Surgical technique

Radial corneal incisions sever collagen fibrils in the corneal stroma. This produces a wound gape with midperipheral bulging of the cornea, compensatory central corneal flattening, and decreased refractive power, thereby decreasing myopia (Fig 3-1).

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. The ideal depth of RK incisions is 85%–90% of the corneal thickness.

Postoperative refraction, visual acuity, and corneal topography

Radial keratotomy changes 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 reduces myopia but increases spherical aberration. The result is less correlation among refraction, central keratometry, and UCVA, presumably because the new corneal curvature creates 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. Also, central corneal flattening affects intraocular lens (IOL) power calculations for cataract surgery (discussed later in this chapter and in Chapter 11).

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, pre-RK.C, After RK the corneal periphery is steepened and the center flattens. 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.)

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 described in the PERK study. Greater hyperopic shift was noted with smaller optical zones. The potential stabilizing effect of corneal crosslinking (CCL) is currently being studied.

• Elbaz U, Yeung SN, Ziai S, Lichtinger AD, et al. Collagen crosslinking after radial keratotomy. Cornea. 2014;33(2):131–136.

• Mazzotta C, Baiocchi S, Denaro R, Tosi GM, Caporossi T. Corneal collagen cross-linking to stop corneal ectasia exacerbated by radial keratotomy. Cornea. 2011;30(2):225–228.

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 3-2), and central clear zones smaller than 3.0 mm.

Many patients reported the appearance of starburst, glare, or halo effects around lights at night after RK. Treatment with drugs that promote pupillary constriction, such as pilocarpine, or decrease pupillary dilation, such as brimonidine may be able to reduce symptoms by keeping the pupillary diameter within the central optical clear zone. Other complications included fluctuation in vision and loss of best-corrected visual acuity (BCVA; also called corrected distance visual acuity, CDVA), induced astigmatism due to epithelial plugs and wound gape (see Fig 3-2), vascularization of stromal scars, and nonprogressive endothelial disruption beneath the incisions.

Figure 3-2 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.)

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 often resulted in vascularization of the incisions, with subsequent scarring and irregular astigmatism. Radial keratotomy incisions remain a point of weakness, and rupture of RK wounds secondary to blunt trauma has been reported up to 13 years after the procedure.

Ocular surgery after radial keratotomy

It is not uncommon for RK patients to present years later with hyperopia. LASIK and PRK 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, splaying of the incisions, epithelial ingrowth, as well as loss of sections of the flap, which can be challenging to treat. Surface ablation avoids the LASIK-related risks after RK but increases the risk of postoperative corneal haze. The off-label use (in the United States) of mitomycin C, 0.02% (0.2 mg/mL) applied to the stroma after laser ablation for 12–30 seconds, has dramatically reduced corneal haze after RK and other prior corneal surgical procedures (eg, corneal transplant and LASIK). The drug should be copiously irrigated from the eye so that toxic effects are reduced.

Patients undergoing laser vision correction for refractive errors after RK need to 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, some patients may still experience continued hyperopic progression.

In patients with endothelial dystrophy, corneal infection, irregular astigmatism, severe visual fluctuations, or starburst effects, keratoplasty may be needed to restore visual functioning. It should be avoided if the patient’s visual problems can be corrected with glasses or contact lenses (see the section Corneal Transplantation After Refractive Surgery in Chapter 11). If keratoplasty is deemed necessary, before trephination the RK incisions may need to be stabilized with sutures outside the trephine cut. This minimizes the chance of their opening and allows adequate suturing of the donor corneal graft to the recipient bed.

Cataract extraction with IOL implantation may lead to variable results after RK. In the early postoperative period, corneal edema may result in temporary hyperopia. In addition, IOL power calculation may be problematic and may result in ametropia. Calculation of implant power for cataract surgery after RK should be done by first using a third-generation formula (eg, Haigis, Hoffer Q, Holladay 2, or SRK/T) rather than a regression formula (eg, SRK I or SRK II) and then choosing the highest resulting IOL power. Keratometric power is determined in 1 of 3 ways: direct measurement using corneal topography; application of pre-RK keratometry value minus the refractive change; or adjustment of the base curve of a plano contact lens by the overrefraction (see the section Eyes With No Preoperative Information in Chapter 11).

A useful online resource for calculating IOL power in a post-RK patient is the post–refractive surgery IOL power calculator available on the website of the American Society of Cataract and Refractive Surgery (ASCRS), www.ascrs.org, and directly at http://iolcalc.ascrs.org (see Chapter 11). In addition, modalities such as intraoperative wavefront aberrometry can be used to obtain real-time IOL calculations that may help improve refractive outcomes.

Incision placement and construction is vital when performing cataract surgery in the post-RK patient. Scleral tunnel incisions are often preferred, because clear corneal incisions increase the risk of the blade transecting the RK incision, which can induce irregular astigmatism. To help reduce preoperative corneal astigmatism, the surgeon may consider placing the incision in the steep astigmatic meridian of the cornea; in addition, toric IOLs can be used in patients with regular astigmatism but multifocal IOLs should be avoided. At the conclusion of surgery, care should be taken to prevent overhydrating the cataract incisions to avoid rupture of the RK incisions.

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• Majmudar PA, Schallhorn SC, Cason JB, et al. Mitomycin-C in corneal surface excimer laser ablation techniques: a report by the American Academy of Ophthalmology. Ophthalmology. 2015;122(6):1085–1095.

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• Shammas. HJ. Intraocular lens power calculation in patients with prior refractive surgery. Focal Points: Clinical Modules for Ophthalmologists. San Francisco: American Academy of Ophthalmology; 2013, module 6.

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Excerpted from BCSC 2020-2021 series: Section 13 - Refractive Surgery. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.