Distribution of Corneal Geometric Landmarks and Relationship Between Their Distances and Biomechanical Parameters in the Development of Keratoconus

Abstract

Purpose: To analyze the changes in coordinates and distances among three typical geometric landmarks of the cornea, namely, the thinnest point (TP), maximum curvature (Kmax), and corneal apex (AP) during the development of keratoconus, and explore the potential relationship between these changes and the abnormalities of corneal biomechanics. Methods: Normal eyes (n = 127), clinical keratoconic eyes (CKC, n = 290), and the eyes of forme fruste keratoconus (FFKC, n = 85) were included; among them, the CKC group was classified into four grades based on the Topographic Keratoconus Classification (TKC) provided by Pentacam. A total of 38 Corvis ST output parameters and three distance parameters of three typical landmarks (D_Kmax-AP, D_TP-AP, and D_Kmax-TP) based on Pentacam were included. The differences of parameters among the abovementioned six groups (Normal, FFKC, and CKC stage I to CKC stage IV) were analyzed. Spearman's rank correlation test was performed to choose several dynamic corneal response (DCR) parameters that could best reflect the changes of corneal biomechanical characteristics during the progression of the disease, and the Pearson's or Spearman's correlation test was conducted to determine the association between the three distances and the selected DCR parameters in each grade. In addition, by flipping the X coordinate of the left eye on the vertical axis to reflect the direction of the right eye, the coordinates of TP and Kmax in different developmental grades were highlighted. Results: From CKC stage II, the three geometric landmark distances commenced to correlate with the corneal DCR parameters (CBI, SPA1, IR, DA Ratio 2, ARTh, MIR, Radius, Pachy, and DA Ratio 1), which could better represent the changes of biomechanical properties from normal cornea to keratoconus. From normal cornea to CKC stage IV, the coordinates of Kmax were gradually tended to the inferior temporal region from dispersion, while TP was always concentrated in the inferior temporal region. Although D_Kmax-AP, D_Kmax-TP, and D_TP-AP all showed a gradual decreasing trend with the progress of the disease, the first two did not change significantly, and only D_TP-AP significantly approached AP in the later stage of disease development. In addition, from the FFKC group, the corresponding values of D_Kmax-TP in each disease development group were smaller than D_Kmax-AP. Conclusions: In the later stage of keratoconus, the relationship between the three typical landmark distance parameters and DCR parameters is stronger, and even the weakening of corneal biomechanical properties may be accompanied by the merger of typical landmark positions.

Keywords: biomechanics; forme fruste keratoconus; geometric landmark; keratoconus; morphology.

FIGURE 1

Spearman correlations of DCR parameters and the developmental grades of keratoconus (strong correlation: 0.6 ≤ |r| < 1 and p < 0.05, weak correlation: |r| < 0.6 and p < 0.05, irrelevant: p ≥ 0.05).

FIGURE 2

The change tendency of DCR parameters, which are highly correlated as disease progression with the disease stage increasing. a, b, c, d, and e indicate that the parameters in a certain stage are statistically different from the corresponding parameters in the previous adjacent stage (p < 0.01); 0 = Normal group, 1 = FFKC group, 2 = CKC Stage I group, 3 = CKC Stage II group, 4 = CKC Stage III group, 5 = CKC Stage IV group.

FIGURE 3

The coordinate distribution of TP and Kmax in different developmental grades of keratoconus, where the X coordinate of the left eye was flipped on the vertical axis to reflect the direction of the right eye (TP = Thinnest point; Kmax = Maximum keratometry of the front surface).

FIGURE 4

The variation trend of three typical landmark distance parameters with the progress of disease stage (0 = Normal group, 1 = FFKC group, 2 = CKC Stage I group, 3 = CKC Stage II group, 4 = CKC Stage III group, 5 = CKC Stage IV group; D_Kmax-AP: the absolute distances from the maximum curvature of the anterior surface to the apex of the cornea; D_Kmax-TP: the absolute distances from the maximum curvature of the anterior surface to the thinnest point; D_TP-AP: the absolute distances from the cornea apex (geometric center of the examination [x = 0; y = 0]) to the thinnest point).

FIGURE 5

Correlation between the three geometric landmark distance parameters and the nine DCR parameters in each grade from normal to CKC Stage IV (the small lattice filled with color indicates that the correlation is statistically significant, in which red indicates that the correlation coefficient is positive, blue indicates that the correlation coefficient is negative, and the depth of color indicates the absolute value of the correlation coefficient).