Astigmatic Keratotomy for the Correction of Astigmatism
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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.
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.
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.
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.
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.
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].
James Hays, MD, MBA, MHA Consulting Staff, Department of Corneal Transplantation and Refractive Surgery, Atlanta Eye Surgery Center
Disclosure: Received salary from Alimera Sciences for consulting.
Debra M Stone, OD, MS Consultative Optometrist and Outcomes Analyst, Woolfson Eye Institute
Debra M Stone, OD, MS is a member of the following medical societies: American Academy of Optometry, Georgia Optometric Association
Disclosure: Nothing to disclose.
Simon K Law, MD, PharmD Clinical Professor of Health Sciences, Department of Ophthalmology, Jules Stein Eye Institute, University of California, Los Angeles, David Geffen School of Medicine
Simon K Law, MD, PharmD is a member of the following medical societies: American Academy of Ophthalmology, Association for Research in Vision and Ophthalmology, American Glaucoma Society
Disclosure: Nothing to disclose.
Louis E Probst, MD, MD Medical Director, TLC Laser Eye Centers
Louis E Probst, MD, MD is a member of the following medical societies: American Academy of Ophthalmology, American Society of Cataract and Refractive Surgery, International Society of Refractive Surgery
Disclosure: Nothing to disclose.
Douglas R Lazzaro, MD, FAAO, FACS Chairman, Professor of Ophthalmology, The Richard C Troutman, MD, Distinguished Chair in Ophthalmology and Ophthalmic Microsurgery, Department of Ophthalmology, State University of New York Downstate Medical Center; Chief of Ophthalmology, Director of Cornea, Director of Surgical Training, Kings County Hospital Center
Douglas R Lazzaro, MD, FAAO, FACS is a member of the following medical societies: American Academy of Ophthalmology, American College of Surgeons, American Society of Cataract and Refractive Surgery, Association for Research in Vision and Ophthalmology, Association of University Professors of Ophthalmology, Brooklyn Ophthalmological Society, Cornea Society, New York Society for Clinical Ophthalmology, Ophthalmic Laser Surgical Society
Disclosure: Nothing to disclose.
Daniel S Durrie, MD Director, Department of Ophthalmology, Division of Refractive Surgery, University of Kansas Medical Center
Daniel S Durrie, MD is a member of the following medical societies: American Academy of Ophthalmology, Association for Research in Vision and Ophthalmology
Disclosure: Received grant/research funds from Alcon Labs for independent contractor; Received grant/research funds from Abbott Medical Optics for independent contractor; Received ownership interest from Acufocus for consulting; Received ownership interest from WaveTec for consulting; Received grant/research funds from Topcon for independent contractor; Received grant/research funds from Avedro for independent contractor; Received grant/research funds from ReVitalVision for independent contractor.
Mauro Fioretto, MD Professor of Ophthalmology, University Eye Clinic of Genova; Head of Ophthalmology Department, Hospital of Casale Monferrato, Italy
Disclosure: Nothing to disclose.
Vincenzo Orfeo, MD Head, Operating Unit, Clinica Mediterranea, Naples, Italy
Vincenzo Orfeo, MD is a member of the following medical societies: American Academy of Ophthalmology
Disclosure: Nothing to disclose.
Antonio Pascotto, MD Consulting Ophthalmologist, Pascotto, Istituto per la Salute degli Occhi, Clinica Mediterranea, Italy
Disclosure: Nothing to disclose.
Sergio Claudio Saccà, PhD Professor of Ophthalmology, Department of Neurological and Visual Sciences, Ospedale San Martino, Italy
Disclosure: Nothing to disclose.
Spencer Thornton, MD Medical Director, Thornton Eye Center
Spencer Thornton, MD is a member of the following medical societies: American Academy of Ophthalmology and American College of Surgeons
Disclosure: Nothing to disclose.
Astigmatic Keratotomy for the Correction of Astigmatism
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