Introduction
The use of radial and astigmatic keratotomy has declined over time as laser vision correction and cataract surgery with the implantation of toric intraocular lenses have gained popularity owing to superior predictability in most cases. Radial keratotomy for myopia correction is an especially abandoned procedure. Nevertheless, it is included in this chapter to illustrate its principles and the reasons for its eventual failure. On the other hand, incisional corneal surgery continues to play a role as a surgical method of correcting high astigmatism that is beyond the range of laser correction, for example, as it may occur iatrogenically after penetrating keratoplasty or for the correction of smaller degrees of astigmatism within the framework of cataract surgery. It may be combined with laser ablation or implant surgery in one session or as a staged procedure. This chapter presents the principles and techniques of incisional keratotomy. This knowledge is particularly valuable when working in developing countries with limited access to refractive laser surgery or toric intraocular implants. Likewise, in developed countries, the introduction of femtosecond (FS) lasers into corneal and cataract surgery rekindled interest in this technique assuming more predictable results by eliminating the surgeon factor.
As early as 1885, Schiötz described corneal incisions as a means of treating astigmatism after cataract extraction. In 1898, L. J. Lans systematically examined the astigmatic effect of incisions in rabbit eyes and described many principles that are still accepted today. In 1981, Luis Ruiz noticed that five transverse incisions bounded by “pseudoradial” incisions on either side of the optical zone (OZ) resulted in an extremely large effect in correcting astigmatism, much larger than could be achieved by either transverse or pseudoradial incisions performed separately. He called this procedure trapezoidal keratotomy for the correction of astigmatism , which was later renamed the Ruiz procedure .
In performing this correction, the meridian perpendicular to the one in which the Ruiz procedure was performed (the uninvolved meridian) became steeper as the involved meridian became flatter ( Fig. 24.1 ). The degree of steepening of the uninvolved meridian was directly proportional to the length of the transverse elements of the Ruiz procedure. In other words, the total correction of astigmatism was equal to the net sum of the flattening of the meridian in which the Ruiz procedure was performed added to the steepening of the flatter (uninvolved) meridian.
Success and Failure of Radial Keratotomy
Radial keratotomy (RK) was the first refractive procedure and thus the beginning of refractive surgery as a subspecialty of ophthalmology ( Fig. 24.2 , ). It was the most common form of surgical correction of myopia from the 1970s through the early 1990s. Important lessons can be learned from its history: Early reports from Fydorov and media coverage were quite enthusiastic. However, a multicenter prospective clinical trial with a long-term follow-up, called the Prospective Evaluation of Radial Keratotomy (PERK) study, revealed severe disadvantages of this technique. Although results were acceptable at 1 year and 5 years after RK, there was a significant loss of uncorrected and corrected visual acuity after 10 years, and a continuous hyperopic shift was observed. This hyperopic shift was also demonstrated in other studies; thus it was established as an inherent side effect of RK. As this side effect has not stopped the use of the procedure, knowing this complication is important to every ophthalmologist. Patients treated many years ago may now present with diurnal fluctuating vision, an irregular and instable hyperopic astigmatism and corresponding loss of visual acuity and changes of the corneal architecture ( Figs. 24.3 and 24.4 ).
Astigmatic Keratotomy
Astigmatic keratotomy (AK) when performed correctly is a lot safer and more predictable than RK. Because the spherical equivalent of a patient who undergoes an AK becomes more hyperopic while the spherical component of the refraction becomes more myopic, a solid understanding of these terms is essential to comprehend the mechanics of AK. Hoffmann coined the descriptive term coupling to describe the ratio of the flattening of the principal (steeper) meridian to the steepening of the flatter meridian.
Fyodorov developed parallel incisions for the correction of astigmatism, which were unpredictable in the higher ranges of astigmatism and gave way to “T-cuts,” or flags that were staggered along the radial incisions on one or either side of the OZ. A T-cut had a predictable effect whether it was performed on only one side or on both sides of the visual axis. The Ruiz procedure always had to be performed on both sides of the visual axis.
The presence of a transverse incision made AK much more predictable, but the intersection of transverse and radial incisions could lead to delayed corneal healing and epithelial recurrent erosions, especially when metal blades were used. Soon, most surgeons tried to avoid joining transverse and radial incisions.
Currently, the most commonly used AK patterns are the arcuate and transverse incisions (T-cut), with or without a peripheral radial incision ( Figs. 24.5A–C ) and an OZ diameter of 6 to 7 mm. Up to about 4 diopters (D) of astigmatism can be corrected by a pair of T-cuts, one on either side of the visual axis.
The correction of astigmatism by means of wedge resection has been investigated since the 1960s and 1970s by two of the great corneal surgeons, Jose Barraquer and Richard Troutman.
Lindstrom et al and Friedlander et al have investigated the effect of the Ruiz procedure on cadaver eyes. These studies tended to show a 150% to 200% greater effect than the in vivo procedure (common for cadaver eyes following incisional keratotomy); any generalizations from cadaver eyes to live patients were difficult, at best.
Merlin suggested that better results are possible with arcuate rather than straight astigmatic incisions (see Fig. 24.5C ). This idea was repopularized by Lindstrom. Arcuate incisions less than 45 degrees tend to act like straight ones, but the excessive gaping and subsequent unpredictability caused by arcuate incisions in the 75- to 90-degree range are undesirable.
Straight transverse incisions appear rectilinear only when viewed perpendicular to the corneal surface ( Fig. 24.6A and B ). Using a ball as a substitute cornea, it is easy to visualize T-cuts in a three-dimensional view and to understand that these incisions are actually curvilinear because they follow the curved corneal surface. Thus these “straight” T-cuts are really a segment of a great circle around the imaginary corneal sphere, imparting a consistency of action and promoting coupling by gaping less at the end of the incisions. Only when viewed in two dimensions, instead of three, do arcuate incisions appear to be a favorable pattern relative to the center of the cornea, the visual axis. Because all points of an arcuate incision are equidistant from the center of the cornea, an equal amount of wound gape occurs. This equal and large degree of gaping tends to increase irregular astigmatism and makes coupling less predictable.
Limbal Relaxing Incisions
Limbal relaxing incisions (LRIs) are a type of arcuate incisions made at the limbus to correct low degrees of astigmatism. These cuts are also called peripheral corneal relaxing incisions (PCRIs). LRIs can be considered safe and effective for correcting astigmatism of up to 2.5 D. A major advantage of LRIs is that night vision problems, which may be associated with incisions in the corneal midperiphery, are avoided. This is because LRIs are made in the periphery near the limbus and most patients’ pupils will not dilate that wide, eliminating night glare. LRIs can be performed with cataract surgery if an astigmatic error was preexisting, allowing the simultaneous correction of a patient’s astigmatism. However, if astigmatism has been induced by cataract or corneal surgery, the incisions can be made months after the initial surgery to improve uncorrected visual acuity (UCVA) to acceptable values.
Different nomograms for LRIs exist. Although corneal thickness varies by patient, the most preferred incision depth is 600 µm. Nevertheless, taking a pachymeter measurement at the site of the cut is recommended to prevent accidental perforation into the anterior chamber. Table 24.1 shows the nomogram of Wang et al. Based on a corneal diameter of 11.5 mm, a chord length of 45 degrees equals 4.5 mm and a chord length of 60 degrees equals 6.0 mm. It is applicable for a refractive cylinder equal to 0.75 D stable within plus or minus 0.50 D on manifest refraction (MR) at least 2 weeks apart, and a mean spherical equivalent cycloplegic refraction within plus or minus 0.75D of emmetropia. Corneal disease that might interfere with corneal wound healing should be ruled out. The length and number of LRIs are chosen based on age and refractive astigmatism. Incisions are centered around the plus axis of the cylinder of the manifest refraction. To ensure correct centration of LRIs, two approaches may be used: (1) When available, the meridional location of prominent landmarks on the conjunctiva or limbus can be noted and drawn relative to the 6 o’clock and 12 o’clock positions. (2) If a clear landmark is not evident by slit lamp biomicroscopy, the corneal and conjunctival epithelia at the limbus at the 90 and 270 semi-meridians are marked with a Sinskey hook stained with gentian-violet dye.
| Astigmatism (D) | Age (Years) | Number | Length (Degrees) |
|---|---|---|---|
| With-the-Rule | |||
| 0.75–1.00 | < 65 | 2 | 45 |
| ≥ 65 | 1 | 45 | |
| < 65 | 2 | 60 | |
| ≥ 65 | 2 | 45 | |
| < 65 | 2 | 80 | |
| ≥ 65 | 2 | 60 | |
| Against-the-Rule/Oblique | |||
| 0.75–1.00 | All | 1 | 45 |
| 1.25–2.00 | < 65 | 2 | 50 |
| ≥ 65 | 2 | 40 | |
| > 2.00 | < 65 | 2 | 55 |
| ≥ 65 | 2 | 45 | |
* Based on a corneal diameter of 11.5 mm; a chord length of 45 degrees = 4.5 mm and 60 degrees = 6.0 mm).
Intraoperatively, a degree gauge is aligned with the 90 and 270 semi-meridians, enabling identification of the surgical meridians. Incisions are placed in the peripheral cornea just inside the anterior insertion of the conjunctival vessels with a guarded diamond knife set at a depth of 600 µm. Care must be taken to avoid the corneal flap in eyes previously treated with laser in situ keratomileusis (LASIK). Patients should be monitored after 1 day; 1 week; and 1, 3, 6, and 12 months.
Mechanism of Action
Although the Ruiz procedure was too complicated for routine astigmatic cases, it allowed a great deal to be learned about the mechanics of AK. To investigate the mechanism of action of an AK, let us consider a mini-Ruiz procedure performed to correct 6 D of astigmatism. The patient’s refraction is plano −6 × 180 and the K readings are 40 D @ 180 and 46D @ 90 ( Fig. 24.7 ). Remember, even though the Ruiz procedure is highlighted in this example, the corneal mechanics described are valid for any form of AK.
The surgeon must confirm that the astigmatism is regular, that is, that the patient does not have irregular astigmatism. Regular astigmatism is an astigmatism that can be corrected by a cylindrical lens, that is, an astigmatism in which the steeper meridian and flatter meridians are perpendicular to each other ( Fig. 24.8 ). The cause of irregular astigmatism may be an abnormal epithelium (with normal stroma), as in punctate keratopathy, or an abnormal stroma (with normal epithelium), as in keratoconus or stromal injury and scarring. AK corrects only regular astigmatism.
Because of its high magnification and very thin mire, manual keratometry was the most accurate method of determining keratoconus and irregular astigmatism for a long time. However, manual keratometry requires an experienced observer. Over the past several years, automated topography and tomography examinations have become the standard of care as screening methods for keratoconus because of their ease of use, colorful permanent display, and technician-oriented approach. In addition to a topographic representation of anterior corneal curvature and its deviation from a perfect sphere, these modern devices also provide a map of the posterior corneal surface (endothelium). Subtle changes in this posterior float may precede anterior surface alterations and may be helpful in determining very early stages of keratoconus ( Fig. 24.9 ).
The coupling ratio for a specific AK has to be derived empirically through experience. A surgeon cannot calculate or divine a coupling ratio. Also, the coupling ratio is unpredictable when an AK is performed following a corneal transplant because the encircling scar can change corneal dynamics significantly.
Let us return to our patient with a refraction of plano −6 × 180. The flatter meridian of the cornea corresponds to the “plano” term of the refraction, and the “6” corresponds to the steeper meridian of the cornea (see Fig. 24.7 ). This should seem correct to the surgeon because a steeper meridian creates myopia by focusing light in front of the retina. The plano term of the refraction indicates that light passing through this meridian of the cornea is focused in a line (not a point, because this is an astigmatic cornea) on the retina.
With a refraction of plano −6 × 180, the spherical equivalent (sph + 1/2 cyl) is −3 D and the spherical component of the refraction is plano. The spherical equivalent represents the single best representation of the patient’s refractive status. A refraction of plano −6 × 180 can be transposed to the plus cylinder form as −6 + 6 × 90. An AK should be performed on the steeper meridian of the cornea, which usually corresponds closely to the plus axis of the refraction.
What happens to the cornea and the patient’s refraction? Because there is a coupling ratio of 2 : 1, we know that 6 D of astigmatism will be corrected with two “units” of flattening for every 1 unit of steepening.
Consider the original K readings. The steeper meridian started at 46 D and will be flattened by 4 D, ending up at 42 D. The flatter meridian started at 40 D and will be steepened by 2 D, ending up at 42 D (see Fig. 24.7 ). The patient has no astigmatism. But how has the patient’s refractive status changed?
Before the surgery, the 40 D (flatter) meridian was focusing light on the retina. Now, it has steepened to 42 D. Therefore the eye must be more myopic by 2 D. Similarly, the steeper meridian of 46 D is now flatter, thereby focusing light closer to the retina by 4 D, the amount it has flattened. The patient’s refraction is −2 sph (see Fig. 24.7B ).
It may seem enigmatic at first, but it is totally predictable that the patient’s spherical equivalent has become more hyperopic (−3 D to −2 D) following AK. The minus cylinder has been removed by flattening the cornea faster than it could steepen—remember, that is what a 2 : 1 coupling ratio means. The myopic component has become more myopic, even though the spherical equivalent has become more hyperopic.
If the coupling ratio were 1 : 1 (which it is not), the principal meridian of the patient’s cornea would flatten at the same rate as the secondary meridian steepened. The K readings would have become 43 D and the postoperative refraction would have ended up −3 sph; that is, the steeper meridian would flatten by 3 D and the flatter meridian would steepen by 3 D in order to correct the 6 D of astigmatism. Postoperatively, the spherical equivalent would remain unchanged from its preoperative value of −3 D.
Similarly, a coupling ratio of 3 : 1 should leave this patient with a refraction after AK of −1.50 D. It is interesting that the variation between an assumption of 2 : 1 coupling and 3 : 1 coupling is only 0.50 D, whereas the difference between a coupling ratio assumption of 2 : 1 and 1 : 1 is 1 D, a twofold increase ( Table 24.2 ).
| Coupling Ratio | Flatter Meridian (D) b | Steeper Meridian (D) | Postoperative Refraction |
|---|---|---|---|
| 1 : 1 | 40 → 43 | 46 → 43 | −3.00 sph |
| 2 : 1 | 40 → 42 | 46 → 42 | −2.00 sph |
| 3 : 1 | 40 → 41.5 | 46 → 41.5 | −1.50 sph |
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