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.

In keratoconus, the corneal elastic modulus is reduced due to pathology in the corneal stroma. From a biomechanical perspective, the resistance to deformation is reduced in relation with the reduction of the elastic modulus that leads to increased strain and protrusion in the cornea. The consequence is increased curvature and corneal thinning, the hallmarks of keratoconus. Since stress is defined as applied force divided by cross-sectional area, stress focally increases in the zone of corneal thinning. The placement of intracorneal ring segments generates both an immediate response that interrupts the biomechanical disease progression in keratoconus, and a time-dependent biomechanical response that allows subsequent improvement of vision over 6 months. The immediate response governed by the elastic properties and the long-term response is by viscoelastic properties. Intracorneal ring placement results in a reduction of astigmatism and improved visual acuity. This is accomplished by shortening the path length of the portion of the collagen lamellae which are central to the segments⁵. Redistribution of corneal curvature leads to a redistribution of corneal stress, interrupting the biomechanical cycle of keratoconus disease progression and, in some cases, reversing the process.

Goals of surgery

Indications for surgery

Preoperative assessment

Anesthesia

The intracorneal ring procedure is usually performed under topical anesthesia with oral sedation depending on patients’ preference.

Operation techniques

Assessment of surgery: self-evaluation; result of surgery

Colin et al.⁹ reported 2-year results in 100 keratoconic eyes by using this standardized nomogram: two symmetric Intacs segment of either 0.45 mm or 0.40 mm were inserted in the cornea. In this study the thickness of the Intacs segments used was based on the preoperative spherical equivalent, with eyes of >−3.00 D receiving 0.45 mm inserts and those of <−3.00 D receiving 0.40 mm inserts.

Using the mechanical dissection method, Colin et al. reported that 78% of eyes gained lines in uncorrected visual acuity (UCVA) with an improvement in mean refractive spherical equivalent (MRSE) and mean K value of 3.1 ± 2.5 D and 4.3 ± 2.8 D, respectively. Alio et al. reported that BCVA increased from 20/50 to 20/30 and remained constant at 36 and 48 months and MRSE improved from −5.40 D to −3.95 D. Although they found fluctuation in K values, refraction remained constant at 36 and 48 months.

Corneal cross-linking treatment is another surgical option to improve visual acuity after intracorneal segments. Cross-linking, unlike intracorneal segments, has been shown experimentally to increase the biomechanical rigidity by 4.5 times. Furthermore, with cross-linking of the collagen lamellae, collagen fibril diameter also increases⁸.

Chan et al. proposed an additional reason for the combination of the two therapies. Their study of 25 eyes found that combining C3-R with Intacs resulted in an augmentation of the flattening effects of Intacs. There was statistically greater reduction in cylinder and K values in the Intacs +C3R group compared with the Intacs alone group. In particular, there was a greater than two-fold reduction in steep and average keratometric values. There are a few possible reasons for the increased effect seen with the addition of cross-linking. Firstly, it may be due to a simple additive effect as Wollensak et al. showed that an average of 2.01 D reduction in mean K occurred with cross-linking alone in about 70% of eyes in their hallmark study. Secondly, the channel created for Intacs insertion may result in localized pooling and concentration of the riboflavin around the Intacs segment. The biomechanical changes also induced by the Intacs may facilitate the changes caused by C3-R.