The Effect of the Back Optic Zone Diameter on the Treatment Zone Area and Axial Elongation in Orthokeratology

Purpose: To investigate the influence of corneal parameters on the treatment zone area (TZA) after Corneal Refractive Therapy (CRT) with a 5.0-mm back optical zone diameter (BOZD) were worn and to compare changes in the axial length (AL) with traditional 6.0-mm BOZD lenses.

Methods: This retrospective study involved 146 subjects (7-12 years) who wore orthokeratology (ortho-K) lenses for one year: 86 subjects were treated with CRT 5.0-mm lenses, and 60 subjects were treated with CRT 6.0-mm lenses. The TZA was measured after one year of ortho-K treatment. Both TZA and AL elongation after wearing the two kinds of lenses was compared. The parameters were recorded in the CRT 5.0 group: flat K, steep K, corneal toricity, e value, and anterior corneal elevation values at the 3-, 4-, and 5-mm chords along the principal meridians of the superior, inferior, nasal, and temporal sides. The relationships between these data and the TZA were analyzed.

Results: The TZA was 12.90 ± 5.15 mm and 20.61 ± 4.54 mm, and the AL elongation was 0.15 ± 0.18 mm and 0.26 ± 0.18 mm in the CRT 5.0 group and the CRT 6.0 group, respectively (all p < 0.001). The one-year AL elongation was significantly associated with initial age and the TZA (r =  - 0.394, 0.393; all p < 0.001) in the CRT 5.0 group. The following corneal parameters were found to have statistically significant correlations with the TZA: the e value, difference in corneal elevation (nasal-temporal at the 3-, 4-, and 5-mm chord), and the absolute value of elevation difference (nasal-temporal at the 3- and 4-mm chord and inferior-superior at the 3-, 4-, and 5-mm chord). The e value was the only relevant factor for the TZA by multiple regression analysis (unstandardized β = 14.219, p = 0.008). In the CRT 6.0 group, the one-year AL elongation was statistically significantly associated only with initial age (r =  - 0.605, p = 0.005), but not with the TZA (p = 0.161).

Conclusions: A smaller TZA induced by a smaller BOZD may be beneficial for retarding AL elongation in children undergoing ortho-K treatment. The morphology and eccentricity of the cornea may show effects on the TZA.