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At Issue: the next generation of microkeratomes
At Issue posed the following question to an international panel of ophthalmologists: “What improvements would you like to see in the next generation of microkeratomes and why?”
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Consistent flap thickness
Hiroko Bissen-Miyajima, MD: First, I appreciate the developments in currently available microkeratomes. Assembling the devices has become easier and the quality of blades has improved.
In the next generation, I would like to see a consistent thickness of the flap. Most current devices cut the flap with a standard deviation of 20 µm to 30 µm. Keratectasia is a severe complication following LASIK, and the residual thickness of the bed is one of the factors in this. When we plan LASIK, we calculate the residual bed using the number written on the microkeratome such as 130 µm and 160 µm, and the ablation depth with the excimer laser. If the microkeratome cuts 30 µm deeper than expected, there is a risk that the residual bed would be less than 250 µm.
Also, I would prefer to have a microkeratome that requires less suction. Although the duration of high intraocular pressure during LASIK is short, this situation is rather critical for some eyes.
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Consistency, handling
Michael C. Knorz, MD: The most significant problem of microkeratomes today are the inconsistent flap dimensions. The flap thickness varies from 20 µm to 100 µm between each cut, and it is not consistent throughout the cut, the center usually being thinner than the periphery. In addition, the flap diameter varies. New microkeratomes must solve these problems, as they present safety hazards in some cases and inconsistent refractive results in most cases because of the corneal biomechanics. I personally expect that laser microkeratomes will be the way to address these issues.
Other improvements should address handling, suction loss, epithelial defects and blade quality. Handling for me means that micro - keratomes should be preassembled. There seems to be no reason to have to put them together on the eye. It also means it should be possible to either sterilize them or at least cover them completely with some autoclavable cover.
Suction loss should be minimized by multiple suction ports and an intelligent software that maintains preselected suction levels.
Epithelial defects must be minimized by careful polishing and an intelligent operating system. It is very sad that the blade still oscillates on the return path in many of our microkeratomes today. There is no need for this; it just traumatizes the eye unnecessarily. I also recommend that the suction should automatically be lowered once the forward path is completed. This seems an easy task, and it would be far safer than turning the suction off, as is frequently practiced today.
Blade quality is another sad issue. Today, we should expect perfect blades and pristine interfaces, but the reality is different. We must continue to work together with industry to improve our microkeratomes.
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Suction, flap cutting
Thomas F. Neuhann, MD: I would like to see the following improvements:
A smaller “footprint” of the suction ring to facilitate cutting in deep-set eyes and small palpebral fissures.
Multiple suction openings in the suction ring instead of just one. One opening can be plugged by conjunctiva; with multiple openings this is almost impossible.
Exact adjustability of the flap diameter, together with the option to cut free flaps. This would greatly enhance the possibilities for therapeutic lamellar keratoplasty. Obviously, this would necessitate a transparent applanation plate.
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Many choices
Ioannis G. Pallikaris, MD: There is a large gamut of microkeratomes on the market today. Some features might be discussed in order to select one keratome over the other.
One of the first features a refractive surgeon might look for is visibility of the cornea during keratectomy.
Automated systems that control the oscillation rate of the microkeratome are important for a surgeon who wants to think about the surgery and not the microkeratome.
The quality and angulation of the blade and the impact of these factors on the reproducibility of flap thickness are essential for an accurate and predictable refractive outcome.
A blade that is either disposable or gear-free is necessary to avoid debris under the flap and obtain good vision quality. High blade quality, along with a smooth cut, would allow good adherence of the flap and help avoid secondary epithelial regrowth. The quality of the cut is much higher if torque is delivered to the cutting blade.
Quickly achieving the necessary suction level and maintaining its stability are useful in creating a smooth and thick corneal flap. Providing a microkeratome with a variety of suction rings and adjustable flap orientation would allow the surgeon to be more flexible with his or her decisions.
The ability to check intraocular pressure intraoperatively is important to avoid excessive — or insufficient — elevation of pressure.
Whether or not the keratome is disposable is important if you lack sterilizing equipment or just want to stay out of the constant cleaning process.
The complexity of the microkeratome, ease of assembly and service (or its availability) and cost is of great importance for the long-term use of the machine.
Is there a perfect keratome today? Hardly. Are there microkeratomes for each taste? Yes. So the thing that is left to be decided is which one makes you feel more confident and which one has a case-after-case dependability and durability.
Looking toward future use of laser epithelial keratomileusis (LASEK) or a subepithelial LASIK approach, probably an epithelial separator is urgently needed.
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Disposable or sterilizable
Matteo Piovella, MD: I would like to see the following qualities in a microkeratome:
A disposable microkeratome, to prevent contamination and improve blade handling. Blade insertion maneuvers can reduce sharpness and lead to contamination. If not disposable, all microkeratome components that reach the surgical field should be easily sterilizable, including engine and cord.
A microkeratome that does not require assembly maneuvers on the eye before use; safe and short surgery is the best goal.
Smaller suction rings that fit very small eyes. Improved suction, but without negative effects on the cornea and retina, to prevent epithelial damage in steep corneas.
A new generation of blades to improve bed surface and provide a flap of uniform thickness and width.
A watertight motor to avoid percolation of toxic fluid onto the blade, which is possible with the Bausch & Lomb Hansatome.
My favorite microkeratome is the Hansatome. However, I do not like the fact that sometimes, after sterilization, the engine gets stuck. It is enough to wet it with a little water to go back to standard performance.
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More control, better blades
Emanuel S. Rosen, MD: I have used the Hansatome now for the past 5 years with great satisfaction. Despite the user’s inability to observe the actual cutting of the flap, problems have been rare. I strongly favor the superior hinge. The vacuum ring that was rather bulky has been redesigned and smaller rings are now available. Cutting the flap is a speedy and efficient process. While it would be preferable to observe the flap during the cutting process, it has certainly not proved to be an essential requirement.
The next generation of microkeratomes need to guarantee a much tighter spread of outcomes regarding thickness of the flap. If 160 µm is to be cut, then it would be nice to know that we are getting 150 µm or 170 µm to add to the precision of the process. At the moment, with the Hansatome we anticipate up to 20% thicker flaps than planned, which is a useful safety factor.
Blade improvements will surely continue. Smoother flaps may mean less diffraction and enhanced outcomes, but that situation remains unproven. Smoother cuts and consistent diameter flaps must be the goals of evolution of microkeratome design.
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Beyond mechanical blades
Julian Stevens, MD: Microkeratomes operate with a mechanical cut aiming to create a flap of defined and consistent thickness with a hinge and minimal stress to the corneal epithelium. Microkeratomes will always be associated with some finite incidence of complications since the process is mechanical.
In Europe, due to concerns about new variant Creutzfelt-Jacob Disease (nvCJD), these worries have led to more stringent cleaning and sterilization regulations for avoiding prion material transfer. These new procedures require vastly increased mechanical cleaning and sterilization turnaround time and cost. An ideal system would involve a disposable head unit that has the reliability and rigidity of current reusable systems.
Other technologies such as waterjet have some advantages, including a lack of heating of tissue, but a mechanical blade simply separates tissue. Though a mechanical blade has a squeegee effect upon the corneal base and flap, altering water content, waterjet may significantly alter hydration and so laser ablation rate.
Effecting refractive change with femtosecond laser technology may eventually produce the “holy grail” procedure of no-flap LASIK. Femtosecond laser systems can create a LASIK flap, but the flap created will differ from that of a mechanical blade. A blade separates tissue, whereas a femtosecond laser ablates and so removes tissue. There will therefore be a different interface and flap healing properties, and so there needs to be careful examination of corneal contour outcomes. Any ablation tunnel at a flap edge may promote epithelial ingrowth as a complication.
Desirable features in future microkeratomes include the following:
- Reliable suction or, in radical technologies, no suction required.
- Disposable component that comes in contact with the patient. avoiding sterilization issues and allowing fast turnaround.
- Minimized mechanical stress on the epithelium and consequent epithelial abrasion.
- Self-checking diagnostics and error reporting with maintenance diagnostics for advising of preventive servicing.
- Blade consistency for metal blades.
- Consistent cut depth, flap size and hinge size.
Despite near-future revolutionary developments with femtosecond lasers, I think it likely that flaps will always be required for the performing of “classic” LASIK, insertion of corneal inlays and also deep lamellar corneal grafts.
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Eliminate metal blade complications
Maurizio Zanini, MD: In the last few years, microkeratome technology has not undergone the same degree of development as excimer lasers. We still rely on metal blades and high vacuum to create the best flap, and even high-volume LASIK surgeons can experience some unpredictability associated with microkeratomes.
The disadvantages of mechanical metal blades are well known. First, although the blade is always inspected before surgery, it is not uncommon to detect metal chips in the interface. Second, the microsaw cutting action tears rather than cleaves the lamellar structure of the cornea and leads to the loss of keratocytes. The mechanical distortion of the tissue during the cut can produce irregular flaps. Finally, even with the most commonly used and reliable microkeratomes, corneal flaps that are supposed to be cut at 130 µm, 160 µm and 180 µm depths show a high degree of variability.
Computer-assisted laser flap creation, such as that achieved with the femtosecond laser, will improve our results.
Three main advantages should be anticipated:
- The disappearance of metal blade-related complications: metal chips, oil from the gears on the device (with possible development of diffuse lamellar keratitis), irregular nonplanar cuts, etc.
- Precise customization of the flap diameter and thickness, hinge location and peripheral bevel.
- The need for a lower vacuum (40 mm Hg).
- The same results may be obtained by waterjet technology as well.
In addition, to reduce the rate of complications, manufacturers should continue focusing their attention on microkeratome assembly. Newer microkeratomes, such as the Hansatome and the Carriazo-Barraquer, cannot function to produce a flap if their components are inserted incorrectly; this is important to avoid serious damages to the flap in the event of erroneous assembly. Preassembled microkeratomes (such as the Nidek, Alcon and Allergan devices) represent an additional option to treat this problem.
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