CHAPTER 3
Incisional Corneal Surgery
Incisional refractive surgery has largely been replaced by other modalities but is still used in limited circumstances for treatment of primary and residual astigmatism after both cataract and keratorefractive surgery (limbal relaxing incisions) and following penetrating keratoplasty (arcuate keratotomy).
The history of incisional keratotomy dates back to the 1890s. Lans examined astigmatic changes induced in rabbits after partial-thickness corneal incisions and thermal cautery. Sato made significant contributions to incisional refractive surgery in the 1930s and 1940s. He observed central corneal flattening and improvement in vision after the healing of spontaneous ruptures of the Descemet membrane (corneal hydrops) in patients with advanced keratoconus, which led him to develop a technique to induce artificial ruptures of the Descemet membrane. His long-term results in humans were poor, because incisions were made posteriorly through the Descemet layer, inducing late corneal edema in 75% of patients. In the 1960s and 1970s, Fyodorov, using radial incisions on the anterior cornea, established that the diameter of the central optical clear zone was inversely related to the amount of refractive correction: smaller central clear zones yield greater myopic corrections.
Incisional Correction of Myopia
Radial Keratotomy in the United States
Radial keratotomy (RK) is now largely considered an obsolete procedure, but it did play an important role in the history of refractive surgery. The excimer laser was originally intended to produce more accurate incisions for RK, not for surface ablation or laser in situ keratomileusis (LASIK), for which the excimer laser is now used. Radial keratotomy differs from surface ablation and LASIK in that it does not involve removal of tissue from the central cornea; rather, there is a redistribution of power from the center to the periphery.
To evaluate the safety and efficacy of RK, the Prospective Evaluation of Radial Keratotomy (PERK) study was undertaken in 1982 and 1983 for patients with myopia from –2.00 D to –8.75 D (mean, –3.875 D). The sole surgical variable was the diameter of the central optical clear zone (3.00, 3.50, or 4.00 mm), based on the level of preoperative myopia. It was later found that the older the patient, the greater the effect achieved with the same surgical technique. In the PERK study, 8 radial incisions were used for all patients; repeat surgery, if necessary, involved an additional 8 incisions. Ten years after the procedure, 53% of the 435 study patients had 20/20 or better uncorrected distance visual acuity (UDVA; also called uncorrected visual acuity, UCVA) and 85% had 20/40 or better. Of the patients who had bilateral surgery, only 30% reported the use of spectacles or contact lenses for distance refractive correction at 10 years. Complications related to the procedure included loss of corrected distance visual acuity (CDVA; also called best-corrected visual acuity, BCVA; 3%), delayed
bacterial keratitis, corneal scarring, irregular astigmatism, and epithelial erosions.
The most important finding in the 10-year PERK study was the continuing long-term instability of the procedure. A hyperopic shift of 1.00 D or greater was found in 43% of eyes between 6 months and 10 years postoperatively. There was an association between length of the incision and hyperopic shift, particularly if the incisions extended into the limbus.
Waring GO III, Lynn MJ, McDonnell PJ; PERK Study Group. Results of the Prospective Evaluation of Radial Keratotomy (PERK) study 10 years after surgery. Arch Ophthalmol. 1994;112(10):1298–1308.
Surgical technique
Radial corneal incisions severed collagen fibrils in the corneal stroma. This produced a wound gape with midperipheral bulging of the cornea, compensatory central corneal flattening, and decreased refractive power, thereby decreasing myopia (Fig 3-1).
Figure 3-1 Schematic diagrams of the effect of radial incisions. A, 8-incision radial keratotomy (RK) with circular central optical zone (dashed circle), which shows the limit of the inner incision length. B, Cross-sectional view of the cornea,
showing RK incisions (shaded areas). C, Flattening is induced in the central cornea. (Modified from Troutman RC, Buzard KA. Corneal Astigmatism: Etiology, Prevention, and Management. St Louis: Mosby-Year Book; 1992.)
The design of the diamond-blade knife (angle and sharpness of cutting edge, width of blade, and design of footplate) influenced both the depth and the contour of incisions (Fig 3-2). The footplates reduced the risk of penetration and stabilized the blade. The guard on the front of the blade prevented inadvertent entry into the central optical zone. The length of the knife blade and the associated depth of the incisions were set according to the corneal thickness, which was usually measured with an ultrasonic pachymeter. The ideal depth of RK incisions was 85%–90% of the corneal thickness.
Figure 3-2 A, Illustration of the guarded diamond knife used in RK surgery. Note the footplates and blade between them. The distance from the tip of the blade to the footplates is adjustable. B, Diagram of RK diamond blade with footplates that rest on the cornea, reducing the risk of penetration into the anterior chamber. (Part A courtesy of KMI Surgical; redrawn by Cyndie C. H. Wooley.)
Postoperative refraction, visual acuity, and corneal topography
Radial keratotomy changed not only the curvature of the central cornea but also its overall topography, creating an oblate cornea—flatter in the center and steeper in the periphery. The procedure reduced myopia but increased spherical aberration. The result was less correlation among refraction, central keratometry, and UDVA, presumably because the new corneal curvature created a more complex, multifocal optical system. The effect is that keratometric readings, which sample a limited number of points approximately 3.0 mm apart, might show degrees of astigmatism that differ from those detected by refraction. Similarly, UDVA might vary, particularly depending on pupil diameter: the smaller the pupil, the less the multifocal effect from postoperative corneal contour and the better the quality of vision. Also, the central corneal flattening may affect intraocular lens (IOL) power calculation for cataract surgery (discussed later in this chapter and in Chapter 11).
Stability of refraction
Most eyes were generally stable by 3 months after RK surgery. However, diurnal fluctuation of vision and a progressive flattening effect after surgery have been known to persist, resulting in refractive instability.
Diurnal fluctuation of vision occurs due to hypoxic edema of the incisions with the eyelids closed during sleep. This edema causes flattening of the cornea (and hyperopic shift) upon awakening, followed by steepening later in the day. In a subset of the PERK study at 10 years, the mean change in the spherical equivalent of refraction between the morning (waking) and evening examinations was an increase of 0.31 ± 0.58 D in minus power.
The progressive flattening effect of surgery was one of the major untoward results with RK. The
refractive error in 43% of eyes in the PERK study changed in the hyperopic direction by 1.00 D or more between 6 months and 10 years postoperatively. The hyperopic shift was statistically associated with decreasing diameter of the central optical clear zone. Corneal lasso sutures were once advocated, but their use has become largely obsolete. The potential stabilizing effect of collagen crosslinking induced with riboflavin and ultraviolet A (UVA) light is currently being studied.
Complications
After RK surgery, 1%–3% of eyes experienced loss of 2 or more lines of Snellen visual acuity. This effect was due to induction of irregular astigmatism from hypertrophic scarring, intersecting radial and transverse incisions (Fig 3-3A, B), and central clear zones smaller than 3.0 mm.
Figure 3-3 A, Crossed RK and arcuate keratotomy incisions with epithelial plugs in a patient who had intraoperative corneal perforation. B, Fluorescein study demonstrates gaping of the incisions, causing persistent ocular irritation. (Courtesy of Jayne S.
Weiss, MD.)
Many patients reported the appearance of starburst, glare, or halo effects around lights at night after RK. Although most patients found the starburst effect comparable to looking through dirty spectacles or contact lenses, some patients could not drive at night because of this complication. Treatment with drugs that promote pupillary constriction, such as brimonidine or pilocarpine, may be able to reduce symptoms. Other complications included postoperative pain, undercorrection and overcorrection, induced astigmatism due to epithelial plugs and wound gape (see Fig 3-3), vascularization of stromal scars, and nonprogressive endothelial disruption beneath the incisions.
Potentially blinding complications occurred only rarely after RK. These included perforation of the cornea, which can lead to endophthalmitis, epithelial downgrowth, and traumatic cataract. The postoperative use of contact lenses may have resulted in vascularization of the incisions, with subsequent scarring and irregular astigmatism.
Radial keratotomy incisions remain a point of weakness, and traumatic rupture of RK wounds has been reported up to 13 years after the procedure (Fig 3-4).
Figure 3-4 Traumatic rupture of an 8-incision RK, showing communication between 2 horizontal RK incisions. Interrupted
10-0 nylon sutures were used to close the incision. (Reprinted with permission from External Disease and Cornea: A Multimedia Collection.
San Francisco: American Academy of Ophthalmology; 2000.)
Ocular surgery after radial keratotomy
It is not uncommon for RK patients to present years later with hyperopia. LASIK and surface ablation have been shown to be effective in correcting hyperopia and myopia after RK. However, surface ablation may be preferred, as creation of a LASIK flap may result in irregular astigmatism due to splaying of the incisions and epithelial ingrowth, which can be challenging to treat. Surface ablation avoids the LASIK-related risks after RK but does increase the risk of postoperative corneal haze. The off-label use (in the United States) of mitomycin C, 0.02% (0.2 mg/mL), has dramatically reduced surface ablation haze after RK and other prior corneal surgeries (eg, corneal transplant and LASIK). The drug should be copiously irrigated from the eye so that toxic effects are reduced. The refractive correction is often reduced by 5%–15% when mitomycin C is used prophylactically.
Patients undergoing laser vision correction for refractive errors after RK should understand that laser correction will not remove scars caused by RK incisions, so glare or fluctuation symptoms may remain after the laser surgery. In addition, obtaining accurate wavefront analysis may not be possible due to complex optical irregularities associated with RK. Because of the progressive hyperopia that can occur with RK, it is prudent to aim for slight myopia with laser vision correction, as some patients may still progress to hyperopia in the future.
In patients with endothelial dystrophy, corneal infection, irregular astigmatism, severe visual