Comparison Of Tomographic And Biomechanical Characteristics Of Keratoconus At Different Stages In A Mediterranean Population

To determine the relationship between corneal biomechanics determined in Scheimpflug-based corneal biomechanical assessments with the Corvis ST and the severity of keratoconus according to the topographic keratoconus classification (TKC).

To investigate the correlations between the tomographic parameters and the biomechanical indices of keratoconus (KC) including the Corvis Biomechanical Factor (CbiF ).

A cross-sectional observational study  conducted in a University-based Moroccan ophthalmology department. 85 eyes from 46 patients with confirmed keratoconus based on clinical and tomographic characteristics were included in the study. According to TKC grading method , keratoconus was divided into stage I (TKC=1, 15 eyes ), stage II (TKC=1–2 , 2 , 31 eyes ) , stage III ( TKC=2–3, 3, 29 eyes ) and stage IV ( TKC = 3–4,4,10 eyes).

Pentacam AXL Wave and Corvis ST (Oculus Optikgeräte GmbH, Wetzlar, Germany) were used to obtain corneal morphological and biomechanical parameters.The Corvis Biomechanical factor was calculated for each patient by using the linear Corvis Biomechanical Index (beta) (CBiF = -0.24294226 × CBI beta + 6.02).The Mann-Whitney-U test was used to detect,whether and which biomechanical parameters differ significantly between stage I,stage II ,stage III,stage IV.Correlation was performed with the Spearman rank correlation coefficient between biomechanical parameters and the tomographic values anterior radius of curvature (ARC),posterior radius of curvature (PRC), and thinnest corneal thickness (TCT)

Between stage I and stage II,all biomechanical parameters evaluated differ significantly:stiffness parameter A1(SP,p<0,001),Ambrosio relational thickness to the horizontal (Arth,p<0,001), Integral radius (p<0,001), stress and strain index (SSI,p < 0,001), DA ratio (p<0,001), CBI beta (p<0,001),and CbiF (p<0,003).The same result between stage II and III with p<0,001 for all parameters.Between stage III and IV, Only CBiF showed significant difference (p<0,006).There was a moderate correlation of the CBI beta and its modification,the CBiF,with ARC (r = -0.635),PRC (r =-0.551), and TCT (r=-0.647) with p<0.001.For other biomechanical parameters: Arth presented the highest correlation with ARC(r=-0.637),PRC(r=-0.743),and TCT(r=-0.608)with p<0.001

Corvis Biomechanical Factor was highly associated with KC severity at different stages  and was correlated with corneal tomographic parameters. Others Biomechanical parameters differs significantly in mild and moderate keratoconus stages.These findings support the possibility in developing  a keratoconus classification that associate  Belin ABCD grading and biomechanical parameters .