The details of the ORA measurement were described previously.
27 Briefly, the ORA records two applanation pressures, before and after an indentation of the cornea with the application of a rapid air jet. Due to its viscoelastic property, the cornea resists the air puff, resulting in a delay in the outward corneal movement, which causes the difference in the pressures at the inward and outward applanation. This difference is called CH.
28 The CRF is also calculated using the difference between the inward and outward pressure, but indicates the elastic property of the cornea.
29 The ORA waveform is composed of two peaks, the in-signal peak (peak 1) and out-signal peak (peak 2); each represents inward and outward applanation events, respectively. The ORA waveform parameters consist of 35 parameters that represent characteristics of the two ORA response wave peaks. These parameters are defined as height (h1 and h2), width (w1, w2, w11, and w21), degree (uslope1, uslope2, uslope11, uslope21, dslope1, dslope2, dslope11, dslope21, slew1, and slew2), length (mslew1, mslew2, dive1, dive2, path1, path2, path11, and path21), area (p1area, p2area, p1area1, and p2area1), aspect ratio (aspect1, aspect2, aspect11, and aspect21), and the degree of non-monotonicity (aindex and bindex) of the two peaks, and frequency noise between the two peaks (aplhf), as illustrated in
Figure 2. The ORA measurements were carried out three times with at least a 5-minute interval between each measurement, and the average value was used in the analysis. All data had a quality index of more than 7.5 as recommended by the manufacturer.