Intracorneal Ring Segments and Keratoconus
In the past, 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 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 glasses (Fig 4-5). The intent was 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 glasses
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
Figure 4-5 Slit-lamp biomicroscopy of the cornea immediately after symmetric intracorneal ring segment implantation for the surgical management of keratoconus.
(Courtesy of George O. Waring IV, MD.)
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 visual acuity (UCVA); also called uncorrected distance visual acuity, UDVA) 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 BCVA also improved, from approximately 20/50 (0.41 logMAR) to 20/32 (0.24 logMAR) (P <.01). Most patients still required optical correction to achieve their best-corrected vision. Eyes with corneal scarring had a similar improvement in UCVA and BCVA. Inferior steepening was reduced on topography as was coma. The dioptric power of the inferior cornea relative to the superior (I–S value) was reduced from a preoperative mean of 25.62 to 6.60 postoperatively.
A study evaluating the long-term stability of Intacs in keratoconus found that in nearly 93% of patients with documented progression of keratoconus pre-Intacs, there was no further progression of keratoconus between 1 and 5 years after Intacs implantation. In addition, no statistically significant differences were noted in mean steep, flat, and average keratometry readings; manifest refraction spherical equivalent; and UCVA and BCVA (P >.05) between 1 and 5 years postimplantation.
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.