This project aims to study the effects of treatment characteristics on myopia control in orthokeratology by investigating the effects of modified treatment zone by reducing the lens back optic zone diameter on the effect of myopia control. The project will be a two-year randomized, double-masked (examiner - mask axial length only) study conducted at The Hong Kong Polytechnic University recruiting sixty Chinese subjects aged between 6-11. Subjects will be randomly assigned to two ortho-k groups. The Control group will wear KATT BeFree lens with a BOZD of 6mm while the Test group will wear KATT MC lens with a modified BOZD of 5mm.

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 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, a study 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, axial and refractive maps. A study compared TZ parameters derived from the three different maps. The study defined the TZ as the area enclosed by zero power change in different subtractive maps; 12 such points equally spaced 30 degree 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. The study 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, the investigators suggested that either refractive or axial map would be of high validity and accuracy in determining TZ characteristics.

The current study would investigate the effect of TZ on myopic control in ortho-k. TZ, defined as 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.

Inclusion Criteria:

• Manifest myopia between 1.00-4.00D in both eyes at screening visit

• Manifest astigmatism ≤2.50D; with-the-rule astigmatism (axes 180 ± 30)

  • 2.50D; astigmatism with other axes ≤0.50D in both eyes at screening visit

• <1.00D difference in manifest spherical equivalent (SE) between the two eyes at screening visit

• Baseline cycloplegic objective refraction between 1.00-4.00D in sphere; astigmatism ≤2.50D; <1.00D difference in manifest SE between the two eyes

• Best-corrected logMAR visual acuity 0.10 or better in both eyes Symmetrical corneal topography with corneal toricity <2.00D in either eye

• Normal ocular health other than myopia

• Agree to be randomized and to attend the scheduled visits and aftercare

Exclusion Criteria:

• Contraindications to atropine: known allergies or cardiovascular disease, epilepsy
• Contraindications to contact lens wear and ortho-k: corneal scar, history of ocular inflammation/infection, limbus-to-limbus corneal cylinder and dislocated corneal apex
• Strabismus or amblyopia
• History of myopia control treatment (e.g. soft contact lenses, progressive add spectacles, atropine eye drops)
• Rigid contact lens (including ortho-k) wear experience
• Systemic condition which might affect refractive development (for example, Down syndrome, Marfan's syndrome)
• Ocular conditions which might affect refractive error (for example, cataract, ptosis)
• Poor response to lens wear including poor lens handling, poor vision and/ocular response after lens modifications
• Poor compliance with schedule visits