BIOMETRY & PEDIATRICS IOL CALCULATIONS

1. BIOMETRY & PEDIATRICS IOL
CALCULATION
BY
DR RAKESH JAISWAL

2. BIOMETRY
Biometry is the process of measuring the
power of the cornea (keratometry) and the
Axial length of the eye, by using this data to
determine the ideal intraocular lens power
Measurement of axial length(AL)
• A-scan
• IOL master

3. A-scan Biometry
• A scan: Amplitude Scan;
utilizes ultrasound waves of
10 - 12 MHz frequency.
• 2 Principles: Piezoelectric
Phenomenon
Acoustic
Impedence.
• Components: Transducer
Amplifier
Display Monitor
• Ultrasound biometry
machines use the formula
Distance = Velocity x Time

4. • In A-scan, thin, parallel sound beam is emitted
from the probe tip, with an echo bouncing
back into the probe tip as the sound beam
strikes each interface.
• An interface is the junction between any two
media of different densities and velocities.
 anterior corneal surface
 aqueous/anterior lens surface
 posterior lens capsule/anterior vitreous
 posterior vitreous/retinal surface
 choroid/anterior scleral surface.

5. • The echoes received back into the probe from
these interfaces are converted by the
biometer to spikes arising from baseline.
• The greater the difference in the two media at
each interface, the stronger the echo and the
higher the spike.

6. • Average Axial Length of Normal Eye 23.06 mm
•Majority 22.0 to 24.5 mm
•Error of 0.4mm in the measurement of axial length
may result in a one diopter change in calculated IOL
power.
•Difference in AL measurement Between both eyes +
0.3 mm

7. • Contact
- Applanation Method
- Hand-Held Method
• Immersion

8. Applanation A-scan Biometry
• A-scan biometry by applanation requires that the
ultrasound probe be placed directly on the corneal
surface. This can either be done at the slit lamp

9. Applanation A-scan Biometry.
• a: Initial spike (probe tip
and cornea)
b: Anterior lens capsule
c: Posterior lens capsule
d: Retina
e: Sclera
f: Orbital fat

10. Applanation A-scan Biometry
• When echoes b through d
are high and steeply rising,
the ultrasound beam is most
likely on visual axis.
• If no scleral or orbital fat
echoes visible, then
ultrasound beam is most
likely aligned with optic
nerve.

11. The five basic limitations of
applanation A-scan biometry are:
1. Variable corneal compression.
2. Broad sound beam without precise localization
3. Limited resolution.
4. Incorrect assumptions regarding sound velocity
5. Potential for incorrect measurement distance.

12. PROCEDURE: Hand-Held Method
A probe is placed on the patient’s cornea.
The probe is attached to a device that delivers
adjustable sound waves.
The measurements are displayed as spikes on the
screen of an oscilloscope (Visual monitor).
The appearance of the spikes and the distance
between them can be correlated to structures within
the eye and the distance between them.

13. Probe positioning:
The probe lightly touches the cornea and is
positioned, such that the barrel of the probe is
aligned with the optical axis or visual axis of the
eye.
The operator aims the probe towards the macula
of the eye.
Alignment with the optical axis will be indicated
by high lens spikes and a high retina spike on the
scan graph.

14. • Spike height is affected by the difference in
density & by the angle of incidence, which
is determined by the probe orientation to
the visual axis.
• If the probe is held nonparallel, part of the
echo is diverted at an angle away from the
probe tip, and is not received by the
machine.
• A perfect high, steeply rising retinal spike
may be impossible when macular
pathology is present (eg, macular edema,
macular degeneration, epiretinal
membranes, posterior staphylomas).

16. Corneal Compression
• If pressure is applied on the cornea, the axial
length measurment may be falsely too short.
• It can be monitored by observing the anterior
chamber depth, read out by an instrument.
• Most eyes will have an ACD readings between
2.5 to 4.0mm.
• The corneal compression error factor can be
avoided by using the immersion technique

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

18. Immersion A-scan Biometry
• The immersion technique is accomplished by placing
a small scleral shell between the patient's lids, filling
it with saline, and immersing the probe into the fluid,
being careful to avoid contact with the cornea.
• More accurate than contact method because corneal
compression is avoided.
• Eyes measured with the immersion method are, on
average, 0.1-0.3 mm longer.

19. Immersion A-scan Biometry
• The immersion
technique requires the
use of a Prager Scleral
Shell .

20. Immersion A-scan Biometry
• . • a: Probe tip. Echo from tip of
probe, now moved away from the
cornea and has become visible.
• b: Cornea. Double-peaked echo
will show both the anterior and
posterior surfaces.
• c: Anterior lens capsule.
• d: Posterior lens capsule.
• e: Retina. This echo needs to have
sharp 90 degree take-off from the
baseline.
• f: Sclera.
• g: Orbital fat.

21. Immersion A-scan Biometry
• When the ultrasound beam is properly aligned
with the center of the macula, all five spikes will
be steeply rising and of maximum height.
• Both the peaks of corneal spike should be equal
in height ideally.
• Other advantage: Easier,
better repeatability.

22. • The gain setting on A-scan is measured
in decibels and affects amplification
and resolution of spikes.
• When on highest gain, spike height
and sensitivity of display screen are
maximized, enabling visualization of
weaker signals, but resolution is
affected adversely.
• When gain is lowered, the spike
amplitude and sensitivity are
decreased, which eliminates the
weaker signals but improves
resolution.
• Error can occur when the gain
is set too high or too low .
• Very high gain short reading
• Very low gain long reading

23. • Resolution: ability to display two interfaces
that lie in close proximity, one directly behind
the other, as separate echoes or spikes.
• The more dense the cataract, the higher the
necessary gain.
• Gain setting may vary not only from patient to
patient but from one eye to the next in the
same patient, depending on cataract density.

24. • Gates are electronic calipers on
the display screen that measure
distance between two points.
• Proper gate placement is on the
ascending edge of each
appropriate spike.
• If the biometer does not allow for
movement of gates, scans must be
repeated until they automatically
align properly.

26. The scan of an aphakic eye
• It will either have no lens
spikes,(or) it will have one lens
spike (A) that represents an
intact posterior lens capsule. ( C
)
• The velocity of sound will be
different because the beam is
not passing through the lens.
• A velocity of sound of 1532m/s
is typically used for aphakic
measurements
• .
• Its required for scondary IOL
• Immersion techique is choice

27. • Biometry of pseudophakic eye performed:
- To compare to the fellow phakic eye for accuracy
- IOL exchange
- Checking an unwanted postoperative refractive error.
• A scan of pseudophakic eye → multiple reverberation echoes in
the vitreous cavity that tend to decrease in amplitude from left
to right.
• Decreasing the gain in pseudophakic eye is helpful.

28. Factors to Consider for IOL material
 The type of material is important because the
velocity of sound is a function of the material that
the sound is passing through.
 A-Scan does not actually measure length, they
measure how long it takes a sound beam to
bounce off an object ( Anterior lens, Posterior
lens,and Retina) & return to the probe.
 The instrument is pre programmed with the
velocity of sound factors for the aqueous, the
lens material, and the vitreous.

29. PMMA protocol : (Polymethylmethacrylate)
•Sound travels faster through PMMA than it does through the
natural lens.
•If a pseudophakic mode is not available,you can measure the
eye in the aphakic mode and add a standard compensating
factor of 0.4mm to the resultant axial length.
Silicon protocol:
•Silicon IOL’s are foldable. Sound travels much slower through
a silicon lens than it does through the natural lens.
•If not taking this in to account could result in a –3.0D post -
op refractive error.
•If your biometer does not have a pseudophakic or silicon
mode, use the aphakic mode and subtract a compensation
factor of 0.8mm.

30. Acrylic protocol:
•Sound travels faster through acrylic than it
does through the natural lens.
•If biometer does not have an acrylic mode,use
the aphakic mode and add a compensation
factor of 0.2mm.

31. 1 Inadequate Patient Fixation
Low Vision
Nystagmus
Blepharospasm
Strabismus
2. Posterior Staphyloma
Posterior staphylomas often cause
resulting in an inability to display a
distinct , high retinal spike , leading
to a significant error in A-scan
measurement .
3. High Hyperopia
Immersion technique
is preferable
4. Macular Lesions
5 Vitreous Lesions
6 Dense Cataract

32. IOL MASTER
WORKING PRINCIPAL
• IOL Master (Zeiss Humphrey system) is a
combined biometric instrument that
measures quickly and preciesely parameters
of human eye needed for IOL power
calculation by non-contact technique.
• It also incorporates the softwere to calculate
IOL power from various formulae

33. WORKING RINCIPAL
It is a non contact optical device that measures the
various parameters based on the following
principal
1. AL measurement is based on patented
interference optical method known as Partial
Coherence Interferometry (PCI).This technique
relies on a laser Doppler technique to measure
the echo delay and intensity of infrared light
reflected back from the tissue interface-cornea
and retinal pigment epithelium .
2. Corneal curvature(K) is determined by
measuring the distance between reflected light
images as in conventional keratometry

34. Anterior chamber depth (ACD) is determined as
the distance between the optical section of
the crystalline lens and cornea produced by
lateral slit illumination
White-to-White is determine from the image of
the iris
Calculation of IOL power by softwere
incorporating internationally accepted
calculation formulae

35. Advantages of the IOL master
• Patient comfort as the technique involves non
contact measurements
• User friendly
• Single instrument for measuring AL,corneal
curvature and ACD
• Cross-infection risk is not there,as technique is
non-contact
• More accurate AL measurement as compared
to A-scan (five times) with AL ranging b/w 14
to 40mm

36. • Specially useful in certain ocular conditions
like small corneal scar,ant cortical
spokes,extream myopia, post staphyloma
• Incorporates IOL power
• This method cannot be used in significant
media opacity (eg. dense cataracts or corneal or
vitreal opacity) due to absorption of light or
inability of the patient to fixate on target.

37. Accuracy of axial length by different
machine
Applanation A -
scan
Immersion A-scan IOL Master
+/- 0.24mm +/- 0.12mm +/- .01mm

38. Immersion
ultrasound
IOL
master
Posterior staphyloma
Silicone oil
Pseudophakia
4++brunescent lens
Central PSC plaque
Vitreous hemorrhage
Central corneal scar
Difficult
Difficult
Variable
•Yes
•Yes
•Yes
•Yes
•Yes
•Yes
•Yes
No
No
No
No

40. Introduction
• Keratometry .
1. Keratometry is the measurement of a patients
corneal curvature .
2. It provides on objective,quantitative measurement
of corneal astigmatism,measuring the curvature in
each meridian as well as the axis.
3. Keratometry is also helpful in determining the
appropriate fit of contact lens .
4. It is use for measurement the corneal dioptric
power.
5. The measurement of the curvature of the anterior
corneal surface by using the first Purkinje image.

41. Keratometer:
• Determines corneal curvature by measuring the size
of a reflected “mire”.
n - 1
• Surface power formula: D = .........
R
• D = the dioptric power of the cornea
n = the refractive index of the cornea used (1.3375)
R = the radius of curvature of the cornea in meters
• Keratometer measures only the central 3mm of the
corneal diameter.

42. Principle of Keratometry
• Cornea is a convex refracting surface
• In order to find the refracting power of the cornea, we
need to reflect an object of a known size at a known
distance off the corneal surface.
• Then determine the size of the reflecting image with
measuring telescope and calculate the refractive power
of the cornea based on the refractive index of n=
1.3375
Doubling Principle
Helps us to measure the size of something that
moves around since the small movements of the eye
makes the measurement impossible

43. Keratometry 43
Bausch & Lomb Keratometer
• One position keratometer: it measures two
meridians at the same time
• It contains two prisms

44. Principle of Keratometry

45. • The object used is an illuminated circle with plus and
minus rings .
• The two prisms inside the instrument give two additional
one displaced horizontally and another displaced
vertically. Three images are seen.
• While taking the reading the pluses and minuses coincide.
This is achieved by moving the keratometer with the
object forward or backward in front of the eye.
• When coincidence takes place the size of the images of
fixed value. The distance of the object is different for
different curvatures. The instrument is calibrated. As the
drum rotate the distance varies.

46. Parts of keratometer

47. Parts of keratometer
1. Telescope
2. Eye piece at one end (near to examiner)
3. Objective (near to patient)
4. Knobs for adjustment of vertical and horizontal
curvature on the telescope.
5. Knobs adjusting height of telescope.
6. Chinrest-adjusting knobs.
7. Bulb for illumination.
8. Knob for focusing of mires.
9. Model cornea with occluder.

48. Performing Keratometry
1. Looking the through the eye piece of the
keratometer,use the eye piece to focus the
reticule (cross hair) in the same way as for the
lens meter.
2. The patient can comfortably put the chin and
forehead on the appropriate rests.
3. Use the occluder attached to the keratometer to
cover the eye not being measured.
4. Then use the height adjustment knob of the
keratometer to position the light reflections at
the level of the cornea.

49. 1. To obtain proper focus, rotate the focus knob until the bottom-right
circles converge to form a fused image.(fig)
1. To locate the proper axis, rotate the keratometer until the pluses between
the two bottom circles are in the same plane.(fig)

50. Range of keratometer
• 36-52D
• Its lower limit can be extented up to 30D by
interposing -1.0D in front of objective of
telescope
• Its upper limit up to 61D by interposing a lens
of +1.25D in front of objective of telescope

51. Source of keratometry errors
• Unfocused eye piece
• Failure to calibrate unit
• Poor patient fixation
• Dry eye
• Drooping eye lids
• Irregular cornea

52. Repeat Keratometery If
• Corneal curvature more than 47D or less than
40D.
• The difference in corneal cylinder is more than
one diopter between eyes.
• The average keratometry (K) → 43.0-44.0D,

53. Difficult Situations
• Post Refractive Surgery
• Corneal Transplantation
• Corneal Scar
• Keratoconus etc.

54. Formula
for
IOL Power Calculation

55. Depending upon the basis of their deviation ,the various
formulae for calculating IOL power have been grouped
into
•Theoretical formulae –these is based on mathematical
principal revolving around the schematic eye
•Regression formulae-these were arrived at by looking
at postoperative outcomes retrospectively

56. IOL Formulae
• First generation
• Second generation
• Third generation
• Fourth generation

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

58. IOL FORMULA Ist generation
• Most are based on regression formula developed
by Sander ,Retzlaff & Kraff
• Known as SRK formula.
• P = A - 2.5(L) - 0.9(K)
P=lens implant power for emetropia
L= Axial length (mm)
K=average keratometric reading (diaopters)
A= lens constant

59. Second generation
Theoretical
Modified Binkhorst
Regression
SRK - II

60. IOL FORMULA 2nd generation
• SRK formula –
works well for average eyes.
less accurate for long, short eyes.
• SRK II formula
Modification of SRK
A-constant is modified on the basis of AL
P = A1 – 2.5L – 0.9K 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

61. Third generation
• Holladay I
• SRK/T
• Hoffer Q

62. IOL FORMULA 3rd generation
• Third generation formulas-
• SRK/T -very long eyes >26mm
• Holladay I -long eyes 24-26 mm
• HofferQ -Short eyes<22mm

63. Fourth generation
• Holladay - II

64. IOL FORMULA 4th generation
• Holladay II
• Haigis formula-
d = a0 + (a1 * ACD) + (a2 * AL)
ACD is the measured anterior chamber depth
AL is the axial length of the eye
The a0, a1 and a2 constants are set by optimizing
a set of surgeon- and IOL-specific outcomes for a wide
range of ALs and ACDs.

65. • SRK/T formula — uses "A-constant“
• Holladay 1 formula — uses "Surgeon Factor“
• Holladay 2 formula — uses "Anterior
Chamber Depth“
• Hoffer Q formula — uses "Anterior Chamber
Depth"

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

67. When capsular tear does not allow bag
placement of the lens → change IOL power
for sulcus placement
• >=28.5 D Decrease by 1.5 D
• +17 To 28 D Decrease by 1.0 D
• +9 To 17 D Decrease by 0.5 D
• <+ 9 D No change

68. Pediatric biometry
• Pediatric eye is not a miniaturized adult eye.
• It has shorter axial length, steeper cornea with
higher keratometry value and smaller anterior
chamber depth.
• Errors in axial length measurement affect IOL
power calculation the most, it increases to 3.75
D per mm in children.

69. • no formula has been proven to have an
advantage over others.
• newer theoretic formulas such as Holladay II
that take anterior segment measurements into
account, may be recommended for pediatric IOL
power calculation or Hoffer Q
• 20% undercorrection if the child is less than age
2 years 10% undercorrection for age 2–8 years .

72. A-SCAN:
A-SCAN We measure AL with an immersion technique,
often under sedation.
KERATOMETRY For the K measurement, we use a handheld
keratometer. Alcon autokeratometer produces accurate
measurements of curvature of the cornea in pediatric eyes.
• Measurements should be taken without the use of an
eyelid speculum
• To avoid the problems associated with corneal dryness,
measurements should be taken as soon as possible
(following IOP measurement) after induction of
anesthesia.
•Balanced salt solution should be instilled as necessary to
maintain a smooth corneal surface

73. Using hand –held instruments , reliable keratometry &
axial length measurement. For each individual case the
IOL power should be customized based on many
characteristics including the age , one eye or both ,
amblyopia status , likely compliance with glasses &
family history of myopia.