Orthokeratology (Ortho-K) vs. Defocus Spectacles: Corneal Reshaping vs. Non-Invasive Optics

Ortho-K lenses do not permanently 'cure' myopia; they exert gentle hydraulic fluid forces beneath a high-Dk reverse-geometry contact lens while the child sleeps:

1. Central Epithelial Thinning: The central corneal epithelium thins by 15 to 20 microns, flattening central corneal curvature ($K_1$) to correct daytime distance vision.
2. Mid-Peripheral Epithelial Thickening: Displaced epithelial cells migrate outward, creating a ring of increased tissue thickness in the mid-periphery.

This mid-peripheral ring functions as an optical zone of relative myopic defocus, mimicking the optical mechanism of DIMS spectacle lenses.

The decisive clinical drawback of Ortho-K in children is corneal infection risk:

$$\text{Microbial Keratitis Incidence: } \approx 13.9 \text{ per } 10,000 \text{ patient-years in pediatric Ortho-K}$$

Children frequently touch lenses with unwashed hands, rinse cases with tap water, or rub their eyes during sleep. A severe bacterial or amoebic corneal ulcer can cause permanent central scarring, irreversible loss of best-corrected visual acuity, or require corneal transplantation.

Multiple head-to-head meta-analyses demonstrate that DIMS and H.A.L.T. defocus spectacle lenses achieve 55% to 65% axial elongation slowing—statistically indistinguishable from Ortho-K—with zero risk of corneal infection. For the vast majority of children, non-invasive defocus spectacles represent the superior safety-first standard of care.