Reliability analysis of successive Corvis ST® measurements in keratoconus 2 years after accelerated corneal crosslinking compared to untreated keratoconus corneas

Abstract

Purpose To assess the reliability of successive Corvis ST® measurements (CST, Oculus, Wetzlar, Germany) in keratoconus (KC)≥2 years after accelerated corneal crosslinking (9 mW/cm2 , 10 min, 5.4 J/cm2 ) compared to untreated KC corneas. Methods Three successive CST measurements per eye were performed in≥2 years after CXL (CXLG, n=20 corneas of 16 patients) and a control group consisting of non-operated, ABC-stage-matched KC corneas according to Belin’s ABCD KC grading (controls, n=20 corneas, 20 patients). Main outcome measures included maximal keratometry (Kmax), the Belin/ Ambrósio-Enhanced-Ectasia-Deviation-Index BAD-D; the biomechanical parameters A1 velocity, deformation amplitude (DA) ratio 2 mm, Ambrósio relational thickness to the horizontal profle (ARTh), integrated radius, stifness parameter A1 (SP-A1), and the Corvis Biomechanical Factor (CBiF, the linearized term of the Corvis Biomechanical Index). Mean values, standard deviations, and Cronbach’s alpha (CA) were calculated. Results Both groups were tomographically comparable (BAD: 11.5±4.7|11.2±3.6, p=0.682, Kmax: 60.5±7.2|60.7±7.7, p=0.868 for controls|CXLG, paired t-test). A1 velocity (mean±SD: 0.176±0.02|0.183±0.02, p=0.090, CA: 0.960|0.960), DA ratio 2 mm (6.04±1.13|6.14±1.03, p=0.490, CA: 0.967|0.967), integrated radius (12.08±2.5|12.42±1.9, p=0.450, CA: 0.976|0.976), and CBiF (4.62±0.6|4.62±0.4, p=0.830, CA: 0.965|0.965) were also comparable (controls|CXLG). ARTh was signifcantly higher in controls (177.1±59, CA: 0.993) than after CXL (155.21±65, p=0.0062, CA: 0.993) and SP-A1 was signifcantly higher after CXL (59.2±13, CA: 0.912) than in controls (52.2±16, p=0.0018, CA: 0.912). Conclusion ARTh and SP-A1 difered signifcantly between controls and CXLG. Biomechanical measurements were generally of excellent reliability in both groups. CXL seems to afect biomechanical measurements of human corneas over more than 2 years.