Intrastromal Corneal Ring Segments

displaces the lamellar bundles, thereby shortening the corneal arc length and reducing the myopia. (Courtesy of Addition

Technology.)

Figure 4-3 Clinical photograph showing ring segments implanted in an eye to treat low myopia. Note the vertical placement of the ring segments with a clear central zone. (Courtesy of Steven C. Schallhorn, MD.)

Treatment using ring segments has several potential advantages over other forms of refractive surgery. The ring segments can be explanted, making the refractive result of the procedure potentially reversible, and they can be replaced with ring segments of a different thickness to titrate the refractive result. Intacs are FDA approved to treat myopia at levels ranging from –1.00 to –3.00 D spherical equivalent; they are not approved for patients with astigmatism. However, Intacs surgery is no longer commonly performed for myopia because the results are not as predictable as are those with ablative corneal surgery.

Intacs are typically contraindicated in

patients with collagen vascular, autoimmune, or immunodeficiency diseases pregnant or breastfeeding women

patients who may be predisposed to future complications because of the presence of ocular conditions (such as herpetic keratitis, recurrent corneal erosion syndrome, and corneal dystrophy)

Instrumentation

Initially, a 1-piece 360° Intacs ring was used in the procedure, but it proved difficult to insert. The design was later changed to 2 segments of 150° arc. The segments have a fixed inner diameter of 6.50 mm and an outer diameter of 8.10 mm, and they are available in various thicknesses: 0.210, 0.250, 0.275, 0.300, 0.325, 0.350, 0.400, and 0.450 mm. The degree of correction achieved is related to the thickness of the ring segments; thicker ring segments are used for greater correction. Manually operated surgical equipment or a femtosecond laser can be used to create the channels.

Technique

The procedure involves creating a lamellar channel at approximately 68%–70% stromal depth, followed by insertion of the ring segments. The geometric center of the cornea is marked with a blunt hook. An ultrasound pachymeter is used to measure the thickness of the cornea over the entry mark. A

diamond knife is set to 68%–70% of the stromal depth and then used to create a 1.0-mm radial incision. Specially designed mechanical instruments are then used to create the channels for the segments by blunt separation of the collagen lamellae (Fig 4-4). Similar entry incisions and channels may be created using a femtosecond laser. The channels are created in an arc pattern at the desired inner and outer diameters. Once the channels are created, specialized forceps are used to insert the first ring segment and rotate it into position, followed by similar insertion and rotation of the second segment. Tissue glue or 1 or 2 10-0 nylon sutures may be used to close the radial incision at the corneal surface.

Figure 4-4 Rendering of the Intacs dissector tool as it is being rotated to create the intrastromal channel. (Courtesy of Addition

Technology.)

Outcomes

FDA clinical trials provided the most complete outcome analysis of Intacs for myopia. A total of 452 patients enrolled in these trials. Patients received 0.25-, 0.30-, or 0.35-mm ring segments to correct an average preoperative mean spherical equivalent of –2.240 D (range, –0.750 to –4.125 D). At 12 months postoperatively, 97% of treated eyes had 20/40 or better uncorrected vision and 74% had achieved 20/20 or better. In addition, 69% and 92% of eyes were within ±0.50 and 1.00 D of emmetropia, respectively. These clinical outcomes were similar to early results with photorefractive keratectomy (PRK) and LASIK, although excimer laser studies generally treated a broader range of preoperative myopia.

Additional FDA approval was later granted to include intermediate segment sizes of 0.275 and 0.325 mm. Internationally, CE (Conformité Européene) marking status (similar in concept to US FDA approval) was extended to thicker segment sizes. In 2000, Colin found that Intacs implantation compared favorably with PRK for treating low myopia, although it induced greater astigmatism.

The removal or exchange rate varies between 3% and 15%. The most common reason for a ring segment exchange is residual myopia. Ring segment removal is most often performed because of disabling vision symptoms such as glare, double vision, and photophobia. Few complications are associated with ring segment removal. In a series of 684 eyes that received Intacs, 46 (6.7%) underwent their removal. Most patients returned to their original preoperative myopia by 3 months postremoval (73% returned to within 0.50 D of preoperative mean spherical equivalent). No patient had a loss of CDVA of more than 2 lines. However, up to 15% of patients reported new or worsening symptoms after removal.

Intacs and Keratoconus

Until recently, very few surgical options other than penetrating and lamellar keratoplasty were available for the treatment of keratoconus. Excimer laser procedures, which correct ametropia by removing tissue, are generally not recommended for treating keratoconus because of the risk of exacerbating corneal structural weakening and ectasia.

In 2004, Intacs received a Humanitarian Device Exemption (HDE; see Appendix 1) from the FDA for use in reducing or eliminating myopia and astigmatism in certain patients with keratoconus, specifically those who can no longer achieve adequate vision with their contact lenses or spectacles. The intent is to restore functional vision and defer the need for a corneal transplant. Labeled selection criteria for patients include

progressive deterioration in vision such that the patient can no longer achieve adequate functional vision on a daily basis with contact lenses or spectacles

age 21 years or older clear central corneas

a corneal thickness of 450 µm or greater at the proposed incision site

a lack of options other than corneal transplantation for improving functional vision

Although these are FDA labeling parameters, many surgeons perform Intacs insertion outside these criteria. In one study of 26 keratoconus patients, the ring segments were oriented horizontally, with a thick ring (0.450 mm) placed in the inferior cornea and a thinner one (0.250 mm) in the superior cornea. In another study of 50 patients (74 eyes), the orientation of the ring segments was adjusted according to the refractive cylinder. On the basis of the level of myopia, either the 0.300-mm ring or the 0.350-mm ring (the largest available in the United States at that time) was placed inferiorly, and the 0.250-mm ring was placed superiorly. Patients had mild to severe keratoconus with or without scarring. A superficial channel with perforation of the Bowman layer in 1 eye was the only operative complication. A total of 6 rings were explanted for segment migration and externalization (1 ring) and foreign-body sensation (5 rings).

The improvement in vision was significant. With an average follow-up period of 9 months, the mean uncorrected distance visual acuity (UDVA; also called uncorrected visual acuity, UCVA) improved from approximately 20/200 (1.05 logMAR [base-10 logarithm of the minimum angle of resolution]) to 20/80 (0.61 logMAR) (P < .01). The mean CDVA also improved, from approximately 20/50 (0.41 logMAR) to 20/32 (0.24 logMAR) (P < .01). Most patients still required optical correction