The concept behind PTK (phototherapeutic keratectomy) is to use the excimer laser to remove superficial corneal opacities and/or to smooth the corneal surfaceThe excimer laser has been used since the late 1980s to reshape the anterior corneal curvature in a procedure known as photorefractive keratectomy (PRK).Not many Doc prefer this treatment but i have assisted in one of the treatment personally
Initially for myopia and later for astigmatism and hyperopia.The concept behind PTK is to use the excimer laser to remove superficial corneal opacities and/or to smooth the corneal surface.
In this surgery, the epithelium is removed and the laser is applied to ablate a specific amount of Bowman’s membrane and stroma. The excimer laser can also be used to remove superficial corneal pathology in a procedure termed phototherapeutic keratectomy (PTK). Unlike PRK and PTK, laser-assisted in-situ keratomileusis (LASIK) is a procedure where a thin flap of corneal tissue, including epithelium, Bowman’s membrane, and stroma, is fash fashioned, and the excimer laser is used to reshape the stroma under the hinged flap. Afterward the flap is repositioned on the corneal surface without sutures.The excimer laser clinically used in ophthalmology utilizes 193-nm wavelength ultraviolet light to break molecular bonds in the cornea to remove tiny amounts of tissue. One pulse of excimer laser light removes approximately 0.25 µm of tissue, depending on the specific laser system.
The concept behind PTK is to use the excimer laser to remove superficial corneal opacities and/or to smooth the corneal surface. This procedure is potentially applicable to a large number of patients, including those with anterior corneal dystrophies, anterior corneal scars, and superficial corneal irregularities. Over the past 7 years, we have learned a great deal about when excimer laser PTK is effective and when it is not.
Excimer laser PTK is procedure to treat anterior corneal pathology affecting visual function, including symptoms of pain and/or decreased vision. It is specifically indicated for conditions such as anterior corneal dystrophies, including anterior basement membrane dystrophy, dystrophies of Bowman’s membrane, and granular and lattice dystrophies. It is also approved to treat anterior stromal scars occurring after corneal ulcers, ethylenediaminetetraacetic acid (EDTA) chelation of band keratopathy, trauma, or surgery (eg, pterygium surgery). Elevated lesions, such as Salzmann’s nodular degeneration, can also be treated but are often more easily and effectively improved with superficial keratectomy. FDA indications state that the bulk of the pathology should be in the anterior 33% of the cornea. Additionally, not more than one third of the cornea should be removed and at least 250 µm of tissue should remain at the end of surgery.
Consequently, contraindications to PTK include patients with deep corneal pathology or thin corneas. Caution should be taken in patients with potential healing abnormalities such as patients with keratitis sicca, neurotrophic corneas (eg, after herpes simplex or herpes zoster keratitis), exposure keratopathy, collagen vascular disorders (eg, rheumatoid arthritis), and diabetes mellitus. Eyes with a history of herpes simplex keratitis are at risk for recurrence of herpes after PTK.
Generally, the best candidates for excimer laser PTK are patients with corneal opacities in the anterior 10% to 20% of the cornea without significant irregularity or thinning. Eyes with localized elevated lesions are also good candidates for this procedure. Complications of PTK include infectious keratitis, corneal haze, and corneal scarring. In addition, opacities and dystrophies can recur after PTK, and induced refractive error, most typically hyperopia, but also myopia and astigmatism, is common.
While one of the goals of the procedure is to decrease corneal irregularity, it is not unusual for PTK to cause worsened irregular astigmatism. Patients should understand that PTK is often being performed in lieu of a more invasive procedure such as lamellar or penetrating keratoplasty. Occasionally, PTK is unsuccessful and corneal grafting is required to improve vision.
PTK PROCEDURE
The exact procedure used to perform PTK depends greatly on the specific corneal pathology being treated. There are three general techniques employed to treat most corneas.
The three approaches are used to treat
(1) relatively smooth central anterior stromal opacities (eg,Reis-Bücklers’ or granular dystrophies),
(2) elevatedcorneal lesions (eg, Salzmann’s nodular degeneration),and
(3) recurrent erosions, most commonly associatedwith anterior basement membrane dystrophy.
Often, more than one of these techniques is used in an eye.
PTK for Stromal Opacities
Eyes with anterior stromal opacities, such as corneal dystrophies of Bowman’s membrane (eg, Reis-Bücklers’ dystrophy) and anterior stromal dystrophies (eg, lattice and granular dystrophies, recurrent dystrophies in a graft), generally respond well to PTK. In most of these cases, the epithelial layer is relatively smooth. Often the superficial stromal opacities extend anteriorly into the posterior aspect of the epithelium. In these cases, the epithelium acts as a smoothing or masking agent. Here, removing the epithelium manually actually creates a more irregular surface in many eyes. Therefore, the epithelium is preferably removed with the excimer laser. Laser epithelial removal potentially creates as smooth a surface in the stroma as existed in the epithelium. A large-diameter ablation zone (eg, 6 to 7 mm) is centered over the entrance pupil, and the ablation is performed through the epithelium and Bowman’s membrane and into the stroma.
Preoperatively, an estimate of the depth of the pathology needs to be determined, typically using a combination of slit-lamp biomicroscopy and ultrasound pachymetry. Only a percentage of this depth (eg, 50% to 75%) should be programmed into the laser system computer for initial delivery. When this amount of ablation has been performed, the patient is brought to a slit lamp and examined. Generally, more ablation is then required to remove the majority of the corneal pathology to improve the patient's symptoms. Not all of the opacity needs to be removed to significantly improve vision. This “ablate and check” technique is essential to remove only the amount of opacity necessary to improve symptoms, but not any additional tissue, which would increase the risks of significant refractive change and corneal haze or scar.
PTK for Elevated Lesions Elevated opacities,
most commonly Salzmann’s nodular degeneration lesions and keratoconus nodules, are often treatable with mechanical superficial keratectomy using a blade. Those lesions not amenable to removal with a blade, generally because of some stromal involvement, can be treated with PTK. In these eyes, the epithelium is removed manually only from the elevated portion of the lesion and left in place adjacent to the lesion. A small-diameter ablation zone (eg, 1 to 2 mm) is used to “shave” down the lesion. Ideally, the lesion is ablated to the level of the surrounding stroma, resulting in a smooth cornea. When there are areas of cornea that do not require ablation in close proximity to more elevated areas, the areas not requiring ablation can be coated with a masking agent to protect them. Different viscosities of masking agents can be employed for different lesions. Most surgeons use a variety of thinner viscosity and thicker viscosity preservative- free tears as masking agents. Once the elevated area is relatively smooth, a larger-diameter ablation zone (eg, 4 to 6 mm) can be used to smooth the entire area.
PTK in Thin Corneas When corneal opacities associated with corneal thinning (eg, corneal ulcer scars) are being treated, it is difficult to create a smooth corneal surface without causing a large area of significant corneal thinning. Masking agents are often necessary to produce even a somewhat smooth surface. When required, such lesions can be treated, but the resulting corneal flattening, which can be dramatic, must then be managed, often with a rigid gas permeable contact lens.
PTK for Recurrent Erosions
The third technique is used to treat recurrent erosions, most commonly associated with anterior basement membrane dystrophy. Most eyes with recurrent erosions can be managed with medical therapy such as lubrication,hypertonic agents, and bandage soft contact lenses. With failure of medical management, surgical options are available, including anterior stromal puncture, diamond burr polishing of Bowman’s membrane, and excimer laser PTK. When PTK is used, the entire area of loose epithelium is removed, and the cornea is treated to ablate 5 to 6 µm of Bowman’s membrane. Care should be taken to remove all areas of loose epithelium and then to treat all of the exposed Bowman’s membrane to prevent recurrences outside the treated area.
PTK-Induced Refractive Error
One of the most frustrating aspects of PTK surgery is induced refractive error. Most of the time ablations are performed centrally, causing central corneal thinning and flattening, resulting in induced hyperopia. When peripheral ablations are performed, induced myopia may occur. Induced astigmatism is not uncommon, because corneal opacities are often not uniform and are difficult to completely smooth out with current techniques, even with the use of masking agents. During the early PTK experience, hyperopic shifts of 5 to 15 D were routinely induced.15-19 As more procedures were performed and patients were followed for longer periods of time, surgeons realized that deep ablations were responsible for this hyperopic shift. Techniques were developed, including the “ablate and check” procedure discussed earlier, to combat this adverse effect. Additionally, an antihyperopia ablation was proposed to decrease corneal flattening and induced hyperopia. The effectiveness of these modalities is uncertain. One problem is that precisely how much tissue needs to be removed in any given patient is not known preoperatively. Also, exactly how much hyperopia is induced per amount of tissue removed during PTK is unknown, as is the best method to counteract induced hyperopia.