The popularity of LASIK (laser-assisted in situ keratomileusis) has created a highly competitive market for microkeratomes. This burgeoning market has encouraged research and development, resulting in microkeratomes with enhanced features, greater safety and better outcomes. Today, more than 20 different microkeratomes with distinct options and features are available.

If you're new to LASIK, or trying to decide whether you want a new microkeratome, you'll need to consider a number of key factors. First and foremost, of course, you want an instrument that will produce few (or no) significant complications and excellent visual outcomes for all of your patients, both myopic and hyperopic.

To reach that goal, a number of features and specifications come into play. The ideal microkeratome should do the following:

�         It should produce a smooth cut.

�         It should give you options for creating the flap, including choices about the diameter and thickness of the flap and the location of the hinge.

�         It should be easy to assemble (easy enough that a surgeon or a technician can assemble it without difficulty).

�         If possible, the instrument should monitor vacuum pressure during the procedure, so that blade excursion stops instantly if suction is lost.

In this article, we'll also discuss some other factors you might want to consider, such as size, flap visibility during the procedure and the importance of the learning curve.

Smoothness of cut

A smooth cut is important for two reasons:

�         First, the eye heals more quickly when the cut made by the microkeratome is more even.

�         Second, an irregular cut makes a perfect alignment of the flap crucial. If the flap is misaligned by 5 to 10 microns, recovery is delayed while the epithelium on the surface undergoes changes to smooth itself out. (A misalignment greater than 10 microns can lead to permanent irregular astigmatism.)

So, how do you determine whether any given microkeratome is likely to produce a smooth cut? Factors that affect the smoothness of the cut include:

�         Sharpness of the blade. This is largely a function of the material the blade is made of. Today almost all keratome blades are metal. (Water jet technology is under development, and it remains to be seen how viable this technology will be. Laser microkeratomes should have the potential to create a very smooth cut and very predictable flap thickness.)
At a practical level, the sharpness of each individual blade can be a factor. A poorly designed microkeratome can allow frequent damaging of the blades during insertion, forcing you to discard them. Before choosing a microkeratome, ask colleagues who already use the one you're considering how often they have this problem.

�         Number of cuts per millimeter. Most microkeratomes offer a blade oscillation speed between 2,000 and 20,000 rotations per minute.
Because metal blades create a smoother cut at higher speeds, many companies will encourage you to choose their microkeratome on the grounds that their blade has a high oscillation rate. However, the real determining factor here is the number of cuts per millimeter, which is determined by combining the oscillation rate with the translation speed.
Consider three hypothetical microkeratomes with blade oscillation rates of 6,000, 12,000 and 24,000 per minute, respectively. Dividing these numbers by 60 gives us their oscillation rates per second: 100, 200 and 400.
As you can see, a rapid excursion minimizes the number of cuts per millimeter (and hence the smoothness of the cut), despite a high oscillation rate. Conversely, a slower excursion can compensate for a slower rate of blade oscillation.
The bottom line? You have to take both speeds into account. All three of these blades can create an effective cutting speed of 100 cuts per millimeter -- and hence produce cuts with the same degree of smoothness -- despite their different oscillation rates. (That's not to say that 100 cuts per millimeter is a magic number. Just don't automatically dismiss a microkeratome because it has a lower oscillation rate. A slower translation speed might make it comparable to one with a higher oscillation rate and faster translation speed.)

�         Stroke length. How far does the blade move side to side? Too small a stroke can lead to irregular cuts.
To visualize this, think of an electric shaver. The first part of a stroke doesn't do any cutting; it just pushes the tissue. For that reason, a very short stroke length is inefficient. Up to about 0.6 mm, the blade just pushes; once the stroke gets longer than 0.6 mm, it begins to cut.
Most microkeratomes have a stroke length of 0.8 to 1.2 mm. Anything shorter is problematic.

Flap options

To treat patients with different problems, you must be able to modify the characteristics of the flap.

�         Flap diameter. Being able to create a larger-diameter flap is important for patients with hyperopia. A flap of at least
9 mm makes it possible to place more of the ablation onto the bed. Also, small diameter flaps usually lead to higher incidence of regression and irregular astigmatism.

�         Flap thickness. The ideal thickness in any given case depends on many factors, including pre-op refractive error, corneal thickness and whether this is a primary or secondary cut. For this reason, being able to vary thickness is obviously important. Many older microkeratomes offer only one or two options. Some of the newer generation instruments offer more.
It's also important that the instrument creates a flap with a thickness that's very close to what you intended. This is difficult to measure in human patients, but you should ask colleagues who have used the instrument you're considering whether they've noticed any obvious problems in this area.

�         Edge thickness. Blade angles vary from 0� to 30�. The steeper the angle, the thicker the edge and deeper the peripheral gutter. This is usually a good thing because a thinner peripheral edge is harder to realign, and a thicker edge means less fragile tissue.
Several microkeratomes in the past had 0� entry. With 0� entry, the periphery of the flap is very thin and tends to be associated with a higher incidence of epithelial ingrowth. Today, most microkeratomes have an entry angle of at least 20�. (Among upcoming options, laser technology has the potential to create an edge with any characteristics you want.)

�         Hinge position. Many microkeratomes are designed to create only a nasal hinge; a few can create both a nasal and a superior hinge. At least one microkeratome on the market can be used obliquely, in any position.
Flexibility here is an advantage, albeit a small one. When a patient has hyperopia and astigmatism, it may be helpful to place the hinge in the plus axis, giving you a better chance of getting the full ablation onto the bed. Flexibility also can allow you to avoid vascularization, if necessary.

Ease of assembly

With a microkeratome, the fewer parts you have to assemble, the quicker your turnaround time will be. In addition, you'll be less likely to encounter trouble because of human error during the assembly.

Disposable microkeratomes are one solution, but most of the surgeons I've spoken to resort to disposable microkeratomes primarily for backup. (If you choose to use disposables in any capacity, quality control is essential. To determine whether the disposable microkeratome you're considering is reliable, talk to other surgeons who've used it.)

Vacuum monitoring

Loss of suction during the cut can cause severe complications. The microkeratome you choose should address this issue. Many microkeratomes have an automatic abort system to stop the blade instantly if the vacuum fails. (Obviously, this is not available with manual systems.)

Into the future

Everyone hopes that the next generation of laser-based microkeratomes will give us greater control and better outcomes, but its too early to know for sure. Many issues, such as expense and ease of use, could undermine the desirability of this technology. Laser-based instruments should become available soon, so you can decide for yourself.

In the meantime, plenty of options, both high-tech and otherwise, are available. Do your homework, talk to others who have experience and choose carefully. If you do, you should have excellent results, an efficient surgical practice and happy patients. And that's the name of the game.

Q & A

Here are some questions that frequently come up when doctors are choosing a microkeratome:

�         What about a manual system? Electronic systems have obvious advantages, but they also can have more complex problems when something doesn't work right. (It's a bit like the difference between a bicycle and a car.) However, most refractive surgeons today are using automated microkeratomes.

�         How important is it to be able to see the flap? Some microkeratomes allow visualization of the flap during the keratectomy; others don't. Being able to see the flap is a theoretical advantage. It certainly makes the surgeon more comfortable; whether it actually improves outcomes isn't so clear. (In my experience, by the time you can visually determine that a problem has occurred, the damage has already been done.)

�         How important is size? A smaller, lighter instrument is advantageous; it's easier to hold and work with, and will allow you to work in smaller or more deep-set eyes.

�         Is there any statistical evidence that choice of microkeratome affects outcomes? Different manufacturers have conducted studies comparing their products to others. You'll have to decide for yourself how valid these comparisons are. It's often more to the point to talk to surgeons who use the instruments and get their opinions based on their experience.

I think it's likely that you won't find a big difference between outcomes when comparing instruments in the hands of experienced surgeons who are well acquainted with their microkeratomes. The differences are likely to be most apparent when comparing the results beginners achieve.

In other words, the size of the learning curve is the most significant element in terms of outcomes. A long learning curve simply means that the surgeon has to learn more in order to avoid potential problems. (Obviously, everything else being equal, an instrument with a short learning curve is preferable to one that takes hundreds of procedures to master.)

-- Raymond Stein, M.D., F.R.C.S.C.

Dr. Stein is co-director of the Bochner Eye Institute in Toronto, chief of ophthalmology at Scarborough Hospital in Toronto, and assistant professor of ophthalmology at the University of Toronto.