The aims of this study are to investigate the effects of ortho-k for astigmatic and myopic reduction and myopic control in children, and the long term effects on corneal curvatures and biomechanics.

Ortho-k has been shown to be effective in correcting low myopia but relatively ineffective for astigmatism, using spherical reverse geometry lens designs. Toric ortho-k lenses have been introduced in recent years but the efficacy for astigmatic reduction and for myopic control in children have not been confirmed. The mechanism of myopic reduction in ortho-k cannot be fully explained by changes to the anterior corneal curvatures. It is therefore possible that other corneal parameters such as posterior corneal curvature and corneal biomechanics may contribute to the mechanism.

The current study aims at investigating the efficacy of toric ortho-k lenses for correcting myopic astigmatism and for retarding myopic progression in children compared to children wearing single-vision spectacles. Long term changes to other corneal parameters such as posterior cornea curvatures, topographical corneal thickness, corneal hysteresis, corneal resistance factor, with and without ortho-k lens wear will also be investigated.

Inclusion Criteria:

• Myopia (refractive sphere): more than -0.50DS to -5.00DS
• Astigmatism: with-the-rule astigmatism more than -1.25DC
• Anisometropia: ≤ 1.50D in both refractive sphere
• Best corrected monocular visual acuity: equal to or better than 0.10 in logMAR scale in both eyes
• Availability for follow-up for at least 2 years

Exclusion Criteria:

• Strabismus at distance or near
• Contraindication for contact lens wear and orthokeratology (e.g. limbus to limbus corneal cylinder and dislocated corneal apex
• Prior experience with the use of rigid lenses (including orthokeratology)
• Prior experience with myopia control treatment (e.g. refractive therapy or progressive spectacles)
• Systemic or ocular conditions which may affect contact lens wear (e.g. allergy and medication)
• Systemic or ocular conditions which may affect refractive development (e.g. Down syndrome, ptosis)