## 4 Discussion
Theoretically, a larger area may contribute to generating a higher dose of defocus, depending on its relationship to the pupil area. Previous OK studies have explored the effects of either the BOZD diameter or the corneal treatment zone diameter on myopia control. It has been shown that pupil diameter may be important and that better results are obtained when the myopic defocus is partiallywithin the pupil area [20; 21]. However, the relative pupil area covered by the treatment zone has not been previously measured. This study adds new knowledge by introducing the concept of plus power ratio (PPR) in orthokeratology. This concept is important when considering lenses with the same optical zone but producing a different ratio depending on their corneal effect vs. the patient's pupil. For a given optical zone, a larger pupil diameter will increase the PPR, allowing for a greater proportion of myopic to hyperopic defocus, thus contributing to better treatment efficacy, as previously suggested by [PERSON] et al. [18]. In light of this understanding, it is also possible to hypothesize that the better results obtained in some studies combining the use of low-dose atropine and OK lenses may have been obtained in part because of the increase in the dose of defocus affecting the retina as a response to the increase in pupil diameter caused by the drug. The results from the Montreal Experience part 1 showed a significantly higher AL progression with OK 1 (0.175 mm at year 1/0.359 mm at year 2) than with OK 2, 3, or 4 (0.102 mm/0.249 mm respectively) [14]. When analyzed, OK 1 had a larger treatment zone diameter, resulting in a lower PPR compared to OK 2, 3, and 4. This implies that it may be necessary to consider the patient's pupil diameter/area when selecting OK lens designs and also how this design translates at the corneal level. The results suggest that the higher ratios are associated with smaller corneal treatment areas, but also with more modest axial length progression. However, the question of the optimal ratio remains, and this retrospective study does not allow us to fully clarify this aspect. The OK 1 lens is not completely ineffective and has a defocus ratio equivalent to 40.1% of the pupil area. Therefore, the minimum threshold to induce a positive effect may be slightly less than 40.1%. Efficacy is improved with other lenses (OK 2-3 and 4) that cover between 53.8 and 60.3% of the pupil without affecting distance vision. Are these the upper limits? Is it possible to increase the coverage even further to achieve greater efficacy? More work is needed to determine these limits, but we can assume that a PPR value between 40% and 60% is associated with increasing efficacy as the percentage of area increases. This is also consistent with the concept of a dose-response at the retinal level. The results of the chicken and guinea pig studies suggest that the information from the two focal planes depends not only on the amplitude and the sign of the defocus, but also on their relative contrast on the retina. For example, in the chicken study using +10 D / -10 D as bifocal powers, varying the area ratio between the powers also affected the resulting refractive change outcome [26]. In guinea pigs and marmosets, the relative effectiveness of the two powers in controlling axial length appears to be linearly related to the relative lens area associated with each power [30; 31]. For example: for +5 D / -5 D powers in the presence of a 50:50 ratio, the eye developed towards hyperopia [31]. In chickens, when the convex power ratio is reduced to 33% for identical powers, the preference for developing hyperopia is completely eliminated and the eyes evolve more toward myopia. In terms of a refractive evolution management, the variation of the PPR would have an effect on the refractive and axial length changes magnitude [26]. It is also important to consider the contrast quality of the retinal image, which is affected by the presence of multiple transition zones.