Corneal Implants in the Treatment of Ectasia

Penetrating keratoplasty (PKP) is necessitated in approximately 20% of keratoconus patients, and while visual acuity is typically improved with a keratoplasty, significant risks are incurred: perioperative complications including endophthalmitis and loss of vision; side effects from long-term use of steroid drops; potential graft rejection; permanent weakening of the globe; and extended periods required for visual rehabilitation. Subtractive techniques such as laser in situ keratomileusis (LASIK) or incisional surgery are considered a contraindication for keratoconus because of the corneal instability conferred by these procedures (J Cataract Refract Surg. 2003;29(2):379-386). Intacs corneal inserts (intrastromal corneal ring segments—ICRS) have emerged as an innovative tool for addressing naturally occurring and iatrogenic keratectasia. Implantation of corneal inserts represents an additive technique in which segments reinforce the cornea and may help prevent further deterioration. The inserts shorten the corneal cord length and flatten the entire corneal curvature, maintaining the natural asphericity of the cornea while preserving corneal tissue. Intacs inserts are removable and their effect is reversible (J Refract Surg. 2001;17(1):25-31). Because Intacs inserts obviate or delay the need for PKP, they are a viable alternative for keratoconus patients with either contact lens intolerance or post-LASIK complications.

Patient Selection

Intacs corneal implants were first approved in 1999 by the U.S. Food and Drug Administration (FDA) for the treatment of myopia, and in July 2004 the implants were approved under a Humanitarian Device Exemption (HDE) to treat keratoconus patients otherwise facing PKP surgery (Ophthalmology. 2001;108(9):1688-1694). Intacs segments are the only corneal implants approved for use in the United States, although the Ferrara rings are available in Europe (J Cataract Refract Surg. 2003;29(2):379-386). While Intacs inserts are currently approved to treat keratoconus and myopia only, positive results have led investigators to try Intacs inserts in other corneal ectasia—both naturally occurring and iatrogenic—following LASIK and other refractive surgeries. The ideal keratoconus patient for Intacs implants is contact lens intolerant, has no central corneal scarring, and has moderate corneal steepening up to about 57 diopters (D). Additionally, eyes with more severe cones and scarring have also been treated with excellent results. During patient selection, the ophthalmologist should discuss the risks and benefits of Intacs inserts, the alternatives, and the probable need for continued contact lens use after placement—albeit with better fitting lenses and a softer lens material.

Surgical Procedure and Techniques

Initially produced by KeraVision and currently manufactured by Addition Technology, Inc., Intacs inserts were designed as single 360-degree PMMA rings that have since evolved into paired 150-degree arc segments. After applying topical anesthetic, Intacs segments are inserted into the peripheral stroma using mechanical dissectors or femtosecond laser to create the channels. There are 3 sizes for use in keratoconus: 0.25, 0.30, and 0.35 mm (0.40 and 0.45 mm sizes are available outside of the United States). Degree of correction is determined by the thickness of the chosen segments. In general, most surgeons place the incision in the steep meridian of the cornea (based on refraction and topography) using 2 segments of the same size, insert the segments at two-thirds the depth of the peripheral cornea, and close the incision with a single 10-0 nylon suture. New developments in this technique include the use of single inserts and asymmetric inserts of varying paired sizes.

Complications and Safety Profile

Intacs inserts have been used safely for over a decade to treat myopia (J Cataract Refract Surg. 2001;27(9):1456-1468), and a similar safety profile has also been established for use in keratoconus (Invest Ophthalmol Vis Sci. 2004;45:E-Abstract 2896). Most unfavorable issues, e.g., incorrect placement of the inserts, glare, and poor visual results, can be treated easily with removal of the implants, and the cornea and visual function return to the preoperative state (J Refract Surg. 2001;17(1):25-31 and Ophthalmology. 2004;111(4):747-751). Infections can occur, but these are treated effectively with topical antibiotics or removal of the implants. Should the inserts be explanted, patients are free to consider other alternatives including contact lens use, laser surgery, and PKP. Unlike other refractive procedures such as LASIK, the implantation of Intacs inserts does not commonly incite dry eye.

Postoperative Results

Keratoconus

Though much of the information on Intacs inserts used in keratoconus and ectasia is reported informally at national and international meetings, to date 19 articles published in the peer-reviewed literature discuss the use of the implants in keratoconus and pellucid marginal degeneration (PMD), reporting safety and efficacy in over 300 eyes. Due to significant variation in the degree of keratoconus from patient to patient, however, results are difficult to analyze or generalize.

Clinical research and most retrospective reviews of case series have demonstrated that the majority of eyes implanted with Intacs inserts typically show improvement of uncorrected visual acuity (UCVA) and best-corrected visual acuity (BCVA) of 2 lines. The reviews also show the average decrease of myopia is 3 D and the average decrease of keratometry is 3 to 4 D. Results, however varied, suggest that patients with moderate keratoconus achieve better clinical improvement than those with severe keratoconus (i.e., K-readings generally over 57 D).^7-11 In a retrospective review of 58 eyes with 1 year follow-up, UCVA improved from an average of 20/200 to 20/50, with a multiple regression analysis suggesting that patients with more moderate keratoconus demonstrate the most pronounced improvement in refractive and visual outcomes (Arch Ophthalmol. 2005;123(10):1308-1314). In a separate study of 50 eyes in 37 patients, Hellstedt et al found that placement of asymmetric Intacs inserts improves BCVA and UCVA and also reduces astigmatism in patients with mild to moderate keratoconus (J Refract Surg. 2005;21(3):236-246). But because such studies are typically conducted at single centers with slightly differing surgical techniques and retrospective reviews of data, assessing their cumulative value is both difficult and imperfect. What's more, these studies do not analyze improvement in quality of life, ease of contact lens use, or other factors that may be of significance in assessing the outcome and benefit of Intacs implants.

After the insertion of Intacs segments, BCVA may be achieved with spectacles or, in most cases, with soft or rigid gas permeable contact lenses that offer a more comfortable fit and greater tolerance (Cont Lens Anterior Eye. 2003;26(4):175-180). Many patients note improvement in vision even if significant changes in topography are not evident. Some reports discuss the use of additional procedures to correct residual refractive errors, such as conductive keratoplasty (CK), photorefractive keratoplasty (PRK), or phakic intraocular lens (IOL) implantation.

Pellucid Marginal Degeneration

Encouraged by the positive results in keratoconus patients, several investigators have evaluated Intacs inserts for the treatment of PMD. In a recent case series of 8 patients with at least 12 months follow-up, all eyes studied showed improved UCVA, and 75% had a BCVA of 20/25 or better. No intra- or postoperative complications were noted (Ophthalmology. 2005;112(4):660-666).

Future Directions

Implantation of Intacs inserts has become an accepted modality for keratoconus patients who have poor vision with spectacles and are contact lens intolerant. The implants offer a minimally invasive alternative to corneal transplant surgery, at least postponing the need for PKP and providing potentially lifelong quality functional vision in patients indicated.

Additional research is certainly needed to examine how variations in surgical technique may optimize improvements in corneal shape and to evaluate the long-term stability and safety of changes induced by corneal ring segments. Studies are also needed to identify how Intacs inserts affect corneal topography, higher order aberrations, BCVA, contact lenses use, and quality of life in addition to corneal clarity, corneal thickness, and the need for further eye surgery. Further studies and longer follow-up will help to define more clearly the benefits of Intacs segments for use in keratoconus, post-LASIK ectasia, PMD, and other corneal disorders.

References

Author Disclosure

The author discloses a financial interest as a member of the Speaker's Bureau and Medical Advisory Board for Addition Technology, Inc. She has no proprietary interest in any of the products or procedures discussed in this article.