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Modification des propriétés biomécaniques du kératocône mesurée avec le Corvis ST

Diagnosis of keratoconus (KC) includes analysis of corneal biomechanics. Aim of this study was to elucidate, which biomechanical parameters measured with the Corneal Visualisation Scheimpflug Technology noncontact tonometer (Corvis® ST, Oculus Optikgeräte, Wetzlar, Germany) differ stage-dependently in KC.

The patients had to pause wearing contact lenses at least 3 days before inclusion. Diabetes or corneal surgery were exclusion criteria. A total of 348 eyes with KC were classified into stages 1, 2, 3 and 4 (n=100|127|103|18) according to the topographic KC classification (TKC) implemented in the Pentacam® software (Oculus). Intermediate TKC stages were excluded. 30 healthy eyes served as control group (CG). After Pentacam® Scheimpflug imaging and Corvis® ST measurement, 36 individual biomechanical parameters measured by the Corvis® ST were extracted separately for each TKC stage. The Mann-Whitney-U-test was used to detect, whether and which biomechanical parameters differ significantly between CG, TKC1, TKC2, TKC3 and TKC4.

For five parameters, significantly higher values were detected in the CG compared to TKC1 (CG|TKC1, mean; stiffness parameter A1 (SPA1): 123|83; Ambrosio relational thickness to the horizontal profile (ARTh): 534|367; A2 velocity:-0.26|-0.28m/s; radius: 7.4|6.8mm; and A1 dArc length: -0.0176|-0.0193mm; p≤0.0044). The same applied for TKC1>TKC2, TKC2>TKC3 and TKC3>TKC4 (p<0.022). Nine other parameters turned out to be significantly lower in the CG compared to TKC1 (DA ratio 1mm: 1.5|1.6; and 2mm: 3.8|4.7; max. inverse radius: 0.16|0.19mm-1; highest concavity (HC) deflection amplitude: 0.88|0.92mm; A1 deformation amplitude: 0.126|0.134mm; A1 deflection amplitude: 0.087|0.104mm; max. deflection amplitude: 0.9|0.94mm; A1 deflection area: 0.17|0.19mm2; and integrated radius: 8.1|8.9mm-1; p≤0.0463). For these nine parameters and five complementary parameters, which, however, were similar in CG and TKC1 (A1 velocity, max. deformation amplitude, HC deflection area, HC deformation amplitude, pachy slope), significantly lower values were found in TKC1 compared to TKC2, TKC2<TKC3 and TKC3<TKC4 (p≤0.0333).

The Corvis Biomechanical Index (CBI, consisting of SPA1, ARTh, DA ratio 1 and 2mm, A1 velocity and max. deformation amplitude) detects KC independent of its stage, but the latter two parameters do not distinguish between healthy controls and TKC1. All the six parameters differ significantly from TKC1 to TKC4.

Overall, 19 out of 36 Corvis® ST parameters differ significantly in KC depending on its stage. Based on this study, different characteristics of the biomechanical Corvis® ST parameters can be assigned to the different KC stages possibly leading to the development of a “biomechanical KC classification“.