Femtosecond lasers are solid-state lasers operating in the form of pulses. By focusing on the inside of a corneal tissue, free electrons and high-temperature plasma are generated to produce photocracking, which vaporizes the tissue near the targeted area, forming high-temperature and high-pressure bubbles. The bubbles expand rapidly to open and cut the corneal tissue. The extreme focusing ability and accurate positioning of a femtosecond laser allow cutting random shapes at any position in the corneal stroma through the use of artificial intelligence, exhibiting high accuracy, good safety, and simple operation. Therefore, femtosecond lasers can be applied in corneal surgeries.

In 2000, the Food and Drug Administration approved the clinical application of femtosecond lasers in ophthalmology. The earliest femtosecond laser was mainly used to make a corneal flap in a corneal refractive surgery. Because the femtosecond laser can cut any shape in the corneal tissue with artificial intelligence, a plane parallel cut to the corneal surface in the anterior corneal surface stroma and then a perpendicular cut to the corneal surface after reserving the surrounding corneal pedicle were made to make the corneal flap for refractive surgery.

Keratoconus is mainly treated by corneal collagen cross-linking surgery, corneal transplantation, and corneal stromal lenticule transplantation. With the continuous evolution of femtosecond laser technology in corneal surgery, it has become a good auxiliary tool for various surgical treatments of keratoconus, thereby improving the predictability and safety of surgery.

Currently, the femtosecond laser systems mainly used in clinics include IntraLase iFS150, Femtec 20/10, Femto LDV, Visumax, and WaveLight FS200. Different femtosecond laser systems have varying physical parameters and different advantages in corneal surgery. With the continuous advancement of femtosecond lasers and the sustained improvements in the theoretical model in ophthalmology, the lasers are widely used in other fields of ophthalmology. IntraLase iFS150, Femtec 20/10, and WaveLight FS200 can accurately make corneal flaps and perform penetration keratoplasty (PK), deep lamellar keratoplasty, corneal stromal ring segments, and so on. In 2009, the first commercial femtosecond laser system, LensX™, designed for cataract surgery was applied in clinical practice, opening new applications of femtosecond lasers in the field of ophthalmic cataract surgery. Subsequently, several similar laser systems were introduced, including Optimedica Catalyst, LENSAR^®, Victus™, and so on.

PK is the main treatment for patients with keratoconus at the last stage. Femtosecond lasers can design various customized incision modes, including top hat, mushroom, and Zig-zag (Fig. 2). Given that the lower width of the top hat corneal incision is larger than the upper width, it is conducive to the adhesion of corneal graft and graft bed and increases the biological stability of the wound under the action of postoperative intraocular pressure. Femtosecond laser-assisted deep anterior lamellar keratoplasty (FS-DALK) can better preserve the integrity of the corneal structure, reduce postoperative corneal rejections, and obtain low long-term transplantation failure rates. Compared with traditional DALK surgery, FS-DALK is less complicated, provides higher interface smoothness, and possesses greater ideal postoperative visual quality. Femtosecond laser-assisted intra-corneal ring segment is widely used in the treatment of keratoconus. Femtosecond lasers are used to make semicircular or circular tunnels in the corneal stroma, and PMMA plastic ring is implanted to reduce the curvature of the anterior corneal surface. With the continuous maturity of small incision lenticule extraction (SMILE), femtosecond lasers can be used to make a corneal stromal lenticule and stromal capsule, realizing femtosecond laser-assisted corneal cross-linking and corneal stromal lenticule covering-assisted corneal cross-linking. By making a capsule in the corneal stroma and then implanting the corneal stromal lenticule that is removed after SMILE operation in hyperopia patients, the thickness and the biomechanics of the cornea can be increased. In addition, by injecting riboflavin into the corneal stromal capsule and performing a corneal cross-linking surgery, the permeability of riboflavin can be increased and the surgical effect can be improved. Thus, the use of femtosecond lasers provides effective surgical schemes for treating keratoconus.

However, various defects and risks exist in actual clinical practice, such as designing laser parameters for different surgical methods and different surgical positions; selecting the size and mode of the negative pressure suction in laser corneal refractive surgery; the high risk of surgical infection when patients are being transferred to the operating room during operation; incomplete corneal cutting caused by equipment failure; poor parameter settings, leading to intraoperative and postoperative complications. Therefore, the application of femtosecond lasers in keratoconus treatment necessitates continuous optimal designing, improvements in the surgical processes, and follow-up of long-term clinical results.

Femtosecond lasers have been used in ophthalmic surgery for over 20 years. The continuous development of laser technology and artificial intelligence control technology has helped achieve a smoother corneal cutting surface, a more regular corneal incision, a more accurate cutting depth, and less tissue damage. Concurrently, femtosecond lasers combined with other surgical methods expands the surgical boundaries of complex corneal diseases, shortens the operation time, improves the operation safety, provides comfortable treatment experience for patients, and introduces new methods for treating keratoconus.