Excimer Laser Phototherapeutic Keratectomy

Introduction

The excimer laser revolutionized corneal refractive surgery in the 1990s. Photorefractive keratectomy (PRK) was approved by the US Food and Drug Administration (FDA) in 1995 and laser in situ keratomileusis (LASIK) was FDA approved in 1998. While these procedures have helped millions of patients achieve improved uncorrected vision, they are not indicated for patients with abnormal corneas. The term phototherapeutic keratectomy (PTK) is applied when the excimer laser is used to treat corneal pathology. PTK was FDA approved in 1995.

The excimer laser uses 193 nm wavelength ultraviolet light to break molecular bonds in the cornea to remove corneal tissue in a process called “photoablative decomposition” or “photoablation.” Two significant benefits of this process are that: (1) tissue can be removed in an extremely accurate fashion, and (2) there is negligible collateral damage to adjacent tissue, resulting in minimal tissue healing response. For refractive purposes, the cornea can be reshaped to correct myopia, hyperopia, and astigmatism. For therapeutic purposes, corneal opacities and irregularities can be treated.

The primary advantage of PTK over lamellar keratectomy with a blade is the precision with which the excimer laser removes tissue. One laser pulse removes approximately 0.25 μm of tissue, or approximately 1/2000 of the corneal thickness. The shape of the laser spots can be adjusted from small circles to large circles (on the order of 0.6–6.5 mm diameter). Rectangular-shaped laser spots of similar dimensions can also be used. This precision allows the surgeon to remove superficial corneal abnormalities while leaving the remaining cornea clear and smooth.

A disadvantage of PTK is that it does not discriminate between abnormal and normal tissue. When the excimer laser is used to remove tissue, once it has ablated the pathologic cornea it continues to remove normal underlying tissue. Alternatively, superficial corneal tissue can be removed with a blade, a microkeratome, or a femtosecond laser. When lamellar keratectomy is performed with a blade, it is often possible to find a tissue plane between the abnormal and normal cornea, which can then be dissected, leaving normal tissue behind. However, if a smooth plane cannot be identified with a blade, this type of lamellar keratectomy generally creates a fairly irregular surface.

The principal advantage of PTK over lamellar keratectomy with a microkeratome or femtosecond laser is the control the physician has during the surgery. With a microkeratome, the depth plate is selected and a portion of superficial cornea is removed. With a femtosecond laser, the size, depth, and shape of the tissue removal are programmed at the beginning of the surgery. For these two procedures, the exact size and depth are then out of the surgeon’s control during the actual surgical procedure. Additionally, when a lamellar plane is removed from an irregular cornea with a microkeratome or femtosecond laser, the remaining surface will often also be irregular. Furthermore, the microkeratome and femtosecond laser generally cannot remove very thin sheets of corneal tissue.

Indications for Phototherapeutic Keratectomy

Excimer laser PTK is used in the management of a large variety of anterior corneal pathology. In general, PTK can be used to treat two types of corneal abnormalities: opacities and irregularities. Needless to say, these two conditions often occur in the same eye.

FDA-approved indications for PTK include: (1) superficial corneal dystrophies (including granular, lattice, and Reis-Bücklers dystrophies [ Video 143.1 ]), (2) epithelial basement membrane dystrophy (EBMD) and irregular corneal surfaces (e.g., secondary to Salzmann nodular degeneration, keratoconus nodules or other irregular surfaces), and (3) corneal scars and opacities (e.g., owing to trauma, surgery, infection, and degeneration). The laser should not remove greater than one-third of the corneal thickness and should leave at least 250 μm of tissue after the procedure.

Video 143.1 Transepithelial Phototherapeutic Keratectomy With an Antihyperopia Treatment for Reis-Bucklers Corneal Dystrophy. Christopher J. Rapuano.

Contraindications

Contraindications to PTK include an immunocompromised host, uncontrolled ocular disease such as uveitis, blepharitis or dry eyes, and any condition thought to adversely affect corneal healing. As with most contraindications, the patient and physician need to take any of these conditions into account and then decide whether the benefits outweigh the risks. Other contraindications are eyes with deep corneal pathology in which removal of greater than 20%–30% of the corneal thickness would be required to clear the bulk of the opacity. Similarly, eyes with areas of significant corneal thinning in the ablation zone are not good candidates for PTK owing to the fact that the ablation could weaken the cornea and predispose to irregular astigmatism or keratectasia. In these cases a lamellar or penetrating keratoplasty might be a better option.

Preoperative Evaluation

As is true prior to most ocular surgeries, a complete history and ophthalmic examination should be performed before excimer laser PTK. One of the most important goals of the preoperative evaluation is to establish whether the patient is a good candidate for the PTK procedure. This includes determining whether the cornea is suitable for PTK, whether the eye is likely to heal well after PTK, and whether the patient’s goals will be achieved after PTK.