Intrastromal corneal ring segments (ICRS) can treat low degrees of myopia by displacing the lamellar bundles and shortening the corneal arc length. However, the main indication for ICRS placement is keratoconus and other forms of ectatic corneal diseases. These circular arcs, made of PMMA, are placed in the posterior midperipheral corneal stroma in a lamellar channel (Figs 4-2, 4-3). The thicker the segment is, the greater will be the flattening of the central cornea and the reduction in myopia. Ferrara rings (Ferrara Ophthalmics, Belo Horizonte, Brazil) or Kerarings (Mediphacos, Belo Horizonte, Brazil) have a smaller optical zone and a greater flattening effect in comparison to Intacs (CorneaGen, Seattle, WA). This section focuses on Intacs because Ferrara-type rings, although commonly used internationally, are not FDA approved for use in the United States.
Figure 4-2 Rendering of a cross section of the cornea with an intrastromal corneal ring segment. The ring segment 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)
Excerpted from BCSC 2020-2021 series: Section 13 - Refractive Surgery. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.