Astigmatic Keratotomy for the Correction of Astigmatism Treatment & Management
- Author: James Hays, MD, MHA, MBA; Chief Editor: Hampton Roy, Sr, MD more...
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 for astigmatic 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.
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 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.
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.
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.
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 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.
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. 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.
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.
Thornton SP, Sanders DR. Graded nonintersecting transverse incisions for correction of idiopathic astigmatism. J Cataract Refract Surg. 1987 Jan. 13(1):27-31. [Medline].
Buzard K, Haight D, Troutman R. Ruiz procedure for postkeratoplasty astigmatism. J Refractive Surgery. 1987. 3:40-5.
Price FW, Grene RB, Marks RG, Gonzales JS. Astigmatism reduction clinical trial: a multicenter prospective evaluation of the predictability of arcuate keratotomy. Evaluation of surgical nomogram predictability. ARC-T Study Group. Arch Ophthalmol. 1995 Mar. 113(3):277-82. [Medline].
Nordan LT. Quantifiable astigmatism correction: concepts and suggestions, 1986. J Cataract Refract Surg. 1986 Sep. 12(5):507-18. [Medline].
Lindstrom RL. The surgical correction of astigmatism: a clinician's perspective. Refract Corneal Surg. 1990 Nov-Dec. 6(6):441-54. [Medline].
Nichamin LD. Astigmatism management for modern phaco surgery. Int Ophthalmol Clin. 2003. 43(3):53-63. [Medline].
Bates WH. A suggestion of an operation to correct astigmatism. 1894. Refract Corneal Surg. 1989 Jan-Feb. 5(1):58-9. [Medline].
Sato T. Posterior incision of cornea; surgical treatment for conical cornea and astigmatism. Am J Ophthalmol. 1950 Jun. 33(6):943-8. [Medline].
Troutman RC, Swinger C. Relaxing incision for control of postoperative astigmatism following keratoplasty. Ophthalmic Surg. 1980 Feb. 11(2):117-20. [Medline].
Waring GO 3rd, Lynn MJ, Gelender H, Laibson PR, Lindstrom RL, Myers WD, et al. Results of the prospective evaluation of radial keratotomy (PERK) study one year after surgery. Ophthalmology. 1985 Feb. 92(2):177-98, 307. [Medline].
Nichamin L. Femtosecond laser technology applied to lens-based surgery. Medscape Ophthalmology. Available at http://medscape.com/viewarticle/723864. Accessed: Sept 29, 2014.
Navarro R. The optical design of the human eye: a critical review. J Optom. Jan-Mar 2009. 2(1):3-18.
Pinnamaneni N, Funderburgh JL. Concise review: Stem cells in the corneal stroma. Stem Cells. 2012 Jun. 30(6):1059-63. [Medline].
Bourne WM. Biology of the corneal endothelium in health and disease. Eye (Lond). 2003 Nov. 17(8):912-8. [Medline].
Rufer F, Sander S, Klettner A, Frimpong-Boateng A, Erb C. Characterization of the thinnest point of the cornea compared with the central corneal thickness in normal subjects. Cornea. 2009 Feb. 28(2):177-80. [Medline].
Mandelbaum S, Waring GO 3rd, Forster RK, Culbertson WW, Rowsey JJ, Espinal ME. Late development of ulcerative keratitis in radial keratotomy scars. Arch Ophthalmol. 1986 Aug. 104(8):1156-60. [Medline].
Rubenstein JB. Today’s peripheral corneal relaxing incisions. Cataract & Refractive Surgery Today. May 2014. 26-28.
Koch DD, Ali SF, Weikert MP, Shirayama M, Jenkins R, Wang L. Contribution of posterior corneal astigmatism to total corneal astigmatism. J Cataract Refract Surg. 2012 Dec. 38(12):2080-7. [Medline].
Wenjing Wu, Yan Wang, Lulu Xu. Meta-analysis of Pentacam vs. ultrasound pachymetry in central corneal thickness measurement in normal, post–LASIK or PRK, and keratoconic or keratoconus-suspect eyes. Graefe's Archive for Clinical and Experimental Ophthalmology. Issue 1. January 2014. Volume 252: 91-99.
Krumeich JH, Kezirian GM. Circular keratotomy to reduce astigmatism and improve vision in stage I and II keratoconus. J Refract Surg. 2009 Apr. 25(4):357-65. [Medline].
Hoffart L, Proust H, Matonti F, Conrath J, Ridings B. Correction of postkeratoplasty astigmatism by femtosecond laser compared with mechanized astigmatic keratotomy. Am J Ophthalmol. 2009 May. 147(5):779-87, 787.e1. [Medline].
Navarro R, Palos F, Lanchares E, Calvo B, Cristóbal JA. Lower- and higher-order aberrations predicted by an optomechanical model of arcuate keratotomy for astigmatism. J Cataract Refract Surg. 2009 Jan. 35(1):158-65. [Medline].
Kymionis GD, Yoo SH, Ide T, Culbertson WW. Femtosecond-assisted astigmatic keratotomy for post-keratoplasty irregular astigmatism. J Cataract Refract Surg. 2009 Jan. 35(1):11-3. [Medline].
Bahar I, Levinger E, Kaiserman I, Sansanayudh W, Rootman DS. IntraLase-enabled astigmatic keratotomy for postkeratoplasty astigmatism. Am J Ophthalmol. 2008 Dec. 146(6):897-904.e1. [Medline].
Kumar NL, Kaiserman I, Shehadeh-Mashor R, Sansanayudh W, Ritenour R, Rootman DS. IntraLase-enabled astigmatic keratotomy for post-keratoplasty astigmatism: on-axis vector analysis. Ophthalmology. 2010 Jun. 117(6):1228-1235.e1. [Medline].