Diagnostic And Refractive Relevance Of Photopic And Mesopic Chord Μ In Keratoconus: A Comparative Cross-Sectional Study
Published 2026 - 44th Congress of the ESCRS
Reference: FP06.05 | Type: Free paper | DOI: 10.82333/zbyy-8a25
Authors: Dr Beyza Gürpınar* 1 , Dr Sevgi Tongal 1 , Dr Betül Dumlu 1 , Dr Şakir Arıman 1 , Dr Dilan Çolak Arıcı 1 , Prof. Dr Ahmet Kırgız 1
1Eye,Beyoglu Eye Training and Research Hospital,Istanbul,Türkiye
Purpose
To compare photopic and mesopic chord μ values between keratoconus (KC) patients and healthy individuals, assess their correlation with biometric and aberrometric parameters, and evaluate their diagnostic value in KC detection and preoperative planning for refractive surgery.
Setting
Beyoglu Eye Training and Research Hospital, Istanbul, Türkiye
Methods
In this cross-sectional study, 51 eyes with keratoconus and 50 healthy eyes were examined using the Topcon Aladdin Biometer and Corneal Topographer HW3.0. Chord μ values were calculated manually under both photopic and mesopic lighting conditions according to the Euclidean formula, based on pupil center and corneal light reflex coordinates. We compared photopic and mesopic chord μ values and their associations with biometric and aberrometric parameters. Statistical analyses were performed using SPSS software version 26.0(IBM, NY, USA), including Mann–Whitney U test, Spearman correlation, ROC curve analysis, and multivariate linear regression to assess diagnostic thresholds and parameters.
Results
Both photopic and mesopic chord μ values were significantly higher in the keratoconus group compared to controls (photopic: 0.309 ± 0.10 mm vs. 0.247 ± 0.08 mm, p < 0.001; mesopic: 0.272 ± 0.10 mm vs. 0.226 ± 0.09 mm, p = 0.003). ROC analysis revealed moderate diagnostic capacity: Photopic chord μ > 0.315 mm; AUC = 0.656, OR = 5.91 Mesopic chord μ > 0.275 mm; AUC = 0.615, OR = 3.33 In healthy eyes, chord μ correlated negatively with anterior chamber depth and axial length, suggesting anatomical variance. In keratoconus, chord μ correlated positively with vertical pupil shift and negatively with coma aberration under mesopic conditions.
Conclusions
Chord μ is significantly increased in keratoconus and reflects both geometric misalignment and optical distortion.While it lacks strong standalone diagnostic power,it may serve as a supportive parameter in borderline KC cases and assist in preoperative refractive surgery planning,especially where pupil-optic axis misalignment is a concern.