Regularise · Chapters 10 → 11

Intracorneal rings
& CAIRS

An intrastromal insert shortens the arc and flattens the cone. The whole decision is a grammar: thickness, arc length and optical zone set the effect; implant symmetry answers cone symmetry. CAIRS replaces PMMA with tissue.

Chapter 10

Intracorneal ring segments (ICRS) — optics & sizing

10.1 Physical principle

The insert, placed in the mid-deep stroma, exerts an arc-shortening effect: it flattens the cornea opposite the segment, recentres the cone apex, reduces irregular astigmatism and the myopic spherical equivalent. The effect is additive, adjustable, reversible/explantable.

10.2 Effect rules — to internalise

Flattening effect ↑ with segment thickness.
Effect ↑ with arc length (longer arc = more flattening, over a wider sector).
Effect ↑ as the optical zone ↓ (small OZ = more effect; large OZ = gentler, more centring).
Segment placed on the steepest / thinnest meridian (at the cone axis, opposite the bulge).

10.3 Astigmatism, irregularity, SE — the decision grammar

Symmetric cone

Regular astigmatism / symmetric bowtie → two symmetric segments (same thickness/arc) → balanced flattening, SE reduction.

Asymmetric cone

Irregular astigmatism / high coma / inferiorly decentred cone → a single segment (on the bulging sector) or two asymmetric segments (different thickness/arc) to "rebalance" the asymmetry.

Spherical equivalent

The more you flatten (thickness ↑, arc ↑, OZ ↓), the more you reduce spherical myopia — size to the refractive target and the primary goal (regularisation vs myopic reduction).

Mnemonic rule

Topographic symmetry → implant symmetry; asymmetry/coma → implant asymmetry (or single segment).

10.4 Dimensions of the main ICRS (reference orders of magnitude)

Main intracorneal ring-segment models
Model	Section	Optical zone (Ø)	Arc	Thickness	Note
Keraring (Mediphacos)	triangular	5.0 mm (SI-5) or 6.0 mm (SI-6)	90-355°	150-300 µm (50 steps)	Wide range of arcs, central cones.
Ferrara Ring	triangular/prismatic	~5.0 mm	90-210°	150-350 µm	Narrow apex, marked effect.
Intacs	hexagonal	6.8-7.0 mm	150°	250-450 µm	Large OZ, more diffuse/peripheral cones.
Intacs SK	oval	6.0 mm	150°	400-450 µm	Severe keratoconus, advanced central cones.
MyoRing (Daxer)	continuous full ring	—	360° (closed)	variable (Ø 5-8 mm)	Implanted in a pocket, centring effect.

Sizing synthesis

Small OZ (5 mm) + thick/long segments = maximal effect (advanced central cones); large OZ (6.8-7 mm) = gentler, centring effect (more diffuse cones). Cone asymmetry is handled by thickness/arc asymmetry or a single segment.

10.5 Tunnels and implantation depth (PMMA)

Depth: 70-80 % of local pachymetry at the tunnel, measured at the thinnest point of the track (neither too superficial → extrusion; nor too deep → Descemet perforation).
Femtosecond creation: regular tunnel, programmed depth, entry incision on the chosen axis — far superior to the manual Suarez dissector for depth reproducibility.
Optical zone: centre the tunnel on the visual axis / pupil centre per nomogram (generally 5.0 / 6.0 / 6.8 mm depending on model).

PMMA complications

Extrusion (if too superficial), stromal melt, migration, peri-segment deposits/haze, channel neovascularisation, interface keratitis — precisely the limits that motivate CAIRS.

Chapter 11

CAIRS — Corneal Allogenic Intrastromal Ring Segments

11.1 Principle (Jacob, 2018)

Ring segments cut from allogeneic corneal stroma (de-epithelialised donor tissue), implanted in a stromal tunnel. The biological additive tissue replaces PMMA.

11.2 Advantages over PMMA

Biocompatibility: no melt, no extrusion, no migration, no deposits, near-zero neovascularisation.
More superficial implantation possible (no melt risk above the segment) → enhanced additive flattening ("tissue addition") and management of corneas where PMMA would be risky.
Additive (adds tissue volume), combinable with CXL, potentially explantable/adjustable; no permanent foreign material.

11.3 Indications

Keratoconus with irregular astigmatism and a cornea too thin or too steep for safe PMMA.
Central/paracentral cones requiring a marked effect.
Wish to avoid PMMA complications (young patients, borderline corneas).
Combination CAIRS + CXL ("CAIRS-plus"), or ± topo-guided PTK for fine regularisation.
Selected PMD (inferior segment).

11.4 Insert preparation & sizing

Allograft cutting: from a de-epithelialised donor corneal button, stromal arcs are harvested using a double trephine (two concentric diameters) or a manual/femto cut; an arc segment of chosen width and thickness is obtained.

Arc length

According to cone extent (ICRS analogue; longer arc = wider flattening).

Width

Typically around 0.5-0.9 mm (modulates the volumetric effect).

Thickness

Depends on donor-button thickness and the cut; adjustable, additive.

Optical zone / tunnel Ø

Around 5.0-6.0 mm, centred on the cone axis.

Mandatory recalibration

The exact sizing tables (width × length × tunnel Ø by K/asymmetry) vary with the preparation method (double-trephine vs free cut) and platform. Always recalibrate on the Jacob/Istanbul source table.

11.5 Tunnel & depth (CAIRS specificity)

Femtosecond tunnel as for ICRS, but a deliberately more superficial depth (typically ~50 % of stromal depth, ~40-60 % range) — the absence of melt risk allows it and enhances the surface additive effect.
Tunnel diameter / OZ adapted to segment length (often 5.0-6.0 mm).
Combined sequence: CXL usually performed after implantation (or deferred); topo-guided PTK as a second step if fine regularisation is needed.

11.6 Points of vigilance

Quality and thickness of donor tissue (cut reproducibility), dimensional segment/tunnel matching, interface (possible but usually transient haze), learning curve of graft preparation.