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Video Report

Sub- Epithelial Gas Breakthrough During Femtosecond Laser Flap Creation for LASIK

Herzig Eye Institute, and Department of Ophthalmology and Vision Sciences, University of Toronto, Toronto, Canada

Correspondence: Sathish Srinivasan
Email: sathish.srinivasan{at}gmail.com Herzig Eye Institute, The Colonnade, 131 Bloor St. West, Suite 201, Toronto, M5S 1R1, Canada. Tel: + 416-929-2020; Fax: + 416-929-0232.

Date of acceptance: July 2007

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Introduction

The femtosecond laser is the first alternative to mechanical microkeratomes, and the IntraLase femtosecond laser (IntraLase ®, Irvine, CA, USA) has been approved by the US Food and Drug Administration for lamellar corneal surgery. The surgical effect of this laser is achieved by photodisruption at the molecular level. The laser induced optical breakdown (LIOB) generates plasma, which expands and displaces the surrounding tissue resulting in the formation of cavitation bubbles. We report a case of myopic LASIK in which a vertical gas break through the surface occurred during IntraLase femtosecond flap creation.

Case Report

A 30-year female was referred for laser vision correction. Medical and ocular history was unremarkable. Best spectacle-corrected visual acuity was 20/20 in both eyes with a manifest refraction of –5.00 in the right and –4.75 dioptre sphere (D) in the left eye. Slit lamp biomicroscopy and applanation tonometry was normal. In particular there was no evidence of anterior basement membrane dystrophy in either eye. Preoperative corneal topography was normal with mean Keratometric readings of 44.85 D and 45.12 D in the right and left eye respectively. Preoperative ultrasound pachymetry was 530 μm in the right eye and 535 μm in the left eye.

Following informed consent, she underwent bilateral Wavefront guided (WaveScan, Visx, USA) LASIK. The IntraLase (FS 60) was used to create a 100- μm flap in both eyes. Laser settings for flap creation were: 9.00 mm diameter, 50°superior hinge angle, 1.10 μJ raster energy, 9- μm spot separation, 9- μm line separation, and 1.10 side-cut energy. A superior pocket depth of 220 μm and width of 0.250 μm was created. The cornea was applanated with a disposable glass contact lens cone docked to the suction ring. In the right eye, during flap creation in a raster mode, subepithelial gas breakthrough was noted in two focal areas superiorly. There was no loss of suction and the flap creation step was completed uneventfully. The surgeon (SH) was able to lift the flap without creating a buttonhole in the flap. The excimer ablation procedure was performed and the flap was repositioned. The LASIK procedure in the left eye was uneventful. On the first postoperative day uncorrected visual acuity was 20/20 in both eyes. Slit lamp biomicroscopy showed two focal areas of subepithelial scarring in the superior area, which indicated the site of subepithelial gas breakthrough during the LASIK procedure.

Discussion

The incidence of flap-related complications associated with the use of motorized microkeratomes for creating corneal flap during LASIK is around 5%.1,2 This includes decentered flaps, free flaps, irregular flap edges and stromal bed surfaces, epithelial abrasions, buttonholes, and flap lacerations. These complications often cause delayed recovery of visual acuity and occasionally lead to permanent vision loss The femtosecond laser is the first alternative to mechanical microkeratomes, and the IntraLase femtosecond laser has been approved by the US Food and Drug Administration for lamellar corneal surgery. The IntraLase system relies on a low-pressure suction ring to align and stabilize the globe. A flat glass contact lens attached to the laser delivery system is used to applanate the cornea within the suction ring. The laser software creates a circular cleavage plane either in a raster or a spiral pattern. A flap edge with a programmable angle is then created by using a circumferential pattern of progressively shallower pulses. A predefined arc along the edge is left uncut to create the hinge.

The femtosecond laser emits infrared radiation in the 1035 nm range. It induces optical breakdown of the target tissue by photodisruption that degenerates plasma. As this plasma expands it displaces the surrounding corneal lamella by creating cavitation bubbles thus resulting in a lamellar dissection. There have been previous reports of these cavitation bubbles migrating to the anterior chamber resulting in poor tracking during subsequent excimer laser ablation.3,4 Vertical subepithelial gas breakthrough during femtosecond laser flap creation is extremely rare and a Pub Med search revealed no previous report of this complication. Vertical gas breakthrough occurs between the dissection plane and the subepithelial space resulting in escape of gas bubbles in to the subepithelial space. The cause is unknown but a thin flap or a focal break in the Bowman's membrane may contribute to this complication. Our patient had a flap of 100 μm thickness and we did not experience suction loss during flap creation. It is very likely that a focal break in the Bowman’s membrane would have contributed to this complication. Careful dissection of the flap at the region of the gas breakthrough was performed as demonstrated in the video. The occurrence of subepithelial gas break-through with a femtosecond laser is comparable to a buttonhole formation during flap creation with a microkeratome. As the gas breakthrough was small, focal and not involving the visual axis we decided to proceed with the procedure. If the breakthrough involves a large area or involves the papillary area, we recommend either recutting the flap at a deeper level or aborting the procedure and converting to a surface ablation procedure on a subsequent day.

References

1. Stulting RD, Carr JD, Thompson KP, et al. Complications of laser in situ keratomileusis for the correction of myopia. Ophthalmology 1999;106:13-20.
2. Gimbel HV, Penno EE, van Westenbrugge JA, et al. Incidence and management of intraoperative and early postoperative complications in 1000 consecutive laser in situ keratomileusis cases. Ophthalmology 1998;105:1839-47.
3. Lifshitz T, Levy J, Klemperer I, et al. Anterior chamber gas bubbles after corneal flap creation with a femtosecond laser. J Cataract Refract Surg. 2005;31:2227-229.
4. Srinivasan S, Rootman DS. Anterior Chamber Gas Bubble Formation During Femtosecond Laser Flap Creation for LASIK. J Refract Surg 2007 (in print)