More than 20 million people in the United States have myopia between 1.00 and 3.00 diopters (D) with astigmatism of 1.00 D or less. Several options for visual rehabilitation are available to each individual, including spectacle correction, contact lens fitting, refractive keratotomy (RK), photorefractive keratectomy (PRK)  , laser-assisted in situ keratomileusis (LASIK) [2, 3] , and intracorneal rings, as shown below. [4, 5] Currently, nonsurgical approaches are still the least expensive and safest. For the subset of patients who desire freedom from glasses and lenses, refractive surgery is an option. [6, 7, 8]
The ideal refractive surgery procedure is one that is effective, predictable, safe, and potentially reversible. [9, 10] The goal of this article is to present a refractive surgical procedure in which implantation of a polymethyl methacrylate (PMMA) ring or ring segments in the peripheral corneal stroma is used to correct myopia and astigmatism.  What initially was limited to the correction of myopia and astigmatism is now being primarily used for the management of postrefractive laser surgery associated corneal ectasia and keratoconus. [12, 13, 14, 15, 16]
The intrastromal corneal ring (ICR) is a device designed to correct mild-to-moderate myopia by flattening the anterior corneal curvature without encroaching on the visual axis. The device is an open-ended PMMA transparent ring with an outer diameter (R2) of 8.1 mm and an inner diameter (R1) of 6.8 mm, and the curvature conforms to that of the cornea. It is precision lathe-cut to ±0.01 mm as a hexagonal-shaped section of a cone with positioning holes for manipulation.  It is inserted through a peripheral radial incision made with a diamond knife at two-thirds corneal depth in to a 360° peripheral intrastromal channel created with specially designed instruments. Anterior corneal curvature is changed by using rings of different thicknesses. 
Intrastromal corneal ring segments (Intacs microthin prescription inserts, as shown below), or ICRS, are a more recent design modification of the ICR. The ring segments split the ring into two 150° arcs.  The use of ring segments simplifies the implantation procedure. Furthermore, ring segments can be placed away from the radial incision, thereby minimizing the potential for incision-related complications. The degree of correction (greater-thicker) is determined by the thickness of the Intacs inserts, which are available in 5 sizes: 0.25 mm, 0.275 mm, 0.30 mm, 0.325 mm, and 0.35 mm. [20, 21, 22]
MediPhacos KeraRing is another intracorneal ring segment developed and marketed in Brazil. It is available in 2 models for 5-mm implantation in optical zones of 5 mm, 5.5 mm, and 6 mm. There are 40 different variations of thicknesses, arc lengths, and diameters, allowing for enhanced customization of corneal remodeling and refractive correction. Over 150,000 implants have been used worldwide.
Ferrara Ring is made of yellow PMMA, an inert and biocompatible acrylic used in intracorneal implants. The apical diameters are 5 mm and 6 mm. It has a flat base with a variable arch length of 90 mm and 210 mm and a variable thickness of 0.15 mm to 0.30 mm.
Addition Technology has surgical instruments specifically for use with Intacs insertion. These instruments include the following: corneal thickness gauges, glides, incision and placement marker, pocketing hook, pocketing lever, ring forceps, stromal spreader, vacuum-centering guide with vacuum system, and clockwise and counterclockwise dissectors. 
Ferrara Ring has its own surgical kit used in the insertion of the intracorneal ring segments. These instruments include the following: Ferrara spatula, Suarez spreader, Bicalho guide, Pre-delaminator, Ferrara marker, Sinskey hook of 0.20 mm, Modified McPherson forceps, and an adjustable Ferrara diamond blade knife. 
How do intracorneal ring segment inserts work?
Imagine the cornea as compressed arcs of fiber covering the eye. If a spacer element is inserted between those fibers, effectively pushing them apart, then the arc must flatten to accommodate the inserted element. The corneal ring complies with Barraquer and Blavatskaya postulates. According to these postulates, an addition in the cornea periphery results in its flattening, and the ring diameter determines how much the cornea will be flattened. In other words, the more tissue is added (increasing ring thickness) and the smaller the diameter, the greater will be the myopia correction obtained. Intracorneal ring segments inserts shorten the corneal cord length, produce flattening across the entire cornea, and maintain the positive asphericity of the cornea. 
Advantages of intracorneal ring segment inserts
Several benefits of using intracorneal ring segment inserts exist. It is a minimally invasive outpatient procedure with low morbidity. Since the surgery is completed in the peripheral cornea, the central optical zone is not disturbed. Results are rapid and predictable. There is a reduced risk of visual adverse effects and a long-term convenient refractive correction. It is removable and exchangeable. There is central and peripheral flattening of the cornea, maintaining corneal asphericity (prolate). 
Intracorneal ring segments may now be used to treat irregular steepening (irregular astigmatism) of the cornea caused by keratoconus. [27, 28] There is a stabilization or delay of cone progression. The intracorneal ring segment is made of acrylic, an inert and biocompatible material; therefore, there is lack of rejection. There is a high patient satisfaction rate. It is possible to combine with other procedures like contact lens fitting, corneal collagen cross-linking, PRK, phakic IOL implantation, and intraocular lenses. There is no evidence of any interference with a corneal transplant.
Disadvantages of intracorneal ring segment inserts
The technology is limited for use in low-to-moderate myopia with low astigmatism, corneal ectasia, and keratoconus.  It is still a relatively new technology with emerging complications (eg, diurnal fluctuations in visual acuity, stromal opacifications). [30, 31, 32, 33] Of further concern is the prolonged intraoperative elevated intraocular pressure. [34, 35]
History of the Procedure
The KeraVision Ring was conceived by A.E. Reynolds, OD, in 1978. Kera Associates formed in 1980 to develop the ICR and other concepts. In 1995, the first preclinical study on Reynolds' product was performed. 
In 1991, the first human clinical trials began in Brazil with 360° ICR. The rings were implanted in corneas of nonfunctional human eyes. Shortly thereafter, a similar study was performed in the United States. Implantation in sighted eyes in the United States was implemented in 1993 as part of a US Food and Drug Administration (FDA) regulated phase II study. 
Implantation in contralateral eyes began in 1994. Preliminary astigmatism study began in Brazil at this time. In 1995, the US phase II myopia trial began for 150° ICRS.  In 1996, the US phase III for -1.00 to -3.50 D of myopia began.  CE (Conformite Europeene) marking was granted, permitting the commercial sale of the ICR in the European Union. A Pan-European myopia study began for ICRS. In 1998, HPB approval was granted in Canada, while preliminary hyperopia studies were started in Mexico. 
A premarket approval (PMA) application was filed with the FDA. In 1999, the trade name Intacs inserts was adopted for commercial sales of the ICRS, and FDA approval was granted for the commercial sale of Intacs inserts for -1.00 to -3.00 D of myopia with less than -1.00 D of astigmatism in the United States. After a promising introduction into the refractive market, KeraVision, the maker of Intacs, experienced financial difficulties and eventually sold its assets to Addition Technology in April 2001. Since its introduction, however, the use of Intacs has expanded from the treatment of patients with low-to-moderate myopia with low astigmatism to the treatment of patients with keratoconus. 
Table 1. Evolution of Intacs Inserts (Open Table in a new window)
|360° ICR||Radial incision|
|360° ICR||Circumferential incision|
|Gapped ICR||Radial incision|
|Intacs inserts||Radial incision|
Addition Technology (Intacs) is the popular intracorneal ring segment used in Australia and the United States. In Europe, Ferrara Ring intracorneal ring segments are popular. Addition Technology was recently acquired by Ferrara Ring. Ferrara Ring has been in development since 1986 and over 100,000 implants have been used with excellent results.
According to the FDA, Intacs are intended for the reduction or elimination of mild myopia (-1.00 to -3.00 D spherical equivalent at the spectacle plane) in patients who are aged 21 years or older, in patients with documented stability of refraction as demonstrated by a change of less than or equal to 0.50 D for at least 12 months prior to the preoperative examination, and in patients where the astigmatic component is +1.00 D or less.
In 2004, Intacs was given humanitarian device approval by the FDA for use in patients with keratoconus. The inserts may now be used to reduce irregular steepening (irregular astigmatism) caused by keratoconus. Intacs inserts help restore clear vision in patients with keratoconus by flattening and repositioning the cornea. Intacs inserts are intended for patients with keratoconus who are no longer able to achieve adequate vision using contact lenses or glasses and for whom corneal transplantation is the only remaining option. 
Table 2. Predicted Nominal Correction and Recommended Prescribing Range (Open Table in a new window)
|Intacs thickness (mm)||Predicted nominal correction (D)||Recommended prescribing range (D)|
|0.25||-1.30||-1.00 to -1.625|
|0.30||-2.00||-1.75 to -2.25|
|0.35||-2.70||-2.375 to -3.00|
The main indication for the Ferrara Ring implant is for the management of progressive keratoconus (ie, with gradual decrease of uncorrected visual acuity [UCVA] and best-corrected visual acuity [BCVA] and progressive corneal steepening). The Ferrara Ring implant is also indicated in patients with unsatisfactory BCVA with glasses and intolerance to contact lenses and in post-LASIK corneal ectasia when there is worsening of the condition.
The cornea is a transparent, avascular tissue that is continuous with the opaque sclera and semitransparent conjunctiva. The cornea is covered by tear film on its anterior surface and bathed by aqueous humor on its posterior surface. 
In adults, the cornea measures 11-12 mm horizontally and 9-11 mm vertically. The average corneal thickness is 0.5 mm (500 µm) centrally and 0.7 mm (700 µm) peripherally.
Intracorneal ring segments are contraindicated in the following patients:
High degrees of myopia, hyperopia, or astigmatism
Patients younger than 21 years, with stable refractive error
Patients with existing collagen vascular, autoimmune, or immunodeficiency disease
Women who are pregnant or breastfeeding
Patients with previous anterior segment trauma 
In the presence of existing ocular conditions, such as recurrent corneal erosion syndrome, corneal dystrophy, or other corneal pathology that may predispose the patient to future complications
Patients who are using one or more of the following prescription medications that may affect corneal healing or vision: isotretinoin (Accutane), amiodarone (Cordarone), and/or sumatriptan (Imitrex)
Patients with corneas that are too thin
Patients with unrealistic expectations
Intracorneal ring segments should not be used in patients with keratoconus who have any of the following characteristics:
Can achieve functional vision on a daily basis by using contact lenses
Are younger than 21 years
Do not have clear central corneas
Have a corneal thickness of less than 450 µm at the proposed incision site
Have other options to improve their functional vision
Presence of apical opacities in very advanced keratoconus, usually with K readings above 75 D; postoperative results in these cases are usually poor and the best treatment for them is lamellar or penetrating keratoplasty
Warnings include the following:
Increased intraocular pressure results from the use of the vacuum-centering guide.
The surgeon should limit continuous application of the vacuum to 3 minutes or less and to no more than 750 mBar. In some instances, reapplication of the vacuum may be necessary, and it is advised that a 5-minute "reperfusion" phase be allowed prior to reestablishing suction.
It is not advisable to use intracorneal ring segments in patients with systemic diseases likely to affect wound healing (eg, insulin-dependent diabetes, severe atopic disease).
It is not advisable to use intracorneal ring segments in patients with a history of ocular manifestations of herpes simplex virus or herpes zoster virus.
Resterilization and/or reuse of intracorneal ring segments are not recommended.
Precautions include the following:
The use of thicker diameter (0.35 mm) ring segments is accompanied by a higher probability for a reduced outcome compared to the use of the thinner diameter (0.25-0.30 mm) ring segments. Dissatisfaction from the reduced outcomes may lead to a higher rate of removal of thicker diameter (0.35 mm) ring segments.
Patients with myopia of -1.00 D carry the higher risk for overcorrection.
The long-term effects of Intacs on endothelial cell counts have not yet been established.  A temporary decrease in corneal sensations may occur in some patients. Patients with large scotopic pupils may experience some visual symptoms.  They have to be properly advised. In some instances, some patients may experience a decrease in contrast sensitivity. 
No studies have been performed to establish the safety and effectiveness of other refractive procedures after the removal of Intacs. 
The safety and effectiveness of Intacs have not been established for the following:
- Patients with progressive myopia or astigmatism, nuclear sclerosis or other crystalline lens opacity, corneal abnormality, or previous corneal surgery or trauma
- Patients younger than 21 years
- Corneas that are flatter than 40 D or steeper than 46 D
- Corneas with a central thickness less than 480 µm or peripheral thickness less than 570 µm
- Patients with greater than -3.50 D of myopia or with astigmatism greater than +1.00 D
- In long-term use 
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