CAIRS Nomogram Review
Comparative analysis of corneal allogenic intrastromal ring segment sizing systems
Overview
Corneal allogenic intrastromal ring segments (CAIRS) use donor corneal tissue implanted into femtosecond laser-created channels to flatten the cornea in keratoconus. Three published nomogram systems guide the selection of segment dimensions and channel parameters. Each takes a fundamentally different approach to sizing.
| BRISBANE | ISTANBUL | AWWAD | |
|---|---|---|---|
| Full name | Brisbane Nomogram 2026 | Istanbul Protocol | Awwad Femto-CAIRS |
| Authors | Gunn | Kılıç (KeraNatural) | Bteich, Awwad et al. |
| Published | 2026 | KeraNatural (VisionGift) | J Refract Surg 2023 |
| Size grades | 7 | 1 | 2 |
| Sizing variable | Peak Kmax (D) | None (fixed) | ΔK threshold |
BRISBANE Brisbane Nomogram 2026
Gunn — Brisbane Nomogram 2026
Sizing Philosophy
The Brisbane nomogram uses a graded, K-stratified approach with 7 sizes spanning the full spectrum of keratoconus severity from mild forme fruste (46 D) through to extreme ectasia (>75 D). Each size grade specifies independent values for segment width, thickness, ring diameters, and channel dimensions.
The key insight is that both the segment cross-section and the channel geometry should scale together — more tissue volume for steeper corneas, delivered through progressively wider and deeper channels. The X Small size provides only 25% of the tissue volume of Large (the historical reference), while 3X Large provides 200%.
Key Features
- 7-grade system from X Small (46–48 D) to 3X Large (>75 D)
- Variable channel depth: 200–600 μm, increasing with severity
- Variable channel width: 1.10–2.40 mm, scaling with segment size
- Cross-section area ranges from 0.140 to 1.100 mm² (8× range)
- Both width and thickness vary: smaller sizes use thinner segments (200 μm) while larger sizes plateau at 550 μm
Size Selection Algorithm
| Size | Peak K Range | Area (mm²) | % of Large |
|---|---|---|---|
| X Small | 46–48 D | 0.140 | 25% |
| Small | 48–50 D | 0.280 | 51% |
| Medium | 50–55 D | 0.405 | 74% |
| Large | 55–63 D | 0.550 | 100% |
| X Large | 63–70 D | 0.688 | 125% |
| 2X Large | 70–75 D | 0.825 | 150% |
| 3X Large | >75 D | 1.100 | 200% |
Considerations: Requires accurate peak K measurement. More complex to implement clinically due to multiple parameter combinations.
ISTANBUL Istanbul Protocol
Kılıç (KeraNatural) — Istanbul Nomogram
Sizing Philosophy
The Istanbul protocol takes a fixed-width, variable-count approach. All segments have a fixed 1 mm width placed in a wide 1.75 mm channel — the only variable is whether 1 or 2 segments are implanted based on cone symmetry. This philosophy prioritises simplicity: one segment width, one channel specification.
The wide channel (1.75 mm for a 1 mm graft) allows the tissue to adopt a fusiform shape once implanted. Tissue thickness (400–650 μm) can be adjusted by the surgeon. Dose modulation comes from segment count and arc length (180° standard).
Key Features
- Fixed segment width: 1000 μm (1 mm) for all cases
- Variable thickness: 400–650 μm (surgeon-adjusted)
- Channel depth: 250 μm
- Wide channel: 1.75 mm (inner 4.0 mm, outer 7.5 mm)
- Dose modulation via count: asymmetric cone = 1, symmetric cone = 2
- Cross-section: 0.400–0.650 mm² per segment (depending on thickness)
Segment Count Algorithm
| Cone Type | Segments | Area per Segment (mm²) |
|---|---|---|
| Asymmetric cone | 1 | 0.400–0.650 |
| Symmetric cone | 2 | 0.400–0.650 each |
Considerations: Limited width titration. The same 1 mm segment width is used for all severities. Relies on thickness and count for dose adjustment.
AWWAD Awwad Femto-CAIRS
Bteich, Awwad et al. — J Refract Surg 2023;39(11):767–776
Sizing Philosophy
The Awwad system uses a two-tier approach based on ΔK (Kmax minus Kcentral). Rather than using absolute keratometry, it uses the relative steepening to determine segment size. This reflects the concept that it is the degree of ectasia (cone height relative to baseline) that matters, not the absolute corneal power.
The system uses notably smaller segments than either Brisbane or Istanbul, placed in a narrower (0.9 mm), more peripheral channel. The channel inner diameter of 6.0 mm is significantly larger than Brisbane (4.0–4.8 mm) or Istanbul (4.0 mm), placing the implant closer to the limbus.
Key Features
- Two sizes: Standard (500 μm) and Large (750 μm) width, both 500 μm thick
- ΔK-based sizing: ΔK < 6 D = Standard, ΔK ≥ 6 D = Large
- Narrow channel: 0.9 mm width (smallest of the three systems)
- Peripheral placement: inner channel 6.0 mm, outer 7.8 mm
- Deeper implantation: 250–275 μm (similar depth to Istanbul’s 250 μm)
- Small cross-sections: 0.250–0.375 mm² (smallest of the three systems)
Size Selection Algorithm
| Size | Criteria | Width (μm) | Area (mm²) |
|---|---|---|---|
| Standard | ΔK < 6 D | 500 | 0.250 |
| Large | ΔK ≥ 6 D | 750 | 0.375 |
Considerations: Smallest tissue volume of the three systems — may require longer arc lengths or paired segments to achieve equivalent flattening. The narrow channel (0.9 mm) limits maximum segment width.
Head-to-Head Comparison
Dimension Ranges
| Parameter | Brisbane | Istanbul | Awwad |
|---|---|---|---|
| Segment width (μm) | 700–2000 | 1000 | 500–750 |
| Segment thickness (μm) | 200–550 | 400–650 | 500 |
| Cross-section area (mm²) | 0.140–1.100 | 0.400–0.650 | 0.250–0.375 |
| Inner ring ∅ (mm) | 4.4–5.2 | 4.0 | 6.53–6.65 |
| Outer ring ∅ (mm) | 6.6–8.4 | 7.5 | 7.15–7.28 |
| Channel inner ∅ (mm) | 4.0–4.8 | 4.0 | 6.0 |
| Channel outer ∅ (mm) | 7.0–8.8 | 7.5 | 7.8 |
| Channel width (mm) | 1.10–2.40 | 1.75 | 0.90 |
| Channel depth (μm) | 200–600 | 250 | 250–275 |
Clinical Scenario Comparison
What each nomogram prescribes for common clinical scenarios:
| Scenario | Brisbane | Istanbul | Awwad |
|---|---|---|---|
| Mild (K=48 D, ΔK=3) | X Small 700×200 μm 0.140 mm² |
×1 (asym) 1000×500 μm 0.500 mm² |
Standard 500×500 μm 0.250 mm² |
| Moderate (K=55 D, ΔK=5) | Large 1000×550 μm 0.550 mm² |
×1–2 1000×500 μm 0.500–1.00 mm² |
Standard 500×500 μm 0.250 mm² |
| Advanced (K=65 D, ΔK=8) | X Large 1250×550 μm 0.688 mm² |
×2 (sym) 1000×500 μm 1.000 mm² |
Large 750×500 μm 0.375 mm² |
| Severe (K=75 D, ΔK=12) | 3X Large 2000×550 μm 1.100 mm² |
×2 (sym) 1000×650 μm 1.300 mm² |
Large 750×500 μm 0.375 mm² |
Key Design Differences
1. Sizing Strategy
Brisbane uses peak K as the primary sizing variable. This directly correlates with disease severity and provides the finest-grained dose titration (7 levels). The assumption is that steeper corneas need proportionally more tissue volume.
Awwad uses ΔK (Kmax − Kcentral), arguing that the relative cone prominence matters more than the absolute power. A naturally steep cornea at 48 D with 3 D of ectasia should be treated differently from a 48 D cornea that was originally 45 D.
2. Channel Geometry
The three systems differ dramatically in channel placement:
| Aspect | Brisbane | Istanbul | Awwad |
|---|---|---|---|
| Inner edge ∅ | 4.0–4.8 mm | 4.0 mm | 6.0 mm |
| Approach | Central/paracentral | Central | Peripheral |
| Tissue width | 1.1–2.4 mm | 1.0 mm | 0.9 mm |
Brisbane and Istanbul place segments centrally (inner diameter 4.0–4.8 mm), spanning the cone apex. Awwad places segments more peripherally (inner diameter 6.0 mm), acting as a reinforcing belt outside the optical zone. These represent fundamentally different biomechanical strategies.
3. Tissue Volume Range
The total tissue volume delivered per segment varies enormously:
- Awwad smallest: 0.250 mm² cross-section
- Brisbane X Small: 0.140 mm² (smallest overall, but ranges up to 1.100 mm²)
- Istanbul standard: 0.400–0.650 mm² (variable by thickness)
For a moderate case (K = 55 D), Brisbane prescribes 0.550 mm², Istanbul prescribes 0.500 mm² (similar), and Awwad prescribes 0.250 mm² (55% less). Istanbul can increase dose by using thicker tissue (up to 0.650 mm²) or adding a second segment.
4. Depth Strategy
Channel depth affects how the implant interacts with the corneal stroma:
- Istanbul: Fixed 250 μm — shallow, simple
- Awwad: 250–275 μm — similar depth, consistent
- Brisbane: 200–600 μm — variable, deepest for larger sizes. Deeper channels place more corneal tissue above the implant, potentially providing greater surface flattening effect
References
- Gunn D. Brisbane Nomogram 2026 for CAIRS segment sizing. Unpublished clinical protocol, 2026.
- Kılıç A. Istanbul Nomogram for KeraNatural CAIRS procedures. VisionGift / KeraNatural, 2025.
- Bteich S, Awwad ST, et al. Femtosecond laser-assisted corneal allogenic intrastromal ring segments implantation for keratoconus. J Refract Surg. 2023;39(11):767–776.
- Jacob S, Patel SR, Agarwal A, et al. Corneal allogenic intrastromal ring segments (CAIRS) combined with corneal cross-linking for keratoconus. J Refract Surg. 2018;34(5):296–303.
- Zhu K, Parker JS, Melles GRJ. Alternative keratoconus treatment: corneal allogenic intrastromal ring segments. Asia Pac J Ophthalmol. 2022;11(6):497–504.