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

<i>Background and Objectives</i>: 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. <i>Materials and Methods</i>: 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. <i>Results</i>: 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], <i>p</i> < 0.001) and best corrected visual acuity (BCVA) (-0.36 logMAR, 95% CI [-0.46 to -0.25], <i>p</i> < 0.001). These improvements remained significant after one year (UCVA: -0.39 logMAR, 95% CI [-0.48 to -0.30], <i>p</i> < 0.001; BCVA: -0.34 logMAR, 95% CI [-0.50 to -0.18], <i>p</i> < 0.001). Similarly, mean simulated keratometry (Kmean) decreased by -4.42 D (95% CI [-5.94 to -2.90], <i>p</i> < 0.001) and maximum keratometry (Kmax) by -3.88 D (95% CI [-6.71 to -1.05], <i>p</i> < 0.001) at one month, with sustained reductions at one year (-3.59 D, 95% CI [-4.35 to -2.84], <i>p</i> < 0.001 and -3.73 D, 95% CI [-4.91 to -2.55], <i>p</i> < 0.001). No significant differences in surgical outcome have been observed between the different surgical techniques. <i>Conclusions</i>: 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.