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. If keratoplasty is deemed necessary, the RK incisions may need to be sutured before trephination in order to minimize the chance of their opening and to allow 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 Chapter 11). Newer modalities such as intraoperative aberrometry may help refine IOL selection.

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, but multifocal IOLs should be avoided. At the conclusion of cataract surgery, care should be taken to prevent overhydrating the cataract incision to avoid rupture of the RK incision.

Anbar R, Malta JB, Barbosa JB, Leoratti MC, Beer S, Campos M. Photorefractive keratectomy with mitomycin-C for consecutive hyperopia after radial keratotomy. Cornea. 2009;28(4):371–374.

Hill WE, Byrne SF. Complex axial length measurements and unusual IOL power calculations. Focal Points: Clinical Modules for Ophthalmologists. San Francisco: American Academy of Ophthalmology; 2004, module 9.

Joyal H, Grégoire J, Faucher A. Photorefractive keratectomy to correct hyperopic shift after radial keratotomy. J Cataract Refract Surg. 2003;29(8):1502– 1506.

Linebarger EJ, Hardten DR, Lindstrom RL. Laser-assisted in situ keratomileusis for correction of secondary hyperopia after radial keratotomy. Int Ophthalmol Clin. 2000;40(3):125–132.

Nassaralla BA, McLeod SD, Nassaralla JJ Jr. Prophylactic mitomycin C to inhibit corneal haze after photorefractive keratectomy for residual myopia following radial keratotomy. J Refract Surg. 2007;23(3):226–232.

Salamon SA, Hjortdal JO, Ehlers N. Refractive results of radial keratotomy: a ten-year retrospective study. Acta Ophthalmol Scand. 2000;78(5):566–568. Seitz B, Langenbucher A. Intraocular lens calculations status after corneal refractive surgery. Curr Opin Ophthalmol. 2000;11(1):35–46.

Waring GO III. Radial keratotomy for myopia. Focal Points: Clinical Modules for Ophthalmologists. San Francisco: American Academy of Ophthalmology; 1992, module 5.

Incisional Correction of Astigmatism

Several techniques of incisional surgery have been used to correct astigmatism, including transverse (straight) keratotomy and arcuate (curved) keratotomy (AK), in which incisions are typically placed in the cornea at the 7-mm optical zone; and limbal relaxing incisions (LRIs), which are placed at the limbus. Transverse keratotomy was frequently used in the past in combination with RK to correct myopic astigmatism, but it is now used only seldomly. Arcuate keratotomy was also used to correct naturally occurring astigmatism, but the procedure is now used primarily to correct postkeratoplasty astigmatism. LRIs are used to help manage astigmatism during or after cataract surgery and IOL implantation and after refractive surgery procedures such as LASIK and photorefractive keratectomy.

Coupling

When 1 meridian is flattened from an astigmatic incision, an amount of steepening occurs in the meridian 90° away (Fig 3-5). This phenomenon is known as coupling. When the coupling ratio (the

amount of flattening in the meridian of the incision divided by the induced steepening in the opposite meridian) is 1.0, the spherical equivalent remains unchanged. When there is a positive coupling ratio (greater than 1.0), a hyperopic shift occurs. The type of incision (arcuate vs tangential) and the length and number of parallel incisions can influence the coupling ratio. Long, straight, and tangential incisions tend to induce more positive coupling (greater than 1.0), and therefore more hyperopia, than do short, arcuate incisions. When a correction is less than 2.00 D of astigmatism, the coupling ratio is typically 1.0, whereas when a correction is greater than 2.00 D of astigmatism, the ratio tends to be greater than 1.0. In general, LRIs do not change the spherical equivalent.

Figure 3-5 Coupling effect of astigmatic incisions. A, A limbal relaxing incision has a coupling ratio of 1.0, and the spherical equivalent and average corneal power are not changed. B, A transverse incision has a coupling ratio greater than 1.0, which causes a hyperopic change in refraction by making the average corneal power flatter. (Illustration by Cyndie C. H. Wooley.)

Rowsey JJ, Fouraker BD. Corneal coupling principles. Int Ophthalmol Clin. 1996;36(4):29–38.

Arcuate Keratotomy and Limbal Relaxing Incisions

Arcuate keratotomy is an incisional surgical procedure in which arcuate incisions of approximately 95% depth are made in the steep meridians of the midperipheral cornea at the 7-mm optical zone. LRIs are incisions set at approximately 600 µm depth, or 50 µm less than the thinnest pachymetry measurement at the limbus, and placed just anterior to the limbus (Fig 3-6). Arcuate keratotomy differs from LRIs by its midperipheral location and its greater relative depth. Due to the concomitant steepening of the orthogonal meridian (coupling), AK and LRIs correct astigmatism without inducing a substantial hyperopic shift of the spherical equivalent of the preoperative refraction. LRIs achieve increased effect primarily by increasing the length of the incision. For AK, cylindrical correction can be increased by increasing the length or depth of the incision, using multiple incisions, or reducing the optical zone (Table 3-1). The longer and deeper the incision and smaller the optical zone, the greater the astigmatic correction.

Figure 3-6 Limbal relaxing incision. A relaxing incision is made at the limbus with the use of a diamond knife. The coupling ratio is typically 1.0 and does not change the spherical equivalent. (Courtesy of Brian S. Boxer Wachler, MD.)

Table 3-1

Instrumentation

The instruments used in AK and LRIs are similar. Front-cutting diamond blades are more often used in AK, and back-cutting diamond blades are more often used in LRI surgery. A mechanized trephine, the Hanna arcuate trephine, has been shown to make smooth, curvilinear AK incisions of specified optical zone and arc length. Recently, the femtosecond laser has been adapted to create peripheral arcuate incisions. These incisions may be titratable, as only part of the incision may be opened initially, followed by a larger area later if there is a need for greater astigmatic correction.

Surgical Techniques

With any astigmatism correction system, accurate determination of the steep meridian is essential. The plus cylinder axis of the manifest refraction is used, as this accounts for corneal and lenticular astigmatism, which are “manifest” in the refraction. If the crystalline lens is to be removed at the time of the astigmatic incisional surgery (ie, LRI), the correction should be based on the steep meridian and magnitude as measured with corneal topography or keratometry. Intraoperative keratoscopy can be helpful in determining incision location and effect. The amount of treatment for a given degree of astigmatism can be determined from a nomogram, such as the one shown in Table 3-1.

It is prudent to make horizontal reference marks using a surgical marking pen, with the patient sitting up, preferably at the slit lamp. Marking with the patient in this position avoids reference-mark error due to cyclotorsion of the eyes. Studies have demonstrated that up to 15° of cyclotorsion can occur when patients move from an upright to a supine position. Arcuate keratotomy incisions may be placed in pairs along the steep meridian and, because of induced glare and aberrations, no closer than 3.5 mm from the center of the pupil. LRIs are placed in the peripheral cornea, near the limbus. They result in lower amounts of astigmatic correction than do AK incisions, presumably due to faster healing because of their proximity to the vascularized limbus. Arcuate keratotomy incisions used to correct post–penetrating keratoplasty astigmatism are often made in the graft or in the graft–host junction, but care must be taken to avoid perforation. When AK incisions are made in the host, the effect is significantly reduced. Arcuate keratotomy incisions in a corneal graft may require compression sutures at the meridian 90° away, and an initial overcorrection is desired in order to compensate for wound healing.

Outcomes

The outcome of AK and LRI surgery depends on several variables, including patient age; the distance separating the incision pairs; and the length, depth, and number of incisions. Few large prospective trials have been performed. The Astigmatism Reduction Clinical Trial (ARC-T) of AK, which used a 7-mm optical zone and varying arc lengths, showed a reduction in astigmatism of 1.6 ± 1.1 D in patients with preoperative, naturally occurring astigmatism of 2.8 ± 1.2 D. Other studies of AK have shown a final UDVA of 20/40 in 65%–80% of eyes. Overcorrections have been reported in 4%–20% of patients.

Studies of LRIs are limited, but these incisions are frequently used with seemingly good results in astigmatic patients undergoing cataract surgery. One study showed an absolute change in refractive astigmatism of 1.72 ± 0.81 D after LRIs in patients with mixed astigmatism. Astigmatism was decreased by 0.91 D, or 44%, in another series of LRIs in 22 eyes of 13 patients. Incisions in the horizontal meridian have been reported to cause approximately twice as much astigmatic correction as those in the vertical meridian (see Table 3-1).

Complications

Irregular astigmatism may occur after either AK or LRIs; however, it is more common with AK than with LRIs, probably because LRIs are farther from the corneal center, thus mitigating any effects of irregular incisions. Off-axis AK can lead to undercorrection or even worsening of preexisting astigmatism. To avoid creating an edge of cornea that swells and cannot be epithelialized, arcuate incisions and LRIs should not intersect other incisions (see Fig 3-3). Corneal infection and perforation have been reported.