Abstract
Purpose :
To investigate algorithm differences in maximum corneal curvature between locating the steepest point vs steepest region in keratoconus and the impact on corneal stiffness.
Methods :
Pentacam tomography and Corvis ST biomechanical exams were prospectively acquired in 61 eyes of 41 subjects with keratoconus. Disease severity was defined by maximum axial curvature, comparing single point Kmax vs magnitude of surrounding 2mm zone (ZKmax) which was manually extracted, vs magnitude of steepest 2mm zone (CSpot_Axi) located by Cone Location and Magnitude Index (CLMI) as the primary search parameter using custom software. Distance from corneal center to Kmax (Kmax dist) was compared to radial distance of CLMI (CRad_Axi). All measurements were analyzed using paired t-tests. Both single-point and CLMI derived zones were compared to biomechanical metrics via univariate regression analyses to stiffness parameter at first applanation (SP-A1) and highest concavity (SP-HC), deformation amplitude ratio at 2mm (DA Ratio Max) and integrated inverse radius (IIR). Statistical analyses were performed in SAS with significance threshold of p < 0.05.
Results :
ZKmax and CSpot_Axi were significantly different (p<0.001) with Cspot_Axi having a 2.67±0.95D greater average magnitude. Kmax dist and CRad_Axi were significantly different (p<0.001) with Crad_Axi showing a 0.20±0.23mm greater distance. Single point Kmax (56.09±8.99D), was significantly greater than ZKmax (51.81±7.50D). Regressions for ZKmax and CSpot_Axi were significantly negative to both SP-A1 (R2=0.417 and R2=0.427) and SP-HC (R2=0.192 vs R2=0.201) and significantly positive to both DA Ratio Max (R2=0.603 vs R2=0.610) and IIR (R2=0.728 vs R2=0.746), respectively. Regressions for Kmax dist and CRad_Axi vs SP-A1 (R2=0.074 vs R2=0.126) were significantly positive while SP-HC was not significant. DA Ratio Max and IIR were significantly negative for Kmax dist (R2=0.098 vs R2=0.206) and CRad_Axi (R2=0.129 vs R2=0.234), respectively.
Conclusions :
In keratoconus single point Kmax did not locate the steepest zone and is likely affected by artifact. Zonal values, ZKmax and Cpsot_axi, were more reliable with lower variability than single point Kmax. Both zonal values had similar relationships to biomechanical metrics. Cspot_axi was more robust than ZKmax as it identified the steepest 2mm zone rather than creating a 2mm zone around the artifact prone single point Kmax (see Fig 1).
This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.