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)[1] , 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]
View Image | Graphic representation of the intracorneal ring segments in place. Reprinted with permission of Addition Technology Inc. |
View Image | Intacs microthin prescription inserts treat mild myopia. Reprinted with permission of Addition Technology Inc. |
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.[11] 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.[17] 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.[18]
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.[19] 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]
View Image | KeraVision Intacs microthin prescription inserts. Reprinted with permission of Addition Technology Inc. |
View Image | KeraVision Intacs microthin prescription inserts. Intracorneal ring segments. Reprinted with permission of Addition Technology Inc. |
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.[23]
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.[24]
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.[25]
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).[26]
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.
The technology is limited for use in low-to-moderate myopia with low astigmatism, corneal ectasia, and keratoconus.[29] 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]
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.[36]
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.[37]
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.[38] In 1996, the US phase III for -1.00 to -3.50 D of myopia began.[39] 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.[40]
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.[41]
Table 1. Evolution of Intacs Inserts
View Table | See Table |
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.[12]
Table 2. Predicted Nominal Correction and Recommended Prescribing Range
View Table | See Table |
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.[42]
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:
Intracorneal ring segments should not be used in patients with keratoconus who have any of the following characteristics:
Warnings include the following:
Precautions include the following:
Corneal topography development provides surgeons with effortlessly understood color-coded maps of corneal curvature in addition to quantitative indexes of irregular astigmatism that correlate with potential visual acuity.
Modern instrumentation produces a videokeratograph, which generally is in the form of a color-coded contour map.
Different manufacturers use different methods (eg, Placido, 40 scanned slits, combination Placido and 40 scanned slits, phase modified laser holography, raster stereography).
The use of video keratography in the preoperative and postoperative evaluation of all patients undergoing refractive surgery is valuable.
This is a high-precision optical system for corneal topography and 3-dimensional analysis of the anterior eye segment, based on a Revolving Dual Channel Scheimpflug Camera and a Placido Disk.
Placido imaging furnishes high accuracy curvature data.
Scheimpflug imaging is optimal for precise elevation data.
The pachymeter (optical, ultrasonic) is used to measure corneal thickness.
Accurate determination of corneal thickness preoperatively allows the surgeon to set the depth of incision to two thirds of the measured result.
Orbscan II by Orbtek uses both 40 scanned slits and Placido methods to provide anterior and posterior corneal curvature in addition to data on corneal thickness.
A manual corneal trephination set to create a corneal tunnel for intrastromal corneal ring segments has been available since the introduction of ICRS. Currently, most femtosecond laser units are able to create similar tunnels using a specific program. The femtosecond laser–created tunnels are more precise and result in less morbidity than the manual approach.
Medical therapy is limited to broad-spectrum topical antibiotics and corticosteroids for uncomplicated cases.
See Postoperative details and/or articles on ocular pharmacology for more information.
The procedure for placement of the intrastromal ring and ring segments is similar, and both can be performed with topical anesthesia. The operative field is prepared, and the patient is prepared and draped in the usual sterile fashion for ophthalmic surgery. A lid speculum is used for globe exposure.
The corneal center is identified and marked with a Sinskey hook.
A 2-mm long epithelial impression is created at the 12-o'clock position, where the ring segments are to be placed at an 8-mm diameter optical zone.
A diamond blade, set at 65% of the peripheral corneal depth, is used to perform a radial incision along this mark. See the image below.
View Image | The 1.2-mm radial incision is made with a diamond knife at the edge of a 7-mm optical zone. The diamond knife is set for approximately two thirds of c.... |
A vacuum-centering guide is positioned relative to the central corneal indentation.
The vacuum is increased, and the guide is circumferentially adhered onto the perilimbal conjunctiva, stabilizing the globe. This device provides a guide for the dissector. The dissector is inserted into the radial incision, and blunt dissection of the cornea at two-thirds depth is performed in the clockwise and counterclockwise directions to create 2 stromal channels. See the image below.
View Image | Rotation of the dissector creates the tunnel in the peripheral cornea into which an Intacs insert will be placed. Reprinted with permission of Additio.... |
The vacuum is released, and the vacuum-centering guide is removed.
Ring segments are inserted through the radial incisions using special forceps and are positioned using a Sinskey hook nasally and temporally, such that the superior ends are approximately 3 mm apart. See the images below.
View Image | The first Intacs insert is placed in the tunnel. Reprinted with permission of Addition Technology Inc. |
The incision is closed with 1 or 2 interrupted 11-0 nylon sutures.
View Image | An Intacs insert in place at approximately two thirds of corneal depth. Reprinted with permission of Addition Technology Inc. |
Postoperatively, antibiotic-corticosteroid combination drops and/or ointment are used, and the speculum is removed. The eye may be covered overnight with a shield. Antibiotic-corticosteroid combination drops are used 4 times daily for 1 week. The sutures are removed 2 weeks postoperatively or longer if the incision is not healed adequately.
Patients are seen postoperatively on day 1, week 1, and months 1, 3, 6, and 12. The surgeon should watch for postoperative complications. An observations timeline has been outlined for potential complications. See Complications.
Complications include localized incision-related epithelial defects, epithelial plug formation, wound dehiscence, superficial neovascularization, surgically induced astigmatism, infiltrates in the channel, transient decreased corneal sensation, and delayed infectious keratitis.[49, 50] Other observations included haze and deposits around the intrastromal channel.
Observations timeline consists of the following:
In selected patients with undercorrection or overcorrection, an exchange procedure can be done. In selected patients who are dissatisfied, an explantation procedure can be done. The procedures can be done with the patient using only topical anesthesia. The surgical steps are listed below:
A summary of the safety and efficacy variables at 12 months after surgery is as follows:
Applications of intrastromal ring segments include the following:
Design Technique 360° ICR Radial incision 360° ICR Circumferential incision Gapped ICR Radial incision Intacs inserts Radial incision
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