Microkeratome Blades and Corneal Flap
Thickness in LASIK

Chi-Wang Yau, MD; Huey-Chuan Cheng, MD, MSc

• BACKGROUND AND OBJECTIVE: To determinethe corneal flap thickness produced by using two different types of blades on the same microkeratome for LASIK, comparing flap thickness in the first and second eyes.

• PATIENTS AND METHODS: and eitheThe corneal flapthickness was measured in two sets of 100 consecutive patients undergoing bilateral LASIK procedures using the Moria M2 microkeratome with a 110 head (Moria, France) r Moria or CLB blades (Med-Logics Inc., Laguna Hills, CA). Corneal flap thickness was determined by intraoperative pachymetry using an ultrasonic pachymeter.

• RESULTS: The mean corneal flap thickness was 138.95 ± 21.6 μm (range: 75–205 μm) with Moria blades and 115.07 ± 16.0 μm (range: 70–153 μm) with CLB blades, which is a significant difference (P < .001). The difference in flap thickness between the first and second eyes was not significant with the Moria blades (141.46 ± 21.46 μm vs 136.45 ± 21.55 μm, P = .965), but it was when the CLB blades were used (123.21 ± 12.07 μm vs 106.93 ± 15.51 μm; P = .013).

• CONCLUSION: In LASIK surgery using a Moria M2 microkeratome, blades from different manufacturers may produce significantly different corneal flap thicknesses, as well as differences between the first and second eyes.

[Ophthalmic Surg Lasers Imaging 2008;39:471-475.]

INTRODUCTION

Laser in-situ keratomileusis (LASIK) is well accepted as effective refractive surgery for myopia, hyperopia, and astigmatism. Although the procedure is safe, there are some complications, including iatrogenic keratectasia.1,2 The cause of keratectasia is not completely known but inadequate residual stromal thickness, a problem that may occur if the corneal flap is unexpectedly thicker than intended, may be an important factor. Several studies have demonstrated inaccuracy in the predictability and reproducibility of corneal flap thickness produced by mechanical microkeratomes.3-10 The Flap Thickness Study Group8 summarized factors influencing flap thickness as microkeratome model, plate thickness, mean preoperative pachymetry, surgery order, head serial number, blade lot number, and surgeon. Consistent use of one microkeratome may improve the precision of flap thickness, but it is possible that some additional factor such as differences in blades may also affect flap thickness. We designed this study to compare the corneal flap thickness created by the same microkeratome with blades from two manufacturers.

______________________________________________________________

From the Department of Ophthalmology (C-WY, H-CC), Mackay Memorial
Hospital; and Mackay Medicine, Nursing and Management College
(H-CC), Taipei, Taiwan.
Accepted for publication April 19, 2008.
Address correspondence to Chi-Wang Yau, MD, 2F, 108 Section 2, Chung
Shan North Road, Taipei, Taiwan.

______________________________________________________________

PATIENTS AND METHODS

We retrospectively assessed flap thickness in a bilateral LASIK procedure to correct myopia and astigmatism in patients older than 18 years. All procedures were routine LASIK operations, the only difference being the use of two different types of blades approved by the government for LASIK surgery. Because the operations were routine procedures and were retrospectively analyzed, institutional ethics review was not required for the study. All patients underwent complete ophthalmic examination before surgery, including best spectacle-corrected visual acuity, manifest and cycloplegic refraction, pneumotonometry, slit-lamp microscopy, detailed fundus examination, tear secretion testing, ultrasonic pachymetry, and Orbscan (Bausch & Lomb, Rochester, NY) topography.

The LASIK procedures were performed under topical anesthesia with 0.5% proparacaine eye drops given 3 minutes before and again just before insertion of the eye speculum. Under an operating microscope, excessive
water was wiped off the corneal surface with a surgical sponge. The central corneal thickness was measured at the papillary area using a Sonogage ultrasonic pachymeter
(Sonogage, Cleveland, OH). An average of three measurements was recorded as the preoperative corneal thickness.

A Moria M2 microkeratome (Moria, France) with a 110-μm head was used to create a corneal flap with a superior hinge. The suction ring was chosen according to the manufacturer’s instructions. The hinge width was set to 7.5mm. One microkeratome blade was used in both eyes. Immediately following the cut, the corneal flap was lifted with forceps holding the flap edge without touching the corneal stromal bed. Ultrasonic pachymetry of the stromal bed was again performed as described above, using an alcohol-disinfected tip for the measurement. The corneal flap thickness was calculated by subtracting the corneal stromal bed thickness from the preoperative corneal thickness. Laser ablation was performed with the VISX S4 system (VISX Incorporated, Santa Ana, CA). The flap was repositioned and the interface was irrigated with balanced salt solution. One surgeon performed all procedures in the study.

For the first 2 years that we used the Moria microkeratomemicrokeratome, we also used Moria blades (Moria, France). We then switched to CLB blades (Med-Logics Inc., Laguna Hills, CA). We retrospectively analyzed the corneal flap thickness in the last 100 consecutive patients using the Moria blades and the first 100 consecutive patients using the CLB blades. The study group thus comprised a total of 200 patients and 400 eyes. The Student’s t test was used to compare the corneal thickness in the Moria and CLB groups and also in the first and second eyes of all patients. Data are reported as mean ± standard deviation (SD).

RESULTS

The two groups of patients had similar demographic and baseline preoperative data (Table 1). With the Moria blade, the mean flap thickness was 138.95 ± 21.6 μm (range: 75 to 205 μm). There was no significant difference in flap thickness between the first and second eyes (P = .965). In the CLB group, the mean flap thickness was 115.07 ± 16.0 μm (range: 70 to 153 μm), but the flap thickness in the first and second eyes differed significantly (123.21 vs 106.93 μm, P = .013). The Moria blades produced a significantly thicker flap than did the CLB blades (138.95 ± 21.6 vs 115.07 ± 16.0 μm; P < .001) (Table 2). There were no intraoperative complications such as button hole, free flap, or epithelial ablation in both groups of patients.

DISCUSSION

In the current study, we found that corneal flap thickness can vary even with use of the same microkeratome if different blades are used. The Moria M2 is an automated microkeratome. It has three suction rings with variable hinge widths to produce the desired flap diameter, hinge width, and ablation zone size. It has 110 and 130 heads to create different flap thicknesses. According to the manufacturer’s information and previous reports, the actual flap thickness is usually 20 to 30 μm thicker than the labeled head thickness.11
 

Several investigators have studied the flap thickness produced by the Moria M2 microkeratome (Table 3).5,6,8,12,13 Solomon et al.8 reported the mean flap thicknesses using the 130 head were 150 and 148 μm in the first and second eyes. Miranda et al.5 used a 110 head in 257 eyes, obtaining a mean flap thickness of 134 μm. Muallem et al.6 found that the 110 head with fast translation velocity created a 136- and 132-μm flap in the first and second eye, respectively. The mean flap thicknesses in those studies were considered close to the desire thicknesses. However, Muallem et al. also found that the head translation velocity significantly affected the flap thickness. With a 110 head with a slow translation velocity, they obtained much thicker flaps of 151 and 148 μm in the first and second eyes, comparable to those produced by the 130 head.
 

The current investigation demonstrated that the choice of blade alone may also affect flap thickness. All other conditions for the LASIK procedure were the same in both groups of patients. Using the same Moria M2 110 head, we found that Moria blades created a significantly thicker flap than did CLB blades. The flap thickness in our series using Moria blades is similar to the results of other studies,5,6 but we were unable to find reports comparing it with the CLB blades. Therefore, one must use the same brand of blades to obtain consistent flap thickness. Alternatively, if changing blade brands for any reason, one should be aware that the results obtained may be different than expected. Our study was not designed to examine why different brands of blades produce different flap thicknesses. Factors such as the design, sharpness, and material of the blades might contribute to the variable performance. This issue needs to be examined from an engineering perspective.
 

We found that the Moria blade yields a wider range of flap thickness than the CLB blade (SD = 21.4 and 21.6 in the first and second eyes for the Moria blade vs 12.1 and 15.5 for the CLB blade). Muallem et al.6 found similar results with the Moria M2 microkeratome (SD = 23.5 to 25.5), but they did not specify what blade(s) they used. This is comparable to results reported with other microkeratomes. The smallest reported SD (13.3) was for the Nidek MK-2000 microkeratome.14 Interestingly, we found a smaller SD with the CLB blades, especially in the first eyes (Table 4). As a goal for future microkeratome designs, Choudhri et al.15 proposed that the mean SD be reduced to below 10 μm. In our hands, the CLB blade in a Moria M2 microkeratome did not achieve that goal, but it came closer to it than the Moria blade.
 

If the same blade is used for both eyes, there is a tendency to cut a thinner flap in the second eye.5,8,16-18 Muallem et al.6 found this was true with the Moria M2 130 head but not so much with the 110 head. The reason for a thinner flap in the second eye is not clear, but most authors suggest it is because the blade is dulled when cutting the first eye.6,19 We found a significantly thinner second flap cut with the CLB blades, but there was no significant difference with the Moria blades. This again demonstrates different performance by different brands of blades during LASIK surgery.
 

The thinner flap cut by the CLB blades may be helpful in treating patients with a thin cornea and high degree myopia. With the thin flap, one can leave thicker stromal tissue to correct a high refractive error. The difference in flap thickness might also be exploited in particular cases. For example, one could deliberately choose the more myopic eye as the second eye, expecting a thinner flap in that eye. There are therefore potential benefits to be derived from the characteristic results of the CLB blade. At the same time, this must be balanced against the risk of an excessively thin or even a buttonhole flap for the second eye. It might be wise to use intraoperative pachymetry to verify the flap thickness. If the flap in the first eye is relatively thin, a new blade for the second eye should be used to avoid such complications.
 

The current study showed that the corneal flap thickness produced by a Moria M2 microkeratome 110 head is variable depending on the blade used. CLB blades produced significantly thinner flaps than Moria blades. This study therefore adds to the list of factors potentially affecting flap thickness. Surgeons performing LASIK should be aware of this factor and be cautious when using a different type of blade than they have had experience with. Under such circumstances, we recommend using intraoperative pachymetry to evaluate corneal flap thickness until one is familiar with the results yielded by the new type of microkeratome blade.

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