KEY CONCEPTS
Mechanism:
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An intrastromal corneal ring segment (ICRS) acts as a spacer, shortens arc length, and increases thickness in the cornea.
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The corrective effect is correlated in direct proportion to the thickness of the implant and in inverse proportion to its diameter (optical zone).
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New, progressive asymmetrical ICRSs have been designed for specific types of keratoconus such as asymmetric bowtie (snowman), oval (duck), or pellucid-like (lobster claw) types, because this design provides a more flattening effect on the steeper part of cornea with its thicker end and vice versa.
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ICRSs with long arc lengths, like 320 degrees or 360 degrees, act as an artificial limbus biomechanically and are effective in nipple-type and advanced keratoconus.
Techniques:
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There are two main techniques for ICRS implantation: manual (mechanical) and femtosecond laser techniques.
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Reference centers for the channel location, channel depth, and proper incision site are the main considerations in ICRS implantation.
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The limbus center, the pupil center, Purkinje reflex, or the point between the pupil center and Purkinje reflex can be considered as the center to determine the channel location.
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The depth of the incision site and the channel track should be measured accurately and 70% to 80% of the thinnest point should be taken into account for ICRS implantation.
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Migration of the segment to the incision site is responsible for most of the complications of extrusion and melting at the incision location. To avoid migration, the incision site should be far from the ring tip. Distance of the incision site to tips of the rings should be approximately 10 degrees in case of implantation of two segments. However, for single ring implantation, the incision site should be created much farther from the tip of the segment.
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In either technique, properly implanted segments should be in a symmetrical position, placed in deep stroma, and with tips far from the incision.
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The femtosecond laser technique allows more accurate stromal dissection and yields good visual and refractive results with much lower complication rates.
Mechanism
Jose I. Barraquer published his article on the thickness law, “The modification of the refraction by means of intracorneal inclusions,” in 1966. He noted that the addition of material to the corneal periphery flattened the central cornea. One of the considered action mechanisms for intracorneal ring segment (ICRS) implantation is related to thickness addition to obtain flattening. As the thickness of the segment increases, the anterior displacement of the cornea with the segment and sequentially central corneal flattening also increase. , Another explanation is the spacer effect. An ICRS forms a new, additional space within collagen fibers of the corneal stroma ( Fig. 23.1 ). This spacer effect of the ICRS shortens the central arc length of the cornea (arc-shortening effect) and induces central corneal flattening. , In a normal cornea, this effect can induce a predictable, direct proportion. However, keratoconic corneas have different biomechanical properties because of the irregular distribution of collagen fibers, unlike the orthogonal alignment of collagen lamellae in the normal cornea. Hence, the effect of ICRSs in eyes with keratoconus is expected to be different from the normal corneas.
ICRSs have a flattening effect on the line that passes through both ends of the segment and a steepening effect on the line perpendicular to the segment arc. It is known that the corrective effect is correlated in direct proportion to the thickness of the implant and in inverse proportion to its diameter (optical zone) ( Fig. 23.2 ). A shorter segment induces a greater astigmatic correction compared with long arc segments, whereas long arc segments decrease the prolate shape in advanced cases and produces flattening in nipple and oval keratoconus. Studies of rings with long arc lengths (320-degree arc segments or the full ring of 360-degree designs) demonstrate that these ring types act as an artificial limbus biomechanically. This effect separates the load on the cornea, separating the intraocular pressure into two independent loads, one inside the inner diameter of the implant and the other one between the outer diameter of the implant and the limbus. ,
Based on the proportional correction effect of the ICRS thickness, progressive, asymmetric ring segments have been developed. , These segments are designed as thicker at one end and thinner at the other end ( Fig. 23.3 ). This design provides a greater flattening effect on the steeper part of cornea with its thicker end and vice versa and, therefore, allows the surgeon to remodel the corneal irregularity in specific types of keratoconus such as asymmetric bowtie (snowman), oval (duck), or pellucid-like (lobster claw) types ( 23.4A,B and Fig. 23.5 ).
The availability of various types of ICRS with different thicknesses, optical zones, and designs enables surgeons to treat several corneal irregularities together with keratoconus. ICRS implantation can be performed to regularize irregular astigmatism after penetrating or anterior lamellar keratoplasty, after radial keratotomy, and in post-LASIK/photorefractive keratectomy (PRK) ectasia.
A novel, customized ring presented by Mediphacos at the XXXIII Congress of European Society of Cataract and Refractive Surgeons in 2015 was based on the idea of a corneal endoskeleton. This segment, called Keraring Second Generation, acts like a tent skeleton or a bra wire after implantation within the cornea and gives a new shape to the cornea according to its own curvature ( Fig. 23.6 ). It is a double arc segment. The optical zone of 5 mm is bound to a surrounding arc of 8 mm. The second arc is considered as an artificial limbus.
Techniques
There are two main techniques for ICRS implantation: manual (mechanical) and femtosecond laser techniques. Three key points should be considered to achieve successful outcomes in ICRS implantation, independent of the performed technique. These are the channel location with respect to the center, the channel depth, and the proper incision site.
The channel location is determined according to the center of the cornea. However, the preferred center may change according to the surgeon’s approach. The limbus center, the pupil center, Purkinje reflex, or the point between the pupil center and Purkinje reflex can be considered as the center to determine the channel location ( Fig. 23.7 ).
The creation of channels at appropriate depth is another important point in implantation. The thickness of the incision site and along the channel track should be measured accurately, and 70% to 80% of the thinnest point should be taken into account for ICRS implantation ( Fig. 23.8 ). Otherwise, inaccurate measurement of the thickness in the channel track and the incision site will lead to improper creation of the channels and consequently, to complications such as perforation or anterior or posterior dislocations of the segments.
Determination of the incision site is the third key point that should be planned before implantation. The steepest keratometric axis is the commonly preferred place for the incision site in case of implantation of two segments. The type of keratoconus may affect this decision. Temporal incision in patients with oval and central cones is reported in some studies. Some authors propose the axis of coma aberration as the reference line for the channel location and the incision site. Wherever the incision site is placed, it should be far away enough from the ends of the segments to minimize complications such as segment migration, extrusion, and corneal melting.
Properly implanted segments in either technique should be:
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in a symmetrical position,
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placed in deep stroma,
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with tips far from the incision.
MANUAL (MECHANICAL) TECHNIQUE
The first ICRS implantations were performed with the manual technique. Special surgical instruments were developed for this technique ( Fig. 23.9 ). Steps are as follows in the manual technique:
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Firstly, the visual axis is determined. The patient is asked to look at the fixation light of the operation microscope, and the Purkinje reflex is marked ( Fig. 23.10 ).
Fig. 23.10
Marking of Purkinje reflex.
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The planned optical zone and incision site are marked using a marker stained with gentian violet and centralizing the determined Purkinje reflex ( Figs. 23.11A,B ).
