Laser in situ keratomileusis (LASIK) is currently the most common form of refractive surgery performed I in the United States.1 Although LASIK is not highrisk, it does carry some inherent risk to the eye because it can weaken the mechanical strength of the cornea.23 As surgeons, we are constantly attempting to make this surgery safer for patients by minimizing corneal mechanical instability following surgery. One such technique to minimize corneal instability is to maximize the amount of residual corneal stroma following laser ablation by intentionally creating a thin flap with the microkeratome.
Perhaps the most dreaded complication of LASIK is corneal ectasia, or iatrogenic keratoconus.4"6 Management of this potentially visually disabling complication is difficult, requiring a rigid gas permeable contact lens or corneal transplantation. Other experimental treatments for iatrogenic keratoconus include intracorneal ring segments and ultraviolet light stiffening of the cornea, but no studies exist to prove the long-term effectiveness of these modalities. Corneal ectasia after LASIK is thought to result from excessive tissue removal from the central cornea, resulting in a residual stromal bed that is too thin to maintain corneal integrity. It is believed that maintaining a residual stromal bed of 250 µm may be adequate to prevent ectasia.6 Patients at highest risk for ectasia are those with thinner corneas (central pachymetry <550 µm), high myopes, and patients with large pupils (>6.0 mm) because of the greater amount of tissue removed with the excimer laser in these situations.
Traditionally, microkeratome manufacturers have offered either a 160- or 180-µm gap between the plate and the blade for creation of the flap. Some newer keratomes have a 130-µm or thinner gap to achieve a thinner flap. Newer laser keratomes can theoretically create flaps thinner than 130 µm. Theoretically, the thinner the microkeratome makes the flap, the less the risk of ectasia because of the higher amount of residual stromal tissue. Previous reports7-9 have implied that thin-flap LASIK (≤130 µm) may be less safe for the patient because of an increased risk of flap complications including irregular flaps, buttonholes, and microstriae, all of which can affect final visual outcomes. If thinner flaps could be created safely (ie, with no increased risk of flap complications as compared to thicker flaps), more patients would become candidates for LASIK. Furthermore, patients undergoing LASIK-particularly those with high myopia, large pupils, or thinner corneas-could have the procedure performed with less risk of corneal ectasia.
PATIENTS AND METHODS
In a recent study,10 LASIK with a 130-µm microkeratome head was performed to determine whether thinflap LASIK is as visually effective and safe as when using a standard 160-µm head. A chart review was performed on 434 eyes of 228 consecutive patients undergoing primary myopic LASIK with a postoperative refractive goal of piano.
The keratectomies were performed using the BD K-3000 microkeratome (BD Ophthalmic Systems, Waltham, Mass) with either a 130-µm (thin flap) or 160-µm (conventional flap) head. Head size was selected to ensure a minimum calculated residual stromal bed of 250 µm following laser ablation. In general, the 130-µm head was used for eyes with preoperative central pachymetry of ≤550 µm and/ or spherical equivalent refraction of ≥-7.00 diopters (D). The average flap thickness by subtraction pachymetry for the 130-pm head of the BD K-3000 is 127.8±21.9 µm.11 The 160-µm head was used for all other eyes. The average flap thickness by subtraction pachymetry for the 160-µm head with the BD K-3000 is 150±20.0 µm (P.J. Dougherty, MD, unpublished data, 2002). All laser ablations were performed with the NIDEK EC-5000 excimer laser (NIDEK Technologies Ltd, Fremont, Calif) with an optical zone of 6.0 or 6.5 mm depending on scotopic pupil size. The 130-µm group was compared with the 160-µm group in terms of geometric mean12 on postoperative day 1. Snellen visual acuity was evaluated using a two-tailed t test. All intra- and postoperative flap complications were documented and compared between groups.
Patients Undergoing LASIK With the 130- or 160-µm Microkeratome Head
No statistically significant difference was noted between the groups with respect to mean age, mean preoperative refractive cylinder, best spectacle-corrected visual acuity (BSCVA), or keratometry (Table). The mean preoperative central corneal pachymetry in the 130-µm group was 526. 8±35.4 µm compared to 574.5±27.8 pm in the 160-µm group. The preoperative spherical equivalent refraction in the 130-µm group of -5. 00±2. 53 D was significantly higher (P<.0001 by ordinal logistic regression analysis) than -3.78±1.73 D in the 160-µm group (Fig 1). Geometric mean uncorrected visual acuity (UCVA) on postoperative day 1 was 20/25 in the 130-µm group and 20/26 in the 160-µm group. A statistically significant parallel t test, P=. 76) was noted between groups (Fig 2). No immediate postoperative flap complications, including microstriae, macrostriae, diffuse lamellar keratitis, or epithelial ingrowth, occurred in either group. A single partial flap due to loss of suction was seen in the 160µm group. No other intraoperative flap complications including thin flaps, epithelial defects, buttonholes, or irregular flaps, were seen in either group.
Based on this previous study, thin flaps can be created as safely as and result in similar, if not better, visual acuity than traditional thickness flaps. For this reason, I have switched to using thin-flap LASIK almost exclusively in my refractive practice. For higher myopia or thinner corneas, I have one particular 130-µm head that cuts, on average, a 100-µm flap. For these more difficult cases, I also routinely use intraoperative bed pachymetry to verify that at least 250 µm of residual stromal bed remains following the laser ablation. If the calculation shows less, I will narrow the optical and transition zones of the laser until adequate residual tissue is calculated, or I will abandon LASIK altogether and put the flap back in place without performing the laser ablation. I then will insert a phakic intraocular lens or wait 3 months and perform laser subepithelial keratomileusis with mitomycin-C on these patients. Although I exclusively use the BD K-4000 microkeratome because of its safety profile and reliability, many manufacturers produce microkeratomes that create thin flaps including NIDEK, Bausch &Lomb, Moria, and Intralase.
Figure 1. Preoperative spherical equivalent refraction in the 130-µm and 160-µm groups.
Figure 2. Uncorrected visual acuity on postoperative day 1 in the 130-µm and 160-µm groups.
The main reason to perform thin -flap LASIK is to decrease the risk of ectasia. The exact incidence of keratectasia after LASIK is difficult to determine given the infrequency of this complication. Pallikaris et al4 reviewed LASIK outcomes on 2873 eyes without evidence of forme fruste or frank keratoconus to determine the incidence of keratectasia. Only 19 (0.66%) of these eyes developed ectasia with a mean follow-up of 16 months. Five of the 14 patients who developed ectasia did so bilaterally. In my practice, I have seen only 2 (0.014%) cases of ectasia in 14,000 eyes. Neither flap was created with the thin-flap head of my keratome.
Multiple factors have been implicated in corneal ectasia and elevation of the posterior corneal surface after LASIK. Forme fruste keratoconus,413"16 increased patient age,4 high refractive corrections,417 high intraocular pressure,5 residual corneal stromal bed following laser ablation,41619 and biomechanical factors919 have all been implicated. Biomechanical factors and high refractive corrections are both risk factors for ectasia and are directly related to residual corneal stromal bed following laser ablation.
Multiple studies have demonstrated that frank or forme fruste keratoconus can lead to ectasia. In a study to determine the incidence of keratectasia, Pallikaris et al4 described six eyes with high cylinder and forme fruste keratoconus by topographic and pachymetric findings20,21 that developed ectasia after LASIK and were excluded from the study. Seiler and Quurke13 described a patient with forme fruste keratoconus with normal central pachymetry who developed keratectasia after LASIK. Schmitt-Bernard et al14 reported a patient with forme fruste keratoconus who underwent bilateral LASIK and developed significant keratectasia and loss of BSCVA.
Controlled ectasia by decreasing corneal biomechanical strength through a deep lamellar keratectomy is the principle behind hyperopic automated lamellar keratoplasty (ALK).2224 Creating a deep keratectomy causes the remaining posterior central cornea to bulge forward causing an increased radius of curvature of the anterior corneal surface and resulting myopia. I have seen cases of progressive ectasia with loss of BSCVA in eyes that have undergone hyperopic ALK.
Although we know that thin-flap LASIK can decrease the risk of ectasia by minimizing the biomechanical weakening of the cornea and maximizing residual corneal tissue following laser ablation, few studies focusing on thin -flap LASIK have been reported. Lin25 demonstrated the safety and effectiveness of thin-flap LASIK when he reported a retrospective study of 1131 eyes that underwent LASIK with a NIDEK EC-5000 excimer laser and NIDEK MK-2000 microkeratome with a 130-µm head. In this study, the average flap thickness by subtraction pachymetry was 87.3±15.4 µm. Seventy percent of eyes had postoperative UCVA of ≥20/25 and 95% saw ≥20/40 despite the large number of high myopes in the study with <20/20 preoperative BSCVA. Nine hundred twenty-two (82%) eyes achieved within one line of their preoperative BSCVA. Only 7 (0.6%) eyes were noted to have flap striae. No irregular flaps or buttonholes were noted.
Chayet26 reported thin-flap LASIK results on 168 eyes using a NIDEK MK-2000 microkeratome with a 120- or 130-µm head. The mean flap thickness in the 120-µm group was 103.1±14.5 µm. The mean flap thickness in the 130-µm group was 110.7±19.3 pm. No flap complications were seen in either group.
My study10 was the first to demonstrate that thin-flap LASIK is as safe and effective as conventional LASIK. In fact, fewer flap complications were noted in the thin flap group than in the conventional flap group. In addition, the postoperative day 1 visual outcomes were slightly better in the 130-µm group despite their having a statistically significant higher level of preoperative myopia than the 160-µm group. This might possibly relate to thinner flaps having less flap edema in the early postoperative period, allowing for faster visual rehabilitation.
Creating thinner flaps without increasing flap complications may benefit two groups of patients. The first group who might benefit from this technique includes patients who may not currently be good candidates for traditional 160- or 180-µm flap LASIK because of corneas that are too thin relative to their level of myopia and pupil size. Some of these patients may now safely undergo LASIK with a thinner flap while maintaining a 250-µm residual stromal bed.
The second group that may benefit from this technique comprises patients who are currently candidates for LASIK performed with traditional 160- or 180-µm microkeratome heads. These patients can also benefit from the added safety of the increased amount of residual stromal tissue following laser ablation in thin-flap LASIK. Thin-flap LASIK is currently recommended for any patient with borderline corneal thickness or myopia >-5.00 D. The Snydacker Curve27 estimates that 3.3% of the US population has myopia of >? 5.00 D. Given the current US population of 285.3 million people,28 up to 9.4 million Americans with myopia >? 5.00 D could potentially benefit from thin-flap technology.
1. Learning DV. Practice styles and preferences of ASCRS members-2000 survey. American Society of Cataract and Refractive Surgery. J Cataract Refract Surg. 2001;2 7:948-955.
2. Roberts C. The cornea is not a piece of plastic. J Refract Surg. 2000;16:407-413.
3. Peacock LW, Slade SG, Martiz J, Chuang A, Yee RW. Ocular integrity after refractive procedures. Ophthalmology. 1997;104:1079-1083.
4. Pallikaris IG, Kymionis GD, Astyrakakis NI. Corneal ectasia induced by laser in situ keratomileusis. / Cataract Refract Surg. 2001;27:1976-1802.
5 . Koch DD. The riddle of iatrogenic keratectasia. / Cataract Refract Surg. 1999;25:453-454.
6. Seiler T, Koufala K, Richter G. Iatrogenic keratectasia after laser in situ keratomileusis. J Refract Surg. 1998;14:312-317.
7. Pallikaris IG, S?ganos DS, Katsanevaki VI. LASIK complications and their management. In: Pallikaris IG, S?ganos DS, eds. LASIK. Thorofare, NJ: SLACK Ine; 1998:257-274.
8. Durairaj VD, Balentine J, Kouyoumdjian G, Tooze JA, Young D, Spivack L, Taravella MJ. The predictability of corneal flap thickness and tissue laser ablation in laser in situ keratomileusis. Ophthalmology. 2000;107:2140-2143.
9. Geggel HS, Talley AR. Delayed onset keratectasia following laser in situ keratomileusis. / Cataract Refract Surg. 1999;25:582-586.
10. Dougherty PJ. Thin flap LASIK. Clinical and Surgical Journal of Ophthalmology. 2003;l/21:326-330.
11. Dougherty PJ. Efficacy and safety of a 130 µm microkeratome head in LASIK. Paper presented at: Summer 2001 ISRS Refractive Symposium; July 28, 2001; Orlando, FIa.
12. Holladay JT, Prager TC. Mean visual acuity. Am J Ophthalmol. 1991;111:372-374.
13. Seiler T, Quurke AW. Iatrogenic keratectasia after LASIK in a case of forme fruste keratoconus. / Cataract Refract Surg. 1998;24:1007-1009.
14. Schmitt -Bernard CF, Lesage C, Arnaud B. Keratectasia induced by laser in situ keratomileusis in keratoconus. / Refract Surg. 2000;16:368-370.
15. Buzar d KA, Tuengler A, Febbraro JL. Treatment of mild to moderate keratoconus with laser in situ keratomileusis. J Cataract Refract Surg. 1999;25:1600-1609.
16. Argento C, Cosentino MJ, Tytiun A, Rap etti G, Zarate J. Corneal ectasia after laser in situ keratomileusis. J Cataract Refract Surg. 2001;27:1440-1448.
17. Seitz B, Torres F, Langenbucher A, Behrens A, Suarez E. Posterior corneal curvature changes after myopic laser in situ keratomileusis. Ophthalmology. 2001;108:666-673.
18. Wang Z, Chen J, Yang B. Posterior corneal surface topographic changes after laser in situ keratomileusis are related to residual corneal bed thickness. Ophthalmology. 1999;106:406-410.
19. Joo CK, Kim TG. Corneal ectasia detected after laser in situ keratomileusis for correction of less than -12 diopters of myopia. J Cataract Refract Surg. 2000;26:292-295.
20. Rabino witz YS, McDonnell PJ. Computer -assisted corneal topography in keratoconus. Refract Corneal Surg. 1989;5:400-408.
21. Rabino witz YS, Garbus J, McDonnell PJ. Computer-assisted corneal topography in family members of patients with keratoconus. Arch Ophthalmol. 1990;108:365-371.
22. Ghiselli G, Manche EE, Maloney RK. Factors influencing the outcome of hyperopic lamellar keratoplasty. / Cataract Refract Surg. 1998;24:35-41.
23. Kezirian GM, Gremillion CM. Automated lamellar keratoplasty for the correction of hyperopia. / Cataract Refract Surg. 1995;21:386-392.
24. Manche EE, Judge A, Maloney RK. Lamellar keratoplasty for hyperopia. / Refract Surg. 1996;12:42-49.
25. Lin RT. Thin flaps may decrease the risk of post-LASIK ectasia. Paper presented at: Fall 2001 ISRS Refractive Symposium; October 10, 2001; New Orleans, La.
26. Chayet A. Ultra- thin flaps may guard against ectasia. Paper presented at: 2002 ASCRS meeting; June 4, 2002; Philadelphia, Pa.
27. Borish IM. The refractive status of the eye ? distribution and influences. In: Borish IM, ed. Clinical Refraction. 3rd ed. Chicago, Ill: Professional Press Ine; 1975:5-20.
28. US Government Census Office. The 2000 US Census. Available at: http://www.census.gov. Accessed March 17, 2005.
Patients Undergoing LASIK With the 130- or 160-µm Microkeratome Head