Femtosecond laser flap creation is rapidly gaining popularity because both surgeons and patients realize the increased accuracy and safety of these instruments over mechanical microkeratomes.1,2 In 2007, we reviewed data from 3,009 eyes that underwent flap creation with a femtosecond laser (IntraLase; Abbott Medical Optics Inc., Santa Ana, California). Our flap-related complication rate was 0.63%,3 whereas complications with mechanical microkeratomes are reportedly as high as 5%.4 As we gained experience with the femtosecond laser, our flap-related complication rate further dropped to 0.55% by the end of 2008. Now, we rarely see complications, including diffuse lamellar keratitis (DLK) and epithelial ingrowth.
There is a learning curve with femtosecond lasers. Table 1 shows the last occurrence of the flap-related complications that I encountered from March 2008 to February 2009. In this article, I discuss some of the complications that occur most often with use of the femtosecond laser for LASIK (also known as Intra-LASIK when the IntraLase femtosecond laser is used), both flap- and nonflap–related, and how to prevent or to treat them when they occur.
Although not a sight-threatening problem, subconjunctival hemorrhage can cause distress and concern to the patient. Evidence suggests that significantly less subconjunctival hemorrhage occurs when brimonidine is applied just before surgery.5 However, there is increased incidence of flap slippage when brimonidine is used before mechanical microkeratome flap creation.6 The same does not appear to occur with Intra-LASIK. In fact, brimonidine may decrease flap retraction in Intra-LASIK.5 When subconjunctival hemorrhage occurs, it is important to immediately tell the patient what has happened and to reassure him it will subside and not affect the end result. If the patient is not informed, he will often be concerned the next day.
DLK was a common occurrence with the IntraLase FS 15 and FS 30 kHz femtosecond laser models, but its incidence has significantly decreased with the FS 60 and iFS 150 kHz models. DLK most often occurs after too much energy is used during the treatment or too much tissue is dissected at a low-energy setting. When surgeons transition from mechanical microkeratomes to femtosecond lasers, they tend to use higher energy for easier flap dissection. With the faster lasers, the energy level required is much lower, and higher bubble density allows less dissection. As soon as I see any DLK, I start the patient on hourly topical dexamethasone while awake and steroid ointment at night. If the DLK is grade 1 or worse, I add 60 mg of oral prednisolone per day and watch the patient daily. Since beginning this regimen, I have not had to take any patient for washout.
VERTICAL GAS BREAKTHROUGH
Vertical gas breakthrough (VGB) is probably the most annoying femtosecond-laser–related complication. Not only does it prevent the flap lift, but also the surgeon must usually wait a few weeks to try again. Alternatively, if a sidecut is not already performed, the surgeon could immediately try a much deeper cut after the initial bubbles have gone away. This is usually not possible with Asian patients because we cannot afford to go much deeper into the cornea with the amount of high myopia that we have to treat. If the VGB is in the periphery and the sidecut has been made, the surgeon could dissect around it, creating a buttonhole and lasering as usual; however, there is risk of epithelial ingrowth. If VBG happens at an early stage of the laser procedure, the surgeon should immediately stop because the flap is still intact and will not move as long as the sidecut is not made. In this case, the surgeon could repeat the laser procedure from the opposite direction. I have managed to complete the cut and lift the flap without a buttonhole.
Another alternative is to switch to a mechanical microkeratome if the sidecut has not been performed. If the sidecut is completed, there is a risk of moving or shredding the flap. To avoid this complication, the surgeon could initiate the cut from the hinge and sweep in the same direction. For example, if the original femtosecond-laser hinge is nasal, the microkeratome cut should start nasally to create a temporal hinge. The original hinge will prevent the flap from folding over. The risk of shredding must be discussed with the patient before proceeding. The key is to stop as soon as the VGB occurs; your options for continuing are then much greater. This has been described in more detail elsewhere.7
From the perspective of corneal biomechanics, sub- Bowman's keratomileusis (SBK) is similar to surface ablation.8-10 This technique is gaining popularity, and more surgeons are cutting thinner flaps. I routinely make 90-µm flaps regardless of the patient's corneal thickness.
Thinner flaps are advantageous, but they are also easier to tear. Therefore, care must be taken when dissecting the flap. The trick is to have the tip of the dissecting instrument facing toward the stromal bed; it should never face toward the flap. Since I have adopted this maneuver, I have not torn any flaps.
If a flap is torn and the bed is even, the surgeon could complete the flap lift and continue with the laser ablation. Careful replacement of the flap is paramount. If it bisects the visual axis, the surgeon should cease immediately and attempt surgery another day. Fortunately, thin torn flaps do not appear to scar much. Extreme care should be taken when dissecting over the visual axis.
Epithelial ingrowth is a common complication with mechanical microkeratomes because the epithelial cells are brought into the flap-stromal bed interface when the blade cuts along the cornea. Haw et al11 suggested that 50% ethanol can be used to treat epithelial ingrowth after LASIK. With that concentration, however, occurrence of DLK is also possible. I prefer to use a lower concentration of ethanol to avoid DLK.
When epithelial ingrowth occurs, I lift the flap and scrape it with a hockey-stick knife. A small cellulose sponge, presoaked with 10% ethanol, is then placed over the involved areas for 20 seconds. Remember to scrape and soak both the flap and stromal bed and to copiously wash away the ethanol. A more convenient and effective way to remove epithelial ingrowth is with Nd:YAG laser treatment.12
There is no doubt that flap creation is safer with a femtosecond laser than with a mechanical microkeratome. The incidence of ectasia with SBK is reduced, and flap complications are less common. Employing the methods outlined above, flap complication and vision loss after LASIK should be even less frequent.
John S. M. Chang, MD, is Director of the GHC Refractive Surgery Centre, the Hong Kong Sanatorium and Hospital, Happy Valley, Hong Kong, China. Dr. Chang states that he has no financial interest in the products or companies mentioned. He may be reached at tel: +852 2835 8885; fax: +852 2835 8887; e-mail: firstname.lastname@example.org.