8 The Future of Laser-Assisted In Situ Keratomileusis: Femtosecond Laser versus Other Technologies

Peter Wu, Clare Kelliher, Joelle Hallak, and Dimitri Azar

Keywords: LASIK, femtosecond laser, technology, excimer

8.1 Introduction

Laser-assisted in situ keratomileusis (LASIK) is one of the most widely performed ophthalmic surgical procedures in the world. The origins of the procedure began in 1949 with Jose Barraquer, one of the pioneers of lamellar corneal refractive surgery, who advanced the concept of altering the refractive power of the eye by the addition or subtraction of corneal tissue. He termed this new concept keratomileusis, derived from the Greek words keras (hornlike = cornea) and smileusis (carving or chiseling). ¹ Lamellar corneal refractive surgery has subsequently undergone an evolutionary process with refinement of the techniques and instruments. Barraquer’s initial technique of myopic keratomileusis involved the freehand creation of a lamellar corneal disc with the Paufique knife, followed by freehand excision of tissue from the residual stromal bed or the disc and replacement of the free lamellar disc. ² Barraquer refined the precision and reliability of lamellar keratectomy using microelectrokeratomes, globe fixation rings, and applanation lenses. ³ Further advancements in lamellar corneal refractive surgery included the introduction of the Barraquer-Krumeich-Swinger (BKS) refractive system in 1985. The lamellar corneal disc was excised by the refined microkeratome, and the second refractive cut was made on the stromal side of the corneal disc. ⁴ Luis Ruiz developed a geared, motorized microkeratome called the automatic cornea shaper (ACS), which utilized a suction device to fix the globe and a foot pedal to control the speed of the lamellar cut. The ACS led to the development of automated lamellar keratoplasty (ALK). In ALK, the first passage of the keratome made the initial lamellar cut. The second passage was the refractive cut, performed on the stromal bed instead of the free corneal cap, which was adjusted by altering the height of the suction ring. ²

The major breakthrough in modern corneal lamellar refractive surgery was the development of the excimer laser. The laser used today is an argon fluoride laser with output at 193 nm, which allows the laser to precisely photoablate tissue with minimal thermal damage to the surrounding tissue. Although the excimer laser was first developed in the 1970s, its surgical potential in precisely removing corneal tissue was not realized until 1983, ⁵ which led to the development of photorefractive keratectomy (PRK). In this procedure, the excimer laser is used to photoablate corneal stromal tissue after manual removal of the corneal epithelium.

In 1990, Pallikaris and colleagues introduced LASIK, ⁶ utilizing the excimer laser to ablate the corneal stromal bed under a corneal lamellar flap. With LASIK, an automated microkeratome was first used to create a corneal lamellar flap. The flap was then lifted by the LASIK surgeon and excimer laser ablation performed on the corneal stromal bed. With LASIK, the patients experienced less regression, less corneal haze, and better predictability as compared to PRK. ⁷ In addition, patients had less postoperative discomfort and faster visual recovery.

Despite this significant advancement in refractive surgery, LASIK was still considered a more challenging procedure due to the need for flap creation. Microkeratomes had potential complications such as partial flaps, buttonholes, thin or irregular flaps, and free caps. ⁸ In the early 2000s, the femtosecond laser was approved for LASIK flap creation by the Food and Drug Administration (FDA). The femtosecond laser produces near-infrared (1,053 nm) pulses, which, unlike excimer laser pulses, were not absorbed by the surrounding tissue. ⁹ The femtosecond laser energy vaporizes corneal stromal tissue into plasma of free electrons and ionized molecules, which rapidly expands to create a cavitation gas bubble. The cavitation bubble then expands to separate the corneal lamellae. Most femtosecond laser systems utilize a suction ring to align and stabilize the eye. A flat contact lens system, attached to a computer-controlled laser delivery system, is then used to applanate the cornea within the suction ring. Laser pulses are then delivered at a preset depth in a rasterized pattern to create a corneal lamellar flap. The parameters of the femtosecond laser are set so that neighboring shots do not completely overlap, resulting in tissue bridges that must be bluntly dissected. The LASIK flap is then manually dissected and lifted, and the excimer laser is then used to perform the refractive treatment to the stromal bed.

8.2 Femtosecond Laser in Primary LASIK Surgery

The femtosecond laser has potential advantages over mechanical microkeratomes in LASIK surgery. Flap size, flap thickness, edge angle, hinge width, and hinge location can now be controlled with the computer-guided femtosecond laser platforms. In addition, suction levels and intraocular pressures are much lower than mechanical systems, ^{10,​ 11} resulting in improved patient comfort. The suction ring and applanation contact lens are disposable, possibly decreasing contamination rates and decreasing the need for sterilization. In addition, eyes treated with femtosecond lasers have improved quality of vision when compared to mechanical microkeratomes with better visual acuity outcomes, ^{12,​ 13,​ 14} improved contrast sensitivity, ^{13,​ 15,​ 16} and less higher-order aberrations. ^{12,​ 13,​ 15,​ 17}

Using the femtosecond lasers to perform LASIK has resulted in significantly more predictable flap thickness and reduced variability when compared to mechanical microkeratomes. ¹⁸ LASIK flaps created by femtosecond lasers had greater flap adhesion strength in animal models ^{19,​ 20} and smoother stromal beds ²¹ when compared to mechanical microkeratomes. Femtosecond LASIK flaps tend to be more homogeneous with a planar morphology profile when compared to the meniscus-shaped flaps of the microkeratome. ^{22,​ 23}

LASIK flap complications were reported to be as high as 5% with mechanical microkeratomes ²⁴ versus femtosecond lasers, which ranged from 0.33 to 0.92%. ^{25,​ 26,​ 27} Femtosecond lasers had lower rates of epithelial defects, ^{28,​ 29} flap displacement, ³⁰ and epithelial ingrowth ³¹ when compared to mechanical microkeratomes. However, despite its advantages, femtosecond lasers have been shown to have a higher incidence of diffuse lamellar keratitis than microkeratomes. ^{15,​ 32}

8.2.1 Types of Femtosecond Lasers Used in LASIK

Several generations of femtosecond lasers have been released. The 6-kHz laser was the first commercially available system and there has been subsequent development, and commercial release, of 10, 15, 30, 60, and 150 kHz and even higher frequency femtosecond laser systems. As the frequency of the laser increases, it creates a flap at a higher speed with smaller spot sizes and decreased energy. Currently available femtosecond laser systems include the Intralase (Abbott Medical Optics, Inc.), the Wavelight & LensX (Novartis), the Visumax (Carl Zeiss Meditec AG), the Victus (Bausch & Lomb), and the Femto LDV (Ziemer Ophthalmic Systems AG).

Since 2010, femtosecond laser technology has also been applied to cataract surgery to create a “bladeless” alternative to traditional cataract surgery. The femtosecond laser can be used to make the corneal incisions, arcuate incisions, capsulotomy, and lens fragmentation. The surgeon can then proceed with phacoemulsification after completion of the laser portion of the surgery. Currently, four femtosecond laser technology platforms are commercially available for cataract surgery: Catalys (Optimedica), LenSx (Novartis), LensAr (Lensar, Inc.), and Victus (Bausch and Lomb).

8.3 Femtosecond LASIK Retreatment

In general, LASIK delivers predictable refractive results. However, a significant number of patients, especially those with high refractive errors, older patients, and those undergoing correction of a hyperopic refractive error, appear prone to residual postoperative refractive error. Estimates on the percentage of patients with visually significant error requiring retreatment vary from 5 to 28%. ³³ Many options are available for retreatment and these can be considered when the postoperative refractive error has stabilized. Options include applying surface laser onto the flap or creating an entirely new flap deeper and larger than the original flap. Most commonly, in cases where the residual stromal bed is of adequate depth, surgeons lift the flap and treat the stromal bed. The flap can be lifted manually or femtosecond laser can be used to facilitate re-lifting of the flap. ^{33,​ 34,​ 35}

One of the main risks of re-lifting the original LASIK flap during enhancement surgery is epithelial ingrowth beneath the flap. ^{33,​ 35} Patients whose original flap was created with a microkeratome, rather than a femtosecond laser, are particularly susceptible to epithelial ingrowth. Microkeratome-created flaps have a sloping edge in comparison to femtosecond laser flaps which have a vertical configuration. It is suggested that the vertical cut may be a more effective barrier to epithelial cell migration. Epithelial ingrowth can reduce the patient’s visual acuity and may, rarely, result in melting of the flap. The incidence of epithelial ingrowth can be as high as 23% in re-lifted flaps. Predisposing factors are patients older than 40 years, those who originally had correction of a hyperopic error, and those with preexisting corneal epithelial disease such as anterior basement membrane dystrophy.

The use of femtosecond laser to assist in LASIK retreatment involves the creation of a new vertical side cut only, within the margins of the original flap. ³⁴ This side cut intersects with the original flap–stromal interface, thus allowing the original interface to be accessed for retreatment. Preoperative optical coherence tomography (OCT) may be helpful to evaluate the diameter and depth of the original interface. ³⁵ A commonly used technique involves creating a side cut with a diameter of 1.2 mm smaller than the original flap. This cut should be at least 10 to 20 µm deeper than the measured flap thickness to ensure that the interface is reached. The original flap margins should be marked preoperatively to avoid the intersection of the new side cut and the original flap.

Some studies have suggested that use of a femtosecond side cut can decrease the incidence of epithelial ingrowth, particularly in those whose original flap was created with a microkeratome. ³³ It is suggested that the clean vertical interface created by the laser minimizes mechanical trauma to the surface epithelium and thus the possibility of pushing epithelial cells beneath the flap during dissection. One drawback of this method is that the treatment zone is limited by the diameter of the original flap. This is of particular significance in hyperopic retreatments. Furthermore, most flaps can be carefully lifted in the first year after LASIK with minimal risk of epithelial ingrowth.

8.4 Femtosecond LASIK after Corneal Surgery

8.4.1 Descemet’s Stripping Automated Endothelial Keratoplasty

Several case reports or small case series regarding patients being treated with femtosecond LASIK following DSAEK have been published. Typically, these patients developed anisometropia due to a hyperopic shift after DSAEK and were intolerant of contact lenses. Published studies describe a good refractive outcome without any complications noted to date. ³⁶

8.5 Penetrating Keratoplasty

Astigmatism is the most common refractive complication following penetrating keratoplasty. ^{37,​ 38} High astigmatism, of greater than 5 D, occurs in approximately one-third of cases. Many factors influence the development of astigmatism including host factors (the preexisting corneal pathology, postoperative wound healing), the donor tissue characteristics, as well as the surgical technique employed. There are limitations to the optical correction of post-keratoplasty refractive error. Refractive surgery is particularly beneficial when the residual astigmatism cannot be corrected with spectacles or rigid gas permables (RGP). Refractive surgery to correct residual astigmatism can be considered 3 to 6 months after all corneal sutures have been removed and topical steroids were discontinued, as long as the refractive error is stable and the graft is healthy.

Traditional surgical management options for the treatment of high degrees of postkeratoplasty astigmatism include manual astigmatic keratotomy (AK), wedge resections, compression sutures, and limbal relaxing incisions. Femtosecond laser-assisted wedge resection has been used with extreme levels of postoperative astigmatism. ³⁹ In 2006, Ghanem and Azar described a standardized technique of femtosecond-laser arcuate wedge-shaped resection (LAR) to correct high astigmatism. ³⁹ A simple formula was used to calculate the relative decentration of the arcuate cuts based on the radii of curvature and desired wedge width to be resected. The first procedure was performed on a patient with 20.0 D of post-penetrating keratoplasty astigmatism. The astigmatism was reversed. Suture removal resulted in reduction of 14.5 D of astigmatism. ³⁹

8.6 Astigmatic Keratotomy

Manual keratotomy was the most commonly employed technique for correction of high astigmatism, greater than 6 D. ³⁹ This procedure involves creating one or paired incisions on the steep corneal meridian just inside the graft-host junction, thereby relaxing the steep meridian. However, predictability has been an issue using this technique and some of the more serious surgical complications include wound gape, epithelial ingrowth into the incision, and intraoperative corneal perforations.

Femtosecond laser arcuate keratotomies (FLAKs) are now considered preferable to surgical AK. FLAK is reported to have enhanced safety, accuracy, and faster visual recovery. Several techniques have been employed. The femtosecond laser corneal incisions can traverse the epithelium or the epithelium can be surgically opened afterward (▶ Fig. 8.1). ^{40,​ 41} The advent of real-time OCT also allows for the creation of purely intrastromal incisions. ^{42,​ 43} Highly accurate assessment of the depth, length, and curvature of incisions is possible using OCT imaging, thereby avoiding the complication of corneal microperforation. Standardized nomograms, which were developed for use during manual AK, have been employed for FLAK. These nomograms vary the angular length and the depths of incisions, depending on the preoperative astigmatism. However, it is very likely that FLAK incisions, especially those that are purely intrastromal, heal differently so that new nomograms are being developed for FLAK. ⁴¹ New techniques that take advantage of the precision of the femtosecond laser have used beveled corneal incisions and skewed incisions on steep corneal meridians in patients with asymmetric astigmatism. ⁴⁴

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