The Spatial Distribution of Relative Corneal Refractive Power Shift and Axial Growth in Myopic Children: Orthokeratology Versus Multifocal Contact Lens

Abstract

Purpose: To determine if the spatial distribution of the relative corneal refractive power shift (RCRPS) explains the retardation of axial length (AL) elongation after treatment by either orthokeratology (OK) or multifocal soft contact lenses (MFCLs).

Methods: Children (8-14 years) were enrolled in the OK (n = 35) or MFCL (n = 36) groups. RCRPS maps were derived by computing the difference between baseline and 12-month corneal topography maps and then subtracting the apex values. Values at the same radius were averaged to obtain the RCRPS profile, from which four parameters were extracted: (1) Half_x and (2) Half_y, i.e., the x- and y-coordinates where each profile first reached the half peak; (3) Sum4 and (4) Sum7, i.e., the summation of powers within a corneal area of 4- and 7-mm diameters. Correlations between AL elongation and these parameters were analyzed by multiple linear regression.

Results: AL elongation in the OK group was significantly smaller than that in the MFCL group (p = 0.040). Half_x and Half_y were also smaller in the OK group than the MFCL group (p < 0.001 each). Half_x was correlated with AL elongation in the OK group (p = 0.005), but not in the MFCL group (p = 0.600). In an analysis that combined eyes of both groups, Half_x was correlated with AL elongation (β = 0.161, p < 0.001).

Conclusions: The OK-induced AL elongation and associated RCRPS Half_x were smaller than for the MFCL. Contact lenses that induce RCRPS closer to the corneal center may exert better myopia control.

Keywords: axial length; corneal refractive power; multifocal soft contact lens; myopia; orthokeratology.

FIGURE 1

Computation of the RCRPS for a representative eye. (A) An axial map at baseline. (B) An axial map at 12 months after treatment. (C) CRPS was derived by subtracting the baseline map from the after-treatment map. (D) RCRPS was derived by subtracting the center point value from each point on the CRPS. (E) RCRPS profile derived by taking the mean of each ring. Data are for an eye wearing the OK lens. Similar determinations were made for eyes wearing the MFCLs. RCRPS, relative corneal refractive power shift; CRPS, corneal refractive power shift; OK, orthokeratology; MFCL, multifocal soft contact lens; Half_x, the corneal eccentric location where RCRPS reached the half peak value; Half_y, half of the RCRPS peak value; Sum4, the integrated value of the RCRPS located within an area of 4-mm diameter; Sum7, the integrated value of the RCRPS located within an area of 7-mm diameter.

FIGURE 2

Representative RCRPS profiles. (A) An eye in the OK group. (B) An eye from the MFCL group. RCRPS, relative corneal refractive power shift; D, diopter; OK, orthokeratology; MFCL, multifocal soft contact lens; Half_x, the corneal eccentric location where RCRPS reached the half peak value; Half_y, half of the RCRPS peak value; Sum4, the integrated value of the RCRPS located within an area of 4-mm diameter; Sum7, the integrated value of the RCRPS located within an area of 7-mm diameter.

FIGURE 3

RCRPS data from all subjects. (A) RCRPS profiles for the OK group (red) and MFCL group (blue). Solid line: mean value; shaded area: 95% confidence interval for the population means. (B) Half_x, (C) Half_y, (D) Sum4, and (E) Sum7. (B–E) OK group, red line; MFCL group, blue bars. RCRPS, relative corneal refractive power shift; D, diopter; OK, orthokeratology; MFCL, multifocal soft contact lens. Half_x, the corneal eccentric location where RCRPS reached the half peak value; Half_y, half of the RCRPS peak value; Sum4, the integrated value of the RCRPS located within an area of 4-mm diameter; Sum7, the integrated value of the RCRPS located within an area of 7-mm diameter.

FIGURE 4

Association between axial length elongation and RCRPS parameters (A) Half_x, (B) Sum4, (C) Half_x – Pupil radius for the OK group (red circles) and the MFCL group (blue triangles). The red solid line is the regression line of the OK data; the black dashed line is the regression line of the combined data. RCRPS, relative corneal refractive power shift; D, diopter; OK, orthokeratology; MFCL, multifocal soft contact lens; combined data: OK and MFCL; Half_x, the corneal eccentric location where RCRPS reached the half peak value; Sum4, the integrated value of the RCRPS located within an area of 4-mm diameter.