Intacs & Ferrara Rings: Intrastromal Corneal Ring Segments (ICRS) Geometrical Optics

The optical mechanism of intrastromal rings is rooted in the Barraquer Thickness Law formulated in 1964:

'When material is added to the periphery of the cornea or subtracted from the center, a flattening effect is produced. When material is added to the center or subtracted from the periphery, a steepening effect is produced.'

By inserting rigid PMMA spacers into the mid-peripheral stroma (at a 5.0 mm to 6.0 mm chord diameter), the overlying anterior corneal lamellae are placed under tensile stretch. This pulls the bulging central cone backward, inducing an immediate central corneal flattening of $2.00\text{ to } 7.00\ \text{Diopters}$ ($K_{\max}$ reduction).

While all ICRS implants flatten the cornea, their geometric cross-sections serve different clinical goals:

• Intacs (Hexagonal Cross-Section): Feature a wide, gentle $1.06\ \text{mm}$ base with a hexagonal profile. Designed for a 6.0 mm optical zone, Intacs provide broad, symmetric flattening ideal for central cones.
• Ferrara Rings & Kerarings (Triangular Prismatic Cross-Section): Feature a sharp triangular base ($0.6\text{--}0.8\ \text{mm}$) placed at a tighter 5.0 mm optical zone. The apical prism reflects incoming peripheral light rays away from the visual axis, drastically reducing night-vision glare and halos while exerting intense focal flattening on asymmetric inferior cones.

Historically, channels were carved manually using mechanical dissection dissectors, carrying high risks of channel misplacement or anterior perforation. Modern surgery utilizes infrared femtosecond lasers ($\approx 1053\ \text{nm}$):

$$\text{Channel Depth: Strict } 75\%\text{ to } 80\% \text{ of local pachymetry } (\pm 5\ \mu\text{m})$$

Creating channels with micron-level precision prevents implant shallowing, extrusion, and late epithelial ulceration.