2020–2021 BCSC Basic and Clinical Science Course™
13 Refractive Surgery
Chapter 4: Onlays and Inlays
Intrastromal Corneal Ring Segments
Number of Segments
Although most surgeons implant 2 Intacs segments, the use of only 1 segment may be indicated. If the steep area is peripheral (similar to pellucid marginal degeneration), it may be preferable to place 1 segment instead of 2 segments because the keratoconic cornea has 2 optical areas of distortion within the pupil: a steep lower area and a flat upper area. For peripheral keratoconus, it is better to flatten the steep area and steepen the flat area than to flatten the entire cornea. Single-segment placement can achieve that result (Fig 4-6). When a single segment is placed, it flattens the adjacent cornea but causes steepening of the cornea 180° away—the “beanbag effect” (ie, when one sits on a beanbag, it flattens in one area and pops up in another area). This effect may yield a more physiologic improvement than would the global flattening effect from the use of double segments. Intacs treatment can also be combined with corneal crosslinking for improved corneal strength and phakic IOL implantation to improve refractive error (see Chapter 7).
Figure 4-6 Corneal topography analysis before and after single-segment Intacs placement. The preoperative topography (lower left) shows oblique steepening, and the postoperative topography (upper left) shows contraction of a steep cone after a single-segment Intacs was placed outside the cone. The difference map (subtraction of preoperative and postoperative topography) (right) shows topography over the ring segment (blue) and steepening in the overly flat area (red). The apex of the cornea has moved more centrally.
(Courtesy of Brian S. Boxer Wachler, MD.)
Bedi R, Touboul D, Pinsard L, Colin J. Refractive and topographic stability of Intacs in eyes with progressive keratoconus: five-year follow-up. J Refract Surg. 2012;28(6):392–396.
Ertan A, Karacal H, Kamburoğlu G. Refractive and topographic results of transepithelial cross-linking treatment in eyes with Intacs. Cornea. 2009;28(7):719–723.
Sharma M, Boxer Wachler BS. Comparison of single-segment and double-segment Intacs for keratoconus and post-LASIK ectasia. Am J Ophthalmol. 2006;141(5):891–895.
Wollensak G, Spörl E, Seiler T. Riboflavin/ultraviolet-A-induced collagen crosslinking for the treatment of keratoconus. Am J Ophthalmol. 2003;135(5):620–627.
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.