Chapter 17 – Radial and Astigmatic Keratotomy
KERRY K. ASSIL
MARK A. ROTHSTEIN
The Dutch ophthalmologist Snellen (1834–1908) proposed, in his 1869 thesis on the surgical correction of astigmatism, that incisions within the steep corneal meridian might affect astigmatic magnitude. In 1885, Schiøtz, a Norwegian ophthalmologist, confirmed Snellen’s hypothesis when he reported a significant degree of corneal flattening in a cataract patient after the placement of a corneal incision tangential to the steep meridian. Nearly a decade later, in 1894, Bates made the seemingly casual observation that in patients who had corneal scars, flattening of the corneal surface occurred along the meridian of tangential scars.
Soon thereafter, in 1896, the Dutch ophthalmologist Lans conducted extensive studies in rabbits and demonstrated that anterior surface corneal incisions resulted in corneal flattening within the meridian of the tangential incisions. Lans further demonstrated that deeper and longer incisions are associated with greater flattening and that steepening occurs in the meridian 90° away from the tangential incision.
The work of these early ophthalmic pioneers was largely ignored until the late 1930s, at which time Sato, in Japan, noted that patients with keratoconus who experienced breaks in Descemet’s membrane developed significant corneal flattening. In 1953, Sato et al., who then recognized only part of the role of the corneal endothelium, reported a comprehensive study of surgical approaches toward corneal flattening by the administration of numerous radial incisions on both the epithelial and endothelial corneal surfaces.
Nearly two decades later, in 1972, the Russian ophthalmologists Beliaev and Ilyina demonstrated that externally placed radial incisions limited to the anterior corneal stroma also resulted in a flattened cornea. Their contemporaries, Durnev and Fyodorov,    reported that variation in the predetermined size of the central clear zone produced surgical variation that appeared to titrate the effect of the incisional procedure. Incisional keratotomy is now typically limited to astigmatic keratotomy at the time of cataract surgery, or two-incision radial keratotomy for patients with low-grade myopic astigmatism who are not good candidates for laser refractive surgery.
Radial Keratotomy Technique
Radial keratotomy predictive factors, surgical technique, instrumentation, and adjunctive technology continued to evolve during the 1980s and 1990s.          Solid-state ultrasonic pachymeters are used widely to provide surgeons with precise corneal thickness measurements, and coaxial microscopes are used to enable more precise determination of the surgical centration site and for diamond blade calibration.
It was recognized that age is a significant variable, accounting, on average, for slightly less than 1D of refractive effect per decade of age. It is also recognized that centripetally directed incisions (uphill, or Russian, method; Fig. 17-1 ) provide consistently deeper incisions than the centrifugal incisions (downhill, or American, method; Fig. 17-2 ) used in the surgical protocol of the Prospective Evaluation of Radial Keratotomy (PERK) study.     As described later in this chapter, a more refined incisional technique combines the safety of the centrifugal method with the efficacy of the centripetal method for radial keratotomy incisions ( Fig. 17-3 ). This radial keratotomy procedure, known as the combined technique (Genesis method), optimizes safety and efficacy for the performance of radial keratotomy.      The ideal combined incision incorporates the advantages of both the Russian and American techniques; results in a safely performed, uniformly deep incision with slight undermining of the optical zone; and yields maximal corneal flattening for a radial incision.
CORNEAL WOUND HEALING AFTER INCISIONAL KERATOTOMY
The basic principles of corneal wound healing help explain many of the resultant effects of incisional keratotomy.
Phases of Normal Epithelial and Stromal Wound Healing
Immediately after an incision into the corneal stroma, an initial wound gape occurs that effectively generates new surface area, with a resultant central corneal flattening. Subsequently, the corneal epithelium and stroma undergo, over a period of a few hours to several years after surgery, a series of physiological and anatomical changes. These characteristic transformations of the normal wound healing process may be classified into one of the following phases: epithelial, stromal, cross-linking, and remodeling. These phases of wound healing occur in a chronological sequence of early postoperative (12–48 hours), intermediate postoperative (1–6 weeks), and late postoperative (2–6 months) periods ( Table 17-1 ).
The early epithelial stage of wound healing lasts 12–48 hours. As new basement membrane is deposited, the surface epithelium begins to slide and replicate, resulting in the formation of an epithelial plug that fills the cavity ( Figs. 17-4 and 17-5 ). This concludes the epithelial phase of wound healing, although maturation of basement membrane adhesion complexes may require up to 6 weeks.
The stromal phase, also known as the keratocyte phase, lasts for several weeks ( Fig. 17-6 ). The most important part of this phase is the migration of activated keratocytes into the wound. These keratocytes transform into myofibroblasts, which behave like smooth muscle cells; they serve to bridge the gap with secondary contraction and thus help to reapproximate the wound margins and pull the wound closed. The transformed myofibroblasts also synthesize and secrete
Figure 17-1 Centripetal (Russian) radial incision technique.
Figure 17-2 Centrifugal (American) radial incision technique.
Figure 17-3 Combined technique (Genesis incision). Initial corneal penetration of the diamond knife tip: It is important to enter at a slightly oblique angle, pause to ensure appropriate epithelial tissue penetration, and slightly undermine the optical zone before orienting the diamond blade perpendicularly and commencing the centrifugal (downhill) incision. Centripetal component of combined incision technique: The completed centrifugal (shallow) incision is followed by reversal of the incision direction in a centripetal motion, which creates a deeper second incision guided within the initial (shallow) groove. Completed combined technique incision: Note the relative depth and limits of the combined centrifugal and centripetal cuts.
TABLE 17-1 — PHASES OF WOUND HEALING IN CORNEAL EPITHELIUM AFTER INCISIONAL KERATOTOMY
Epithelial Phase (keratocyte)
Stromal Phase and Remodeling Phase
Early (12–48 hours)
Intermediate (1–6 weeks)
Late (2–6 months and beyond)
Basement membrane synthesis
Nonsteroidal anti-inflammatory drugs and corticosteroids diminish wound healing by blunting the inflammatory response
Dipivefrin and pilocarpine stimulate regression of effect while pressure patching diminishes regression of effect
Vitamin C promotes collagen cross-linking
new collagen. This new tissue gradually extrudes and displaces the epithelial plug and serves as a permanent spacer; this maintains the corneal flattening achieved by the original wound gape at the time of the radial incision.
CROSS-LINKING AND INITIAL STABILIZATION PHASE.
The collagen forms cross-links over a period of several months, which stabilizes and strengthens the wound and secures it in its partially contracted position.
REMODELING AND STRENGTHENING PHASE.
The final phase of wound healing, the remodeling phase, lasts for many months after surgery ( Fig. 17-7 ). Residual collagen synthesis and breakdown, as well as continued collagen cross-linking, occur during this phase. These processes help to further strengthen the wound. Over the ensuing years, if mechanical trauma occurs, such as contact lens wear, chronic eye rubbing, or insufficient collagen cross-linking, the collagen layers may slip past one another, stretching the wounds and causing a hyperopic shift.
Postoperative Side Effects Related to Wound Healing
The previous section helps explain many of the side effects that follow radial keratotomy ( Table 17-2 ). A common side effect is early visual fluctuation in the perioperative period, possibly as a result of overnight wound edema associated with lid closure and
Figure 17-4 Early epithelial wound healing, 12–24 hours. After an incisional wound to the epithelium and stroma, the epithelial cells migrate and replicate and move down into the groove during the first 12–24 hours after the incision.
Figure 17-5 Early epithelial wound healing, 24–48 hours. A complete epithelial plug forms within the wound 24–48 hours postincision.
wound expansion (relative gaping) and a greater degree of corneal flattening during the early-morning hours. As the corneal wound becomes more detumescent over the ensuing hours, the cornea steepens, with a regression toward myopia. Besides irregular astigmatism and altered asphericity, the phenomenon of glare may, in part, be associated with both edema and the prominent nature of the wound in the early period.
The early regression of effect is probably secondary to the keratocyte phase, as myofibroblast contraction provides partial reapproximation of the wound margins. These early responses tend to resolve as the wound matures. Late progression of effect may be secondary to collagen slippage. The interindividual variability in outcome after radial keratotomy thus may be accounted for, in part, by the variability in wound healing and the effect of age on the wounds.
To maximize surgical outcome predictability, surgeons should adopt a standardized surgical protocol based on adjunctive technologies, standardized equipment, and instrumentation.
To achieve reliable corneal pachymetry readings, intraoperative, real-time, solid-state ultrasonography is invaluable. The optimal ultrasonic pachymeter design includes a continuous display monitor, data memory, and solid-state probe tip that can provide
Figure 17-6 Stromal phase of wound healing, 2–6 weeks. Keratocytes migrate into the wound cavity and then transform into myofibroblasts, which help to pull the wound closed while collagen is deposited and the epithelial plug is displaced.
Figure 17-7 Remodeling phase of wound healing, 2–6 months. This late phase of stromal healing includes the synthesis, breakdown, and cross-linking of collagen, which results in overall wound remodeling and strengthening.
sequential measurements without a foot pedal. It is advisable to conduct paracentral screening pachymetry during the preoperative evaluation to supplement intraoperative measurements and thus establish a rough map of corneal thickness.
Furthermore, to reduce perforation risk, intraoperative pachymetry for radial keratotomy is conducted at a 3?mm central clear zone (1.5?mm from the surgical centration point), over both the temporal and the thinnest paracentral cornea (as determined by screening pachymetry). These two loci often coincide.
Modern diamond knife blades generally are of high quality. However, it may be valuable to select a knife design that includes a protective housing to prevent blade chipping. Although diamond blades of thinner width are preferred because of less tissue resistance, a diamond blade that is too thin may be more prone to damage. Thicker diamond blades (=200?mm) may generate greater resistance and more variable depth of tissue penetration over the incision course. Additionally, incisions made using thicker diamonds could theoretically result in more prominent scarring. Although the authors’ clinical experience supports this concept, it has not been documented clinically in a formal study.
The angle formed by the radial and enhancement diamond tip is 35–45°. A more acute angle using a pointed diamond blade renders the centrifugal incision susceptible to meander and the blade susceptible to chipping. More important,
TABLE 17-2 — POSTOPERATIVE OBSERVATIONS RELATED TO PRINCIPLES OF WOUND HEALING
Wound Healing Principle
Cross-linked collagen not yet present in the wound
Fresh wounds are associated with edema, which may cause light scatter
Partial reversal of wound gape
Myofibroblasts partially reapproximate wound margins
Progressive wound gape associated with mechanical trauma
Insufficient collagen cross-linking
Varied degree of postoperative regression
Diverse biomechanical properties of individual cornea
Variations in overall wound healing response
diamond knife blades used at angles greater than 45° meet increased resistance on the initial insertion into corneal stroma, making rapid penetration to the desired depth more difficult to achieve.
The diamond blade design employed in the combined radial keratotomy technique ( Fig. 17-8 ) enables the surgeon to start the incision at the central clear zone, extend out toward the limbus, and then reverse direction back toward the central clear zone without invading that zone (see Fig. 17-3 ). The 45° angled margin (the “front” surface) that serves as the centrifugal cutting component is sharpened along its entire length. The vertical margin (the “back” surface) is ground sharp to a cutting edge for a distance of only 250?µm from the blade tip (see Fig. 17-8 ). The blunt, superficial segment of the knife’s vertical margin prevents unwanted invasion of the central clear zone margin during the final centripetal return to the central clear zone (see Fig. 17-3 ). This diamond knife design was selected based on vector force analysis to optimize each of the two separate incision components, allowing the surgeon to incorporate the benefits of each into the combined incision technique. The ideal features of the centrifugal method—a linear groove and minimal risk of central clear zone invasion—are thus combined with the ideal features of the centripetal method—a uniformly deeper incision (for reproducible corneal flattening) and reduced risk of globe penetration.
The diamond knife footplate is designed to maintain the diamond blade tip at a relatively uniform stromal depth over a broad range of angular deviation of the knife (relative to the corneal surface), which minimizes incision depth variations that can result from undesirable movement of the surgeon’s hand. With knife footplates that make minimal radial corneal contact, the blade tends to maintain a constant and uniform depth within the stroma, even if the surgeon’s hand rocks radially ( Fig. 17-9 ). The footplates also are relatively broad to provide maximal lateral support ( Fig. 17-10 ). As the surgeon’s hand rocks from side to side, the footplates exert lateral compression on the cornea and thus maintain constant and uniform stromal penetration (see Fig. 17-10 ).
Footplate tips are curved and smoothly polished and thus offer minimal resistance against a dry epithelium. Finally, diamond knife footplates enable the diamond and cornea to be viewed simultaneously, a feature referred to as having a low profile. Footplates with such a combination of features are referred to as being of universal design.
Figure 17-8 Diamond knife blade design for combined (Genesis) radial incision technique. The sharpened cutting edge extends only 250?µm along the vertical edge from the pointed tip; the remainder of the vertical margin remains blunt. The angled margin of the knife is sharpened to provide a cutting edge along its entire length.
Footplate spacing is a significant variable that affects incision depth. A relatively wide space between footplates may cause anterior bowing of the cornea beneath the diamond, which results in relatively deeper blade penetration. Conversely, more closely spaced footplates may cause posterior corneal displacement, which results in relatively shallow incisions. Thus, footplate design features are significant determinants of incision efficiency and precision.
When an experienced keratorefractive surgeon encounters a patient who has a significant degree of astigmatism (>1.75D), it is reasonable to perform simultaneous radial and astigmatic keratotomy. In such a case, the spherical equivalent may be used to determine the extent of radial incisions (aiming for mild undercorrection), and the tangential (or arcuate) incisions are chosen to eliminate approximately two thirds of the astigmatism. In patients who have less than 1D of astigmatism, the radial marks are positioned to include the astigmatic meridian, because even a radial incision tends to provide slightly preferential flattening along a steep axis. In patients who have 1D or more of astigmatism, the steep axis of astigmatism is not incised radially so that if simultaneous or subsequent astigmatic keratotomy is required, that meridian is readily available.
Numerous valid protocols are available for the safe and effective delivery of incisional keratotomy. Since a review of each is beyond the scope of this chapter, the authors’ personal perspective is offered to provide the reader with a clear overview.
IMMEDIATE PREOPERATIVE PROTOCOL
Although there is no conclusive clinical evidence that prophylactic topical antibiotics diminish or prevent the incidence of keratitis, the low cost and convenience of these agents make them a good investment for the possible prevention of infectious complications. The selected antibiotics should have broad-spectrum efficacy against both gram-negative and gram-positive organisms. They may be administered approximately 30 minutes before the operation. Prophylactic antibiotic drops should not be given before the day of the operation, because they may enhance the survival and proliferation of resistant microorganisms, including fungi.
It may be beneficial to administer pilocarpine 1%, because the miotic pupil enhances the patient’s ability to view the operating microscope relatively undisturbed by the intensity of the filament light. Although the miotic pupil is often displaced nasally,
Figure 17-9 Diamond knife footplate. Footplate designed for minimal epithelial contact in the radial direction. These footplates counteract the effects of radial rocking of the surgeon’s hand (as shown by the directional arrow), such that depth of stromal penetration of the diamond blade is unaffected.
Figure 17-10 The cornea, diamond knife, and footplate. The broad area of epithelial contact in the lateral direction is shown. Action of a diamond knife footplate: the knife position shows that greater epithelial contact laterally counteracts the effects of radial rocking of the surgeon’s hand (as shown by the directional arrow), such that a uniform depth of stromal diamond blade penetration is maintained.
this location may align more closely with the physiological visual axis. Hence, the miotic pupil may serve to reassure the surgeon of the relative accuracy of visual axis determination.
Topical lidocaine (lignocaine) 4%, two successive drops with a 10-minute interval, plus tetracaine (amethocaine) 0.5%, also two successive drops with a 10-minute interval, provide effective topical anesthesia. To minimize epithelial toxicity and desiccation, direct application of the anesthetic drops onto the corneal epithelial surface is avoided and the corneal surface is periodically moistened.
A sedative (e.g., diazepam 5?mg orally, which may be repeated once if needed) serves not only as a significant anxiolytic but also as a muscle relaxant so that the eyelid speculum does not become unduly bothersome. It is optimal to administer the oral sedative approximately 20 minutes before the surgical procedure.
PREOPERATIVE PATIENT PREPARATION
Visual Axis Determination and Marking.
A corneal light reflex used to guide procedure centration serves only to approximate the physiological visual axis location, since a coaxially aligned light reflex corresponds to the center of the corneal optical system and not the true visual axis. Studies have demonstrated this site to be associated most closely with the physiological visual axis. To overcome errors introduced by parallax, the PERK study group recommended that the patient fixate on the microscope light filament while the surgeon looks through one eyepiece and marks the epithelial surface that overlies the cornea inferior to the light filament reflex, opposite to the surgeon’s sighting (open) eye. For example, if the surgeon views through the right eye, the mark is placed on the left lower border of the microscope filament reflex ( Fig. 17-11 ). Globe decentration within the operative field also may introduce significant parallax-associated central clear zone marking decentration. An alternative technique is to use a coaxial light source mounted on or within the operating microscope optical system. This provides the patient with a light target that is nearly coaxial with the viewer. A third alternative is to center the epithelial mark over the entrance pupil.
For the visual axis to be marked, the administration of a drop of fluid over the corneal apex may enhance an otherwise dull corneal light reflex. A Sinskey hook is used to indent gently the epithelium that overlies the visual axis. If the epithelial indentation is not visualized readily, a Weck cell may be applied to the central epithelium, which enhances the central epithelial mark.
INTRAOPERATIVE CORNEAL PACHYMETRY.
The paracentral corneal thickness (1.5?mm from the visual center, at the 3?mm central clear zone) is measured at both the temporal site and the thinnest paracentral site, as previously established by pachymetry at the screening examination. Most often, this thinnest paracentral site coincides with the paracentral temporal (or inferotemporal)
Figure 17-11 Visual axis marked by light filament. Approximation of the visual axis relative to a paraxial light microscope filament image, which is the recommended method of visual axis marking adopted by the Prospective Evaluation of Radial Keratotomy study group.
site as the region closest to the anatomical corneal center. If the two sites do not coincide, the diamond blade is set to 100% of the thinner of the two intraoperatively measured sites using a calibration microscope.
If the range of screening paracentral pachymetry values between the thinnest and the thickest sites exceeds 75?mm, less refractive change may be achieved than predicted by the nomogram. In such cases, the incisions are lengthened, either by extension to slightly closer to the limbus or by reduction of the recommended central clear zone by 0.25?mm.
Diamond Knife Calibration and Adjustment.
The sterilized diamond knife blade is mounted onto the sterile mounting block of the calibration microscope with the knife footplates set to zero and the diamond either extended to 550?mm or set at the thinnest paracentral screening pachymetry reading. Once real-time, intraoperative ultrasonic pachymetry is obtained, the diamond-tip extension is adjusted to the newly selected level. In this way, a minor adjustment is generally all that is needed, and it can be carried out in only a few seconds.
POSSIBLE ADVANTAGES OF THE COMBINED TECHNIQUE.
The combined (Genesis) technique of radial incision was created to use the previously described special diamond knife design, which enables the surgeon to combine the ideal safety features of the centrifugally directed incision with the ideal efficacy of the centripetally directed incision (see Fig. 17-3 ). This centrifugal motion results in the formation of a groove of inconsistent depth that is relatively shallow at multiple locations. However, the critical function of this incision is to provide a linear safety groove to guide and confine the ensuing centripetal incision. At the termination of this centrifugal motion, the blade is maintained within the groove and the direction is reversed; the diamond blade is returned toward the central clear zone (see Fig. 17-3 ). This second motion deepens the groove evenly, but the blade design greatly reduces the risk of central zone invasion.
Incision sites are not routinely irrigated. When deemed necessary, irrigation may be carried out using a bent 27-gauge cannula with the stream directed from the central clear zone to the limbus; care is taken not to challenge Descemet’s membrane. Self-sealing perforations are not irrigated.
IMMEDIATE POSTOPERATIVE PROTOCOL.
At the termination of the procedure, diclofenac sodium and a triple anti-infective agent (dexamethasone–neomycin–polymyxin B) drops can be given, in addition to tobramycin, ciprofloxacin, ofloxacin, or norfloxacin (gram-negative coverage). This combination provides analgesic (diclofenac) and broad-spectrum antimicrobial coverage and a mild corticosteroid effect. The diclofenac drops are discontinued on postoperative day 2 to avoid masking early keratitis and to minimize the risk of a toxic response.
Astigmatic keratotomy has remained the modality of choice for the correction of astigmatism in patients undergoing cataract surgery. Although limbal relaxing incisions have become popular as well, they do not have the tensile strength of Descemet’s membrane that fortifies a corneal incision.
In selecting patients for astigmatic keratotomy, the surgeon must screen surgical candidates for myopic or planospherical equivalents. Astigmatic keratotomy does not, as a general rule, benefit patients who have hyperopic astigmatism in which the spherical equivalents are relatively unaffected or associated with a further hyperopic shift. Hyperopic subjects who experience extreme degrees of astigmatism and cannot tolerate both spectacles and contact lenses may, however, be candidates for astigmatic keratotomy.
Ideal candidates for astigmatic keratotomy, besides having a myopic or planospherical equivalent, are intolerant of contact lenses, because high-cylinder spectacles produce meridional magnification and distorted peripheral vision.
Astigmatic incisions, either arcuate or tangential, produce maximal flattening in the meridian of the incision when they are placed within 2.5–3.5?mm of the visual axis, which also is within the 5–7?mm optical zone. Incisions made closer than the 5?mm optical zone begin to simulate the effect of radial incisions, and incisions beyond 7?mm have a diminished effect on central corneal flattening. A 3?mm tangential incision is approximately equivalent in magnitude of meridional corneal flattening to a 45° arcuate incision at a 6?mm optical zone. As both types of incisions are lengthened beyond 3?mm, the arcuate incision provides progressively greater reduction of meridional corneal flattening relative to tangential incisions because a progressively greater disparity in effective optical zone size is encountered.
When arcuate incisions are lengthened, increasingly greater degrees of astigmatic correction are provided, up to an arc length of 90°. Beyond an arc length of 90°, no reliable additional flattening occurs. Stacking multiple rows of astigmatic incisions is neither productive nor advised. A pair of tangential incisions yields approximately 75% of maximal flattening, and two additional incisions yield approximately 25% additional flattening. Further incisions carried out at progressively smaller optical zones may result in global corneal flattening. Such incisions also may be associated with an increased incidence of irregular astigmatism.
VARIABLES AFFECTING SURGICAL OUTCOME.
Just as with radial keratotomy, the effect of astigmatic keratotomy is influenced by wound healing and patient age. Some studies do not support this observation, but they generally have not accounted for other major variables such as control for apparent cyclotorsion or centration of the surgery with respect to the visual axis.
The recommended nomograms for the performance of tangential keratotomy, with some modifications, are those constructed by Dr. Richard Lindstrom, Dr. Lee Nordan, and Dr. Spencer Thornton. One modification to Lindstrom’s nomogram (as recommended by Dr. Miles Friedlander) reduces the optical zone from 7 to 6?mm, which provides more predictable results. Lindstrom’s nomogram predicts the effect on a 30-year-old patient, with a calculated 2% per year age-related change in effect. Thus, on average, compared with the predicted effect for a 30-year-old patient, a 31-year-old patient experiences a 2% greater effect from the same incision, and a 29-year-old patient experiences a 2% smaller effect. Similarly, a 30-year-old patient achieves, on average, about half as much effect from the same incision as does an 80-year-old patient.
The standard distribution of effect that results from astigmatic keratotomy is greater than that observed for radial keratotomy.
For this reason, the keratorefractive surgeon generally does not aim to eliminate the entire astigmatic error using a single procedure. The surgical objective of correcting approximately two thirds of the astigmatic error is advised, because this avoids a 90° rotation in the astigmatic axis induced by overcorrection, a condition that patients find annoying.
When astigmatic keratotomy is planned, careful attention to quantitative keratography (corneal topography) may be valuable, because paired incisions based on either the standard keratometer measurements or the refraction alone may be inaccurate. If a manifest refraction fails to yield the patient’s potential acuity, irregular astigmatism may be present, and a topographical analysis is useful.
The preoperative surgical protocol for astigmatic keratotomy is similar to that for radial keratotomy. The visual axis is determined first, followed by selection of the appropriate optical zone, incision number, and length, as directed by the nomogram.
Axis of Astigmatism.
When disparity occurs between corneal topography and clinical refraction, the surgeon must rely on sound clinical judgment on a case-by-case basis. In such circumstances, if topographical analysis demonstrates orthogonal astigmatism, the surgeon may abide by the manifest refraction, as this provides the physiological combined (lenticular plus corneal) astigmatism. Alternatively, in cases of nonorthogonal astigmatism (when the two steep hemimeridians differ by any angle other than 180°), if the spherocylindrical reconstruction of the topographical pattern is consistent with the refraction, the incisions are placed as indicated by the topographical map.
For example, if refraction indicates a steep axis at 90°, and the topographical map demonstrates one steep hemimeridian at 75° and the other at 285°, this “bent bow-tie” pattern of true astigmatism has as its spherocylindrical counterpart a refractive axis of 90°. In such a case, the most precise surgical outcome is achieved by the placement of incisions centered at 75° and 285°.
Alternatively, if in this example the surgeon cannot reconcile the manifest refraction with the topographical analysis, the cycloplegic refraction is evaluated for confirmation of either the manifest refraction or the topographical analysis. The cycloplegic refraction is not used as a primary source for surgical decisions in astigmatic keratotomy; however, because in the setting of astigmatism, a cycloplegic refraction may include peripheral (nonphysiological) corneal and lenticular astigmatism.
Once the desired axis of astigmatic correction has been determined, this needs to be translated onto the cornea. Because the astigmatic axis is defined so carefully with the patient in an upright position without sedation or a lid speculum, one must not estimate the surgical axis intraoperatively with the patient in a supine position or sedated or with a lid speculum in place. Cyclotorsional rotation of the globe may occur and introduce significant error.
To control for these sources of error, with the patient seated at the slip lamp, epithelial marks are placed on either the vertical or horizontal axis. Using the slit beam for centration and with the contralateral eye covered, the patient fixates first on the slit-lamp light source from straight ahead. Fixation on the slit-lamp filament at eye level and from head-on provides a virtual image of the light filament, which falls at the center of the corneal optical system and closely approximates the visual axis. The patient is asked to view straight ahead through the light and onto the horizon to minimize accommodation.