Intrastromal Corneal Ring Segments (ICRS)
Intrastromal Corneal Ring Segments (ICRS)
Corneal thinning disorders often lead to protrusion, irregular astigmatism, and even perforation. Keratoconus, PMD (Pellucid Marginal Degeneration), and Keratoglobus do not have a known cause. However, corneal ectasia can also be acquired after the LASIK (Laser Assisted In Situ Keratomileusis) procedure. Corneal ectatic disease may impair vision which can be restored by optical means such as glasses, soft, or rigid gas permeable contact lenses. However, severe cases may require correction or restoration of tectonic integrity of the cornea by surgical means. Previously, PKP (Penetrating Keratoplasty) was the treatment of choice  . Although PKP can be very successful in this subset of patients, limitations of PKP can include high post-op cylinder, side effects from chronic topical steroids, corneal rejection, and recurrence of keratoconus in the donor cornea. Intracorneal ring segments are a new modality in the treatment of corneal ectactic disorders.
Intrastromal corneal ring segments (ICRSs) are made of PMMA (polymethylmethacrylate). They are implanted in the deep corneal stroma to modify the corneal curvature. This procedure does not involve corneal tissue nor does it invade the central optical zone. The intrastromal ring concept was proposed by Reynolds in 1978. It was first evaluated as a treatment for myopia by Keravision. INTACS® technology for myopia received European CE Certification in 1996 and Food and Drug Administration (FDA) approval in 1999 . This technology was then adopted for the treatment of keratoconus. In 2004, INTACS (sizes 0.25 -0.35mm) were given an FDA Humanitarian Device Exemption for their use in keratoconus. In 2010, the exemption was extended to include the 0.4 and 0.45 mm rings.
Colin et al were the first to report the use of these rings in keratoconus . They found INTACS reduced the corneal steepening and astigmatism associated with keratoconus.
There are four types of ICRS which differ in geometrical profile and diameter: 1) INTACS (Addition Tecnology Inc., Sunnyvlae, CA, USA), 2) Ferrara rings (Mediphacos, Belo Horizonte, Brazil) 3) Bisantis segments (Opticon 2000 SpA Soleko SpA) and 4) Myoring (DIOPTEX). Specifications for the INTACS and Ferrara Rings are shown in Table 1.
|Arc length (degrees)||150||160|
|Thickness (mm)||0.25 – 0.45 (0.05 increments)||0.20-0.35 (0.05 increments)|
|Inner Radius (mm)||6.77||4.40|
|Outer Radius (mm)||8.10||5.60|
Other ring segments such as Bisantis and Myoring (DIOPTEX) were developed but there are no studies reporting outcomes with these implants. 
Intracorneal ring segments may be indicated for patients with low myopia (-1.00D -3.00D), keratoconus, and post-LASIK keratectasia . The indications for implantation of ICRS are clear central cornea and contact lens intolerance. The cornea should be at least 400 μ thick at the insertion site for implantation of INTACS .
INTACS should not be used in keratoconus patients who can achieve functional vision on a daily basis using contact lenses, are younger than 21 years of age, do not have clear central corneas, and have a corneal thickness less than 450 μ at the proposed incision site. (www.fda.gov/MedicalDevices/ProductsandMedicalProcedures)
The insertion is performed under topical anesthesia. The pupillary center or the geometric center of the cornea is marked with a Sinsky hook. This is the reference point. For the mechanical procedure, a 1 mm radial incision is created at 70-80% of corneal thickness with a calibrated diamond knife. This is facilitated by intra-operative ultrasonic pachymetry. Pocketing hooks are used to create corneal pockets on each side of the radial incision. Then a semi-automated suction ring is placed around the limbus. After the suction is increased to 630 mbar, two semicircular dissectors are advanced into the lamellar pocket by rotating clockwise and counterclockwise. Two semicircular tunnels are then created. The suction device is removed and the ring segments are inserted into the channels . The channels can also be prepared with a femtosecond laser (Intralase, Irvine, CA, USA). The infrared Nd:Glass laser (wavelength 1053nm) can create channels at desired depth with a high degree of precision. Carrasquillo and Rabinowitz showed similar visual and refractive outcomes with either mechanical or laser-assisted procedures in keratoconus and post LASIK ectasia.
There are no prospective randomized controlled studies with ICRS. This is associated with the low incidence of these conditions and there are no well-established surgical nomograms. Different approaches in ICRS implantation in keratoconus were based on either spherical equivalent refraction or topographic values. In all of the studies in keratoconic eyes, statistically significant central flattening of the cornea were reported .   Mean keratometric change after ICRS insertion was variable from 2.14 to 9.60 D. ICRS reduced the sphere, cylinder and the spherical equivalent in keratoconus.   However, there were differences in the reported magnitude of outcome. A regression in the spherical correction was observed in the medium and long-term.This showed that ICRS did not stop cone progression. There are conflicting results in terms of reduction of corneal higher order aberrations, especially of the coma type. The favorable effect of ICRS is supported by the fact that most studies showed an improvement of best spectacle corrected visual acuity (BSCVA) in 50% of cases. More studies are needed to delineate the aberrometric effects of ICRS. ICRS is also found to be useful in improving the contact lens tolerance for the residual refractive error. Carrasquillo et al found an 81% improvement in contact lens tolerance after Intacs implantation in keratoconus and post-LASIK ectasia. There is limited data on the efficacy of ICRS in eyes with PMD. Ertan et al showed a mean reduction of 1.59D and 1.47 D in sphere and cylinder, respectively in PMD. As in eyes with keratoconus, patients with PMD were able to tolerate hybrid contact lenses better, after Intacs. A mean central flattening of 3.00D and a mean reduction in spherical equivalent of 2.00D has been achieved in post-LASIK ectasia.  This may be evidence for partial correction of higher order aberrations by ICRS.
Complications are rare as ICRS implantation is reversible, no corneal tissue is removed, and the central optical zone is not invaded. Intraoperative complications are epithelial defects at the keratotomy site, extension of the incision towards the central visual axis or towards the limbus, inadequate channel depth, superficial channel dissection, segment decentration, asymmetry of implants, and anterior chamber perforation. Postoperatively, segment extrusion, corneal neovascularization, segment migration, epithelial plug at the initial incison site, infectious keratitis, channel deposits, corneal haze, corneal melting, persistent incisional gaping, night halos, chronic pain, and focal edema were reported.  ICRS can easily be explanted and visual, refractive, and topographic features can return to pre-implantation values.  Adjustment surgery can also be performed to improve visual acuity and corneal regularity.  ICRS implantation is a safe and successful option in keratoconus, PMD and post-LASIK ectasia. Patients should be presented with realistic expectations to avoid disappointment. Well-designed, prospective, comparative studies are needed to establish accurate nomograms for ICRS implantation.
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