Slides are regarding A scan biometry in ophthalmology and its basic principles and updated knowledge about the same.

1. A Scan-
Basics and Update
Dr. Pravin Madhukar Bhat
Director,Prathamesh Eye Clinic, Pune

2. Wel Come all……..

3. INTRODUCTION
 Innovative techniques and advanced technology have
greatly improved cataract surgery over the past few
years.
 There is an increased quest for accuracy, and patients
are now seeking better results.
 As a result, accurate biometry and power precision of
IOLs have gained greater importance.
 Several factors influence the refractive outcome.
 Axial length (AL) measurements are essential for
determining the accuracy of the IOL calculation and are
probably the element with the largest potential for error.
 Methods are still evolving, but ultrasound (US) biometry
and partial coherence interferometry (PCI) are the most
commonly used methods for determining the IOL power.

4.  Traditionally, axial lengths have been measured using
ultrasound biometry, a time consuming exam that
requires skilled biometrists.
 Contact A scan ultrasonography is a well established
method for measuring axial lengths but immersion A-
scan technique is potentially more accurate, since it
does not require indentation of the cornea.
 More recently, PCI has emerged as a new modality
for biometry with the advantages of being fast, non
invasive and less dependent on technician expertise.
 The precision of IOL power calculations depends on
more than just accurate biometry, or the correct
formula, but in reality is a collection of interconnected
nuances. If one item is inaccurate, the final outcome
will be less than optimal.

5. Components to Calculate IOL Power

6. Axial Length Measurement
 Axial length is defined as the distance from the anterior
corneal surface to the retinal pigment epithelium.
 By A-scan biometry, errors in axial length measurement
account for 54% of IOL power error when using two-variable
formulas.
 Optical coherence biometry has been shown to be
significantly more accurate and reproducible and is rapidly
becoming the prevalent methodology for the measurement of
axial length.
 Ultrasound:
 Axial length has traditionally been measured using
ultrasound biometry.
 By measuring the time required for a portion of the sound
beam to return to the ultrasound probe, the distance can be
calculated (d = v × t)/2.
 Clinically, applanation and immersion techniques have been
most commonly used.

7.  Applanation Technique
 With the applanation technique, the ultrasound probe
is placed in direct contact with the cornea.
 After the sound waves exit the transducer, they
encounter each acoustic interface within the eye and
produce a series of echoes that are received by the
probe.

9.  The axial length is the summation of the anterior
chamber depth, the lens thickness, and the vitreous
cavity.
 The applanation technique requires direct contact with
the cornea, compression will typically cause the axial
length to be falsely shortened.
 This method of ultrasound biometry is highly operator-
dependent.

10.  Immersion Technique
 The currently preferred A-scan method is the
immersion technique.
 Unlike the applanation echogram, the immersion
technique produces an additional spike corresponding
to the probe tip. This spike is produced from the tip of
the probe within the coupling fluid.
 Axial length is calculated from the measured time and
the assumed average speed that sound waves travel
through the eye.
 Length correction can be performed simply using the
following formula:
True length = [corrected velocity/measured velocity] ×
measured length

12.  Optical Coherence Biometry
Introduced in 2000, and has proved to be an
exceptionally accurate and reliable method
of measuring axial length.
Principle is similar to conventional ultrasound
A-scan which uses ultrasonic pulse echo-
imaging technique.
The patient is asked to fixate on an internal
light source to ensure axiality with the fovea.
When the reflected light is received by the
instrument, the axial length is calculated
using a modified Michelson interferometer.

14.  Advantages of optical coherence biometry:
 It can measure pseudophakic, aphakic,and phakic
IOL eyes. It can also measure through silicone oil.
 It uses a much shorter wavelength than ultrasound,
so axial length can be more accurately obtained.
 It permits accurate measurements when posterior
staphyloma are present.
 The IOL Master also provides measurements of
corneal power and anterior chamber depth, enabling
the device to perform IOL calculations using newer
generation formulas, such as Haigis and Holladay 2.
 Both the IOL Master and immersion ultrasound
biometry have been shown to produce a
postoperative refractive error close to targeted values

15.  Since optical biometry measures to the center of the macula, it
gives the refractive axial length versus the anatomic axial length
achieved with ultrasound biometry.
 Optical biometry also incorporates actual thickness of the retina,
whereas ultrasound adds a standard 200um to the axial length.
 Partial coherence interferometry based biometry presents an
alternative for precise ocular measurements, used not only for axial
length, but anterior chamber depth, pachymetry and lens and
retinal thickness measurements.
 Limitations:
 Inability to measure through dense cataracts and other media
opacities that obscure the macula; due to such opacities or fixation
difficulties, approximately 10% of eyes cannot be accurately
measured using the IOL Master .
 Measurements not possible in patients with associated nystagmus,
tremors or lid abnormalities cannot fixate well.
 Needs vision of atleast 20/200 for fixation

17.  Keratometry
 Errors in corneal power measurement can be an equally
important source of IOL power calculation error, as a 0.50 D
error in keratometry will result in a 0.50 D postoperative error
at the spectacle plane.
 A variety of technologies are available,including manual
keratometry, automated keratometry, and corneal
topography.
 These devices measure the radius of curvature and provide
the corneal power in the form of keratometric diopters using
an assumed index of refraction of 1.3375.
 Important sources of error are corneal scars or dystrophies
that create an irregular anterior corneal surface.
 These lesions can often be seen with slit lamp
biomicroscopy, their impact on corneal power measurements
can best be assessed by examining keratometric or
topographic mires.

18. Anterior Chamber Depth Measurement
A-scan biometers and the IOL Master
calculate anterior chamber depth as the
distance from the anterior surface of the
cornea to the anterior surface of the
crystalline lens.
 In some IOL calculation formulas, the
measured anterior chamber depth is used
to aid in the prediction of the final
postoperative position of the IOL (known
as the effective lens position, or the ELP).

19. Which Formula should use?
?????

20. IOL Calculation Formulas
 The first IOL power formula was published by Fyodorov and
Kolonko in 1967 and was based on schematic eyes.
 The other formulas from Colenbrander, Hoffer, and Binkhorst
incorporated ultrasound data.
 In 1978, a regression formula was developed by Gills,
followed by Retzlaff, then Sanders and Kraff, based on
analysis of their previous IOL cases. This work was
amalgamated in 1980 to yield the SRK I formula.
 In the 1980s, further refinement of IOL formulas occurred
with the incorporation of relationships between the position of
an IOL and the axial length as well as the central power of
the cornea.
 There are two major types of IOL formulas. One is
theoretical, derived from a mathematical consideration of the
optics of the eye, while the other is empirically derived from
linear regression analysis of a large number of cases.

21.  IOL FORMULA
There are two major categories of IOL formulae
Theoretical Formula
 This formula is based on an optical model of the eye. An optics equation is solved to
determine the IOL power needed to focus light from a distant object onto the retina.
 The most popular formula in this group is the Binkhorst formula. This is based on
sound theory. All the theoretical formulae can be algebrically transformed into the
following
P=[N/(L−C)]—[NK/(N−KC)]
where, P=Dioptric power of the lens for emmetropia,
N=Aqueous and vitreous refractive index, L=Axial length (mm),
C=Estimated postoperative anterior chamber depth (mm), and
K=Corneal curvature [D].
 Binkhorst has made a correction in his formula for surgically induced flattening of the
cornea, using a corneal index of refraction of 1.333.
D=1336 (4r−a)/(a−d) (4r−d)
where, D=Dioptric power of IOL in aqueous humor,
1336=Index of refraction of vitreous and aqueous,
r=Radius of curvature of the anterior surface of the cornea,
a=Axial length of the globe (mm), and d=Distance between the anterior cornea and the
IOL.
 Theoretical formulae help the surgeon to anticipate what should result, not what
will result from implantation.

22.  Regression Formula (Empirical Formula)
 The regression formulae or empirical formulae are derived from empirical
data and are based on retrospective analysis of postoperative refraction
after IOL implantation.
The most popular regression formula is the SRK formula which was
developed by Sanders, Retzlaff and Kraff in 1980.
This is P=A−2.5 L−0.9 K
 Where, P=Implant power to produce emmetropia, L=Axial length
(mm),K=Average keratometer reading, and A=Specific constant for each
lens type and manufacture.
 The SRK formula calculates the IOL power by linearly regressing the
results of previous implants.
 If the Binkhorst formula predicts that a 28- diopter lens should be used,
the SRK formula will predict that a 26-diopter lens should be used. In
lenses with low power, if the Binkhorst formula predicts that a 10-diopter
lens is necessary, the SRK will predict that a 12-diopter lens should be
used.

23.  The new generation formulas:
Formulas to be detailed in the following include: SRK(I,II), Hoffer Q, Holladay(I,II),
Olson and the more recently formulas of Haigis d-formula and Lin’s S-formula.
A. SRK formula:
1. SRK I formula: It is basic regression formula. It is given by P=A−2.5 L−0.9 K
Where, P=Implant power to produce emmetropia, L=Axial length
(mm),K=Average keratometer reading, and A=Specific constant for each lens
type and manufacture.
2. SRK II formula: Here A- constant is adjusted to different axial length ranges.
It is given as P=A1−2.5 L−0.9 K Where A1= Adjusted constant
A1 = A+3 if L<20mm; A1 = A+2 if L=20-21mm
A1 = A+1 if L=21-22mm; A1 = A if L= 22-24.5mm; A1 = A-0.5 if L>24.5mm
3. SRK/T formula: Regression formula for ACD is used to calculate IOL power
based on Fyodorov formula. This formula is more accurate than SRK I and II.
ACD post = ACD*3.336+Corneal height(H),
Where ACD is related to the manufacturer’s A constant by
ACD= 0.62467*A*68.747
B. Hoffer Q formula (1993): P=f(A,K,Rx,pACD) which is a function of
A= Axial length, K= avarage corneal refractive power, Rx= refraction,
pACD= personalized ACD constant.

24. C. Holladay formula:
Holladay I: The initial formula uses basic Surgeon Factor. It can be calculated from the A
constant provided by lens manufacturer.
Holladay II: The IOL power is calculated based on the Binkhorst formula as in Holladay I.
D. Olson formula (2003): Regression formula
ACD post= ACD mean+ 0.12H+0.33 ACD pre+0.3T’+0.1L*5.18
Where H is the corneal height, T’ is the natural lens thickness. This formula only apply to
phakic eyes, for aphakic or pseudophakic eyes the coefficients will change.
E. Haigis formula:
It uses three constant to set both the position and shape of a power prediction curve.
d= a0+(a1*ACD)+(a2*AL)
Where d= the ELP, ACD= Anterior Chamber Depth of the eye, AL= Axial length of the eye,
a0 constant= same as lens constant for the different formulas given before, a1 constant=
tied to ACD, a2 constant= Measured axial length. The a0, a1, a2 constants are derived by
multivariable regression analysis.
F. Lin’s S-Formula
It includes the effect due to natural lens and primary IOL which are totally neglected by
other formulas
S=ELP+gT
g=1/[1+Z”(P1/P2)
Z”=1-T(P2/1336)
Where T is the IOL thickness, and the geometry factor is determined by the ratio of the IOL
front and back surface power P1/P2. g could be positive(P1/P2>0) or negative (P1/P2<0)

25. Confused?????

26. Here is the solution…….
 The Holladay 1 formula, works well for eyes of normal to
moderately long axial lengths, while the Hoffer Q has been
reported to be better suited to normal and shorter axial
lengths.

27.  The Second and Third Generation of IOL Formulas
 Commonly used lens constants do not take variations into account.
These include:
SRK/T formula—uses an “A-constant,”
Holladay 1 formula—uses a “Surgeon Factor,”
Hoffer Q formula—uses a “Pseudophakic Anterior Chamber Depth”
(pACD).
 Optimized constants for each formula for positive diopter and
negative-diopter IOLs.
Constant Positive-Diopter IOL Negative-Diopter
IOL
Haigis a0 5.74 -4.01
SRK/T A 126.63 104.43
Holladay 1 sf 10.46 -6.48
Hoffer Q pACD 16.15 -4.86

30. Special circumstances and solution…….

31.  Capsular Bag to Ciliary Sulcus IOL Power Conversion
 A reduction in an IOL power is typically required with an
unanticipated intraoperative tear in the posterior lens capsule.
 The power adjustment necessary between the capsular bag and
the ciliary sulcus will depend on the power of the capsular bag IOL .
 The important concept is that for stronger intraocular lenses, the
reduction in power must be greater. For very low IOL powers, no
reduction in IOL power is required.
Capsular bag
IOL power
Ciliary sulcus power
adjustment
+35.00 D to +27.50 D –1.50 D
+27.00 D to +17.50 D –1.00 D
+17.00 D to +9.50 D –0.50 D
+9.00 D to -5.00 D No change

32.  Determining IOL Power Following Corneal Refractive Surgery
 The true corneal power following corneal refractive surgery is difficult to obtain
by any form of direct measurement.
 The measurement method can be divided into those that require preoperative
data and those that do not.
 Methods requiring Historical Data
 Clinical History Method:
 Feiz-Mannis IOL Power Adjustment Method
 Masket IOL Power Adjustment Method
 Topographic Corneal Power Adjustment Method
 Methods Requiring No Historical Data:
 Hard Contact Lens Method
 Modified Maloney Method
 Hyperopic Corneal Refractive Surgery
 Corneal Transplantation
There is presently no reliable method for calculating IOL power for eyes
undergoing combined corneal transplantation and cataract removal with IOL
implantation. This is because it is impossible to accurately predict the central
power of the donor graft.

33.  IOL Power Calculation in patients with Silicone Oil
 Best carried out with the patient seated as upright as
possible, especially if the vitreous cavity is partially filled with
silicone oil.
 In the recumbent position, the less dense silicone oil will shift
away from the retina, toward the anterior segment. This can
lead to confusion as to the correct interpretation of the
position of the retinal spike.
 The refractive index of silicone oil is much less than that of
vitreous. Hence, it acts as negative lens in the eye.
 To prevent the silicone oil from altering the refractive power
of the posterior surface of the IOL, it is preferable to implant
polymethyl methacrylate (PMMA) convexplano lenses, with
the plano side oriented toward the vitreous cavity and
preferably over an intact posterior capsule.
 For patients who may possibly undergo a silicone oil
procedure at some point in the future, it is recommended that
bilateral baseline axial length measurements be carried out.

34.  IOL Power Calculation in Pediatric Cataract
 In infants, there is rapid growth of the eye and thus increase in
axial length during the first 2 years of life.
 In toddlers and older children, the eye continues to grow,
although at a slower pace. As against the axial length, the corneal
power drops considerably during the first 2 years of life.
 Between the ages 2 to 5 years axial length growth slows to about
0.4 mm per year and only increases another 1 mm from 5 to 10
years, while corneal power remains stable.
 It is recommended that childrens’ eyes should be undercorrected
at the time of surgery, to offset the myopic shift that occurs in
there growing eyes.
 For children under 2 years they advice to undercorrect the IOL
Power by 20%, since axial length and keratometry readings
change rapidly.
 For children older than 2 years they advice to undercorrect IOL
power by 10%. This helps in minimizing the need for an IOL
exchange later in life, when myopic shift occurs.

35.  IOL Power Calculation in Patients with High Myopia
 Posterior pole staphyloma temporal to the fovea is
commonly present in eyes with axial lengths longer
than 30 mm. The distance from corneal vertex to
fovea is 0.5 to 1.5 mm shorter than the distance from
corneal vertex to the bottom of the staphyloma.
 Hence, current third and forth generation IOL power
calculation formulas have a tendency to give the IOL
power lower than what is necessary, leaving patients
with postoperative hyperopia.
 The use of B-scan ultrasonography to identify the
location of a posterior pole staphyloma is necessary.
 Few refinements in preoperative measurement
techniques helps to improve the accuracy of IOL
calculation in eyes with extreme myopia.

37.  Accuracy of IOL power: Common pitfalls
The factors which significantly affects the accuracy of
IOL power calculations are:
1.The error in preoperative biometry with regards to the
difference between post and preoperative axial length
measurement.
2.The position of the implantation of intraocular lens.
3. The style of intraocular lens
4.The preoperative corneal astigmatism
5.Surgically induced corneal astigmatism
6. The post operative asigmatism
7. The true corneal power (Post- LASIK)
8. The formulas used to find IOL power
9. Assumptions of thin layer or 2-optics system

39.  Future Trends
 As technology and patient expectation increases, one must continue to look to
improve our own precision.
 Formulas: Holladay IOL consultant (HIC) program uses Holladay 2 formula
and performs complex power calculation for the surgeon. It has various other
new features, but it still not widely available. Oslens’s formula uses pre-
operative refraction and lens thickness for increasing accuracy.
 Software based: Okulix
A new biometric computer program to stimulate whole pseudophakic eye aims
to reduce calculation error and ensure a more reliable estimation of IOL
strength. This approach separates the errors due to measurement and those
due to calculation, helping us in correcting them better.
 Newer Machines: The Pentacam and Orbscan have already been used
widely for corneal ectasia, but are now beginning to be utilized for precise
corneal power measurements. (28)
 IOL with tolerance
It has been suggested that manufacturers reduce the internal – tolerance levels
of IOLs to +/- 0.25D, thereby increasing accuracy. However, these parameters
are not routinely provided by manufacturers to clinicians.

40. Now its time to conclude……

41. Takehome message
 The refractive results of cataract surgery has greatly improved
due to recent advances in biometry. Accurate measurement of
preoperative axial length is required for accurate IOL power
calculation.
 The methodology for accurately calculating IOL power in normal
and complex eyes has improved dramatically in recent years.
 Future advances are needed in all areas, including methods of
measuring corneal power, predicting effective lens position, and
perhaps even measuring axial length.
 The ultimate solution may be an IOL whose spherical and
astigmatic power and higher order aberrations can be modified
postoperatively.
 Ideally, such an IOL could be modified multiple times to adapt to
the patient’s changing visual needs and to compensate for aging
changes of the cornea.

42. THANK YOU