Comparison of Biomechanical and Topographic Parameters in Normal and Pathologic Corneas

Abstract

Purpose:: To investigate biomechanical parameters derived from the applanation and air pressure signals of the Ocular Response Analyzer (ORA), and compare them to severity of disease in pathologic corneas.

Methods:: A retrospective study of 87 eyes of 56 subjects who received Orbscan II corneal topography and ORA measurements was conducted. Three patient groups were analyzed: normal (n = 44), keratoconus and keratoconus suspect (n = 36), and pellucid marginal dystrophy (n = 7). Orbscan data were processed using The Ohio State University Corneal Topography Tool to calculate the Cone Location and Magnitude Index (CLMI) and average thinnest pachymetry (PACH) over a 2 mm zone. CLMI finds the steepest area of the axial map and compares it to the rest of the map, as an indicator of irregularity. Custom software was used to derive 10 parameters from ORA signals: applanation peak magnitudes (Peak1 and Peak2), time to applanation peaks (Time1 and Time2), width of each peak at half maximum, the slopes of the air pressure curve within each peak, the maximum of the air pressure curve (pmax), and the time to pmax. Linear regression analyses were performed to analyze the relationships between CLMI and PACH to the ORA parameters for each subject group. An ANOVA analyses was performed to determine which parameters differed between subject groups.

Results:: In the Keratoconic group, CLMI, not PACH, correlated with Peak1 (p=0.0001, R²=0.36), Peak2 (p=0.0048, R²=0.22). In the Pellucid group, CLMI, not PACH, correlated with Peak1 (p=0.0002, R²=0.94). The ANOVA analyses showed that the CLMI of the normal group was statistically different than the other two groups and Peak1 of the Keratoconic group was statistically different than normals and Pellucids.

Conclusions:: Peak1 and 2 are directly related to the amount of light detected during an applanation event. Therefore, the Peak amplitude may be an indication of corneal properties manifested in the characteristics of corneal deformation. The negative correlation of Peak1 with disease severity, as defined by CLMI, demonstrates that as Keratoconus and Pellucid progress, Peak1 decreases in amplitude. This may be due to a small area of applanation or a nonuniform deformation, both of which would lead to lower applanation peaks, and may be an indication of a softer cornea or nonhomogeneous corneal biomechanical properties.

Keywords: keratoconus • refractive surgery: corneal topography • intraocular pressure