Orthokeratology Reverse Geometry Optics: Corneal Remodeling & Myopia Defocus Physics

Unlike standard RGP lenses whose curves flatten toward the edge, an Orthokeratology lens utilizes a Reverse Geometry Design:

1. Base Curve (Treatment Zone, $5.0\text{--}6.0\ \text{mm}$): Flatter than the central flat keratometry reading ($K$) by the exact amount of myopia to be corrected plus a $+0.75\ \text{D}$ Jessen factor.
2. Reverse Curve: A dramatically steeper annular zone ($5.00\ \text{D}$ to $10.00\ \text{D}$ steeper than base curve) that forms a fluid reservoir to hold displaced epithelial cells.
3. Alignment Curve: Parallels the peripheral corneal profile ($35^\circ\text{--}45^\circ$) to center the lens securely over the pupil overnight.
4. Peripheral Relief Curve: Provides edge lift to promote tear exchange beneath the lens during sleep.

A common misconception is that Ortho-K lenses 'press' or 'crush' the cornea. High-resolution OCT proves that the rigid lens never touches the central cornea:

$$\text{Sub-Lens Tear Film Pressure: } P = \frac{6 \mu U}{h^3} \left( x - x_0 \right)$$

The lens works through hydrodynamic squeeze-film fluid dynamics:

• Thin tear layers beneath the flat base curve generate positive hydrostatic compression forces, thinning the central corneal epithelium by $15\text{ to } 20\ \mu\text{m}$ (without loss of cells; cells compress and lose fluid).
• Thick tear layers beneath the steep reverse curve generate powerful negative suction forces, pulling fluid and cells into the mid-periphery, thickening the annular mid-periphery by $15\text{ to } 20\ \mu\text{m}$.

The flattening of the central cornea corrects axial distance refraction, while the mid-peripheral annular steepening creates a ring of plus power ($+2.00\ \text{D}$ to $+3.50\ \text{D}$):

• In standard single-vision glasses, off-axis peripheral light focuses behind the retina (Hyperopic Peripheral Defocus), sending molecular signals via retinal dopamine downregulation that drive pathological axial eye elongation.
• Ortho-K shifts peripheral light focal planes to fall in front of the peripheral retina (Myopic Peripheral Defocus).
• Clinical trials confirm this optical stop-signal slows pediatric axial length growth by $45\%\text{ to } 60\%$ compared to conventional spectacles.