210° Vs 160° Symmetric Intracorneal Ring Segments In Snowman Keratoconus: Does Arc Length Determine Visual And Optical Success?

To evaluate visual, refractive, tomographic and aberrometric outcomes after implantation of a single symmetric intracorneal ring segment (ICRS) in patients with snowman phenotype keratoconus, and to compare the 4-month clinical evolution between 160° and 210° arc-length segments.

Cornea and Keratoconus Unit, Department of Ophthalmology, Hospital Universitario Virgen del Rocío, Seville, Spain.

Retrospective descriptive study including 16 eyes with snowman phenotype keratoconus, defined by low corneal asphericity, irregular topographic astigmatism, and 30°–60° divergence between topographic and comatic axes. All eyes underwent implantation of a single symmetric ICRS. Group 1 (n=5) received a 160° segment and Group 2 (n=11) a 210° segment. Assessed variables were visual acuity, spherical equivalent, K1, K2, mean keratometry (Km), Kmax, topographic astigmatism (AST), corneal asphericity (Q), spherical aberration (Z4⁰) and coma (Z3⁻1). Measurements were obtained preoperatively and at 1 and 4 months. Intra- and inter-group comparisons were performed (p<0.05).

In the 210° group, visual acuity improved significantly from 0.477 preoperatively to 0.758 at 4 months (p=0.002), with additional gain between months 1 and 4 (p=0.045); no significant change occurred in the 160° group. Spherical equivalent showed a non-significant improving trend in both groups. K1 decreased significantly at 1 month in both groups, but remained significant at 4 months only in the 210° group. K2, Km and Kmax significantly decreased only in the 210° group. Asphericity improved significantly in the 210° group. Coma decreased significantly in both groups. No significant changes were observed in AST or spherical aberration.

In snowman phenotype keratoconus, implantation of a single 210° symmetric ICRS resulted in significant visual improvement, greater corneal flattening (K1, K2, Km, Kmax), improved asphericity, and sustained coma reduction at 4 months, whereas the 160° segment showed more limited and mainly transient effects. Segment arc length appears to play a key role in biomechanical and optical regularization in this specific morphological pattern. In snowman phenotypes, a single 210° segment may represent the preferred option. Larger prospective studies are needed to confirm these findings.