ICRS: Corneal biomechanics effects
Corneal thinning disorders often lead to protrusion, irregular astigmatism, and in rare cases to corneal perforation. Pellucid Marginal Degeneration (PMD), and Keratoglobus do not have a known cause and eye rubbing is nowadays recognized as one of the major risk factor of keratoconus. In addition, corneal ectasia can also be acquired after LASIK (Laser Assisted In Situ Keratomileusis) procedure when LASIK is performed on preoperative unrecognized abnormal cornea. 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. Penetrating Keratoplasty (PKP) was for long time the treatment of choice then replaced by Deep Anterior Lamellar Keratoplasty (DALK). Although PKP and DALK can be very successful in this subset of patients, limitations can include high post-op cylinder, side effects from chronic topical steroids and corneal rejection. Intracorneal ring segments are another modality in the treatment of corneal ectactic disorders.
Introduction and Mechanism of Action
In 1987, intrastromal rings were introduced as synthetic, intracorneal implants for the correction of various degrees of myopia. An elastic modulus, or modulus of elasticity, is the mathematical description of an object's or substance's tendency to be deformed elastically (i.e., nonpermanently) when a force is applied to it. In keratoconus, the corneal elastic modulus is reduced due to pathology in the corneal stroma. From a biomechanical perspective, the resistance to deformation is reduced in relation to the reduction of the elastic modulus that leads to increased strain and protrusion in the cornea. The consequence is increased curvature and corneal thinning, the hallmarks of keratoconus. Since stress is defined as applied force divided by cross-sectional area, stress focally increases in the zone of corneal thinning.
Intrastromal corneal ring segments (ICRSs) are placed in the mid-corneal peripheral stroma at approximately two-thirds of depth outside the central optical zone to reshape the anterior corneal surface while maintaining the prolate profile (positive asphericity) of the cornea. Intracorneal ring segment acts as spacer elements between the bundles of corneal lamellae producing a shortening of the central arc length (arc shortening effect) that is proportional to the thickness of the device. As a consequence of this effect, the central portion of the anterior corneal surface tends to flatten and the peripheral area adjacent to ring insertion is displaced forward. Corneal changes induced by the ICRS must be in relation to the structural properties of the collagen framework in the corneal stroma. The stroma accounts for 90% of corneal thickness, and evidently its mechanical properties define for the most part the mechanical properties of the whole corneal structure.
According to Barraquer, when material is removed from the central area of the cornea or added to the periphery, a central flattening effect is achieved. In contrast, when material is added to the center or removed from the corneal periphery, the central surface curvature is steepened. The corrective results vary in direct proportion to the thickness of the implant and in inverse proportion to its diameter. The thicker and smaller the device, the higher the refractive result achieved. The diameter of the ring is proportionally inverse to the flattening intensity. Thus, the smaller the diameter, the more tissue is added (ring thickness) with higher myopic correction.
ICRS create an arc-shortening effect on the corneal lamellae, flattening the central cornea. For the correction of astigmatism, the end point of each segment may produce a traction force on the surface, producing additional flattening on this reference axis. In addition, the presence of a corneal inlay may provide biomechanical support for this ocular tissue. Topographic interpretation of eyes that underwent ICRS implantation shows overall flattening of the cornea, dislocation of the corneal apex towards the center, preservation of corneal asphericity, and decreased surface irregularity. ICRS change the arc length of the anterior corneal curvature. The refractive effect achieved is directly related to the thickness of the segments. Placing the ring segments in the periphery of the cornea causes local separation of the corneal lamellae which results in shortening of the corneal arc length. This flattening effect on the cornea reduces myopia by lowering its optical power. Increasing the thickness of ICRS also causes greater degrees of local separation and increased corneal flattening. Thus, the degree of corneal flattening (correction) achieved by ICRS is directly related to their thickness.
The initial enthusiasm regarding ICRS for the correction of myopia has faded for multiple reasons, including a limited range of correction, induced astigmatism, and slow visual recovery. Nowadays, the use of ICRS has evolved into an important therapeutic intervention for corneal ectatic diseases such as keratoconus and post-laser surgery-induced ectasia. Redistribution of corneal curvature leads to a redistribution of corneal stress, interrupting the biomechanical cycle of keratoconus disease progression, in some cases.
Apart from INTACS, many other types of ICRS have are available, including Ferrara rings, Kerarings, Cornealring, which are different in geometrical design and diameter.
- Intacs have a hexagonal transverse shape with 8.1 mm external diameter and 6.8 mm internal diameter. The refractive effect is modulated by the Intacs' thickness (0.25-0.45 mm, with 0.05-mm increments) and current designs have a predicted myopic range of correction from -1.00 to -4.10 D. Recently, a new Intacs segment (Intacs SK) design, with inner diameter of 6 mm and oval cross-section shape, was produced (Addition technology Information, AAO, NV, USA). Two different Intacs SK thicknesses can be obtained: 400 µm (for steep K-value of 57-62 D and cylinder > 5 D) and 450 µm (for steep K-value > 62 D and cylinder > 5 D).
- Ferrara: In 1986, Ferrara started implanting modified PMMA rings in rabbit corneas and, in 1994, developed a technique of corneal stroma tunnel construction for implanting the rings. Ferrara rings are available in European countries and South America. The cross-section of a Ferrara ring is triangular in shape to reduce the photic phenomena by inducing a prismatic effect.In 1996, he replaced the single ring with two segments, each having 160° of arc and obtained improved results for high myopia. Furthermore, he began to implant the segments into corneas with keratoconus and after keratoplasty. The Ferrara ring segments, manufactured by Mediphakos, Inc., Belo Horizonte, Brazil, are made as computer-lathed PMMA and camphorquinone (CQ)-acrylic segments, available in two diameters (6.0 mm for myopia up to 7.00 D, 5.0 mm for higher degrees of myopia) and thickness varying from 150 to 350 µm. Typical Ferrara ring characteristics include :
- The internal and external diameters are 4.4 mm and 5.4 mm for the 5.0-mm optical zone and 5.4 mm and 6.4 mm for the 6.0 mm optical zone, respectively;
- The segment cross-section is triangular;
- Constant 600 µm base for every thickness and diameter;
- An arc of 160°;
- KeraRings (Mediphacos) are mainly identical in design to Ferrara rings, but different options of arc length are available. KeraRings were developed mainly for refractive correction in eyes with keratoconus. Each segment is available with an internal diameter of 4.40 mm and an external diameter of 5.60 mm. Intacs SK and KeraRings have a greater flattening effect on the central cornea due to their close position to the visual axis. These are made of PMMA and are characterized by a triangular cross-section. Their optical zones are 5.0, 5.5 and 6.0 mm, and the flat basis width is 0.6 mm with variable thickness (0.15–0.30 mm thickness with 0.5 mm steps) and arc lengths (90°, 120°, 160°, 150°, 210, and 355°). The new model Keraring 355° is used only for nipple cones and is available with 0.20 and 0.30 mm thickness.
- MyoRing (DIOPTEX GmBh, Linz, Austria): This ICR is a 360° continuous full-ring implant for myopia and keratoconus. Its diameter ranges from 5–8 mm and the thickness ranges from 200–320 microns. See more information at www.dioptex.com
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)
For selection of the ICRS size and position, the surgeon must consider refraction, keratometry, and corneal thickness. Implant manufacturers usually suggest a nomogram to be followed for the implantation of the ICRS.
The surgical planning is based on the location of the steepest axis, the extent of ectatic area, the type of keratoconus and the refraction. Suggested sites for complete nomogram descriptions are: www.mediphacos.com, www.ferrararing.com.br, www.additiontechnology.com and www.dioptex.com.
There are some differences among surgeons in choosing the implantation depth of the ICRS. Ertan and Kamburoglu used Intacs segments implanted at about 70% of the corneal depth. Segment extrusion occurred in three eyes of patients with severe keratoconus. Alió et al. also used Intacs implanted at 70% of the corneal depth without intraoperative complications. In another study, Wachler et al. inserted Intacs at a depth of 66%. During surgery, one eye experienced a superficial channel dissection with anterior Bowman’s layer perforation. Two eyes had a transient superficial inflammatory reaction, which resolved within one week. Segment migration and externalization were found in one eye on the first postoperative day. Coskunseven et al. and Wijdh and Rij reported implantation at 75% of the corneal depth for corneal ring segments. Other surgeons have used 80%.
The aim of ICRS implantation is not to treat or eliminate the existing disease, or should at least not be considered as a traditional refractive surgical procedure. However, ICRS is a surgical alternative to decrease astigmatisms and corneal abnormalities and, thus, increase the visual acuity to acceptable limits aiming to at least delay the need of corneal grafting.
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. 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.
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