Expanding the ICRS toolkit: Keraring for high regular astigmatism

A prospective pilot case series published in Scientific Reports evaluated whether intracorneal ring segment (ICRS) implantation using the SI-5 Keraring model could effectively manage high regular astigmatism exceeding 3.00 D in eyes with topographically normal, nonectatic corneas.¹ Twenty eyes of 20 patients were enrolled, with postoperative assessments conducted at 1 week, 1 month, and 6 months.¹

Background and rationale

High astigmatism remains difficult to manage across available treatment options. Spectacle correction can distort and magnify images at high refractive errors, while toric contact lenses depend on rotational stability that is not always achievable in practice. Toric phakic IOLs, though effective, are sensitive to postoperative axis misalignment, which can leave clinically meaningful residual refractive error. Corneal laser procedures such as PRK and LASIK perform reliably in mild-to-moderate astigmatism but tend toward reduced predictability as cylinder magnitude increases, and residual astigmatism after treatment is not uncommon.

ICRSs are an established tool in the management of keratoconus and other ectatic corneal conditions, where they reshape the cornea, reduce aberrations, and improve visual performance through a tissue-preserving, reversible mechanism.¹ Whether this approach could be applied usefully to nonectatic corneas carrying high regular astigmatism remained an open clinical question at the time this study was undertaken.¹

Study design and methods

All procedures were performed at Future Femto-Laser Center, Sohag, Egypt. The study received Institutional Review Board approval from the Faculty of Medicine, Sohag University (Approval No. Soh-Med-25-4-2PD), was registered at ClinicalTrials.gov (NCT06963099), and was conducted in accordance with the Declaration of Helsinki.¹ Written informed consent was obtained from each participant prior to enrollment.¹

To be eligible, patients were required to be at least 18 years of age, to have demonstrated refractive and topographic stability for a minimum of 12 months, and to show no tomographic features of ectatic disease on Sirius anterior segment analysis.¹ Comprehensive evaluation of anterior and posterior elevation maps and pachymetric progression was used to rule out ectasia.¹ Patients were excluded if they had any active ocular pathology, a history of corneal surgery, pregnancy, systemic healing impairment, or a confirmed diagnosis of a corneal ectatic condition.¹

Each of the 20 eyes received two asymmetrical SI-5 Keraring segments (Mediphacos Inc.)—polymethyl methacrylate implants with a triangular cross-section and a 5.0-mm optical zone—inserted via femtosecond laser–created intrastromal tunnels at 80% corneal depth using a 60-kHz femtosecond laser (Abbott Medical Optics).¹ Because no validated nomogram exists for Keraring use in nonectatic corneas, segment selection followed a keratoconus-based nomogram as the closest available clinical framework, guided by cylinder magnitude and axis, keratometric readings, corneal thickness, and topographic characteristics.¹

Outcome measures included UDVA, CDVA, manifest refraction, flat and steep keratometry (K1, K2), maximum keratometry (Kmax), mean keratometry, corneal thickness at the thinnest point, and anterior corneal HOAs—including total HOA root mean square (RMS), coma, spherical aberration, and trefoil—assessed over 4.0-mm and 6.0-mm optical zones via Zernike polynomial analysis.¹ Repeated-measures ANOVA and Pearson correlation coefficients were used for statistical analysis.¹

Results

The cohort had a mean age of 32.11 ± 8.70 years; 12 patients (60%) were male.¹ Mean preoperative refractive cylinder was −8.19 ± 2.50 D and mean spherical equivalent was −7.67 ± 1.92 D.¹

UDVA improved significantly at every postoperative visit (P < .001).¹ CDVA gains reached statistical significance at 1 week and 1 month (P < .001) but not at 6 months (P = .103).¹ At the 6-month mark, 20% of eyes gained two or more lines of CDVA, 35% gained one line, 30% were unchanged, and 15% lost one line; no eye lost two or more lines.¹

Mean refractive cylinder fell from 8.19 ± 2.50 D preoperatively to 3.14 ± 1.55 D at 6 months, a mean absolute reduction of 5.06 D corresponding to 61.3% (P < .001).¹ Spherical error, spherical equivalent, K1, K2, and mean keratometry all decreased significantly across the follow-up period.¹ Kmax rose postoperatively, a finding the authors interpret as reflecting focal stromal remodeling around the implant sites rather than any increase in overall corneal steepening, since Kmax captures a single localized point rather than global curvature.¹

Total HOA RMS and coma both declined significantly across follow-up (P < .001 and P = 0.011, respectively).¹ Spherical aberration and corneal thickness at the thinnest location remained statistically stable throughout.¹ Efficacy indices were 1.00, 1.17, and 0.90 at 1 week, 1 month, and 6 months, respectively; safety indices were 1.26, 1.21, and 1.10.¹ No complications arose during the study period, including no instances of segment migration, extrusion, infectious keratitis, stromal haze, or need for surgical revision.¹

Limitations

The authors identify several constraints on interpretation.¹ The 20-eye sample with no comparator arm limits the extent to which findings can be generalized or benchmarked against alternative treatments.¹ Six months of follow-up is insufficient to characterize long-term refractive stability.¹ Reliance on a keratoconus-derived nomogram introduces uncertainty about whether segment parameters were optimally selected for this distinct patient population.¹ Missing axis data across the cohort prevented vector analysis via the Alpins method, precluding calculation of surgically induced astigmatism and related indices.¹ Differences in individual corneal biomechanics and healing capacity represent an additional source of outcome variability.¹

Conclusions

Mounir and colleagues noted that SI-5 Keraring implantation produced meaningful gains in visual acuity, refractive cylinder reduction, corneal curvature regularization, and selected HOA reduction in patients with high regular astigmatism and nonectatic corneas, without safety events over the 6-month observation window.¹ The authors propose that the procedure may offer a clinically relevant option for patients in whom standard corneal refractive surgery is unsuitable, and call for larger controlled studies with extended follow-up to validate these results and clarify optimal selection criteria.¹

Reference

1. Mounir A, Mossa EAM, Mahmoud A. Outcomes of keraring implantation for high regular astigmatism in non-ectatic corneas: A prospective pilot case series. Sci Rep. 2026;16(1):18859. Published 2026 Jun 17. doi:10.1038/s41598-026-57128-7