Incisional Refractive Techniques: Radial Keratotomy, Astigmatic Keratotomy, and Limbal Relaxing Incisions

Kerry K. Assill, Andrea D. Border, John F. Doane, Scot Morris, and James A. Denning

CHAPTER CONTENTS

General Preoperative Considerations

General Surgical Considerations

General Postoperative Considerations

Radial Keratotomy

Astigmatic Keratotomy

Limbal Relaxing Incisions

Suggested Readings

Radial keratotomy (RK) and astigmatic keratotomy (AK) were the first refractive procedures to be practiced on a wide scale in North America. Although photorefractive RK and laser in situ keratomileusis have largely replaced these two procedures, they are still effective for low myopia and astigmatism and are occasionally used by refractive surgeons. Most of the concepts now used for refractive surgery were developed originally for RK and AK.

GENERAL PREOPERATIVE CONSIDERATIONS

Indications

low to moderate myopia and/or astigmatism
realistic patient expectations of surgical effect
patient age greater than 21 years

Patient Evaluation

documented medical history (see list of contraindications on pages 69–70)
family history (especially of keratoconus or rheumatoid arthritis)
measurement of distance and near uncorrected visual acuity (for statistical documentation, postoperative patient comparison, and legal considerations)
manifest refraction and, ideally, cycloplegic refraction [after discontinuation of rigid gas-permeable (RGP) contact lens wear for 3 weeks and soft contact lens (SCL) wear for 3 days]
evaluate for potential problems using corneal topography (e.g., keratoconus, irregular astigmatism, and contact lens warpage)
- Retest every 2 to 3 weeks until stable using corneal topography, after discontinuing contact lens wear (also helps determine the meridian and zone for arcuate and tangential incisions)
slit-lamp examination (rules out any preexisting corneal disease)
dilated fundus examination (rules out any macular or peripheral retinal disease)
serial pachymetry at the corneal apex and at eight paracentral meridia (from superotemporal to superior) to determine the thinnest paracentral zone [Be alert for serial readings differing more than 75 μm (Fig. 8-1).]
tonometry [establishes preoperative baseline intraocular pressure (IOP); increased IOP may result in greater than expected refractive results]
determination of ocular dominance (see Chapter 5)
patient education
- one-on-one conferences
- educational brochures
- videos
- ample opportunity for patient questions (and answers from knowledgeable personnel)
informed consent read by patient in its entirety and signed in ink; personally reviewed by surgeon with the patient and signed in the patient’s presence by surgeon

Figure 8–1 The eight paracentral meridia viewed serially by pachymetry from superotemporal to superior.

Patient Preparation

Administer broad-spectrum antibiotics for prophylaxis approximately 25 minutes before surgery.
Instill 1% pilocarpine.
- ensures maximum patient focus intraoperatively with high illumination
- better defines patient’s visual axis
Apply topical anesthetic but not directly to the corneal epithelium [recommended regime: 1 gtt of 4% lidocaine (Xylocaine; Abbott) every 10 minutes (×2), followed by 1 gtt of 0.5% tetracaine every 10 minutes (X2)].
Administer up to 20 mg of diazepam 20 minutes before surgery (acts as an anxiolytic and relaxes the muscles of the eye to make the lid speculum more comfortable).

GENERAL SURGICAL CONSIDERATIONS

Relative Contraindications

tendency toward aggressive wound healing due to potential minimized effect and a greater potential for future hyperopic drift
collagen vascular disease [propensity for stromal melt (keratolysis) following the procedure]
active infectious or inflammatory ocular conditions (e.g., blepharoconjunctivitis or ocular rosacea, which should be fully controlled before surgery)

Absolute Contraindications

unrealistic patient expectations of perfect vision or improved best corrected visual acuity (BCVA)
patient age less than 21 years
systemic disease (e.g., diabetes mellitus because of possible poor healing of corneal incisions)
recent unstable refraction greater than 0.5 D sphere or cylinder (within the previous 24 months)
keratoconus because of progressive ectasia
corneal warpage from contact lens wear or large, chronic eyelid masses like chalazia (discontinue lens wear and topographically document stable curvature before surgery)
pre-existing irregular astigmatism
severe retinopathy
- increased risk for severely compromised acuity in the future
- possible adverse effects on low contrast acuity from increased depth of focus
severe dry eye (keratoconjunctivitis sicca)
- increased risk for keratolysis
- greatly prolonged visual recovery
history of chronic herpes simplex or zoster keratitis because of the. increased risk for keratolysis (ideally should not be considered for surgery)

Equipment

a user-friendly computerized topographer

an operating microscope (with certain features)

a para-axial illumination source
an infrared filter (prevents corneal desiccation)
an ultraviolet filter (eliminates retinal phototoxicity)
a coaxial fixation target

durable optical zone (OZ) and radial markers machined from a single sheet of titanium (without soldered cross hairs or gold plating)

a corneal pachymeter (with certain features)

a solid-state probe
continuous readout
contact activation
a display monitor
enough memory for multiple readings

a combined-style diamond knife (with certain features)

a 45-degree-angle head

80 to 120 μm thick

retractable for safe handling

universal design footplates

a high quality diamond blade (Fig. 8-2)
a computer-programmable diamond knife with computer-driven motor (allows the pachymeter to communicate with the diamond to reduce microperforations and improve topographic profiles; now superceded by the excimer laser)

Figure 8–2 Diagram of a combined-style diamond knife.

Methods

Position the patient comfortably on the operating table with a folded towel or foam pillow under the neck to facilitate the chin-up position required for placement of superior incisions.

Patch the nonsurgical eye.

A trained, gloved, and masked technician may administer periocular povidone iodine solution (Betadine).

Wipe away excess solution.

Position the open, sterile eye drape over the patient’s face.

Immediately before beginning the actual procedure, instill 1 final drop of 0.5% tetracaine into the conjunctiva.

Position a lid speculum to allow adequate ocular exposure and avoid undue eyelid tension.

Use one of the three methods that follow to mark the exact center of the pupil (i.e., the visual axis).

Use the miotic pupil centering method.

Instill a miotic agent, such as 1% pilocarpine.
Mark the exact center of the pupil (miotic pupil centering).

Use the light filament reflex method (recommended by the PERK Study Group).

Physically mark the midpoint along the lower margin of the reflex on the cornea while the patient fixates on the microscope light filament.

Look through one ocular and mark along the opposite inferior border of the light filament reflex [e.g., if you fixate through the left ocular, the mark should be positioned along the lower right border of the filament reflex on the cornea (Fig. 8-3)].
Use the coaxial light source method.
- This method allows the patient to fixate on a less intense light during the procedure than does the light filament reflex method and is more precise.
- Coaxially mount a fixation light source between the patient and the surgeon within the operating microscope optical system.
Mark the visual axis as follows.
- When the lid speculum is in place, administer 1 drop of balanced salt solution (BSS) over the corneal apex to enhance the corneal light reflex.
- Use a Sinskey hook to carefully indent the epithelium overlying the visual axis.
- Mark the axis carefully because any deep epithelial defect can leave scarring in the visual axis.
- When the epithelial indentation is not easily visible, use a Week cell sponge to dry the central epithelium and enhance visualization.
- Clear excess pooling in the canthal region with a Week cell sponge (a dry field is important for detection of early microperforation).
- Avoid excessive dryness to decrease friction between knife footplates and corneal epithelium.
Identify and mark the OZ.
Perform corneal pachymetry.
- Soak the probe tip in hydrogen peroxide.
- Dry the tip with an alcohol wipe.
- Allow the tip to air dry.
- Position the probe tip exactly perpendicular to the epithelium at 1.5 mm temporal to the visual axis and at the thinnest paracentral point (determined by preoperative pachymetery screening).
- If pachymetry readings do not register, ensure that the probe tip is exactly perpendicular with the corneal epithelium or moisten the epithelium with a micro-drop of BSS.
- Usually the thinnest paracentral site corresponds with the paracentral temporal site, but if these two sites differ in thickness, set the diamond blade to 100% of the thinner of the two (use a calibrating microscope to set the blade).
- When the range of paracentral screening values (from thickest to thinnest) is greater than 75 μm, expect less refractive change than predicted by the nomogram, and lengthen the incisions slightly toward the limbus or reduce the recommended optical zone by 0.25 mm.
Calibrate and adjust the diamond knife.
- Sterilize and mount the diamond knife into the sterile mounting block of the calibrating microscope at the beginning of each procedure.
- Zero the knife footplates and either extend the diamond to 550 μm or set it to correspond with the thinnest paracentral screening pachymetery reading.
- After obtaining real-time intraoperative pachymetry readings, adjust the diamond tip accordingly.
- Make the final setting of the diamond knife blade 100% of the thinnest intraoperative pachymetery reading.
If consistent undercorrections result but without microperforations or incisions approaching the pre-Descemet’s region, set the diamond blade to a longer setting (in 10-μm or 2% increments) for subsequent incisional procedures on other patients.
- If consistent undercorrections occur with deep incisions or incisions that extend within 1 mm of the limbus, decrease the OZ diameter by 0.25 mm.

Figure 8-3 When looking through the left ocular, the surgeon marks the visual axis along the opposite inferior border of the filament reflex on the cornea.

GENERAL POSTOPERATIVE CONSIDERATIONS

Medications

1 drop neomycin/poly/dexamethasone (Maxitrol); 1 drop diclofenac sodium (Voltaren; Cibavison), 1 drop aminoglycoside, or 1 drop fluoroquinolone

1 drop Voltaren every 6 hours as needed for pain (first evening only)

1 drop Maxitrol every 6 hours for 5 days and then every 12 hours for 2 days only (same schedule for aminoglycoside or fluoroquinolone, which may be taken several minutes after Maxitrol)

2 oxycodone/acetaminophen (Percocet) tabs every 6 hours for immediate postoperative pain (optional)

Patient Instructions

Provide written instructions to the patient.
Instruct patient to go home and sleep for several hours and to take the drops as directed (see above).
Instruct patient not to rub his or her eyes.
Instruct patient not to use eye make-up for 2 weeks.
Instruct patient to avoid contaminated water (e.g., pools, hot tubs, showers) for 2 weeks.
Instruct patient to refrain from reading for the first day after surgery.
Instruct patient to call emergency telephone numbers if experiencing severe pain, discharge, increased redness, or swelling.
Inform patient about first follow-up visit.

Follow-Up

first postoperative visit (day 2)
- Measure uncorrected acuity.
- Perform a slit-lamp examination.
subsequent postoperative visits (weeks 1, 3, and 6 and months 3, 6, and 12)
- Schedule for late afternoon for corneal stability.
- Measure uncorrected visual acuity.
- Measure manifest refraction.
- Measure BCVA.
- Perform slit-lamp examination.
- Review patient progress and symptoms.

Enhancements

nonsurgical
- Nonsurgical intervention is possible if the degree of myopia is within 0.5 D of target.
- Begin with pressure patching (preferred) beginning at 1 week and continuing for 6 weeks only at night or continuously both day and night.
surgical
- Surgery is required if residual myopia is greater than 0.5 D.
- Provide additional incisions (cumulative total, ≤ 8) if the undercorrection is greater than 1.25 D, the OZ is a minimum of 2.75 mm, the limbal zone is a minimum of 0.5 mm, and there are fewer than eight original incisions.
- Lengthen existing incisions by decreasing the limbal clear zone or by decreasing the optical clear zone (if the undercorrection is 0.50-1.25 D, the OZ is >2.75 mm, the limbal zone is >0.5 mm, and there is significant astigmatism and eight original incisions).
- Deepen some or all of the existing incisions if they appear to be shallow at the slit lamp, the OZ is at the minimum, and there are at least eight original incisions.

RADIAL KERATOTOMY