Incisional keratotomy patients are far less abundant than LASIK and PRK patients, but they're out there. Unlike the latter, the numbers of RK patients is not increasing. Hexagonal keratotomy, a procedure to treat mild hyperopia, was uncommon even in its heyday, and the number of patients who will come to cataract surgery is small. The number of RK patients coming to cataract surgery, on the other hand, will be larger, and many of them are seeking cataract surgery now.
Lens power calculation is difficult following radial keratotomy, just as it is after LASIK and PRK, but there are differences. LASIK and PRK change the anterior curve of the cornea without changing the posterior curve, absent ectasia. Radial keratotomy, on the other hand, produces changes in the curvatures of both surfaces. Lens power formulas that are useful for LASIK and PRK cannot be applied to radial keratotomy.
The ASCRS website has a post-refractive surgery IOL calculator (iol.ascrs.org) that is useful for the three most common types of keratorefractive patients: myopic LASIK/PRK, hyperopic LASIK/PRK, and RK.
In his article, Dr. Packer discusses the two most important challenges facing the cataract surgeon who operates on post-RK patients, the challenge of calculating IOL power accurately and the challenge of staying clear of the radial corneal incisions. As he notes, difficulty in avoiding RK incisions goes up as the incision number increases.
As with all post-keratorefractive patients, counseling is critical. These patients need to be warned about the likelihood of a lens power calculation error and the greater surgical difficulty that the corneal incisions impose. It's important not to plan a lens exchange, piggyback IOL, or keratorefractive enhancement until any hyperopic shift in the cornea that cataract surgery might have caused has settled. Hyperopic shifts can be minimized by not overpressurizing the eye during surgery, especially at the time of incision closure.
Kevin Miller, M.D.
complicated cataract cases editor
Watch this video on your smartphone or iPad using your QR code reader.
(Scanner available for free at your app store.)
Or view the video of the procedure at www.eyeworld.org/ replay.php
Figure 1. The left-hand clear corneal 1.2-1.4 trapezoidal incision is placed between two RK incisions
Figure 2. The right-hand incision is placed between RK incisions in this 16-cut cornea
Figure 3. Utilizing the 20-gauge Tsuneoka front-end irrigating chopper in the left hand and a 30-degree bevel 20-gauge straight phaco needle in the right hand, a pie-shaped segment is mobilized with high vacuum (270 mm Hg) and zero ultrasound power
Figure 4. The slit beam retroillumination feature of the operating microscope (Carl Zeiss Meditec) highlights a posterior subcapsular plaque while the front-end irrigation stream from the chopper puts the posterior capsule on stretch and facilitates polishing with the silicone-coated 0.3 mm aspiration tip
Figure 5. The 2.7 mm temporal incision for IOL insertion is constructed at the limbus, posterior to the RK incisions; the incision is sized generously to avoid pressure on the roof during IOL insertion
As radial keratotomy patients are aging and developing progressive hyperopia and cataracts, it is increasingly important to master safe and effective ways to treat these patients. Eyes that have undergone radial keratotomy (RK) present two separate challenges to the refractive cataract surgeon. First, the altered and often irregular curvature of the cornea after RK renders standard keratometry and routine IOL calculations inaccurate. Second, the altered anatomy of the post-RK cornea with its compromised structural integrity leads to technical challenges during phaco surgery. Linear RK incisions were typically made in a spoke-like pattern extending from a 3-4 mm central optical zone peripherally to within 1-2 mm of the limbus. The incisions were made to 90-95% corneal stromal depth and numbered from 2 up to 32 or higher in some extreme cases. Most spherical myopic RK treatments involved 4-16 radial incisions arranged in a symmetric wagon wheel spoke pattern over the cornea. Various incisions for the treatment of astigmatism were often added as T-cuts or other small concentrically arranged incisions placed between and perpendicular to the radial cuts. At the time that RK was in widespread use, cataract surgery was still mainly performed via scleral incisions either in the form of extracapsular cataract extraction or scleral tunnel phaco surgery. Consequently, there was little worry at the time about how these peripherally located corneal RK incisions would interfere with future clear corneal cataract surgery. Unfortunately, we now find ourselves dealing with this unintended legacy as we take care of an increasing number of post-RK presbyopic hyperopes and cataract patients. The development of computed corneal topography in the 1980s facilitated understanding of the effects of radial keratotomy on corneal refractive power. The resultant flattening of the central cornea violated the central assumption of keratometry, relatively perfect sphericity of the anterior corneal surface within the 3 mm optical zone. Fortunately, corneal topography has also enabled improved measurement of the central corneal power so that a measurement of the effective refractive power can be used in IOL power calculation formulas. The latest generation of formulas, such as the Holladay II, allow use of the "double K" method, in which average or pre-keratorefractive values are used to help determine the effective lens position while topographically measured post-keratorefractive values are used for the corneal power. The key to clear corneal incisions in these post-RK patients is to avoid the crossing of phaco incisions and RK incisions. If a clear corneal phaco incision is made through an RK incision, there is a high likelihood that the roof of the phaco incision will split open along the RK incision due to manipulation during the course of the procedure. The split roof of the clear corneal incision will prevent a good seal at the incision and allow excessive outflow of fluid and consequent chamber instability during phacoemulsification. An unstable chamber can lead to multiple complications including iris damage, endothelial damage, capsule rupture, and vitreous loss. A split incision roof can also lead to difficulty in closing the corneal incision at the end of the case. Often multiple sutures are required to achieve a watertight closure. The added sutures can create astigmatism and patient discomfort. A poorly sealing corneal incision may also increase the risk of endophthalmitis. When seeking to optimize safety and outcomes, there are multiple surgical options to consider for post-RK phaco patients. Standard clear cornea (2.5-3.0 mm): If the number of RK incisions is small, there may be enough space between incisions to create a standard 2.5-3.0 mm clear corneal incision without crossing any RK incision. Careful location of the main and paracentesis incisions may be all that is necessary. However, if the RK incisions are too numerous to allow a standard phaco incision to be made in the peripheral clear cornea without violating an RK incision, then alternative phaco incisions should be considered.
Scleral tunnel: A posteriorly placed scleral tunnel incision with a posterior entry into the anterior chamber will avoid the peripheral ends of most RK incisions. This type of incision would, however, require a peritomy and possibly a suture. There remains a risk that the anterior aspect of the corneal tunnel would incorporate a deep peripheral RK incision, resulting in splitting of the internal opening of the incision and resultant decreased water-tightness. The posterior aspect of the incision, however, would be expected to remain intact. Biaxial micro-incisions (<1.5 mm): Using clear corneal micro-incisions in conjunction with biaxial phacoemulsification is a technique that effectively avoids manipulation of the RK incisions and also avoids the need for peritomy and cautery. The small incisions can frequently be placed entirely between old RK incisions in the peripheral clear cornea and can therefore allow the surgeon to perform the procedure in the standard fashion while incurring minimal increased risk (Figures 1-4). In the absence of micro-incision IOLs, it is still necessary to create a larger incision for the insertion of the lens implant. This IOL incision should be made separately from the two micro-incisions and should be made in the far-peripheral (near-clear) cornea (Figure 5). In patients with many RK incisions, it may not be possible to avoid incising a meridian that contains an RK incision. It is therefore of utmost importance to avoid stretching or torquing this incision. Stretching can be avoided by making a slightly generous incision for a given IOL injector; the tip can then be inserted through the incision with minimal distortion or stress. If the incision does split at this point in the procedure, it will cause less difficulty and pose less risk than at earlier points in the case. Once the lens is injected, I/A and all further intraocular manipulations should be performed through the micro-incisions to avoid torquing or other manipulation and potential splitting of the delicate IOL incision.
Capsular tension rings: Axially myopic eyes (ALM>27 mm) tend to have extremely deep anterior chambers and large, floppy lens capsules. In addition, these eyes have often undergone extreme versions of radial keratotomy, with a high number of very deep incisions. They therefore carry a high likelihood of having had a microperforation or frank perforation during the RK procedure with resultant anterior chamber shallowing and zonular stress. In this setting the adjunctive use of a capsular tension ring to stabilize the capsule during phacoemulsification helps to reduce the risk of rupture and dialysis. With careful pre-op planning for IOL calculation, incision placement, and technique, surgeons and their post-RK patients can enjoy excellent results.
Editors' note: Dr. Packer has financial interests with Bausch + Lomb (Rochester, N.Y.).