Intrastromal corneal ring segments (ICRSs) are a proven treatment option for keratoconus. Implantation of these crescent-shaped rings between layers of the corneal stroma flattens the cornea and changes its refraction. Considering the wide range of available models, it is essential not only to understand how each affects corneal topography but to also select the most appropriate model for the patient. This article focuses on one available type of ICRS, the Keraring (Mediphacos).
The Keraring has two models, the SI5, which has a diameter of 5 mm, and the SI6, which has a diameter of 6 mm. Each model is available in a variety of thicknesses (150, 200, 250, 300, and 350 μm) and arc lengths (90°, 120°, 160°, 210°, and 355°), allowing up to 50 possible combinations for personalized treatment. Additionally, the surgeon can choose between implanting a single ICRS or multiple ICRSs.
Understanding the mechanism of action of the ICRS in different corneal topographies can aid in the task of choosing the right Keraring; however, little is known about the final topographic effect of the ICRS. What we do know is that the ring’s thickness, diameter, and arc length affect its primary mechanism of action, which is corneal flattening. Essentially, the thicker the ICRS and smaller its diameter, the greater the flattening effect. Longer arc lengths improve spherical refraction whereas shorter arc lengths improve cylinder.
Over the past 2 years, I have been studying the corneal topography characteristics of keratoconic eyes implanted with the Keraring in order to understand how the various models affect corneal topography and improve refraction. Through this work, I have developed ring simulator software that includes a database of the topographic action of each Keraring model. When the patient’s preoperative keratometry (K), refraction, and topography are entered into this software, it will simulate the effects of the various Keraring models on corneal topography. This tool can allow ophthalmologists to make an informed decision about the best Keraring model for an individual patient.
In developing this software, I studied 420 eyes with stage 2 or 3 keratoconus that were implanted with the Keraring. All eyes were evaluated using topography and elevation differential maps to understand the effect of the Keraring on topographic and visual outcomes; follow-up ranged from 6 to 36 months.
The primary observation of this study was that, although the corneal flattening profile of each model of the Keraring was independent of the shape of the keratoconus, the achieved change in topographic refraction depended on the biomechanics of each cornea (Figure 1). Based on our theoretical models of the flattening profiles of the two main Keraring models, we found that the SI5 created peripheral and central flattening, with its greatest flattening effect in the periphery, and the SI6 created primary peripheral flattening accompanied by central steepening on the opposite axis (Figure 2).
These observations explain the refractive changes caused by the SI5 and SI6 models. With the SI5, we observed an average sphere reduction of 4.20 D (range, 0.75–9.00 D) and an average cylinder reduction of 4.02 D (range, 1.25–7.50 D). This is a result of the peripheral and central flattening associated with this ICRS model. With the SI6, we observed a smaller average spherical reduction (2.00 D; range, 0.75–3.50 D) and a greater average cylinder reduction (6.50 D; range, 2.00–9.00 D). The astigmatism correction achieved with the SI6 can be attributed to the central steepening effect on the opposite axis.
Arc length also played a role in refractive outcomes. The longer the arc length, the greater the area of flattening and the spherical correction. Conversely, the shorter the arc length, the smaller the area of flattening, thus causing a couple of effects: flattening in one axis and steepening on the opposite axis that contributes to cylindrical correction (Figure 3).
CHOOSING BETWEEN THE SI5 AND SI6
In practice, the choice between the Keraring SI5 and SI6 depends on the patient’s K reading and the shape and surface of his or her ectasia. For instance, in eyes with early keratoconus and central K less than 52.00 D, where the ectasia surface area is still small and does not reach or affect the central cornea, the desired effect of the ICRS is astigmatism reduction, not central flattening. In this scenario, the ideal Keraring model is the SI6. On the other hand, in eyes with advanced keratoconus and K readings greater than 52.00 D, where the surface area of the ectasia is large and affects the central cornea, the preferred Keraring model is the SI5, which induces central corneal flattening and astigmatism reduction.
K readings less than 60.00 D, pachymetry greater than 400 μm, and BCVA greater than 2/10 are good prognostic factors in the treatment of keratoconus. Another strong determinant of the success of keratoconus treatment, I believe, is ectasia shape. Hence, ophthalmologists should aim to match the flattening profile of each ICRS model with the patient’s ectasia shape.
The results of my study provide several guidelines for use of ICRSs in the management of keratoconus. These guiding principles are outlined in the Take-Home Message box on page 31.
Additionally, the ring simulator software and database of topography actions of the Keraring models developed during this study may eventually lead to surgeons more easily choosing the best Keraring model for their patients’ specific needs.
Heykel Kamoun, MD, is Head of the Cornea and Refractive Surgery Department at the International Eye Clinic, in Tunis, Tunisia. Dr. Kamoun states that he has no financial interest in the products or companies mentioned. He may be reached at e-mail: firstname.lastname@example.org.