Intrastromal corneal ring segments for keratoconus: a comprehensive review of different types

Abstract

This comprehensive review analyzes different types of intrastromal corneal ring segments (ICRS) used in the treatment of keratoconus, focusing on visual outcomes, complication rates, and patient selection criteria for INTACS, KeraRing, Ferrara Ring, MyoRing, and corneal allogenic intrastromal ring segments (CAIRS). We reviewed clinical studies, case reports, and long-term follow-ups to compare visual outcomes, corneal stability, and safety profiles of these ICRS types, with specific emphasis on parameters such as uncorrected distance visual acuity (UDVA), best-corrected distance visual acuity (CDVA), keratometry (Kmax reduction), and complication rates, including migration, extrusion, and postoperative visual disturbances. Each type of ICRS exhibits distinct advantages, with efficacy varying according to disease severity and corneal irregularity. INTACS demonstrated reliable visual improvements for moderate keratoconus with minimal complication rates. KeraRing provided customizable options that significantly improved UDVA and CDVA in cases with irregular astigmatism, although segment migration was more common. The Ferrara Ring was highly effective in central keratoconus, offering substantial corneal flattening with a moderate risk of visual disturbances. MyoRing effectively reduced higher-order aberrations in advanced keratoconus but was associated with a higher reoperation rate. CAIRS, combined with corneal crosslinking, showed promising outcomes with enhanced biocompatibility and minimal complications, particularly for patients sensitive to synthetic materials. ICRS types offer tailored options for keratoconus management. INTACS remains effective for moderate cases, while KeraRing and Ferrara Ring are suitable for advanced stages, especially where customization and flattening are needed. MyoRing offers significant benefits for severe ectasia, and CAIRS presents a novel, biocompatible alternative. Optimizing outcomes and minimizing complications requires tailored selection based on patient-specific corneal characteristics and disease stage. Further comparative studies are needed to refine patient selection criteria and assess the long-term efficacy of each ICRS type.

Plain language summary

Improving vision in keratoconus: how different types of corneal ring implants help

Keratoconus is an eye condition that causes the cornea, the clear front part of the eye, to thin and bulge into a cone-like shape. This leads to blurry vision, difficulty with light, and challenges in using glasses or contact lenses. When the condition becomes severe, more advanced treatments are required to reshape the cornea and improve vision. Intrastromal Corneal Ring Segments (ICRS) are small devices inserted into the cornea to flatten its shape and correct irregularities. These segments are made from materials that are safe for the eye and provide an alternative to corneal transplant surgery. There are several types of ICRS available, including INTACS, KeraRing, Ferrara Ring, MyoRing, and CAIRS (Corneal Allogenic Intrastromal Ring Segments). Each type has unique features and is selected based on the severity of keratoconus and the patient's individual eye condition. This review explains how these different types of ICRS work, compares their effectiveness, and discusses potential risks, such as device migration or infection. The findings show that each ICRS type can improve vision and delay the need for a corneal transplant, but their success depends on careful selection and surgical expertise. Newer options, such as CAIRS, offer promising results with better compatibility and fewer risks. This study helps eye specialists choose the most appropriate treatment for patients with keratoconus and highlights the need for further research to improve outcomes and reduce complications.

Introduction

Keratoconus is a progressive disease-causing corneal thinning and anterior protrusion, resulting in a cone-like shape. This deformation leads to a decline in visual acuity, changes in refractive power, and the development of irregular astigmatism. ¹ Although keratoconus affects both eyes, it often does so asymmetrically, with one eye typically more severely impacted. ² The disease typically manifests in adolescence or early adulthood, progressing throughout the second and third decades of life.

Epidemiological studies show significant variability in the prevalence of keratoconus depending on ethnicity and geographic location. In the Middle East and parts of Asia, prevalence rates range from 1500 to 5000 per 100,000 individuals, while incidence rates are approximately 20 per 100,000 annually. ³ In contrast, studies conducted in Caucasian populations report lower prevalence, generally under 1000 per 100,000. ⁴ In addition, ethnic factors seem to play a more significant role than geographic location, as indicated by studies in the United Kingdom that show a much higher prevalence among Asian populations compared to Caucasians. ⁵

There are conflicting reports regarding gender distribution in keratoconus, with some studies suggesting a male predominance, ⁶ others indicating a female predominance, ⁷ and some finding no significant gender difference at all. ⁸ These variations underscore the complexity of keratoconus, suggesting a combination of genetic, environmental, and potentially hormonal influences.

Emerging evidence has suggested that hormonal factors, particularly thyroid dysfunction, may contribute to the pathophysiology of keratoconus. A study by Thanos et al. found a higher prevalence of thyroid gland dysfunction among keratoconus patients compared to the general population, as well as elevated tear thyroxine levels and thyroxine receptor expression in keratoconic corneas. ⁹ These findings support a possible systemic influence on corneal metabolism and collagen integrity, warranting further investigation into hormonal contributions to disease progression. ⁹

Anatomy and pathophysiology of keratoconus

The cornea consists of six distinct layers: the outer non-keratinized squamous epithelium, Bowman’s layer, the stroma, Dua’s layer, Descemet’s membrane, and the single-layered endothelium. ¹⁰ Keratoconus affects these layers to varying degrees, with histopathological changes most prominent in the central cornea. Early in the disease, only the anterior corneal layers may be involved, but as the disease progresses, deeper structures are compromised. Sandali et al. formulated a staging system that describes the progression of keratoconus through the corneal layers. ¹¹ This staging highlights corneal thinning, disruptions in Bowman’s layer, stromal scarring, and, in advanced cases, ruptures in Descemet’s membrane leading to acute hydrops and scarring. ¹

Genetic factors play a crucial role in keratoconus development. The disease has been linked to genetic syndromes such as Ehlers-Danlos, Noonan syndrome, and Down syndrome.^12,13 There is also a strong hereditary component, with a higher prevalence in consanguineous families, monozygotic twins, and first-degree relatives.^14,15 Although the genetic basis of keratoconus is complex and still under investigation, some studies have identified mutations in gene loci such as 5q21.2 as being associated with the disease. ¹⁶ The roles of the VSX1 and SOD1 genes have also been studied, but their involvement remains inconclusive. ¹⁷

In addition to genetic predisposition, environmental factors such as frequent eye rubbing, allergies, and atopic conditions (e.g., asthma and eczema) contribute to the onset and progression of keratoconus. ¹⁸ Elevated levels of Immunoglobulin E IgE) have been observed in keratoconus patients, further implicating allergic responses in the disease’s pathogenesis. ¹⁹ While keratoconus has historically been considered a non-inflammatory disorder, recent studies have identified elevated levels of pro-inflammatory cytokines and markers of oxidative stress in the tears of patients, suggesting that inflammation may play a more significant role than previously thought.^20,21 This combination of oxidative stress and dysregulated antioxidant pathways leads to the degeneration of collagen fibrils, contributing to the characteristic cone-shaped cornea seen in keratoconus. ²²

Management of keratoconus

The management of keratoconus is multifaceted and depends on the severity of the disease. Conservative approaches include the use of spectacles and contact lenses, particularly in the early stages of the disease. ²³ However, as the disease progresses and higher-order aberrations develop, surgical interventions become necessary. Intrastromal corneal ring segments (ICRS) are among the surgical options used to improve corneal shape and visual acuity by flattening the steep corneal surface. ICRS are small, arc-shaped implants made from polymethyl methacrylate (PMMA) that are inserted into the mid-peripheral corneal stroma. Their primary mechanism involves mechanically flattening the central cornea by exerting an outward force on the surrounding corneal tissue, creating a shortening of the arc length and reducing anterior corneal protrusion. This structural redistribution leads to a more regular corneal shape, which in turn decreases irregular astigmatism and improves the quality of the retinal image. ICRS are particularly effective in keratoconus because they reduce the cone’s steepness, improve corneal symmetry, and help centralize the visual axis. By modifying the corneal curvature, ICRS enhance both uncorrected distance visual acuity (UDVA) and best-corrected visual acuity (BCVA), and in many cases can delay or obviate the need for corneal transplantation.^24,25

Recent advancements in laser-assisted techniques, particularly femtosecond laser technology, have improved the precision and safety of ICRS implantation, further solidifying their role in the treatment of moderate-to-severe keratoconus. ²⁶ In this review, we will examine the various ICRS types, their associated outcomes and complications, and explore their use in combination with corneal crosslinking (CXL) as a means of enhancing long-term corneal stability. By doing so, we aim to offer a clearer understanding of the optimal treatment strategies for keratoconus and improve patient outcomes.

Methods

A comprehensive literature review was conducted using the following databases: PubMed, Scopus, and Web of Science. The search was limited to articles published in English between January 2000 and December 2024. A total of 437 articles were retrieved, and 108 were selected for inclusion based on predefined criteria: clinical studies, case series, and long-term follow-ups evaluating outcomes of INTACS, KeraRing, Ferrara Ring, MyoRing, or Corneal Allogenic Intrastromal Ring Segments (CAIRS) in keratoconus patients, with a minimum follow-up of 6 months. Exclusion criteria were non-peer-reviewed articles, studies with sample sizes of fewer than 10 patients, and those without sufficient follow-up data (minimum 6 months). The number of references included per ICRS type was as follows: INTACS (5), KeraRing (5), Ferrara Ring (5), MyoRing (3), and CAIRS (1). Search terms included combinations of “Intrastromal Corneal Ring Segments,” “ICRS,” “keratoconus,” “INTACS,” “KeraRing,” “Ferrara Ring,” “MyoRing,” and “CAIRS.” Boolean operators (AND, OR) were applied to refine the results.

Types of ICRS

INTACS

Overview and technical characteristics

INTACS^® are FDA-approved ICRS composed of PMMA, featuring a hexagonal cross-section. ²⁷ These segments are implanted at approximately 70%–80% depth within the mid-peripheral corneal stroma, typically within a 6.77 mm optical zone. ²⁸ They come in varying thicknesses, ranging from 150 to 450 µm, and can be inserted as either a single segment or a paired configuration, depending on the severity and location of the keratoconic cone. ²⁷ INTACS may be implanted using mechanical dissection or, more commonly now, with femtosecond laser-assisted techniques to enhance precision and minimize intraoperative complications. ²⁸

Mechanism of action

The mechanism of action for INTACS relies on biomechanical redistribution of the corneal lamellae. ²⁹ By creating an arc-shortening effect in the peripheral cornea, INTACS reduce central steepening and promote corneal flattening. This mechanical modulation helps restore a more regular corneal shape, reduces irregular astigmatism, and realigns the visual axis, resulting in improved image quality on the retina. These changes contribute significantly to enhancements in both uncorrected and best-corrected distance visual acuity (CDVA). ²⁹

Indications and patient selection criteria

INTACS are particularly suitable for patients with mild-to-moderate keratoconus, especially when the maximum keratometry (Kmax) value is less than 55–58 diopters and the cone is located centrally or paracentrally. ²⁹ A minimum corneal thickness of 400 µm at the intended implantation site is typically required to reduce the risk of complications. Ideal candidates should also have a clear central cornea without significant scarring or opacities. On the other hand, INTACS are generally contraindicated in cases with Kmax exceeding 70 diopters, severe thinning at the tunnel site, a history of corneal hydrops, or the presence of central stromal scars. ²⁸

Clinical outcomes and evidence

A comprehensive multicenter study by Warrak et al. involving 932 eyes provided robust evidence for the efficacy and safety of INTACS. ²⁹ The study population had a mean age of 27.4 ± 6.5 years, and patients were followed for a mean duration of 3 years, with a subset of 41 eyes observed for over a decade. Significant visual improvements were noted, with uncorrected visual acuity improving from 0.75 to 0.53 logMAR and BCVA improving from 0.27 to 0.20 logMAR. ²⁹ In addition, the average Kmax was reduced by 3.8 diopters. The complication rate was low, with extrusion reported in only 0.4% of cases, all related to trauma. Halos and glare were the most commonly reported visual disturbances, occurring in approximately 10%–15% of patients, typically due to decentration or segment positioning. Fewer than 1% of eyes required additional procedures such as re-implantation or explantation. ²⁹ A separate long-term study by Kang et al. further corroborated these findings, demonstrating continued stability in BCVA and significant reduction in higher-order aberrations, particularly coma, over a 5-year follow-up period. ²⁸

INTACS offer a safe and effective treatment option for patients with moderate keratoconus who are not yet candidates for corneal transplantation. When appropriately selected based on cone location and corneal thickness, INTACS can provide long-term visual and topographic stability. The technique is further enhanced by femtosecond laser assistance, which improves implantation accuracy and reduces postoperative complications. While not as customizable as other ring designs, INTACS remain a dependable option with one of the most favorable safety profiles among synthetic ICRS.

KeraRing

Overview and technical characteristics

The KeraRing is an ICRS with a triangular cross-sectional profile, manufactured from PMMA. ³⁰ It is designed for implantation at approximately 70% corneal depth within a 5.0 mm optical zone, which allows for greater centration and influence over corneal curvature. ³⁰ Available in arc lengths of 90°, 120°, 160°, and 210°, and in thicknesses ranging from 150 to 300 µm, the KeraRing allows for a high degree of customization. ³⁰ One or two segments may be implanted depending on the topographic pattern and cone location. Femtosecond laser-assisted tunnel creation has largely replaced mechanical dissection, enabling more precise placement.

Mechanism of action

By inducing an arc-shortening effect in the mid-peripheral cornea, KeraRing segments lead to central corneal flattening and normalization of irregular astigmatism. ³⁰ Their triangular design enhances the biomechanical effect, particularly in ectatic zones, thereby improving the optical quality and visual acuity.

Indications and patient selection criteria

KeraRing is particularly indicated for patients with moderate-to-advanced keratoconus, especially those with irregular astigmatism or paracentral cones. Ideal candidates typically have a Kmax under 65 D, a minimum corneal thickness of 400 µm at the tunnel site, and no central stromal opacities. ³⁰ It is often used when more pronounced flattening is needed or when INTACS are deemed suboptimal due to cone location. Patients with severe thinning, apical scarring, or decentered cones may not be suitable candidates.

Clinical outcomes and evidence

Coskunseven et al. conducted a prospective study involving 50 eyes implanted with KeraRing using a femtosecond laser. ³⁰ The mean patient age was 29.3 years. After one year, uncorrected visual acuity improved in 78% of eyes, and 68% gained between one and four lines of BCVA. The spherical equivalent improved from −5.62 to −2.50 D, and Kmax was reduced from 50.63 to 47.56 D. ³⁰ The study reported a follow-up period of 12 months, with a complication rate of 6%, primarily segment migration, which was managed successfully with repositioning.

Another retrospective study by Gatzioufas et al. focused on 33 eyes of patients aged over 40 years, showing that UDVA improved from 0.94 to 0.76 logMAR and CDVA from 0.41 to 0.20 logMAR at 6 months. ³¹ The spherical equivalent dropped from −11.65 to −4.44 D, and mean Kmax decreased from 63.24 to 60.15 D, highlighting its effectiveness even in stiffer, older corneas. ³¹

Complications and safety profile

KeraRing implantation is generally safe, particularly when assisted by a femtosecond laser. ²⁷ The use of lasers significantly lowers the risk of decentration and epithelial trauma compared to manual dissection. In a comparative study by Monteiro et al., complication rates dropped from 18.1% with manual technique to 3.6% with laser-assisted surgery. ²⁷ Early segment migration was noted in 6% of eyes in the Coskunseven study, but was not associated with long-term adverse effects. No cases of extrusion or infection were reported. ³⁰

The KeraRing offers a versatile and effective solution for the management of moderate-to-advanced keratoconus. Its triangular design and flexible arc lengths enable targeted correction of irregular astigmatism, particularly in eyes with asymmetric cones or high keratometric values. Outcomes are enhanced with laser-assisted placement, and complication rates are low when proper patient selection criteria are followed. KeraRing remains a preferred choice in cases requiring more aggressive reshaping or customized arc planning.

Ferrara Ring

Overview and technical characteristics

The Ferrara Ring is an ICRS developed in the late 1990s specifically to manage keratoconus and post-LASIK ectasia. ³² It is made of PMMA and features a triangular cross-section, which is designed to maximize the arc-shortening effect and deliver greater flattening of the central cornea. Ferrara Rings are available in arc lengths of 120°, 160°, and 210° and thicknesses ranging from 150 to 300 µm. ³² They are typically implanted at a depth of 75%–80% of the corneal thickness in the 4.4 mm optical zone, allowing for enhanced centration and biomechanical effect. ³² Implantation may be done manually or with femtosecond laser assistance, with the latter offering better accuracy and fewer complications.

Mechanism of action

The Ferrara Ring works by displacing corneal lamellae, leading to central flattening and improved corneal symmetry. Its triangular profile exerts greater force on the cornea than hexagonal designs, making it especially suitable for advanced or steep keratoconus cases. This biomechanical redistribution reduces higher-order aberrations and improves both uncorrected and BCVA. ³³

Indications and patient selection criteria

The Ferrara Ring is most suitable for patients with moderate-to-advanced keratoconus, particularly those with central or paracentral cones and Kmax values up to 65–68 D. Adequate corneal thickness⩾400 µm at the intended tunnel depth is required. It is not recommended in eyes with central stromal scarring, apical opacities, or hydrops history. The Ferrara Ring’s design makes it especially effective in younger patients with steeper corneal profiles, where aggressive central flattening is necessary.^32,33

Clinical outcomes and evidence

A long-term prospective study by Torquetti et al. evaluated 35 eyes over 5 years, reporting improvements in UDVA from 0.15 to 0.31 logMAR and CDVA from 0.41 to 0.62 logMAR. ³³ Both changes were statistically significant (p = 0.003 and p = 0.002, respectively). Keratometric readings also showed a substantial improvement, with Kmin decreasing from 48.99 to 44.45 D and Kmax from 54.07 to 48.09 D. ³³

A broader study by Costa et al. included 124 eyes with an average follow-up of 5 years. ³⁴ The mean UDVA improved from 0.91 to 0.46 logMAR, and CDVA improved from 0.40 to 0.22 logMAR. Impressively, 90.3% of patients maintained or improved CDVA, while only 9.7% experienced a slight decline. Mean Kmax dropped from 55.23 to 53.43 D, confirming the ring’s ability to achieve long-term corneal reshaping and stability. ³⁴

Complications and safety profile

While generally safe, the Ferrara Ring has a slightly higher complication rate than INTACS or KeraRing, especially when implanted manually. In the Costa et al. study, 4.8% of eyes experienced extrusion, and 12.9% required ICRS exchange or repositioning for suboptimal refractive outcomes. ³⁴ Visual disturbances such as halos and glare occurred in about 10% of cases, though these symptoms typically diminished with time. No cases of infectious keratitis or vision-threatening complications were reported. ³⁴

The Ferrara Ring is a highly effective option for treating central keratoconus, especially in patients requiring more aggressive flattening. Its biomechanical profile and available arc/thickness options allow for precise customization. Although it may present a slightly higher risk of complications than other ICRS types, these risks are generally manageable with appropriate surgical technique and patient selection. With proven long-term efficacy, the Ferrara Ring remains a preferred choice in advanced and steep keratoconus cases.

MyoRing

Overview and technical characteristics

The MyoRing is a full 360° intrastromal corneal implant made from PMMA, designed as a continuous ring rather than segmented arcs. ³⁵ It is typically implanted within a full stromal pocket at a depth of approximately 300–320 µm, which is created using either a femtosecond laser or a mechanical microkeratome. Available in diameters of 5.0 mm and thicknesses ranging from 200 to 400 µm, the MyoRing provides uniform corneal flattening and structural support, making it particularly suitable for advanced keratoconus with generalized ectasia. ³⁵

Mechanism of action

Unlike segmental ICRS, the MyoRing exerts a circular compressive force throughout the entire corneal circumference. This leads to even biomechanical redistribution, flattening the central cornea and improving symmetry in both meridians. Its closed-loop architecture also contributes to greater structural support and resistance to further ectatic progression, which is advantageous in severe keratoconus cases. ³⁵

Indications and patient selection criteria

MyoRing is indicated for moderate-to-severe keratoconus, particularly when a more global reshaping of the cornea is needed. ³⁵ It is well suited for patients with generalized ectasia or irregularity not limited to one axis. Ideal candidates have a minimum corneal thickness of 400 µm, a Kmax below 70 D, and an intact central cornea without scarring. ³⁵ It is less suitable in cases where cone decentration is highly asymmetric, or where localized correction would be more beneficial than uniform flattening.

Clinical outcomes and evidence

In a prospective 5-year study by Naderi et al., 48 eyes were implanted with the MyoRing. ³⁵ The UDVA improved from 1.20 to 0.42 logMAR, and the CDVA improved from 0.63 to 0.20 logMAR. The spherical equivalent was reduced from −6.53 to −2.23 D, and mean keratometry values decreased by 2.82 D. These improvements were statistically significant (p < 0.001), and 81% of patients reported moderate to high satisfaction. ³⁵

A comparative study by Al-Tuwairqi et al. analyzed outcomes of MyoRing versus KeraRing. ³⁶ In the MyoRing group, the mean refractive spherical equivalent reduced by 3.60 D after 6 months, and Kmax decreased by 6.51 D, a larger reduction than observed with KeraRing. The study also reported a greater reduction in coma aberrations in the MyoRing group, which contributed to improved optical quality and patient satisfaction. ³⁶

Complications and safety profile

While MyoRing is generally safe, its implantation is more invasive due to the creation of a full stromal pocket. In the Naderi study, 15.8% of patients required additional procedures, including MyoRing exchange, removal, or subsequent deep anterior lamellar keratoplasty. ³⁵ However, no cases of infection or long-term visual loss were reported. Postoperative complications such as ring decentration or epithelial ingrowth are rare but have been described in isolated case reports. ³⁶

The MyoRing is a powerful option for treating advanced keratoconus, especially in patients requiring uniform flattening and biomechanical stabilization. Its closed-loop design sets it apart from other ICRS types, offering strong structural support and notable reductions in higher-order aberrations. Though more invasive, its visual and topographic benefits are well-documented. Proper patient selection and surgical expertise are essential to minimize reoperation rates and maximize long-term success.

Corneal allogenic intrastromal ring segments

Overview and technical characteristics

CAIRS are a novel form of ICRS developed to address limitations associated with synthetic implants. ³⁷ Unlike PMMA-based rings, CAIRS are derived from donor corneal tissue, offering a biocompatible and customizable alternative. These allogenic segments are tailored intraoperatively using a double-bladed trephine to produce uniform or tapered shapes suited to the patient’s unique topography. CAIRS are implanted at a depth of 50% of the corneal stroma, typically within femtosecond laser-created tunnels. ³⁷ Due to their biological origin, CAIRS are expected to integrate more naturally with the host cornea and minimize inflammatory responses.

Mechanism of action

CAIRS segments exert mechanical tension similar to synthetic ICRS, reshaping the cornea through arc shortening. ³⁷ However, they are less rigid, providing a more physiologic redistribution of biomechanical forces. When combined with CXL, a strategy often employed alongside CAIRS, the procedure achieves both geometric flattening and biomechanical stiffening, offering dual benefit in halting disease progression and improving vision. ³⁷

Indications and patient selection criteria

CAIRS are suitable for patients with early-to-advanced keratoconus, particularly those who may not tolerate synthetic materials or have had complications with PMMA segments. Ideal candidates include individuals with sufficient corneal thickness (⩾400 µm) at the intended tunnel depth, intact epithelium, and no central scarring. CAIRS are also useful in eyes where irregular, asymmetric cones may benefit from customized segment architecture. Limitations include the availability of donor tissue, as well as the current lack of large-scale, long-term data. ³⁷

Clinical outcomes and evidence

The primary clinical evidence for CAIRS comes from a pilot study by Jacob et al., involving 20 patients (24 eyes) with keratoconus stages 1 to 4, treated with CAIRS combined with accelerated corneal crosslinking (A-CXL). ³⁷ Results showed a mean improvement in UDVA of 2.79 ± 2.65 lines, with a maximum gain of 8 lines, and CDVA improvement of 1.29 ± 1.33 lines. The study observed meaningful flattening in keratometric readings and reduction in topographic astigmatism across 3 and 5 mm optical zones. No cases of keratoconus progression were observed during the 6–18-month follow-up period. ³⁷

Complications and safety profile

In the Jacob et al. study, no segment-related complications such as extrusion, necrosis, or migration were reported. ³⁷ Mild opacification of the implanted tissue occurred in seven patients, though it did not impact visual acuity and was not visible under normal lighting. The biocompatibility of the allogenic material likely reduces the risk of corneal melt, infection, and immunologic reaction compared to synthetic options. Nonetheless, the long-term durability and integration of CAIRS require further investigation through larger studies and longer follow-up durations. ³⁷

CAIRS represent a promising advancement in keratoconus management, offering customized, biocompatible reshaping of the cornea with low complication rates. Their ability to be combined with crosslinking makes them especially appealing in patients at risk for progression. While current data are limited to early trials, the results are encouraging, particularly for individuals who are not candidates for synthetic implants. Broader clinical adoption will depend on future studies establishing long-term efficacy, surgical standardization, and access to donor tissue.

A summary of key clinical studies evaluating each ICRS type, including baseline characteristics, visual outcomes, and complication rates, is presented in Table 1 to facilitate direct comparison and clinical decision-making. Figure 1 provides a visual representation of the different ICRS designs discussed in this review. Not all studies identified in section “Methods” are included here to provide a concise overview.

Table 1.

Summary of main clinical studies on ICRS types.

ICRS type	Study (author, year)	Patients (eyes)	Mean age	Baseline UDVA (logMAR)	Baseline CDVA (logMAR)	Kmax (preop/postop)	Corneal thickness (min)	Visual outcome (CDVA gain)	Complications
INTACS	Warrak et al., 2020	932 eyes	27.4 ± 6.5 years	0.75	0.27	Not stated/−3.8 D	⩾400 µm	+1.8 lines	0.4% extrusion, halos/glare (~10%–15%)
KeraRing	Coskunseven et al., 2008	50 eyes	29.3 years	Not specified	0.41	50.63 → 47.56 D	⩾400 µm	+1–4 lines	6% migration, no extrusions
Ferrara Ring	Costa et al., 2021	124 eyes	Not specified	0.91	0.40	55.23 → 53.43 D	⩾400 µm	+0.18 logMAR	4.8% extrusion, 12.9% required ICRS exchange
MyoRing	Naderi et al., 2021	48 eyes	Not specified	1.20	0.63	Not specified/−6.51 D	⩾400 µm	+2 lines	15.8% reoperation rate
CAIRS	Jacob et al., 2023	20 patients (24 eyes)	Not specified	Not specified	Not specified	Reduced (not quantified)	⩾400 µm	+1.29 ± 1.33 lines	No major complications; 7 with opacification

CDVA, corrected distance visual acuity; ICRS, intrastromal corneal ring segments; UDVA, uncorrected distance visual acuity.

Figure 1.

The different types of ICRS.

ICRS, intrastromal corneal ring segments.

Each ICRS type reviewed in this manuscript offers unique advantages and presents specific limitations that influence their suitability for individual patients. INTACS are widely regarded for their excellent safety profile, ease of implantation, and long-term stability, but they offer more limited flattening compared to other designs and are less effective in highly irregular or advanced keratoconus. KeraRing provides greater customization through variable arc lengths and is especially beneficial for patients with asymmetric cones or high irregular astigmatism; however, it carries a moderate risk of segment migration. Ferrara Rings deliver substantial central flattening and are ideal for advanced keratoconus with steep corneas, but slightly higher extrusion and reoperation rates have been reported, particularly when using manual techniques. MyoRing, with its full-ring design, offers uniform biomechanical reinforcement and excellent reduction in coma aberrations, making it suitable for severe ectasia, although its more invasive nature and higher reintervention rate must be considered. Finally, CAIRS represents a novel biocompatible solution with promising visual outcomes and minimal complications, though it remains limited by donor tissue availability and a current lack of long-term data.

This review is not without limitations. Included studies varied in sample size, patient demographics, follow-up duration, and surgical techniques, particularly between manual and femtosecond-assisted methods. In addition, the absence of randomized head-to-head comparisons and long-term multicenter trials, especially for emerging techniques such as CAIRS, limits the ability to generalize outcomes across populations. Despite these constraints, the present synthesis provides a clinically relevant comparative overview that may aid ophthalmologists in tailoring ICRS selection based on individual corneal topographies and disease stages.

Conclusion

The diverse range of ICRS types offers clinicians the flexibility to choose the most appropriate treatment based on each patient’s specific keratoconic profile. While INTACS continues to provide a stable and effective solution for moderate keratoconus, options like KeraRing, Ferrara Ring, and MyoRing cater to more advanced cases and irregular corneal topographies. The introduction of CAIRS adds a new dimension to keratoconus management, offering a biocompatible and customizable solution. As more studies emerge, particularly regarding the long-term outcomes of CAIRS, the landscape of keratoconus treatment will continue to evolve, providing patients with safer and more effective options to preserve their vision.