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News
Flap Dimensions Created with the IntraLase laserFlap dimensions created with the IntraLase FS laser
Perry S. Binder, MD
Purpose: To assess the safety and predictability of the IntraLase femtosecond laser to create accurate flap thickness and diameter.
Setting: Clinical office-based practice.
Methods: In the first 103 eyes in which flaps were created with the IntraLase laser, the flap thickness was measured by the ultrasonic difference between the pre-operative and post-flap-creation central corneal thickness and the flap diameter was measured with calipers.
Results: As the attempted flap thickness decreased from 140.0 |o,m to 110.0 u,m in 10.0 (im increments, the mean flap thickness decreased from 132.5 u.m to 125.0 u.m, with standard deviations decreasing from ±18.5 to ±12.0 urn. The mean flap diameter differed from the attempted diameter by less than 0.03 (im in all but the 130.0 |xm group. Two slipped flaps and 20 cases of interface inflammation occurred early in the series.
Conclusions: The IntraLase laser, while adding technical complexity to the laser in situ keratomileusis procedure, is able to predictably create flap diameters, hinge location, and flap thickness while eliminating the risk for cap perforations. The technique of flap elevation affects rapidity of visual recovery.
J Cataract Refract Surg 2004; 30:26-32 © 2004 ASCRS and ESCRS
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'""T'he thickness of the laser in situ keratomileusis J. (LASIK) flap determines how much corneal tissue will remain for laser ablation. Because stromal anatomy,1 ultrasound absorbance,2 and possibly hydration may vary with corneal depth, the effectiveness of an ultraviolet laser beam will vary with stromal depth. Predicting flap thickness is important to avoid corneal ectasia.3 In addition to thickness, certain refractive errors necessitate predictable flap diameters of at least 9.0 mm.
Current microkeratome systems are limited in their ability to create flaps in eyes with different corneal and/or orbital configurations.4 The flap complications associated with mechanical microkeratomes include de-
Accepted for publication May 29, 2003.
From the Gordon Binder Vision Institute, San Diego, California, USA.
Dr. Binder is the owner of Outcomes Analysis Software and is a consultant to IntraLase.
Reprint requests to Perry S. Binder, Gordon Binder Vision Institute, 8910 University Center Lane, Suits 800, San Diego, California 92122, USA. E-mail:garrett@aol.com.
© 2004 ASCRS and ESCRS Published by Elsevier Inc.
centered and free flaps, irregular edges and surfaces (chattering), epithelial abrasions, buttonhole perforations, cap lacerations, and inadequate diameters for a given correction. Most systems are limited to producing a single hinge location and create hinge sizes that are highly variable. For some refractive errors, it is necessary to decenter the flap, but current microkeratome systems do not allow predictable decentrations. Most systems produce meniscus flap shapes that are thinner in the center and thicker toward the periphery, which increases the risk for a buttonhole perforation.5 Preoperative thin and/or steep corneas can limit the use of most systems because thin and steep corneas create thinner-than-pre-dicted flaps.3 To adjust flap thickness, one must use a different microkeratome head or a different microkeratome system.
Current microkeratome systems produce variable flap thicknesses with a standard deviation (SD) greater than ±25 to ±40 (xm.6"8 The mean flap diameter SD is ±0.3 mm.9 The ability to create predictable flaps depends on the blade, microkeratome system, and sur-
0886-3350/03/$-see front matter doi:10.1016/S0886-3350(03)00578-9
geon. Ideally, surgeons would have a microkeratome system that allows surgeon control of flap dimensions and the ability to operate most orbital and corneal configurations while producing minimal, predictable wavefront abnormalities. The limitations of current mechanical microkeratomes include poor predictability of flap thickness and flap diameter, microkeratome-spe-cific hinge location, flap thickness determined by pre-operative corneal thickness, flap diameter determined by corneal power, flap thickness adjusted by changing microkeratome heads, flap diameter changed by suction rings, and poor predictability of hinge size and location relative to the visual axis; in addition, the quality of the flap cut is blade dependent, most flaps have a meniscus shape (thinnest in the center), and many systems are restricted to certain orbital dimensions.
The IntraLase femtosecond mode laser was designed to address most limitations of current systems. The system creates focused cavitation spots within the stroma delivered in a raster pattern that begins at the hinge and progresses temporally in depths theoretically predictable to ±4 |xm using a 1053 nm (infrared) wavelength (Nd:Glass).'° When the laser beam is focused at the desired corneal depth, laser-induced optical Breakdown occurs at low energy without creating rher-maj.or shockwavejlamage to the surrounding tissue."'12 Thg spots are placed 5 to 12 fjirn apart; as the microcavita-tion bubbles expand, the spots coalesce, forming a resection plane.13'14 This process does not remove corneal tissue^After the horizontal cleavage plane is created, the pattern changes to a vertical one, exiting through Bowman's layer and the epithelium. The computer's software controls the attempted flap diameter and thick-ness, the angle of the_side_cut. hinge size and locadon^ and all the energy settings_taj".re,afe these dimensions.
In preliminary clinical studies, the IntraLase has produced excellent refractive results.10'15 The current study reports the flap dimension measurements in my initial cases using the pulsion laser.
Patients and Methods
All surgery was performed by me in a clinical private practice setting. Patients received video informed consent and a written informed consent before they had surgery. Indications for the IntraLase procedure included eyes with cornea] thickness greater than 500 u,m and refractive errors clinically indicated to be correctable with standard LASIK. Before surgery, all eyes had at least 2 manifest or cycloplegic refractions 2 or more weeks apart.
The preoperative examination included a cmngletejlit-lamp examination, central keratometry (Bausch & Lomb), comeal topography (Technomed,). waverront aberrometrv (Wavescan, Visx), central ultrasound pachymetry (Sonnogage Cornea Scan II 5). applanation intraocular pressure (IOP), and a dilated fundus examination. The laser surgery was performed with the Visx S3 or the LadarVision 4000 excimer laser using my nomogram (Outcomes Analysis Software).
Flap Dimensions
Before the IntraLase was used, a prospective study of the predictability of flap thickness was performed. The initial series attempted to create a flap thickness of 140 u/m; the next attempted flap thickness was 130 u,m and the final attempted flap thickness, 110 u,m.
Surgical Procedure
The patient reclined on the Visx excimer laser bed, which was positioned between the Visx S3 laser and the IntraLase laser. After the eyelid speculum was placed under the Visx operating microscope, the central corneal thickness was measured ultrasonically with the unit that was used preopera-tively. This measurement served as the preoperative corneal thickness reading. The eye was then fixed with a disposable suction fixation ring (Figure 1, top), centering the cornea within the ring whose outer silicone flange diameter was 22.0 mm. The outer diameter of the fixed poly(methyl methacrylate) ring internal to the flexible silicone flange was 19.0 mm. The Visx bed was rotated to the left under the IntraLase laser and locked in position. The IOP measured with a standard Barraquer tonometer when the suction ring was affixed to the globe was less than 65 mm Hg, and the patients reported they could see. As soon as the applanation cone of the IntraLase was applied, patients stated their vision blacked out.
The computer was preprogrammed for each procedure. The initial settings were ? planned flap diameter of 9.3 mm, ajlanned^ flap_thickness^of 140 u,m. a hinge angle of 45 degrees, flap energy of 4 microjoules fu.J), beam sepgrarinn oTTT^UTLby; 12 jlm,'jtndside cut energy "f R |'J After the cornea was applanated with the disposable glass contact lens cone attached to the IntraLase (Figure 1, bottom), the laser was lowered into position over the eye. The cone was fixed to the disposable suction ring via an adjustable internal clamp on the ring.
The laser focused the beam relative to the bottom of the glass cone. The alignment of the applanation was observed through the IntraLase and/or through the computer screen of the IntraLase untiljhe cornea was applanated guisjde_the. desired cap djameter Rising a diagram on the computer screen. The applanated cornga was aligned_via software to center^he planned flap diameter on the pupil. (The software assesses whetherjhe_planned diameter^ will_fit within the newly de-renrf-rprLHirqejisions and asks the surgeon to accept the decentration or to select a smaller flap,diarrif'rpr.LA safety light on the computer screen warns the surgeon if excessive pressure is being placed on the eye.
27
J CATARACT REFRACT SURG—VOL 30, JANUARY 2004
INTRA1ASE LASER
When the settings were accepted, the footpedal was pressed and the ablation began from the hinge in a raster pattern going back and forth vertically, the beam moved farther from the hinge with each vertical pass. The hinge location was nasal
In all but 2 eyes in which a temporal flap was selected for a hyperopic ablation. All flap data regardless of hinge location were included. The entire flap creation was observed by the surgeon. The beam was pulsed in a mode-locked fundamental mode using spot sizes of 5um to 6 um, a pulse duration of 600 femtoseconds a repetition rate of 6000Hz, and a cone angle of 36 degrees.
When the resection plane was completed (the time varied depending on the planned diameter), the software directed the laser to make a vertical flap edge cut with an initial angle of 75 degrees (compared to 25 to 30 degrees with standard microkeratomes); this required 25 seconds to complete. The time from placement of the suction ring to removal of the applanation cone varied from 1 minute 37 seconds to 3 minutes 30 seconds; in most cases the time was less than 2.5 minutes.Following the laser procedure, 1 drop of (Ocuflox®) and 1 drop of ketorolac tromethamine: (Acular®) were placed in the eye. The same procedure was performed in the fellow eye. The patient was then placed in the surgical waiting room for 15 to 30 minutes to allow absorption of the microcavitaion bubbles.
Flap Elevation
The patient was placed under the laser, and the lid margins were covered with a sterile drape. The surgeon wore surgical gloves. An eyelid speculum was placed, and 2 disparate-diameter radial keratotomy optical zone markers dipped in gentian violet were used to demarcate the nasal superior and inferior quadrants. The central pachymetry was measured again. Any difference from the presurgical readings was noted, but-the flap dimensions were calculated based on the in vivo surgical measurements.Under X10 magnification, a Maloney spatula (Katena) was used to enter the laser wound in the superior quadrant near the hinge. Since there was no flap separation, a spatula was used to break the microadhesions. The spatula was moved back and forth gently until it entered the interface about 3.0 mm. The patient was asked to look at the fixation light and resist eye movement. The spatula was then moved temporally until it exited the temporal flap edge. The spatula was wiped free of epithelium, placed in the initial location, advanced 3.0 mm, and then swept temporally. The interface was opened in 1 to 3 such passes, and the flap was then reflected nasally.
Flap Dimensions
Immediately after nasal reflection of the flap, central pachymetry was recorded through the entrance pupil without the use of eyedrops. (The stromal bed is dry compared to
that seen when a mechanical microkeratome is used.) The difference between the preoperative reading and this reading, was recorded as the flap thickness."' After the laser procedure and flap repositioning, the vertical flap diameter was measured with standard calipers chat was read at X 10 magnification to the nearest O.I mm. The hinge length was not recorded.
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1 CATARACT KIT-KACT SURG—VOl. 30, JANUARY 2004
INTRALASK LASER
Flap Repositioning
The interface was cleaned with a dry cellulose sponge after the laser ablation. One drop of Ocuflox was placed in the interface, and the flap was repositioned using, forceps (Katena). The interface was next irrigated with a sterile balanced salt solution with a Millipore filter attached to a 30-gauge cannula. The flap was gently swept nasally to temporally several times with a cellulose sponge moistened with Ocuflox. The surface of the flap was then dried with a 10-second blast of oxygen through a sterile plastic tube. The surface was moistened with Ocuflox after 1 minute, and the lid speculum was removed. The patient was asked to blink several times and after flap stability was confirmed, a plastic shield was applied to the eye.
After the ablation, the interface was cloudy and patients reported their vision was cloudy. I he cavitation bubbles were seen in the interface and the cornea! periphery (Figure 4).
Statistical Analysis
The data were exported to Microsoft® Excel. The Student t test was used to detect differences between flap thick nesses in each group.
Results
Flap Thickness
One hundred three consecutive eyes were operated on from the first day the laser was used. In the first 10 eyes, the cornea! Thickness was remeasured immediately before the flap was elevated. It deviated from the previous measurement by up to 5.0 um. As the-attempted flap thickness decreased, the achieved mean thickness decreased, and as experience was gained with
The laser, the SD for each attempted flap thickness
decreased from ±9.0um in the 140um flap thickness group to ±12.0um in the 110um flap thickness group. In the initial 140um group, the flap was difficult to raise because of user inexperience with the new approach and because the spot separation was 12.0um by 14.0um, which created a wide separation between each focused ablation. When the spot separator was reduced by 10.0um by 10.0um, the flap was easier to lift so less trauma was inflicted. There was no statistical difference between the 110um and the 120um flap thickness groups. The range of achieved flap thickness also decreased as the attempted flap thickness decreased.
Flap Diameter
The attempted diameter was between 9.0 mm and 9.3 mm in eyes with attempted flap thick nesses of 1 10 to 140 um. In some cases, the planned diameter had to be reduced from the attempted diameter because the applanation cone decentered. A diameter of 9.3 mm was achieved in 26 of the 87 eyes in which it was attempted; in the remaining eyes, the attempted flap diameters were between 8.7 mm and 9.2 mm. fable 2 shows the difference between the achieved diameter and the attempted diameter. The standard deviation of the diameter varied from 0.12 to 0.26 mm. The largest standard deviation was due to 2 eyes in the 120 p.m group whose flap diameter was 0.30 mm smaller than the intended. When these eyes were excluded, the standard deviation was 0.24 mm. 1 here was no correlation
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j CATARACT KKI KACT SUKC;~vol. .)o, JANUARY 21104
INTRALASE LASER
between the flap diameter and the attempted flap
thickness.
Flap and Stromal Bed Clinical Quality
The flap edges were crisp along the entire diameter. The hinge height and location were where the software predicted.
The interface and stroma were drier than after a
standard microkeratome pass (Figure 5), but the dryness
did not interfere with pachymetry. There was no fluid
on the stroma] surface, which appeared opaque com
pared to the stromal bed created with a conventional
microkeratome. It was difficult to obtain tracking with
either laser as the dryness obscured the view of the
pupil in some cases. A cellulose sponge moistened with
Ocuflox was swept across the bed once or twice to
permit tracking. Laser treatment of the 2 temporal flaps
was performed without difficulty. No flap decentrations
or epithelial defects occurred. Flap clarity at the end of
surgery appeared no different than that after flap cre
ation by standard microkeratomes.
Clinical Postoperative Appearance
On the first postoperative day, Bowman's layer in some flaps appeared slightly more granular than flaps created with other microkeratomes. The flap edge gap was prominent. By the second week, the next scheduled examination, the flaps appeared no different than those created by current microkeratomes.
Complications
In 1 eye, suction was lost when the ablation was 60% complete. Examination of the eye immediately after the aborted procedure revealed no evidence of a vertical cut through the pupil. By postoperative day 1, the eye appeared as if no surgery had been performed.
One hour after surgery, 1 patient presented with an interiorly displaced flap, which was immediately reposi-tioned. The next day, the flap was well oriented without complications. The fellow eye had no flap complications.
A second patient presented with an irferiorly displaced flap in the operated eye on the first postoperative day. I he flap had deep folds emanating from the supe-
Attempted Flap Thickness (um) Number of Eyes Mean Flap Diameter Standard Deviation Range Mean Difference from (mm) (mm) (mm) Attempted (mm)
110 34 8.98 0.12 8.7-9.1 -0.02
120 22 9.07 0.17 8.7-9.3 0-03
130 21 3.1 0.24 8.4-9.4 0.37
140 26 9.1 0.26 8.5-9.4 -0.02
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J CATARACT RF.FRACT SURCI—VOI 30. JANUARY 2004
INTRALASE LASER
J CATARACT REFRACT SURG—VOL 30, JANUARY 2004
ergy imparted to the cap and stroma may affect vision recovery, which can require up to 2 weeks after surgery. As more eyes were operated on and laser energy settings were modified (2.8 fij to 1.9 (xj), and the spot separation decreased from 14 jjum by 14 (xm to 10 |xm by 10 jjum, there was less trauma to die flap and consequendy die UCVA on the first postoperative day improved to die levels seen with conventional microkeratomes. The decrease in die side-cut energy and side-cut angle eliminated the interface inflammation.
In a series of 150 eyes completed after this study (unpublished observations), I decreased the side cut energy to 4.9 mj and decreased the side cut angle to 30 degrees, which eliminated the interface inflammation. Subcconjunctival hemorrhage was more common than with the SKBM or the Automated Corneal Shaper mi-crokeratome, but the slow release of the suction decreased the incidence:
As with any new technology, surgeons will have j to assess all factors and direct the companies toward improvement in hardware and software. In the development of the excimer laser, it took about 5 years of clinical trials and U.S. Food and Drug Administration approval for the industry to develop systems and algorithms that provide the excellent results we have come to expect. With minor modifications of the computer software and the suction ring and increased speed in the laser procedure, the IntraLase FS laser has the potential to replace current microkeratome systems and reduce the need to perform photorefractive keratectomy or laser-assisted subepithelial keratectomy.
References
1. Bron AJ. The architecture of the corneal stroma [editorial]. Br J Ophthalmol 2001; 85:379-383
2. Kolozsvari L, Nogradi A, Hopp B, Bor Z. UV absorbance of the human cornea in the 240- to 400-nm range. Invest Ophthalmol Vis Sci 2002; 43: 2165-2168
3. Seiler T, Koufala K, Richter G. latrogenic keratectasia after laser in situ keratomileusis. J Refract Surg 1998; 14:312-317
4. Binder PS, Lambert RW. A comparison of current instrumentation for lamellar refractive surgery. In: Long DA, ed, Cornea and Refractive Surgery; Proceedings of the 46th Annual Symposium of die New Orleans Academy of Ophthalmology. The Hague, Kugler, 1997; 189-218
5. Walker MB, Wilson SE. Lower intraoperative flap complication rate with ihe Hansatome microkeratome com-
32
pared to the automated corneal shaper. J Refract Surg 2000; 16:79-82
6. Arbelaez MC. Nidek MK 2000 microkeratome clinical evaluation. J Refract Surg 2002; 18:S357-S360
7. Ucakhan OO. Corneal flap thickness in laser in situ keratomileusis using the Summit Krumeich-Barraquer microkeratome. J Cataract Refract Surg 2002; 28:798-804
8. 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
9. Binder PS. Defining the profile of the Automated Corneal Shaper microkeratome. Ophthalmic Pract 1999; 17:308-313
10. Nordan LT, Slade SG, Baker RN, et al. Femtosecond laser flap creation for laser in situ keratomileusis: six-month follow-up of initial US clinical series. J Refract Surg 2003; 19:8-14
11. Kurtz R, Liu X, Elner VM. Photodisruption in the human cornea as a function of laser pulse width. J Refract Surg 1997; 13:653-658
12. Juhasz T, Loesel F, Kurtz R, et al. Femtosecond laser refractive corneal surgery. IEEE J Spec Top Quant Electron 1999; 5:902-910
13. Vogel A, Gtinther T, Asiyo-Vogel M, Birngruber R. Factors determining the refractive effects of imrastromal photorefractive keratectomy with the picosecond laser. J Cataract Refract Surg 1997; 23:1301-1310
14. Kurtz RM, Horvath C, Liu H-H, et al. Lamellar refractive surgery with scanned picosecond and femtosecond laser pulses. J Refract Surg 1998; 14:541-548
15. Ratkay-Taub I, Juhasz T, Horvath C, et al. Ultra-short pulse (femtosecond) laser surgery; initial use in LASIK flap creation. Ophthalmol Clin North Am 2001; 14(2): 347-355
16. Binder PS, Flannagan GF. The precision of flap measurements for laser in situ keratomileusis. In press, J Refract Surg
17. Yildirim R, Aras C, Ozdamar A, et al. Reproducibility of corneal flap thickness in laser in situ keratomileusis using the Hansatome microkeratome. J Cataract Refract Surg 2000; 26:1729-1732
18. Yi W-M, Joo C-K. Corneal flap thickness in laser in situ keratomileusis using an SCMD manual microkeratome. J Cataract Refract Surg 1999; 25:1087-1092
19. Muller L, Pels E, Vrensen GFJM. The specific architecture of the anterior stroma accounts for maintenance of corneal curvature. Br J Ophthalmol 2001; 85:437-443
20. Tham VM-B, Maloney RK. Microkeratome complications of laser in situ keratomileusis. Ophthalmology 2000; 107:920-924
21. Binder PS, Akers PH, Deg JK, Zavala EY. Refractive keratoplasty; microkeratome evaluation. Arch Ophthalmol 1982; 100:802-806
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