BIOMETRY & PEDIATRICS IOL
DR RAKESH JAISWAL
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)
• IOL master
• A scan: Amplitude Scan;
utilizes ultrasound waves of
10 - 12 MHz frequency.
• 2 Principles: Piezoelectric
• Components: Transducer
• Ultrasound biometry
machines use the formula
Distance = Velocity x Time
• 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.
• 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.
• 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
•Difference in AL measurement Between both eyes +
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
Applanation A-scan Biometry.
• a: Initial spike (probe tip
b: Anterior lens capsule
c: Posterior lens capsule
f: Orbital fat
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
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.
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.
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
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
• 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
• A perfect high, steeply rising retinal spike
may be impossible when macular
pathology is present (eg, macular edema,
macular degeneration, epiretinal
membranes, posterior staphylomas).
• 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
Error caused by
1 mm Corneal Compression
Average eye 2.5 D
Long eye 1.75 D
Short eye 3.75 D
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.
Immersion A-scan Biometry
• The immersion
technique requires the
use of a Prager Scleral
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
• c: Anterior lens capsule.
• d: Posterior lens capsule.
• e: Retina. This echo needs to have
sharp 90 degree take-off from the
• f: Sclera.
• g: Orbital fat.
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,
• 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
• When gain is lowered, the spike
amplitude and sensitivity are
decreased, which eliminates the
weaker signals but improves
• Error can occur when the gain
is set too high or too low .
• Very high gain short reading
• Very low gain long reading
• 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
• 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.
• Gates are electronic calipers on
the display screen that measure
distance between two points.
• Proper gate placement is on the
ascending edge of each
• If the biometer does not allow for
movement of gates, scans must be
repeated until they automatically
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
• Its required for scondary IOL
• Immersion techique is choice
• 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
• Decreasing the gain in pseudophakic eye is helpful.
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.
PMMA protocol : (Polymethylmethacrylate)
•Sound travels faster through PMMA than it does through the
•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 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.
•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.
1 Inadequate Patient Fixation
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
3. High Hyperopia
4. Macular Lesions
5 Vitreous Lesions
6 Dense Cataract
• 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
It is a non contact optical device that measures the
various parameters based on the following
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
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
Calculation of IOL power by softwere
incorporating internationally accepted
Advantages of the IOL master
• Patient comfort as the technique involves non
• User friendly
• Single instrument for measuring AL,corneal
curvature and ACD
• Cross-infection risk is not there,as technique is
• More accurate AL measurement as compared
to A-scan (five times) with AL ranging b/w 14
• 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.
Accuracy of axial length by different
Applanation A -
Immersion A-scan IOL Master
+/- 0.24mm +/- 0.12mm +/- .01mm
Central PSC plaque
Central corneal scar
• 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
5. The measurement of the curvature of the anterior
corneal surface by using the first Purkinje image.
• Determines corneal curvature by measuring the size
of a reflected “mire”.
n - 1
• Surface power formula: D = .........
• 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
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=
Helps us to measure the size of something that
moves around since the small movements of the eye
makes the measurement impossible
Bausch & Lomb Keratometer
• One position keratometer: it measures two
meridians at the same time
• It contains two prisms
• 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.
Parts of keratometer
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.
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
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.
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)
Range of keratometer
• Its lower limit can be extented up to 30D by
interposing -1.0D in front of objective of
• Its upper limit up to 61D by interposing a lens
of +1.25D in front of objective of telescope
Source of keratometry errors
• Unfocused eye piece
• Failure to calibrate unit
• Poor patient fixation
• Dry eye
• Drooping eye lids
• Irregular cornea
Repeat Keratometery If
• Corneal curvature more than 47D or less than
• The difference in corneal cylinder is more than
one diopter between eyes.
• The average keratometry (K) → 43.0-44.0D,
• Post Refractive Surgery
• Corneal Transplantation
• Corneal Scar
• Keratoconus etc.
Depending upon the basis of their deviation ,the various
formulae for calculating IOL power have been grouped
•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
• First generation
• Second generation
• Third generation
• Fourth generation
SRK - I
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
SRK - II
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
• Holladay I
• Hoffer Q
IOL FORMULA 3rd generation
• Third generation formulas-
• SRK/T -very long eyes >26mm
• Holladay I -long eyes 24-26 mm
• HofferQ -Short eyes<22mm
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.
• SRK/T formula — uses "A-constant“
• Holladay 1 formula — uses "Surgeon Factor“
• Holladay 2 formula — uses "Anterior
• Hoffer Q formula — uses "Anterior Chamber
AL < 19 mm (<0.1%)
AL 19-22 mm (8%)
AL 22-24.5 mm (72%)
AL 24.5-26 mm (15%)
Holladay 1 ,
AL > 26 mm ( 15%)
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
• Pediatric eye is not a miniaturized adult eye.
• It has shorter axial length, steeper cornea with
higher keratometry value and smaller anterior
• Errors in axial length measurement affect IOL
power calculation the most, it increases to 3.75
D per mm in children.
• 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 .
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
• To avoid the problems associated with corneal dryness,
measurements should be taken as soon as possible
(following IOP measurement) after induction of
•Balanced salt solution should be instilled as necessary to
maintain a smooth corneal surface
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.
Notas do Editor
with an average error of 2.5 D per millimeter of axial length in adults;
Parece que tem um bloqueador de anúncios ativo. Ao listar o SlideShare no seu bloqueador de anúncios, está a apoiar a nossa comunidade de criadores de conteúdo.
Atualizámos a nossa política de privacidade.
Atualizámos a nossa política de privacidade de modo a estarmos em conformidade com os regulamentos de privacidade em constante mutação a nível mundial e para lhe fornecer uma visão sobre as formas limitadas de utilização dos seus dados.
Pode ler os detalhes abaixo. Ao aceitar, está a concordar com a política de privacidade atualizada.