Corneal asymmetry contributes decentration in both spherical and toric orthokeratology lenses

Purpose: To discuss the characteristics of anterior corneal elevation asymmetry in myopic eyes and clarify which kind of asymmetry most influenced lens position. Methods: In this retrospective study of 199 consecutive myopic participants, corneal topography was used to analyse asymmetry in anterior corneal elevation. Amongst them, 65 participants (65 eyes) who underwent orthokeratology (31 and 34 with spherical and toric lenses, respectively) were re-evaluated. Stepwise multiple linear regression analysis was used to identify the contributing factors that influenced lens decentration. Receiver operating characteristic curve (ROC) analysis was employed to assess how the corneal asymmetry vector could predict decentration. Results: There were no significant differences in treatment zone decentration (TZDec) between participants wearing toric and spherical lenses (p = 0.60 and 0.64 for 1 week and 1 month of wear, respectively). Amongst the underlying factors, the magnitude of TZDec was only correlated with the amount of corneal asymmetry vector (standardised beta = 0.44, 0.48, p < 0.001 for all) after 1 week and 1 month of wear, and the direction of TZDec after 1 month of lens wear was associated with the angle of the asymmetry vector (r = 0.25, p = 0.04). ROC analysis showed that the magnitude of corneal asymmetry vector produced accurate discrimination between non-severe and severe decentration for 1 week and 1 month of wear (area under the curve was 0.93 +/- 0.04 and 0.89 +/- 0.05, respectively, p < 0.001). Amongst participants whose corneal asymmetry vector exceeded 41.06 mu m and was oriented inferiorly, 35.29% showed severe decentration after 1 month of lens wear. Conclusions: In myopic participants, corneal asymmetry existed in the 8.0 mm chord diameter. If the asymmetry vector >41.06 mu m and the direction was oriented inferiorly, then practitioners must be vigilant about severe decentration which would not be alleviated by a toric design.