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Corneal cross-linking (CXL) is a procedure combining corneal saturation with riboflavin and exposure to UV light to increase the amount of links between collagen fibers in the corneal stroma, thereby increasing its tensile strength.
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Intracorneal ring segments (ICRS) are semi circularly shaped polymethyl methacrylate (PMMA) segments that are inserted into the paracentral region of the anterior corneal stroma to flatten the center of the cornea and induce a hyperopic shift.
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CXL is used to halt progression of corneal ectasias, whereas ICRS are used in patients with moderate to advanced stable keratoconus to counterbalance the myopic shift induced by corneal steepening.
Key Features
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Preoperative evaluation and topographical assessment is necessary to select the appropriate ICRS size and positioning.
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The flattening effect of ICRS can be potentiated by the combination with cross-linking.
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Careful selection of patients for combined ICRS and CXL is imperative to increase the chances of success and reduce the risk of postoperative complications.
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There is currently no consensus regarding timing of combined procedures. Inserting the ICRS before CXL may optimize the impact of ring segment insertion, whereas CXL alone may be sufficient to halt progression and improve corneal shape, thereby obviating the need for a combined procedure.
Introduction
For the majority of patients seeking refractive surgery, the best, most effective treatment will be laser ablation as highlighted in the previous chapters. However, for patients with ectatic corneal disease, ICRS and CXL play a prominent role in halting disease progression and improving acuity.
Introduction
For the majority of patients seeking refractive surgery, the best, most effective treatment will be laser ablation as highlighted in the previous chapters. However, for patients with ectatic corneal disease, ICRS and CXL play a prominent role in halting disease progression and improving acuity.
Intracorneal Ring Segments
Intracorneal ring segments add volume peripherally by changing the arc length of the anterior corneal curvature and redistributing the posterior corneal tissue. This shortens the posterior lamellae and flattens the cornea centrally, correcting myopia.
Intracorneal ring segments received European Conformité Européene certification in 1996 and US Food and Drug Administration (FDA) approval in 1999. However, the initial enthusiasm for the correction of myopia faded due to a limited range of correction, less predictability, induced astigmatism, and slower visual recovery.
The idea of using intrastromal rings in the treatment of keratoconus was proposed in 2000 by Colin et al. Since then their use has evolved into an important therapeutic intervention in corneal ectatic diseases. Intacs received FDA approval for the use in the treatment of keratoconus in 2004 for rings 0.25–0.35 mm and in 2010 for 0.4–0.45 mm.
In patients with ectatic disease, ICRS induce morphological changes by flattening the steepest meridian and decreasing the cone location magnitude index (CLMI) and the mean curvature in the 2-mm area over the cone. They do not induce significant changes in biomechanical corneal parameters such as corneal hysteresis and corneal resistance factor, and in younger patients ICRS outcomes have regressed in long-term follow-up, suggesting the underlying biomechanical failure in keratoconus has been left untreated.
Corneal Cross-Linking (CXL)
In 1998, Spoerl et al. published their findings after collagen cross-linking of porcine corneas, establishing riboflavin as a safe and effective component of the treatment. The first clinical results after CXL in human keratoconic corneas were published in 2003. In the photochemical reaction, riboflavin acts as a photosensitizer that absorbs UV-A energy and excites into a triplet state. This triplet can undergo an aerobic (type 2) or anaerobic (type 1) reaction, both of which create oxygen species that induce covalent bonds between collagen molecules and between proteoglycans and collagen. The depth of this effect is thought to be seen in the demarcation line on optical coherence tomography (OCT) images of post-cross-linking corneas, usually at 300–350 µm of depth. Long-term follow-up of cross-linked corneas has demonstrated that in contrast to ICRS, CXL affects the biomechanical properties of the cornea and halts the progression of the disease with only mild and refractive changes.
CXL Plus
The lack of significant refractive impact of CXL stimulated the development of treatment protocols combining CXL with a variety of techniques (CXL Plus), including phakic intraocular lens (IOL) implantation and selective excimer laser ablation and CXL combined with ICRS.
The rationale behind the combination of ICRS and CXL seems to be the symbiosis that occurs when combining the mechanical and refractive changes induced by the segments with the biomechanical changes induced by CXL and their benefits in halting disease progression.
Corneal Cross-Linking (CXL)
In 1998, Spoerl et al. published their findings after collagen cross-linking of porcine corneas, establishing riboflavin as a safe and effective component of the treatment. The first clinical results after CXL in human keratoconic corneas were published in 2003. In the photochemical reaction, riboflavin acts as a photosensitizer that absorbs UV-A energy and excites into a triplet state. This triplet can undergo an aerobic (type 2) or anaerobic (type 1) reaction, both of which create oxygen species that induce covalent bonds between collagen molecules and between proteoglycans and collagen. The depth of this effect is thought to be seen in the demarcation line on optical coherence tomography (OCT) images of post-cross-linking corneas, usually at 300–350 µm of depth. Long-term follow-up of cross-linked corneas has demonstrated that in contrast to ICRS, CXL affects the biomechanical properties of the cornea and halts the progression of the disease with only mild and refractive changes.
CXL Plus
The lack of significant refractive impact of CXL stimulated the development of treatment protocols combining CXL with a variety of techniques (CXL Plus), including phakic intraocular lens (IOL) implantation and selective excimer laser ablation and CXL combined with ICRS.
The rationale behind the combination of ICRS and CXL seems to be the symbiosis that occurs when combining the mechanical and refractive changes induced by the segments with the biomechanical changes induced by CXL and their benefits in halting disease progression.
CXL Plus
The lack of significant refractive impact of CXL stimulated the development of treatment protocols combining CXL with a variety of techniques (CXL Plus), including phakic intraocular lens (IOL) implantation and selective excimer laser ablation and CXL combined with ICRS.
The rationale behind the combination of ICRS and CXL seems to be the symbiosis that occurs when combining the mechanical and refractive changes induced by the segments with the biomechanical changes induced by CXL and their benefits in halting disease progression.
Surgical Procedure: ICRS
Patient Selection
The primary indication for ICRS implantation in ectatic patients include moderate to advanced stages of disease, with clear corneas and unsatisfactory corrected near visual acuity (CDVA) or contact lens intolerance, steepest keratometry less than 58 diopters (D), no corneal scarring or opacities, and at least 450 µm thickness at the 7-mm diameter where the segments are to be placed. The flattening effect of ICRS becomes more robust in advanced stages of ectasia.
Preoperative Considerations
Preoperative refraction, aberrometry, and corneal topography are important factors to consider in surgical planning. Topographical features are relevant in the selection of the segments and their location in asymmetrical corneas, with thicker and longer segments placed in the area just below the cone. Single versus paired segment implantation appear to be equivalent in terms of refractive results, and the decision should be made on a case-by-case basis using the manufacturer’s ICRS nomograms.
ICRS Selection
There are currently three main types of ICRS: Intacs (Addition Technology, Sunnyvale, CA), with a hexagonal transverse shape, and Intacs SK, with an oval cross-section shape; Ferrara , with triangular shape, and Kerarings (both from Mediphacos, Belo Horizonte, Brazil), with almost identical design to Ferrara rings, but different arc lengths and internal diameters. Intacs are the only FDA-approved ICRS for the reduction or elimination of myopia and astigmatism in normal patients or those with keratoconus and are available in 0.25, 0.30, and 0.35 mm. The 0.4-mm Intacs and the Ferrara rings are available only in European countries and South America. The effect of the implantation of an ICRS correlates directly to its thickness and inversely to its distance from the visual axis, or optical zone, with thicker implants and smaller optical zones resulting in greater flattening effect and reduction of coma-like aberrations.
Single Versus Paired ICRS
In cases of peripheral steepening, where the main issue is irregular astigmatism, implanting a single segment may provide better results than two segments, whereas in central cones, where the high myopic refractive error is the main issue, two segments or a longer continuous segment with a simple nomogram using keratometry readings only are a better alternatives because they target a larger change in spherical equivalent.
ICRS Surgical Technique
Intrastromal corneal ring segments are placed within the peripheral stroma at approximately two-thirds of the stromal depth, outside the central optical zone, to reshape the anterior corneal surface while maintaining the positive asphericity of the cornea.
There are no significant differences in refractive outcomes of ICRS between femtosecond laser and manual spreader. When using femtosecond laser, the efficacy in creation of an intrastromal channels can be affected by previous cross-linking treatment, perhaps due to changes in the optical quality of the anterior stroma that interfere with laser tracking. Therefore even though femtosecond laser is associated with fewer complications compared with the mechanical spreader, manual dissection of the channels may be considered in patients that have been previously cross-linked.
After channel formation, the segments are introduced with forceps into the channels. In their final position, the segments should be located 3 mm apart superiorly. Once the segments have been inserted, each segment has a small positioning hole at the superior end to aid with surgical manipulation for proper positioning, after which the incision site is hydrated and closed with 10–0 nylon sutures.
Clinical Outcomes
ICRS flatten the cornea permanently, but the major shifts in refraction and topographical findings after ICRS implantation usually occur in the first 6 months of postoperative period with some fluctuations during the first month ( Fig. 3.9.1 ). Even though late postoperative variations probably occur due to intrinsic biomechanical changes in those with unstable disease, the difference in CDVA and keratometry between month 6 and month 36 is not significant.
The results of ICRS in patients with corneal ectasia are encouraging, especially in decreasing astigmatism and improving topographical abnormalities, with reported significant changes in spherical equivalent (SE), (average 4.0 D reduction in SE), manifest astigmatic error and UCVA, with the most significant improvement in the first 6 and 9 months.
Postoperative Complications
The overall reported adverse event rate after ICRS is 1.1% including infectious keratitis (0.2%), shallow placement (0.2%), loss of two lines of best spectacle-corrected visual acuity (BSCVA) (0.2%), and anterior chamber perforation during initial and exchange procedures (0.4%). Reduced central corneal sensation can occur up to 6 months after surgery and can persist up to 12 months in 5.5% of cases. Some patients experience difficulty with night vision (4.4%), blurry vision (2.9%), diplopia (1.6%), or glare and halos (1.3%), which accounts for an explantation rate of 8.7%, mostly due to glare, halos, and difficulty with night vision as well as dissatisfaction with refractive results. Late onset corneal haze is another potential complication ( Fig. 3.9.2 ).
Patient Selection
The primary indication for ICRS implantation in ectatic patients include moderate to advanced stages of disease, with clear corneas and unsatisfactory corrected near visual acuity (CDVA) or contact lens intolerance, steepest keratometry less than 58 diopters (D), no corneal scarring or opacities, and at least 450 µm thickness at the 7-mm diameter where the segments are to be placed. The flattening effect of ICRS becomes more robust in advanced stages of ectasia.
Preoperative Considerations
Preoperative refraction, aberrometry, and corneal topography are important factors to consider in surgical planning. Topographical features are relevant in the selection of the segments and their location in asymmetrical corneas, with thicker and longer segments placed in the area just below the cone. Single versus paired segment implantation appear to be equivalent in terms of refractive results, and the decision should be made on a case-by-case basis using the manufacturer’s ICRS nomograms.
ICRS Selection
There are currently three main types of ICRS: Intacs (Addition Technology, Sunnyvale, CA), with a hexagonal transverse shape, and Intacs SK, with an oval cross-section shape; Ferrara , with triangular shape, and Kerarings (both from Mediphacos, Belo Horizonte, Brazil), with almost identical design to Ferrara rings, but different arc lengths and internal diameters. Intacs are the only FDA-approved ICRS for the reduction or elimination of myopia and astigmatism in normal patients or those with keratoconus and are available in 0.25, 0.30, and 0.35 mm. The 0.4-mm Intacs and the Ferrara rings are available only in European countries and South America. The effect of the implantation of an ICRS correlates directly to its thickness and inversely to its distance from the visual axis, or optical zone, with thicker implants and smaller optical zones resulting in greater flattening effect and reduction of coma-like aberrations.
Single Versus Paired ICRS
In cases of peripheral steepening, where the main issue is irregular astigmatism, implanting a single segment may provide better results than two segments, whereas in central cones, where the high myopic refractive error is the main issue, two segments or a longer continuous segment with a simple nomogram using keratometry readings only are a better alternatives because they target a larger change in spherical equivalent.
ICRS Surgical Technique
Intrastromal corneal ring segments are placed within the peripheral stroma at approximately two-thirds of the stromal depth, outside the central optical zone, to reshape the anterior corneal surface while maintaining the positive asphericity of the cornea.
There are no significant differences in refractive outcomes of ICRS between femtosecond laser and manual spreader. When using femtosecond laser, the efficacy in creation of an intrastromal channels can be affected by previous cross-linking treatment, perhaps due to changes in the optical quality of the anterior stroma that interfere with laser tracking. Therefore even though femtosecond laser is associated with fewer complications compared with the mechanical spreader, manual dissection of the channels may be considered in patients that have been previously cross-linked.
After channel formation, the segments are introduced with forceps into the channels. In their final position, the segments should be located 3 mm apart superiorly. Once the segments have been inserted, each segment has a small positioning hole at the superior end to aid with surgical manipulation for proper positioning, after which the incision site is hydrated and closed with 10–0 nylon sutures.
Clinical Outcomes
ICRS flatten the cornea permanently, but the major shifts in refraction and topographical findings after ICRS implantation usually occur in the first 6 months of postoperative period with some fluctuations during the first month ( Fig. 3.9.1 ). Even though late postoperative variations probably occur due to intrinsic biomechanical changes in those with unstable disease, the difference in CDVA and keratometry between month 6 and month 36 is not significant.
