Keratoconus classification rules, nomograms, and sizing algorithms
Keratoconus Subtypes
Pattern
Description
Peaks (2-4mm)
Segments
Taper
CROISSANT
Symmetric inferior steepening, crescent-shaped. Most common (~60%)
1 peak, width ≥90°, symmetric falloff
2 symmetric
No
DUCK
Asymmetric oval inferior steepening, peak offset within steepened arc
1 peak, width ≥90°, asymmetric falloff
1 or 2 asymmetric
Yes
SNOWMAN
Two-lobed, one dominant lobe (>2D difference between peaks)
2 peaks, unequal magnitude
2 asymmetric
Variable
NIPPLE
Small central steep zone (<5mm diameter), round
1 peak, width <90°
2 symmetric or full ring
No
BOWTIE
Symmetric two-lobed pattern, peaks ~180° apart
2 peaks, similar magnitude (within 2D)
2 symmetric
No
Classification Decision Tree
Based on sagittal keratometry peaks in the 2.0–4.0mm zones (weighted toward 2.5–3.5mm)
Count peaks above 44D
0 peaks →SUBCLINICAL / NO CAIRS
1 peak
Width <90° →NIPPLE
Width ≥90°, symmetric falloff →CROISSANT
Width ≥90°, asymmetric falloff →DUCK
2 peaks
Similar magnitude (within 2D) →BOWTIE
One dominant (>2D difference) →SNOWMAN
Taper Rule
Compare the slope of K falloff on each side of the peak within the steepened arc.
If the ratio of slopes exceeds 1.5:1, the CAIRS segment tapers toward the gentler slope
(more tissue volume at the steeper side, less at the flatter side).
If slopes are similar (≤1.5:1), the segment has uniform thickness.
Brisbane Nomogram 2026
Dimensions define femtosecond laser channel and CAIRS segment parameters.
Femto Cut Dimensions
X Small
Small
Medium
Large
X Large
2X Large
3X Large
Segment width (μm)
700
800
900
1000
1250
1500
2000
Segment thickness (μm)
200
350
450
550
550
550
550
Cross-section area (mm²)
0.140
0.280
0.405
0.550
0.688
0.825
1.100
Compared to Large
25%
51%
74%
100%
125%
150%
200%
Ring Settings
X Small
Small
Medium
Large
X Large
2X Large
3X Large
Inner ring diameter (mm)
5.2
5.0
5.0
5.0
4.8
4.6
4.4
Inner ring depth (μm)
250
350
450
600
600
600
600
Outer ring diameter (mm)
6.6
6.6
6.8
7.0
7.3
7.6
8.4
Outer ring depth (μm)
200
300
400
550
550
550
550
Resulting ring width (mm)
0.70
0.80
0.90
1.00
1.25
1.50
2.00
Channel Settings
X Small
Small
Medium
Large
X Large
2X Large
3X Large
Inner channel diameter (mm)
4.8
4.6
4.5
4.4
4.2
4.1
4.0
Outer channel diameter (mm)
7.0
7.1
7.3
7.5
7.8
8.1
8.8
Channel width (mm)
1.10
1.25
1.40
1.55
1.80
2.00
2.40
Platform length
2.0
2.0
2.0
2.0
2.0
2.0
2.0
Platform width
1.2
1.4
1.5
1.7
1.9
2.1
2.1
Channel–implant diff
0.40
0.45
0.50
0.55
0.55
0.50
0.40
Keratometry Guide
Size
K Range (D)
X Small
46 – 48
Small
48 – 50
Medium
50 – 55
Large
55 – 63
X Large
63 – 70
2X Large
70 – 75
3X Large
>75
Istanbul Nomogram
Fixed-dimension approach using uniform segment parameters for all grades of keratoconus.
Segment number varies by cone symmetry. Arc length 180°.
Kılıç (KeraNatural) — Istanbul Nomogram
Segment Dimensions (Fixed)
Parameter
Value
Segment width (μm)
1000
Segment thickness (μm)
400–650
Cross-section area (mm²)
0.400–0.650
Inner channel diameter (mm)
4.00
Outer channel diameter (mm)
7.50
Channel width (mm)
1.75
Channel depth (μm)
250
Segment Count by Cone Type
Cone
Segments
Asymmetric cone
1
Symmetric cone
2
Philosophy: Simplifies planning by using a single segment width (1 mm) for all cases
in a wide 1.75 mm channel. The wider pocket allows the graft to adopt a fusiform shape.
Variation comes from the number of segments (1 vs 2) based on cone symmetry, and
tissue thickness (400–650 μm) can be adjusted by the surgeon.
Awwad Femto-CAIRS Nomogram
Two-size system based on ΔK (Kmax − Kcentral) threshold.
Uses a narrower channel with deeper implant placement.
Bteich, Awwad et al. — J Refract Surg 2023;39(11):767–776
Segment Dimensions
Standard
Large
Indication
ΔK < 6 D
ΔK ≥ 6 D
Segment width (μm)
500
750
Segment thickness (μm)
500
500
Cross-section area (mm²)
0.250
0.375
Inner ring diameter (mm)
6.65
6.53
Outer ring diameter (mm)
7.15
7.28
Channel inner diameter (mm)
6.0
Channel outer diameter (mm)
7.8
Channel width (mm)
0.90
Channel depth (μm)
250–275
Philosophy: Uses smaller cross-section segments placed in a narrower, more peripheral channel.
Sizing is based on ΔK (> 6D = large, ≤ 6D = standard) rather than absolute K.
The deeper implant placement (250–275 μm) and narrower channel (0.9 mm) differ
significantly from the Brisbane and Istanbul approaches.
Nomogram Comparison
Key differences between the three CAIRS nomogram systems