FLAP THICKNESS
My colleagues and I conducted a study of flap thickness predictability with four microkeratomes (the Hansatome [Bausch & Lomb Surgical, Inc., San Dimas, CA]; the CB [Moria Inc.]; the M2; and the SKBM [Alcon Laboratories, Inc., Fort Worth, TX]).1 The first three units create a superior hinge, and the last produces a nasal hinge. Both the M2 and the SKBM feature two motors, one for oscillation and one for advancement. The CB has a motor for oscillation but is advanced manually. The Hansatome, meanwhile, uses one motor for both oscillation and advancement.
When comparing the standard deviation in flap thickness of these microkeratomes in a cohort of 490 eyes, we found that the M2 produced the most consistent results. The units with dual motors produced more predictable flap thicknesses than the Hansatome, which performed better than the CB. We concluded that microkeratomes using one motor for oscillation and another for advancement create more predictable, reproducible flap thicknesses.
We hypothesize that a single motor that performs both functions may experience a power drain during advancement. This problem was obvious in the early CB unit, which offered surgeons the alternative of a single motor for both oscillation and advancement. Sometimes, the microkeratome pass was labored and inconsistent in speed. As a result, the company opted to recommend manual advancement for the M2 and to incorporate a new design of two motors into the unit.
THE SUPERIORITY OF SUPERIOR HINGES
I prefer superior-hinge microkeratomes because the flaps they produce appear to remain in alignment, whereas gravity sometimes causes nasal hinges to shift downward and develop striae. With a superior-hinge microkeratome, I simply load the unit onto a post, slide it down, lock it into the right position (to ensure the microkeratome cannot pop up from the post), engage suction, verify the IOP, and then step on the footswitch to advance the microkeratome head. I was not as fond of the SKBM, because I had to fit the unit into the grooves of its track, and snapping it into place sometimes took additional effort. That is true of some other nasal-hinge microkeratomes as well.
I have found both the M2 and Hansatome simple to use. I prefer the former for the reasons just expressed regarding flap thickness and also because I had a statistically higher rate of epithelial defects with the Hansatome compared with the M2.2
DRAWBACKS
Flap complications are still possible with the M2. I have found them to be infrequent, however, with both the Hansatome and the M2, but probably more so with the former in terms of buttonholes.
Generally, the M2 creates flaps of smaller diameter than does the Hansatome, but the hinges are also smaller, whereas the amount of stroma available for ablation is nearly equivalent. I perform many customized ablations using LADARVision with CustomCornea (Alcon Laboratories, Inc.), which creates a 9-mm ablation zone. Sometimes, although set to create a 9.0-mm flap, the M2 actually produces a flap 8.5 mm in diameter. In this case, the laser pulses spill over the edge of the flap. To me, losing 0.5 mm of pulses because they hit the epithelium outside the flap is not problematic, provided I have centered the flap well over the treatment area. I do so via nasal decentration when applying the ring to the eye. I continue with the procedure in the presence of an 8.5-mm flap diameter because the spillover will not affect the overall result. If the flap's diameter were small and the flap became decentered, I would abort the treatment, but this has not been an issue to date.
ADVICEBlades and Rings
I personally use the blades manufactured by Moria. In addition, I must emphasize the importance of employing the company's established nomogram for ring selection based on the eye's keratometry reading. I understand that some surgeons employ the same ring regardless of the keratometry measurements. For instance, on steeper corneas, they may use a -1 ring, which is generally used for flatter corneas. This practice will produce a large flap but may also create a buttonhole.
Suction
I train fellows and occasionally see them experience difficulty obtaining suction with a microkeratome. Sometimes, when they hold the ring on the eye, they produce a hissing sound, rather than suction. At this point, I usually encourage them to press down on the eye with slightly more force, because oftentimes some of the fluid on the eye will disperse and allow the suction to take hold. At that point, the device should be held comfortably, not pulled upward or pressed downward. Overall, I find that suction lasts throughout the entire pass of the M2 microkeratome. Occasionally, however, a fellow may pull upward too hard and lose suction when trying to load the microkeratome onto the ring.
Lubrication
If a patient undergoing a bilateral procedure develops epithelial sloughing or an epithelial slider on his first eye, I recommend applying Celluvisc (Allergan, Inc., Irvine, CA) to the cornea of his second eye to ensure that it is well lubricated prior to microkeratome advancement.
CONCLUSION
The M2 creates flaps of reproducible thickness, a quality that I value most in a microkeratome next to safety. In addition, I find Moria's service department excellent in its response to any question or concern I have. For me, this microkeratome is the best choice.
1. Miranda D, Smith S, Krueger R. Comparison of flap thickness reproducibility using microkeratomes with a second motor for advancement. Ophthalmology. In press.
2. Miranda D, Iacobucci M, Krueger RR. Flap reproducibility and complication rate using the Bausch & Lomb Hansatome, the Moria Microtech CB, and the Alcon SKBM microkeratome. Ophthalmic Practice. 2002;20:3:96-99.
