The document provides information on axial length measurement techniques using ultrasound (A-scan) biometry. It discusses average axial lengths, accuracy of measurements, examination procedure, potential sources of error for different techniques, instrument settings, and special measurement considerations. Key points include: - The average axial length of a normal eye is 23.06mm, ranging mostly from 22-24.5mm. - Accuracy of A-scan ultrasound is ±0.1mm. Differences between eyes should be ≤0.3mm. - Potential sources of error include corneal compression, fluid excess, misalignment, inappropriate eye type settings. - Gates, gain, and eye type settings impact accuracy and must be optimized. - Special

1. Axial Length Measurement
( Biometry )
Mohammad Reza ZARRIN (optometrist- MsC)
Tehran University of Medical Sciences

2. IOL Power Calculation
1. Keratometry
2. A-Scan Biometry
3. IOL Formula

3. A-Scan Biometry
Measurement of Axial Eye Length
by Ultrasound

4. Average Axial Length of
Normal Eye
23.06 mm
Majority 22.0 to 24.5 mm

5. Accuracy of AL measurement
using A-scan ultrasound is
+ 0.1 mm

6. Difference in AL measurement
Between both eyes
+ 0.3 mm

8. Instrumentation

10. Examination Procedure
1. History Taking
2. Patient Preparation
3. Biometry Technique

11. Biometry Technique
 Contact
- Applanation Method
- Hand-Held Method
 Immersion

14. Values are 0.14 to 0.36 mm
longer with immersion technique
than with contact method

22. Potential Sources of Error
with Contact Method
1.Corneal Compression
2. Fluid Excess
3. Misalignment of Sound Beam
4. Inappropriate Eye type

24. Error caused by
1 mm Corneal Compression
Average eye 2.5 D
Long eye 1.75 D
Short eye 3.75 D

26. Potential Sources of Error
with Immersion Method
1. Air bubbles within fluid
2. Inappropriate eye type

27. Instrument Setting
1. Measurement Mode
2. Gates
3. Gain
4. Eye Type

28. Measurement Mode
 Automatic
 Semiautomatic
 Manual

29. Gates
Gates are electronic markers on the
screen that provide measurement of
distance between 2 or more anatomic
interfaces .

32. Gain Setting
Initially high gain setting should be
used to assess the overall appearance
of the echogram , then gain should
be reduced to a medium level to
improve resolution of spikes .

34. Error can occur when the gain
is set too high or too low .
Very high gain short reading
Very low gain long reading

35. Eye Type
( Sound Velocity )
1. Phakic
2. Aphakic
3. Pseudophakic

41. Use of average sound velocity ,although
sufficient in normal phakic eye , may
result in slight error when the lens is
inordinately thin or thick or when the
eye is very short or very long .

43. The use of individual sound velocity
may provide more consistent and
accurate AL reading .

45. Aphakia & Pseudophakia
Manual measurement mode is
better to help ensure alignment
of sound beam .

46. If an incorrect eye type is used
an erroneous measurement will
occur .
For determination of correct value
Velocity Conversion Equation
should be used .

47. Velocity Conversion Equation
True AL = V c /Vm x Apparent AL

48. Biometry
in
Special Cases

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

50. 2. Posterior Staphyloma
Posterior staphylomas often causes
an irregular shape of the ocular wall
resulting in an inability to display a
distinct , high retinal spike , leading
to a significant error in A-scan
measurement .

51. Deepest portion of the staphyloma
may be located eccentric to macula
thus te measurement may be longer
than true AL along the visual axis .

52. B-scan can be used to demonstrate
the shape of posterior ocular wall
and the relationship of macula to
the staphyloma .

56. Probes with fixation light
are preferable

57. 3. High Hyperopia
Immersion technique
is preferable .

58.  Edema
 DMS
 Tumor
4. Macular Lesions
 RD

59. The presence of an elevated macular
lesion may prevent the display of a
distinct retinal spike and often causes
a shortened AL measurement .

62. 5. Vitreous Lesions
 Asteroid Hyalosis
 Vitreous Hemorrhage
 Gas Bubble

64. 6. Dense Cataract
Strong sound attenuation produced
by a very dense cataract can
significantly impair the ability to
display spikes from the various
interfaces along the visual axis .

65. Maximum gain setting may be
required to obtain spikes of
sufficient height from the
posterior lens capsule and retina .

66. Semiautomatic mode should be
used in eyes with dense cataract

68. 7. Silicone Oil
Sound velocity in silicone oil
1040 m/s 5000 cs
980 m/s 1000cs

69. This low sound velocity can result
in pronounced sound attenuation
and difficulty in identifying the
retinal spikes .

71. If proper sound velocity are not
used , erroneously long AL
measurement will be obtained .

72. For accurate AL measurement ,
various ocular components should
be measured separately with
appropriate sound velocity .

73. If biometer provides only preset
sound velocity , AL measurement
can be obtained using velocity
conversion equation .

74. The least preferred method is
use of average sound velocity
Average sound velocity in eyes with
average length (23.5 mm)
1,139 m/s phakic eye
1,052 m/s aphakic eye

75. Due to strong sound attenuation
AL measurement often can not be
obtained from an eye containing
emulsified silicone oil .

76. IOL Master

79. Zeiss IOL Master
 Axial Length
 ACD
 Corneal Power
 IOL Power Calculation
Hoffer-Q , SRK/T ,Holladay 1, Haigis

82. Keratometry
A second person should confirm measurements prior to A-scan
ultrasonography if: The corneal power is less than 40.0 diopters, or
greater than 47.0 diopters.
If there has been prior keratorefractive surgery. In this case the corneal
power will need to be estimated by either the historical, or the contact lens
method.
The average corneal power difference between the two eyes is greater
than 1.00 diopter.
The patient cannot fixate, as seen with a mature cataract, or macular hole.
The amount of corneal astigmatism by keratometry, or
topography, correlates poorly with the amount of astigmatism on the most
recent manifest refraction.
The corneal diameter is less than 11.00 mm.
There is any problem with patient cooperation, or understanding.

83. Immersion A-scan Ultrasonography
A second person should re-measure both eyes if: The axial length is less
than 22.00 mm, or greater than 25.00 mm in either eye.
The axial length is greater than 26.0 mm, and there is a poor retinal
spike, or wide variability in the readings.
There is a difference in axial length between the two eyes of greater than
0.33 mm that cannot be correlated with the patient's oldest refraction.
Axial length measurements do not correlate with the patient's refractive
error. In general, myopes should have eyes longer than 24.0 mm and
hyperopes should have eyes shorter than 24.0 mm. Exceptions to this rule
involve steep, or flat corneas. Be sure to use the oldest refractive data.
There is difficulty obtaining correctly positioned, high, steeply rising
echoes, or wide variability in individual axial length readings for either
eye.

84. There is a difference in axial length between the two eyes of greater
than 0.33 mm that cannot be correlated with the patient's oldest refraction.
Axial length measurements do not correlate with the patient's refractive error.
In general, myopes should have eyes longer than 24.0 mm and hyperopes
should have eyes shorter than 24.0 mm. Exceptions to this rule involve steep,
or flat corneas. Be sure to use the oldest refractive data.
There is difficulty obtaining correctly positioned, high, steeply rising echoes,
or wide variability in individual axial length readings for either eye.

85. Intraocular Lens Power
A second person should repeat the axial length measurements, keratometry
readings and re-run the IOL power calculations for both eyes if: The IOL
power for emmetropia is greater than 3.00 diopters different than
anticipated.
There is a difference in IOL power of greater than 1.00 diopter between the
two eyes.
If the patient has had prior keratorefractive surgery and the calculated IOL
power for standard phacoemulsification is less than +20.0 D or greater than
+23.0 D.

86. Formula
for
IOL Power Calculation

87. IOL Power Formula
 Theoretical
 Regression
 Refractive

88. Theoretical Formulas
These are derived from
geometrical optics

89. Regression Formulas
Actual postop refractive results
of many lens implantations are
used to predict IOL power

90. Theoretical Formula
These formulas contain many
assumptions including values of
postop ACD , refractive index of
cornea and ocular humors , retinal
thickness

91. Theoretical Formula
These formulas are reliable for
average AL , but overestimates
in short eyes and underestimates
in long eyes

92. Refractive Formulas
IOL power calculation without
determination of axial length

98. SRK I
(Sanders,Retzlaff,Kraff)
P = A – 2.5L – 0.9K
It generally undercorrects short eyes
and overcorrects long eyes

99. SRK II
A1 = A + 3 AL < 20mm
A1 = A + 2 AL 20-21
A1 = A + 1 AL 21-22
A1 = A AL 22-24.5
A1 = A – 0.5 AL >24.5

100. SRK/T
It is a nonlinear theoretical optical
formula empirically optimized for
postop ACD , retinal thickness ,
corneal refractive index .
It combines advantages of theoretical
and regression formulas .

101. Generations of IOL Formulas
1st Generation
Fyodorov , Colenbrander ,Hoffer , SRK I
2nd Generation
Binkhorst , SRK II
3rd Generation
Holladay 1 , Hoffer-Q , SRK/T
4th Generation
Holladay 2 , Haigis

105. There are currently three IOL constants in use: The
SRK/T formula uses an "A-constant."
The Holladay 1 formula uses a "Surgeon Factor."
The Holladay 2 formula, and the Hoffer Q formula,
both use an "Anterior Chamber Depth." aka: ACD.

106. d = the effective lens position, where ...
d = a0 + (a1 * ACD) + (a2 * AL)
Haigis Formula

107. * The a0 constant basically moves the curve up,
or down, in much the same way that the A-
constant, Surgeon Factor, or ACD does for the
Holladay 1, Holladay 2, Hoffer Q and SRK/T
formulas.
* The a1 constant is tied to the measured
anterior chamber depth.
* The a2 constant is tied to the measured axial
length. The way the a0, a1 and a2 constants are
derived is by generating a set of surgeon, and
IOL-specific

108. Formula Choice

109. AL < 19 mm (<0.1%)
Holladay 2
AL 19-22 mm (8%)
Holladay 2 , Hoffer-Q
AL 22-24.5 mm (72%)
SRK II , Hoffer-Q ,Holladay 1
AL 24.5-26 mm (15%)
Holladay 1 , Hoffer-Q
AL > 26 mm ( 15%)
SRK/T

111. Axial Length in mm Haigis
unoptimized
Hoffer Q Holladay 1 Holladay 2 SRK/T
20.00 to 21.99 0.25 D 0.25 D 0.25 - 0.50 D 0.25 D 0.51 - 1.0 D
22.00 to 24.49 0.25 D 0.25 D 0.25 D 0.25 D 0.25 D
24.50 to 25.99 0.25 D 0.25 D 0.25 D 0.25 D 0.25 D
26.00 to 28.00 0.25 - 0.50 D 0.25 - 0.50 D 0.25 D 0.25 D 0.25 D
28.00 to 30.00 0.25 - 0.50 D 0.25 - 0.50 D 0.25 D 0.25 D 0.25 - 0.50 D
Minus power IOLs 0.51 - 1.0 D 0.51 - 1.0 D 0.25 - 0.50 D 0.25 D 0.25 - 0.50 D

112. Haigis formula
may be appropriate for all
ranges of axial lengths