Replacement of Synthetic Intracorneal Ring Segments by Corneal Allogenic Intrastromal Ring Segments (CAIRS) in Keratoconus: A Narrative Review with Systematic Search

Replacement of Synthetic Intracorneal Ring Segments by Corneal Allogenic Intrastromal Ring Segments (CAIRS) in Keratoconus: A Narrative Review with Systematic Search | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Systematic Review Replacement of Synthetic Intracorneal Ring Segments by Corneal Allogenic Intrastromal Ring Segments (CAIRS) in Keratoconus: A Narrative Review with Systematic Search Adel Barbara This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-9320902/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Purpose To identify, synthesize, and critically appraise all published series and case reports specifically reporting corneal allogenic intrastromal ring segments (CAIRS) implantation as a replacement for failed synthetic polymethyl methacrylate (PMMA) intrastromal corneal ring segments (ICRS) in keratoconus (KC). Methods A literature search was conducted in PubMed/MEDLINE, Google Scholar, and Research Square using predefined search terms related to CAIRS rescue implantation after synthetic ICRS failure. Additional records were identified through hand-searching reference lists of included studies. All study designs were eligible. Studies reporting primary CAIRS implantation without prior synthetic ICRS were excluded. The search was concluded in March 2026. Results Nine publications were identified, comprising 1 controlled comparative study, 1 prospective case series, 1 retrospective case series, and 6 case reports, reporting on approximately 40 eyes across four failure scenarios: anterior stromal necrosis/melt, ICRS extrusion, anterior chamber intrusion, or unsatisfactory refractive outcome. CAIRS implantation preserved or improved visual and topographic outcomes in 8 of 9 publications. The single documented failure occurred in a cornea with extensively diseased stroma following 20 years of synthetic ICRS implantation. Conclusion Available evidence suggests that CAIRS may represent an effective and biologically rational rescue strategy for failed synthetic CAIRS exchange appears preferable to explantation alone in anterior stromal necrosis when structurally feasible; delayed implantation after extrusion may allow accurate topographic planning; and the indication may be cautiously extended to include unsatisfactory refractive outcome. The evidence base consists predominantly of case reports and small series (~ 40 eyes), and all conclusions should be interpreted as hypothesis-generating pending prospective multicenter validation. Keratoconus corneal allogenic intrastromal ring segments CAIRS intracorneal ring segments ICRS extrusion ICRS rescue PMMA replacement Figures Figure 1 INTRODUCTION Corneal allogenic intrastromal ring segments (CAIRS) represent an emerging therapeutic option for the management of keratoconus (KC), offering reduction of both irregular and regular astigmatism as well as higher-order aberrations (HOA). CAIRS implantation has been shown to improve uncorrected visual acuity (UCVA) and best spectacle-corrected visual acuity (BSCVA) while inducing corneal flattening. [ 1 – 3 ] While polymethyl methacrylate (PMMA) intrastromal corneal ring segments (ICRS) have a longer history of use in keratoconus and achieve outcomes comparable to CAIRS, segment extrusion remains a recognized complication occurring at variable rates. [ 4 ] The accepted management in such cases is removal of the ICRS, which in most instances leads to reversal of the previously achieved biomechanical and visual benefits. Beyond extrusion, the spectrum of synthetic ICRS failure encompasses anterior stromal necrosis and melt, segment migration, anterior chamber intrusion and unsatisfactory refractive outcomes after technically uneventful implantation. In all these scenarios, simple explantation leaves the patient with topographic and visual regression, often returning to pre-implantation baseline. CAIRS, by virtue of their allogenic nature, offer a biologically rational response to failed synthetic implantation. Unlike synthetic reimplantation — which reintroduces the same foreign-body dynamics responsible for the original failure — CAIRS integrate with the host corneal stroma, provide tectonic structural support to thinned or necrotic corneal beds, and can be implanted into pre-existing synthetic ICRS tunnels without new tunneling. Their superficial implantation capability, refractive index match with host tissue, and reduced intrusion risk collectively make them uniquely suited to the rescue setting. Despite the clinical importance of this indication, the published literature on ICRS replacement by CAIRS consists exclusively of case reports and small series. The present narrative review with systematic search identifies, summarizes, and synthesizes all published case reports and series on this specific topic, organized by failure scenario, with a comprehensive summary table and analysis of the surgical principles and outcomes emerging from the literature. METHODS A literature search was conducted in PubMed/MEDLINE, Google Scholar, and Research Square using the following search terms in combination: "CAIRS," "corneal allogenic intrastromal ring segments," "ICRS exchange," "ICRS replacement," "ICRS extrusion CAIRS," "ICRS melt CAIRS," "ICRS intrusion CAIRS," "allogenic ring segment rescue," and "synthetic ICRS CAIRS rescue." No date restrictions were applied. Only studies reporting CAIRS implantation specifically in the setting of prior synthetic ICRS failure — whether due to extrusion, anterior stromal necrosis (ASN), anterior chamber intrusion, or unsatisfactory refractive outcome — were included. Studies reporting primary CAIRS implantation without prior synthetic ICRS were excluded. All study designs were eligible, including case reports, retrospective series, and prospective series. Reference lists of all identified publications were hand-searched for additional relevant reports not captured by database searching. The search was concluded in March 2026. The search and screening were performed by the two co-authors. No formal risk-of-bias assessment was applied, as validated instruments are not well suited to the predominant study type in this review (single case reports); this is acknowledged as a limitation. Outcomes are reported and interpreted per failure scenario rather than pooled, given the significant heterogeneity of surgical techniques, CAIRS preparation methods (manual trimming, femtosecond laser (FSL)-assisted preparation, and commercially processed KeraNatural segments), implantation depths (ranging from 200 µm to 50% of minimum stromal thickness), and clinical indications across the included studies. During the preparation of this work, the authors used Claude (Anthropic) for assistance with data organization, literature synthesis, and English language editing. All clinical content, interpretation, inclusion decisions, and final text were authored, reviewed, and approved by the two physician co-authors, who take full responsibility for the accuracy and integrity of the published article. Figure 1. Flow diagram of the literature search and study selection process. RESULTS: SYNTHESIS OF PUBLISHED EVIDENCE The search identified 9 publications meeting inclusion criteria, comprising 1 controlled comparative series, [ 5 ] 1 prospective case series, [ 6 ] 1 retrospective case series, [ 7 ] and 6 case reports. [ 8 – 14 ] Together these publications report on approximately 40 eyes across four distinct failure scenarios: anterior stromal necrosis/melt, ICRS extrusion, anterior chamber intrusion, and unsatisfactory refractive outcome. A structured summary of outcomes across the extrusion subgroup shows Kmax reduction ranging from − 2.13 D to − 7.0 D, corrected distance visual acuity (CDVA) gain ranging from + 2.13 to + 2.75 lines in series with up to 7 lines in individual cases, and a complication/failure rate of 1 in approximately 40 eyes. Table 1 provides a comprehensive summary of all identified publications. 1. Anterior Stromal Necrosis and Stromal Melt The principle of replacing a failed synthetic ICRS with allogenic tissue at a melt site was first described by Jarade et al. in 2019, [ 8 ] who reported a single case in which a biologic stromal ring was inserted into the ICRS tunnel overlying the melt site and secured with temporary anchoring sutures, with restoration of keratometric values and visual acuity. This concept was formalized and placed on comparative footing by Kozhaya et al. in 2022 [ 5 ] — the first and only controlled comparative study in the rescue literature. Among 643 eyes implanted with Intacs SK at a single center, 16 eyes (15 patients) developed spontaneous ASN over 10 years (5.5% incidence). Eight eyes underwent ICRS removal only; eight had the synthetic segment exchanged with CAIRS at the same sitting. In the explantation-only group, CDVA worsened from 0.14 to 0.28 logMAR, Kmax regressed from 49.23 to 52.26 D, and coma worsened significantly from 0.87 to 1.52 D (p = 0.02). In the CAIRS exchange group, CDVA improved from 0.16 to 0.11 logMAR, Kmax remained stable (49.25 to 49.64 D), and coma mildly improved from 0.87 to 0.81 D — all differences non-significant (p > 0.99), reflecting preserved stability rather than regression. The tectonic benefit of CAIRS was quantified as a 163 µm greater residual pachymetry at the melt site (490 µm vs 327 µm), and the benefit was proportionally greater with thicker segments. Less satisfactory results were reported in cases with advanced melting. A cautionary counterpoint was provided by Asfar et al. in 2025, [ 9 ] who reported a 72-year-old woman with bilateral keratoconus and ICRS implanted 20 years prior. After stromal thinning was identified, the lower ring was explanted and a 550 µm CAIRS was inserted into the same tunnel. Topography showed mild initial improvement; however, four months later, a new linear stromal defect appeared overlying the CAIRS at a different arc location. Despite management with antibiotics and loteprednol, the CAIRS was removed, and final visual acuity and topography were worse than the pre-intervention baseline. This is the only failure case in the rescue literature and demonstrates that in extensively diseased stroma with longstanding synthetic implants, the underlying necrosis process may be irreversible and may continue despite CAIRS exchange. This case further confirms that CAIRS can extrude, as reported in several primary CAIRS series, and refutes the previously held assumption that extrusion does not occur with allogenic segments. [ 15 , 16 ] 2. ICRS Extrusion The concept of substituting extruded ICRS with donor corneal tissue was established by Daoud et al. in 2019 [ 7 ] in a case series of four eyes. The synthetic ring was removed through the exposed area, and donor corneal tissue tailored to the size and shape of the ICRS was inserted into the empty tunnel and fixed with 10 − 0 nylon sutures. At 3-month follow-up, all four cases had healed and Kmax, BCVA, and corneal topography were stable and comparable to pre-extrusion values — the first published series describing donor tissue substitution for extruded ICRS. The longest follow-up case in the rescue literature was reported by Barbara and Barbara [ 14 ] — a woman with advanced bilateral keratoconus whose right eye had undergone a pair of Ferrara ring implantation (350 µm, 160°, 5.0 mm optical zone) in 2002. After 22 years of uneventful implantation with significant improvement of UCVA and BSCVA, the patient presented in June 2024 with extrusion of the nasal segment. The extruded ICRS was replaced with a full-thickness KeraNatural CAIRS (VisionGift, USA) while the non-extruded temporal PMMA segment was deliberately left in situ. The CAIRS was manually trimmed with scissors after partial dehydration to fit the pre-existing narrow Ferrara ring tunnel — a technically demanding step the authors describe as producing an imperfectly contoured segment. Despite suboptimal shaping conditions, UCVA and BSCVA remained stable at all follow-up visits through 18 months, with no change in refraction. Kmax reduced from 64.50 D at the time of extrusion to 60.68 D at 18 months. No extrusion, migration, or adverse events were recorded. This case is the first to report ICRS extrusion after 22 years of implantation. Susanna et al. published the first prospective series specifically addressing CAIRS in eyes with a history of ICRS explantation, [ 6 ] including 8 patients with stable keratoconus at a mean of 12 ± 2.8 months after ICRS removal. Segments were fully individualized in arc length, thickness, and optical zone (OZ) based on keratometry, posterior elevation, asphericity, and the Alfonso-Ferrara morphological phenotype, using the LDV Z8 FSL (Ziemer Ophthalmic Systems). Tunnel depth was set at 50% of the minimum stromal thickness, with deliberate avoidance of residual ICRS tunnel scars. At a mean follow-up of 7.9 ± 2.6 months, Kmax decreased by 3.03 ± 1.35 D (p = 0.01), spherical equivalent (SE) by 6.28 ± 2.71 D (p = 0.01), and CDVA improved by a mean of 2.75 ± 1.7 lines; 7 of 8 eyes gained ≥ 2 lines of CDVA, 5 achieved ≥ 20/40. No intraoperative or postoperative complications were observed. Two patients retained a non-extruded synthetic segment with CAIRS placed parallel to it, confirming feasibility of the combined approach. The authors proposed that delayed implantation — at least 6 months after explantation — allows more accurate topographic planning than simultaneous exchange. 3. Anterior Chamber Intrusion Gendy et al. reported the first case of CAIRS used as rescue after synthetic ICRS anterior chamber intrusion. [ 10 ] A 43-year-old man with bilateral keratoconus presented with tearing and light sensitivity 2 weeks after KeraRing implantation; AS-OCT confirmed intrusion of the inferotemporal segment into the anterior chamber. The synthetic segment was explanted and, after stromal oedema resolved, a CAIRS was implanted at the same location but at the substantially shallower depth of 250 µm. At 6 months, CDVA improved from 20/80 to 20/20−, coma decreased from 1.54 to 0.60 D, and Kmax fell from 53.30 to 47.59 D. The authors noted that the 750 µm allogenic segment compressed to 355 µm intrastromally — equivalent in profile to the original synthetic segment placed at a much deeper level — yet produced greater flattening by virtue of its anterior position. A second intrusion case was reported by Alabbasi et al. in 2025, [ 11 ] who inserted CAIRS into the pre-existing synthetic ICRS tunnel without creating a new FSL channel. A 24-year-old man developed segment migration, stromal necrosis, and intrusion 6 weeks after bilateral ICRS implantation. Following explantation, a CAIRS was placed into the same tunnel, fully filling the stromal defect and covering the necrosis site. At 6 months, CDVA improved from 20/60 to 20/25, astigmatism decreased from − 6.50 to − 4.00 D, and keratometry fell from 49.00 to 45.00 D with no signs of re-necrosis or migration. Barbara and Barbara [ 14 ] likewise implanted CAIRS into the pre-existing Ferrara ring tunnel at the site of the extruded nasal segment, avoiding new stromal dissection in a cornea with a 22-year implantation history. In all three cases, favorable visual and topographic outcomes were achieved with no recurrence of extrusion or migration, confirming that pre-existing synthetic ICRS tunnels represent a viable and tissue-sparing implantation channel for CAIRS across different failure scenarios. 4. CAIRS–PMMA Coexistence and Unsatisfactory Refractive Outcome Bteich et al. described the first case of CAIRS implanted alongside a retained synthetic ICRS at a different OZ, [ 12 ] in a 29-year-old man with severe keratoconus who had moderate improvement from a PMMA ring at a 7 mm OZ but persistent residual irregular astigmatism. A CAIRS was inserted at a smaller OZ without removing the existing synthetic segment. At 6 months, 3 lines of CDVA were gained, Kmax decreased from 50.02 to 47.89 D (p = 0.03), and coma fell from 1.05 to 0.21 D (p = 0.01). This case demonstrated that PMMA and CAIRS can coexist productively in the same cornea at different optical zones, with each contributing independently to the refractive and topographic outcome. AlQahtani and Alsulami used unsatisfactory refractive outcome as a stand-alone indication for ICRS-to-CAIRS exchange. [ 13 ] In Case 1, a 49-year-old woman with unimproved vision 10 years after ICRS underwent synthetic segment removal followed by a single KeraNatural CAIRS at the deliberately shallow depth of 200 µm to maximize flattening in a steep cone. UCVA improved from 20/400 to 20/30, Kmax reduced by 7.0 D, and astigmatism from 4.2 to 0.6 D. In Case 2, CAIRS was implanted alongside a retained broken-eyelet ICRS that could not safely be removed; UCVA improved from 20/60 to 20/25, BCVA to 20/20, and Kmax fell 3.4 D. Table 1 Comprehensive summary of all published case reports and series on ICRS replacement by CAIRS, organized by failure scenario. # Author, Year Journal n / Eyes Failure Scenario Key Visual & Refractive Outcomes Kmax Change Key Finding / Surgical Note STROMAL MELT / ANTERIOR STROMAL NECROSIS (ASN) 1 Jarade et al., 2019 [ 8 ] J Cataract Refract Surg 1 eye Biologic ring inserted into ICRS tunnel overlying melt site; temporary anchoring sutures Visual acuity and keratometry restored to pre-melt levels Improvement (unquantified) First description of allogenic tissue to manage ICRS-related stromal melt 2 Kozhaya et al., 2022 [ 5 ] J Refract Surg 16 eyes (8 CAIRS vs 8 explant) ASN overlying Intacs SK; 10-year incidence 5.5% in 643 eyes; simultaneous CAIRS exchange vs explantation alone CAIRS: CDVA 0.16→0.11 logMAR; coma 0.87→0.81 D. Explant: CDVA 0.14→0.28 logMAR; coma 0.87→1.52 D (p = 0.02). HOA stable in CAIRS; regressed to pre-ICRS levels after explantation alone CAIRS: stable (49.25→49.64 D). Explant: regressed (49.23→52.26 D) First and only controlled comparative study in rescue literature. Tectonic pachymetry: 490 µm (CAIRS) vs 327 µm (explant alone). Greater benefit with thicker segments; less satisfactory results with large melt areas 3 Asfar et al., 2025 [ 9 ] JCRS Online Case Rep 1 eye 20-year-old ICRS; stromal thinning → lower ICRS explanted → 550 µm CAIRS in same tunnel Initial mild topographic improvement. At 4 months: new melt at different arc overlying CAIRS. CAIRS removed. Final VA and topography worse than pre-intervention Worsened Only documented failure in rescue literature. Longstanding stromal disease may be irreversible; CAIRS exchange does not guarantee arrest of underlying melt process ICRS EXTRUSION 4 Daoud et al., 2019 [ 7 ] J Fr Ophtalmol 4 eyes Extruded ICRS in 4 KC patients; donor tissue tailored to ICRS shape inserted into empty tunnel; fixed with 10 − 0 nylon sutures All 4 grafts healed at 3 months. Kmax, BCVA, and topography stable and comparable to pre-extrusion values Stable (unquantified) First published series describing donor tissue substitution for extruded ICRS 5 Barbara & Barbara, 2026 [ 14 ] Int Ophthalmol (in press) 1 eye Ferrara ring extrusion after 22 years; selective nasal CAIRS replacement (KeraNatural, manually trimmed); non-extruded temporal PMMA retained in situ; pre-existing tunnel reused UCVA and BSCVA stable throughout 18 months. No change in refraction. PMMA–CAIRS coexistence well tolerated. No extrusion, migration, or adverse events 64.50→60.68 D at 18 months (− 3.82 D) Longest follow-up in rescue literature (18 months). First extrusion after 22 years. Selective single-segment replacement preserving functioning temporal synthetic. Manual trimming to fit pre-existing narrow tunnel feasible but technically demanding 6 Susanna et al., 2025 [ 6 ] J Cataract Refract Surg 8 eyes KC with ICRS explantation ≥ 6 months prior (mean 12 months); customized FSL CAIRS (LDV Z8); Alfonso-Ferrara phenotype planning; tunnel depth 50% of min stromal thickness Kmax: −3.03 ± 1.35 D (p = 0.01). SE: −6.28 ± 2.71 D (p = 0.01). CDVA: +2.75 ± 1.7 lines. 7/8 gained ≥ 2 lines. 5/8 achieved ≥ 20/40. No complications −3.03 ± 1.35 D (p = 0.01) First prospective series after ICRS explantation. Delayed implantation (≥ 6 months) appears to allow more accurate planning. Prior CXL or older age may reduce flattening. 2 patients: CAIRS + retained nasal ICRS ANTERIOR CHAMBER INTRUSION 7 Gendy et al. (AUBMC), 2023 [ 10 ] J Refract Surg Case Rep 1 eye KeraRing intrusion into anterior chamber; explanted; CAIRS implanted at same site at 250 µm depth after oedema resolved; new FSL tunnel created CDVA: 20/80→20/20−. Coma: 1.54→0.60 D (− 61%). Trefoil: 2.74→0.99 D. Stable at 6 months 53.30→47.59 D (− 5.71 D) at 1 week; stable at 6 months First rescue of ICRS intrusion with CAIRS. 750 µm CAIRS compressed to 355 µm at 250 µm depth — greater flattening superficial position 8 Alabbasi et al., 2025 [ 11 ] J Refract Surg 1 eye ICRS migration + stromal necrosis + intrusion at 6 weeks; CAIRS inserted into pre-existing ICRS tunnel — no new FSL channel created CDVA: 20/60→20/25. Astigmatism: −6.50→−4.00 D. Stable at 6 months. No re-necrosis or migration 49.00→45.00 D (− 4.00 D) First report of pre-existing tunnel reuse for CAIRS — eliminates additional stromal dissection in compromised cornea. CAIRS fully filled stromal defect CAIRS–PMMA COEXISTENCE / UNSATISFACTORY REFRACTIVE OUTCOME 9 Bteich et al. (AUBMC), 2023 [ 12 ] J Refract Surg Case Rep 1 eye Functioning PMMA ring at 7 mm OZ; residual irregular astigmatism; CAIRS added at smaller OZ without removing existing PMMA — first coexistence report CDVA: +3 lines. Coma: 1.05→0.21 D (p = 0.01). Topographic astigmatism improved 50.02→47.89 D (− 2.13 D, p = 0.03) First CAIRS + retained PMMA in same cornea at different OZ. Additive effect without synthetic removal 10 AlQahtani & Alsulami, 2025 [ 13 ] Am J Ophthalmol Case Rep 2 eyes Case 1: Unsatisfactory outcome 10 years post-ICRS; both removed; CAIRS at 200 µm. Case 2: broken-eyelet ICRS + poor outcome; CAIRS alongside retained ICRS Case 1: UCVA 20/400→20/30; astigmatism 4.2→0.6 D; HOA 1.384→1.095 µm. Case 2: UCVA 20/60→20/25; BCVA→20/20; coma 1.42→0.861 µm; SA 1.04→0.030 µm Case 1: 68.9→61.9 D (− 7.0 D). Case 2: 51.3→47.9 D (− 3.4 D) Expands indication: first to report unsatisfactory refractive outcome as exchange indication. Shallower depth in steep cones OZ = optical zone (OZ); KC = keratoconus; FSL = femtosecond laser; HOA = higher-order aberrations; SE = spherical equivalent; SA = spherical aberration; ASN = anterior stromal necrosis; CDVA = corrected distance visual acuity; UCVA = uncorrected distance visual acuity; BSCVA = best spectacle-corrected visual acuity; CXL = corneal collagen cross-linking; PMMA = polymethyl methacrylate. DISCUSSION The collective evidence synthesized in this review, while limited in volume and consisting predominantly of case reports and small series, is internally consistent and clinically coherent. Across approximately 40 eyes reported in 9 publications spanning 2019 to 2026, CAIRS implantation after synthetic ICRS failure produces favorable visual and topographic outcomes in the large majority of cases, with a single documented failure. The dominant theme across all scenarios is the biological advantage of allogenic tissue in a setting where PMMA ICRS has already failed: CAIRS integrate where PMMA could not persist, provide structural support where stroma has been thinned, and offer a refractive benefit that explantation alone irreversibly forfeits. Kozhaya et al. [ 5 ] — reporting the first and only controlled comparison in this literature — documented 163 µm greater residual stromal thickness at the melt site following CAIRS exchange compared with explantation alone, a clinically meaningful difference with direct implications for corneal structural integrity. CAIRS exchange preserved the refractive gain originally achieved by synthetic implantation, whereas explantation alone returned higher-order aberrations toward pre-ICRS levels. These findings suggest CAIRS exchange may represent a preferable management strategy for synthetic ICRS anterior stromal necrosis whenever technically feasible, though the limited evidence base warrants caution in generalizing this conclusion. The cautionary case of Asfar et al. [ 9 ] introduces an essential caveat: not all melt scenarios are salvageable with CAIRS. In a cornea with 20-year-old ICRS and extensive pre-existing stromal disease, the underlying ischemic and inflammatory process may have progressed beyond the point at which allogenic tissue substitution can provide durable structural support. It should also be noted that CAIRS are not entirely free of the complications associated with synthetic rings; extrusion and anterior stromal melting have been reported in primary CAIRS implantation cases in eyes with no prior synthetic ICRS. [ 15 , 16 ] The extrusion cases collectively suggest that Kmax reduction — not merely stabilization — is achievable with CAIRS in post-extrusion corneas. The Susanna series [ 6 ] reports a mean Kmax reduction of 3.03 D in corneas with established ICRS tunnel scarring, confirming that allogenic segments retain meaningful biomechanical efficacy even in the scarred stromal environment left by prior synthetic implantation. The Barbara and Barbara case [ 14 ] documents progressive Kmax reduction from 64.50 D to 60.68 D over 18 months — the longest individual case follow-up in this literature — suggesting ongoing corneal remodeling as the CAIRS integrates biologically. An important confounding variable not systematically reported or controlled across the included studies is the concurrent or prior use of corneal collagen cross-linking (CXL). CXL stiffens the corneal stroma and stabilizes KCs independently of any intrastromal implant, and its presence or absence could meaningfully influence the topographic outcomes attributed to CAIRS in the rescue setting. In particular, progressive Kmax reduction over extended follow-up cannot be attributed solely to biological CAIRS integration without knowledge of CXL status. None of the included studies were specifically designed to isolate the contribution of CAIRS from that of concurrent or prior CXL. Future prospective studies should report CXL status as a mandatory stratification variable to allow meaningful interpretation of the independent biomechanical contribution of allogenic segment implantation in the rescue setting. The intrusion cases merit separate attention because anterior chamber intrusion — arguably the most feared acute complication of synthetic ICRS, although rare — has not been reported with CAIRS in the published literature to date. An allogenic segment that integrates biologically into the host stroma is, by its nature, far less likely to migrate as a rigid foreign body through the endothelial layer. Gendy et al. [ 10 ] demonstrated that CAIRS can be implanted safely at the exact site of a previous intrusion via a newly created FSL tunnel at a substantially shallower depth than the original synthetic segment, achieving keratometric outcomes superior to the pre-intrusion state. Alabbasi et al. [ 11 ] similarly replaced the failed synthetic segment with CAIRS using the pre-existing ICRS tunnel without creating a new FSL channel, eliminating the need for additional stromal dissection in an already-compromised cornea. Barbara and Barbara [ 14 ] likewise implanted CAIRS into the pre-existing Ferrara ring tunnel at the site of the extruded nasal segment, avoiding new stromal dissection in a cornea with a 22-year implantation history. Favorable visual and topographic outcomes were achieved in all three cases with no recurrence of extrusion or migration, confirming that pre-existing synthetic ICRS tunnels represent a viable and tissue-sparing implantation channel for CAIRS across different failure scenarios. The expansion of the indication to include unsatisfactory refractive outcome — independent of any complication — as a stand-alone reason for ICRS-to-CAIRS exchange represents an important conceptual development. This approach was first reported by Bteich et al., [ 12 ] who added a CAIRS at a smaller OZ alongside a functioning retained synthetic ICRS in a patient with persistent irregular astigmatism despite technically successful ICRS implantation. AlQahtani and Alsulami [ 13 ] subsequently extended this concept further, reporting two cases in which CAIRS was used after bilateral ICRS failure due to unsatisfactory refractive outcome alone — including one case where the existing synthetic segment was retained in situ. Together, these cases raise the possibility that CAIRS may be considered not merely as a rescue for complication-driven failure but as an option in any clinical scenario where synthetic ICRS have produced suboptimal results, though prospective validation is required before this can be considered established practice. Critical Appraisal of the Evidence Base The evidence synthesized in this review is subject to several quality limitations. First, publication bias is likely: favorable outcomes are substantially more likely to reach publication than failures, making the single documented failure [ 9 ] particularly valuable as a counterpoint. Second, all included studies were performed by experienced corneal surgeons at tertiary referral centers, limiting generalizability to less experienced settings. Third, follow-up duration is short across most publications, with the longest controlled comparison extending to only 6 months in most of the reported cases. Fourth, the heterogeneity of surgical techniques — spanning manual tissue trimming, FSL-assisted preparation, and commercially processed KeraNatural segments, with implantation depths ranging from 200 µm to 50% of minimum stromal thickness — prevents standardized interpretation and limits conclusions about any specific CAIRS protocol. Fifth, the search and screening were performed by the two co-authors without an independent third reviewer, representing a methodological limitation inherent to a two-author review. Several additional limitations must be acknowledged. No standardized outcome reporting exists across publications, limiting cross-study quantitative comparison. The variety of ICRS types involved (Intacs, Ferrara rings, Kerarings) and the heterogeneity of failure scenarios prevent any meaningful pooled analysis. These limitations collectively define the research agenda for this field: prospective multicenter registries with standardized CAIRS preparation protocols, mandatory CXL status reporting, standardized outcome measures, and longer follow-up. CONCLUSIONS The published evidence on ICRS replacement by CAIRS, while limited in volume and consisting predominantly of case reports and small series, is consistent in demonstrating that allogenic tissue substitution for failed synthetic ICRS is feasible across most of failure scenarios — anterior stromal necrosis, extrusion, anterior chamber intrusion, persistent unsatisfactory refractive outcome, and coexistence with retained synthetic segments. CAIRS preserves or improves visual and topographic outcomes where explantation alone produces regression, provides tectonic structural support to thinned and necrotic corneal beds, and is associated with a lower risk of re-extrusion and re-intrusion that would accompany synthetic reimplantation. All conclusions below should be interpreted as hypothesis-generating, given the low level of evidence, and require prospective validation. The available evidence supports the following provisional conclusions. First, available evidence suggests CAIRS exchange may represent a preferable strategy over explantation alone when ASN is managed surgically, provided the extent of stromal damage does not preclude adequate structural support — as the single documented failure in this literature demonstrates. Second, in post-extrusion corneas, delayed CAIRS implantation (≥ 6 months after explantation) may allow more accurate topographic planning and is associated with meaningful Kmax reduction and CDVA improvement in the available series. Third, pre-existing ICRS tunnels appear to represent a viable and tissue-sparing implantation channel for CAIRS, based on three independent reports. Fourth, available evidence suggests CAIRS and retained functioning synthetic segments can coexist in the same cornea at different optical zones with favorable short-term outcomes. Fifth, the currently available evidence raises the possibility that the indication for CAIRS exchange may be cautiously extended beyond complication-driven failure to include unsatisfactory refractive outcome, though prospective validation is required before this can be considered established practice. Prospective multicenter studies with standardized CAIRS preparation protocols, mandatory reporting of CXL status, standardized outcome measures, and longer follow-up are needed to consolidate these findings and establish evidence-based guidelines for CAIRS rescue implantation after synthetic ICRS failure. Declarations Competing interests: The authors have no relevant financial or non-financial interests to disclose. Ethics approval: This is a narrative review article. No ethical approval was required as it does not involve human participants, animal subjects, or identifiable patient data. Consent to participate: Not applicable. Consent to publish: Not applicable. Data availability All data analyzed in this review are available in the published papers cited in the reference list. Funding: The authors did not receive support from any organization for the submitted work. Author contributions: Both authors contributed equally to the conception, literature search, data synthesis, and preparation of this manuscript. Both authors read and approved the final version. References Friedrich M, Auffarth GU, Soiberman U, Augustin VA, Khoramnia R, Son H-S (2025) Visual and topographic outcomes after corneal allogeneic intrastromal ring segments (CAIRS) for keratoconus: a systematic review and meta-analysis. Am J Ophthalmol. https://doi.org/10.1016/j.ajo.2025.03.028 Bteich Y, Assaf JF, Mrad AA, Jacob S, Hafezi F, Awwad ST (2023) Corneal allogenic intrastromal ring segments (CAIRS) for corneal ectasia: a comprehensive segmental tomography evaluation. J Refract Surg 39(11):767–776. https://doi.org/10.3928/1081597X-20231011-01 Jacob S, Agarwal A, Awwad ST, Mazzotta C, Parashar P, Jambulingam S (2023) Customized corneal allogenic intrastromal ring segments (CAIRS) for keratoconus with decentered asymmetric cone. Indian J Ophthalmol 71(12):3723–3729. https://doi.org/10.4103/IJO.IJO_1988_23 Bautista-Llamas MJ, Sánchez-González MC, López-Izquierdo I et al (2019) Complications and explantation reasons in intracorneal ring segments (ICRS) implantation: a systematic review. J Refract Surg 35(11):740–751. https://doi.org/10.3928/1081597X-20191010-02 Kozhaya K, Mehanna CJ, Jacob S, Saad A, Jabbur NS, Awwad ST (2022) Management of anterior stromal necrosis after polymethylmethacrylate ICRS: explantation versus exchange with corneal allogenic intrastromal ring segments. J Refract Surg 38(4):256–263. https://doi.org/10.3928/1081597X-20220223-01 Susanna BN, Forseto AS, Moriyama AS et al (2025) Femtosecond laser-assisted customized corneal allogenic intrastromal ring segments in patients with keratoconus after intrastromal corneal ring segment explantation: prospective case series. J Cataract Refract Surg 51(4):267–273. https://doi.org/10.1097/j.jcrs.0000000000001600 Daoud RC, Sammouh FK, Baban TA, Warrak JE, Warrak EL (2019) Allogenic corneal tissue transplantation in substitution for extruded intracorneal rings: a case series. J Fr Ophtalmol 42(10):1090–1093. https://doi.org/10.1016/j.jfo.2019.05.028 Jarade E, Issa M, Chanbour W, Warhekar P (2019) Biologic stromal ring to manage stromal melting after intrastromal corneal ring segment implantation. J Cataract Refract Surg 45(9):1222–1225. https://doi.org/10.1016/j.jcrs.2019.05.022 Asfar K et al (2025) Ongoing stromal melt after corneal allogenic segment implantation in exchange for an explanted synthetic segment due to melt. JCRS Online Case Reports Gendy J, Bteich Y, Ibrahim P et al (2023) Corneal allogenic intrastromal ring segments as a therapeutic method for intrusion of synthetic intrastromal corneal ring segment: a case report. J Refract Surg Case Rep 3(3):e33–e37. https://doi.org/10.3928/jrscr-20230814-01 Alabbasi OM, Alahmadi MW, Alhejaili RA, Alharbi AK (2025) Use of preexisting ICRS tunnels for implantation of CAIRS following ICRS migration and corneal tissue necrosis: a case report. J Refract Surg 41(9):e1022–e1026 Bteich Y, Ibrahim P, Gendy J et al (2023) Polymethylmethacrylate and corneal allogenic intrastromal ring segments: same cornea, different optical zones. J Refract Surg Case Rep 3(3):e38–e41 AlQahtani BS, Alsulami RA (2025) The role of corneal allogenic intrastromal ring segments (CAIRS) implantation after failed synthetic intracorneal ring segments (ICRS): a rescuer. Am J Ophthalmol Case Rep 38:102287. https://doi.org/10.1016/j.ajoc.2025.102287 Barbara A, Barbara R (2026) From synthetic to biological rings: corneal allogenic intrastromal ring segments for visual preservation after PMMA intrastromal corneal ring segment extrusion in keratoconus. Int Ophthalmol (in press) Awwad ST, Asfar KE, Gendy J, Jacob S, Assaf JF (2025) Anterior stromal melt with corneal allogeneic intrastromal ring segments in a patient with severe atopy. Cornea. https://pubmed.ncbi.nlm.nih.gov/40095577/ Barbara R, Gunn D, Khayat B, Barbara A (2025) Complications after corneal allografts intrastromal ring segments (CAIRS) in keratoconus and post LASIK ectasia: a case series. Res Square. https://doi.org/10.21203/rs.3.rs-8991375/v1 Additional Declarations The authors declare no competing interests. Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-9320902","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Systematic Review","associatedPublications":[],"authors":[{"id":617551211,"identity":"979b256f-2dfb-4646-8cb6-a397257eaa01","order_by":0,"name":"Adel Barbara","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA50lEQVRIie3PsQrCMBCA4dRCupx0bRHqK1QKfZ4UwamokFEHQdDNWRHxFeobFA50Cc4FRaxCJwen4iTaQXCqcRPMB7npfsIRoig/iWlYzOfTjoTEUgl5Jbr7dUItqcQcN2Ps9g9tc45ZD/p7xyR6ekpKEktkDKdrzq1ty9/BOvPsAfW8sOybJHQRKAsGgjwTikEUA62VJfWkc0W4s2ApjJzDXSJxk5BgdcSCSICvV0cSSUNkLs4mjDcEcHsxQc8efrjF2TTTczdnbUcYq+slR8c0hum59Px3FSimLrte0G7fbCuKovyNBwDiVKIZRpCVAAAAAElFTkSuQmCC","orcid":"","institution":"IVISION","correspondingAuthor":true,"prefix":"","firstName":"Adel","middleName":"","lastName":"Barbara","suffix":""}],"badges":[],"createdAt":"2026-04-04 13:47:25","currentVersionCode":1,"declarations":{"humanSubjects":true,"vertebrateSubjects":false,"conflictsOfInterestStatement":false,"humanSubjectEthicalGuidelines":true,"humanSubjectConsent":true,"humanSubjectClinicalTrial":false,"humanSubjectCaseReport":false,"vertebrateSubjectEthicalGuidelines":false},"doi":"10.21203/rs.3.rs-9320902/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-9320902/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":106540874,"identity":"b97ebc39-df7b-422a-856f-a96c6ec29838","added_by":"auto","created_at":"2026-04-09 16:02:51","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":83813,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003eFlow diagram of the literature search and study selection process.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-9320902/v1/1488b5fd5fe1d4f839712f17.png"},{"id":106724670,"identity":"71431325-a9e0-41d8-96f2-9d6dce376bd3","added_by":"auto","created_at":"2026-04-12 18:29:04","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":928989,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-9320902/v1/b5cf0394-5eca-4e2d-aeb7-b374a51952d9.pdf"}],"financialInterests":"The authors declare no competing interests.","formattedTitle":"\u003cp\u003e\u003cstrong\u003eReplacement of Synthetic Intracorneal Ring Segments by Corneal Allogenic Intrastromal Ring Segments (CAIRS) in Keratoconus: A Narrative Review with Systematic Search\u003c/strong\u003e\u003c/p\u003e","fulltext":[{"header":"INTRODUCTION","content":"\u003cp\u003eCorneal allogenic intrastromal ring segments (CAIRS) represent an emerging therapeutic option for the management of keratoconus (KC), offering reduction of both irregular and regular astigmatism as well as higher-order aberrations (HOA). CAIRS implantation has been shown to improve uncorrected visual acuity (UCVA) and best spectacle-corrected visual acuity (BSCVA) while inducing corneal flattening. [\u003cspan additionalcitationids=\"CR2\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e] While polymethyl methacrylate (PMMA) intrastromal corneal ring segments (ICRS) have a longer history of use in keratoconus and achieve outcomes comparable to CAIRS, segment extrusion remains a recognized complication occurring at variable rates. [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e] The accepted management in such cases is removal of the ICRS, which in most instances leads to reversal of the previously achieved biomechanical and visual benefits.\u003c/p\u003e \u003cp\u003eBeyond extrusion, the spectrum of synthetic ICRS failure encompasses anterior stromal necrosis and melt, segment migration, anterior chamber intrusion and unsatisfactory refractive outcomes after technically uneventful implantation. In all these scenarios, simple explantation leaves the patient with topographic and visual regression, often returning to pre-implantation baseline.\u003c/p\u003e \u003cp\u003eCAIRS, by virtue of their allogenic nature, offer a biologically rational response to failed synthetic implantation. Unlike synthetic reimplantation \u0026mdash; which reintroduces the same foreign-body dynamics responsible for the original failure \u0026mdash; CAIRS integrate with the host corneal stroma, provide tectonic structural support to thinned or necrotic corneal beds, and can be implanted into pre-existing synthetic ICRS tunnels without new tunneling. Their superficial implantation capability, refractive index match with host tissue, and reduced intrusion risk collectively make them uniquely suited to the rescue setting.\u003c/p\u003e \u003cp\u003eDespite the clinical importance of this indication, the published literature on ICRS replacement by CAIRS consists exclusively of case reports and small series. The present narrative review with systematic search identifies, summarizes, and synthesizes all published case reports and series on this specific topic, organized by failure scenario, with a comprehensive summary table and analysis of the surgical principles and outcomes emerging from the literature.\u003c/p\u003e"},{"header":"METHODS","content":"\u003cp\u003eA literature search was conducted in PubMed/MEDLINE, Google Scholar, and Research Square using the following search terms in combination: \u0026quot;CAIRS,\u0026quot; \u0026quot;corneal allogenic intrastromal ring segments,\u0026quot; \u0026quot;ICRS exchange,\u0026quot; \u0026quot;ICRS replacement,\u0026quot; \u0026quot;ICRS extrusion CAIRS,\u0026quot; \u0026quot;ICRS melt CAIRS,\u0026quot; \u0026quot;ICRS intrusion CAIRS,\u0026quot; \u0026quot;allogenic ring segment rescue,\u0026quot; and \u0026quot;synthetic ICRS CAIRS rescue.\u0026quot; No date restrictions were applied. Only studies reporting CAIRS implantation specifically in the setting of prior synthetic ICRS failure \u0026mdash; whether due to extrusion, anterior stromal necrosis (ASN), anterior chamber intrusion, or unsatisfactory refractive outcome \u0026mdash; were included. Studies reporting primary CAIRS implantation without prior synthetic ICRS were excluded. All study designs were eligible, including case reports, retrospective series, and prospective series. Reference lists of all identified publications were hand-searched for additional relevant reports not captured by database searching. The search was concluded in March 2026.\u003c/p\u003e\n\u003cp\u003eThe search and screening were performed by the two co-authors. No formal risk-of-bias assessment was applied, as validated instruments are not well suited to the predominant study type in this review (single case reports); this is acknowledged as a limitation. Outcomes are reported and interpreted per failure scenario rather than pooled, given the significant heterogeneity of surgical techniques, CAIRS preparation methods (manual trimming, femtosecond laser (FSL)-assisted preparation, and commercially processed KeraNatural segments), implantation depths (ranging from 200 \u0026micro;m to 50% of minimum stromal thickness), and clinical indications across the included studies.\u003c/p\u003e\n\u003cp\u003eDuring the preparation of this work, the authors used Claude (Anthropic) for assistance with data organization, literature synthesis, and English language editing. All clinical content, interpretation, inclusion decisions, and final text were authored, reviewed, and approved by the two physician co-authors, who take full responsibility for the accuracy and integrity of the published article.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFigure 1.\u003c/strong\u003e \u003cem\u003eFlow diagram of the literature search and study selection process.\u003c/em\u003e\u003c/p\u003e"},{"header":"RESULTS: SYNTHESIS OF PUBLISHED EVIDENCE","content":"\u003cp\u003e\u003c/p\u003e\n\u003cp\u003eThe search identified 9 publications meeting inclusion criteria, comprising 1 controlled comparative series, [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e] 1 prospective case series, [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e] 1 retrospective case series, [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e] and 6 case reports. [\u003cspan additionalcitationids=\"CR9 CR10 CR11 CR12 CR13\" citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e] Together these publications report on approximately 40 eyes across four distinct failure scenarios: anterior stromal necrosis/melt, ICRS extrusion, anterior chamber intrusion, and unsatisfactory refractive outcome. A structured summary of outcomes across the extrusion subgroup shows Kmax reduction ranging from \u0026minus;\u0026thinsp;2.13 D to \u0026minus;\u0026thinsp;7.0 D, corrected distance visual acuity (CDVA) gain ranging from +\u0026thinsp;2.13 to +\u0026thinsp;2.75 lines in series with up to 7 lines in individual cases, and a complication/failure rate of 1 in approximately 40 eyes. Table \u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e provides a comprehensive summary of all identified publications.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e1. Anterior Stromal Necrosis and Stromal Melt\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe principle of replacing a failed synthetic ICRS with allogenic tissue at a melt site was first described by Jarade et al. in 2019, [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e] who reported a single case in which a biologic stromal ring was inserted into the ICRS tunnel overlying the melt site and secured with temporary anchoring sutures, with restoration of keratometric values and visual acuity. This concept was formalized and placed on comparative footing by Kozhaya et al. in 2022 [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e] \u0026mdash; the first and only controlled comparative study in the rescue literature. Among 643 eyes implanted with Intacs SK at a single center, 16 eyes (15 patients) developed spontaneous ASN over 10 years (5.5% incidence). Eight eyes underwent ICRS removal only; eight had the synthetic segment exchanged with CAIRS at the same sitting. In the explantation-only group, CDVA worsened from 0.14 to 0.28 logMAR, Kmax regressed from 49.23 to 52.26 D, and coma worsened significantly from 0.87 to 1.52 D (p\u0026thinsp;=\u0026thinsp;0.02). In the CAIRS exchange group, CDVA improved from 0.16 to 0.11 logMAR, Kmax remained stable (49.25 to 49.64 D), and coma mildly improved from 0.87 to 0.81 D \u0026mdash; all differences non-significant (p\u0026thinsp;\u0026gt;\u0026thinsp;0.99), reflecting preserved stability rather than regression. The tectonic benefit of CAIRS was quantified as a 163 \u0026micro;m greater residual pachymetry at the melt site (490 \u0026micro;m vs 327 \u0026micro;m), and the benefit was proportionally greater with thicker segments. Less satisfactory results were reported in cases with advanced melting.\u003c/p\u003e\n\u003cp\u003eA cautionary counterpoint was provided by Asfar et al. in 2025, [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e] who reported a 72-year-old woman with bilateral keratoconus and ICRS implanted 20 years prior. After stromal thinning was identified, the lower ring was explanted and a 550 \u0026micro;m CAIRS was inserted into the same tunnel. Topography showed mild initial improvement; however, four months later, a new linear stromal defect appeared overlying the CAIRS at a different arc location. Despite management with antibiotics and loteprednol, the CAIRS was removed, and final visual acuity and topography were worse than the pre-intervention baseline. This is the only failure case in the rescue literature and demonstrates that in extensively diseased stroma with longstanding synthetic implants, the underlying necrosis process may be irreversible and may continue despite CAIRS exchange. This case further confirms that CAIRS can extrude, as reported in several primary CAIRS series, and refutes the previously held assumption that extrusion does not occur with allogenic segments. [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e2. ICRS Extrusion\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe concept of substituting extruded ICRS with donor corneal tissue was established by Daoud et al. in 2019 [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e] in a case series of four eyes. The synthetic ring was removed through the exposed area, and donor corneal tissue tailored to the size and shape of the ICRS was inserted into the empty tunnel and fixed with 10\u0026thinsp;\u0026minus;\u0026thinsp;0 nylon sutures. At 3-month follow-up, all four cases had healed and Kmax, BCVA, and corneal topography were stable and comparable to pre-extrusion values \u0026mdash; the first published series describing donor tissue substitution for extruded ICRS.\u003c/p\u003e\n\u003cp\u003eThe longest follow-up case in the rescue literature was reported by Barbara and Barbara [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e] \u0026mdash; a woman with advanced bilateral keratoconus whose right eye had undergone a pair of Ferrara ring implantation (350 \u0026micro;m, 160\u0026deg;, 5.0 mm optical zone) in 2002. After 22 years of uneventful implantation with significant improvement of UCVA and BSCVA, the patient presented in June 2024 with extrusion of the nasal segment. The extruded ICRS was replaced with a full-thickness KeraNatural CAIRS (VisionGift, USA) while the non-extruded temporal PMMA segment was deliberately left in situ. The CAIRS was manually trimmed with scissors after partial dehydration to fit the pre-existing narrow Ferrara ring tunnel \u0026mdash; a technically demanding step the authors describe as producing an imperfectly contoured segment. Despite suboptimal shaping conditions, UCVA and BSCVA remained stable at all follow-up visits through 18 months, with no change in refraction. Kmax reduced from 64.50 D at the time of extrusion to 60.68 D at 18 months. No extrusion, migration, or adverse events were recorded. This case is the first to report ICRS extrusion after 22 years of implantation.\u003c/p\u003e\n\u003cp\u003eSusanna et al. published the first prospective series specifically addressing CAIRS in eyes with a history of ICRS explantation, [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e] including 8 patients with stable keratoconus at a mean of 12\u0026thinsp;\u0026plusmn;\u0026thinsp;2.8 months after ICRS removal. Segments were fully individualized in arc length, thickness, and optical zone (OZ) based on keratometry, posterior elevation, asphericity, and the Alfonso-Ferrara morphological phenotype, using the LDV Z8 FSL (Ziemer Ophthalmic Systems). Tunnel depth was set at 50% of the minimum stromal thickness, with deliberate avoidance of residual ICRS tunnel scars. At a mean follow-up of 7.9\u0026thinsp;\u0026plusmn;\u0026thinsp;2.6 months, Kmax decreased by 3.03\u0026thinsp;\u0026plusmn;\u0026thinsp;1.35 D (p\u0026thinsp;=\u0026thinsp;0.01), spherical equivalent (SE) by 6.28\u0026thinsp;\u0026plusmn;\u0026thinsp;2.71 D (p\u0026thinsp;=\u0026thinsp;0.01), and CDVA improved by a mean of 2.75\u0026thinsp;\u0026plusmn;\u0026thinsp;1.7 lines; 7 of 8 eyes gained\u0026thinsp;\u0026ge;\u0026thinsp;2 lines of CDVA, 5 achieved\u0026thinsp;\u0026ge;\u0026thinsp;20/40. No intraoperative or postoperative complications were observed. Two patients retained a non-extruded synthetic segment with CAIRS placed parallel to it, confirming feasibility of the combined approach. The authors proposed that delayed implantation \u0026mdash; at least 6 months after explantation \u0026mdash; allows more accurate topographic planning than simultaneous exchange.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e3. Anterior Chamber Intrusion\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eGendy et al. reported the first case of CAIRS used as rescue after synthetic ICRS anterior chamber intrusion. [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e] A 43-year-old man with bilateral keratoconus presented with tearing and light sensitivity 2 weeks after KeraRing implantation; AS-OCT confirmed intrusion of the inferotemporal segment into the anterior chamber. The synthetic segment was explanted and, after stromal oedema resolved, a CAIRS was implanted at the same location but at the substantially shallower depth of 250 \u0026micro;m. At 6 months, CDVA improved from 20/80 to 20/20\u0026minus;, coma decreased from 1.54 to 0.60 D, and Kmax fell from 53.30 to 47.59 D. The authors noted that the 750 \u0026micro;m allogenic segment compressed to 355 \u0026micro;m intrastromally \u0026mdash; equivalent in profile to the original synthetic segment placed at a much deeper level \u0026mdash; yet produced greater flattening by virtue of its anterior position.\u003c/p\u003e\n\u003cp\u003eA second intrusion case was reported by Alabbasi et al. in 2025, [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e] who inserted CAIRS into the pre-existing synthetic ICRS tunnel without creating a new FSL channel. A 24-year-old man developed segment migration, stromal necrosis, and intrusion 6 weeks after bilateral ICRS implantation. Following explantation, a CAIRS was placed into the same tunnel, fully filling the stromal defect and covering the necrosis site. At 6 months, CDVA improved from 20/60 to 20/25, astigmatism decreased from \u0026minus;\u0026thinsp;6.50 to \u0026minus;\u0026thinsp;4.00 D, and keratometry fell from 49.00 to 45.00 D with no signs of re-necrosis or migration. Barbara and Barbara [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e] likewise implanted CAIRS into the pre-existing Ferrara ring tunnel at the site of the extruded nasal segment, avoiding new stromal dissection in a cornea with a 22-year implantation history. In all three cases, favorable visual and topographic outcomes were achieved with no recurrence of extrusion or migration, confirming that pre-existing synthetic ICRS tunnels represent a viable and tissue-sparing implantation channel for CAIRS across different failure scenarios.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e4. CAIRS\u0026ndash;PMMA Coexistence and Unsatisfactory Refractive Outcome\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eBteich et al. described the first case of CAIRS implanted alongside a retained synthetic ICRS at a different OZ, [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e] in a 29-year-old man with severe keratoconus who had moderate improvement from a PMMA ring at a 7 mm OZ but persistent residual irregular astigmatism. A CAIRS was inserted at a smaller OZ without removing the existing synthetic segment. At 6 months, 3 lines of CDVA were gained, Kmax decreased from 50.02 to 47.89 D (p\u0026thinsp;=\u0026thinsp;0.03), and coma fell from 1.05 to 0.21 D (p\u0026thinsp;=\u0026thinsp;0.01). This case demonstrated that PMMA and CAIRS can coexist productively in the same cornea at different optical zones, with each contributing independently to the refractive and topographic outcome.\u003c/p\u003e\n\u003cp\u003eAlQahtani and Alsulami used \u003cem\u003eunsatisfactory refractive outcome\u003c/em\u003e as a stand-alone indication for ICRS-to-CAIRS exchange. [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e] In Case 1, a 49-year-old woman with unimproved vision 10 years after ICRS underwent synthetic segment removal followed by a single KeraNatural CAIRS at the deliberately shallow depth of 200 \u0026micro;m to maximize flattening in a steep cone. UCVA improved from 20/400 to 20/30, Kmax reduced by 7.0 D, and astigmatism from 4.2 to 0.6 D. In Case 2, CAIRS was implanted alongside a retained broken-eyelet ICRS that could not safely be removed; UCVA improved from 20/60 to 20/25, BCVA to 20/20, and Kmax fell 3.4 D.\u003c/p\u003e\n\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003e\u003cem\u003eComprehensive summary of all published case reports and series on ICRS replacement by CAIRS, organized by failure scenario.\u003c/em\u003e\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003e#\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c2\"\u003e\n \u003cp\u003eAuthor, Year\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c3\"\u003e\n \u003cp\u003eJournal\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c4\"\u003e\n \u003cp\u003en / Eyes\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c5\"\u003e\n \u003cp\u003eFailure Scenario\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c6\"\u003e\n \u003cp\u003eKey Visual \u0026amp; Refractive Outcomes\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c7\"\u003e\n \u003cp\u003eKmax Change\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c8\"\u003e\n \u003cp\u003eKey Finding / Surgical Note\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003cth align=\"left\" colspan=\"8\" nameend=\"c8\" namest=\"c1\"\u003e\n \u003cp\u003eSTROMAL MELT / ANTERIOR STROMAL NECROSIS (ASN)\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003e\u003cstrong\u003e1\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c2\"\u003e\n \u003cp\u003e\u003cstrong\u003eJarade et al., 2019\u003c/strong\u003e [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c3\"\u003e\n \u003cp\u003e\u003cem\u003eJ Cataract Refract Surg\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c4\"\u003e\n \u003cp\u003e1 eye\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c5\"\u003e\n \u003cp\u003eBiologic ring inserted into ICRS tunnel overlying melt site; temporary anchoring sutures\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c6\"\u003e\n \u003cp\u003eVisual acuity and keratometry restored to pre-melt levels\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c7\"\u003e\n \u003cp\u003eImprovement (unquantified)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c8\"\u003e\n \u003cp\u003eFirst description of allogenic tissue to manage ICRS-related stromal melt\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c2\"\u003e\n \u003cp\u003e\u003cstrong\u003eKozhaya et al., 2022\u003c/strong\u003e [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c3\"\u003e\n \u003cp\u003e\u003cem\u003eJ Refract Surg\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c4\"\u003e\n \u003cp\u003e16 eyes (8 CAIRS vs 8 explant)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c5\"\u003e\n \u003cp\u003eASN overlying Intacs SK; 10-year incidence 5.5% in 643 eyes; simultaneous CAIRS exchange vs explantation alone\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c6\"\u003e\n \u003cp\u003eCAIRS: CDVA 0.16\u0026rarr;0.11 logMAR; coma 0.87\u0026rarr;0.81 D. Explant: CDVA 0.14\u0026rarr;0.28 logMAR; coma 0.87\u0026rarr;1.52 D (p\u0026thinsp;=\u0026thinsp;0.02). HOA stable in CAIRS; regressed to pre-ICRS levels after explantation alone\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c7\"\u003e\n \u003cp\u003eCAIRS: stable (49.25\u0026rarr;49.64 D). Explant: regressed (49.23\u0026rarr;52.26 D)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c8\"\u003e\n \u003cp\u003eFirst and only controlled comparative study in rescue literature. Tectonic pachymetry: 490 \u0026micro;m (CAIRS) vs 327 \u0026micro;m (explant alone). Greater benefit with thicker segments; less satisfactory results with large melt areas\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003e\u003cstrong\u003e3\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c2\"\u003e\n \u003cp\u003e\u003cstrong\u003eAsfar et al., 2025\u003c/strong\u003e [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c3\"\u003e\n \u003cp\u003e\u003cem\u003eJCRS Online Case Rep\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c4\"\u003e\n \u003cp\u003e1 eye\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c5\"\u003e\n \u003cp\u003e20-year-old ICRS; stromal thinning \u0026rarr; lower ICRS explanted \u0026rarr; 550 \u0026micro;m CAIRS in same tunnel\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c6\"\u003e\n \u003cp\u003eInitial mild topographic improvement. At 4 months: new melt at different arc overlying CAIRS. CAIRS removed. Final VA and topography worse than pre-intervention\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c7\"\u003e\n \u003cp\u003eWorsened\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c8\"\u003e\n \u003cp\u003eOnly documented failure in rescue literature. Longstanding stromal disease may be irreversible; CAIRS exchange does not guarantee arrest of underlying melt process\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colspan=\"8\" nameend=\"c8\" namest=\"c1\"\u003e\n \u003cp\u003e\u003cstrong\u003eICRS EXTRUSION\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003e\u003cstrong\u003e4\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c2\"\u003e\n \u003cp\u003e\u003cstrong\u003eDaoud et al., 2019\u003c/strong\u003e [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c3\"\u003e\n \u003cp\u003e\u003cem\u003eJ Fr Ophtalmol\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c4\"\u003e\n \u003cp\u003e4 eyes\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c5\"\u003e\n \u003cp\u003eExtruded ICRS in 4 KC patients; donor tissue tailored to ICRS shape inserted into empty tunnel; fixed with 10\u0026thinsp;\u0026minus;\u0026thinsp;0 nylon sutures\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c6\"\u003e\n \u003cp\u003eAll 4 grafts healed at 3 months. Kmax, BCVA, and topography stable and comparable to pre-extrusion values\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c7\"\u003e\n \u003cp\u003eStable (unquantified)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c8\"\u003e\n \u003cp\u003eFirst published series describing donor tissue substitution for extruded ICRS\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003e\u003cstrong\u003e5\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c2\"\u003e\n \u003cp\u003e\u003cstrong\u003eBarbara \u0026amp; Barbara, 2026\u003c/strong\u003e [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c3\"\u003e\n \u003cp\u003e\u003cem\u003eInt Ophthalmol (in press)\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c4\"\u003e\n \u003cp\u003e1 eye\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c5\"\u003e\n \u003cp\u003eFerrara ring extrusion after 22 years; selective nasal CAIRS replacement (KeraNatural, manually trimmed); non-extruded temporal PMMA retained in situ; pre-existing tunnel reused\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c6\"\u003e\n \u003cp\u003eUCVA and BSCVA stable throughout 18 months. No change in refraction. PMMA\u0026ndash;CAIRS coexistence well tolerated. No extrusion, migration, or adverse events\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c7\"\u003e\n \u003cp\u003e64.50\u0026rarr;60.68 D at 18 months (\u0026minus;\u0026thinsp;3.82 D)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c8\"\u003e\n \u003cp\u003eLongest follow-up in rescue literature (18 months). First extrusion after 22 years. Selective single-segment replacement preserving functioning temporal synthetic. Manual trimming to fit pre-existing narrow tunnel feasible but technically demanding\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003e\u003cstrong\u003e6\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c2\"\u003e\n \u003cp\u003e\u003cstrong\u003eSusanna et al., 2025\u003c/strong\u003e [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c3\"\u003e\n \u003cp\u003e\u003cem\u003eJ Cataract Refract Surg\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c4\"\u003e\n \u003cp\u003e8 eyes\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c5\"\u003e\n \u003cp\u003eKC with ICRS explantation\u0026thinsp;\u0026ge;\u0026thinsp;6 months prior (mean 12 months); customized FSL CAIRS (LDV Z8); Alfonso-Ferrara phenotype planning; tunnel depth 50% of min stromal thickness\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c6\"\u003e\n \u003cp\u003eKmax: \u0026minus;3.03\u0026thinsp;\u0026plusmn;\u0026thinsp;1.35 D (p\u0026thinsp;=\u0026thinsp;0.01). SE: \u0026minus;6.28\u0026thinsp;\u0026plusmn;\u0026thinsp;2.71 D (p\u0026thinsp;=\u0026thinsp;0.01). CDVA: +2.75\u0026thinsp;\u0026plusmn;\u0026thinsp;1.7 lines. 7/8 gained\u0026thinsp;\u0026ge;\u0026thinsp;2 lines. 5/8 achieved\u0026thinsp;\u0026ge;\u0026thinsp;20/40. No complications\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c7\"\u003e\n \u003cp\u003e\u0026minus;3.03\u0026thinsp;\u0026plusmn;\u0026thinsp;1.35 D (p\u0026thinsp;=\u0026thinsp;0.01)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c8\"\u003e\n \u003cp\u003eFirst prospective series after ICRS explantation. Delayed implantation (\u0026ge;\u0026thinsp;6 months) appears to allow more accurate planning. Prior CXL or older age may reduce flattening. 2 patients: CAIRS\u0026thinsp;+\u0026thinsp;retained nasal ICRS\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colspan=\"8\" nameend=\"c8\" namest=\"c1\"\u003e\n \u003cp\u003e\u003cstrong\u003eANTERIOR CHAMBER INTRUSION\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003e\u003cstrong\u003e7\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c2\"\u003e\n \u003cp\u003e\u003cstrong\u003eGendy et al. (AUBMC), 2023\u003c/strong\u003e [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c3\"\u003e\n \u003cp\u003e\u003cem\u003eJ Refract Surg Case Rep\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c4\"\u003e\n \u003cp\u003e1 eye\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c5\"\u003e\n \u003cp\u003eKeraRing intrusion into anterior chamber; explanted; CAIRS implanted at same site at 250 \u0026micro;m depth after oedema resolved; new FSL tunnel created\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c6\"\u003e\n \u003cp\u003eCDVA: 20/80\u0026rarr;20/20\u0026minus;. Coma: 1.54\u0026rarr;0.60 D (\u0026minus;\u0026thinsp;61%). Trefoil: 2.74\u0026rarr;0.99 D. Stable at 6 months\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c7\"\u003e\n \u003cp\u003e53.30\u0026rarr;47.59 D (\u0026minus;\u0026thinsp;5.71 D) at 1 week; stable at 6 months\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c8\"\u003e\n \u003cp\u003eFirst rescue of ICRS intrusion with CAIRS. 750 \u0026micro;m CAIRS compressed to 355 \u0026micro;m at 250 \u0026micro;m depth \u0026mdash; greater flattening superficial position\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003e\u003cstrong\u003e8\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c2\"\u003e\n \u003cp\u003e\u003cstrong\u003eAlabbasi et al., 2025\u003c/strong\u003e [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c3\"\u003e\n \u003cp\u003e\u003cem\u003eJ Refract Surg\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c4\"\u003e\n \u003cp\u003e1 eye\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c5\"\u003e\n \u003cp\u003eICRS migration\u0026thinsp;+\u0026thinsp;stromal necrosis\u0026thinsp;+\u0026thinsp;intrusion at 6 weeks; CAIRS inserted into pre-existing ICRS tunnel \u0026mdash; no new FSL channel created\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c6\"\u003e\n \u003cp\u003eCDVA: 20/60\u0026rarr;20/25. Astigmatism: \u0026minus;6.50\u0026rarr;\u0026minus;4.00 D. Stable at 6 months. No re-necrosis or migration\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c7\"\u003e\n \u003cp\u003e49.00\u0026rarr;45.00 D (\u0026minus;\u0026thinsp;4.00 D)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c8\"\u003e\n \u003cp\u003eFirst report of pre-existing tunnel reuse for CAIRS \u0026mdash; eliminates additional stromal dissection in compromised cornea. CAIRS fully filled stromal defect\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colspan=\"8\" nameend=\"c8\" namest=\"c1\"\u003e\n \u003cp\u003e\u003cstrong\u003eCAIRS\u0026ndash;PMMA COEXISTENCE / UNSATISFACTORY REFRACTIVE OUTCOME\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003e\u003cstrong\u003e9\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c2\"\u003e\n \u003cp\u003e\u003cstrong\u003eBteich et al. (AUBMC), 2023\u003c/strong\u003e [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c3\"\u003e\n \u003cp\u003e\u003cem\u003eJ Refract Surg Case Rep\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c4\"\u003e\n \u003cp\u003e1 eye\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c5\"\u003e\n \u003cp\u003eFunctioning PMMA ring at 7 mm OZ; residual irregular astigmatism; CAIRS added at smaller OZ without removing existing PMMA \u0026mdash; first coexistence report\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c6\"\u003e\n \u003cp\u003eCDVA: +3 lines. Coma: 1.05\u0026rarr;0.21 D (p\u0026thinsp;=\u0026thinsp;0.01). Topographic astigmatism improved\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c7\"\u003e\n \u003cp\u003e50.02\u0026rarr;47.89 D (\u0026minus;\u0026thinsp;2.13 D, p\u0026thinsp;=\u0026thinsp;0.03)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c8\"\u003e\n \u003cp\u003eFirst CAIRS\u0026thinsp;+\u0026thinsp;retained PMMA in same cornea at different OZ. Additive effect without synthetic removal\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003e\u003cstrong\u003e10\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c2\"\u003e\n \u003cp\u003e\u003cstrong\u003eAlQahtani \u0026amp; Alsulami, 2025\u003c/strong\u003e [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c3\"\u003e\n \u003cp\u003e\u003cem\u003eAm J Ophthalmol Case Rep\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c4\"\u003e\n \u003cp\u003e2 eyes\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c5\"\u003e\n \u003cp\u003eCase 1: Unsatisfactory outcome 10 years post-ICRS; both removed; CAIRS at 200 \u0026micro;m. Case 2: broken-eyelet ICRS\u0026thinsp;+\u0026thinsp;poor outcome; CAIRS alongside retained ICRS\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c6\"\u003e\n \u003cp\u003eCase 1: UCVA 20/400\u0026rarr;20/30; astigmatism 4.2\u0026rarr;0.6 D; HOA 1.384\u0026rarr;1.095 \u0026micro;m. Case 2: UCVA 20/60\u0026rarr;20/25; BCVA\u0026rarr;20/20; coma 1.42\u0026rarr;0.861 \u0026micro;m; SA 1.04\u0026rarr;0.030 \u0026micro;m\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c7\"\u003e\n \u003cp\u003eCase 1: 68.9\u0026rarr;61.9 D (\u0026minus;\u0026thinsp;7.0 D). Case 2: 51.3\u0026rarr;47.9 D (\u0026minus;\u0026thinsp;3.4 D)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c8\"\u003e\n \u003cp\u003eExpands indication: first to report unsatisfactory refractive outcome as exchange indication. Shallower depth in steep cones\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e\n\u003cp\u003e\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eOZ\u0026thinsp;=\u0026thinsp;optical zone (OZ); KC\u0026thinsp;=\u0026thinsp;keratoconus; FSL\u0026thinsp;=\u0026thinsp;femtosecond laser; HOA\u0026thinsp;=\u0026thinsp;higher-order aberrations; SE\u0026thinsp;=\u0026thinsp;spherical equivalent; SA\u0026thinsp;=\u0026thinsp;spherical aberration; ASN\u0026thinsp;=\u0026thinsp;anterior stromal necrosis; CDVA\u0026thinsp;=\u0026thinsp;corrected distance visual acuity; UCVA\u0026thinsp;=\u0026thinsp;uncorrected distance visual acuity; BSCVA\u0026thinsp;=\u0026thinsp;best spectacle-corrected visual acuity; CXL\u0026thinsp;=\u0026thinsp;corneal collagen cross-linking; PMMA\u0026thinsp;=\u0026thinsp;polymethyl methacrylate.\u003c/em\u003e\u003c/p\u003e"},{"header":"DISCUSSION","content":"\u003cp\u003eThe collective evidence synthesized in this review, while limited in volume and consisting predominantly of case reports and small series, is internally consistent and clinically coherent. Across approximately 40 eyes reported in 9 publications spanning 2019 to 2026, CAIRS implantation after synthetic ICRS failure produces favorable visual and topographic outcomes in the large majority of cases, with a single documented failure. The dominant theme across all scenarios is the biological advantage of allogenic tissue in a setting where PMMA ICRS has already failed: CAIRS integrate where PMMA could not persist, provide structural support where stroma has been thinned, and offer a refractive benefit that explantation alone irreversibly forfeits.\u003c/p\u003e \u003cp\u003eKozhaya et al. [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e] \u0026mdash; reporting the first and only controlled comparison in this literature \u0026mdash; documented 163 \u0026micro;m greater residual stromal thickness at the melt site following CAIRS exchange compared with explantation alone, a clinically meaningful difference with direct implications for corneal structural integrity. CAIRS exchange preserved the refractive gain originally achieved by synthetic implantation, whereas explantation alone returned higher-order aberrations toward pre-ICRS levels. These findings suggest CAIRS exchange may represent a preferable management strategy for synthetic ICRS anterior stromal necrosis whenever technically feasible, though the limited evidence base warrants caution in generalizing this conclusion.\u003c/p\u003e \u003cp\u003eThe cautionary case of Asfar et al. [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e] introduces an essential caveat: not all melt scenarios are salvageable with CAIRS. In a cornea with 20-year-old ICRS and extensive pre-existing stromal disease, the underlying ischemic and inflammatory process may have progressed beyond the point at which allogenic tissue substitution can provide durable structural support. It should also be noted that CAIRS are not entirely free of the complications associated with synthetic rings; extrusion and anterior stromal melting have been reported in primary CAIRS implantation cases in eyes with no prior synthetic ICRS. [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]\u003c/p\u003e \u003cp\u003eThe extrusion cases collectively suggest that Kmax reduction \u0026mdash; not merely stabilization \u0026mdash; is achievable with CAIRS in post-extrusion corneas. The Susanna series [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e] reports a mean Kmax reduction of 3.03 D in corneas with established ICRS tunnel scarring, confirming that allogenic segments retain meaningful biomechanical efficacy even in the scarred stromal environment left by prior synthetic implantation. The Barbara and Barbara case [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e] documents progressive Kmax reduction from 64.50 D to 60.68 D over 18 months \u0026mdash; the longest individual case follow-up in this literature \u0026mdash; suggesting ongoing corneal remodeling as the CAIRS integrates biologically.\u003c/p\u003e \u003cp\u003eAn important confounding variable not systematically reported or controlled across the included studies is the concurrent or prior use of corneal collagen cross-linking (CXL). CXL stiffens the corneal stroma and stabilizes KCs independently of any intrastromal implant, and its presence or absence could meaningfully influence the topographic outcomes attributed to CAIRS in the rescue setting. In particular, progressive Kmax reduction over extended follow-up cannot be attributed solely to biological CAIRS integration without knowledge of CXL status. None of the included studies were specifically designed to isolate the contribution of CAIRS from that of concurrent or prior CXL. Future prospective studies should report CXL status as a mandatory stratification variable to allow meaningful interpretation of the independent biomechanical contribution of allogenic segment implantation in the rescue setting.\u003c/p\u003e \u003cp\u003eThe intrusion cases merit separate attention because anterior chamber intrusion \u0026mdash; arguably the most feared acute complication of synthetic ICRS, although rare \u0026mdash; has not been reported with CAIRS in the published literature to date. An allogenic segment that integrates biologically into the host stroma is, by its nature, far less likely to migrate as a rigid foreign body through the endothelial layer. Gendy et al. [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e] demonstrated that CAIRS can be implanted safely at the exact site of a previous intrusion via a newly created FSL tunnel at a substantially shallower depth than the original synthetic segment, achieving keratometric outcomes superior to the pre-intrusion state. Alabbasi et al. [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e] similarly replaced the failed synthetic segment with CAIRS using the pre-existing ICRS tunnel without creating a new FSL channel, eliminating the need for additional stromal dissection in an already-compromised cornea. Barbara and Barbara [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e] likewise implanted CAIRS into the pre-existing Ferrara ring tunnel at the site of the extruded nasal segment, avoiding new stromal dissection in a cornea with a 22-year implantation history. Favorable visual and topographic outcomes were achieved in all three cases with no recurrence of extrusion or migration, confirming that pre-existing synthetic ICRS tunnels represent a viable and tissue-sparing implantation channel for CAIRS across different failure scenarios.\u003c/p\u003e \u003cp\u003eThe expansion of the indication to include unsatisfactory refractive outcome \u0026mdash; independent of any complication \u0026mdash; as a stand-alone reason for ICRS-to-CAIRS exchange represents an important conceptual development. This approach was first reported by Bteich et al., [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e] who added a CAIRS at a smaller OZ alongside a functioning retained synthetic ICRS in a patient with persistent irregular astigmatism despite technically successful ICRS implantation. AlQahtani and Alsulami [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e] subsequently extended this concept further, reporting two cases in which CAIRS was used after bilateral ICRS failure due to unsatisfactory refractive outcome alone \u0026mdash; including one case where the existing synthetic segment was retained in situ. Together, these cases raise the possibility that CAIRS may be considered not merely as a rescue for complication-driven failure but as an option in any clinical scenario where synthetic ICRS have produced suboptimal results, though prospective validation is required before this can be considered established practice.\u003c/p\u003e\n\u003ch3\u003eCritical Appraisal of the Evidence Base\u003c/h3\u003e\n\u003cp\u003eThe evidence synthesized in this review is subject to several quality limitations. First, publication bias is likely: favorable outcomes are substantially more likely to reach publication than failures, making the single documented failure [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e] particularly valuable as a counterpoint. Second, all included studies were performed by experienced corneal surgeons at tertiary referral centers, limiting generalizability to less experienced settings. Third, follow-up duration is short across most publications, with the longest controlled comparison extending to only 6 months in most of the reported cases. Fourth, the heterogeneity of surgical techniques \u0026mdash; spanning manual tissue trimming, FSL-assisted preparation, and commercially processed KeraNatural segments, with implantation depths ranging from 200 \u0026micro;m to 50% of minimum stromal thickness \u0026mdash; prevents standardized interpretation and limits conclusions about any specific CAIRS protocol. Fifth, the search and screening were performed by the two co-authors without an independent third reviewer, representing a methodological limitation inherent to a two-author review.\u003c/p\u003e \u003cp\u003eSeveral additional limitations must be acknowledged. No standardized outcome reporting exists across publications, limiting cross-study quantitative comparison. The variety of ICRS types involved (Intacs, Ferrara rings, Kerarings) and the heterogeneity of failure scenarios prevent any meaningful pooled analysis. These limitations collectively define the research agenda for this field: prospective multicenter registries with standardized CAIRS preparation protocols, mandatory CXL status reporting, standardized outcome measures, and longer follow-up.\u003c/p\u003e"},{"header":"CONCLUSIONS","content":"\u003cp\u003eThe published evidence on ICRS replacement by CAIRS, while limited in volume and consisting predominantly of case reports and small series, is consistent in demonstrating that allogenic tissue substitution for failed synthetic ICRS is feasible across most of failure scenarios \u0026mdash; anterior stromal necrosis, extrusion, anterior chamber intrusion, persistent unsatisfactory refractive outcome, and coexistence with retained synthetic segments. CAIRS preserves or improves visual and topographic outcomes where explantation alone produces regression, provides tectonic structural support to thinned and necrotic corneal beds, and is associated with a lower risk of re-extrusion and re-intrusion that would accompany synthetic reimplantation. All conclusions below should be interpreted as hypothesis-generating, given the low level of evidence, and require prospective validation.\u003c/p\u003e \u003cp\u003eThe available evidence supports the following provisional conclusions. First, available evidence suggests CAIRS exchange may represent a preferable strategy over explantation alone when ASN is managed surgically, provided the extent of stromal damage does not preclude adequate structural support \u0026mdash; as the single documented failure in this literature demonstrates. Second, in post-extrusion corneas, delayed CAIRS implantation (\u0026ge;\u0026thinsp;6 months after explantation) may allow more accurate topographic planning and is associated with meaningful Kmax reduction and CDVA improvement in the available series. Third, pre-existing ICRS tunnels appear to represent a viable and tissue-sparing implantation channel for CAIRS, based on three independent reports. Fourth, available evidence suggests CAIRS and retained functioning synthetic segments can coexist in the same cornea at different optical zones with favorable short-term outcomes. Fifth, the currently available evidence raises the possibility that the indication for CAIRS exchange may be cautiously extended beyond complication-driven failure to include unsatisfactory refractive outcome, though prospective validation is required before this can be considered established practice.\u003c/p\u003e \u003cp\u003eProspective multicenter studies with standardized CAIRS preparation protocols, mandatory reporting of CXL status, standardized outcome measures, and longer follow-up are needed to consolidate these findings and establish evidence-based guidelines for CAIRS rescue implantation after synthetic ICRS failure.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e \u003ch2\u003eCompeting interests:\u003c/h2\u003e \u003cp\u003eThe authors have no relevant financial or non-financial interests to disclose.\u003c/p\u003e \u003c/p\u003e\u003cp\u003e \u003ch2\u003eEthics approval:\u003c/h2\u003e \u003cp\u003eThis is a narrative review article. No ethical approval was required as it does not involve human participants, animal subjects, or identifiable patient data.\u003c/p\u003e \u003c/p\u003e \u003cp\u003e \u003cstrong\u003eConsent to participate:\u003c/strong\u003e \u003cp\u003eNot applicable.\u003c/p\u003e \u003c/p\u003e \u003cp\u003e \u003cstrong\u003eConsent to publish:\u003c/strong\u003e \u003cp\u003eNot applicable.\u003c/p\u003e \u003c/p\u003e \u003cp\u003e \u003cstrong\u003eData availability\u003c/strong\u003e \u003cp\u003eAll data analyzed in this review are available in the published papers cited in the reference list.\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eFunding:\u003c/h2\u003e \u003cp\u003eThe authors did not receive support from any organization for the submitted work.\u003c/p\u003e\u003ch2\u003eAuthor contributions:\u003c/h2\u003e \u003cp\u003eBoth authors contributed equally to the conception, literature search, data synthesis, and preparation of this manuscript. Both authors read and approved the final version.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eFriedrich M, Auffarth GU, Soiberman U, Augustin VA, Khoramnia R, Son H-S (2025) Visual and topographic outcomes after corneal allogeneic intrastromal ring segments (CAIRS) for keratoconus: a systematic review and meta-analysis. Am J Ophthalmol. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.ajo.2025.03.028\u003c/span\u003e\u003cspan address=\"10.1016/j.ajo.2025.03.028\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBteich Y, Assaf JF, Mrad AA, Jacob S, Hafezi F, Awwad ST (2023) Corneal allogenic intrastromal ring segments (CAIRS) for corneal ectasia: a comprehensive segmental tomography evaluation. J Refract Surg 39(11):767\u0026ndash;776. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.3928/1081597X-20231011-01\u003c/span\u003e\u003cspan address=\"10.3928/1081597X-20231011-01\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eJacob S, Agarwal A, Awwad ST, Mazzotta C, Parashar P, Jambulingam S (2023) Customized corneal allogenic intrastromal ring segments (CAIRS) for keratoconus with decentered asymmetric cone. Indian J Ophthalmol 71(12):3723\u0026ndash;3729. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.4103/IJO.IJO_1988_23\u003c/span\u003e\u003cspan address=\"10.4103/IJO.IJO_1988_23\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBautista-Llamas MJ, S\u0026aacute;nchez-Gonz\u0026aacute;lez MC, L\u0026oacute;pez-Izquierdo I et al (2019) Complications and explantation reasons in intracorneal ring segments (ICRS) implantation: a systematic review. J Refract Surg 35(11):740\u0026ndash;751. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.3928/1081597X-20191010-02\u003c/span\u003e\u003cspan address=\"10.3928/1081597X-20191010-02\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eKozhaya K, Mehanna CJ, Jacob S, Saad A, Jabbur NS, Awwad ST (2022) Management of anterior stromal necrosis after polymethylmethacrylate ICRS: explantation versus exchange with corneal allogenic intrastromal ring segments. J Refract Surg 38(4):256\u0026ndash;263. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.3928/1081597X-20220223-01\u003c/span\u003e\u003cspan address=\"10.3928/1081597X-20220223-01\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSusanna BN, Forseto AS, Moriyama AS et al (2025) Femtosecond laser-assisted customized corneal allogenic intrastromal ring segments in patients with keratoconus after intrastromal corneal ring segment explantation: prospective case series. J Cataract Refract Surg 51(4):267\u0026ndash;273. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1097/j.jcrs.0000000000001600\u003c/span\u003e\u003cspan address=\"10.1097/j.jcrs.0000000000001600\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eDaoud RC, Sammouh FK, Baban TA, Warrak JE, Warrak EL (2019) Allogenic corneal tissue transplantation in substitution for extruded intracorneal rings: a case series. J Fr Ophtalmol 42(10):1090\u0026ndash;1093. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.jfo.2019.05.028\u003c/span\u003e\u003cspan address=\"10.1016/j.jfo.2019.05.028\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eJarade E, Issa M, Chanbour W, Warhekar P (2019) Biologic stromal ring to manage stromal melting after intrastromal corneal ring segment implantation. J Cataract Refract Surg 45(9):1222\u0026ndash;1225. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.jcrs.2019.05.022\u003c/span\u003e\u003cspan address=\"10.1016/j.jcrs.2019.05.022\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eAsfar K et al (2025) Ongoing stromal melt after corneal allogenic segment implantation in exchange for an explanted synthetic segment due to melt. JCRS Online Case Reports\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eGendy J, Bteich Y, Ibrahim P et al (2023) Corneal allogenic intrastromal ring segments as a therapeutic method for intrusion of synthetic intrastromal corneal ring segment: a case report. J Refract Surg Case Rep 3(3):e33\u0026ndash;e37. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.3928/jrscr-20230814-01\u003c/span\u003e\u003cspan address=\"10.3928/jrscr-20230814-01\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eAlabbasi OM, Alahmadi MW, Alhejaili RA, Alharbi AK (2025) Use of preexisting ICRS tunnels for implantation of CAIRS following ICRS migration and corneal tissue necrosis: a case report. J Refract Surg 41(9):e1022\u0026ndash;e1026\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBteich Y, Ibrahim P, Gendy J et al (2023) Polymethylmethacrylate and corneal allogenic intrastromal ring segments: same cornea, different optical zones. J Refract Surg Case Rep 3(3):e38\u0026ndash;e41\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eAlQahtani BS, Alsulami RA (2025) The role of corneal allogenic intrastromal ring segments (CAIRS) implantation after failed synthetic intracorneal ring segments (ICRS): a rescuer. Am J Ophthalmol Case Rep 38:102287. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.ajoc.2025.102287\u003c/span\u003e\u003cspan address=\"10.1016/j.ajoc.2025.102287\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBarbara A, Barbara R (2026) From synthetic to biological rings: corneal allogenic intrastromal ring segments for visual preservation after PMMA intrastromal corneal ring segment extrusion in keratoconus. Int Ophthalmol (in press)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eAwwad ST, Asfar KE, Gendy J, Jacob S, Assaf JF (2025) Anterior stromal melt with corneal allogeneic intrastromal ring segments in a patient with severe atopy. Cornea. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://pubmed.ncbi.nlm.nih.gov/40095577/\u003c/span\u003e\u003cspan address=\"https://pubmed.ncbi.nlm.nih.gov/40095577/\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBarbara R, Gunn D, Khayat B, Barbara A (2025) Complications after corneal allografts intrastromal ring segments (CAIRS) in keratoconus and post LASIK ectasia: a case series. Res Square. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.21203/rs.3.rs-8991375/v1\u003c/span\u003e\u003cspan address=\"10.21203/rs.3.rs-8991375/v1\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Keratoconus, corneal allogenic intrastromal ring segments, CAIRS, intracorneal ring segments, ICRS extrusion, ICRS rescue, PMMA replacement","lastPublishedDoi":"10.21203/rs.3.rs-9320902/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-9320902/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003ch2\u003ePurpose\u003c/h2\u003e \u003cp\u003eTo identify, synthesize, and critically appraise all published series and case reports specifically reporting corneal allogenic intrastromal ring segments (CAIRS) implantation as a replacement for failed synthetic polymethyl methacrylate (PMMA) intrastromal corneal ring segments (ICRS) in keratoconus (KC).\u003c/p\u003e\u003ch2\u003eMethods\u003c/h2\u003e \u003cp\u003eA literature search was conducted in PubMed/MEDLINE, Google Scholar, and Research Square using predefined search terms related to CAIRS rescue implantation after synthetic ICRS failure. Additional records were identified through hand-searching reference lists of included studies. All study designs were eligible. Studies reporting primary CAIRS implantation without prior synthetic ICRS were excluded. The search was concluded in March 2026.\u003c/p\u003e\u003ch2\u003eResults\u003c/h2\u003e \u003cp\u003eNine publications were identified, comprising 1 controlled comparative study, 1 prospective case series, 1 retrospective case series, and 6 case reports, reporting on approximately 40 eyes across four failure scenarios: anterior stromal necrosis/melt, ICRS extrusion, anterior chamber intrusion, or unsatisfactory refractive outcome. CAIRS implantation preserved or improved visual and topographic outcomes in 8 of 9 publications. The single documented failure occurred in a cornea with extensively diseased stroma following 20 years of synthetic ICRS implantation.\u003c/p\u003e\u003ch2\u003eConclusion\u003c/h2\u003e \u003cp\u003eAvailable evidence suggests that CAIRS may represent an effective and biologically rational rescue strategy for failed synthetic CAIRS exchange appears preferable to explantation alone in anterior stromal necrosis when structurally feasible; delayed implantation after extrusion may allow accurate topographic planning; and the indication may be cautiously extended to include unsatisfactory refractive outcome. The evidence base consists predominantly of case reports and small series (~\u0026thinsp;40 eyes), and all conclusions should be interpreted as hypothesis-generating pending prospective multicenter validation.\u003c/p\u003e","manuscriptTitle":"Replacement of Synthetic Intracorneal Ring Segments by Corneal Allogenic Intrastromal Ring Segments (CAIRS) in Keratoconus: A Narrative Review with Systematic Search","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-04-09 16:02:47","doi":"10.21203/rs.3.rs-9320902/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"26f979c4-1ca0-4dfc-919c-491029367e03","owner":[],"postedDate":"April 9th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2026-04-09T16:02:47+00:00","versionOfRecord":[],"versionCreatedAt":"2026-04-09 16:02:47","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-9320902","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-9320902","identity":"rs-9320902","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}