Combining corneal presbyopia solutions with LASIK

Combined treatment

I use the WaveLight FS200 femtosecond laser (Alcon Surgical) and find it preferable to other lasers I have used primarily as a result of the docking system. With many systems, the intensity of suction can cause the eye to rotate slightly, making it very easy to end up with an eccentric flap. The joystick and the computer control guidance system of the Wavelight FS200 allow the surgeon to lower the applanation plate very gradually and with full control. In addition, the large size of the applanation plate means that it typically sits beyond the limbus, allowing me to view the anatomical centre of the cornea.

For thick flaps a tighter spot and line separation with lower energy is required than normal femtolaser settings. I cut a flap with 4 microns of spot separation and 4 microns of line separation. I set the flap thickness at 200 microns with a 130 degree edge cut, and use 4.5 µJ of energy to produce a nice, clean flap that lifts easily. I lift the flap and perform the LASIK treatment as required for the patient, aiming for –0.5 D for the final result. In older individuals, I find that I need to aim for a slightly more myopic post-op refraction to achieve the best results.

A flap thickness of 200 microns does not create as smooth a bed as surgeons are used to seeing. The superficial corneal lamellae are more densely packed than deeper in the cornea. The same femtosecond laser settings used to cut a smooth flap in the superficial layers can be a rough bed in deeper layers. The rougher the surface will be due to the characteristics of the tissue. Corneal sclera tissue is not as densely packed as the more superficial layer, and fibres are very densely bundled at the subBowman level in particular. The rougher the surface, the longer it takes for the interface to disappear and clarity to return. An understanding of this anatomy is particularly important when cutting a pocket because you can't see the actual quality of the bed as you can with a flap. Therefore, the physician has to make assumptions on the quality of the bed based on experience with flaps of the same depth. This is one reason why it is important to start with a flap-based procedure before moving to a lamellar pocket procedure. When performing a pocket incision, I also mark the cornea with a ring marker to indicate where the inlay should be placed.

To help with inlay placement, I use the AcuTarget Diagnostic System. Preoperatively the AcuTarget images the patient's eye and identifies and correlates important ocular landmarks such as the pupil centroid and first Purkinje image. The device then calculates the most appropriate location for the inlay based on the patient's first Purkinje image. Using this overlay information and the ability to use the laser lights on my Wavelight laser to detect and follow the corneal apex after lifting the flap, I am able to centre the inlay precisely where it needs to be. On rare occasions, I need to make a small adjustment to the positioning. The WaveLight laser can also be used to check how well the inlay is centred with respect to the central corneal light reflex by asking the patient to look toward the fixation light after the flap is replaced.

Once the flap is closed and the inlay is in place, the flap must be evaluated for any edge gap. In a procedure that is this efficient, a 200 microns flap will almost always close back in position with a very minimal gap, leading to rapid visual recovery and significant decrease in patient discomfort. An edge gap occurs when there is excessive shrinkage with the flap drying out or flap swelling by excessive irrigation. A bandage contact lens can be used if the edge gap cannot be closed adequately.