Biometry is the process of measuring the
power of the cornea (keratometry) and the
Axial length of the eye, and using this data to
determine the ideal intraocular lens power.
Measuring the axial length of the eye
Axial length of the eye is measured with A scan
An 1-mm error in measurement of axial length produces
an error of 3 D in IOL calculation.
For accurate measurement of axial length (using
ensure the machine is calibrated and set for the correct
velocity setting (e.g. cataract, aphakia, pseudophakia)
take care with axial alignment, especially with a hand-held
probe and a moving patient.
the gain should be set at the lowest level at which a good reading
don’t push too hard – corneal compression commonly causes
the echoes from cornea, anterior lens, posterior lens, and retina
should be present and of good amplitude misalignment along the
optic nerve is recognised by an absent scleral spike.
average the 5-10 most consistent results giving the lowest
standard deviation (ideally < 0.06 mm)
errors may arise from an insufficient or greasy corneal meniscus
due to ointment or methylcellulose used previously.
Measuring the power of the cornea
Keratometry may be carried out manually or using an
automated or hand-held device.
Accuracy is essential, as an error of 0.75 D in the
keratometry will result in a similar post-operative error.
For accurate manual keratometry :
ensure a good tear film
adjust the eyepiece to bring the central cross-hairs into
make sure that the patient’s other eye is occluded and
that the cornea is centred
take an average of three readings.
if high or low results are encountered (< 40.00 D or >
48.00 D), it is advisable to have a second check.
repeat if the difference in total keratometric power
between the eyes exceeds 1.50 D
in a scarred cornea, use the fellow eye or average the
SRK - I
SRK - II
Holladay - II
Biometry after keratorefractive surgery
Anterior surface is flattened with no change in the
This causes error in keratometry leading to hyperopic
The methods to calculate K values in a Post LASIK eye:
Contact lens correction
It is considered the gold standard.
For this method, keratometric values and the
manifest refraction (MR) from before the
refractive surgery as well as MR at appropriate
intervals after the refractive procedure are
The data is used to calculate the change in MR
from pre to post refractive surgery
That value is subtracted from the original
keratometric value to obtain the “effective
Keratometric Value” for use in IOL power
In this method the post-op MR value that
should be used is the most recent one
obtained before Cataract development as
refraction is effected by the lens change.
Contact lens correction
It is based on the principle that if a contact
lens of base curve(B) is equal effective power
of the cornea then there would be no change
in the refractive error with/without the
This is not very accurate.
This is used when Preop ‘K’ is not known in a
dense cataract patient.
A CVK derived mean anterior corneal curvature
value calculated for a specific 3 mm central
region of cornea, modified based on the amount
of surgicallyinduced refraction change, is used.
But this is not as accurate as other techniques;
because ablative procedure changes
standardised index of Refraction (SIR).
There is 1D overestimation for change of 7D
during the refractive surgery.
Pediatric eye is not a miniaturized adult eye.
It has shorter axial length, steeper cornea with
higher keratometry value and smaller anterior
Errors in axial length measurement affect IOL
power calculation the most, it increases to 3.75 D
per mm in children.
In small eyes with congenital cataract the lens is a
greater proportion of the total axial length and
therefore the average velocity would be faster.
no formula has been proven to have an advantage
newer theoretic formulas such as Holladay II that
take anterior segment measurements into account,
may be recommended for pediatric IOL power
20% undercorrection if the child is less than age 2
years 10% undercorrection for age 2–8 years .
Biometry in aphakia
Two lens spikes are absent
Velocity is decreased
Biometry in pseudophakia
High spike at the lens
TAL = 0.988(AAL) + T[1-(1532/V)]
Secondary piggyback IOL
Only refractive error is required.
Holladay Refractive formula
Hyperopic error : piggyback IOL = 1.5 x R
Myopic error : piggyback IOL = 1 x R
Where R = postoperative spherical
equivalent refractive error.
with an average error of 2.5 D per millimeter of axial length in adults;