Corneal surgery includes curative or therapeutic procedures and refractive procedures (Fig. 5.18).
❖ Curative corneal procedures are intended to improve vision by eliminatingcorneal opacification.
❖ Refractive corneal procedures change the refractive power of aclearcornea.
Principle: This involves replacement of diseased corneal tissue with a full-thickness donor graft of corneal tissue of varying diameter. A clear, regularly refracting button of donor cornea is placed in an opacified or irregularly refracting cornea. The corneal button is sutured with a continuous single or double suture (Fig. 5.19) or with interrupted sutures. (For special considera-tions in corneal transplants, see also Morphology and healing.)
Penetrating keratoplasty can be performed as an elective procedure to improve visual acuity or as an emergency procedure (emergency kerato-plasty). Emergency keratoplasty is indicated to treat a perforated or nonheal-ing corneal ulcer to remove the perforation site and save the eye (tectonic ker-atoplasty).
Indications: Corneal diseases that affect the full thickness of the cornealstroma (corneal scars, dystrophy, or degeneration) or protrusion anomalies such as keratoconus or keratoglobus with or without central corneal opacification.
Allograft Rejection (Complications): The body’s immune system canrespond with a chronic focal allograft rejection (Fig. 5.20) or a diffuse allograft rejection (Fig. 5.21). The graft will be become opacified.
Principle: This involves replacement of a superficial stromal opacificationwith a partial-thickness donor graft of clear corneal tissue.
This surgery requires the corneal epithelium, Descemet’s membrane, and the deeper layers of the cornea to be intact and healthy as it is only suitable for removing superficial opacifications down to about the middle of the cornea. The donor corneal button is then sutured with one or two continuous sutures or with interrupted sutures.
Indications: Corneal opacifications and scars affecting the superficial cornealstroma (post-traumatic, degenerative, dystrophic, or postinflammatory opacifications). This method is not suitable for treating corneal ulcers.
Allograft Rejection (Complications): Allograft rejection is less frequent thanin the case of penetrating keratoplasty. There is also less danger of infection as lamellar keratoplasty does not involve opening the globe.
Principle: Superficial corneal scars can be ablated with an excimer laser(wavelength of 193 nm). The lesion is excised parallel to the surface of the cor-nea to avoid refractive effects. The edges of the ablated area are merged smoothly with the rest of the corneal surface, eliminating any irregularities.
Indications: Indications are identical to those for lamellar keratoplasty.However, this method is only suitable for ablation of relatively superficial cor-neal opacifications, i.e., in the upper 20% of the corneal stroma.
Disadvantage: Despite attempting ablation parallel to the surface of the cor-nea, phototherapeutic keratectomy often creates a hyperopic effect.
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.