Orthokeratology (ortho-k) is a clinical technique that uses reverse geometry rigid gas permeable contact lens exerting positive pressure on the central cornea to temporary reduce refractive error. Researchers have shown that this treatment is effective for myopia control in low to high myopes, with and without astigmatism. Most designs of ortho-k lenses in the market are fitted based on the Jessen formula. The compression factor was introduced to compensate for the regression of the ortho-k effect during the no lens-wear period, so that the wearer can obtain clear distance vision throughout the day and most lens designs use a compression factor of 0.50-0.75 D. However, in a retrospective study (mixed brands of ortho-k lenses), it showed that most patients did not achieve an over-correction of 0.75 D. In order to achieve an over-correction of 0.75 D, an extra flattening power of about 1.50 D instead of 0.75 D should be be targeted. The increased compression factor is expected to increase the target reduction and it may play a role in myopic control and providing a higher successful rate in fitting ortho-k lenses.

Subjects were fitted with ortho-k lenses of conventional (0.75 D) and increased (1.75 D) compression factor. The laterality of the compression factor for each subject was randomised.

Subjects were instructed and trained with proper lens handling and disinfection procedures. Lenses were given to the subjects only when they demonstrated proper techniques.

All subjects were required to attend regular follow-ups (baseline, first overnight, and weekly over one-month period). The follow-ups were scheduled (except for the first overnight which was scheduled in the early morning) at a similar time to the baseline visit (+/- 2 hours) to minimise any potential influence of diurnal variation on ocular biometrics.

Additional unscheduled visits were provided when necessary to ensure good ocular health and vision throughout the study period.

Inclusion Criteria:

1. 6 to 10 years old
2. Myopia: between 0.50 D and 4.00 D in both eyes
3. Astigmatism: <1.50 D; ≤ 1.25 D for with-the-rule astigmatism (axes 180 ± 30); ≤ 0.50 D for astigmatism of other axes in both eyes
4. Anisometropia: ≤ 1.50 D
5. Symmetrical corneal topography with corneal toricity <2.00 D in both eyes
6. Agree for randomization

Exclusion Criteria:

1. Contraindications for orthokeratology wear (e.g. limbus-to-limbus corneal cylinder and dislocated corneal apex)
2. Any type of strabismus or amblyopia
3. Myopic treatment (e.g. refractive surgery and progressive lens wear for myopic control) before and during the study period
4. Rigid contact lenses (including orthokeratology lenses) experience
5. Systemic condition which might affect refractive development (for example, Down syndrome, Marfan's syndrome)
6. Ocular conditions which might affect the refractive error (for example, cataract, ptosis)
7. Poor compliance for lens wear or follow-up

Data of subjects who completed the study were reported

one month