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12 Biometry andIntraocular Lens Power Calculation
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formulae are superior to the Holladay 1, Holladay
2, and Hoffer Q formulae in longer eyes with silicon oil [26, 27].
Among the biometry machines, the IOL
Master 700 measures all biometric indices (ACD,
K values, and AL) and has specic formulas for
IOL power assessment [28]. In the thirdgeneration formulae, including the Holladay 1,
SRK/T, and Hoffer Q formulae, postsurgical
ACD depends on the AL of the eye and corneal
curvature [26].
12.5.6 Calculation oftheIOL Power
After Refractive Surgery
The advent of corneal refractive surgery in the
1990s and its increasing popularity have resulted
in those patients now presenting for cataract surgery. The altered corneal prole adds further
complexity to accurately estimating the power of
the IOLs for these eyes. This has been largely
overcome by multiple new-generation calculation formulae. These formulae mainly depend on
the net corneal power based on the direct anterior
and posterior corneal curvature measurement by
advanced topography techniques [29, 30]. This
overcomes the difculty of calculating the keratometric index of the cornea due to changes in the
anterior corneal shape. Some formulae use the
pre- and post-LASIK corneal measurements for
calculating the correct IOL power [9].
The challenge of accurately determining the
IOL power is increased in radial keratotomy
patients due to the unpredictable biomechanical
properties of the cornea. The consensus is to use
the central 3 mm corneal power in such eyes.
Studies have shown that using the Barrett True
K formula produces minimum postoperative
refractive surprises in people with radial keratotomy [9].
12.5.6.1 Clinical History Method
• Holladay rst introduced this method; it is
theoretically easy but unreliable when relevant
data cannot be provided [31].
• The formula for calculating the power for the
IOL is K=KPRE−RCC.
where K Calculated corneal power, KPRE
Corneal power before refractive surgery, RCC
Change in manifest refraction at the corneal
plane
12.5.6.2 Contact Lens OverRefraction Method
• This method, introduced by Riddley, is practi-
cal and inexpensive [32]. However, this
method did not gain popularity as it was inaccurate in eyes with poor visual acuity, such as
in dense cataracts.
• The formula used is K = BCL + PCL
+RCL−RNoCL.
where BCL Contact lens base curve, PCL
Contact lens power, RCL Contact lens overrefraction, RnoCL The spherical equivalent
of the manifest refraction without a contact
lens
12.5.6.3 Topography-Based PostLASIK Adjusted Keratometry
These regression formulae are based on the analysis of post-LASIK corneal topography central K
values (TK) in LASIK eyes. The true corneal
power is predicted using only the single central
postoperative TK.A few popular formulae used
are the Wang Koch Maloney, adjusted Atlas,
Double K, Masket, Shammas, Haigis-L, Hoffer
Q, and Galilei formulae.
The Wang Koch Maloney method uses the
pre- and post-LASIK/PRK anterior and posterior
corneal power to predict post-refractive corneal
power. If pre-surgery corneal power is not available, then a mean posterior corneal power is used
for the calculation [33].
The Shammas formula uses post-refractive
surgery keratometry values for power calculation
[9]. This reduces the dependency on pre-surgery
measurements for calculations.
The Masket formula determines the surgi-
cally induced refractive power along with SRK/T
or Hoffer Q for myopic or hyperopic patients,
respectively. This enables the adjustment of the
predicted IOL power [34].
The Haigis-L formula utilizes the ACD
which does not change with refractive surgery
and corrects for myopic or hyperopic eyes dur-
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ing IOL power calculation [13]. This formula
is incorporated into the IOL Master 700
platform.
The Double K formula uses the pre-refrac-
tive surgery corneal power estimate for predicting the ELP, while the post-refractive
power is used for vergence calculation. This
helps in the better prediction of ELP after cataract surgery. This formula is incorporated in
the SRK/T, Holladay II, and Hofer Q online
calculations [35].
The Barrett True K formula holds promise
for accurate calculation of IOL power in postLASIK eyes [9].
The American Society of Cataract and
Refractive Surgery (ASCRS) website has an
online calculator incorporating all these formulae
for IOL power calculation in post-refractive surgery eyes [9].
The net corneal power for all these newer formulae can be measured using the Pentacam,
Orbscan, or anterior segment OCT.
12.5.7 Piggyback IOLs
Piggyback IOLs are necessary due to residual
high refractive errors caused by inaccurate
biometry, errors in IOL power calculation, and
placement of incorrectly powered IOLs. While
lower residual powers can be corrected by
refractive surgery, larger errors will need either
an IOL exchange or the placement of an additional IOL in the ciliary sulcus to obtain refractive neutrality [19, 36]. The adjustments
required for this secondary IOL placement in
the ciliary sulcus include calculations for:
• Myopic correction: P=1.0×Error
• Hyperopic correction: P=1.5×Error
where P The needed power in the piggyback lens, Error The residual refractive error
that needs to be corrected
The implantation of piggyback IOLs may
lead to intralenticular opacication, iris chang
with pigment release, and a higher risk of
glaucoma.
12.5.8 High Myopia
A study by MacLaren etal. showed that postoperative refraction is overestimated when the
SRK/T formula was used for IOL power calculation with estimates of the AL in extreme myopia
with both ultrasound and optical biometry [37].
Wang etal. evaluated the accuracy of refractive prediction of four IOL power calculation formulae in eyes with ALs >25.0mm and proposed
a method to optimize AL measurements to
improve the accuracy of such calculations.
According to this study, the proposed method
signicantly reduced the percentage of long eyes
with a hyperopic outcome [38].
Holladay1 2-center optimized AL = 0:8814
×IOLMaster AL+2:8701
Haigis 2-center optimized AL = 0:9621
×IOLMaster AL+0:6763
SRK/T 2-center optimized AL = 0:8981
×IOLMaster AL+2:5637
Hoffer Q 2-center optimized AL = 0:8776
×IOLMaster AL+2:9269
In another study, Wang et al. proposed the
Holladay Nonlinear AL adjustment method to
optimize AL calculations in long eyes (AL
>24.00mm) with the Holladay 2 formula [39].
12.5.9 Toric IOL
Toric IOL is a technology that meets the increasing expectations of spectacle-free vision after
cataract surgery in people with astigmatism.
However, refractive surprises can sometimes
happen even after the implantation of toric IOLs.
Repositioning or rotating the IOL is indicated
when the IOL is placed in an incorrect position
during surgery but not when the rotation occurs
in the postoperative period. The Toric Results
Analyzer (https://www.astigmatismx.com),
developed by Berdahl and Hardten, helps analyze
the alignment of an already placed toric IOL.This
toric analyzer helps determine if the refractive
surprise can be decreased by rotating the IOL by
comparing the current location of the toric IOL
12 Biometry andIntraocular Lens Power Calculation
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and a patient's refraction. It simultaneously helps
calculate the required IOL rotation and the
expected residual refraction [40].
12.6 Errors inIol Power
Calculation
Errors in IOL power calculations have three
sources: operator-induced, machine-induced, or
surgeon-induced.
• Operator-induced: This may include biome-
try on the wrong patient, incorrect calculation
formulae or IOL selection, or inaccurate IOL
labelling. In ultrasound biometry, an excessive
indentation of the cornea or improper alignment of the probe may cause wrong IOL
power calculation. The operator may also miss
noting dryness in the eyes during the recording, which may contribute to errors. Manual
entries also are prone to error.
• Machine-induced: Improper calibration of the
machine can cause errors in the IOL power
calculation.
• Surgeon-induced: Incorrect selection of an
IOL and/or improper placement of the IOL
during surgery can lead to postoperative
refractive surprises.
12.7 Techniques Used inIOL
Power Measurement
The pre-operative workup for cataract surgery
includes biometry to obtain measurements for
calculating the most accurate intraocular lens
power for each patient.
12.7.1 Ultrasound Biometry Must
BePerformed by Following
These Steps
• Ask the patient to sit comfortably and look
straight at a xation point to measure the AL
along the visual axis.
• Select the appropriate mode based on the
patient’s eye status, such as cataract, aphakia,
pseudophakia, or silicone oil-lled globe, as
the velocity of ultrasound waves varies in each
medium.
• For contact ultrasound biometry, instill a topical anesthetic drop in the eye, place the ultrasound probe over the cornea without
compression, and align it along the visual
axis. The probe should be sterilized before
use.
• A scleral shell lled with saline is placed
between the eye and the probe for immersion
ultrasound biometry. Proper alignment of the
probe is necessary for accurate AL
measurement.
• The AL value and keratometry readings (measured using a keratometer) must be used in an
appropriate IOL power calculation formula to
calculate the power of the IOL to be implanted.
12.7.2 Optical Biometry Must
BePerformed by Following
These Steps
• Ask the patient to sit in front of the machine
with their chin resting comfortably on the chin
rest.
• Ask the patient to blink their eyes to ensure
the even spreading of the tear lm on the cornea and look at the internal xation target in
the machine.
• The instrument will automatically track the
test eye, and the scanner will move.
• The machine will scan the eyes and retrieve the
data needed for IOL power calculation, including corneal curvature measurements, anterior
chamber depth, WTW diameter, and AL.
• IOL power will be calculated and displayed
according to different lens-specic constants
and the formulae selected. Other data, like
astigmatism measurements, will also help the
surgeon plan the surgery.
Different modes are available in eyes with pre-
vious cataract or retinal surgeries, such as apha-
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kic, pseudophakic, silicone oil-lled phakic/
aphakic/pseudophakic eye, phakic IOL, and piggyback lens.
12.8 Conclusion
The accuracy of IOL power selection for cataract
surgery has advanced signicantly in the past
decade. With the increasing expectations of
patients and constantly evolving IOL designs,
accurate biometry measurements and calculation
of IOL power using appropriate formulae are crucial for successful surgeries with the desired
refractive outcomes and patient satisfaction.
Newer IOL formulae have been created to generate a single algorithm that can accurately determine the appropriate IOL power for a wide range
of eye dimensions. The Barrett Universal II formula has achieved consistently accurate results
across a large range of ALs. The current biometers and formulae have made it possible to achieve
outcomes within the range of ±0.50D with minimum astigmatism for most patients. However,
achieving the target postoperative refraction is
still challenging in silicone oil-lled eyes, irregular corneas, and post-refractive surgery eyes.
Disclosure None.
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Imaging inCataract
https://t.me/med1917
SupriyaSharma , ManeckNicholson ,
PrashantGupta , PrajaktaDandekar ,
andSomasheilaI.Murthy
13
13.1 Introduction
In the quest for improved quality of vision and to
match the increasing expectations of people,
cataract surgical techniques and designs for
intraocular lenses (IOLs) have undergone a
remarkable evolution in the past decade.
Improvement in the quality of vision has been
possible through our improved understanding of
optical aberrations, which can often cause postoperative dissatisfaction. While the introduction
of aspheric IOLs addresses this problem, assessment of pre- operative aberrations is necessary
and is made possible by aberrometry-measuring
devices. Imaging the lens using Scheimpug
technology objectively documents the changes in
the lens and detects various types of cataracts,
which helps in surgical planning. The anterior
segment optical coherence tomography
(AS-OCT) and ultrasound brightness scan (B
scan) have added new dimensions to managing
complex cases. Over the past two decades, IOL
power calculations have been modied using
data from corneal topography, resulting in
increased precision in achieving the targeted
refraction [1, 2]. Common sources of error in calculating refraction include imprecise axial length
measurements and incorrect keratometry [3].
Many keratometry devices do not measure posterior corneal astigmatism, often resulting in under
or over-estimating the total corneal astigmatism,
particularly in eyes where toric IOL implantation
is indicated. Obtaining accurate biometric data is
one of the major challenges in the preoperative
planning of cataract surgery.
Of the various preoperative imaging modalities explored in the past to improve surgical
outcomes, in this chapter we have highlighted
four imaging modalities that help the surgeon
anticipate intraoperative complications and
achieve better postoperative results and patient
satisfaction.
13.2 Aberrometry (iTRACE, Tracey
Technologies, Houston, TX)
S. Sharma (*) · M. Nicholson · P. Dandekar ·
S. I. Murthy
Shantilal Shanghvi Eye Institute, Mumbai, India
e-mail: supriya.sharma@ssei.ind.in;
maneck.nicholson@ssei.ind.in;
prajakta.dandekar@ssei.ind.in;
somasheila.murthy@ssei.ind.in
P. Gupta
Kallam Anji Reddy Campus, L V Prasad Eye
Institute, Hyderabad, India
e-mail: prashant.gupta@lvpei.org
© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2024
T. Das, P. Satgunam (eds.), Ophthalmic Diagnostics, https://doi.org/10.1007/978-981-97-0138-4_13
Aberrations occur when light from a point source
does not converge to or diverges from a single
point after transmission through an optical system [4]. Aberrations are a major cause of poor
quality of vision. The various refracting surfaces
like the tear lm, cornea, and lens are primarily
responsible for inducing ocular aberrations in the
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eye [5]. Several factors affect the quality of
vision, such as refractive/wavefront error, light
scatter, binocular summation, illumination
(pupillary size), and neuro adaptation [6]. The
commonly performed diagnostic procedures to
evaluate the quality of vision include contrast
sensitivity and vision testing in different illumination levels by glare test and aberrometry.
Different aberrometers work on various principles such as Hartmann-Shack (outgoing reection aberrometry), Tscherning, Ray tracing
(retinal imaging aberrometry), and automatic
retinoscopy (slit sciascopy). Aberrationcorrecting IOLs have largely replaced traditional
IOLs, which has helped select the appropriate
IOL for customized cataract surgery.
13.2.1 Principle oftheiTrace
The iTrace uses ray-tracing aberrometry to
dynamically measure and analyze wavefront
aberrometry while also analyzing corneal topography, allowing the surgeon to separate visual
function into a corneal component and an internal optic component to calculate higher order
aberrations and refractive data [7]. This system
combines ray tracing aberrometry wavefront
analysis, advanced corneal topography, autorefraction, pupillometry, and auto keratometry in a
single device. In an emmetropic eye, the 64 laser
beams would focus on the fovea, but due to ocular aberrations at the level of the cornea and lens,
these points shift to other retinal areas with
respect to the reference. This deviation of the
emergent wavefront from the original wavefront,
which entered the eye, is measured as an ocular
aberration.
13.2.2 Image Acquisition Technique
This is a non-contact test. Once the patient is
seated comfortably, the machine takes an image
in about 30s (Fig.13.1). It sends 64 laser beams
four times sequentially, making 256 individual
rays enter through the pupil. The aberrations in
the path of the laser beam (cornea and internal
S. Sharma et al.
Fig. 13.1 Aberrometry using iTrace testing
structures) cause a shift in the location of the
laser beam when it reaches the retina. These laser
beams pass through multiple points on the pupil.
By analysing the process of using the entry and
exit of laser beams, a real wavefront (WF) error is
obtained. This method obtains the forward aberrations of light that go through the pupil.
13.2.3 Clinical Applications
13.2.3.1 Localization oftheOrigin
oftheAberration
The “wavefront and corneal topography” display
on the iTrace help localize the origin of the aberrations [6, 8]. The aberration may originate from
the lens (depicted by the internal optics map),
cornea, or the entire eye. Localizing the exact
source helps in targeted treatment, thus eliminating the aberrations from that source and contributing to better post-operative quality of vision
(Fig.13.2).
13.2.3.2 Selecting Spherical vs.
Aspherical IOL [3, 9]
Asphericity (Q value) is a geometric measure of
ellipsoid surfaces, with a change in curvature
from the center toward the edge. A normal cornea
is prolate with negative asphericity (−0.23μm),
as the center is more curved than the periphery,
inducing a positive spherical aberration of
+0.28μm. However, in young adults, the negative
spherical aberration of the crystalline lens
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155
Fig. 13.2 Chang analysis display map on iTrace. These
maps show color-coded wavefront aberrations of the eye
(internal aberrations [lenticular and retinal] of the eye—
top left corner, yellow dotted box; corneal aberrations—
lower left corner, red dotted box; and total eye
aberrations—top middle, yellow dotted box) measured in
microns of error. The error can be positive or negative (the
red color indicates that the wavefront is in front of the
compensates for the corneal aberration, thereby
maintaining a good quality of vision. This
changes with age and cataract formation. The
residual aberration of the eye after cataract surgery is planned by the visual need of the patient
and preoperative aberration of the cornea. The
gure represents Dr. Devgan’s decision-making
tree for selecting aspheric lenses (Fig.13.3) [10].
13.2.3.3 Preoperative Assessments
forPremium IOLs [11, 12]
13.2.3.3.1 Measurement ofHigher Order
Aberrations (HOAs)
Addressing the HOAs before surgery is required
to prevent post- operative dysphotopsia, glare,
and haloes. These phenomena occur due to the
splitting of the image, resulting in loss of contrast
sensitivity, regardless of the type of multifocal
IOL (diffractive or refractive) implanted. Among
the HOAs, coma (asymmetrical aberration) is
commonly associated with these ocular symp-
reference plane, and the blue color indicates that the
wavefront is retarded in relation to this plane).
Decomposition in each of the Zernike polynomials is represented in red and blue bars depending on the value of the
sign. The map in this gure represents the Chang analysis
display of the right eye showing minimal corneal aberrations (red dotted box); thus, the cause of the higher order
aberrations is likely to be lenticular
toms, and a value of more than 0.32μm is consid-
ered a contraindication for multifocal IOL
implantation. The iTrace helps to separate aberrations from the anterior cornea from those in the
entire eye. If there are minimal HOAs in the cornea, and the overall optical quality of the cornea
is good, the patient will likely benet from a premium IOL (assuming optical alignment, as discussed subsequently, is not an issue). Conversely,
if the cornea shows signicant HOAs and poor
optical quality, the patient may not achieve the
best vision after implanting a premium IOL.
13.2.3.3.2 Measurement ofAngle Kappa
andAngle Alpha
Angle kappa is the angle between the visual and
pupillary axis. Angle alpha is the angle between
the visual axis and the center of the limbus or the
optical axis (Fig.13.4). If the multifocal IOL is
signicantly decentered in the eye due to a large
angle alpha or angle kappa (>0.5 mm), the
patient may look through the diffractive rings
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S. Sharma et al.
Fig. 13.3 Dr. Devgan's decision-making tree for selecting an aspheric intraocular lens based on the patient's ocular characteristics and visual requirements. IOL Intraocular
lens, LASIK Laser-assisted in situ keratomileusis, SA
Spherical aberration
Fig. 13.4 The angle kappa and alpha display of the
iTrace. The visual axis is marked with the red cross; the
center of the pupil (green circle) is marked with the green
cross; the center of the limbus (blue circle) is marked with
rather than the central optical zone as intended;
this induces higher order aberrations and
decreases postoperative visual function. For similar reasons, these angles are also important
while planning a toric IOL, particularly in eyes
the blue cross. The screen displays the measurements of
these distances in both polar and rectilinear coordinates in
the bottom right of the eye image (yellow dotted box)
with against-the-rule astigmatism. Hence, preoperative assessment of the angle alpha and
angle kappa is important, and a value of >0.5mm
is considered a contraindication for multifocal
IOL placement (Fig.13.5).
13 Imaging inCataract
https://t.me/med1917
Fig. 13.5 The Chang analysis display map on the iTrace depicting a high angle alpha (0.67mm @ 346°, yellow dotted
box) in the right eye. This patient may not be an ideal candidate for multifocal IOL implantation
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13.2.3.3.3 Planning forToric IOLs [13]
The iTrace helps one to decide if the patient’s
internal optics compensates for any corneal astigmatism and accordingly decide the need for a
toric IOL.Implantation of toric IOLs should be
preferred in people with high corneal astigmatism but who never required astigmatism correction previously due to the compensation of their
natural lens. The iTrace also offers integrated
tools, allowing surgeons to increase the precision
with which the characteristics of the toric IOLs
are planned. The Hoya Toric Calculator, surgically induced astigmatism analysis, and the
Zaldivar Toric Caliper help to enhance the accuracy of power selection, placement, and postoperative assessment of toric IOLs. The toric
calculator provides several power alternatives
and expected residual cylinder outcomes to help
surgeons choose the appropriate toric power for
the intended result. The precise location of the
placement axis with reference to the usual surgical markings is determined as the last step.
13.2.3.4 Dysfunctional Lens Index
(DLI)
The iTrace visual function analyzer provides
objective lens outcomes such as the dysfunctional lens index (DLI) and opacity grade to
measure crystalline lens degeneration
(Fig.13.6) [14]. DLI values vary from 0 to 10,
with a lower value associated with a more dysfunctional lens suggestive of cataract, while a
higher number is associated with improved lens
performance or optically clear crystalline lens.
The opacity grade metric assesses how much
energy reaches the retina from 128 sequential
and independent beams that enter the pupil. The
device reads the variance and intensity of the
energy reaching the retina generating a map of
the opacity or scatter graded from 0 to 5. This
grading is similar to the lens opacication classication system (LOCS) cataract grading.
Thus, DLI and opacity grade are parameters
which assist in the diagnosis and the optimal
timing of cataract surgery.
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