Refractive Corneal Procedures
Principle: Tissue is ablatedto change the corneal curvature and to achieve arefractive correction. Flattening the corneal curvature corrects myopia, whereas steepening the curvature corrects hyperopia. The amount of tissue removed at different sites can be varied with layer-by-layer excimer laser ablation and the use of apertures. This makes it possible to correct for myopia, by removing more tissue from the center of the cornea, or for hyperopia, by removing more tissue from the periphery.
Indications: Best results are achieved in correcting myopia of less than 6diopters. At present stable correction can be achieved in 85 – 95% of all cases of myopia up to !6 diopters, with deviation of !1 diopter from the target within one year. Correction of hyperopia has also been attempted.
Principle: Correction of myopia byflattening the central dome of the corneawith four to sixteen radial incisions extending through as much as 90% of the thickness of the cornea. This increases the steepness of the corneal periphery and lowers the center of the cornea, reducing its refractive power. This method does not influence the optical center of the cornea (Fig. 5.22).
Indications and prognosis: The method is suitable for moderate myopia(less than 6 diopters). The effect achieved is influenced by the initial refrac-tion, intraocular pressure, corneal thickness, and the patient’s age and sex. A disadvantage is refractive fluctuations of up to 1.5 diopters during the course of the day. In one-fifth of all cases refraction becomes unstable within a year.
Principle: Surgicalreduction of severe regular astigmatismby flattening thesteep meridian in the center of the cornea by increasing the steepness of the corneal periphery. Irregular astigmatism cannot be corrected.
Principle: The laser is focused on the corneal stroma to create shrinkageeffects. Placing these areas symmetrically steepens the central cornea, which can correct severe hyperopia.
Indication: Hyperopia correction up to 8 diopters.
Principle: Severe myopia and hyperopia are corrected by suturing speciallyprepared hyperopic or myopic partial-thickness corneal grafts on to the recipient’s cornea. This involves special trephination and preparation of the recipient’s cornea. The donor graft is then fitted into the prepared cornea and sutured in place. The donor corneal button is prepared as a frozen section and shaped to the required refractive power; these implants can be ordered from eye banks.
Indications: Any severity of hyperopia or myopia can be corrected.
Principle: Myopia is corrected with preservation of Bowman’s layer. A super-ficial corneal flap (approx. 160 µm) is created with a microkeratome. The ker-atome is withdrawn, the flap is reflected, and the exposed underlying corneal stroma is ablated with an excimer laser to correct the myopia. Then the flap is repositioned on the corneal bed and fixed in place by force of its own adhe-sion.
Indication: Even severe myopia (up to 10 – 12 diopters) can be corrected withthis method.