IOL : POWER CALCULATION & SELECTION - Chapter
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Chapter - 3
IOL : POWER CALCULATION & SELECTION
Precise IOL power calculation is essential for optimal benefits of implant
surgery. Prior to1975, IOL power was calculated on the basis of clinical history, i.e.
pre-operative refractive error prior to development of cataract. This led to errors in
over 50% of cases. However, a number of formulae are now available to accurately
calculate the IOL power required in a patient. All these formulae are based on an
accurate measurement of the corneal power and the axial length.
FORMULAE IN USE
The original formulae were developed prior to 1980. They include the
theoretical formulae and regression formulae. The commonly used formulae are the
regression formulae, of which the most popular one is the SRK formula described by
Sanders D, Retzlaff J. and Kraff M. The formula is based on the following equation:
P = A – BL - CK
where P is the implant power for emmetropia, L the axial length in millimeters, and K
the average keratometric reading in diopters. A, Band C are constants. The value of
B is 2.5 and that of C is 0.9
Thus P = A - 2. 5L - 0.9K
The constant A varies with the implant design and the manufacturer. Be sure
of the constant value of the IOL you are using while making the calculations. The
SRK formula has been found to be reasonably accurate for eyes with axial lengths
between 22mm and 24.5mm. These eyes constitute approximately 75% of cases,
while 14% of cases have axial lengths greater than 24.5 mm, and 10% have axial
lengths less than 22mm. The modified formulae were developed to correct for errors
in these formulae occurring in long and short eyes.
It is for such 'too long' and 'too short' eyeballs that SRK II formula was
introduced. The SRK II formula is a modification of the original SRK formula with the
addition of a correction factor that increases the lens power in short eyes and
decreases it in long eyes.
21The suggested method of modification of SRK to SRK II is shown below:
L (mm) Add to 'A' constant
Less than 20.00 +3
20.00 - 20.99 +2
21.00 - 21.99 +1
Greater than 24.50 -0.5
Modern formulae for emmetropia:
These formulae are more complex than the original and the modified
formulae. The most striking difference is the manner in which the estimated anterior
chamber depth (ACD) value is calculated. The ACD value is a constant value in the
original formulae. It varies with the axial length in the modified formulae (decreases
in the shorter eye and increases in the longer eye). In the modern formulae, ACD
value varies not only with axial length, but also with corneal curvature (being more
with steeper cornea and deep AC and vice versa). The commonly used modern
formulae are the Holladay formula, the SRK-T formula and the Hoffer-Q formula.
KERATOMETRY
Manual keratometry is the most commonly used method to measure corneal
curvature. It is fast, easy and is very accurate in most cases. Keratometry should be
done before axial length measurement, and for both eyes. Remember to calibrate
the eyepiece for your refraction before recording measurements. The procedure of
keratometry using the common Bausch and Lomb keratometer is given here. The
patient is seated behind the keratometer, with the chin well positioned in the chin rest
and the head resting on the head band. The keratometer is directed towards the eye
to be examined and the other eye is occluded. The keratometer is focused on the
central portion of the cornea using the focusing knobs. The instrument is now rotated
to align the (-) signs in the same vertical meridian and the (+) signs in the same
horizontal meridian. This will determine the axis of the pre-existing astigmatism. The
left drum is rotated to superimpose the (+) signs and the horizontal measurement is
read out. The right drum is now rotated to superimpose the (-) signs and the vertical
measurement reading is recorded. The Javal-Shiötz keratometer utilizes two mires to
achieve the end point. IOL power calculation formulae use the average corneal
22power, K = average of the horizontal and the vertical readings. It is important to
remember that the keratometer has to be calibrated every 6 months.
It is advisable to repeat measurement if the -
a. Average keratometry (K) in either eye is less than 40 D or greater than 47 D.
b. Difference in K between the two eyes is greater than 1 D.
c. Corneal cylinder does not correlate well with the refractive cylinder.
In certain situations, like irregular corneal contour or previous refractive surgery, or
when the surgeon wants to better evaluate the astigmatism, corneal topography may
be utilized.
AXIAL LENGTH MEASUREMENTS
The measurement of the axial length is best done with A-scan ultrasonography. It
can be performed by an immersion technique or a contact technique. The machine
should have a screen showing the spikes for ensuring correct measurement. Always
take measurement for both eyes.
Technique
With the contact technique, a drop of local anesthetic is instilled into each eye.
The patient is examined in the seated position. The probe is positioned in front of the
eye and the patient is asked to fixate on the red light in the probe. The probe is then
brought forward to gently touch the cornea. Particular attention and care must be
taken to ensure that the probe is not indenting the cornea. The probe is moved
slightly up and down or to the side to optimize the echospikes displayed on the
machine. Either the operator or the machine selects the optimum pattern and the
reading is obtained.
The immersion technique is
performed with the patient in the
supine position. Topical anesthetic is
instilled and a proper scleral shell is
chosen. The 20 mm shell fits most
eyes. The flared edges of the scleral
Good A-scan. Echos from left to right : cornea,
shell are placed between the lids and anterior lens capsule, posterior lens capsule,
retina, sclera, orbital fat
23the cup is filled with fluid, preferably gonioscopic solution. The ultrasound probe is
immersed in the solution but kept 5-10 mm away from the cornea. The patient is
asked to look with the fellow eye at a fixation point on the ceiling. The probe is then
gently moved till it is aligned with the optical axis of the eye and the a-scan
echogram on the panel is adequate. The reading is then taken.
The contact technique usually yields shorter measurement than the
immersion technique for various reasons. Most modern biometers calculate the axial
length based on separate sound velocities for different eye components (cornea,
anterior chamber, lens, vitreous cavity).
It is recommended that measurements be repeated if the -
a. Measured axial length is less than 22.0 mm or more than 25.0mm
b. Difference between the two eyes is more than 0.5mm.
c. Axial length value seems wrong when compared with refraction.
All measurements should be repeated if following exist:
a. Calculated emmetropic implant power is more than 3D from the average for the
specific lens style used.
b. Difference in emmetropic implant power between the two eyes is more than 1D.
A new device, the IOL Master, yields accurate axial length measurements using
optical coherence techniques.
The A constant
Formulae in use currently utilize constants, which are based on various
factors that affect the refractive state of the eye post-operatively. The Binkhorst and
the Hoffer formulae use the post-operative AC depth, the SRK II and SRK-T
formulae use the A-constant and the Holladay formula uses the S-factor.
The A- constant encompasses multiple variables including the implant
manufacturer, implant style, surgeon’s technique, implant placement within the eye,
and measuring equipment. Because of its simplicity, the A constant has become the
value by which an implant is characterized. The most common A constants used are-
24! Anterior chamber lenses - 115.0-115.3
! Posterior chamber lenses in the sulcus - 115.9-117.2
! Posterior chamber lenses in the bag - 117.5-118.8
In most cases the power of the IOL for emmetropia varies in a 1:1 relationship
with the A constant.
The S-factor used in the Holladay formula is the distance between the iris
plane and the IOL optic plane. The S-factor should be personalized by solving the
formula in reverse. A change in the true post-operative AC depth will affect the
refractive status of the eye. A change in 1 mm causes a 1.5 D change in the final
refraction. Hence, these constants must be personalized to accommodate any
consistent shift that might affect IOL power calculation. Each constant has to be back
calculated for at least 20 cases, with care to ensure that the same person takes the
measurements.
SPECIAL CASES
Intumescent cataracts will yield a 0.15 mm longer axial length resulting in a
+0.4 -+0.5 hyperopia postoperatively. For aphakic eyes being planned for ACIOL or
scleral fixated IOL, the appropriate A constant must be used and the mode of the
machine changed to compensate for the change in speed of the sound waves. In
eyes with silicone filled vitreous, the sensitivity of the system should be increased
to visualize the retinal echospike and the components of the eye must be measured
separately to reach an accurate result. The usage of a standard sound velocity can
lead to an error of upto 8 mm in such eyes. Usually a factor of 0.72 gives a rough
estimate of the IOL power. It is better to refer the patient to a centre capable of
separate measurements for more accurate assessment.
After corneal refractive surgery, the K reading may not truly reflect the
corneal power. Hence the refractive history method or the contact lens method must
be used to obtain corrected K value. In eyes with high myopia, a B-scan
examination is recommended to rule out a posterior staphyloma or other retinal
pathologies. Identification of the posterior pole may be difficult. The problems are
compounded in unilateral cases. While selecting the IOL power for a myope several
factors are to be kept in mind. The surgeon should aim for a -0.50 D to -1.00 D
25postoperative refraction as most sedentary elderly will prefer being near sighted. In
the presence of monocular cataract in a myopic eye when the other eye is
emmetropic, emmetropia should be aimed for if the myopia was induced by the
cataract. However, if the patient has been functioning with monocular vision using
the emmetropic eye for distance and the myopic eye for near, it is better to leave the
operative eye myopic. In patients with hypermetropia the aim should be to achieve
emmetropia. Here, the use of linear formulae can result in large errors in IOL power
calculation in small eyes. In children, it is wise practice to remove the cataract and
use contact lens correction if the surgery is being performed within the first two years
of life, because growth of the eye will result in a large myopic shift if IOL has been
implanted with intraoperative K and axial length measurements. When surgery is
being performed after the age of two years, a myopic shift of 4-6 D is expected
depending upon the age. Undercorrecting the IOL power by around 3 D partially
compensates for this. A greater undercorrection can lead to anisometropia and
difficulty in amblyopia correction. Residual myopia in adulthood can easily be
corrected by spectacles, contact lenses or refractive surgery. As expected, biometry
in children is difficult and may require general anesthesia.
Postoperative refraction (R) for a given IOL power (I) can be
computed as given below:
• For P less than 14.00 R = P-I
• For P greater than 14.00 R = (P-I)/1.25
To calculate the IOL power which would produce a given refraction:
• For P less than 14.00 I = P-R
• For P greater than 14.00 I = P - (R x 1.25)
Choice of IOL Power
The following factors should be considered:-
• The refraction and presence/absence of cataract in the fellow eye.
• Relevance of emmetropia, isometropia & iseikonia.
• Lifestyle of patient: active patients prefer near emmetropia; sedentary patients
may prefer myopia.
• Hedging: it has been found from experience that it is preferable to hedge
towards myopia.
26It is important to remember that a myopic patient would be very unhappy if he
is made hypermetropic. Also, the final refraction results may be +/- 1D either way
from the calculated power.
IOL DESIGN FEATURES:
A variety of design features incorporated in modern IOLs make them very
safe and reduce adverse phenomena and late complications after cataract surgery.
The modern modified C-loop design ensures maintenance of centration and the
square edge design significantly retards the opacification of the posterior capsule.
Plate haptic lens manufacturing has improved and now lenses with a very good
surface can be fashioned. Various modifications of the edge have been tried to
reduce glare and improve contrast sensitivity. A recent development has been the
introduction of multi-focal lenses which are designed to give three zones (distance,
intermediate and near) of clear vision. Still in the research stage are accommodative
lenses which mimic the change in refractive status of the natural lens with
accommodation.
IOL MATERIALS:
IOL materials Advantages Disadvantages
PMMA High optical quality Large incision wound
Large optical centre Not autoclavable
Proven biocompatibility Mild foreign body
Possibility of surface reaction
modification
Good laser resistance
Soft acrylic Foldable Limited experience
Controlled unfolding Possible damage during
Good laser resistance implantation
Good biocompatibility Sticky surface can
Good optical quality adhere to instruments
Hydrogel Good laser resistance Lack of long term
Good biocompatibility experience
Good optical quality
Easy handling
Silicone Good biocompatibility Irreversible adherence to
Less CME silicone oil
Can tear
Slippery when wet
Limited control during
implantation
?Long term discoloration
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