1. Deepak Vayalambrone FRCOphth
BIOMETRY
Deepak Vayalambrone FRCOphth

2. it can be a bit overwhelming
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3. NOVEMBER 29 1949 St. Thomas Hospital London
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4. IOL as thick as natural lens Power of 24 D in water
First ever IOL implant
In a 49 y.o. nurse
IOL as thick as natural lens
Power of 24 D in water
Resultant myopia of -18 D
Due to higher R I of Perspex CQ.
Biometry?
‘Ridley met John Pike, an optical scientist with Rayner in a parked car and ‘discussed’ the Implant’
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5. Stewart Duke-Elder ‘’an ophthalmic surgeon's job is to take material out of the eye, not to put it in’’ Harold Ridley "I am the only man to have invented his own operation"
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6. Basic refraction based formula
IOL power = x refraction.
i.e. Refraction = 0  IOL power 18 D.
Binkhorst RD. Ophthalmic Surg Fall;7(3):69-82
‘’The rule is simple......careful history and an examination of old glasses and past refraction When the basic refraction cannot be ascertained, capital errors may occur.’’
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7. Lens power tables
‘’Ophthalmologists who do not own a personal computer will find the lens power table a valuable adjunct to their present repertoire of analytical skills’’.
Estimates of primary implant power using an intraocular lens table.
J Cataract Refract Surg Jul;12(4):401-7
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8. The human eye Geometric thin lens optics not applicable
Lens does not have a uniform RI
Variable lens position
Interplay of AL, K, Lens Power, RI of lens.
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9. The human eye
42 D
24 mm
18 D
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10. A few words on terminology
Axial length
Keratometry
Anterior Chamber Depth
Effective Lens Position
Position of principal plane of IOL behind cornea
1st gen- constant of 4 mm
2nd gen- AL as a scaling factor Binkhorst
3rd gen- AL and K
4th gen- AL, K, pre-op ACD and LT
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11. Standardised pseudophakic eye
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12. Theoretical formulae
IOL POWER = Required vergence in the plane of the IOL minus Effective power of the cornea in the same plane
Problems
Prediction of the Lens position
Theoretical ‘thin lens’
Inaccuracy of Keratometry
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13. Theoretical formulae Thijssen Colenbrader Fyodorov Van der Heijde
Binkhorst
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14. Regression formulae Post-op refractive results analysed
Correction factors applied
SRK formula
IOL power= A- 2.5L – 0.9K
A- constant reflecting and affected by
position of lens
angulation of haptics
Lens shape
L- axial length
K- Corneal power
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15. II generation formulae
AL and IOL position related
SRK-2 and Binkhorst.
Correction for longer and shorter eyes.
Axial lengths
< 20mm Al = A + 3
20mm to 21 mm Al = A + 2
21 mm to 22 mm Al = A + 1
22 mm to 24.5 mm Al = A >24.5 mm Al = A - 0.5
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16. Koch suggested classification of IOL formulae1
Historical
Refraction based 2
Regression analysis based
SRK/ SRK II/ BINKHORST 3
Vergence formula based
2 variable
Holladay 1, SRK-T, Hoffer-Q
3 variable
Haigis, Ladas super formula
5 variable
Barett Universal 2
7 variable
Holladay-2 4
AI based
Hill RBF, Clarke neural network 5
Ray tracing
Okulix, Phacoptix

17. 9 types of eyes* AXIAL LENGTH SHORT NORMAL LONG ANT. SEG SIZE LARGE
Megalocornea +
Axial Hypermetropia 0%
Megalocornea 2%
Large eye
Buphthalmos
Megalocornea+ axial myopia
10%
Axial hypermetropia
80%
Normal
96%
Axial myopia
90%
SMALL
Small eye
Nanophthalmos
20%
Microcornea
Microcornea+ axial myopia
*Essential optics for the cataract and refractive surgeon. I ed JT Holladay
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18. Current state of play
SRK-T is the most widely used (RCOphth recommended)
Hoffer –Q for eyes shorter than 22 mm
Barrett Universal 2
Good agreement with SRK- T when 3 variables used
Changes when 5 variables are used
Hill RBK
Wang-Koch correction for high axial lengths
Haigis-L for post LASIK eyes
85% of eyes expected to be within 1 D SE

19. Axial length measurement
Ultrasonography
Cornea to ILM- anatomic AL
Partial coherence interferometry
Cornea to photoreceptor- optical AL
Swept source OCT
Optical low coherence reflectometry

20. Keratometry Measure the anterior cornea
Keratometers have a ‘central scotoma’ of 3.2 mm
Post refractive surgery patients need correction
If entering manually in the IOL Master-
Check RI that the Keratometer uses.
Contact lenses
Discontinue RGP for 3 weeks
Discontinue soft lenses for 1 week
1 D error  nearly 1 D error in post op ref
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21. It’s a bit like choosing a cereal…..
IOLMaster 500
IOLMaster 700
AL scan
Lenstar LS900
Aladdin/Aladdin LT
Argos (Movu)
Galilei G6

22. IOL MASTER 500 Gold standard of biometry
Use 780 nm semiconductor diode laser- PCI
Interference at tear film- cornea interface
Interference at RPE
AL accurately to within ±0.02 mm
Inability to measure through dense cataract
Tear film abnormalities can cause problems

23. IOL master 700 SS OCT - 2000 scans/s.
Can identify unusual ocular geometry
(e.g., crystalline lens tilt/decentration)
More accurate than IOLmaster 500
Fixation check using the foveal pit
The fixation check helps identify macular pathologies
Telecentric K and distance-independent
Restless patients, pupil size independent.
Can perform biometry even through dense cataracts

24. Lenstar OLCR Superluminescent diode laser (820 nm)
Keratometry- dual zone 1.63mm and 2.3 mm
T cone – topography using Placido disc
measured at 32 points
Directly measures ACD
Measures retinal thickness
Hill RBF included
Eye tracking
Cannot measure through dense cataract
AL range only upto 32 mm

25. Special circumstances
Every other eye!

26. Refractive surgery-1 Standard ‘K’
3.2 mm ‘central scotoma’
Measures outside central ablation zone
Corneal standardised index of refraction (n=1.3375) invalid
1 - 2 r
RK flattens both post and ant curves
Ablative flattens ant curve
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27. Refractive surgery-2 Historical ‘K’
Spectacle correction
Pre- LASIK
42.0
− 11.25
Post- LASIK
36.0
− 2.25
Assuming a vertex distance of 12 mm.
Pre LASIK refraction at corneal plane= −9.91 D
Post LASIK refraction at corneal plane= −2.19 D
Amount of correction
−2.19 − (− 9.91) = − = 7.72 D
Corrected K  42.0 – 7.72 = D.
Source: Intraocular Lens Power Calculations H. John Shammas
20/02/2019

28. Refractive surgery-3 Historical RefractionK
Spectacle correction
Pre- LASIK
NOT AVAILABLE
− 11.25
Post- LASIK
36.0
− 2.25
Assuming a vertex distance of 12 mm.
Pre LASIK refraction at corneal plane= −9.91 D
Post LASIK refraction at corneal plane= −2.19 D
Amount of correction
−2.19 − (− 9.91) = − = 7.72 D
Corrected K
Post LASIK K − (0.23 x Correction at cornea)
=36.0 – (0.23 x 7.72)= D
Source: Intraocular Lens Power Calculations H. John Shammas
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29. Refractive surgery-4 Clinically derived method
Regression formula
Shammas-2, Haigis-L
K = (1.14 x KpostPRK) – 6.8
Used when historical data not available
Disadvantage
Does not relate to amount of correction by refractive surgery
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30. Refractive surgery-5 Contact lens methodK
Spectacle correction
Pre- LASIK
NOT AVAILABLE
Post- LASIK
36.0
− 2.25
Refract eye
Fit plano Rigid CL with known BC e.g. 41D
Refract e.g. −8.75 D
Difference added algebraically to BC
41+ [−8.75 D-(-2.25)]= 34.50D
Source: Intraocular Lens Power Calculations H. John Shammas
20/02/2019

31. Refractive surgery-6 Other possible options
Nomogram based assessment *
63% within 0.5D
84% within 1D
100% within 1.5D
Better than ‘Historical method’
but..... Data from 19 eyes
Lenstar
Probably the best currently
IOL master 700
Pentacam
With correcting software
Orbscan II
Measures central1.5 mm
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32. Penetrating keratoplasty
Triple procedure not ideal.
Keratoplasty + Cataract extraction
Planned Aphakic IOL later.
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33. PIGGY BACK IOL MYOPIC ERROR HYPERMETROPIC ERROR
Implant power same as error
HYPERMETROPIC ERROR
Implant power 1.5 x error

34. Post vitrectomy Convex- plano PMMA lenses better? *117 eyes
Biconvex IOLs could lose 1/3- ½ refractive effect
*117 eyes
85% within 1 D
Retro silicone space important if AL in supine position
Myopic shift (Not OIL filled eyes)
Ranges to from predicted refraction
*Graefe's Archive for Clinical and Experimental Ophthalmology Vol 243;10 / 2005
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35. Post Vitrectomy eyes Silicone oil Additional IOL power
Change in AL on USG due to change in velocity
Not an issue with LI techniques
Hypermetropic shift with retained oil
Degree of shift varies depending on IOL design
Additional IOL power
(Ns - Nv) / (AL - ACD) x 1,000
Ns – RI of Silicone oil.(14034)
Nv- RI of vitreous. (1.336)
3.0 – 3.5 D additional power usually
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36. Royal college guidelines
SRK/T and Holladay recommended
SRK-1/ SRK-2 not to be used
Post refractive surgery
Corrected ‘K’ by one of the methods
Use highest IOL power obtained
Axial length
Formula
<22 mm
Hoffer Q or SRK/T mm
SRK/T or Holladay or Haigis
>24.6 mm
SRK/T
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37. And finally Check the glasses! Surgeons need to customise constants
Do focimetry on both lenses
Surgeons need to customise constants
For each IOL that they use
Standard surgical technique
Rhexis
less than IOL size
round
Well centred
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38. Useful material http://www.doctor-hill.com
Intraocular lens power calculations (Slack)
H J Shammas.
Royal College guidelines
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39. Thank you
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