2. Introduction
• Precise IOL power calculation is essential for
optimal benefits.
• IOL Power Calculation Errors can arise from
– Keratometry – 1.0D = 0.9D error in IOL power
– Axial length – 1mm = 2.5D error in IOL power
– IOL Formula
3. KERATOMETRY
• Used to measure the corneal curvature.
• Done before axial length measurement.
• Types of keratometer:
– Manual keratometer
– Auto keratometer, keratometers incorporated in
IOL master and lenstar
– Topography – placido disc based or elevation
based topography
4. • Anterior surface of cornea – CONVEX MIRROR
• ↑ Curvature – ↓ Image Size.
• From Image Size formed by anterior surface of
cornea (1st Purkinje image) – radius of
curvature of cornea can be calculated
5. MANUAL KERATOMETRY
• PRINCIPLE
– FIXED OBJECT AND VARIABLE IMAGE SIZE
• BAUSCH AND LOMB
– FIXED IMAGE AND VARIABLE OBJECT SIZE
• JAVAL SCHIOTZ
6. Bausch and Lomb Keratometer
• Gives readings in dioptric
form.
• Object size is fixed
• Can measure both principal
meridians without having to
reposition instrument
• Principle meridians are
assumed to be at right angles
to each other
7. JAVAL SCHIOTZ
• Require rotation about the axis to
measure each other principal
meridians
• Fixed doubling image size
• Object size adjusted
• Only measures one meridian at a
time
8. Tips for accurate manual keratometry
• No other contact procedure before keratometry
• Use topical anaesthetic before scan
• Calibrate keratometer before scan
• Take an average of 3 readings
• If discrepancy, ask a second person to cross check
• Compare refraction, especially the axis with K values
• Repeat if difference is ˃ 1.0D between two eyes.
9. Automated Keratometer
• Principle- focuses the reflected corneal image on to
an electronic photosensitive device, which instantly
records the size and computes the radius of
curvature.
• Zone of measurement- central 3mm zone.
10. A-scan ultrasound biometry
Contact Applanation biometry:
• Convenient way to determine axial length for normal eyes
• Errors in measurement almost invariably result from the probe
indenting the cornea and shallowing the anterior chamber
• Compression error is variable & IOL power calculations using
these measurements will lead to an overestimation of the IOL
power. In shorter eyes, this effect is amplified
Immersion A-scan biometry:
• Placing a saline filled scleral shell which acts as a coupling fluid
and avoids direct pressure on the cornea
• A mean shortening of 0.25-0.33 mm has been reported
between applanation and immersion
11.
12. Good A-Scan graph
• Corneal echo is seen as a tall single spike
• No echoes from Aqueous humour
• Anterior and Posterior lens capsule produce tall echoes
• No echoes from Vitreous cavity
• Retina produces tall sharply rising spike with no
staircase at origin
• Orbital fat produces medium to low echoes
• Gain should be the minimum possible which gives
good spikes.
• Average of 8-10 measurements increases accuracy
13. High quality contact A-scan of the phakic eye. Note the 5 high-
amplitude spikes and the steeply rising retinal spike, good
resolution of the separate retinal and scleral spikes.
14. Immersion scan of the phakic eye.
The probe and cornea are now separate spikes since they are
not in contact with each other, and the corneal spike
demonstrates 2 peaks (see arrow), representing the
epithelium and endothelium.
15. When the gain setting is too
high, the resolution of the
separate retinal and scleral
spikes is lost, resulting in 1
thick, flattened spike.
16. Gates are electronic calipers on the display (see arrows) that
between each pair render a measurement.
4-gate system, each of 3 sections is measured individually at
its correct velocity, then added for total eye length.
17. Corneal compression
Note the more shallow anterior
chamber depth of 2.63 mm vs
3.20 mm, indicating 0.57 mm of
corneal compression.
18. Reverberation artifacts in the
vitreous cavity resulting from
intraocular lens.
Artifact spikes from polymethyl
methacrylate implants.
19.
20. Optical Methods
• PRINCIPLE OF IOL MASTER – Based on ‘Partial Coherence
Interferometry (PCI)’. Measures echo delay and intensity of
infrared light reflected back from tissue interfaces– Cornea &
RPE.
• PRINCIPLE OF LENSTAR – Based on ‘Low coherence optical
reflectometry (LCOR)’.
• PRINCIPLE OF IOL MASTER 700 – Based on swept source OCT
technology.
21. IOL Formulae
• 3rd and 4th generation formulae are superior
to earlier formulae.
• These are a merger of linear regression
methods with theoretical eye models.
• They predict better ELP.
• Haigis uses multiple A-constant.
• Holladay uses multiple parameters including
lens thickness measured by LENSTAR.
22.
23.
24. Velocity Conversion
Velocity (correct)/Velocity (measured) X Apparent Length = True Length
• The average speed of sound through the phakic eye is 1550 m/s (meters
per second), whereas the velocity through the aphakic eye is slowed to
1532 m/s.
• For eyes with silicone oil, this formula is used to for true vitreous length.
• The velocity conversion equation is necessary because velocity through
silicone oil is only 980 m/s, much slower than the 1532 m/s the biometer
uses in determining vitreous length.
• Therefore, biometer measures the vitreous erroneously long, and,
consequently, the total length also is erroneously long.
• In a 4-gate system with silicone oil, the ACD and lens thickness are
accurate, so they should be subtracted from the total length to isolate the
erroneous apparent vitreous length.
• Then, the formula used is as follows:
980/1532 X Apparent Vitreous Length = True Vitreous Length
• The corrected vitreous length is now added back to the anterior chamber
depth and lens thickness for an accurate total eye length.
25.
26.
27. • There are presently two viscosities of silicone oil in use:
– 1,000 mPa.s. silicone oil slows sound waves to a little more
than half the speed (980 m/sec) of normal vitreous and
can attenuate the returning sound wave during
ultrasonography.
– 5,000 mPa.s. silicone oil has a somewhat higher density,
and slows sound waves to approximately 1,040 m/sec.
• Typically, when ultrasound measurements are made
through silicone oil, hugely erroneous axial lengths
(such as 35 mm) are displayed
28. • PMMA lenses are a first choice, and silicone lenses should be
avoided.
• The additional power that must be added to the original IOL
calculation for a convex-plano IOL (with the plano side facing
towards the vitreous cavity) is determined by the following
relationship, as described in 1995 by Patel and confirmed by
Meldrum:
• Ns = refractive index of silicone oil (1.4034).
• Nv = refractive index of vitreous (1.336).
• AL = axial length in mm.
• ACD = anterior chamber depth in mm.
• Additional IOL power (diopters) = ((Ns - Nv) / (AL - ACD)) x 1,000
• This is because the silicone oil results in a hyperopic shift.
• For an eye of average dimensions, and with the vitreous cavity filled
with silicone oil, the additional power needed for a convex-plano
PMMA intraocular lens is typically between +3.0 D to +3.5 D.
29.
30. Aphakia
• Two lens spikes are replaced by a single spike -
Anterior vitreous face and Posterior lens capsule.
• Immersion technique is the method of choice.
• In present Biometers, options are available for
aphakic mode where in the calculation compensate
for the change in speed of the sound waves in
cataract lens or aqueous or vitreous.
• In ACIOL or Scleral fixated IOL, the appropriate A
constant is used.
31. Pseudophakia
• Important for IOL exchange or for comparison with other eye.
• Eyes with IOL have extremely high spike,followed by an artificial chain of
reduplication echoes which can be confused with retinal spikes.
• Avoided by reducing gain to decrease artificial spikes and make retinal
ones more prominent.
• Speed of sound depends on type of IOL. In newer devices prefilled data is
available in accordance with the type of implant.
• Optical Biometry is preferred, more accurate correction of AL by
correction factor (CF) according to lens type and thickness.
• PMMA, the conversion factor is +0.45; for silicone it is either -0.56 or -0.41
(depending on the style and manufacturer); and for acrylic it is +0.30.
These must be applied to the formulas
TAL = AAL 1532 + (cf x t); Lens thickness
(t) must be obtained from the manufacturer.
32. Pediatric age group
• AL and K value must be measured under GA.
• IOL power chosen should allow good vision to prevent amblyopia
and ideally also give emmetropia in adult age.
• Currently all infants above two months are advised IOL
implantation. The greatest concern at such an early age is to
prevent amblyopia.
• Development of eye necessitates under-correction to avoid later
myopic shift.
33. Eyes with high myopia
• Accurate axial measurement is critical for IOL power
calculation
• Paraxial measurement in ultrasonic measurement due to
posterior staphyloma is likely to give a refractive surprise
• It is partly overcome by optical biometer using a fixation
target
• Most of it is taken care in IOL master 700 by directly
visualising fovea on the OCT image during measurement
• IOL formula needs to be chosen accordingly
• The surgeon should aim for a -0.50 D to -1.00 D
postoperative refraction as most elderly will prefer being
near-sighted
34. • Adjust the optical biometry axial length as
recommended by Wang and Koch in the JCRS.
• Use the Barrett Universal II formula with
optical biometry
• Use the Hill-RBF method for IOL powers of
+6.00 D and above with an in-bounds
indication. After August of 2017, the range of
IOL powers will extend down to -5.00 D.
Wang L, et al. Optimizing intraocular lens power calculations in eyes with axial lengths above 25.0 mm.
JCRS 2011; 37:2018-2027.
35. Posterior staphyloma
• Significant errors in A-scan because the anatomic axial length
(the distance from the corneal vertex to the posterior pole)
may differ from the refractive axial length (the distance from
the corneal vertex to the fovea).
• Simplest method is by optical biometry using either the Haag-
Streit Lenstar or the Zeiss IOL Master.
• If patient's vision is good, look directly at the red fixation light,
and the axial length measurement will typically be to the
center of the macula.
• Immersion vector-A / B-scan
36.
37. Piggyback
• Primary piggyback
– Haigis or Hoffer Q
– Ideally 1 acrylic and 1 silicon IOL to avoid interlenticular
opacification
– Usually single piece in the bag and 3 piece in the sulcus
– Divide the power between the IOL and reduce 1 D for sulcus
placed IOL
• Secondary piggyback IOL for pseudophakia
– Patients with refractive error following the primary IOL
implantation.
– Calculated based on refractive error.
– Holladay’s refractive formula.
– No knowledge of primary implant or the AL is required.
38. After Refractive Surgery
• The true corneal power following RK, ALK, PRK and LASIK is
difficult to measure by any form of direct measurement, such
as keratometry, or corneal topography.
• Keratometry and topography assume a normal relationship
between the anterior and posterior corneal curvatures, and
measure the anterior corneal radius.
39. • Incisional keratorefractive surgery for myopia flattens both
anterior corneal radius and posterior corneal radius. Ablative
keratorefractive surgery for myopia flattens anterior corneal
radius but leaves posterior corneal radius mostly unchanged.
• Standard keratometry measures an intermediate area and
extrapolates central power based on broad assumptions.
• Typically over-estimate central corneal power, following
keratorefractive surgery for myopia.
• Unexpected post-operative hyperopic surprise.
40. Corneal Transplants
• It is extremely difficult to accurately predict corneal
power in transplant patients.
• If a triple procedure is planned it is suggested that K
readings of other eye be used.
• An alternative option is to use the average k readings
from a series of previous transplants.
• In corneal scar, but no graft is planned, other eye
readings can be used.
41. • Or the eye may be left aphakic
– After four to eight months, when the corneal curvature has
stabilized, and corneal astigmatism has been minimized, a
careful aphakic refraction is performed and simulated
keratometry by topography is used to estimate central
corneal power.
– Power of secondary IOL is calculated by means of
Refractive Vergence Formula
– IOL is then placed into ciliary sulcus, and over intact
posterior capsule, via a small scleral tunnel
42. Recent advances
• Formulas
– Holladay IOL consultant (HIC) program uses Holladay 2 formula.
– Oslens’s formula uses pre-operative refraction and lens thickness.
• Software based: Okulix
– Biometric computer program to stimulate whole pseudophakic eye aims to
reduce calculation error and ensure a more reliable estimation of IOL
strength.
• Newer Machines:
– Pentacam and Orbscan are utilized for precise corneal power
measurements.
• IOL with tolerance
– Manufacturers reduce internal tolerance levels of IOL to +/- 0.25D,
increasing accuracy.
