Introduction

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Non-inflammatory corneal thinning disorders (keratoconus, pellucid marginal degeneration (PMCD), and keratoglobus) are characterized by progressive corneal thinning. With the advance of thinning, the cornea becomes more irregular and ectatic. To date, the therapeutic options for patients with corneal ectasia are limited to spectacles and contact lenses, while in advanced stages of the disease the accepted approach is penetrating keratoplasty (PKP). Several attempts at alternative methods in order to treat keratoconus are reported in the literature, such as thermal keratoplasty, epikeratoplasty, photorefractive keratectomy, laser in situ keratomileusis (LASIK), and deep lamellar keratoplasty. Recently intrastromal corneal ring segments (ICRS) have been designed to achieve a refractive adjustment by flattening the cornea.

Epidemiologic consideration and terminology

Keratoconus is a progressive disorder and usually its progression rate is higher before the fourth decade. The treatment of keratoconus, up to date, was mostly focused on visual rehabilitation. In the future, controlling the progression of the disease will be another important goal for treatment modalities to achieve. This would allow more patients to be in the older ages when the progression of the keratoconus slows down, without significant complication and visual deprivation. Although long-term results with controlled studies are not known, depending on early results, ICRS not only provide better visual quality but also may help in controlling the progression of keratoconus^1,2. With the advent of cross-linking, and perhaps with the combination of both modalities, the need for keratoplasty for keratoconus, which has an approximately 20% rejection rate, will decrease. Moreover, it will be possible to help patients with advanced disease with stronger ICRS treatments and state of the art corneal lamellar surgery³.

Post-LASIK ectasia is a rare, but vision threatening complication. The incidence of ectasia after refractive surgery is not known precisely, but has been estimated to be 0.2%–0.66% in two studies. Ectasia can be defined as progressive non-inflammatory corneal thinning after surgery resulting in irregular topographic steepening and resultant irregular astigmatism. Ectasia can occur after any keratorefractive procedure but, for the sake of this chapter, we will be addressing only laser in situ keratomileusis (LASIK) and surface ablation.

Anatomical considerations

The changes in corneal structure induced by additive technologies can be roughly predicted by the Barraquer ‘thickness law’: when you add material to the periphery of the cornea or remove an equal amount of material from the central area, you achieve a resultant flattening effect. Vice versa, when you add material to the center or remove it from the corneal periphery, you obtain a steeped surface curvature. The corrective result varies in direct proportion with the thickness of the implant and in inverse proportion with its diameter. The thicker and the smaller the device the higher is the corrective result obtained.

Fundamental principles

According to the postulates of Barraquer and Blavatskaya, intracorneal ring acts as tissue addition leading to a flattening in the cornea periphery. The diameter of the ring is proportionally inverse to the flattening intensity thus, the smaller the diameter, the more tissue added (ring thickness) with the higher myopic correction⁴.

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