## 4 Discussion  
This retrospective study shows that different OK lens designs mold the cornea differently and that these differences may be related to the relative progression of the axial length of the eye. Thus, smaller treatment zone diameters on the cornea created by the lenses allow for a larger relative area of myopic defocus in the pupil area. Knowing that there is a dose-response mechanism [21; 25], this increase in the area of positive power/myopic defocus in the pupil is likely to result in better control of the axial length progression, which could explain the difference in efficiency found between the large and small BOZD orthokeratology in the Montreal Experience part 1 analysis [14]. The OK corneal molding generates optical defocus from two powers: a first central power, to compensate for the refractive error, and a second, surrounding the central zone, to generate a more convex power associated with myopic defocus. As mentioned above, the myopic and hyperopic defocus signals can be interpreted simultaneously at the retinal level. The proportion of each signal determines the overall direction of refractive development and the effectiveness of a simultaneous optical strategy for myopia management. However, at equal proportions, the myopic defocus tends to be predominant [26; 27]. The amplitude of the myopic defocus represents the distance of anterior focal plane in front of the retina. Consequently, a larger dose of myopic defocus will elicit a larger response, resulting in a reduction of axial elongation [28; 29]. These two elements make up the \"dose\" that reaches the retina: the amplitude (power) of the defocus and its relative area of effect on the retina. Another study [25] compared two lenses. The first lens was designed with a larger BOZD and produced a higher amplitude of defocus. The second lens was designed with a smaller central BOZD, which was associated with a lower amplitude of defocus. However, the smaller BOZD lens provided the better axial length management, suggesting that the combination of the two factors (zone diameter and amplitude of defocus generated) may influence the outcome, rather than either of them considered individually. In the case of this study, the 4 lenses produced similar plus power, approximately 9 D, as measured by RPP variation. If the Montreal experience study shows that the defocus produced by the 4 OK lenses is statistically equivalent, then something else must explain the differences found between the 4 OK lens designs in terms of their effect on axial length. The location of this myopic defocus area must be considered, especially in relation to the pupil. If the myopic defocus area lies outside the pupil, little of this beneficial optical signal will reach the retina, considering the contribution of direct and oblique rays that characterize the entry of light into the eye. On the other hand, if this defocus is partially superimposed on the pupil area, the chances of getting a higher dose of defocus reaching the peripheral macular area (10-20 degrees) and thus positively influencing axial length growth are increased. However, designing a lens with a smaller BOZD does not automatically translate into an equal variation on the corneal surface. This was previously observed in a study that analyzed the corneal optical effect produced by two different orthokeratology designs [19]. The effect on the corneal surface must be evaluated by analyzing the tangential topographical map, which was performed in this study. OK 1 produced a significantly larger treatment zone compared to the OK 2-3 or 4 designs. The distance treatment zone diameter (DTZD) when wearing OK 2-3 and 4 is considered similar, as no statistically significant differences were found between them. OK 1 is associated with higher AL growth [14]. This finding is consistent with the results of different studies [20; 22], reporting better control of axial elongation when a smaller treatment zone is present on the cornea. The width of the area producing this defocus is the second factor to consider.