## 1 Introduction
AL progression at 1 year was higher with MSCLs (0.176 mm; 95%CI [0.143, 0.209] and low-dose atropine (0.197 mm; 95%CI [0.144, 0.251]) than with orthokeratology using larger (OK 1) (0.175 mm; 95%CI [0.128, 0.223]) and smaller (OK 2,3,4) (0.102 mm; 95%CI [0.066, 0.139]) treatment zones. In the latter case, this progression was similar to that of emmetropic children. Having made this observation, it is important to try to understand why there could be such a difference between OK lens designs, even though on paper the lenses could be considered similar since they are based on the same optical principles. It should be noted that OK is a treatment that uses a reverse geometry gas-permeable lens that is worn overnight. It uses the hydraulic forces of the tear film layer to reshape the cornea, temporarily altering the optics of the ocular surface. The modified corneal optics will include a central zone of flattening as well as a zone of reverse curvature forming a midperipheral ring [15; 16]. Changes are easily observed with corneal topography. Axial maps provide data on the original corneal shape from which lens parameters are determined, while tangential maps reflect the behavior of the lens on the cornea [17]. The optical effect of these corneal shape changes will be similar to some soft lens bifocal lens designs, except that the plus power generated at the midperiphery will typically be much greater, resulting in greater myopic defocus hitting the peripheral retina, provided the convex power zone is partially contained within the pupil area [18]. It is important to note the differences in lens design to appreciate the potential impact on myopia management. The only real way to analyze the effect of the lens is to study the change in corneal profile induced by the lens. In fact, the same lens designed to compensate for the same dioptric power will behave very differently in two different subjects. Individual parameters such as corneal profile, curvature, eccentricity, Q-value, lid rigidity, sleep position, etc. will impact the final topographic changes. The effect on the progression of myopia can also vary depending on the size of the pupil diameter [18]. Topography is essential to determine the effect of the lens on the eye, its centration and the power changes produced. This was demonstrated in a retrospective study of topographic data comparing the corneal optical effect of two different orthokeratology designs. A significant difference was found in the central zone, aiming to correct distance vision [19]. It is also possible to achieve the same conclusion by using polynomial mathematical functions [20]. The efficiency of axial length management seems to be influenced by this optical zone difference [21; 22]. A larger back optical zone diameter (BOZD) results in a significantly larger treatment zone diameter molded on the cornea, which is associated with higher axial elongation after 2 years of treatment [22]. This article aims to analyze the changes in the corneal profile before and after lens wear in the Montreal Experience orthokeratology cohort using four OK lens designs, in order to better understand the optical differences that led to this efficiency gap found in the Montreal Experience part 1 [14].