- Treatment zone characteristics [ Time Frame: 2 years ]
Corneal topography will be determined by Medmont topographer and the treatment zone characteristics will be determined from the subtractive maps
- Pupil sizes [ Time Frame: 2 years ]
Photopic and mesotopic pupil sizes
- Choroidal thickness [ Time Frame: 2 years ]
Choroidal thickness measurement will be determined by both LenStar and posterior OCT
Orthokeratology (Ortho-k) treatment reduces the degree of myopia of the eye by flattening the central cornea. This central flattened zone is referred to as the treatment zone (TZ). Munnerlyn’s formula (Munnerlyn et al. 1988) describes the negative correlation between refractive correction and treatment zone size given a maximum ablation depth limited by the patient’s corneal thickness in refractive surgery. This implies smaller TZ for higher myopic reduction and, indeed, Owens et al. (2004) found marginally smaller TZ of 0.3mm in higher myopic group. In ortho-k, it has been speculated that TZ may be associated with the effect of the treatment. The TZ is defined as the central flattened area enclosed by points with zero power/curvature changes comparing to pre-treatment condition on various topography maps.
Previous studies determine the TZ characteristics based on different types of topographical subtractive maps including tangential (Lu et al. 2007; Gifford and Swarbrick 2009), axial (Tahhan et al. 2003) and refractive maps (Sridharan and Swarbrick, 2003). Maseedupally et al. (2015), however, compared TZ parameters derived from the three different maps. They defined the TZ as the area enclosed by zero power change in different subtractive maps; 12 such points equally spaced 30o apart to construct a best-fit ellipse. Geometrical center of this ellipse was used to determine the TZ decentration and TZ diameter along vertical and horizontal axes. They found significantly less TZ decentration and diameter in tangential map comparing to axial and refractive maps and no significant difference between the latter two. Based on these findings, they suggested that either refractive or axial map would be of high validity and accuracy in determining TZ characteristics.
In the current study, we propose to conduct both retrospective and prospective studies to investigate the effect of TZ on myopic control in ortho-k. TZ, as defined by Maseedupally et al. (2015), that is, the area enclosed by zero power change in subjective maps will be employed. The tangential and refractive subtractive maps will be used to determine the TZ. Characteristics of the TZ, including diameter, slope, depth, volume, would be analyzed based on topographical subtractive maps and the effect of each of these characteristics on myopia control will be evaluated. The role of pupil size during normal viewing condition (distance and near) will also be investigated.