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

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.