The "Half-Kappa" Strategy: Optimizing Wavefront Outcomes And Ocular Axis Stability In Femtosecond-Laser Assisted Cairs For Keratoconus

To evaluate the clinical efficacy of a customized centration strategy—positioned at 50% of the preoperative Kappa distance—on high-order aberrations (HOAs) and the longitudinal stability of the pupillary-visual axis relationship following Corneal Allogenic Intrastromal Ring Segment (CAIRS) implantation.

Private Specialized Ophthalmic Center and Surgical Unit SwissLaser Clinics (Warsaw, Krakow/Poland) utilizing the Schwind ATOS Femtosecond Laser platform.

This prospective study analyzed eyes with progressive keratoconus undergoing CAIRS implantation. All intrastromal tunnels were created using the Schwind ATOS femtosecond laser. A specific centration protocol was applied: the treatment was offset to half the distance between the pupillary center and the corneal vertex (50% Kappa).

Comprehensive corneal wavefront analysis (MS39, CSO) was performed preoperatively and at 1-month postoperatively. Key metrics included:

High-Order Aberrations (HOAs): Total RMS, Coma (Z3,-1), Z3,1)
Spherical Aberration (SA) at 3mm, 4mm, 5mm, and 6mm zones

Axis Stability: Quantitative shifts in Angle Kappa (X, Y coordinates).

Analysis of the 1-month postoperative data revealed a significant improvement in central optical quality. At the 3mm pupil zone, there was a consistent reduction in both Total RMS and Coma.

Mean Wavefront Changes (3mm)- Mean

Preoperative | 1-Month Post-Op | % Change

Total RMS 1.04 µm | 0.88 µm (-15.4%) 

Coma (3,-1) -0.11 µm | -0.08 µm (-27.3%)

Coma (3,1) |0.08 µm | 0.13 µm | Redistribution

Angle Kappa Stability:

The Kappa distance remained remarkably stable postoperatively. The mean coordinate shift was only 0.03 to 0.02 mm (X-axis) and 0.04 to 0.03 mm (Y-axis). Despite significant corneal flattening (evidenced by shifts toward negative Spherical Aberration at 6mm), the relationship between the visual axis and the pupillary center did not deviate.

Centering CAIRS at "Half-Kappa" using the Schwind ATOS laser provides an ideal "optical compromise" for corneal remodeling in keratoconus. This approach effectively reduces asymmetric aberrations (Coma) while maintaining the eye's natural visual axis alignment. The high stability of Angle Kappa suggests that this centration strategy avoids "axis-tilt" and provides more predictable refractive outcomes than standard pupillary centration.