keratometry, a scan, iol calculation

1. BIOMETRY
DR BINESH TYAGI

2. INTRODUCTION
• Measuring various dimensions of the eye, its
components and their interrelationships, and
using this data to determine the ideal intraocular
lens power.
• Essentially consists of a keratometric reading
together with an ultrasonic measurement of axial
length of the eye.
2

3. CLAIMS RELATING TO BIOMETRY ERRORS/WRONG
INTRAOCULAR LENS POWER WERE THE SECOND
MOST FREQUENT CAUSE OF CLAIMS AND RESULT IN
PAYMENT OF DAMAGES IN 62% OF CLOSED CASES ! ! !
!
*CAUSES OF CATARACT SURGERY MALPRACTICE CLAIMS IN ENGLAND 1995–2008. NADEEM ALI,
BRIAN C LITTLE BR J OPHTHALMOL DOI:10.1136/BJO.2010.182774

4. KERATOMETRY
• Used to measure the corneal curvature.
• Done before axial length measurement.
• Types of keratometer:
1. Manual keratometer
2. Auto keratometer, keratometers incorporated in IOL master
and lenstar
3. Topography – placido disc based or elevation based
topography

5. MANUAL KERATOMETRY
• PRINCIPLE
1. FIXED OBJECT AND VARIABLE IMAGE SIZE
2. FIXED IMAGE AND VARIABLE OBJECT SIZE
• TWO TYPES
1. BAUSCH AND LOMB
2. JAVAL SCHIOTZ

6. AUTOMATED KERATOMETER
• Principle- focuses the reflected corneal image on to an
electronic photosensitive device, which instantly records
the size and computes the radius of curvature.
• Zone of measurement- central 3mm zone.

7. AXIAL LENGTH MEASUREMENT
• 2 TYPES
1. OPTICAL
2. ULTRASOUND

8. ULTRASOUND BASED A-SCAN
• PRINCIPLE- the ultrasound probe has a piezoelectric crystal that
electrically emits and receive high frequency (10Mz) sound
waves.
• Measurement is from anterior corneal surface to internal
limiting membrane.
• 1 mm error leads to 2.5D error in postoperative refraction
• 50% of a surgeon’s post operative surprises are A-scan errors
(Olsen)
• Errors of 2.00D or more are almost always A-scan related
(Holladay)

9. TIPS FOR ACCURATE MEASUREMENT OF AXIAL LENGTH
(USING APPLANATION):
• Ensure the machine is calibrated and set for the correct
velocity setting
• The echoes from cornea, anterior lens, posterior lens, and
retina should be present and of good amplitude
• Average the 5–10 most consistent results giving the lowest
standard deviation (ideally < 0.06 mm)
• Misalignment along the optic nerve is recognized by an
absent scleral spike.
• The gain should be set at the lowest level at which a good
reading is obtained
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10. 10

11. • High quality contact A-scan of the phakic eye.
•5 high-amplitude spikes
•Steeply rising retinal spike, as well as the good resolution of
the separate retinal and scleral spikes.
•Little or no “stair steps” in leading edge of retinal echo.

12. • Higher the gain, better the sensitivity, but the resolution gets
compromised
• At low gains the sensitivity is less, but the resolution is good.
• Use of gain in difficult situations- gain refers to electronic
amplification of the sound waves received by the transducer.
• Increase in gain is required when height of echoes achieved is
inadequate as in dense cataracts.
• Decrease in gain is required when artefacts are seen near the retinal
echoes as in silicone filled eyes, pseudophakic eyes.

13. IMMERSION TECHNIQUE
• Time consuming, expensive, messy and requires the
patient to be supine.
• The immersion technique minimizes technician variables
leading to more reproducible results
• Can be performed in the standard ophthalmic chair
reclined at 45°
• Ossoinig shell / prager shell

15. Immersion scan of the phakic eye.
The corneal spike demonstrates 2 peaks, representing the epithelium and
endothelium.

16. LIMITATIONS OF IMMERSION
 After a single immersion biometry measurement, 18 of 34 samples
(53%) grew organisms from either the probe/shell or the tubing.
 Positive cultures were found in 32% of the immersion shell/probes
(11 of 34) and in 31% of the infusion tubing samples (10 of 32).*
 Recommendations: The shell and probe should be soaked in alcohol
or hydrogen peroxide for at least 5 minutes. The immersion shell
should be allowed to dry completely and flushed with balanced saline
solution (BSS).
*VELAZQUEZ-ESTADES LJ, WANGER A, KELLAWAY J, ET AL. MICROBIAL CONTAMINATION OF IMMERSION BIOMETRY ULTRASOUND EQUIPMENT.
OPHTHALMOLOGY 2005;112: E13–8.

17. A scan Biometry in
Special Cases

18. 1.Inadequate Patient Fixation
 Low Vision
 Nystagmus
 Blepharospasm
 Strabismus

19. • Causes an irregular shape of
the ocular wall
• Inability to display a distinct ,
high retinal spike
• Deepest portion of the
staphyloma may be located
eccentric to macula, thus the
measurement may be longer
than true AL along the visual
axis.
2. Posterior
Staphyloma

20. RD
Edema
Tumor
3. Macular
Lesions
An elevated macular lesion
may prevent the display of
a distinct retinal spike and
often causes a shortened
AL measurement.

21.  Asteroid Hyalosis
 Vitreous Hemorrhage
 Gas Bubble
4. Vitreous Lesions

22. 5. Dense Cataract
 Strong sound attenuation can
significantly impair the ability to
display spikes from the various
interfaces.
 Maximum gain setting may be
required to obtain spikes of
sufficient height from the posterior
lens capsule and retina.

23. 6. Silicone Oil
Sound velocity in silicone oil
1040 m/s 5000 cs
980 m/s 1000 cs
This low sound velocity can result in
pronounced sound attenuation and
difficulty in identifying the retinal spikes .

24. OPTICAL BIOMETER
• PRINCIPLE OF IOL MASTER – Based on
‘partial coherence interferometry (PCI)’. Diode laser (780nm)
measures echo delay and intensity of infrared light reflected
back from tissue interfaces– cornea & RPE.
• PRINCIPLE OF LENSTAR – Based on ‘low coherence
optical reflectometry (LCOR)’. Superluminescent
Diode laser of 820nm is used.

25. • IOL MASTER –
1. NO OF POINTS TESTED – 6 POINTS IN HEXAGONAL
PATTERN
2. ZONE OF CORNEA TESTED – DIAMETER OF 2.3MM
• LENSTAR –
1. NO OF POINTS TESTED – 32 POINTS IN TWO
CIRCLES (16 EACH)
2. ZONE OF CORNEA TESTED – INNER CIRCLE
DIAMETER – 1.65MM
OUTER CIRCLE DIAMETER – 2.3MM
BETTER IN TERMS OF MEASURING TRUE
CENTRAL CORNEAL POWER

26. • ADVANTAGES OF LENSTAR OVER IOL MASTER –
• Lenstar measures ACD using optical biometry as compared to
IOLMaster which measures ACD using slit imagery.
• Measures ACD from posterior surface of cornea which is
important in short anterior chambers and thicker corneas.
• Takes two sets of readings so covers more central area.
• Pachymetry, Pupillometry, Retinal Thickness.
Hill W, Angeles R, Otani T. Evaluation of a new IOLMaster algorithm to measure axial length
J Cataract Refract Surg 2008;34:6:920-4.

27. • ADVANTAGES OF IOL MASTER 700 OVER LENSTAR-
• In PSC cataracts, opacity are located nearer to the nodal
point of the lens, so more light rays are affected and thus
AL measurement becomes difficult with LENSTAR.
IOLMaster 700 overcomes this problem.
• Unusual eye geometries like tilt or decentration of lens can
be detected.

28. OPTICAL BIOMETER
• PRINCIPLE OF IOL MASTER 700 – Based on swept source OCT
technology. It provides an image-based measurement, allowing
to view the complete longitudinal section of eyeball.
Akman A, Asena L. Evaluation and comparison of the new swept source OCT-based IOLMaster
with the IOLMaster500. Br J Ophthalmol . 2015-307779.

32. • Reflected into the eye by mirrors
• Two equal coaxial beams by beam splitter .
• Both coaxial beams enter the eye, where reflections
take place at the corneal and retinal interfaces.
• On leaving the eye, the difference in frequency
detected by a photodetector.
32

33. • A spatial resolution of 0.01 mm for axial length measurement
• Measures the axial length of the eye in approximately 0.4
seconds
• Designed to measure the axial length along the visual axis
• It also uses lateral slit illumination of the crystalline lens and
the cornea to determine anterior chamber depth and
autokeratometry to estimate corneal curvature.
33

34. LIMITATIONS
• Positioning patients with mobility problems on the
IOLmaster machine may occasionally be a
problem.
• Dense ocular media—that is, corneal scarring,
mature or posterior subcapsular cataracts, prevent
acquisition of Optical AL measurements.
• It may be inaccurate for patients with axial or
dense cataracts or gross astigmatism. It is also
expensive.
34

36. • Depending upon the basis of their derivation:
• THEORETICAL &
• REGRESSION FORMULAE
• Grouped into various generation
36

37. • THEORETICAL FORMULAE
• Derived from the geometric optics as applied to the
schematic eyes, using theoretical constants.
• Based on 3 variables – AL, K reading and estimated
postoperative ACD.
• REGRESSION FORMULAE
• Based on regression analysis of the actual postop
results of implant power as a function of the variables
of corneal power and AL
37

38. First generation
Theoretical
1. Binkhorst
2. Colenbrander-Hoffer
3. Gilll’s
4. Clayman
5. Fyodorov
Regression
SRK - I

39. Second
generation
Theoretical
Modified Binkhorst
Regression
SRK - II

40. THIRD GENERATION
• HOLLADAY I
• SRK/T
• HOFFER Q
FOURTH GENERATION
• HOLLADAY - II

41. FIRST GENERATION
• EARLIEST FORMULAE
• BINKHORST FORMULA
P= 1336(4R-A)/(A-D)(4R-D)
Where P - power or IOL
R – corneal radius
A – AL
D – assumed postop ACD plus CT 41

42. • COLENBRANDER- HOFFER FORMULA
P = 1336/A-D-0.05 – 1336/(1336/K –D-0.05)
• K- Average keratometry in dioptres
42

43. • GILLS FORMULA
P= 129.40+(-108K)
= +(-2.79XLEYE)
= + (0.26X LCL)
= + (-0.38XREF)
• LCL – Distance of apex of anterior corneal surface to apex of
IOL in mm
• Ref- desired postop refraction
43

44. • CLAYMAN’S FORMULA
• Assume, emmetropizing IOL=18D
• Emmetropic AL=24mm
• Keratometer reading = 42 D
• If IOL power>21d, deduct 0.25 for every dioptre > 18 D
44

45. • FYODOROV FORMULA
• P = 1336- LK/(L-C) – CK/1336
• C- Estimated postop ACD
45

46. • These seemingly different formulae are in fact identical
• Can be transformed algebraically
• P = N/L-C - NK/N-KC
• P= implant power
• N=aqueous and vitreous refractive index
• C= estimated postop AC depth
46

47. • Reliable for eyes with AL between 22 and 24.5mm
• Tend to predict too large value in short eyes (<22mm) and too
small value in long eyes(>24.5mm)
• Assumption about the optics of the eye
• Still requires a guess about AC depth
47

48. REGRESSION FORMULAE
• SRK – I formula
• Introduced by Sanders, Retzlaff and Kraff
• P = A- 2.5L-0.9K
• Tends to predict too small value in short eyes and too large
value in long eyes
48

49. SRK –II FORMULA
•P = A-2.5L-0.9K
• A constant is modified on the basis of axial length
as follows:
• If L is < 20mm : A+3.0
• If L is 20-20.99 : A + 2.0
• If L is 21-21.99 : A+1.0
• If L is 22-24.5 : A
• If L is > 24.5 : A-0.5 49

50. MODIFIED SRK II FORMULA
• Based on axial length, A constant is modified
as
• If L is < 20mm : A+ 1.5
• If L is 20-21 : A + 1.0
• If L is 21-22 : A+0.5
• If L is 22-24.5 : A
• If L is 24.5 – 26 :a-1.0
• If L is >26mm :A-1.5 50

51. OTHER FORMULAE
SRK/T FORMULA
• Nonlinear theoretical optical formula empirically optimized
for postop AC depth, retinal thickness, and corneal
refractive index
• Significantly more accurate for extremely long
eyes(>28mm)
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52. •HOLLADAY FORMULA
• Third generation formula
• Its enhanced ability to predict the position of the
implants
• Various constant and equations used in this formula
52

53. HOFFER’S FORMULA
• Third generation theoretical formula
• Optimized with regression techniques for ACD
• Performs best for short eyes
•P= 1336/A-C-0.05 – 1.336/{(1.336/K+R) –
(C+0.05/1000)}
53

54. HAIGIS FORMULA
• This formula was published as early as 1970 by GERNET,
OSTHOLT and WERNER
•DL = N/L-D - N/(N/Z-D
• Z = DC + REF/1-REFDBC DC = NC-1/RC
• D : refractive power of IOL
• DC : refractive corneal power
• RC : corneal radius
• NC : (fictitious) refractive index of cornea
54

55. • Ref : desired refraction
• DBC : vertex distance between cornea and glasses
• D : optical ACD
• L : axial length
• N : refractive index of aequeous and vitreous
(1.336)
55

56. IOL FORMULA DEPENDING
ON AL
Circumstance Choice of formula
AL < 20 mm Holladay II/ Hoffer Q
20- 22 mm Hoffer Q
22- 24.5 mm SRK/ T; Holladay
24.5- 26 mm Holladay I
> 26 mm SRK / T ; Holladay I

57. IOL POWER IN SPECIFIC
SITUATION
Myopic refractive surgery Haigis L
Nanophthalmos Hoffer Q ; Haigis
Post refractive surgery Topography with true net k
and flattest k with formula
according to the AL

58. NEWER GENERATION: ONLINE
CALCULATORS
• FOR TORIC IOL:
• http://eyecryltoriccalculator.com/
• http://www.ascrs.org/barrett-toric-calculator
• https://www.myalcon-iolcalc.com/#/calculator
• FOR ERV IOL:
•
https://www.amoeasy.com/toric2(bd1lbizjpta1ma==)/toric.h
tm

59. BIOMETRY IN SPECIFIC SITUATIONS

60. APHAKIC EYES
• Aphakic eyes – sound travel at slower speed,
1532m/s
• Two lens spike – replaced by a single spike of
variable height – obtained from anterior vitreous
face or posterior lens capsule
• Immersion technique/ modern biometers is the
method of choice.
60

61. KERATOCONUS EYES
• Using K reading of such eyes will yield an overestimated reading due to ELP
calculation error.
• Formulas which consider only axial length and not keratometry to calculate
ELP gives better prediction of true IOL power.
• K reading has less of this effect in the Hoffer Q formula.
• Overestimation is not a factor with the haigis formula as it does not use the
k reading in estimating the lens power.

62. EYES WITH SILICONE FILLED EYES
• The refractive index of the oil is much less than that of the vitreous.
• Usage of a standard sound velocity can give an error of upto 8 mm.
• Difficulty of measuring the al can be overcome by increasing the
‘system gain’.
• Usually a factor of (0.72) gives a rough estimate of the power.
• Tal = 1133/1550 × al.

63. HIGH MYOPIA
• Accurate axial measurement is critical for IOL power calculation
• Paraxial measurement in ultrasonic measurement likely to give a refractive
surprise
• It is partly overcome by optical biometer using a fixation target
• Most of it is taken care in IOL master 700 by directly visualising fovea on
the oct image during measurement
• Minus IOL powers have to be chosen carefully by reducing amount of
minus power
• The surgeon should aim for a -0.50 D to -1.00 D postoperative refraction

64. PEDIATRIC BIOMETRY
• As myopia increases rapidly in pediatric age group, goal should be
undercorrection.
• Undercorrection of 60-75% is recommended depending upon the
child’s age.
• Younger the age, more is the undercorrection.
• Undercorrection has to be done with care especially in unilateral
cases, as amblyopia risk is high.

65. PRIMARY PIGGYBACK
• Haigis or Hoffer Q
• Ideally 1 acrylic and 1 silicon IOL to avoid interlenticular
opacification
• Usually single piece in the bag and 3 piece in the sulcus
• Divide the power between the IOL and reduce 1 D for sulcus
placed IOL

66. SECONDARY PIGGYBACK IOL
• Patients with refractive error following the primary IOL
implantation.
• Calculated based on refractive error.
• Holladay’s refractive formula.

67. SOURCES OF ERROR IN POST
REFRACTIVE CASES (LASIK/PRK)
• Radius measurement error/ keratometer error
• Keratometry index error (altered gullstrand ratio)
• Formula algorithm error (ELP)
• These 3 errors cause hyperopic error in myopic LASIK and myopic
error in hyperopic LASIK.

68. METHODS OF ESTIMATING TRUE
POSTOPERATIVE CORNEAL POWER
1. CLINICAL HISTORY METHOD
K=K PRE + R PRE – R PO
2. CONTACT LENS METHOD
K= B CL + P CL + R CL – R NO CL
3. MALONEY” CENTRAL K METHOD
K= 1.1141 * TK PO-CTRI
4. SHAMMAS METHOD
K = 1.14 * KMANUAL PO – 6.8
5. HAIGIS METHOD
K = -5.1625 * K IOL MASTER + 82.2603 - 0.35

69. CHOICE OF IOL
• IOL with negative spherical aberration like technis or acrysof IQ
would correct spherical aberration induced by myopic LASIK.
• Refractive multifocal IOLs not recommended.
• Diffractive multifocal IOL is not preferred.

70. HOW TO AVOID COMMON ERRORS
KERATOMETRY
 Use a single instrument for all measurements
 Javal-schiotz keratometer preferred
 Autokeratometry needs multiple readings to be reliable
 Use topography when indicated.

71. HOW TO AVOID COMMON ERRORS
IOL CALCULATION FORMULAS
 Most 2-variable formulas assume that short eyes produce a shallower
effective lens position(ELP) and longer eyes will result in a deeper
effective lens position.
 Also assume that flat k readings will result in a more shallow effective
lens position and steeper Ks will result in a deeper effective lens
position
 This is the main reason why the accuracy of 2- variable formulas
decreases at the extremes of axial length and corneal power,
especially in the setting of axial hyperopia.

72. HOW TO AVOID COMMON ERRORS
IOL CALCULATION FORMULAS
 Holladay 2 formula is the best “off-the shelf” tool for improving
the accuracy of IOL power calculations for all axial lengths.
 SRK-t formula for long eyes
 Hoffer-q / haigis formula for extremely short eyes

73. HOW TO AVOID COMMON ERRORS
IOL CONSTANT OPTIMIZATION
 After maintaining a record of your own biometries and
carefully calibrating with the refractory outcomes one
should develop his as her own personal a-constant to
minimize the surgeon’s factor.

74. 74