Murat V. Kalayoglu, M.D., Ph.D.
Contributing Editor
Creation of the flap in corneal refractive surgery is perhaps the most important component of the operation. Many of the most serious LASIK complications occur during the keratectomy or are a consequence of the keratectomy. The first generation of microkeratomes caused many of these complications; however, a new generation of microkeratomes makes LASIK and its variants an exceedingly safe and satisfying procedure.
Perhaps the most serious keratectomy – related complication is corneal perforation. This complication is particularly dangerous during LASIK given that the eye is pressurized to 3 – 4 times the normal pressure through suction; when the globe is inadvertently perforated at such high pressures, there is a real risk of expulsive hemorrhage or loss of intraocular tissue through the perforation. The true incidence of corneal perforation during LASIK is not known, in part because it is (and always has been) a rare complication. Globe perforations that do occur usually result from incorrect assembly of the microkeratome, such as forgetting to include the depth plate in the assembly. However, the design of newer microkeratomes virtually eliminates any chance that this complication will occur: newer designs have built in depth plates and automatic shut-off features to prevent the keratome from cutting too thick of a flap. Some of the new generation microkeratomes even have the capability to perform intraoperative pachymetry to ensure proper depth of the flap.
A more common complication is creation of a flap that is either too thick or too thin. The former complication can lead to compromise of corneal stability in some patients. There is the small but real possibility of inducing progressive corneal thinning in patients with unrecognized ectasia undergoing LASIK. Indeed, no matter how advanced the microkeratome may be, inadequate preoperative screening of the LASIK candidate may lead to iatrogenic keratectasia. Corneal topography and serial pachymetry can help screen out patients with early keratroconus and pellucid marginal degeneration. Leaving a minimum of 250 microns of posterior cornea intact may also help reduce this dreaded complication.
Flaps that are too thin run the risk of developing a myriad of complications. A thin flap may perforate if suction is lost, especially if the cornea is steep (diopters in the high 40s range). Such buttonholing can usually be avoided by deferring surgery to another day at the first sign of poor suction. However, while poor suction explains the majority of complications due to buttonhole flaps, another reason may be a malfunction or defect in the microkeratome. Simple preoperative microkeratome maintenance and cleaning are usually sufficient to avoid this complication. Newer generation microkeratomes are high precision instruments that minimize the risk of thin or buttonhole flaps, but general preoperative inspection of the device is still important to ensure the perfect flap.
An interrupted or jerky pass through the cornea greatly increases the risk of an irregular flap. A smooth pass is critically important, and the surgeon and ancillary staff must be vigilant in ensuring that the blade is not defective (i.e. without breaks or notches). Forcing the blade through despite a jam may lead to a ripped flap. Finally, another serious complication is a free flap, which may lead to loss of the flap. If this should occur, the surgeon must take care to realign the flap onto the cornea for reattachment and ensure appropriate A-P orientation. While free flaps did occur with earlier generation microkeratomes, the newer generation machines are so precise and well automated that they have reduced this complication to exceedingly low levels.
The newer generation microkeratomes feature a diverse array of options. The surgeon may find, depending on the device, such options as variable hinge locations (temporal, nasal and superior), different depth settings, variable head and ring sizes, and disposable microkeratome heads. Most feature sophisticated software, and some machines allow intraoperative pachymetry and tonometry built into the device. Today’s microkeratomes have advanced safety features to prevent many of the flap complications seen with the older generation devices. One may expect to see such safety features as voice confirmation, battery power backup, rigorous automated internal safety checks, automatic vacuum loss shutoff, software driven flap centering, audible suction indicators, and real time monitors to detect motor resistance. Overall, the newer generation devices are much easier to use and cut with smaller standard deviations, yielding consistent results for better outcomes.