Despite extensive investigation, the etiology and underlying mechanism of stromal thinning in keratoconus is not well understood. Recent research suggests that genetic factors may play a role; increased levels and activities of proteases or decreased levels of the inhibitors of protease activity may cause the loss of corneal stroma. It has also been shown that epithelial injury, such as occurs from trauma or refractive surgery, can result in a loss of anterior stromal keratocytes via apoptosis modulated by interleukin-1.1,2
Spectacles and contact lenses are the common treatment modalities for the early stages of keratoconus.3-5 In more advanced cases with severe corneal irregular astigmatism and stromal opacities, contact lenses cannot improve patients' visual acuity, and a penetrating keratoplasty (PKP) is necessary to restore their visual function. Currently, PKP is the primary treatment for patients with keratoconus who are contact lens intolerant,6-9 but this surgical intervention is invasive and can lead to serious complications. In some cases in which the cornea is still transparent and the patient is contact lens intolerant, he and the surgeon are reluctant to pursue PKP. If a less invasive surgical intervention could improve patients' BSCVA or UCVA, the risks of PKP could be deferred or avoided. This article examines the treatment with intrastromal corneal rings of keratoconic patients who have transparent corneas.
REINFORCING THE CORNEA
Incisional or excimer laser refractive surgery is contraindicated in the keratoconic eye. RK and astigmatic keratotomy weaken corneal strength with incisions, while PRK and LASIK do so by removing corneal tissue. In patients with keratoconus, these procedures may induce complications as well as offer poor refractive predictability and stability.10-23 When treating keratoconus, it is far more logical to reinforce the cornea with additive technology.
INTRASTROMAL CORNEAL RINGS
Intrastromal corneal rings act as passive spacing elements that shorten the arc length of the anterior corneal surface and thereby flatten the central cornea.24,25 Dr. Colin performed the first implantation of Intacs corneal rings (Addition Technology, Inc., Des Plaines, IL) into keratoconic eyes in June 1997.26 Applying two Intacs segments to the eye lifts the inferior ectasia and flattens the soft, keratoconic corneal tissue—changes that may decrease the asymmetric astigmatism induced by keratoconus. It does not involve removing corneal tissue or touching the central cornea. The Intacs inserts do not eliminate the corneal disease but rather decrease its associated corneal abnormality and improve patients' visual acuity to satisfactory levels. A principal benefit of treating keratoconus with Intacs inserts is that they delay or eliminate the need for a corneal graft. Another positive aspect of the procedure is that the rings may be removed if necessary.
CANDIDATES
Candidates for Intacs include keratoconic patients who cannot tolerate contact lenses and possess clear central corneas.26-28 If the patient's corneal opacities are apical and superficial, the surgeon may consider performing a phototherapeutic ablation prior to implanting the inserts. Determining candidacy for Intacs involves a complete ophthalmologic evaluation, a biomicroscopic exam in order to describe the corneal opacities and folds, and topography to determine the location and height of the cone. Ultrasound and corneal pachymetry using the Orbscan topographer (Bausch & Lomb Surgical, San Dimas, CA) can also provide valuable information.
SURGICAL PROCEDURE
The surgical procedures for treating keratoconus and correcting low myopia with Intacs inserts are similar except for the location of the incision site. After preparing the patient for normal anterior segment surgery and administering topical anesthesia, the surgeon creates a temporal small corneal incision (approximately 1.8 mm in length) with a diamond knife at the edge of the 7-mm optical zone to two-thirds of the corneal thickness at that location. The location of the incision depends on the morphology of the keratoconus, particularly the axes of the steepest and flattest meridians. The surgeon creates two intrastromal tunnels (clockwise and counterclockwise) using specialized instruments developed for inserting the segments. It is important to exercise care when making the inferior tunnel, which is located where the cornea is relatively thin. Postoperative care includes treatment with a combination steroid/antibiotic ointment and the application of topical corticosteroids. The patient should wear a plastic eyeshield for the first 2 postoperative days and use ocular lubricants for 2 weeks following surgery. The suture may be removed 10 to 15 days postoperatively. Patients should be instructed not to rub their eyes.
RESULTS
In a study of 10 subjects,26-28 Dr. Colin and his colleagues placed a 0.45-mm Intacs insert inferiorly in the subjects' first eye to lift the conus, and a 0.35-mm Intacs insert superiorly to flatten the cornea and decrease baseline keratoconic asymmetric astigmatism (Tables 1 and 2). No intraoperative complications occurred, and the postoperative results demonstrated a significant reduction in spherical equivalent error, a decrease in keratometric and refractive astigmatism, an increase in topographic regularity, and an improvement at all examined time points in UCVA for almost all patients (P&Mac178;.05) (Table 3). The average, postoperative mean keratometry was reduced by approximately 3.00 D. A related, 74-eye study conducted by Brian S. Boxer Wachler, MD, of Los Angeles and his colleagues29,30 concluded that the asymmetric implantation of Intacs can improve both UCVA and BCVA, as well as reduce irregular astigmatism, in keratoconic corneas with and without corneal scarring. The investigators used the three ring segment sizes available in the US: 0.25 mm, 0.30 mm, and 0.35 mm. Study participants who had lower degrees of preoperative cylinder tended to experience better improvement in UCVA when compared with other subjects. Forty-one percent of the eyes experienced a two-line improvement in BCVA, and 66% achieved similar results in UCVA postoperatively. Eyes with better preoperative BCVA attained superior BCVA postoperatively, but it is important to note that eyes with worse preoperative acuity often experienced greater improvements in postoperative acuity.
Inaccurate Correction
In addition to potentially unacceptable visual side effects such as glare or halos, after the implantation of the two Intacs segments, the shape of the cornea may still be too asymmetrical. In those cases, the surgeon may consider performing an adjustment if thicker rings are available. If the corneal shape has improved, but the patient still has some degree of myopia, the surgeon may implant a phakic refractive IOL. If the patient becomes hyperopic, the surgeon should exchange the implanted rings for thinner segments. If the surgery increases the corneal asymmetry (ie, overly flat superiorly) and astigmatism, the physician should remove the superior segment.
Neovascularization Toward the Incision
This condition may occur when the incision is placed at the 12-o'clock position in patients with a history of long-term contact lens wear and superior corneal neovascularization. Vessels are uncommon when the incision is placed at the temporal meridian, and, due to the dimensions of the cornea, the incision is farther from the limbus.
Migration and Extrusion
When one or two segments are left too close to the wound intraoperatively, the natural tendency of the synthetic ring is to move toward the incision, a complication that can lead to corneal stromal melting. If the rings are implanted too superficially into thin corneas, especially if placed vertically, progressive stromal thinning and melting may occur.
STABILITY
Stability is obviously a critical issue of any surgical intervention for keratoconus, which may continue to progress even with the implantation of rigid PMMA rings into the stroma. Four eyes of 20 that we implanted with Intacs have been followed for at least 24 months; during this time, keratometry readings were stable in all eyes, and the UCVA and BCVA progressively improved over time. Another question investigators must answer is whether intrastromal corneal rings accelerate the progression of corneal thinning and ectasia. Four years after performing our first Intacs implantation for keratoconus, the patient had a stable UCVA of 20/40 and a BCVA of 20/30. We currently do not know whether implanting intrastromal corneal rings will prevent the progression of keratoconus. If not, and a PKP becomes necessary, we would advise removing the Intacs inserts as a separate procedure performed prior to the PKP. Concurrently executing the procedures might induce some degree of undesirable astigmatism postoperatively. The scar caused by the channels will not alter the 8-mm graft.
CONCLUSION
The predictability of the procedure is not yet high. Soft contact lenses may be fitted postoperatively to correct the residual spherical and cylindrical refractive error. In some cases, combined surgical options may yield the best results for a patient. For instance, a surgeon might (1) combine Intacs with phakic IOLs for keratoconus associated with a high degree of myopia, or (2) couple phototherapeutic keratectomy with Intacs implantation in cases of apical corneal scarring. New technologies may improve the results of implanting intrastromal corneal rings in keratoconic eyes. Wavefront analysis will augment surgeons' understanding and treatment of these eyes' aberrations. Additionally, creating the channels with a femtosecond laser should be more precise than manual dissection. Further research will produce essential information for improving our nomograms and our outcomes.
Sylvie Simonpoli-Velou, MD, is Clinical Assistant at CHU Université de Bordeaux in Bordeaux, France. She holds no financial interest in any product or company mentioned herein. Dr. Simonpoli-Velou may be reached at +33 55 679 56 08 ; sylvie-simonpoli@chu-bordeaux.fr.
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