This document discusses various biometry instruments and equipment used to calculate intraocular lens (IOL) power for cataract surgery. It describes how keratometry, A-scan ultrasound biometry, and non-contact devices like the IOLMaster measure important ocular dimensions needed for IOL power calculations, including corneal power, axial length, and anterior chamber depth. It also discusses IOL power calculation formulas from first to fourth generation and factors that influence formula choice, such as eye length, anterior chamber depth, and IOL placement in the eye. Accurate biometry is emphasized as key to achieving the desired postoperative refractive outcome.

1. Biometry Instruments
& Equipment
Dr Devdutta Nayak
Fellow Ant. Segment
Biratnagar Eye Hospital

2. Several values are required to
calculate IOL Power
• · Accurate Corneal power
• · Actual axial length
• · Accurate prediction of estimated lens
position
(half a mm shift in lens position can have
a
dramatic effect on final vision)
• · Desired post op refraction
• · A good understanding of the various IOL
power calculation formulas is also required.

3. Keratometery
• Keratometry by - Manual
Topography
Autokeratometer
IOL master/Lenstar 900

5. Hand-held Autorefractometer and Keratometer

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

7. 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, with one eye typically within 1D of
each other.

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

9. A-Scan biometry/laser
interferometry
• A-Scan ultrasound
by applanation method
by immersion method
• Laser interferometry
IOL Master (Zeiss)
Lenstar LS 900 (Haag-
Streit)

10. • A scan: Amplitude Scan;
utilizes ultrasound waves of 10
- 12 MHz frequency.
• 2 Principles: Piezoelectric
Phenomenon
Acoustic Impedence.
• Pulsed-echo system.
• Components: Transducer
Amplifier
Display Monitor

11. • 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.

12. • 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.

13. • 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

14. • The gain setting on
biometers 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

15. • 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.

16. • 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.

17. • Ultrasound biometry machines use the formula
Distance = Velocity x Time.
• Sound velocity through different media:
 Phakic – 1550 m/s
 Aphakic – 1532 m/s
 Pseudophakic – 1532 + Correction factor for IOL
• Velocity through PMMA is 2718 m/s, through
acrylic is 2120 m/s, and through silicone is 980-
1107 m/s.
• Correction factor is +0.4 mm for PMMA, +0.2

18. • 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.

19. A-scan facts
• 50% of a surgeon’s post-operative
surprises are A-scan errors (Olsen).
• Error of 2.0D or more are always A scan
related (Holladay).
• All A-scan unit make mistake in echo
interpretation.

20. 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, or by holding the ultrasound probe by
hand.
• Even in the most experienced hands, some
compression of the cornea is unavoidable; this
typically being 0.14 mm - 0.28 mm.

21. 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

22. 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.

23. 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.

24. 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. 6 spikes instead
of 5.

25. 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.

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

27. 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.

29. NON CONTACT
• The Zeiss IOLMaster (1999)-
non- contact optical device
that measures the distance
from corneal vertex to the RPE
by dual beam partial
coherence laser
interferometry.
• Uses 780 nm infrared light &
Michelson Interferometer.
• The IOL Master is consistently
accurate to within ±0.02 mm or
better.
• Haag-Streit launched similar

30. • IOL Master provides following measurements:
 AC depth
 Lens thickness
 Axial Length
 Keratometry
 White to white distance
• In-built IOL power calculation by diff. formula:
SRK II, SRK – T, Holladay II, Hoffer Q, Haigis L.
• 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.

31. • IOLMaster produces a
primary maxima (narrow,
well-defined, centered peak
identified by a circle above
it), secondary maxima
(discrete lower peaks,
sometimes disappearing
into the baseline), and a
baseline (which is low and
even, but may become high
and uneven with
decreasing signal-to-noise
ratio (SNR)).

32. • SNR is a measure of accuracy and
decreases with increasing cataract
density.
• SNR > 2.0 is valid and good if repeatable,
SNR between 1.6-2.0 is borderline but
usable if repeatable, and SNR < 1.6 is not
usable.
• However, a proper waveform is more
important than the SNR value.

33. Advantages of IOLMaster
• Easy & technician independent
• Noncontact
• No water bath is needed
• Can measure through glasses
• Accurate for silicone oil filled eyes and posterior
staphyloma.
• Accurate (Holladay II)
• Haigis L formula incorporated for post-LVC pts.
• For Piggyback IOLs

34. • Lenstar LS 900 measures CCT, ACD,
Lens thickness, Retinal thickness, AL,
Keratometry, White to white distance,
Pupillometry & eccentricity of optical axis.
• Lenstar measures keratometry & ACD
more accurately than IOL Master.

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

36. Do not throw away old
ultrasound machine
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

37. 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

38. IOL FORMULA 2nd generation
• SRK formula –
works well for average eyes.
less accurate for long, short
eyes.
• SRK II formula
modification of SRK
works on ELP
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

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

40. 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.

41. • 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"

42. 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

43. IOL calculation after Refractive
surgery
• Clinical History Method
• Shammas Equation
corrected K = 1.14 (average K) - 6.8
• Topography Method (Wang et al)
corrected K = 1.114K – 6.1
• Corneal Bypass Method (Wake Forest
Univ.)
• Masket Formula
• Online Calculators (doctor-hill.com,
ASCRS)

44. Summary
• Use IOL master or immersion ultrasound
for most accurate axial length
measurement.
• Use fourth generation IOL formulas.
• Examine and reevaluate your result
periodically.

45. THANK YOU