Orthokeratology (Ortho-K) Reverse-Geometry Lenses: Epithelial Hydraulic Redistribution & Peripheral Myopic Defocus Ring

Unlike traditional RGP lenses that parallel corneal contour, Ortho-K lenses feature a revolutionary Reverse-Geometry profile:

1. Base Curve (Central Optical Zone, $5.0\text{--}6.0\ \text{mm}$): Flatter than central corneal $K$ by the exact amount of myopia to be corrected plus a $+0.75\ \text{D}$ 'Jessen factor'.
2. Reverse Curve ($0.5\text{ to } 1.0\ \text{mm}$ wide): Crucially steeper than the base curve by $5.00\text{ to } 10.00\ \text{D}$, creating a deep annular reservoir for fluid and displaced cells.
3. Alignment Curve: Parallels the peripheral cornea to seal the tear reservoir and center the lens.
4. Peripheral Curve: Flared edge lift for tear exchange and debris clearance.

Ortho-K lenses do NOT physically press or crush corneal tissue. Instead, reshaping is governed by squeeze-film fluid dynamics within the post-lens tear layer ($h(r)$):

$$P_{\text{hydraulic}} \propto \frac{\mu}{h^3(r)} \cdot \frac{\partial h}{\partial t} \implies \begin{cases} \text{Thin Central Tear Layer } (5\ \mu\text{m}): & \mathbf{\text{Positive Compressive Force (+)}} \\ \text{Deep Reverse Curve Reservoir } (60\ \mu\text{m}): & \mathbf{\text{Negative Suction Force (-)}} \end{cases}$$

Overnight, positive pressure squeezes intercellular water out of central epithelial cells (thinning the apex by $15\text{ to } 20\ \mu\text{m}$), while negative suction draws cells into the mid-periphery (thickening the annular ring by $+10\text{ to }+15\ \mu\text{m}$).

Standard single-vision glasses correct central vision but push peripheral light rays behind the curved retina (Peripheral Hyperopic Defocus), stimulating the eye to stretch longer. Ortho-K's mid-peripheral steep ring refracts peripheral rays in front of the peripheral retina (Peripheral Myopic Defocus), eliminating the anatomical stimulus for axial growth.