Hyperguides

Ophthalmology / Refractive surgery

Principles of Radial Keratotomy and Astigmatic Keratotomy

Peter J. McDonnell, MD
Article


Radial Keratotomy

Myopia affects approximately 25% of adults in the United States. Approximately 10% of the myopic population can be expected to have visually significant astigmatism of more than 1 D. With recent technological advances, interest in corneal surgical procedures to correct refractive errors has grown to include radial keratotomy (RK) and astigmatic keratotomy (AK), excimer laser photorefractive keratotomy, laser in situ keratomileusis (LASIK), and intrastromal corneal rings. Although each of these different procedures has relative advantages and disadvantages, at this time, the laser procedures have largely replaced incisional surgery.

Bores and colleagues performed the first RK procedure in the United States in 1978.1 Subsequently, there has been substantial laboratory and clinical research, resulting in technological advances in ultrasonic pachymeters, diamond micrometer knives, and other areas that have made the procedure more predictable. In 1981, the National Eye Institute funded the Prospective Evaluation of Radial Keratotomy (PERK) study, which attempted to assess the efficacy, safety, predictability, and stability of RK.2-5 Results after 4, 5, and 10 years have been published and have helped define the limits of the procedure, as well as its risks and benefits.

Principles
In RK, there are four surgical variables that can be adjusted to achieve the desired result:6

  1. The diameter of the central clear zone may be adjusted between 3 mm and 5.5 mm. The smaller the diameter of the central clear zone, the greater the degree of flattening that is achieved. Review of nomograms suggests that the largest central clear zone with any efficacy is 5.5 mm. RK incisions with an optical zone of 6 mm or more have virtually no effect on the refractive error. Clear zones of less than 3 mm may be associated with increased starbursts or glare. Most modern incisional keratotomies, therefore, involve clear zones of between 3 mm and 5 mm.
  2. The number of incisions usually varies between two and eight. Placement of more than eight incisions is generally not warranted, because succeeding incisions have progressively less effect. Placement of four radial incisions achieves more than 70% of the effect achieved with eight incisions. For this reason, several surgeons have recommended a staged approach to RK in an attempt to minimize overcorrection.
  3. The deeper the incision, the greater the degree of central flattening. Centrifugally placed incisions (e.g., the American technique) incise the cornea from the optical zone outward toward the limbus. This provides greater safety with regard to unintended incisions crossing the clear optical zone; however, these incisions may have a variable depth and tend to have a beveled incision profile at the optical zone. Incisions placed in a centripetal manner (e.g., the Russian technique) incise the cornea from the limbal area toward the optical zone mark, making deep incisions but carrying the risk of inadvertently entering the clear optical zone. Centripetal incisions are often not perfectly straight because of the natural tendency of the knife to stray when cutting with the vertical edge. In an effort to consistently provide deep incisions without the risk of inadvertently entering the clear optical zone, a double-pass technique has been devised that offers the benefits of both the Russian and American techniques.7
  4. It is not necessary for the incisions to be carried to the limbus to achieve a near maximal effect.8 Lindstrom has shown that a similar effect can be achieved with radial incisions from a 3-mm clear zone carried to a 7-mm to 8-mm optical zone (mini-RK) as can be achieved when carried to an 11 mm optical zone.9 In fact, a mean increase of only 7.7% was seen when incisions were extended from the 7-mm to 11-mm optical zone. Thus, doubling the length of the RK incision achieved an additional effect of only 7.7%. The advantage of the shorter incisions is that the structural integrity of the cornea is better maintained. This may minimize the more serious complications and adverse effects of RK, including persistent diurnal fluctuation and long-term refractive instability with progressive hyperopic shift, as well as the potential for traumatic rupture of the keratotomy scars. Thus, the benefits of RK may be retained for the majority of low to moderate myopes while significantly reducing the risks by using a system of minimally invasive RK or mini-RK.9

There are two patient variables that affect the outcome of RK:

  1. With higher degrees of myopia, smaller optical zones are required, more incisions are required, and a greater depth of incisions is necessary to achieve full correction.
  2. As patients age, greater effects will be achieved with an identical surgical technique.

The PERK study determined the safety and short- and long-term efficacies of RK and validated the predictability of the procedure as determined by a simple nomogram. The study involved 10 surgeons in nine centers. The surgical protocol called for only three optical zones: 4 mm for lower errors (-2 D to -3.12 D); 3.5 mm for mid range errors (-3.25 D to -4.37 D); and 3 mm for high range errors (-4.5 D to -8 D). All eyes had eight centripetal incisions at a blade length setting of 100% of the thinnest paracentral cornea depth as measured by ultrasonic pachymetry. No consideration was given for other variables such as age, gender, or intraocular pressure (IOP). All errors were determined by the spherical equivalent of the cycloplegic refraction.

The PERK study involved 793 eyes in 435 patients ranging in age from 21 to 58 years old, with a mean age of 33 years. The procedures were performed over an 18-month period, from March 1982 to October 1983. Of the 793 eyes, 12% underwent repeat surgery with eight additional incisions between the original incisions because of inadequate initial response. A remarkable 95% of the patients were followed for more than 5 years.

The results of the PERK study showed that 64% of eyes were corrected to within 1 D of emmetropia, with some variation by category: in the lower group, 76%; in the middle group 67%; and in the higher group, 49%; 17% were overcorrected and 19% were undercorrected. In the lower group, 92% attained 20/40 vision without correction and 75% attained 20/20; in the middle group, 86% attained 20/40 and 57% attained 20/20 without correction; in the higher group, 72% attained 20/40 and 29% attained 20/20 without correction.2-5

The relative absence of complications was reassuring. There were no ruptured globes in the series. Only two eyes developed delayed bacterial keratitis, which was successfully treated; both patients had better than 20/20 vision. One eye had an epithelial erosion. Subjective complaints of fluctuation were reported in 33% of patients. Nondisabling glare that was described in a starburst fashion was reported; however, only 0.6% of patients in the PERK study discontinued driving secondary to glare.2 The patient satisfaction rating was good, with 95% of patients stating that they would repeat the surgery. Although with this method, success is difficult to measure and to evaluate, there appeared to be overall patient satisfaction with the procedure and resulting vision. After RK, 65% of patients no longer required spectacle or contact lens correction.

Because RK is an elective procedure that attempts to permanently correct myopia and is a procedure that may result in vision-threatening complications, it is essential that the standards of safety and efficacy of this procedure be higher than surgeries for treatment of potentially blinding disorders. Also, patients considering RK must be informed regarding the risks, benefits, and alternatives including spectacles, contact lenses, or other refractive procedures such as LASIK or photorefractive keratectomy (PRK). Patients considering RK should have stable, nonprogressive myopia confirmed by examination of previous refractions. Contact lens wear must be discontinued before consideration of RK to detect any possible corneal warpage associated with contact lens use. Slit lamp examination should reveal an essentially normal cornea. Patients with ocular conditions such as glaucoma or uveitis may be excluded as candidates for RK because the effects of these disorders on the surgical outcomes are not well defined. Patients with systemic conditions that may affect corneal healing, including patients with connective tissue diseases or herpes simplex keratitis, or patients using systemic corticosteroids, may be unsuitable candidates for RK. Radial keratotomy should not be performed on women who are pregnant or nursing.

RK achieves the best uncorrected visual acuity in patients who have low to moderate myopia, up to -4 D to -6 D. Patients with higher degrees of myopia, -6.0 D to -10 D, tend to have a more variable response to surgery. Surgeons have performed RK to correct the refractive error in some patients with myopia of -8 D to 10 D, although the refractive errors of most eyes with this amount of myopia remain undercorrected. However, some patients with high myopia may welcome even partial reduction of their refractive error; partial reduction allows them to read comfortably without glasses. Other refractive corneal procedures are available, including excimer laser PRK, LASIK, and intrastromal corneal rings. Some patients with myopia may be candidates for one of these procedures if their myopia is outside the range of RK.

Patients must understand that whereas nomograms for RK are based on averages for large numbers of procedures, the outcome of surgery cannot be precisely predicted for an individual eye. Additional RK incisions may be required to achieve the desired result, and spectacle and/or contact lenses may still be required for best corrected visual acuity after surgery.

Complications
Corneal perforation during RK may be either a microperforation in which minimal egress of aqueous humor is noted when the eye pressure is raised during the incision or a macroperforation that is large enough to shallow the anterior chamber. A macroperforation usually requires termination of the procedure and possible closure of the perforation with suture. The PERK study showed a 2.3% microperforation rate; no cases required suturing.

RK incisions can decrease central endothelial cell density to less than 10% when measured centrally; however, a higher degree of endothelial cell injury is noted when corneal perforation occurs during RK.

The incisions should not cross the limbus because this may cause bleeding into the wound and incite neovascularization into the wound.

Most patients experience pain, throbbing, or foreign body sensation for 24 to 48 hours postoperatively and frequently require analgesics. Glare or starbursts caused by scattering of light intraocularly commonly occurs for a few months after surgery. Glare or starbursts may persist for 1 year or more. In 9% of patients, these complications may cause diminished vision, particularly at night or on hazy or bright days.2

After 1 year, epithelial inclusion cysts were seen in the incisional scars in 8.6% of patients in the PERK study, but these cysts do not seem to affect the visual results. In the majority of patients, the scars tend to fade years after the surgery as collagen remodeling occurs.

A brown, stellate, epithelial iron line, which is usually insignificant, appears in the majority of eyes after RK. In the PERK study, 81% of eyes had a stellate epithelial iron line 6 months postoperatively. The iron line appears at the junction of the middle and inferior third of the cornea, with fingers extending from the ends of the incisions.

Anterior corneal epithelium basement membrane changes, similar to those seen in epithelial basement membrane dystrophy, are often observed after RK. These changes tend to be transient, lasting less than 3 months in the majority of eyes, and are infrequently associated with clinical symptoms or recurrent epithelial erosions. A single case of epithelial ingrowth after RK has been reported.

Ptosis has been reported to be a complication of RK. Ptosis is believed to be due to the lid speculum's damaging the levator aponeurosis.

Multiple published case studies report endophthalmitis after RK. Pathogens range from the relatively nonvirulent Staphylococcus epidermidis to fulminant cases of pseudomonal endophthalmitis. Most patients recovered some visual acuity if the problem was recognized early and treated aggressively, and if the organism was not an extremely virulent pathogen. Corneal perforation appears to be responsible for allowing ingress of organisms intraoperatively.10

Bacterial or fungal keratitis can occur in the immediate postoperative period, but it can be delayed several years after RK.11,12 The use of contact lenses, especially extended-wear lenses, after RK is associated with a higher incidence of keratitis. A persistent epithelial plug in the keratotomy wound is implicated in delayed keratitis. There are several reports of traumatic rupture of the keratotomy scars after blunt trauma.13,14

The most frequently reported complication of RK is an unexpected surgical outcome. The principal complications include overcorrection, undercorrection, increased astigmatism, irregular astigmatism, a progressive postoperative shift toward hyperopia, and diurnal fluctuation of vision.

Many patients who have undergone RK report fluctuation of vision during the course of the day. This effect may last several years. In the PERK study, patients who complained of fluctuating vision were examined twice in the same day at 1 year, 3½ years, and 11 years postoperatively. Patients were examined before 8 AM and after 7 PM. At 1 year, 42% of the eyes demonstrated a manifest refraction that changed from 0.5 D to 1.25 D. At approximately 3½ years after RK, 31% of eyes experienced a change of 0.5 D or more during the course of the day. At 11 years after RK, 54% of eyes showed a change in refractive error of 0.5 D or more. Thus, diurnal fluctuation may be a permanent sequela of RK in some patients. Keratometry and refraction demonstrated that the majority of the corneas underwent a gradual steepening during the course of the day, which ranged from 0.30 D to 1.25 D. Therefore, patients with undercorrected vision had best visual acuity in the morning, whereas patients whose vision was overcorrected experienced an increase in visual acuity during the day. Although the exact cause of this diurnal fluctuation is unknown, it is most likely related to structural changes induced by the keratotomy incisions and scars. Corneal edema, as occurs during sleep or contact lens wear, has been shown to flatten the cornea after RK. The unsutured wounds in the avascular cornea may require more than 4 to 5 years to eject the epithelial plug completely and to remodel the corneal stroma adjacent to the incisional scar. The constant pressures of the eyelids and mild epithelial and stromal edema during sleep are believed to induce a nocturnal flattening of the cornea. The diurnal fluctuation is seldom significant enough to require different optical corrections during the course of the day.15

Consecutive hyperopia after RK may result from an initial overcorrection or from a continued effect of the procedure with time.4 Five-year follow-up of the PERK study showed that 22% of the eyes had a refractive change of 1 D or more in the hyperopic direction between 6 months and 5 years after RK. In this group, the refractive error was within 1 D of emmetropia for 64% of eyes. However, 17% of patients were hyperopic by more than 1 D. In the PERK study, 43% of eyes changed in the hyperopic direction by 1 D or more during 6 months to 10 years after RK. The average rate of change was +0.21 D per year between 6 months and 2 years and +0.06 D per year between 2 and 10 years. Thus, progression toward hyperopia continues over time with the highest rate of change occurring in the first 2 years after RK. A progressive effect in the hyperopic direction has also been seen after two- to four-incision RK. Because of the progressive effect of RK with time, many surgeons are now intentionally leaving patients slightly myopic. In addition, many RK procedures are now staged such that two to four incisions are placed initially, and additional incisions are added at some point in the future, if necessary.

Spectacles may manage overcorrection of the refractive error after RK, or, if anisometropia is present, contact lenses may be used. However, contact lens fitting after RK can be complicated. The flat central cornea and the relatively steeper paracentral cornea cause the contact lens to decenter as it moves across the paracentral point. Neovascularization of perilimbal incisional scars has limited the use of soft contact lenses in patients who received RK.

Most surgeons consider treating patients with LASIK to correct postoperative refractive complications following RK. Generally, the same considerations regarding automated lamellar keratoplasty (ALK) are applicable to LASIK after RK. A challenge lies in creating the flap with the microkeratome without disrupting the radial incisions to result in multiple “pizza-pie” wedges rather than a single circular flap. To avoid this disruption, most surgeons insist that the RK had been performed a minimum of 6 months to 1 year prior to LASIK. Also, surgeons will use a thicker footplate to create a thicker flap to avoid disruptung the radial incisions.

Five-year results of the PERK study revealed that 15% of eyes had increased astigmatism of 1 D or more, postoperatively. The change in refractive astigmatism ranged from a decrease of 1.5 D to an increase of 4.5 D. After RK, all corneas have some irregular astigmatism that can be detected by keratoscopy. Minimal irregular astigmatism appears to have little visual effect under daylight conditions, presumably because it lies outside the optical clear zone. Under dim light, more glare and distortion result when the pupil dilates.4,16

Astigmatic Keratotomy

The following principles of AK apply to all types of astigmatism.

  1. AK incisions are placed in the steep corneal meridian. This meridian is parallel to the plus-cylinder refraction axis and has the highest power on central keratometry.
  2. When incisions are placed in the axis of the steep meridian, “coupling” can occur. When the coupling ratio, or flattening-steepening ratio, is 1:1, the spherical equivalent is unchanged. Coupling refers to changes of the incised corneal curvature in relation to the unincised cornea 90° away. Therefore, transverse incisions placed across the steep meridian will flatten the meridian while also steepening the unincised meridian 90° away. The coupling ratio is defined by the amount of flattening in the steep meridian caused by a transverse incision versus the compensatory steepening seen in the unincised meridian 90° away. When the amount of flattening is equivalent to the amount of steepening, the coupling ratio is 1:1. The coupling ratio depends on the length of the incision, its location, and its depth. Short transverse incisions measuring 1 mm to 1.5 mm or arcuate incisions approximating 20° tend to flatten the steep meridian more than steepen the unincised meridian 90° away, thereby creating a coupling ratio of 1.5:2. Transverse incisions measuring 2 mm to 5 mm in length, or arcuate incisions of 90° to 120°, tend to induce greater steepening in the orthogonal meridian than flattening in the incised meridian, thereby creating a coupling ratio less than 1.
  3. The maximum efficacy per unit length incision for a straight or arcuate keratotomy incision occurs when the incision is placed coincident with a 5- to 8-mm diameter zone. Generally, transverse incisions closer to the 5-mm clear optical zone produce greater astigmatic correction.
  4. Incisions of an optical zone of less than 5 mm are contraindicated because of irregular astigmatism and glare. The majority of the effect of transverse or arcuate incisions is achieved with the first symmetrical pair of keratotomy incisions, accounting for approximately two thirds of the effect. Placement of an additional pair will increase efficacy by 25% to 33%. No significant additional effect is achieved by placing more than two paired keratotomy incisions on the cornea.
  5. Semiradial incisions induce overall corneal flattening and reduce astigmatism. Generally, paired semiradial incisions flatten the meridian of the incisions twice that compared with the 90° away. Varying the amount of the optical clear zone will affect the degree of myopic correction, as well as the degree of astigmatic correction.
  6. Transverse or arcuate incisions should not cross radial or semiradial incisions. Previous clinical studies have shown that when two incisions intersect, the point of intersection undergoes poor wound healing, which often leads to epithelial inclusion cysts, exaggerated scar formation, and early and late wound dehiscence.

Astigmatisms greater than 0.50 D to 1 D generally require some form of optical correction. An astigmatic refractive error in the range of 1 D to 2 D may be expected to reduce uncorrected visual acuity to the 20/30 to 20/50 range, and 2 D to 3 D should reduce uncorrected visual acuity from the 20/70 to 20/100 range. Naturally occurring astigmatism is common - up to 95% of eyes may have natural astigmatism.

Patients considering astigmatic refractive keratotomy should have stable refractions. This must be confirmed by careful preoperative refractions. Keratometry is a better measure of the true corneal astigmatism than refraction. Computer-assisted corneal topography instruments are sensitive enough to delineate patients who may be poor surgical candidates due to preclinical keratoconus, irregular astigmatism, or other corneal abnormality. Computer-assisted corneal topography maps can be helpful in preoperative assessment, surgical planning, and postoperative monitoring of patients undergoing incisional keratotomy.

The patient's age is an important criterion in screening patients for incisional keratotomy. Patients younger than 21 years of age may not have a stable refraction. Simple and intermediate myopes also have the highest rates of progression during the first and second decades of life. This progression may not stabilize until the patient is in his or her early 20s. Younger persons may also be more likely than older persons to have unrealistic expectations of refractive surgery. Young patients must be aware that older patients receive more surgical correction in the same level of surgery. This may necessitate smaller optical zones and more incisions in the eyes of younger patients, thereby increasing the risk of possible adverse effects. The surgical correction of naturally occurring astigmatism is considerably more predictable than the surgical correction of postkeratoplasty or postcataract astigmatism, in which the patient's wound healing may affect the outcome.

AK can be used to manage astigmatism after cataract surgery. Limbal relaxing incisions, compression sutures, and wedge resections are all techniques reserved for large degrees of postkeratoplasty astigmatism.

Complications
Transverse, arcuate, and semiradial incisions share a common problem of limited predictability. Undercorrections are better tolerated than overcorrections. Additional surgery may be considered for undercorrections to achieve the desired result.

Wound gape and scarring can occur with intersecting incisions. This in turn may induce irregular astigmatism.

Summary

Incisional techniques have proven to be effective in correcting low degrees of myopia and wide ranges of astigmatism. Although technological advances have introduced newer methods for correcting refractive error, modern incisional surgery will continue to have a role for correcting low degrees of residual refractive error after either excimer laser, lamellar, or intraocular refractive procedures.16

References

  1. Bores LD, Myers W, Cowden J. Radial keratotomy. An analysis of the American experience. Ann Ophthalmol. 1981;12:941.
  2. Waring GO, Moffitt SD, Gelender H, et al. Rationale for and design of the National Eye Institute Prospective Evaluation of Radial Keratotomy (PERK) study. Ophthalmology. 1983;90:40.
  3. Waring GO, Lynn MJ, Fielding B, et al. Results of the Prospective Evaluation of Radial Keratotomy (PERK) study four years after surgery for myopia. JAMA. 1990;363:1083.
  4. Waring GO, Lynn MJ, Nizam A, et al. Results of the evaluation of radial keratotomy (PERK) study five years after surgery. Ophthalmology. 1991;98:1164.
  5. Waring GO, Lynn MJ, McDonnell PJ, et al. Results of the Prospective Evaluation of Radial Keratotomy (PERK) study 10 years after surgery. Arch Ophthalmol. 1994;112:1298-1308.
  6. Arrowsmith PN, Marks RG. Evaluating the predictability of radial keratotomy. Ophthalmology. 1985;92:331.
  7. Verity M, Talamo JH, Choyet A, et al. The combined (Genesis) technique of radial keratotomy: Prospective, multicenter study. Ophthalmology. 1995;102:1908-1917.
  8. O'Donnell FE. Short-incision radial keratotomy: A comparative study in rabbits. J Refract Surg. 1987;3:22.
  9. Lindstrom RL. Minimally invasive radial keratotomy: Mini-RK. J Cataract Refract Surg. 1995;21:27-34.
  10. McLeod SD, Flowers CW, Lopez PF, Marx J, McDonnell PJ. Endophthalmitis and orbital after radial keratotomy. Ophthalmology. 1995;102:1902-1907.
  11. Marmor RH. Radial keratotomy complications. Ann Ophthalmol. 1987;19:409.
  12. Robin JB, Beatty RF, Dunn S, et al. Mycobacteria chelonei keratitis after radial keratotomy. Am J Ophthalmol. 1986;102:72.
  13. Pearlstein ES, Agoptos PJ, Cantrill HL, et al. Ruptured globe after radial keratotomy. Am J Ophthalmol. 1988;106:755-756.
  14. McDermott ML, Wilkinson WS, Tukel DB, et al. Corneal scleral rupture ten years after radial keratotomy. Am J Ophthalmol. 1990;110:575-577.
  15. McDonnell PJ, Nizam A, Lynn MJ, Waring GO, the PERK Study Group. Morning-to-evening change in refraction, corneal curvature, and visual acuity 11 years after radial keratotomy in the PERK study. Ophthalmology. 1996;103:233-239.
  16. Wu HK, Thompson VM, Steinert RF, Slade SG, Hersh PS, eds. In: Wu HK, et al, eds. Refractive Surgery. New York, NY: Thieme Inc; 1999:127-213.