KEY CONCEPTS
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There is an inverse relationship between the diameter of the ring and its effect on the cornea.
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The width of the intrastromal corneal ring segments is related to their diameter and this is of fundamental importance to the effect of the implant.
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Intrastromal corneal ring segments with a flat profiles have more effect at their ends and in the center than segments with conical profiles.
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Conical profile segments will release less tension compared with flat profile segments because their inclination is parallel to the cornea.
Anatomy of an Intracorneal Segment
The effect that intrastromal corneal ring segments (ISCRs) produce on the cornea can be explained in part by Blavatskaia et al.’s first law which states that the smaller the diameter, the greater the effect. For example, small-diameter segments, such as the Ferrara type (5 mm diameter), have a greater effect on the visual axis than large-diameter segments such as Intacs 7 mm ISCRs are available in various diameters: 5, 6, and 7 mm ( Fig. 24.1A ).
MATERIAL
The material used to manufacture the segments is polymethyl methacrylate (PMMA) (see Fig. 24.1B ). This material is a PMMA-Perspex QC that absorbs ultraviolet (UV) radiation and is manufactured to the specifications of the US Food and Drug Administration (FDA), ISO 9001, and EN46001.
APICAL DIAMETER
The apical diameter (see Fig. 24.1B ) is described by the line that passes through the center of the body of the segments, thus indicating the diameter of the optical zone that the segments enclose (5-, 6-, or 7-mm optical zone). Although this is not the actual optical zone, this system simplifies the method of classification by segment diameter.
WIDTH
The width of the ISCRs is the measure of the side where the segment rests ( Fig. 24.2 ), toward the corneal endothelium. The width of segments is related to their diameter and this is of fundamental importance to the effect of the implant. The width determines the profile of the segment, along with its angulation.
Older, flat-bottomed 5-mm diameter segments were 600 microns in length, whereas the 7-mm diameter conical segments required base length up to 1330 microns to produce an effect. Fig. 24.1 demonstrates the most frequent sizes found in the market.
SIZES
Some segment manufacturers use millimeters to indicate the characteristics of their implants (Ferrara Ring) ; however, refractive surgeons are used to describing them in microns, for example, a 0.25-mm segment is equivalent to 250 microns.
ARC
The segment of a circle is called an arc. ISCRs are described according to their central angle, the arcs of segments found on the market range from 60 degrees to 340 degrees, but the most frequently used are 90, 120, and 150 degrees. The surgeon selects them according to the pattern of the ectasia to be treated (see Figs. 24.1 and 24.2 ).
INNER EDGE
The inner edge of the segment delimits the optical zone (see Fig. 24.1 ). The placement of the smallest segments (5 mm of the optic zone) must be precise to avoid causing glare symptoms, which occur if slightly off-center. Segments of 6-mm diameter are used in the majority of cases, with 5-mm segments used only for special cases such as severe asymmetries or high post-penetrating keratoplasty (PK) astigmatism.
EXTREMES (END POINT)
Segment terminals ( Figs. 24.1A and 24.3 [1,2]) can be rounded as in the Ferrara type ISCRs or have a flat configuration as in Intacs segments. Some ISCRs have a notch, which allows locating the correct insertion position of the ring.
SECTION
There are two sections : profiles or slopes of the segments (see Figs. 24.2 and 24.3 ).
Flat Segments
Flat segments (Ferrara type) have a 0-degree inclination at their base. These segments are more difficult to implant within a conical structure, because of the tension created by the structure that houses them (flat segment in conical cornea) (see Fig. 24.2 ). This tension translates into a decrease in the keratometric values just above the body of the segment and elevation of the tissue at its ends.
These flat segments, originally designed by Ferrara and currently marketed by several companies (Ferrara Rings, Keraring, Mediphacos, Intraseg), present a base with a flat design, which must be parallel to the corneal endothelium, without angulation.
Conical
Intacs, Intacs SK, and Adapter are conical section segments designed with an inclination of their base of approximately 26 to 34 degrees (see Figs. 24.2 and 24.3 ). These designs mimic corneal curvature making them easier to insert, and they generate very little intracorneal tension. They generate an increase in volume, which produces their effect.
Conical segments maintain a prolate corneal shape without generating astigmatism.
Thickness
Blavatskaia et al. described the direct relationship between the effect of thickness and the effect of the ring: the greater the thickness of the implant, the greater the effect, and the smaller the thickness, the smaller the effect (see Figs. 24.2 and 24.3 ).
Fig. 24.3 shows the associated complications related to the thickness of the ISCRs (minor complications are shown in green, major complications are shown in red, and contraindications are shown in violet).
Asymmetric Segments
Asymmetrical segments have different profiles on each of their ends. The profile in Fig. 24.3 (4) demonstrates the gradual increase in segment height from the proximal to the distal end. The segment that is 150 microns at its proximal end is 250 microns at its distal end, the segment that is 200 microns at proximal end is 300 microns at its distal end.
Profiles of ISCRs
The ISCRs modify the corneal architecture in three main areas: (1) the area where the ring is located where an increase in corneal thickness occurs, (2) at the end of the extremities of the ring (where the tension vectors are released), and (3) in the center of the cornea, where corneal flattening occurs. Fig. 24.4 .
To demonstrate the different behavior of these implants according to their profile, Fig. 24.5 uses a balloon and an acrylic model that resembles an ICRS. It is easy to see that the conical segment (A) will offer less resistance, as it has a similar inclination to the structure represented by the balloon, whereas it is difficult to imagine the flat segment (B) being part of the structure because significant resistance will be generated once inside. These relationships are evident during surgery when the rings are inserted. The conical ring segments do not offer resistance whereas the flat segments do, especially if they have large arcs. In Fig. 24.5 , A-1 demonstrates presurgical spherical topography before the insertion of two conical segments of 150-degrees arc and 6 mm in diameter, and A-2 demonstrates the effect of the segments once implanted. The corneal structure maintains its prolate shape, but with lower keratometric values and without induced astigmatism. The same figure, shows the presurgical spherical topographic image (B-1) of a cornea in which two segments with a flat base of 150-degrees arc and 6-mm diameter were implanted, B-2 shows induced high central astigmatism as a result of the flat design of the segments.
