Corneal Allogeneic Intrastromal Ring Segments for Treating Keratoconus—Systematic Review and Meta-Analysis

Background and Objectives: Corneal allogeneic intrastromal ring segments (CAIRS) are designed to decrease and stabilize the extent of corneal ectasia in keratoconus patients. This systematic review and meta-analysis evaluate the effectiveness of different surgical techniques for CAIRS preparation and the adjunctive use of corneal cross-linking. Materials and Methods: Following the PRISMA statement and checklist, a comprehensive search was conducted in Embase, Medline, and the Cochrane Controlled Trials Register, through the use of a systematic search approach in accordance with the Cochrane Collaboration guidelines. Results: Eighteen studies, involving 567 eyes of 459 patients, met the inclusion criteria. At one month postoperatively, CAIRS implantation significantly improved uncorrected visual acuity (UCVA) (−0.45 logMAR, 95% CI −0.59 to −0.31, p < 0.001) and best corrected visual acuity (BCVA) (−0.36 logMAR, 95% CI −0.46 to −0.25, p < 0.001). These improvements remained significant after one year (UCVA: −0.39 logMAR, 95% CI −0.48 to −0.30, p < 0.001; BCVA: −0.34 logMAR, 95% CI −0.50 to −0.18, p < 0.001). Similarly, mean simulated keratometry (Kmean) decreased by −4.42 D (95% CI −5.94 to −2.90, p < 0.001) and maximum keratometry (Kmax) by −3.88 D (95% CI −6.71 to −1.05, p < 0.001) at one month, with sustained reductions at one year (−3.59 D, 95% CI −4.35 to −2.84, p < 0.001 and −3.73 D, 95% CI −4.91 to −2.55, p < 0.001). No significant differences in surgical outcome have been observed between the different surgical techniques. Conclusions: CAIRS implantation appears to be an effective treatment option for keratoconus, regardless of the technique used for segment preparation or the addition of corneal cross-linking. No approach demonstrated clear clinical superiority over others in the first year after surgery.