Is the Mechanical Microkeratome Dead?
Since Pallikaris et al1 first introduced LASIK in 1988, several mechanical microkeratome systems have been developed in the aims of achieving better safety, efficacy, and predictability. The ideal microkeratome should provide a flap with predictable and reproducible thickness and diameter, epithelial continuity, excellent stromal bed quality, and minimally affected biomechanics. The combination of these components should lead to an ideal and stable refractive outcome.
Recently, the introduction of femtosecond laser technology has initiated competition in both the scientific and marketing arenas. Refractive surgeons are already burdened with rapidly escalating surgical costs, including the purchase and service of new and more sophisticated excimer laser platforms that need replaced every few years. Surgeons are now faced with another dilemma: Does it make sense to get rid of a mechanical microkeratome and invest in a new but expensive technology that utilizes the femtosecond laser?
USEFUL CONCLUSIONS
In order to answer the above question, one has to know whether (1) femtosecond lasers are equipped to solve or handle all currently existing problems better than the mechanical microkeratome and (2) patients are achievinging the best possible results with one technology over the other. An unbiased look at scientific evidence—without any influence from the industry—may help us draw some useful conclusions.
One of the most important characteristics of a microkeratome, and a crucial factor for the overall success of LASIK, is the flap thickness that it achieves. A flap that is significantly thicker than originally planned may lead to corneal ectasia.2 Flap thickness unpredictability has been one major drawback of the mechanical microkeratome; it is, however, one claimed advantage of the femtosecond laser. Solomon et al3 compared the accuracy of the actual flap thickness from six mechanical microkeratomes. The investigators were able to demonstrate that device labeling is often not indicative of the mean obtained flap thickness. Large standard deviation and wide ranges have been reported elsewhere.4 Additionally, the thickness of the mechanically created flap has been dependent on patient factors including preoperative corneal thickness, keratometric power, and age.5
CLEAR ADVANTAGE
Femtosecond technology has a clear advantage in its parameter for flap thickness, as data have shown that accuracy with mechanical microkeratomes is problematic, and, obviously, femtosecond laser manufacturers used such data to refine their own systems. Femtosecond lasers had a head start commencing with thinner flaps, tighter standard deviations, and narrower ranges in the created flaps. IntraLase (Advanced Medical Optics, Inc., Santa Ana, California) has shown a mean actual flap thickness of 114 µm (range, 78–155 µm) for a preset predicted thickness of 130 µm4 and a standard deviation of 12 to 14 µm.4,6
The evolution of the mechanical microkeratome has led to the introduction of new systems that can achieve thinner flaps with smaller standard deviations and narrower ranges, independent of patient characteristics. The Moria M2 (90-µm head; Moria, Antony, France) creates flaps with a mean thickness of 115.4 ±12.5 µm,7 whereas the Moria LSK (100-µm head) can create flaps with a mean thickness of 107 ±14 µm.8
We have clinical experience with five mechanical microkeratomes in the past 10 years, and we have treated more than 12,000 eyes. We are currently using the Carriazo-Pendular (Schwind eye-tech-solutions, Kleinostheim, Germany), which has allowed us to claim that a modern mechanical microkeratome can produce equally accurate, predictable, and reproducible flaps compared with the femtosecond laser. Using the Carriazo-Pendular mechanical microkeratome and a 130-µm head for approximately 2 years, our average flap thickness is 110 ±13.92 µm (range, 72–44 µm). Flap thickness with this mechanical microkeratome was only slightly dependent on preoperative corneal pachymetry (root mean square [RMS]=0.06, P=.002), with an increase of 6.5 µm in actual flap thickness for every 50 µm of preoperative thicker cornea. Age did affect the flap thickness minimally and negatively (RMS=0.05, P=.007), and the microkeratome proved to be independent of preoperative average keratometric power (P=.93).9 Comparing our results to those of Kezirian and Stonecipher,4 we do not recognize any significant differences in terms of accuracy, predictability, or variability between the Carriazo-Pendular (130-µm head) and IntraLase. Both systems can create thin flaps with small standard deviations and similar ranges, and both are equally independent of patient characteristics.
PROCEDURE CUT AND SUCTION TIME
Other important characteristics when evaluating the performance of a microkeratome are the time it takes (1) to perform the cut and (2) for the eye to be kept under suction. The initial femtosecond procedures were performed with a 2- and later a 6-kHz platform. With these lasers, flap creation lasted between 2 and 3 minutes. The 10-kHz engine became commercially available in 2001, and the spot energy was reduced between 3 and 4 µJ to make a 9-mm flap possible in 90 seconds. In 2003, the 15-kHz femtosecond laser was introduced, and in 2005, the 30-kHz laser was introduced. With the currently used 60-kHz platform, a 9-mm flap can be created in 20 to 22 seconds, whereas the 30-kHz platform required 60 seconds for a 9-mm flap.10
Regardless of the reductions in spot energy, flap creation with a femtosecond laser still lasts significantly longer than that of a mechanical microkeratome; the actual cut time with the Carriazo-Pendular microkeratome lasts less than 2 seconds, and the suction time is rarely more than 15 seconds in our personal experience (Figure 1). Longer flap creation and suction times mean that elevated intraocular pressure is extended over a longer period, thus increasing the potential risk for intraocular complications. In 2003, a macular hemorrhage was described after LASIK and flap creation using the IntraLase; there were no preexisting risk factors.11
A third characteristic of the microkeratome is the flap design profile it creates. Planar flap geometry—when the flap is uniformly thick throughout its surface—is preferred and has been shown to offer advantages over meniscus-based flaps. Uniform flap thickness and shape have minimized surgically induced astigmatism4,12 and may yield more optimal visual outcomes with wavefront-guided LASIK. Femtosecond lasers were once considered the only way to achieve planar geometry flaps, which was the main reason that they were considered a good choice for wavefront-guided LASIK.13 Planar flap geometry is, however, now feasible with a mechanical microkeratome as well, allowing it to benefit from the same advantages (Figure 2).
FLAP EFFECTS ON BIOMECHANICS
Another point that has attracted significant interest is the effect of a flap on corneal biomechanics. Jaycock et al14 showed that the anterior-peripheral regions of the cornea are more resistant to stress and strain compared with the central-posterior regions, because they have a more optimal density and arrangement of collagen fibers. Therefore, thicker flaps affect the resistance of corneal stress and strain more than thinner or more superficial flaps, as we know that the flap does not contribute to the postoperative corneal stability.
Many surgeons have recently adopted thin-flap LASIK, or sub-Bowman's keratomileusis (SBK). Because the flaps are significantly thinner than traditional LASIK (ranges, 90–110 µm vs 130–180 µm, respectively), the corneal integrity and strength are maintained better than with traditional LASIK. Although this has initially been in favor of femtosecond lasers due to their ability to create thinner flaps, the latest published data on mechanical microkeratomes show that these devices can create consistently thin flaps,7-9 which allows us to conclude that the biomechanic burden on the cornea is not significantly different when comparing the femtosecond laser and mechanical microkeratome.
The quality of the stromal bed after flap creation is important, and it may have an impact on the final visual outcome; a rougher stromal interface may negatively influence the final quality of vision.15 Initial use of the femtosecond laser has led to rougher stromal beds and, to improve stromal bed quality, lower pulse energies and spot separation settings have been used.16 Although evidence exists that stromal beds have been made smoother,10,16 time is necessary for these data to be proved repeatedly and over time.
The speed of visual rehabilitation is an important parameter that needs to be considered. Although recently published studies have found no significant differences in the visual outcomes 3 and 6 months after surgery,4,17 lower visual acuities have been reported in eyes treated with the femtosecond laser on the first postoperative day, when compared with eyes treated with a mechanical microkeratome.4
LIFTING THE FLAP
One important advantage of LASIK over superficial ablations is that retreatments are easily and safely performed by lifting the flap. It is crucial to lift the flap as atraumatically and with as little tissue manipulation as possible. This is true for mechanical microkeratomes that have stood the test of time and continue to make retreatment an uncomplicated task. For femtosecond-created flaps, however, there is increased corneal stromal leukocytic infiltration in the early postoperative period, which is believed to increase later flap adhesion strength. Leukocytes are a source of growth factors and cytokines, all of which are necessary for initiation and propagation of new tissue formation in wounds.18 This may make flap lifting during a retreatment more challenging for the surgeon and more traumatic for the cornea. Although flap lifting has been made easier with newer femtosecond models, there is some evidence that stromal beds have been made smoother,10,16 time is necessary for these data to be proven repeatedly and consistently, as is the case with mechanical microkeratomes. Finally, there have been concerns regarding a flap recut with the femtosecond laser.
When evaluating whether a new medical apparatus should replace current existing technology, it should, show at the very least, significantly less shortcomings and demonstrate clinical superiority. The truth is, however, that femtosecond lasers are not free of complications. This technology not only shares many complications that mechanical microkeratomes have, but it has some unique complications of its own, including suction breaks, flap folds,19 buttonholes, dislocated flaps,20 diffuse lamellar keratitis (DLK),21 and intraocular complications such as macular hemorrhage.11 Although DLK after LASIK with the femtosecond laser has been frequently reported at ophthalmology meetings, the issue was only recently published for the first time. The reality is that DLK still exists, even with the use of femtosecond lasers,21 but the true rates of occurrence are currently unknown. Interface gas bubbles,22 transient light sensitivity syndrome,23 and anterior chamber gas bubbles24 have been reported and are complications unique to femtosecond lasers, which have not been eliminated and are potentially dangerous. Safety issues regarding the energy transmitted into the intraocular structures are pending but should not be neglected. It has been shown that at least 20% of the femtosecond near-infrared photons reach the retina, and there is concern regarding the potential photochemical, photodisruptive, and thermal effects on the endothelium and photoreceptors.25
FEMTOSECOND COMPLICATIONS
Industry-driven and marketing pressures have introduced the femtosecond laser as a promising technology that can eliminate all existing problems with the mechanical microkeratome. Although it has also been said to be safe in nonvirgin eyes, it has been shown that this is not always true.
CHALLENGING
The use of femtosecond technology in postradial keratotomy eyes can be, at the best, challenging or problematic.26 Despite the recent market surge for the development and promotion of new femtosecond systems, mechanical microkeratome technology continues to evolve and improve. Optimal blade packing, tightening of the tolerances of the microkeratome head gap, and monitoring of significant intraoperative parameters (blade current, blade oscillation, and vacuum progression as provided by the Carriazo-Pendular system) can provide better safety and ergonomics over previous mechanical microkeratomes.
CONTINUING TO IMPROVE
History has shown that competition leads to improvement. Characteristics of the mechanical microkeratome are continuing to improve since their initial introduction more than 2 decades ago. Mechanical microkeratomes are by no means inferior to femtosecond lasers when flap creation is considered. Certainly, when lamellar graft surgery is considered, femtosecond laser technology is an exciting and promising tool.
The decision to choose one system over another is governed by several factors including personal experience, clinical indications, and by analyzing data provided by unbiased and well-designed clinical studies. We should always keep in mind that what matters in the end is having a satisfied patient. The proven long-term, highly efficient, and safe track record of the mechanical microkeratome faces the promises that femtosecond technology wants to deliver, and the game is by no means over. The $400,000 investment in an early-generation technology needs thoughtful consideration. Do not throw your trusty mechanical microkeratome in the wastebasket yet!
George C. Charonis, MD, is Medical Director of Athens Vision Eye Institute, in Athens, Greece. Dr. Charonis states that he has no financial interest in the products or companies mentioned. He may be reached at tel: +30 210 9595215; fax: +30 210 9515120; or gcharonis@athensvision.gr.
Evgenia G. Konstantakopoulou, MSc, is the Excimer Laser Coordinator and Clinical Optometrist, Department of Refractive Surgery, Athens Vision Eye Institute, in Athens, Greece. Ms. Konstantakopoulou states that she has no financial interest in the products or companies mentioned. She may be reached at tel: +30 210 9595215; fax: +30 210 9515120; or evgeniakonst@athensvision.gr.
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