Astigmatic Keratotomy for the Correction of Astigmatism 

Updated: Apr 06, 2017
Author: James Hays, MD, MBA, MHA; Chief Editor: Douglas R Lazzaro, MD, FAAO, FACS 

Overview

Background

The ideal refractive surgical procedure is simple to perform, inexpensive, and applicable to a wide range of ametropias. Astigmatic keratotomy (AK) is one such procedure. Astigmatic keratotomy is used to treat numerous refractive disorders, including congenital astigmatism, residual corneal astigmatism at the time of or following cataract surgery, post-traumatic astigmatism, and astigmatism after corneal transplantation.

Even with the extensive use of excimer laser vision correction platforms to treat refractive error (eg, photorefractive keratoplasty [PRK], LASIK), astigmatic keratotomy continues to be a valuable and versatile tool for the treatment of many eyes.

Early investigative surgeons of astigmatic keratotomy, Thornton, Buzard, Price, Grene, Nordan, and Lindstrom, demonstrated the efficacy, safety and reproducibility of refractive outcomes, and led, albeit over more than a decade, to the adoption of the procedure by the broader ophthalmological community.[1, 2, 3, 4, 5]

Within the past few years, much consideration has been given to an evolutionary variant of the procedure, the limbal relaxing incision (LRI). By moving the incision farther to the periphery, cataract surgeons can safely and predictably remediate mild to moderate amounts of regular astigmatism at the time of cataract surgery by performing this incisional technique, either by hand or by application of femtosecond laser technology.[6] Femtosecond laser offers several potential advantages over manual incision, including fully customizable and reproducible incision parameters, as well as increased safety and titration of effect via intrastromal ablations.

History of the Procedure

Astigmatic keratotomy procedures were first reported in the 1890s. Observations of scar-induced corneal flattening and attempts to rid postsurgical corneal transplant and patients with cataracts of unintended astigmatism[7] sparked the imaginations of early refractive surgeons.

The study of ametropia correction continued over the next 60 years, as surgeons learned to employ radial keratotomy (RK) incisions to decrease myopia and discovered that these radial incisions had little appreciable effect on the amount of astigmatism. However, when the direction of the incisions was turned 90°, a profound astigmatic effect occurred. Sato introduced the idea of coupling: tangential (or arcuate) incisions flatten the steep meridian while steepening the flatter meridian, following Gauss’ law of elastic domes.[8] This combination of flattening the steeper axis and subsequently steepening the flatter axis yields the total amount of astigmatism correction.

In the 1970s, Troutman extended the applicability of coupling in corneal transplant recipients by demonstrating that the donor-recipient interface acted, in essence, like new limbal architecture. Therefore, the same rules apply to incisions made inside the donor-recipient interface as to an untouched normal limbus.[9] Troutman’s work also included the development of the wedge resection for the treatment of very high astigmatism.

No discussion about keratotomies would be complete without reference to the Prospective Evaluation of Radial Keratotomy (PERK) study, which addressed only the effects of symmetrically placed radial incisions. In fact, no astigmatism correction was attempted. The PERK study demonstrated that radial incisions do not change astigmatism in a reproducible way.[10] It was not until later studies that the effects of transverse or arcuate incisions were investigated.

In the 1980s, Nordan’s early approach to astigmatic keratotomy was simply to employ straight transverse incisions that produced targeted corrections in the range of 1-4 D.[4]

Lindstrom developed his own arcuate transverse keratotomy technique and added a nomogram, which took into consideration the number and length of incisions, as well as the patient’s sex and age.[5] The Astigmatism Reduction Clinical Trial (ARC-T) study was born from these activities and showed that Lindstrom’s nomogram could produce predictable results.[3]

Thornton’s technique involved titrating results even further by placing paired arcuate incisions on a curve on the cornea, dictated by either a 7- or 8-mm optical zone size.[1] Others, including Chayez, Chayat, Celikkol, Parker, and Feldman, had recommended optical zone sizes as small as 5 mm at the expense of creating debilitating glare, asthenopia, and monocular diplopia.

Nichamin later developed an extensive nomogram for astigmatic keratotomy to be used at the time of cataract surgery; however, its utility has diminished owing to the rise in popularity of the toric intraocular lens (IOL).

More recently, femtosecond lasers have been used to create astigmatic keratotomy incisions after corneal transplantation and to create LRIs at the time of cataract surgery. These lasers employ sophisticated imaging systems that allow for precise control of incision locations and parameters (ie, depth, length, angle).[11] Further investigations will determine if the lasers prove superior to manual astigmatic keratotomy/LRI procedures.

Indications

Astigmatic keratotomy/LRI procedures can remediate or lessen astigmatism in numerous refractive presentations as either a stand-alone procedure or one that can be easily combined with other forms of surgery. This versatility, the simple surgical setup, and the production of predictable outcomes make this procedure a useful tool for all refractive surgeons, even with the advancement of laser surgery.

Astigmatic keratotomy/LRI surgery combined with or following cataract surgery is frequently used as an ancillary refractive procedure in patients presenting with topographical regular astigmatism at the time of cataract surgery. The addition of an arcuate incision or incisions when the patient exhibits 0.75-2.75 D of regular astigmatism improves the likelihood of attaining excellent uncorrected vision postoperatively.

An astigmatic keratotomy/LRI procedure becomes even more important when multifocal IOLs are chosen because satisfactory simultaneous uncorrected vision at distance and near can be obtained only with a nearly spherical cornea. At the time of this writing, toric multifocal IOLs are not available in the United States, so the need for concurrent astigmatic correction must rely on either manual or laser-assisted incisional placement to mitigate mild to moderate amounts of astigmatism.

A more traditional keratorefractive approach outside of cataract surgery involves using astigmatic keratotomy/LRI surgery in patients who exhibit mixed astigmatism. When a patient requests vision correction surgery and has a refractive error with a spherical equivalent approaching zero (eg, -1.00 + 2.00 X [any axis]), PRK or LASIK may not be necessary.

While an astigmatic keratotomy procedure may appear redundant to PRK or LASIK treatment before or after refractive surgery, synergy between the techniques may benefit some patients. For instance, patients who present with high astigmatism may find that the combined treatment of PRK/LASIK with astigmatic keratotomy may provide a more satisfactory visual result than PRK or LASIK alone. By first reducing high amounts of cylinder by 2-3 D with an LRI, lesser amounts of astigmatic laser correction are needed, allowing for the use of larger optical zone sizes, which ultimately provides for a smoother optical zone transition. This enhanced transition lessens the degree of nighttime glare and ghosting and provides for overall better vision quality. With regard to post-LASIK astigmatic keratotomy surgical interventions, performing an astigmatic keratotomy may be preferable to lifting a well-healed LASIK flap in patients who go on to develop significant astigmatism.

Astigmatic keratotomy also proves useful when treating irregular astigmatism following corneal transplant surgery. While most congenital astigmatism appears as regular (ie, the steep and flat meridians of astigmatism lie 90° away from each other), after corneal transplantation, one quadrant may be especially steep or flat in relation to its reflective counterpart. This is known as nonorthogonal astigmatism and can occur when a segment of the donor-recipient interface has healed too tightly or when the interface has inadvertently slipped. Astigmatic keratotomy, used in conjunction with high-quality corneal topography, allows for an individualized approach as surgeons identify and specifically treat these steep areas.

Relevant Anatomy

The cornea is a clear, dome-shaped surface that covers the front of the eye. The cornea acts as the eye's main refracting surface, supplying two thirds of the focusing power of the eye, or the equivalent of about 43 D[12] of power in the average human. While this transparent surface appears “simple” in nature, the cornea is actually a highly organized avascular tissue composed of the epithelium, the Bowman membrane, the substantia propria, the Descemet membrane, and the endothelium.

The epithelium, the outermost layer of the cornea, is composed of 5-6 layers of stratified squamous, nonkeratinized cells. This layer, which includes a basement membrane, makes up about 10% of the total corneal thickness; it is highly sensitive owing to thousands of nerve endings located within this layer. The epithelium exhibits excellent regenerative power.

The Bowman membrane lies directly beneath the epithelial basement membrane. The Bowman layer is acellular, containing randomly oriented collagen fibrils, which, when damaged, create scar tissue formation.

The central, and by far thickest, layer of the cornea, the substantia propria (stroma), makes up nearly 90% of the cornea's thickness. This layer is primarily composed of water (78%) and regularly arranged collagen I, III, V, and VII fibrils (16%). The unique size of the collagen fibrils, as well as their spacing and layer arrangement within the water substrate, allows for corneal transparency. Disruptions to this delicate architecture can cause loss of transparency and, subsequently, poor vision.[13]

The Descemet membrane is a thin basement membrane, measuring just 3-10 μm, and lies just below the stroma. Despite its thin presentation, it is a tough membrane, rich in type IV collagen fibers. The Descemet membrane acts as a defensive barrier against injury and infection. This layer is produced by the underlying endothelial cells and can be regenerated if injured.

The endothelium is composed of a single layer of simple, cuboidal, and hexagonal cells that line the inner surface of the cornea. Endothelial cells are derived from the neural crest during development and are thought to be nonregenerative in humans. The natural tendency of nutrient-rich aqueous fluid is to seep into the cornea stroma; the primary function of the endothelial layer is to transport stromal fluid back to the anterior chamber. While these cells have tremendous “engines” for doing so, endothelial cell loss occurs naturally over years, stressing the remaining cells. If disease, trauma, or dystrophy is introduced, the layer’s pumping capacity can be greatly reduced, causing a build-up of fluid in the stromal layer and affecting corneal clarity.[14]

The average central thickness of the human cornea is approximately 555 μm.[15] Normal corneas become thicker toward the limbus, with average values greater than 600 μm. Thickness can be measured with devices such as a pachymeter or optical coherence tomographer (OCT). Many astigmatic keratotomy/LRI nomograms advocate penetrance with a diamond knife or femtosecond laser to 85%-90% of total corneal thickness, as calculated intraoperatively from the thinnest corneal thickness measurement to be traversed by an arcuate incision.

For more information about the relevant anatomy, see Eye Globe Anatomy.

Contraindications

All surgical procedures may provide suboptimal outcomes. While the LRI procedure is thought to create less glare and optical artifacts than its predecessor, astigmatic keratotomy, the most common side effects remain overcorrection and undercorrection of astigmatism. Infection,[16] corneal perforation, and decreased corneal sensation are possible sequelae.

Patients with high astigmatism due to Terrien degeneration, Mooren ulcer, or any disease or dystrophy that produces peripheral corneal thinning should not undergo astigmatic keratotomy/LRI incisions owing to the progressive risk of corneal thinning and evolving astigmatism, potentially leading to perforation.[17]

Patients with chronic diabetes, chemical burn, or other causes of ocular surface disease should be approached with increased caution, as re-epithelialization problems after corneal surgery may ensue.

Caution should be exercised when considering an astigmatic keratotomy/LRI procedure in patients with connective-tissue diseases (eg, rheumatoid arthritis). Patients with extreme dry eye, whether related to rheumatoid arthritis or not , require close follow-up care if undergoing this procedure, as they are more prone to ocular discomfort, dryness, poor healing and potential thinning due to corneal melting.

Patients with astigmatism who previously underwent radial keratotomy may later present for astigmatic "enhancement." Astigmatic keratotomy/LRI surgery is a reasonable option in these patients, but the surgeon should take care when orienting the new incisions. The crossing of a radial incision with a transverse incision, even years after the initial procedure, may produce excessive and unwanted overcorrection. It is recommended to preoperatively map the faded RK incisions, identifying their location with useful landmarks. Since most RK incisions approach the limbus, surgeons should avoid crossing the RK incision with a long, uninterrupted astigmatic keratotomy/LRI incision. Instead, they should use multiple smaller astigmatic keratotomy/LRI incisions straddling the RK incisions to obtain the desired effect. As can be imagined, it is especially difficult to perform astigmatic correction through astigmatic keratotomy/LRI incisions in a patient who has undergone a 16-incision RK.

The potential benefits of astigmatism reduction must be weighed against the risks of the procedure on a case-by-case basis.

 

Workup

Imaging Studies

In order to produce accurate arcuate incisions, the use of corneal topographers and/or corneal tomographers is essential when visualizing the corneal landscape. Elevation maps can identify regular versus irregular astigmatism, the magnitude of the astigmatism, and its relationship with the overall contour of the anterior corneal surface. Fortunately, over the past ten years, both topographers and tomographers have become commonplace in ophthalmic practices, allowing for better preoperative and postoperative assessment of corneal contour.

Combined elevation and thickness measurements can be obtained from machines such as the Orbscan and Pentacam. Both technologies offer an added advantage over standard topographers: mapping the posterior corneal curvature, an area whose relevance had been overlooked for many years. Recently, some ophthalmic investigators have attributed improved refractive results when considering the contribution of the posterior corneal curvature to total corneal power.[18]

Since traditional astigmatic keratotomy (AK) nomograms are based on ultrasonic pachymetry measurements, one must correlate corneal thickness values measured with the Orbscan or Pentacam with those obtained from a standard ultrasonic pachymeter. Statistical differences between thickness readings from these machines have been reported.[19]

 

Treatment

Surgical Therapy

While surgical techniques for astigmatic keratotomy (AK) vary based on surgeon preference (eg, blade type, cut depth, cut length, location and number of cuts), the basic principles remain the same. The following description of a standard astigmatic keratotomy procedure is provided for the less-experienced surgeon. Individual modifications certainly are expected as the surgeon's experience increases.[20, 21, 22, 23, 24, 25]

Developing the surgical plan

A surgical plan includes a combination of patient data (eg, refractive history, corneal topography/tomography findings, pachymetry findings, patient goals) with appropriate nomogram selection.

The following astigmatic keratotomy tenets help to explain how nomograms are adapted for each patient: the longer the incision, the greater the effect; the smaller the optical zone, the greater the effect; the less uncut tissue under the knife tip, the greater the effect.

The nomogram to follow is intended for limbal incisions (an approximate 11-mm optical zone) when the astigmatic target is 2.75 D or less. The addition of an 8-mm optical zone is reserved for attempted corrections of more than 3.0 D.

The depth of each incision should be calculated as 0.02 mm less than the thinnest depth measured by pachymetry in the area of the intended cut. Each incision requires the surgeon to perform pachymetry in the affected area and to reset the blade for each incision accordingly.

Incisions are generally paired and placed across the steep axis, but the surgeon may vary the relative length of each half of the pair based on topography. For example, if the nomogram calls for an arcuate incision that traverses 100°, one incision may traverse a 60° arc and one incision may traverse a 40° arc, each straddling the steep axis, if topography shows asymmetrical astigmatism.

While most RK nomograms assume a nominal patient age of 30 years, it is appropriate for astigmatic keratotomy nomograms to have an older nominal age set at 50 years. As many astigmatic keratotomy patients also have cataract, this nominal age works for almost everyone. Revise incision lengths based on this metric: Decrease the total length of the cut by 1° for each year after 50 years and increase the total length of the cut by 1° for each year before 50 years.

The following nomogram is applicable for patients who present with congenital astigmatism, with astigmatism at the time of cataract surgery, or revision of astigmatism following cataract surgery. Do not use this nomogram in patients who present with astigmatism following corneal transplant surgery because posttransplant astigmatism should not be treated with a standard nomogram. Instead, posttransplant astigmatism should be corrected based on individualized corneal healing patterns and topographical analysis.

Standard nomogram forastigmatic keratotomy

Based on the correction of corneal astigmatism at the limbus (an approximate 11-mm optical zone) in a 50 year-old patient:

  • 1 D - 80°

  • 1.5 D - 100°

  • 2 D - 120°

  • 2.5 D - 140°

  • 3 D - 160°

Standardastigmatic keratotomysurgical plan examples

Example 1. A 35-year-old patient presents with 2 D of congenital corneal astigmatism. Topography demonstrates an asymmetrical bow tie, with greater steepness in one hemi-meridian. The standard nomogram advises 120° of incision length, which is increased by 15° because of the patient’s age, totaling 135°. A 60° incision may be paired with a 75° incision, with the longer incision placed in the steeper hemi-meridian.

Example 2. A 50-year-old patient presents with 2 D of congenital corneal astigmatism. On topography, a symmetrical bow tie appearance is present. A pair of 60° incisions are placed, based on the nomogram presented. See the image below.

Example 2. Routine astigmatic keratotomy. In this Example 2. Routine astigmatic keratotomy. In this image, a nasal astigmatic keratotomy is shown. This image was taken minutes after performing the incision. Note that the area around the cut stains lightly with fluorescein.

Higher levels of astigmatism

To correct greater than 3 D of corneal astigmatism, an additional 1 D of astigmatic correction may be achieved in some cases by adding a pair of 40° incisions at the 8-mm optical zone. Because refractive variability increases as the optical zone is decreased, correction of higher astigmatic levels may be better approached in conjunction with LASIK.

Combining astigmatic keratotomy with cataract surgery

When treating against-the-rule astigmatism at the time of cataract surgery, it may be adequate to perform a standard cataract clear-corneal temporal incision and to place an extended astigmatic keratotomy incision nasally. If greater correction is required, make the cataract incision in the Langerman style using a 600-µm groove. At the end of the surgery, simply extend the groove to the appropriate length.

Astigmatic keratotomy after corneal transplantation

The architecture of astigmatism after corneal transplantation is quite different than with naturally occurring corneal astigmatism. The donor-recipient interface creates a new “artificial” limbus, which is typically 8 mm in diameter. Variable wound healing and scar contracture at the new limbal interface can induce high levels of cylinder. Mismatch of the donor and recipient tissues can also create high levels of permanent astigmatism. Moreover, the recipient bed of some transplanted eyes may be quite variable, as in the case of progressive keratoconus. Transplants performed because of herpes simplex infection and chemical burns may involve sewing normal elastic tissue into relatively scarred and inflexible beds, causing unpredictable amounts of postoperative astigmatism.

For these and other reasons, posttransplant cases do not conform to routine nomogram assessment. Astigmatism reduction in corneal transplant recipients should be based completely on physical inspection of the tissue and the topographical appearance of the ocular surface, since transplant recipients often present with asymmetrical astigmatism, irregular astigmatism, and/or high astigmatism.

To further confound results, refraction may be difficult or imprecise in these patients owing to high amounts of optical aberration and coexistent ocular morbidities. The most objective measurement is likely to be topographical astigmatism.

The first step when preparing a surgical plan for posttransplant astigmatism is to determine its cause. For instance, a patient with occult wound dehiscence might show excessive flattening in one hemi-meridian, producing severe astigmatism. In this case, a wedge resection would be a more appropriate procedure for excess flattening than astigmatic keratotomy surgery.

In areas of excessively dense wound contraction, be mindful that a small relaxing incision may go a long way, creating a large refractive effect. As a counterpoint to this, when recipient bed scarring exists, a relaxing incision may have relatively little effect. If there is no "give" in the cornea (eg, in a cornea scarred by herpes simplex), a relaxing incision may not budge the topographical shape or the patient’s refractive error.

Some form of intraoperative keratotomy or wavefront data capture is very helpful when correcting posttransplant astigmatism. A Morcher ring, an ocular response analyzer (ORA), or some similar intraoperative keratometric/wavefront device can provide the real-time effect of each incision as it is rendered.

Posttransplant astigmatic keratotomy surgical plan examples

Example 3. A 30-year-old patient presents with 3 D of refractive astigmatism following corneal graft surgery. Topography shows a somewhat symmetrical bowtie appearance, demonstrating even greater corneal astigmatism than the patient’s subjective response. The goal was to achieve a reduction of astigmatism using these values: A 100° incision was specified by the nomogram, which would then be increased by 20° owing to the patient's age. While a pair of 60° incisions was indicated on the standard nomogram, the surgeon’s prior experience led to a revision of the total arc to 90°.

Example 3. Preoperative topography in a patient ab Example 3. Preoperative topography in a patient about to undergo astigmatic keratotomy (AK) following corneal transplantation. This 30-year-old woman underwent corneal transplantation for keratoconus 6 years ago. She presented with a clear graft but with excessive astigmatism. Based on this pre-AK topography and prior surgical experience, an incision was placed just inside the donor-recipient interface from 250° to 300°. A second incision was placed between 30° and 70°. Incisions were not paired at 180° apart, but were localized based on topography. Refraction improved from +0.75 + 3.00 X 58° to +0.50 + 1.50 X 130°. Best-corrected vision remained 20/20.
Example 3. Postoperative topography following asti Example 3. Postoperative topography following astigmatic keratotomy for the 30-year-old woman with penetrating keratoplasty for keratoconus.

Example 4. A 75-year-old patient with prior history of penetrating keratoplasty presents with 2.5 D of refractive astigmatism. Topography shows a symmetrical bowtie appearance, yielding 6.5 D of corneal astigmatism. According to the nomogram, the 140° total arcuate incision indicated by the chart must be reduced by 25° based on the patient's age. A pair of 67.5° incisions is specified; however, this amount was titrated downward based on surgical experience.

Example 4. Topography shows preoperative and posto Example 4. Topography shows preoperative and postoperative readings with differential analysis in a corneal transplant recipient (performed for Fuchs dystrophy) who underwent astigmatic keratotomy (AK). After all sutures were removed, she had approximately 6.5 D of keratometric astigmatism. The previous sutures seemed to have been tighter temporally than nasally. She underwent AK with a 50° incision temporally and a 30° incision nasally. The alteration in arcuate size values was related to surgeon preference. Following AK, residual astigmatism was slightly less than 1 D. The 5.5 D of astigmatism alteration from an 80° relaxing incision in a normal cornea is not expected. Posttransplant corneas are variable, and individual results can vary widely. Monitor the results as best as possible intraoperatively and be willing to go back and suture an overcorrection if one should occur.

Preoperative Details

The patient's record should reflect a fully documented surgical plan, accompanied by corneal topography of the operative eye for easy reference. Either affix these to the operating microscope or place them on a small cart or table next to the microscope. The surgeon must be able to reference the topography and plan at all times to avoid disorientation.

Administer 2 drops of topical anesthetic, such as proparacaine, into the operative eye 5 minutes apart followed by 1 drop of topical antibiotic. Skin should be prepared with betadine.

Intraoperative Details

The patient enters the minor procedure room and sits upright in the chair. Instill a drop of topical anesthetic into the preoperative eye and, with the patient fixating at distance, mark the 6-o'clock and 12-o'clock locations at the limbus with a gentian violet skin scribe. The patient may then lie down under the microscope. Remember that, owing to cyclotorsion of the eye when the patient is reclined, the 6- and 12-o’clock marks may not appear in the same meridian as when sitting up and marked. All astigmatic keratotomy measurements should be based on the scribe marks.

The tip of the ultrasonic pachymeter should be cleaned with an alcohol pad. Wipe the tip dry with a sterile 4 x 4 pad or rinse with balanced salt solution (BSS), as any residual alcohol on the pachymeter tip can cause an abrasion when touched to the cornea. With manual opening of the lids (rather than with a lid speculum), measure and record the thickness of the cornea in the areas of incision placement.

To determine the diamond micrometer blade depth, take the thinnest pachymetry reading in the area where the first astigmatic keratotomy incision will be placed and subtract 0.02 mm from that measurement. Record this value on the surgical plan and set the depth of the knife blade under the operating microscope. Remember that each incision requires the surgeon to perform pachymetry in the affected area and to reset the blade for each incision accordingly.

Set the knife down on a protective block and insert the lid speculum.

An additional drop of anesthetic may be placed on the eye. Remove excess fluid from the cornea using a slightly moist Weck-Cel sponge. The 360° Thornton astigmatic ruler is used to mark the cornea.

Thornton astigmatic keratotomy ruler. Thornton astigmatic keratotomy ruler.

Line up the flanges with the previously placed 6-o'clock and 12-o'clock marks and lightly press the ruler onto the cornea.

The Thornton ring or Thornton-Fine ring may be used to fixate the globe. Identify the correct optical zone from the nomogram and locate the incision starting point using the 10° marks on the cornea. Firmly and perpendicularly enter the cornea with the diamond blade. Slowly cut along the desired arc and be vigilant for pooling, which may indicate inadvertent perforation. If a perforation occurs, remove the knife immediately and assess the incision. Microperforations require immediate pachymetry and blade depth check. When all is satisfactory, proceed. However, if a macro-perforation occurs, stop the procedure and consider suture of the incision if warranted. Perform no further incisions until the patient is fully healed and refractive stability is achieved.

Barring perforation (a rare phenomenon if careful pachymetry is used), when the desired arc is achieved, remove the knife and reset it for the appropriate depth of the next cut. Repeat this procedure until all cuts have been made. Then, remove the lid speculum.

Do not irrigate the cuts, especially if a small microperforation is present. Place a drop of topical antibiotic on the eye followed by 1-2 drops of a topical nonsteroid anti-inflammatory drug (NSAID). If a bilateral procedure is planned, move to the other eye.

Corneal transplant recipients

The technique differs somewhat in patients who have previously undergone corneal transplant surgery. Preoperatively, when marking the patient, use a skin scribe to precisely mark the limbal area that correlates to the steepest axis on topography. Again, this technique negates any possible cyclotorsion of the eye or misplacement of the 12-o'clock and 6-o'clock positions. It also negates any possible error in translation from topography to the eye.

In transplant recipients, astigmatic keratotomy incisions should be placed 0.5 mm within the transplanted tissue, rather than along the donor-recipient interface, because the interface may have variations in thickness, leading to inadvertent perforation. Measure the thickness of the cornea 0.5 mm central to the interface. Since the donor-recipient interface is the weakest spot in the cornea, do not plan to cut too deeply. Bias the knife at 85% of the measured pachymetry thickness for each area to be incised. It is better to use a slightly longer cut at slightly less depth, rather than a shorter and deeper cut, because the longer cut will provide smoother topography.

Moreover, use a triple-edged arcuate knife to create the arcuate incision parallel to the interface. After the initial incision, perform keratometry to determine its effect. When the cornea appears almost spherical, stop cutting, even if it means reducing the size of the arc compared with the presurgical plan. It is better to cut too little than too much. Remember, it is not necessary to completely neutralize astigmatism in the minor procedure room, as a continued flattening effect may be observed over the next week or so. If a significant overcorrection is noticed in the postoperative period, return to the operating room and suture the cut. The suture may be removed after 8 weeks.

Postoperatively, corneal transplant recipients are treated more frequently with topical antibiotics and steroids than used for routine astigmatic keratotomy. Remember that the eye is neurotrophic and epithelial healing may be problematic. Use adequate lubrication, especially if the patient has any tendency toward dry eye.

Postoperative Details

Postoperative medications vary according to surgeon choice. Ofloxacin 0.3% used 4 times per day for 1 week provides adequate antibacterial coverage as the corneal re-epithelializes. A steroid drop, such as prednisolone acetate 1%, used 4 times per day for 1-2 weeks may help to stop or slow regression. A topical NSAID such as bromfenac 0.07% used 4 times per day for 4-7 days may reduce patient discomfort.

Follow-up

Astigmatism is an ever-changing condition. Grene advises never trusting the results of a refractive surgical procedure until the 12-month postoperative gate is reached.[3] Be conservative when it comes to enhancement; patients may present years later for repeat or enhancement surgery, and surgery performed at the limbus may be repeated as necessary.

For excellent patient education resources, visit eMedicineHealth's Eye and Vision Center. Also, see eMedicineHealth's patient education article Vision Correction Surgery.

Complications

A well-performed astigmatic keratotomy has relatively few complications, as it is one of the safest procedures in ophthalmology.

If overcorrection is noted on topography and/or refraction within a month of astigmatic keratotomy surgery, close the incision by placing one or two 10-0 Vicryl sutures in the excessively flattened hemi-meridian. The sutures do not require removal, as they will dissolve in time after their function is complete.

Undercorrection can be addressed easily with one of two methods. First, retake ultrasonic pachymetry measurements in the area of the incisions. If pachymetry values measure significantly thicker after astigmatic keratotomy surgery, recut the original incisions at 90% of the greater depth. If, however, the depth seems to be adequate based on similar pre– and post–astigmatic keratotomy pachymetry measurements, recut the incisions and extend the length of each incision by increments of 10° to enhance the effect.

Occasionally, late regression of the astigmatic result can be observed. Astigmatic keratotomy incisions placed at the limbus are closer to the blood supply than incisions at a smaller optical zone and thus may have a greater tendency to regress. If late regression occurs, recut the same incision. Treat the patient with topical steroids 4 times per day for 4-6 weeks after the procedure to inhibit the tendency to heal too aggressively.

One of the most serious and avoidable complications of astigmatic keratotomy surgery is using a small optical zone for arcuate incision placement. In the early days of astigmatic keratotomy, some surgeons advocated using optical zones as small as 4 mm in diameter, which frequently caused a decrease in best-corrected vision and created disabling optical aberrations. These complications have essentially been rendered obsolete with the move of the incision to the limbus.