Intracorneal ring segments (ICRS) are synthetic arcuate implants placed at approximately two-thirds depth of the corneal stroma (outside the central optical zone). Introduced in 1987 for myopia correction, they are now positioned as a therapeutic intervention for corneal ectatic diseases such as keratoconus and post-LASIK ectasia.
ICRS function as spacers between corneal lamellae. They shorten the arc length of the central cornea in proportion to the device thickness (arc shortening effect). As a result, the central anterior cornea flattens, and the peripheral area adjacent to the ring insertion is pushed forward.
According to Barraquer’s law, adding tissue to the corneal periphery flattens the central cornea. ICRS is a treatment based on this principle. The thicker the device and the smaller its diameter, the greater the refractive correction achieved.
ICRS is not a curative treatment for keratoconus, but rather a surgical alternative to delay the need for corneal transplantation. Combining ICRS with corneal cross-linking (CXL) can halt disease progression and provide synergistic effects.
ICRS is not a curative treatment for keratoconus. It is a surgical intervention aimed at reducing irregular astigmatism and improving vision, thereby at least delaying the need for corneal transplantation. Combining with CXL can halt disease progression.
Corneal ectasia, the condition for which ICRS is indicated, presents with the following symptoms:
Symptoms typically begin in adolescence or the 20s and often slow in progression around age 302).
Keratoconus is characterized by protrusion and thinning of the central to paracentral cornea. Corneal topography shows a steepening pattern 1). Changes in corneal biomechanics occur before morphological changes 1).
Characteristic findings include Vogt’s striae (fine vertical lines in the deep stroma), Fleischer ring (iron deposition in the epithelium at the base of the cone), and corneal scarring 2). Slit-lamp microscopy reveals forward protrusion and thinning of the cornea, slightly inferior to the center.
Topography after ICRS insertion shows overall corneal flattening, centralization of the corneal apex, maintenance of corneal asphericity, and reduction of surface irregularity.
The etiology of corneal ectasia, for which ICRS is indicated, is multifactorial.
Collagen degradation in the cornea is the essence of thinning 1). Increased matrix metalloproteinases (MMPs) and decreased TIMP are observed 1). Increased IL-6, TNF-α, and MMP-9 in tears induce keratocyte apoptosis 1).
Eye rubbing is a major risk factor for keratoconus 1). It is associated with atopic diseases (hay fever, asthma, eczema, vernal keratoconjunctivitis) 1).
Keratoconus is usually bilateral but often asymmetric in severity. It typically onsets in adolescence and tends to stabilize or progress slowly around age 30 2). Without treatment, about 20% of cases require corneal transplantation 2).
Post-LASIK ectasia can occur when laser refractive surgery is performed on a cornea with unrecognized subclinical keratoconus 1). Thinning of the residual stromal bed and weakening of corneal structure are involved.
For the diagnosis of corneal ectasia, combined use of corneal tomography (Scheimpflug imaging or OCT) and corneal biomechanical assessment is recommended 3).
Key indices required for preoperative evaluation are shown below.
| Index | Characteristics |
|---|---|
| TBI (Tomographic Biomechanical Index) | Integrated index of morphology and biomechanics. High diagnostic performance 3) |
| CBI (Corvis Biomechanical Index) | Index of corneal deformation response to air puff 3) |
| SimK1/Ks | Corneal curvature of the steep meridian |
| SAI / SRI | Symmetry and regularity of corneal shape |
| CRF (Corneal Resistance Factor) | Reflects overall corneal stiffness 3) |
Since a single index may yield false negatives, comprehensive screening combining corneal tomography and biomechanical assessment is recommended 3). In keratoconus, biomechanical changes precede morphological changes, making this useful for early detection 1).
Anterior segment OCT (e.g., CASIA) can acquire tomographic images and corneal topography with high resolution (approximately 10 μm) and has excellent accuracy in detecting opacities. It is also useful for evaluating the depth and position after ICRS implantation.
In children and young adults with keratoconus, the progression may be rapid 4). The KERALINK study evaluated the efficacy of cross-linking in slowing progression in young patients 4).
Preoperatively, corneal tomography (e.g., Pentacam) should be performed to obtain anterior and posterior curvature, corneal thickness map, and elevation map. Comprehensive evaluation combined with corneal biomechanical assessment (TBI, CBI, CRF) is recommended. It is important to measure corneal thickness at the ring insertion site using anterior segment OCT and confirm that it is 450 μm or more.
The characteristics of major ICRS products are shown below.
Intacs
Shape: Hexagonal cross-section, outer diameter 8.0 mm, inner diameter 6.8 mm.
Thickness: 0.21–0.45 mm (in 0.05 mm increments) to adjust the refractive effect.
Intacs SK: New type with inner diameter 6 mm and elliptical cross-section. Indicated for severe cases with steep K value ≥57 D.
Approval: The only synthetic ICRS product approved by the US FDA.
Ferrara / KeraRings
Cross-section: Triangular (reduces photophobia through prism effect).
Optical zone: 4.5–6.0 mm, smaller than Intacs, resulting in a stronger flattening effect.
Arc length: Various options from 90° to 355°.
Indications: Primarily used for refractive correction of keratoconus.
CAIRS / CTAK
Material: Derived from donor corneal tissue (allograft).
Features: High biocompatibility, can be inserted at shallow depths (35–70%).
Advantages: Lower risk of exposure, melting, and corneal neovascularization compared to synthetic ICRS.
CTAK: Gamma-irradiated donor segments from CorneaGen.
Channel creation methods include mechanical dissection and femtosecond laser 1). The implantation depth of 70–80% of corneal thickness is the general standard for synthetic ICRS. With femtosecond laser, channels are created at precise depth and diameter based on pachymetry maps 1).
CAIRS can be inserted at shallower depths (35–70%) and is expected to provide a greater flattening effect 5).
The mean change in corneal curvature after ICRS insertion ranges from 2.14 to 9.60 D. Reductions in spherical power, astigmatic power, and spherical equivalent have been reported. It is considered most effective for moderate keratoconus (Kmax < 58.0 D)1). However, changes in astigmatism can sometimes be unpredictable1).
| Procedure | Kmax Improvement | CDVA Improvement | Notes |
|---|---|---|---|
| Synthetic ICRS alone | 2–10 D flattening | 1–2 line improvement | May be insufficient in severe cases |
| ICRS + CXL simultaneous | Superior to ICRS alone | Improvement also in spherical error | Recommended by meta-analysis1) |
| CAIRS | Kmax 57.8 → 53.6 D | CDVA 0.52 → 0.19 logMAR | Low risk of exposure/melting5) |
A meta-analysis (12-month follow-up, 6 studies) of simultaneous ICRS and CXL showed that simultaneous surgery yielded better results in spherical refractive error and steep-K than either CXL-first or ICRS-first approaches 1).
| Complication | Notes |
|---|---|
| ICRS extrusion | Accounts for 48.2% of all explantation cases. Average onset is approximately 10 years postoperatively. |
| Segment migration | More likely to occur with shallow implantation or width mismatch. |
| Infectious keratitis | Reported with both methods. Severe cases may progress to endophthalmitis. |
| Corneal scarring/melting | Rare but affects vision. |
| Intralamellar deposits | Occur in up to 74% of cases. Minimal impact on visual function 1). |
| Intraoperative perforation | Mechanical: anterior perforation. Laser: incomplete channel. |
ICRS alone may not stop the progression of keratoconus. Simultaneous performance with CXL showed superior results in spherical refractive error and steep-K compared to CXL-first or ICRS-first approaches 1). Chan et al. reported that Intacs + CXL combination was more effective than Intacs alone for improving keratoconus 6). In a three-step treatment of ICRS + CXL + tPRK (topography-guided PRK) by Hashemian et al., sustained improvement in Kmax values was confirmed at 6-month follow-up 8).
Faria-Correia et al. (2023) reported outcomes of ICRS implantation via a limbal incision approach 9). The limbal approach avoids the optical zone of the cornea and reduces the risk of postoperative corneal scarring. The AAO Corneal Ectasia PPP (2024) states that both mechanical dissection and femtosecond laser methods show equivalent outcomes, with the femtosecond laser providing more precise channel depth control 1).
The prevalence of keratoconus is reported as 50–230 per 100,000 in the general population (138 per 100,000 in a recent large meta-analysis) 15). Without treatment intervention, approximately 20% require corneal transplantation, making early intervention with CXL and ICRS important 2). A meta-analysis by Pédretti et al. (2022) confirmed significant improvements in mean UDVA and CDVA after ICRS insertion 11). A 5-year long-term follow-up by Vega-Estrada et al. showed that refractive and visual improvements after ICRS insertion were maintained for 5 years, confirming long-term efficacy and predictability 12). A 1998 review by Rabinowitz reported a prevalence of 0.05% 15), but the latest AAO Corneal Ectasia PPP (2024) suggests this value may be significantly higher 1).
Nuzzi et al. (2025) reported a technique for replacing synthetic ICRS with CAIRS after long-term complications 7). In a case with subepithelial stromal fibrosis at the temporal pocket entrance assessed by anterior segment OCT, the synthetic ICRS was explanted and CAIRS was inserted 3 months later. Postoperative Kmax improved from 68.9 to 61.9 D, and UCVA improved from 20/400 to 20/30. CAIRS can be an effective salvage option even in cases where synthetic ICRS has failed.
In the treatment of corneal ectasia, ICRS is indicated for cases with contact lens intolerance and progressive visual impairment but with a clear central cornea. It is positioned as a precursor to full-thickness penetrating keratoplasty (PKP) and deep anterior lamellar keratoplasty (DALK). The AAO PPP notes that DALK has the advantages of no risk of endothelial rejection and a lower risk of globe rupture compared to PKP 1). The recent decline in the number of corneal transplant surgeries is thought to reflect the widespread adoption of early intervention with CXL and ICRS. The Refractive Surgery Guidelines (8th edition) clearly state that keratoconus is a contraindication for excimer laser surgery and SMILE surgery 10), emphasizing the importance of appropriate preoperative screening.
Key points of postoperative guidance for patients who have undergone ICRS implantation:
Diagnostic criteria and evaluation indices based on the AAO Corneal Ectasia PPP (2024) 1):
The ABCD classification (integrated with Scheimpflug camera) evaluates the following parameters with scores from 0 to 4:
Based on the AAO Corneal Ectasia PPP (2024) and clinical practice, a general treatment algorithm is shown below1).
ICRS improves corneal shape and visual acuity, but alone may not halt progression. CXL increases corneal rigidity through collagen cross-linking, providing a progression-stopping effect. Simultaneous performance has been reported to show the best results, yielding synergistic effects of shape improvement and progression arrest.
ICRS extrusion is a complication in which the ring protrudes from the corneal surface due to progressive corneal stromal thinning and epithelial breakdown. It accounts for 48.2% of all removal cases, with an average onset time of about 10 years, but it can occur at any time from 1 month to 20 years postoperatively.
Early postoperative risk stratification using CAS-OCT is useful. Cases with an average depth percentage of less than 60% or tunnel depth of less than 70% at the first week and first month after implantation are considered high risk.
| Risk Factors | Details |
|---|---|
| Ring migration | ICRS width too wide for thin cornea, tunnel too shallow or narrow |
| Corneal melting | Ring placement near incision, inflammation due to infection, trauma, or stromal dissection |
| Simultaneous CXL | Eccentric cone + grade 3 keratoconus increases risk of migration and melting |
Removal is the definitive treatment for ICRS extrusion. Indications include extrusion, decreased or fluctuating visual quality, corneal melting, infectious keratitis, contact lens intolerance, and corneal perforation.
There are two incision methods depending on the situation. If the extrusion is near the incision site, open the previous entry with a sinskey hook. If the incision site has healed, create an incision window under the extruded ring with a diamond knife set to the original depth. In either case, close the defect with 10-0 nylon sutures.
When Complicated by Infectious Keratitis
Culture testing: Perform culture of conjunctival and corneal scrapings in all cases.
Treatment: High-concentration antibiotic eye drops combined with explantation. Antibiotics alone without explantation may also be effective in some cases.
Severe cases: Progression to endophthalmitis requires systemic antibiotics. Early full-thickness corneal transplantation may be an alternative.
Postoperative Management
Eye drops: Use antibiotic-steroid combination eye drops for 5 days.
Artificial tears: Use for 1–3 months, and instruct the patient to avoid eye rubbing.
Adjunctive therapy: Amniotic membrane transplantation or bandage contact lenses promote epithelial healing.
After ring explantation, no worsening of corrected visual acuity, astigmatism, or myopia was observed. Traces of lamellar opacity decrease over time.
There are three main preventive measures: Adhere to the “pachymetry rule” and keep ring thickness less than half the corneal thickness at the implantation site. Use femtosecond laser to accurately create a tunnel with 80% channel depth. Place the ring tip away from the incision. Postoperatively, confirm early ring position stability with CAS-OCT.
The elastic modulus of the cornea is a quantitative measure of its tendency to deform elastically under force. In keratoconus, the elastic modulus is reduced due to pathological changes in the stroma.
The decrease in elastic modulus results from degradation and degeneration of collagen fibers 3). This initiates a biomechanical failure cycle. Stress levels increase and redistribute, leading to corneal steepening and thinning 3). In the thinned area, local stress further increases, forming a vicious cycle that worsens the protrusion.
ICRS intervenes in this vicious cycle through the following mechanisms.
Andreassen et al. reported that the elastic modulus of keratoconic stroma is reduced to approximately 60% of that of normal cornea 13). This reduction in elasticity is the primary biomechanical background of corneal ectasia, and ICRS functions as structural reinforcement.
The effect of ICRS is closely related to the structural properties of the collagen framework of the corneal stroma. The stroma accounts for 90% of corneal thickness, and its mechanical properties determine the overall biomechanics of the cornea.
The pathology of ICRS extrusion is broadly divided into two mechanisms: ring migration and corneal melting.
Ring migration results from shallow implantation. Placing ICRS too shallowly increases anterior tensile strain, leading to stromal compression. Progression of anterior stromal compression causes epithelial and stromal breakdown, leading to corneal thinning and eventual spontaneous extrusion.
Corneal melting reflects an underlying inflammatory process. Surgical trauma during incision and tunnel creation induces progressive keratocyte apoptosis and tissue degeneration. Involvement of MMPs has been suggested, and introduction of exogenous stimuli leads to degradation and thinning of the corneal matrix.
CAIRS (Corneal Allogenic Intrastromal Ring Segments) involves inserting stromal segments derived from donor corneal tissue into the stroma. They reduce the curvature of the cone area through an arch-shortening effect similar to synthetic ICRS.
While synthetic ICRS requires insertion into the deep stroma (70–80% depth), CAIRS can be inserted at shallower depths (35–70%), potentially producing a greater flattening effect 5).
Allogenic segments are implanted into the avascular, low-cell-density corneal stromal layer. This environment minimizes fibrous adhesion and preserves surgical reversibility. The risks of corneal melting, acute stromal necrosis, and corneal neovascularization associated with synthetic ICRS are also reduced 5).
Advances in corneal biomechanics evaluation are attracting attention. New indices such as TBI and CBI complement traditional morphological indices and improve the detection accuracy of early keratoconus 3). Integrated evaluation of biomechanical indices and corneal tomography has been reported to improve the prediction accuracy of refractive surgery by more than 25% 3).
A meta-analysis on the combination of CXL and ICRS, including 6 studies with 12-month follow-up, showed that simultaneous surgery was superior to CXL-first in spherical refractive error and flat-K, and superior to both CXL-first and ICRS-first in steep-K. 1)
According to a systematic review (AlQahtani et al., 2025), after CAIRS implantation, mean UDVA improved from 0.83 to 0.40 logMAR, CDVA from 0.52 to 0.19 logMAR. Spherical equivalent decreased from −7.09 D to −2.34 D, Kmax from 57.8 to 53.6 D, and Kmean from 49.3 to 45.3 D 5). Regarding long-term effects of CXL, the Siena Eye Cross Study by Caporossi et al. (mean follow-up 6 years) reported that riboflavin-UV-A corneal crosslinking halted or improved progression in 74% of keratoconus cases 14).
Similar improvements have been reported for CTAK. Mean UDVA improved from 1.21 to 0.61 logMAR, and CDVA from 0.63 to 0.34 logMAR 5).
Complications of CAIRS are few and minor. Transient dry eye and intrastromal deposits are most common but not clinically significant. Glare and halos were significantly less frequent compared to synthetic ICRS 5).
The complication rate of synthetic ICRS has been reported to be up to 30% 5). Multiple successful cases of CAIRS rescue for complications such as ICRS exposure, anterior chamber migration, and corneal melting have been reported 5). In a 49-year-old female with failed ICRS (UCVA 20/400), the synthetic ICRS was removed and CAIRS was inserted 3 months later, improving Kmax from 68.9 to 61.9 D and UCVA to 20/30 5).
Key future challenges include the following:
