This document discusses biometry and intraocular lens (IOL) power calculation. It begins by defining biometry as the analysis of biological data using mathematical and statistical methods. It then describes various biometry techniques including A-scan ultrasound to measure axial length, keratometry to measure corneal curvature, and different formulas used to calculate IOL power. Over generations, the formulas have evolved from theoretical to regression-based approaches using parameters like axial length, keratometry readings, and A-constants specific to IOL designs. Proper technique and quality checks are important for accurate biometry and IOL power calculation to achieve the desired refractive outcome.

1. BIOMETRY
BY
DR.NIKITA JAISWAL
1ST YEAR RESIDENT
IMS & SUM HOSPITAL

2. Biometry :The analysis of
biological data using
mathematical &
statistical methods.

3. Glossary
 Measurement of axial length
 Keratometry{ measurement of k reading}
 Iol power calculation

4. A-SCAN
 Axial length-it is the distance between the anterior surface of the cornea and the
fovea.
 A-scan is used to measure axial length.
 It measures the time required for a sound pulse to travel from the cornea to retina.
 Normal axial length of an eye is –----22-25mm
 1 mm error of IOL = 2.88 D 0r 3.0 D of IOL POWER

5. Ultrasonography
 Crystal oscillates—high frequency sound wave penetrates into eye—sound wave
encounters a media interface—part of the sound wave is reflected back to the
probe
 USG doesn’t measure the distance rather it measures the TIME required for a sound
pulse to travel from cornea to retina.
cornea 1620m/s
Anterior
chamber
1532m/s
Lens thickness 1641m/s
Vitreous cavity 1532m/s
1555m/s

6. Types of a scan {methods of
measurement}
1- ultrasonic measurement –this comprises of –applanationn method
immersion technique
2- optical measurement –this uses partial coherence laser ,the iol master measures
time required for infrared light to travel to the retina & this technique doesn’t
requires contact with the globe. {iol master}

7. Applanation
Things required:
A scan machine
Probe
Anaesthetic drops
Chair
Patient

8. Procedure:
 Anaesthetize the eye
 Touch the probe
 “DO NOT PRESS OVER THE CORNEA”
 NOTE THE READING
 a=initial spike(probe tip& cornea)
 b=ant lens capsule
 c=post lens capsule
 d=retina
 e=sclera
 f=orbital fat

9. Immersion A scan biometry
 The ultrasonic beam is coupled to the eye through fluid.
 Because of no corneal compression the results display true axial length.
 Procedure:pt lies supine
looking up at the ceiling
scleral shell is placed between the eyelids centered over the
cornea.

10. Scleral shell
Filled with 40-60
mixture of goniosol &
dacriose & the probe
tip into the solution
Align the ultrasound
with macula by asking
the pt to look to the
fixation light of the
probe.

11. Reading
a-probe tip
b- cornea- double peaked
echo will show both ant &
post surfaces
c-anterior lens capsule
d-posterior lens capsule
e- retina this echo needs to
havre sharp 90’ take off from
baseline
f-sclera
g-orbital fat

12. Essentials :
 Calibration
 Gain or sensitivity
 Sound velocity

13. Calibration: it is done with the help of model eye.
Required time to time
Instructions are specific and the model eye is provided
with the manufacturers

14. Gain/sensitivity:
 Gain = electronic amplification of the sound waves received by the transducer …{
is called as a decibel(db)}
• Normal is 70% in most of the
biometers
Normal
• When the echo height is inadequate.
• High myopia,dense cataracts,ocular opacities
Increase
• When artifacts are seen
• Eg: silicone oil, pseudophakic eye
decrease

15. Features of a good scan:
 One dimensional image in which spikes of variable hts are seen:
 Cornea: single tall peak
 Aqueous chamber: does not produce any echo
 Ant & post lens capsule : produce tall echoes
 Vitreous: no to few echoes
 Retina: tall,sharp echoes with staircase at the origin
 Orbital fat produces : medium to low echoes

16. Keratometry : It is used to measure the corneal
curvature.
In the optical area i.e 2-3 mm

17. History :
Helmholtz—javal & schiotz---Bausch & laumb{reichert}
{1854} {1881} {1932}
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18. HELMHOLTZ: This keratometer consist of two plates which
displaces the image through half of its length & the total displacement
gives the size of image.

19. Javal schiotz

20. Javal schiotz- This is on the principle of “variable object size
constant image size”
The object
Objective lens & doubling prism
The eyepiece lens

22. The object: 2 mires A & B mounted on an arc
A+B= object thus of variable size.
“Stepped mire
rectangular mire”
--divided horizontally through the center.
Image of two mires=object.

24. Bausch & laumb
keratometer (1932)
“constant object size
variable image size”

25. B & L:- THE OBJECT {CIRCULAR MIRE}
The imag eof the circular mires appears on the patients cornea
appears diminished & serves as object.
The objective lens : from the image of the mire (new object)

26. DIAPHRAGM & DOUBLING PRISMS: -A 4 APERTURE DIAPHRAGM
-A 2 DOUBLING PRISMS{ONE WITH
BASE UP AND ONE WITH BASE OUT}
moved independently
parallel to the central axis
UNIQUE– Image doubling mechanism is unique
image is produced side by side or at 90’
thus also k/as “one position keratometer”

28. Procedure:
Instrument:calibration is done with the a steel ball along with the
machine with a known radius of curvature mires formed correctly machine is
calibrated.
PATIENT: chin on chin rest
head on head rest h
occlude occludes the non examining eye
patients pupil & projective knob at the same level

29. Focusing of mire:center of cornea
patients view
examiners view

30. Reading:
perfectly spherical
mires.
Oval mires
horizontal-with the rule astig
Vertical –against with the rule
Keratoconus:irregular
Jumping of mires
Wavy mires

32. Uses:
--It helps to assess the radius of curvature of cornea.
--To monitor the shape of cornea in keratoglobus & keratoconus.
--The K readings have been taken to measure the iol power with axial
length by SRK formula.

33. Limitations:
refractive status of very small central area of cornea is
measured{3-4 mm}
It loses its accuracy when measuring very flat and steep
cornea.
Small corneal irregularities would preclude the use due
to irregular astigmatism.

34. Sources of error:
Improper calibration
Position of the patient at fault
Any corneal pathology
Examiners fault
Any lid position abnormality
tearing.

35. Recent advances :
automated keratometer

36. IOL calculation

37. Formulaes
 Theoretical formulaes:
 This measures IOL based on principles
from schematic eyes.
 Regression formulaes:
 These formulaes arrived after
postoperative outcomes.
With the age the formulaes changed to
-First generations
-Second generations
-Third generations
-Fourth generations

38. First generation
Theoretical
formulaes
Binkhorst formula:
P=1136(4r-a)/(a-d)(4r-d)
P =iol power
r=corneal radius in mm
a=axial length in mm
d=assumed post op acd plus corneal
thickness
Colenbrander –hoffer:
P={1336/a-d-.05}-{1336/1336/k-d-.05}
Gill’s formula
P=129.40+(-108*k)+(-2.79*L
eye)+(0.26*LCL)+(-0.38*ref)
K= ref power in D
L eye=AL in mm
LCL=dist of apex of ant corneal
surface
Ref=desired post op refraction
FYODOROV:
P=1336-LK/(l-c)-CK/1336
CLAYMAN’S FORMULA:
ASSUME
EMMETROPIZING IOL=18D
EMMETROPIC AL=24mm
Emm avg kertometr reading=42.0 D

39. Drawback :
- cumbersome
- guess work
- less accurate
- wrong prediction
- based on simplistic assumptions
about the optics of the eye

40. Regression formula
 SRK 1 (sanders ,retzlaff & kraff)
 a breakthrough in calculating iol: they analysed the post op results and found that the
theoretical formulaes can be odified
 They replaced ACD with A constant which was unique for diff types of iols
 P=A-2.5L-0.9K
 P= IOL POWER
 A=A CONSTANT
 L=AXIAL LENGTH
 K=AVERAGE KERATOMETRY IN DIOPTRES

41. SECOND GENERATION
THEORETICAL FORMULAES
 BINKHORST IN 1981 IMPROVISED IT
BY USING A SINGL EVARIABLE
PREDICTOR the AXL and presented
with a formula to predict better ACD
REGRESSION FORMULAE
 The basic were same
 A-const was modified
<20m
m
A+3.0
20-
20.99
A+2.0
21-
21.00
A+1.0
22-24.5 A
>24.5 A-0.5
<20 mm A+1.5
20-21mm A+1.0
21-22mm A+0.5
22-24.5mm A
24.5-26 mm A-1.0
>26 mm A-1.5
MOD
SRK II
SRK II

42. A-CONSTANT:
The concept was originated for the SRK equation & depends on multiple
variables including IOL manufacturer,style & placement within the eye.
-Theoretical value that relates
the lens power to AL.
-used directly in SRK II
-it is not expressed in units.
Specific to the design of the
iol
THE POWER OF THE LENS VARIES 1:1 relationship with the A-
constant
If A decreases by 1 D,IOL power decreases by 1 dioptre also.

43. THIRD
GENERATION
HOLLADAY 1 FORMULA
HOLLADAY II FORMULA
HOFER Φ FORMMULA
FOURTH
GENERATION
HOLLADAY II FORMULA
HOLLADAY CONST. IOL
PROGRAMME

44. FORMULAES AXIAL LENGTH
SRK1 22.0-24.5MM
HOFFER Q <24.5MM
SRK/ T >26.0MM
HOLLADAY I NORMAL as well as AL 24.5-26.0mm
4th generation More universal application
A recent study in 2011 showed
this formulaes on 8000 eyes

45. IOL : THE SECOND CHANCE OF VISION
 A secondary IOL back up in the OT.
 The staff should be aware of the power to be used.
 Proper labelling of the iol along with patient’s name.
 ACIOL should be calculated & to be kept ready in any eventful condition.