1. Presenter- Rajeshwori Ngakhushi
IOM, Maharajgunj
1

2. INTRODUCTIONINTRODUCTION
• the process of measuring the power
of the cornea (keratometry) and
the length of the eye, and using
this data to determine the ideal
intraocular lens power.
2

3. • measurement of the various
dimensions of the eye and of its
components and their
interrelationships.
• If this calculation is not performed,
or if it is inaccurate, then patients
may be left with a significant
refractive error.
3

4. • IOL implantation is the method of
choice for correcting aphakia
• Major classes of IOLs based on the
method of fixation in the eye are
1.Anterior chamber IOL
2.Iris supported lenses
3.Posterior chamber lenses 4

5. • In 1967, Fyodorov described the
first ultrasonic measurement of
axial eye length for the purpose of
calculating IOL power.
5

6. • Binkhorst, Colaenbrander, Thijssen
and Van der Heijde added no of
formulae, including the calculation
of axial length, keratometry
reading and estimated post
operative AC depth
6

7. • In 1982, a regression analysis
method was introduced by Sanders,
Retzlaff and Kraff (SRK formula)
7

8. • IOL calculation
• Congenital glaucoma
• Psuedoexophthalmos
• Posterior pole coloboma
• Phthisis bulbi
8

9. • Biometry essentially consists of a
keratometric reading together with
an ultrasonic measurement of axial
length of the eye, perhaps with the
measurement of ACD.
9

10. –Required Measurements for
calculating emmetropic lens power[P]
•[L] axial length in mm, defined as
distance between anterior surface
of cornea and anterior surface of
retina
•[K] corneal power in diopters or
•[R] corneal radius of curvature in
mm.
10

11. • [C] estimated postoperative
anterior chamber depth in mm,
defined as distance between
anterior surface of cornea and
anterior surface of pseudophakos
lens.
11

12. Estimation of IOL power based
on Basic Refraction of theEye
• First widely accepted method of
estimating the proper implant
power
• Made use of basic refraction, which
is the refraction a patient would
have if he or she had a crystalline
lens of average thickness
12

13. • Used at a time when iris fixated
IOLs were in vogue
• For each diopter of IOL power
change only 0.8D change in the
basic refraction results
13

14. Basic refraction ( D ) Emmetropizing
pseudophakos power
+ 3.00 21.75
+ 2.00 20.50
+ 1.00 19.25
0.00 18.00
- 1.00 16.75
-2.00
- 3.00
15.50
14.25 14

15. Estimation of IOL power based
on sophisticated measurements
• It is theoretically possible to reach
a highly accurate estimate of IOL
power by making sophisticated
measurements of the components
of the basis refraction and
incorporating the data into one of
several formulas
15

16. • Depended on two measurement to
predict the implant power
- refracting powe of the cornea
- axial length of the eye or the
distance from the anterior surface
of the cornea to the fovea
16

17. • The depth of AC or more
accurately , the postoperative
distance between the anterior
surface of the cornea and the
anterior surface of the
pseudophakos at the optic axis was
used by these formulas as a
constant
17

18. Refracting power of thecornea
• Keratometry has gained a special
place in IOL power calculation.
• Measured with the keratometer or
ophthalmometer
• Two principal meridians are
averaged to obtain a spheric
equivalent
18

19. PRINCIPLEPRINCIPLE
• Based on the fact that the anterior
surface of the cornea acts as a
convex mirror and the size of the
image formed varies with its
curvature.
• Greater the curvature , lesser the
image size
19

20. • Average of at least three readings
in each principal meridian is
recorded
• An error of 0.1 mm in radius
curvature results in a refractive
error of approximately 0.5 D
20

21. • the Javal-Schiötz keratometer
requires the user to align the
keratometer mires along the
principal meridians and corneal
curvature is measured by
subjective alignment of the mires,
reflected from the central 3.4 mm
of the cornea.
21

22. Tips for accurate manual
keratometry:
• calibrate and check the accuracy of the
keratometer
• use a dedicated single instrument that
is known to be accurate
• don't touch the cornea beforehand and
ensure a good tear film
• adjust the eyepiece to bring the central
cross-hairs into focus
22

23. • make sure that the patient's other
eye is occluded and that the cornea
is centred
• take an average of three readings,
including the axes
• if high or low results are
encountered (< 40.00 D or > 48.00
D), it is advisable to have a second
person check the measurements
23

24. • repeat if the difference in total
keratometric power between the
eyes exceeds 1.50 D
• in a scarred cornea, use the fellow
eye or average the results.
24

25. • For each
dioptre
change in k
reading 0.9 D
change in lens
implant power
occurs in
opposite
direction 25

26. Axial length
• Currently employ ultrasonography
• High degree of accuracy in
estimating axial length
• An error of 1mm leads to a
miscalculation of 2.5 to 3.5 D
• If ultrasound frequency of 20 MHz
or more is used, it may be possible
to achieve an accuracy of about
0.1mm
26

27. • Two types of A
scan
ultrasound
biometry
• Applanation A
scan biometry
• Immersion A
scan biometry
27

28. 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
28

29. • don't push too hard
• average the 5–10 most consistent
results giving the lowest standard
deviation (ideally < 0.06 mm)
• errors may arise from an
insufficient or greasy corneal
meniscus due to ointment or
methylcellulose used previously.
29

30. • misalignment along the optic nerve
is recognised by an absent scleral
spike
• the gain should be set at the lowest
level at which a good reading is
obtained
• take care with axial alignment,
especially with a hand-held probe
and a moving patient 30

31. 31

32. • The A-scan applanation device
calculates axial length from the
time taken for ultrasound waves to
reflect back to its receiver from
the internal limiting membrane
• For each mm change in axial length
a 2.5 D change in lens implant power
occurs in the opposite direction
32

33. • Techniques currently available to
measure the ACD include the
1. A-scan ultrasonography,
2. partial coherence interferometry,
3. slit-scanning videokeratography,
4.Scheimpflug imaging and
5. anterior segment optical coherence
tomography (OCT).
33

34. • Repeat measurement if
- axial length is less than 22mm or
greater than 25mm
-average corneal power less than
40 Dor greater than 47 D
34

35. - Calculated implant power is more
than 3.00D from the average
- between eyes the difference in
average corneal power greater
than 2.00 D
axial length greater than 0.3mm
implant power greater than
1.00D 35

36. • Depending upon the basis of their
deviation:
Theoretical and 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
37

38. Regression formulae
• tendency to use the simpler
empirical formulae in clinical
practice
• based on regression analysis of the
actual postop results of implant
power as a function of the variables
of corneal power and AL
• Commonly used are the SRK formula
and its modification 38

39. 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
39

40. • Colenbrander- Hoffer formula
P = 1336/a-d-0.05 – 1336/(1336/k –d-
0.05)
• K- average keratometry in dioptres
40

41. • 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
41

42. • 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
42

43. • For eg AL= 22mm , k= 43 D
• It leads to 6D of hyperopia in
length , 1 D myopia in keratometry
• Hence, IOL power = 18+6-1 = 23 D
• = 23- (23-18)x0.25 = 21.75D
43

44. • Fyodorov formula
• P = 1336- LK/(L-C) – CK/1336
• C- estimated postop ACD
44

45. • 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 45

46. • 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
46

47. 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
47

48. 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 48

49. 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
49

50. Other formulae
SRK/T formula
Nonlinear theoretical optical formula
empirically optimized for postop AC
depth, retinaal thickness, and
corneal refractive index
Significantly more accurate for
extremely long eyes(>28mm)
50

51. • Holladay formula
• Third generation formula
• Its enhanced ability to predict the
position of the implants
• Various constant and equations used
in this formula
51

52. • Designed to improve determination
of the final effective lens position
ELP
• Horizontal corneal white-to-white,
Phakic lens thickness, Anterior
chamber depth, Axial length, Age,
Refraction and Keratometry
52

53. P = 1000na[naR - (nc-1)Alm - 0.001Ref{V(naR-
(nc-1)Alm)+AlmR}]
(Alm-ACD-SF){naR-(nc-1)(ACD+SF)-
0.001Ref{V(naR-(nc-1)(ACD+SF)}+(ACD+SF)R}
R-corneal radius na- n of aqueous=1.336
Alm- modified AL nc- n of cornea=4/3
Ref- desired postop ref
V – vertex distance of pseudophakic spectacles
SF – distance from aphakic anterior iris plane to
optical plane of IOL
53

54. 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)}
54

55. Haigis formula
• Recent addition in the list of IOL
power calculating formula
• based on the elementary IOL
formula for thin lenses
• this formula has first been
published as early as 1970 by
GERNET, OSTHOLT and WERNER
55

56. • 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
56

57. • 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)
57

58. • In 1999, Carl Zeiss introduced a
noncontact partial coherence laser
interferometer (IOL Master; Carl
Zeiss Meditec, Jena, Germany) as an
alternative technique to measure the
axial length of the eye
58

59. • provides high
resolution non-
contact
measurements of
axial length (using
partial coherent
interferometry),
anterior chamber
depth, and corneal
radius (using image
analysis).
59

60. • Combined biometric measurements (
Zeiss Humphrey system) measures
quickly and precisely parameters of
human eye needed for IOL power
calculation by a non contact
technique
• Also incorporates the software to
calculate IOL power from various
formulae 60

61. Working principle
• Non contact
optical device
that measures
the various
parameters
based on the
following
principles:
61

62. • reflected into the eye by mirrors M1 and
M2
• two equal coaxial beams CB1 and CB2 by
the beam splitter B1.
• separation of the two coaxial beams is
twice the displacement d of the mirror
M1.
• Both coaxial beams enter the eye, where
reflections take place at the corneal (C)
and retinal (R) interfaces. 62

63. • On leaving the eye, the difference in
frequency between the coaxial beams is
detected by a photodetector (PHD),
after passing through a second beam
splitter (BS2).
• displacement d of the mirror M1 can be
precisely determined and related to the
reflected signals detected at the
photodetector, allowing accurate
measurements of the length AL between
the cornea and the retina.
63

64. Operating principle
64

65. AL measurement
• Based on a patented interference
optical method known as partial
coherence interferometry ( PCI )
• Relies on a laser Doppler technique to
measure the echo delay and intensity of
infrared light reflected back from the
tissue interfaces- cornea and RPE
65

66. • A spatial resolution of 0.01 mm for axial
length measurement
• Measures the axial length of the eye in
approximately 0.4 seconds
• uses infrared light (λ = 780 nm)
• IOLMaster is designed to measure the
axial length along the visual axis
66

67. • It also uses lateral slit illumination
of the crystalline lens and the
cornea to determine anterior
chamber depth and
autokeratometry to estimate
corneal curvature.
67

68. AC depth
• directs a 0.7 mm width slit beam of
light through the anterior segment of
the eye at an angle of 38 degrees to the
visual axis.
• instrument camera is aligned so that the
light beam forms an optical section and
the internal software measures the
distance between the anterior corneal
pole and the anterior crystalline lens
surface to calculate the anterior
chamber 68

69. comparision
• Anterior chamber depth, as
measured with the IOLMaster, was
significantly shorter (by −0.06
(0.25) mm, p <0.02) than that
measured by applanation ultrasound
• The IOLMaster could be expected
to read as much as 0.43 mm above
or 0.54 mm below ultrasound for
anterior chamber depth. 69

70. 70

71. • Axial length, as measured with the
IOLMaster, was similar to that
measured by applanation ultrasound
(difference 0.02 (0.32) mm, p = 0.47;
• The IOLMaster could be expected to
read as much as 0.65 mm above or 0.61
mm below ultrasound for axial length.
71

72. 72

73. 73

74. limitations
• Positioning patients with mobility
problems on the IOLMaster
machine may occasionally be a
problem.
• its inability to measure the lens
thickness, which is required for the
Holladay II formula
74

75. • Dense ocular media—that is,
corneal scarring, mature or
posterior subcapsular cataracts,
prevent acquisition of optical AL
measurements.
75

76. • it may be inaccurate for patients
with axial or dense cataracts or
gross astigmatism. It is also
expensive.
76

77. Biometry in aphakic eyes
• Phakic eyes – standard sound
velocity of 1550 m/s
• 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
77

78. Biometry in pseudophakia
• Velocity depends on the implant
materials
• velocity of sound through the
pseudophakic eye is 1532 m/s plus
the correction factor for the
implant material
78

79. • velocity through PMMA is 2718
m/s, through acrylic is 2120 m/s,
and through silicone is 980-1107
m/s, depending on the silicone used.
79

80. • to achieve accurate measurements
is to use the aphakic setting, which
uses a sound velocity of 1532 m/s.
Then, the examiner should manually
add the correction factor for the
IOL material to the results
obtained on aphakic mode.
80

81. • The correction factor is +0.4 mm
for PMMA, +0.2 mm for acrylic, and
-0.4 mm to -0.8 mm for silicone,
depending on the silicone velocity.
81

82. • Therefore, if an eye measured
23.32 mm on aphakic mode and the
IOL is made of PMMA, the correct
axial length is 23.72 mm. If the
IOL is acrylic, the correct axial
length is 23.52 mm. If it is low-
velocity silicone, the correct length
is 22.52 mm.
82

83. Choice of formula
• In short eyes, the Hoffer Q had
the lowest mean absolute error
(MAE) for ALs from 20.00 to 20.99
mm.
• The Hoffer Q and Holladay 1 had a
lower MAE than the SRK/T for ALs
from 21.00 to 21.49 mm.
83

84. • For ALs from 23.50 to 25.99 mm, there
was a trend toward lower MAEs for the
Holladay 1,
• In long eyes, the SRK/T had the lowest
MAE, with statistically significant
differences for ALs of 27.00 mm or
longer.
84

85. 85

86. • SRK I formula inaccurate for long
AL
• Hoffer formula performs best for
short eye (<22mm)
• SRK/T formula is best for very long
eyes
• SRK/T formula is probably most
accurate in majority of cases
• Haigis formula is also reported to
give accurate result in recent
studies 86

87. Pitfalls in IOL power calculation
• Inaccurate preoperative
measurements
• Individual variations
• Inherent source of error
87

88. References
• Optics and refraction - A K Khurana
• Ophthalmic ultrasound – Hates R Atta
• Cataract surgery and its complications –
Norman S. Jaffe 4th
edition
• Cataract and IOLs surgery vol-1 –
Stephen P. Ginsberg
• internet
88

89. Thank you!!!
89