The femtosecond laser is a focusable infrared laser similar to the Nd-YAG (neodymium-yttrium-aluminum-garnet) laser used for the posterior capsulotomy procedure; however, this laser delivers ultrashort laser pulses, firing in the femtosecond duration range (100 × 10^-15 second). The advantage of a femtosecond laser is that the extremely short laser pulse of focused energy causes minimal thermal damage or disruption in adjacent tissues. This adjacent disruption in the corneal stroma has been measured and extends out on the order of 1 µm. The femtosecond laser pulse vaporizes small amounts of tissue by the process of photodisruption. The vaporized tissue forms an intrastromal cavitation bubble of microplasma, which is composed of water and carbon dioxide. Focusing the laser energy at a specified tissue depth and placing successive laser spots in close proximity to one another, multiple bubbles are created in a lamellar corneal dissection plane. Some lasers create the dissection plane using a raster (zigzag) line pattern, while others use a spiral pattern. The laser-controlled software can be programmed by the surgeon to create lamellar, axial (side cut), or pocket cuts in a wide range of depths and diameters. Typically after the lamellar cut has been completed, the side cut is created with a series of vertically placed spots along the flap edge.

Significant advances have been made with femtosecond technology over the last decade. First generation femtosecond lasers had repetition rates of approximately 15 kHz, and flap creation could take 60 to 90 seconds. Some current generation lasers operate in the 150 to 200 kHz range and have flap creation time of <10 seconds. Other lasers such as the Ziemer Femto LDV use very high pulse rate in the MHz range with lower energy per pulse. Despite the differences in laser dynamics and parameters from one laser to another, there
are basic principles and practices that are applicable to all femtosecond lasers. The rest of the chapter will focus on the important clinical concepts that can be applied to any femtosecond laser platform used for LASIK flap creation.

The selection of candidates for LASIK with a femtosecond laser is similar in many respects to that for the blade microkeratome. First and foremost is patient safety, followed by the ability to meet patient expectations. However, femtosecond technology may expand the limits of candidacy for LASIK surgery. The advantages and disadvantages of the femtosecond laser for flap creation are summarized in Table 5.1.

Various studies have demonstrated that when compared to mechanical microkeratomes, femtosecond lasers create flaps with more predictable thickness. Femtosecond lasers can also create thinner flaps. The ability to create thinner flaps combined with tighter variance between expected and actual flap thickness may allow more patients with relatively thin corneas or highly myopic patients to undergo the procedure. Intraoperative pachymetry in such cases is still recommended prior to excimer ablation to ensure adequate residual stromal bed thickness.

Femtosecond lasers also create more predictable flap diameters and shapes compared to flaps created with a microkeratome. With a mechanical microkeratome, patients with steeper corneas are at a higher risk for a buttonhole flap or a wider than anticipated flap diameter. Patients with flat corneas are at a higher risk for free flap or a smaller than anticipated flap diameter. With femtosecond lasers, the above mentioned complications are unlikely to occur even in patients with contours at the upper and lower ends of the normal range. Patients with smaller corneas can be treated with less risk of invading the sclera, because of the ability to better adjust flap diameter. Additionally, femtosecond lasers create flaps with a more desirable planar configuration and uniform thickness. Flaps created with a microkeratome have a meniscus shape with the flap periphery being thicker than the center (see Fig. 5.1). Most blade microkeratomes create thinner flaps on the second eye when the same blade is used for both eyes. They also create thinner flaps in thinner corneas and thicker flaps in thicker corneas. This is not the case with the femtosecond laser.

Femtosecond lasers are capable of creating more predictable and desirable side-cut
profiles. With mechanical microkeratomes, the side-cut angle is more tapered or sloped. Femtosecond lasers allow the surgeon to specify the side-cut angle. The side-cut angle range varies with different laser manufacturers, but vertical, near-vertical, or reverse beveled angles can be created (refer to Fig. 5.1).

Proposed mechanisms of epithelial ingrowth include a defect or gap at the flap margin facilitating epithelial migration under the flap, as well as seeding of the stromal bed during the microkeratome pass. Because sidecut angles created with the femtosecond laser are more vertical, the epithelium is less likely to migrate beneath the flap. This may explain the reduced likelihood of significant epithelial ingrowth following LASIK surgery using the femtosecond laser.

The angle on the side cut theoretically may make the flap more resistant to displacement soon after surgery, and now can be varied over a wide range, with some potential strength advantages to an inverted side cut that has an angle of more than 90 degrees. The flap is definitely more resistant to displacement 1 year after surgery; in fact, it may be difficult to lift the flap for retreatment.

The surgeon has control over many of the flap parameters. Each type of laser has a particular range of adjustable parameters that can be manipulated by the surgeon. Despite the differences between lasers, an understanding of the common parameters and their impact on flap creation is necessary for a surgeon to optimize the femtosecond laser settings. The important parameters are flap thickness, bed energy, spot size and separation, flap diameter, hinge location, pocket profile, and side-cut angle/energy. It is wise to heed the recommendations of the representatives of the laser manufacturer and other more experienced surgeons when adjusting settings. As the surgeon gains more intraoperative experience, for example, trouble lifting the flap, and postoperative experience, such as inflammation or associated photophobia, fine-tuning adjustments can be made with various parameters to minimize side effects and facilitate flap creation. A summary of some of the parameters for current generation lasers is in Table 5.2.