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Thin flaps may decrease the risk of post-LASIK ectasia
While the etiology of keratectasia after LASIK remains unknown, maximizing the amount of residual corneal stromal tissue may minimize the risk.
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Historically, thin flaps have the stigma of being classified as a LASIK complication and are considered by many surgeons to be striae-prone and a cause of postoperative irregular astigmatism.
Conversely, an increasing number of surgeons are intentionally creating thin, regular flaps (between 80-110 µm) using various microkeratomes, including the MK-2000 (Nidek), Automated Corneal Shaper (Bausch & Lomb), SKBM (Alcon Surgical) and LSK-One (Moria). The ability to create a thin but regular flap that does not predispose the patient to flap complications is advantageous.
Several studies indicate that the amount of residual corneal thickness after ablation is a factor in the development of post-LASIK ectasia. Therefore, most surgeons are striving to leave at least 250 to 300 µm of residual cornea tissue beneath the flap. As a result, patients with corneas too thin for their degree of myopia and pupil size are unable to undergo LASIK because of a significant risk of corneal ectasia.
LASIK-induced corneal ectasia
LASIK-induced corneal ectasia is likely dependent on several factors besides residual corneal bed thickness. Because it is relatively rare, it is difficult to ascertain its incidence. In the November 2001 issue of the Journal of Cataract and Refractive Surgery, Ioannis Pallikaris, MD, and colleagues reported that 19 of 2,873 eyes (0.66%) had developed post-LASIK ectasia, after a follow-up period of over 4 years.
Preoperatively, these eyes had shown no evidence of keratoconus or subclinical keratoconus. In the eyes that developed post-LASIK ectasia, one eye had a residual stromal thickness of 325 µm. While the etiology and biomechanical changes that induce keratectasia after LASIK remain unknown, it may be prudent to minimize one of the risk factors we can control by maximizing the amount of residual corneal stromal tissue.
From my experience of over 3,000 eyes, I believe that ultrathin flaps in the range of 75 µm to 100 µm are safe and effective. At the fall 2001 symposium of the International Society of Refractive Surgery, I presented a retrospective study of 1,131 eyes that demonstrated the safety of ultrathin-flap LASIK.
In this study, all patients underwent keratectomies with the Nidek MK-2000 microkeratome and ablation with the Nidek EC-5000 excimer laser. For 175 eyes, intraoperative ultrasonic pachymetry (Corneo-Gauge Plus, Sonogauge Inc.) was used to measure central corneal thickness and central bed thickness. The flap thickness was calculated by subtracting bed thickness prior to laser ablation from central corneal thickness. All 175 of these eyes had keratectomies using one specific MK-2000 and its corresponding 130 µm head and 8.5-mm ring (each microkeratome, head and ring is individually handmade) and appropriate lots of blades.
The mean follow-up in the study was from 1 to 544 days. At last follow-up 455 eyes (40%) achieved 20/20 or better, 798 eyes (70%) achieved 20/25 or better and 1,077 eyes (95%) achieved 20/40 or better uncorrected visual acuity. Because of the large number of patients with high myopia many of the patients did not have 20/20 best corrected vision preoperatively. However, 922 eyes (82%) achieved within one line of their best corrected visual acuity postoperatively. The following clinically significant complications were noted: four epithelial defects (3.5%), seven striae (0.6%), one diffuse lamellar keratitis (0.1%) and one epithelial ingrowth (0.1%). There were no infections and no irregular flaps. Two eyes (0.2%) with striae had 20/40 best corrected visual acuity but lost more than two lines. The average measured flap thickness was 87.3 µm ± 15.4 µm, and ranged from 45 µm to 130 µm. Although, the epithelium is reported to be 50 µm on average, we did not observe any buttonholes in any eyes.
The incidence of flap complications and the visual results in this study seems comparable to those reported in the literature for conventional LASIK. Overall, patients were satisfied with the surgery and had few complaints that would suggest there was significant irregular astigmatism. Comparing postoperative irregular astigmatism between ultrathin-flap LASIK and conventional LASIK will be difficult unless a controlled study is performed.
Currently, I routinely create LASIK flaps between 80 µm and 110 µm for my patients in order to minimize potential flap complications and LASIK-induced keratectasia. However, because I know it is safe to create flaps ranging from 60 µm to 70 µm, I will create a thinner flap or perform LASEK if indicated by the clinical situation.
For Your Information:
- Robert T. Lin, MD, is a private practitioner specializing in refractive surgery. He can be reached at IQ Medical Center, 18725 E. Gale Ave., Suite 140, City of Industry, CA 91748; (626) 854-2020; fax: (626) 854-2021; rlin@IQMedicalCenter.com. Dr. Lin has no direct financial interest in the products mentioned in this article, nor is he a paid consultant for any companies mentioned.
- Nidek, manufacturer of the MK-2000 Keratome System and the Nidek EC-5000 excimer laser, can be reached at 47651 Westinghouse Drive, Fremont, CA 94539; (800) 223-9044; fax: (510) 226-5750.