Microkeratomes keep competitive edge because of versatility, lower cost

Femtosecond lasers are positioned to overtake microkeratomes because of their superior accuracy and predictability, one surgeon says.

Dan Z. Reinstein

For several years, the microkeratome has reigned supreme as the preferred tool for creating LASIK flaps. The cutting device played a key role in making LASIK both feasible and practical.

Microkeratomes have a strong safety profile but create flaps that are inconsistent in shape and thickness when compared to femtosecond laser flaps. Flap thickness irregularities may lead to lower predictability of visual outcomes and can increase the risk of postoperative complications.

Femtosecond laser technology enables surgeons to create flaps with more accuracy, predictability and safety than is possible with microkeratomes. Flap geometry can be customized to minimize dry eye and enhance corneal sensation.

“Femtosecond lasers have dramatically improved the flap thickness reproducibility, enabling us to consistently create very thin flaps while also reducing the potential for flap complications,” Dan Z. Reinstein, MD, MA, FRCSC, DABO, FRCOphth, told Ocular Surgery News. “If there are any issues during the creation of the flap interface, the surgeon can simply choose not to lift the flap and return later to create a new flap once the bubbles have gone away.”

Aside from creating LASIK flaps, mechanical microkeratomes are used in corneal lamellar procedures such as anterior lamellar keratoplasty. Femtosecond lasers are also used to cut donor tissue for various corneal transplantation procedures.

One femtosecond laser, the VisuMax (Carl Zeiss Meditec), is used to perform the ReLEx procedure, in which stromal tissue is removed to correct myopia and myopic astigmatism. Femtosecond lasers are also making steady gains in cataract surgery.

Still, despite the apparent advantages of femtosecond lasers, microkeratomes will likely maintain a strong foothold largely because of their lower cost compared to lasers, Dr. Reinstein said.

“In the future, femtosecond lasers will eventually dominate. However, the extra cost compared with manual microkeratomes will mean that manual microkeratomes will have a place for a while yet,” he said. “The actual safety of mechanical microkeratomes in expert hands is extremely high, so I do not believe that the cost differential will justify the disappearance of mechanical microkeratomes from the world of refractive surgery.”

Irregular flap thickness

While microkeratomes and femtosecond lasers differ dramatically in terms of accuracy, two generations of one microkeratome offered similar flap profiles but with varying predictability, according to a recent study.

In the Journal of Refractive Surgery, Dr. Reinstein and colleagues compared mean thickness, accuracy and reproducibility of LASIK flaps created with standard and zero-compression Hansatome microkeratomes (Bausch + Lomb).

Both microkeratomes produced similar non-meniscus-shaped flap thickness profiles. However, the zero-compression model created thinner, more reproducible flaps, the authors said.

The retrospective study included 99 eyes of 51 patients: 60 eyes treated with the standard microkeratome and 39 eyes treated with the zero-compression microkeratome. Device-labeled flap thickness for both microkeratomes used was 160 µm. Artemis very high-frequency digital ultrasound (ArcScan) was performed preoperatively and at least 3 months after surgery.

Topographic maps of the mean and standard deviation of flap thickness centered on the corneal vertex for the standard Hansatome and zero-compression Hansatome groups. The color scale represents the epithelial thickness in microns. Image: Reinstein DZ

Results showed mean central flap thickness of 140.3 µm with the standard microkeratome (range: 102.9 µm to 177.9 µm).

Mean central flap thickness was 119 µm with the zero-compression microkeratome (range: 86.9 µm to 143.9 µm).

The flaps were thinnest in the inferotemporal paracentral region and thickest at the periphery of the inferotemporal edge and nasally, unlike the meniscus shape previously suggested. Virtually all flaps had a systematic irregular shape.

“Our study demonstrated that there is a correlation between patient vacuum pressure setting and flap thickness for the Hansatome. Barraquer described the effect of corneal compression, intraocular pressure and speed of passage of the microkeratome head influencing flap thickness. We can infer from this study that these factors result in a specific flap profile that is non-regular but consistent,” Dr. Reinstein said.

Irregular flap thickness may change the corneal biomechanical response, affecting the refractive outcome and altering higher-order aberrations, Dr. Reinstein and colleagues said.

The femtosecond laser can produce flaps that are more uniform in profile, therefore likely increasing the predictability of the refractive result, Dr. Reinstein said.

“Last but not least, the extraordinarily low compression required by the VisuMax means that the patient experience is amazing: The patient literally feels nothing during the procedure. In addition, the footprint of the device on the eye is so small that you can create flaps in eyes that would not be physically possible with a manual microkeratome,” he said. – by Matt Hasson and Caitlin Langley

Reference:

• Reinstein DZ, Archer TJ, Gobbe M. LASIK flap thickness profile and reproducibility of the standard vs. zero compression Hansatome Microkeratomes: Three-dimensional display with Artemis VHF digital ultrasound. J Refract Surg. 2011;27(6):417-426.

• Dan Z. Reinstein, MD, MA, FRCSC, DABO, FRCOphth, can be reached at the London Vision Clinic; email: dzr@londonvisionclinic.com.
• Disclosure: Dr. Reinstein is a consultant for Carl Zeiss Meditec and has a small equity share and patent interest through Cornell University in the Artemis technology.

Sidebar: Femtosecond laser outperforms microkeratome in flap design for hyperopic LASIK

Hyperopic LASIK performed with a femtosecond laser offered better refractive outcomes at 3 months than did procedures performed with microkeratomes, a study found.

The study authors attributed the difference in refractive outcomes to flap design. Femtosecond flaps were planar and more predictable, while microkeratome flaps were meniscus-shaped and less predictable, they said.

“Cutting the peripheral stroma produces by itself changes in the curvature of the central cornea attributable to interlamellar forces,” the authors said. “Different depths and different shapes of the stromal cut have different effects on the resultant central changes, and thus on the secondary refractive changes. Hence, the less predictable and meniscus-shaped flap obtained with a mechanical microkeratome would be expected to cause a less predictable refractive change than a planar femtosecond laser flap.”

The retrospective study included 144 eyes: 72 eyes treated with the IntraLase FS femtosecond laser (Abbott Medical Optics) and 72 eyes treated with the Moria M2 microkeratome. The Schwind Esiris excimer laser was used to perform ablation on all 144 eyes.

Mean preoperative sphere was +3.45 D in the femtosecond laser group and +3.18 D in the microkeratome group.

At 3 months postop, mean residual sphere was +0.44 D in the femtosecond laser group and +0.72 D in the microkeratome group.

Mean logMAR uncorrected visual acuity was 0.89 in the femtosecond laser group and 0.80 in the microkeratome group. Best corrected visual acuity was 0.96 in the laser group and 0.92 in the microkeratome group.

The femtosecond laser and microkeratome procedures had similar safety profiles, the authors said. – by Matt Hasson and Caitlin Langley

Reference:

• Gil-Cazorla R, Teus MA, de Benito-Llopis L, Mikropoulos DG. Femtosecond laser vs mechanical microkeratome for hyperopic laser in situ keratomileusis. Am J Ophthalmol. 2011;152(1):16-21.

• Disclosure: The authors indicate no financial support for the development of this study and have no proprietary interest in any aspect of this study.