LT

AL

Axial length and Lens Thickness

→The longer the eye, the smaller the ratio of lens to AL, i.e.

Distance from posterior lens to retina more dominant for longer AL

-> Reduced composite refractive index for whole eye.

→Conventional optical biometer uses

composite refractive index weighing this ratio of

Lens to AL which is AL dependent

However, longer eye outside of

normal eye population are

disregarded significantly when

extrapolating with linear fit because

of small population of longer eye

and some people actually have large

ratio LT not obeying the linear trend

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

10%

15%

20%

25%

30%

35%

40%

20.00 22.00 24.00 26.00 28.00

LT [

mm

]

LT/A

L ra

tio

Axial Length [mm]

AL and LT correlation

Ratio LT Linear (Ratio)

y = -0.0011x + 1.3761

1.346

1.347

1.348

1.349

1.35

1.351

1.352

1.353

1.354

1.355

20.00 21.00 22.00 23.00 24.00 25.00 26.00

Composite Refractive index

AL [mm]

• Conventional Optical Biometer：Optical length from cornea to retina / composite refractive index (e.x. na=1.3761-0.0011xAL)

• ARGOS：Sum of all segments(CCT/1.375+AD/1.336+LT/1.41+PosLens-Retina/1.336 )

Error of +/-0.05mm(1.96SD) is added to

AL for IOL calculation.

Difference of how AL is calculated

Especially for longer eye(>26mm), conventional biometer

deviates with the overestimation of AL because of dividing by too

small refractive index even for the eye with large LT which is not

true to the physical size.

LT

ALo

pt/n

a

AD

CC

T Difference between two methods

-0.20

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

0.20

20.00 21.00 22.00 23.00 24.00 25.00 26.00 27.00 28.00

[mm]

AL [mm]

63%

85%

94%

53%

80%

96%

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

Accurate prediction without AL adjustment

Barrett Universal II

*Hill-RBF based on the curated data with Alcon SN60WF only

and generally works only with bioconvex lens(+6-+30D) type

i.e. it is not as accurate if the other model is used.

Accuracy of predicted refraction with multifocal intraocular lenses using two biometry measurement

devices and multiple intraocular lens power calculation formulas

Article · Dec 2014 · Clinical and Experimental Ophthalmology

Reitblat O1,2, Assia EI1,3,4, Kleinmann G1,2,5, Levy A1, Barrett GD6, Abulafia A1

Barrett Universal II

Considering Lens Factor for better lens position estimation

N=73 eyes

Comparison of Methods to Predict Residual Astigmatism After Intraocular Lens Implantation

Adi Abulafia, MD; Warren E. Hill, MD; Maria Franchina, MD; Graham D. Barrett, MD

Journal of Refractive Surgery

October 2015 - Volume 31 · Issue 10: 699-707

Barrett Toric Calculator

• Considering Posterior curvature of cornea

• Better prediction including Against-the-rule astigmatism

• Alcon online calculator now uses Barrett Toric

Prediction of refractive outcomes with toric intraocular lens implantation

Adi Abulafia, MD, Graham D. Barrett, MD, et al., J Cataract Refract Surg 2015; 41:936–944

Barrett Toric Calculator

No need for Baylor monogram shift for WTR, ATR astigmatism

Barrett True-K

0

20

40

60

80

100

60.3

43.1

55.248.3

67.2

84.581 82.8 81

94.8

Within ±0.5D Within±1.0D

0

20

40

60

80

100

ShammasPL Haigis-L Barrett True-K

5045.7

63.3

8076.7

80

Within ±0.5D Within±1.0D

With History of Pre-/Post-Lasik refraction

n=58 n=30

No history

% %

Accuracy of the Barrett True-K formula

for intraocular lens power prediction after

laser in situ keratomileusis or photorefractive

keratectomy for myopia

Adi Abulafia, MD, Warren E. Hill, MD, Douglas D.

Koch, MD, Li Wang, MD, PhD,

Graham D. Barrett, MD

• Based on accurate Barrett Universal II with internally

modified K value with/without refractive surgery history

Barrett True-K

n=58

n=30

Accuracy of the Barrett True-K formula

for intraocular lens power prediction after

laser in situ keratomileusis or photorefractive

keratectomy for myopia

Adi Abulafia, MD, Warren E. Hill, MD, Douglas D.

Koch, MD, Li Wang, MD, PhD,

Graham D. Barrett, MD

With History of Pre-/Post-Lasik refraction

No history