyclosporine and punctum plugs were used at the examiner s discretion. Flap Parameters The target horizontal flap diameter was 9.5 mm and the target vertical flap diameter, 9.1 mm. The hinge was located superiorly, and its size was defined as the width of the uncut portion of the flap. The intended flap thickness was 110 mm and was determined by the thickness of the plastic spacer positioned between the eye and the femtosecond laser system. These parameters were not altered according to corneal thickness or curvature. Intraoperative flap thickness was calculated using a subtraction method. 11,13 The stromal bed thickness was calculated as the lowest of at least 5 consecutive central corneal measurements. The difference between the corneal thickness before flap creation and the stromal bed thickness immediately after flap creation (before laser ablation) was considered the flap thickness. Flap decentration was defined as peripheral excimer laser spots hitting the epithelium at the margin of the flap. The ablation profiles of the excimer laser used can extend up to 9.2 mm. Statistical Analysis Data were stored in an Excel spreadsheet (Microsoft Corp.) and analyzed with Datagraph-med outcomes analysis software for refractive surgery (Ingenieurbüro Pieger GmbH). RESULTS Preoperative Parameters The study comprised 63 patients (111 eyes) with a mean age of 37.2 years (range 22 to 69 years). The mean corneal thickness was mm G 34 (SD) (range 474 to 641 mm). Table 1 shows the preoperative refractive parameters. Intraoperative Parameters It took fewer than 40 seconds to make the corneal cut. The vacuum time was fewer than 50 seconds. Flap Dimensions Table 2 shows the intraoperative flap dimensions. The mean flap thickness increased slightly with preoperative corneal thickness; however, this finding was not statistically significant. The difference between the mean horizontal flap diameter and

4 CLINICAL RESULTS OF FEMTOSECOND LASER LASIK 445 Table 1. Preoperative refractive parameters. Value CDVA Sphere (D) Cylinder (D) MRSE (D) Mean SD Minimum Maximum CDVA Z corrected distance visual acuity (decimal); MRSE Z manifest refraction spherical equivalent print & web 4C=FPO the mean vertical flap diameter was dependent on the superior hinge. Figure 3 shows the observed flap thickness plotted as a function of preoperative corneal thickness. The influence of corneal thickness on flap thickness was not statistically significant. Epithelial Integrity In 12 eyes (10.8%) of 10 patients (4 men, 6 women; mean age 35.8 G 6.21 years), mild epithelial sloughing was noted at the end of the procedure. No large epithelial defects occurred. Complications The cut was slightly decentered in 4 eyes (3.6%). A larger decentration involving the superior limbus occurred in 1 eye (0.9%). Minor flap adhesions, all involving the central stromal bed, occurred in 4 eyes (3.6%). Strong adhesions, which made it difficult to lift the flap, occurred in 6 eyes (5.4%); 2 involved the central stromal bed. One strong adhesion was caused by air under the suction ring. One central strong adhesion was caused by air under the plastic spacer; the air bubble became trapped under the plastic spacer when the nurse applied the spacer on the applanation window of the laser head. The cut was still performed because at that time it was unclear to the surgeon whether the adhesion would affect the cut quality. In eyes with strong adhesions, the flap was lifted with a spatula in 4 cases, a diamond blade in 1 case, and an additional laser cut in 1 case. An additional laser cut was also required in 1 eye with intraepithelial flap cutting; a plastic spacer targeting a 140 mm flap thickness was used for the second laser cut. Air bubbles were seen in the corneal stroma in 2 eyes Table 2. Intraoperative operative flap dimensions. Value Diameter (mm) Pachymetry (mm) Horizontal Vertical Hinge (mm) Mean SD Minimum Maximum Figure 3. Influence of corneal thickness on flap thickness. (1.8%). There was 1 case of free flap. There were no other sight-threatening intraoperative complications, including buttonholed or transected flaps. Postoperative Parameters Complications One case of mild diffuse lamellar keratitis (DLK) occurred in the eye with a free flap. The eye recovered, with no loss of CDVA at the 6-month follow-up. A slightly irregular flap border caused by adhesion at the edge was noted in 6 eyes (5.4%). Microstriae were noted in 6 eyes (5.4%) on the first day after surgery; the microstriae had no further clinical implications and did not result in a loss of CDVA. A smoothing procedure was not performed. Two eyes (1.8%) had epithelial cells underneath the flap, and 1 eye (0.9%) had mild edema of the central flap. Efficacy Table 3 shows the refractive outcomes at 6 weeks and 6 months. At 6 months, the UDVA was 20/25 or better in 109 eyes (98.2%) and 20/20 or better in 105 eyes (94.6%). The MRSE was within G0.50 D in 106 eyes (95.5%) and within G1.00 D in 110 eyes (99.1%). The cylinder was 0.25 D or less in 100 eyes (90.1%) and 0.50 D or less in 110 eyes (99.1%). One eye had a repeat ablation procedure, after which the MRSE decreased from D to D. Safety By 6 weeks, 3 eyes had lost 2 lines of CDVA; the eyes recovered the lines over the follow-up. At 6 months, no eye had a significant loss of CDVA. DISCUSSION Although there are several studies of corneal flap creation with femtosecond lasers, to our knowledge this is one of the first to evaluate the Ziemer LDV femtosecond laser. The flap creation process is different between this laser and most mechanical microkeratomes. With the latter, the eye is fixed with a suction ring and the cornea is not applanated before

5 446 CLINICAL RESULTS OF FEMTOSECOND LASER LASIK Table 3. Postoperative refractive parameters. 6 Weeks Postoperative 6 Months Postoperative Value CDVA Sphere (D) Cylinder (D) MRSE (D) CDVA UDVA Sphere (D) Cylinder (D) MRSE (D) Mean SD Minimum Maximum CDVA Z corrected distance visual acuity (decimal); MRSE Z manifest refraction spherical equivalent; UDVA Z uncorrected distance visual acuity (decimal) the cut is made. During the cutting process, the cornea bulges out of the ring and is compressed by the moving applanation surface. The corneal cut is made with a moving blade. In this case, biomechanical parameters (eg, corneal rigidity, total corneal thickness) can influence the parallelism and thickness of the flap. There are exceptions. For example, during the applanating process with the Lasitome microkeratome (Gebauer Medizintechnik GmbH), the cornea is applanated before the cut is made; therefore, parallel cuts can be made independent of corneal thickness. The Schwind Carriazo-Barraquer microkeratome (Schwind eye-tech-solutions GmbH & Co. KG) has a concave applanating process, which minimizes the influence of biomechanical parameters. In addition, the suction of the femtosecond laser head is less (700 mbar) than with a mechanical microkeratome, which is more comfortable for the patient. The diameter of a LASIK flap depends on corneal curvature because a given suction ring dimension applanates less in eyes with a low corneal curvature and slightly more in eyes with a steep cornea. The variance is much less than with a standard mechanical microkeratome. With the femtosecond laser we used, the applanation area, which approximately corresponds to the cutting diameter, is visible before the cutting process in the applanation window. A smaller or larger applanation area can be corrected by using a bigger or smaller suction ring. Several studies have found that the flap thickness with femtosecond lasers is more predictable than with most mechanical microkeratomes. With femtosecond lasers, unlike with the latter, corneal thickness has little influence on the obtained flap thickness. 9,16 Measurement of the corneal and stromal bed thickness can be performed by intraoperative pachymetry. This can cause measurement-related variability related to nonperpendicular positioning of the pachymeter probe, impingement of the cornea on application of the pachymeter, or fluid between the pachymeter tip and the stromal bed. Thus, we considered correlating ultrasound pachymetry values with Visante optical coherence tomography (OCT) (Carl ZEISS Meditec AG) measurements. However, the resolution is between 10 mm and 12 mm, and the measurement cursor is positioned subjectively; moreover, the device often incorrectly marks the corneal edges and the central corneal flap thickness is often not visible. Therefore, it is difficult to make a correlation between ultrasound pachymetry and the OCT measurements and we decided to rely solely on pachymetry measurements to calculate flap thickness. Histologic sections and stromal bed photographs documenting a smooth stromal bed have been published (P.F. Titze, MD, et al. IOVS 2006; 47:ARVO E-Abstract 4332). The flap thickness is homogeneous throughout the cutting plane, and the margins have a smooth transition to the peripheral cornea. The cutting optic of the femtosecond laser we used is integrated into the handpiece; thus, the distance between the last lens of the cutting objective and the focal point is short. This mechanism creates a high numerical aperture and thus a small focal area. 9 Because of the small focal area, the energy necessary for photodisruption is low and the cavitation bubbles created are small and located in the cutting plane and disappear as soon as the flap is opened. As the flap is elevated, moisture can be present on the stromal bed because during the photodisruption process, a cavitation bubble is created. Analysis of the gas in the bubble shows that it mostly consists of hydrogen and oxygen (water vapor). 17 Because the pulse energy is very small (low nano Joule range), the water vapor can still be present in the bubble when the bubble opens during flap lifting. Other available femtosecond lasers have much higher pulse energies, ranging from approximately 500 to 3000 nj. With these lasers, the vapor at the surface is dried by the higher thermal effect during the photodisruption. Furthermore, after cutting, many bubbles are captured in the stroma creating an opaque bubble layer in which the vapor is caught. 17 This might be

6 CLINICAL RESULTS OF FEMTOSECOND LASER LASIK 447 why the stromal surface with the laser we used, with its pulse energy of less than 100 nj, is sometimes slightly wettish, unlike the surface when other femtosecond lasers are used. In our study, we used a preproduction femtosecond laser. The time needed to make the corneal cut was almost 40 seconds. Several adjustments have made the cutting speed faster in subsequent versions of the laser. Improved coatings have elevated transmission performance of the optical parts. The spinning speed of the polygon mirror deviating the laser beam has been augmented. With the current commercial versions, cutting is fewer than 20 seconds for an intended flap diameter of 9.5 mm. The mean hinge width in our study was 4.88 G 1.04 mm (range 0.0 to 6.5 mm), while the expected mean was 4.0 mm. We have no explanation for this difference. Tissue bridges caused the flap to adhere to the bed in 10 eyes (9.0%); the adhesions were minor in 4 eyes (3.6%) and strong in 6 eyes (5.4%). In most cases, the adhesions were overcome by repeated mechanical cleavage. One case required a recut with the femtosecond laser; the cut was performed at the same depth, immediately after the initial cut. This cannot be done when the corneal flap is created with a mechanical microkeratome. 2 The tissue bridges could be a consequence of the learning curve with the preproduction laser system. The current version of the laser has an improved vacuum sensor that is more sensitive to suction loss, which should lower the incidence of adhesions. Another origin of tissue bridges could be air trapped under the plastic spacer during preparation of the femtosecond laser head. The surgeon can see the air bubbles through the operating microscope of the excimer laser during applanation of the cornea. The applanation can be interrupted to reposition the plastic spacer on the applanation window of the laser head. Additional centration guidance in the current version of the femtosecond laser should help prevent eccentric ablations. Several other minor complications occurred, some the result of the surgeon s learning curve. The sole case of free flap developed mild DLK. The free flap likely occurred as a result of insufficient applanation before suction. The mild epithelial sloughing in 12 eyes (10.8%) of 10 patients at the end of the procedure was likely the result of the friction of the applanation window before suction was achieved. Extra applanation may have been needed during the learning curve. A slightly irregular flap border caused by adhesion at the edge occurred in 6 eyes (5.4%), possibly because the laser energy levels were too low. Six weeks postoperatively, 3 eyes presented with a significant loss of CDVA (R2 lines) from dryness or DLK. However, 6 months postoperatively, no eye had significant CDVA loss. The mean spherical equivalent and mean residual astigmatism at 6 months compare favorably with results in earlier femtosecond laser studies. 10,12 In conclusion, classic parameters (eg, safety, efficacy, predictability, stability) depend on femtosecond laser flap cutting and excimer laser ablation. Overall, LASIK performed using the preproduction femtosecond laser yielded predictable flap dimensions and refractive results and an acceptable complication rate. REFERENCES 1. Jacobs JM, Taravella MJ. Incidence of intraoperative flap complications in laser in situ keratomileusis. J Cataract Refract Surg 2002; 28: Tham VM-B, Maloney RK. Microkeratome complications of laser in situ keratomileusis. Ophthalmology 2000; 107: Gimbel HV, Anderson Penno EE, van Westenbrugge JA, Ferensowicz M, Furlong MT. Incidence and management of intraoperative and early postoperative complications in 1000 consecutive laser in situ keratomileusis cases. Ophthalmology 1998; 105: ; discussion by TE Clinch, Flanagan GW, Binder PS. Precision of flap measurements for laser in situ keratomileusis in 4428 eyes. J Refract Surg 2003; 19: Taneri S. Laser in situ keratomileusis flap thickness using the Hansatome microkeratome with zero compression heads. J Cataract Refract Surg 2006; 32: 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 S 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; Flap Thickness Study Group. J Cataract Refract Surg 2004; 30: Lubatschowski H, Maatz G, Heistercamp A, Hetzel U, Drommer W, Welling H, Ertmer W. Application of ultrashort laser pulses for intrastromal refractive surgery. Graefes Arch Clin Exp Ophthalmol 2000; 238: Lubatschowski H. Overview of commercially available femtosecond lasers in refractive surgery. J Refract Surg 2008; 24:S102 S Nordan LT, Slade SG, Baker RN, Suarez C, Juhasz T, Kurtz R. Femtosecond laser flap creation for laser in situ keratomileusis: six-month follow-up of initial U.S. clinical series. J Refract Surg 2003; 19: Kezirian GM, Stonecipher KG. Comparison of the IntraLase femtosecond laser and mechanical keratomes for laser in situ keratomileusis. J Cataract Refract Surg 2004; 30: Durrie DS, Kezirian GM. Femtosecond laser versus mechanical keratome flaps in wavefront-guided laser in situ keratomileusis; prospective contralateral eye study. J Cataract Refract Surg 2005; 31: Binder PS. One thousand consecutive IntraLase laser in situ keratomileusis flaps. J Cataract Refract Surg 2006; 32: Binder PS. Flap dimensions created with the IntraLase FS laser. 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7 448 CLINICAL RESULTS OF FEMTOSECOND LASER LASIK 16. Aslanides IM, Tsiklis NS, Astyrakakis NI, Pallikaris IG, Jankov MR. LASIK flap characteristics using the Moria M2 microkeratome with the 90-mm single use head. J Refract Surg 2007; 23: Heisterkamp A, Ripken T, Mamom T, Drommer W, Welling H, Ertmer W, Lubatschowski H. Nonlinear side effects of fs pulses inside corneal tissue during photodisruption. Appl Phys B Lasers Opt 2002; 74: Available at com/content/06l4qdyl9jpwug7y/fulltext.pdf. Accessed November 22, 2009 First author Jérôme C. Vryghem, MD Private practice, Brussels, Belgium print & web 4C=FPO