1. Jagdish Dukre

2. 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.

3. Measuring the axial length of the eye
Axial length of the eye is measured with A scan
ultrasonography.
An 1-mm error in measurement of axial length produces
an error of 3 D in IOL calculation.

4. For accurate measurement of axial length (using
applanation):
 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.

5.  the gain should be set at the lowest level at which a good reading
is obtained.
 don’t push too hard – corneal compression commonly causes
errors.
 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.

6. 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.

7. For accurate manual keratometry :
 ensure a good tear film
 adjust the eyepiece to bring the central cross-hairs into
focus.
 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
results.

8. IOL Formulae
 First generation
 Second generation
 Third generation
 Fourth generation

9. First generation
Theoretical
1. Binkhorst
2. Colenbrander-Hoffer
3. Gilll’s
4. Clayman
5. Fyodorov
Regression
SRK - I

10. Second
generation
Theoretical
Modified Binkhorst
Regression
SRK - II

11. Third generation
 Holladay I
 SRK/T
 Hoffer Q

12. Fourth generation
 Holladay - II

13. Biometry after keratorefractive surgery
 Anterior surface is flattened with no change in the
posterior radius.
 This causes error in keratometry leading to hyperopic
IOL estimation.

14. The methods to calculate K values in a Post LASIK eye:
 Clinical history
 Contact lens correction
 Topography

15. Clinical history
 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
needed.
 The data is used to calculate the change in MR
from pre to post refractive surgery

16.  That value is subtracted from the original
keratometric value to obtain the “effective
Keratometric Value” for use in IOL power
calculation.
 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.

17. 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
contact lens.

18. Topography
 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.

19. Pediatric biometry
 Pediatric eye is not a miniaturized adult eye.
 It has shorter axial length, steeper cornea with
higher keratometry value and smaller anterior
chamber depth.
 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.

20.  no formula has been proven to have an advantage
over others.
 newer theoretic formulas such as Holladay II that
take anterior segment measurements into account,
may be recommended for pediatric IOL power
calculation.
 20% undercorrection if the child is less than age 2
years 10% undercorrection for age 2–8 years .

22. Biometry in aphakia
 Two lens spikes are absent
 Velocity is decreased

23. Biometry in pseudophakia
 High spike at the lens
 Holladay formula
 TAL = 0.988(AAL) + T[1-(1532/V)]

24. 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.

25. THANK
YOU