Figure 1.
Elevation data obtained from the Pentacam.
O, denoting the apex of anterior or posterior corneal surface, is the origin point of the Cartesian coordinate system, XOY is the plane that passes through X and Y axes and Point O. Elevation zi is the sagittal distance between a general point, i, and the XOY plane.
Figure 2.
The Cartesian coordinate system of bilateral corneas.
Points A to D were chosen to test instrument measurement repeatability and analyze interocular symmetry. Rotation angles α, β and γ could be positive or negative according to right hand spiral law.
Table 1.
Values of refractive error, corneal curvature, astigmatism and central thickness.
Table 2.
Intraobserver repeatability outcomes for elevation measurements obtained at four points on corneal surface.
Table 3.
Bilateral differences of elevation measurements at two symmetrically positioned points on corneal surface.
Table 4.
Differences between corneal shape parameters determined for the right and left eyes.
Figure 3.
Bland-Altman plots for analysis of the agreement between bilateral eyes in Rx, Ry, Qx and Qy of anterior (A, C, E, G) and posterior surfaces (B, D, F, H).
Figure 4.
Values of X, Y and Z translation displacements estimated for all 342 participants.
With the right eye viewed from the measuring instrument the coordinate convention is positive X toward the nasal side, while for the left eye positive X is towards the temporal side. In both eyes, the coordinate convention has positive Y towards the superior side and positive Z towards the frontward side.
Table 5.
Translational and rotational displacement results of general quadratic model.
Figure 5.
Values of α, β rotation angles estimated for all 342 participants.
With the right eye viewed from the measuring instrument the coordinate convention is positive β towards the nasal side, while for the left eye, positive X is towards the temporal side. In both eyes, the coordinate convention has positive α toward the inferior side.
Figure 6.
Values of γ rotational angles estimated for all 342 participants.
For clarity of presentation, participants were split into two groups according to whether γ of right corneas was greater than 0 (A, C) or less than 0 (B, D).
Figure 7.
Bilateral eyes fixed on a long-distance target (A). When eyes are fixed on a near-distance target during measurement, bilateral eyes turn inward (B). Corneal geometric axis turns towards the nasal side and corneal apex provided by the instrument is located on the nasal side of the corneal geometric center.