Intraocular Lens (IOL) power calculation is a crucial step in cataract surgery and certain refractive surgeries like phakic IOL implantation. The goal is to determine the appropriate power of the IOL to be implanted into the eye, ensuring that the patient achieves their desired postoperative visual outcome. Several formulas and methods are available for IOL power calculation, and the choice of formula depends on various factors, including the patient's eye measurements and the surgeon's preference. Here, we describe the basic principles and some commonly used formulas. Ocular Biometry: Ocular biometry is the process of measuring various dimensions of the eye, primarily the axial length, corneal power, and anterior chamber depth. These measurements are essential for accurate IOL power calculation and achieving the desired post-surgical refractive outcome. Here are the key components of ocular biometry: Axial Length: This measurement determines the overall length of the eye, from the cornea's front surface to the retina's back surface. Axial length is a critical factor in IOL power calculation because it helps determine the eye's focusing power. Corneal Power: The cornea is the transparent front surface of the eye, and its curvature affects the eye's refractive power. Corneal power is typically measured using techniques like keratometry or corneal topography. It helps account for the eye's astigmatism and assists in selecting the appropriate IOL. some basic notions on how they are measured is explored here.

1. BY
Ayienwi Frubih Wilson
Ophthalmic nurse tutor. RN.COA
BIOMETRY AND IOL
CALCULATION

2. Outline of presentation
PRE TEST
I- INTRODUCTION
A.DEFINITION OF OCULAR BIOMETRY
II-ANATOMY OF THE EYE
Key Components Related To Ocular Biometry
III- TECHNIQUES IN OCULAR BIOMETRY
IV- MEASUREMENT OF AXIAL LENGTH (AL)
V-MEASUREMENT OF CORNEA POWER
VI-FORMULA FOR IOL POWER CALCULATION
VII- BIOMETRY IN PEDIATRIC CASES
VII- QUESTIONS

3. PRE-TEST
1). which ocular biometry technique is most suitable for
measuring the corneal radius of curvature and corneal
astigmatism?
a) A-scan ultrasound
b) Keratometry
c) B-scan ultrasound
d) Applanation tonometry
2). what does A-scan ultrasound measure?
a) Corneal curvature
b) Anterior chamber depth
c) Axial length
d) Lens power
3). which type of error can occur if the eye is not properly
aligned during optical biometry?
a) Spherical aberration
b) Iris capture
c) Astigmatism
d) Parallax error
4). A patient's axial length measures 23 mm in one eye and 25
mm in the other eye. What condition might this difference
suggest?
a) Amblyopia (lazy eye)
b) Presbyopia
c) Anisometropia
d) Cataracts

4. INTRODUCTION
Ocular biometry :
This is the process of measuring the power of the cornea
(keratometry) and the Axial length of the eye, by using this data
to determine the ideal intraocular lens power

5. Anatomy of the eye

6. Key components related to Ocular Biometry
A)AXIAL Length
B)ACD
C)Lens thickness
D)Cornea
curvature

7. Axial Length:
Distance from the corneal surface to the retina at the back of the eye.
Range: In adults, is typically between 22 to 24 millimeters.
Anterior Chamber Depth:
The distance between the corneal endothelium and the front surface of the lens.
Range: In adults, is around 2.5 to 3.5 millimeters.
Lens Thickness:
In adults, lens thickness is typically between 4 to 5 millimeters.

8. TECHNIQUES IN OCULAR BIOMETRY

9. Techniques in Ocular Biometry
Ocular biometry involves various techniques for measuring
different eye structures and parameters.
Some of the commonly used techniques:
1). A-Scan Ultrasound (contact and immersion)
2). B-Scan Ultrasound
3). Optical Biometry (IOL Master,lenstar)
4). Keratometry

11. MEASUREMENT OF AXIAL LENGTH (AL)

12. Techniques of Axial Length Measurement
1).A-Scan Ultrasound Biometry
A).Contact
B).Immersion
2). `B-Scan
3). Optical Biometry
A).Lenstar
B).IOL MASTER 500/700

13. A-scan Biometry
A scan: Amplitude Scan; utilizes ultrasound waves of 10 - 12 MHz frequency.
In A-scan, thin, parallel sound beam is emitted from the probe tip, with an echo bouncing
back into the probe tip as the sound beam strikes each interface.
An interface is the junction between any two media of different densities and velocities.

14. Techniques of A-Scan
Contact
Applanation Method
Immersion

15. Applanation/contact A-scan Biometry
A-scan biometry by applanation requires that the ultrasound probe be placed directly on the corneal surface

16. PROCEDURE
◦A probe is placed on the patient’s cornea.
◦The probe is attached to a device that delivers
adjustable sound waves
◦The measurements are displayed as spikes on the
screen of an oscilloscope (Visual monitor)
◦The appearance of the spikes and the distance
between them can be correlated to structures within
the eye and the distance between them

17. A:Initial spike
(probe tip and
cornea)
B: Anterior lens
capsule
C: Posterior lens
capsule
D: Retina
e: Sclera
f: Orbital fat
A
B
C D E
F

18. When echoes B through D are high and steeply rising, the ultrasound beam is most likely on
visual axis.
A
B
C D E

19. Errors resulting from Probe positioning
Spike height is affected by the difference in
density & by the angle of incidence, which is
determined by the probe orientation to the
visual axis
If the probe is held nonparallel, part of the
echo is diverted at an angle away from the
probe tip, and is not received by the
machine.Induces a Paralax Error.

20. The sound beam is directed at an angle through the lens
rather than through the center of both the front and back
surfaces. When the beam is not going through the center of
the lens, it is not on the visual axis or center of the macula
Posterior lens spike too short

21. A perfect high, steeply rising retinal spike
may be impossible when macular
pathology is present (eg, macular edema,
macular degeneration, epiretinal
membranes, posterior staphylomas).

22. Retinal Spikes Not steeply rising
This common error is caused when the beam is not
perpendicular to the macular surface

23. No Orbital Fat
If no scleral or orbital fat
echoes visible, then ultrasound
beam is mostlikely aligned with
optic nerve

24. Summary

25. limitations of Contact Aplanation biometry
Variable corneal compression
Broad sound beam without precise localization
Limited resolution.
Potential for incorrect measurement distance

26. Corneal Compression
If pressure is applied on the cornea, the axial length measurement
may be falsely too short.
It can be monitored by observing the anterior chamber depth, read
out by an instrument.
Most eyes will have an ACD readings between
2.5 to 4.0mm.
The corneal compression error factor can be avoided by using the
immersion technique

27. Error caused by
1 mm Corneal Compression
Average eye 2.5 D
Long eye 1.75 D
Short eye 3.75 D

28. Immersion A-scan Biometry
The immersion technique is accomplished by placing a small
scleral shell between the patient's lids, filling it with saline, and
immersing the probe into the fluid, being careful to avoid contact
with the cornea.
More accurate than contact method because corneal
compression is avoided.
Eyes measured with the immersion method are, on average, 0.1-
0.3 mm longer

30. Scan produced by Immersion
Immersion graph contact graph
1 2 3 4 5 6 1 2 3 4 5

31. Immersion A-scan Biometry
When the ultrasound beam is properly aligned with the center of the
macula, all five spikes will be steeply rising and of maximum height.
Both the peaks of corneal spike should be equal in height ideally

32. A scan in special cases
1). Inadequate Patient Fixation
Low Vision
Nystagmus
2).Posterior Staphyloma
Blepharospasm
Strabismus
A circumscribed outpouching of the
wall of the globe which could result in
long AL

33. Cont…
3). Macular Lesions
RD
Edema
Tumor
An elevated macular lesion may prevent
the display of a distinct retinal spike and
often causes a shortened AL.
A wide space between Retina spike and
Sclera.

34. Cont…
4. Dense Cataract
Strong sound attenuation
Maximum gain setting may be required

35. Factors to Consider for type of eye and IOL material
•The type of material or the type of eye is important because the velocity of
sound is a function of the material that the sound is passing through.
•A-Scan does not actually measure length, they measure how long it takes a
sound beam to bounce off an object ( Anterior lens, Posterior lens, and
Retina) & return to the probe.
•The instrument is pre programmed with the velocity of sound factors for the
aqueous, the lens material, and the vitreous

36. Cont…
Type of EYE
Phakic (with natural lens)
Aphakic (without lens)
Pseudophakic (with artificial lens)
Silicone Oil Filled
Lens Material:
Acrylic
PMMA
Silicone IOL

37. Cont…
Lens material Velocity
Phakic 1641
Aqueous 1532
Acrylic 2120
Pmma 2660
Silicone 980

38. PMMA protocol : (Polymethylmethacrylate)
Sound travels faster through PMMA than it does through the
natural lens.
If a pseudophakic mode is not available, use the aphakic mode
and add a standard compensating factor of 0.4mm to the
resultant axial length.
Compensations When the eye is
pseudophakic

39. Silicon protocol:
Sound travels much slower through a silicon lens than it does through the
natural lens.
If not taken in to account could result in a –3.0D post - op refractive error.
If your biometer does not have a pseudophakic silicon mode, use the aphakic
mode and subtract a compensation factor of 0.8mm.
Acrylic protocol:
Sound travels faster through acrylic than it does through the natural lens.
If biometer does not have an acrylic mode, use the aphakic mode and add a compensation
factor of 0.2mm.
Cont…

40. Compensation Factors
Lens material Velocity Compensation on
Axial Length
Phakic 1641
Acrylic 2120 +0.2 mm
Pmma 2660 +0.4 mm
Silicone 980 -0.8 mm

41. Important Terms
Gain: The gain setting on A-scan is measured in decibels and affects amplification
and resolution of spikes. In cases of dense opacities(cataract) the gain can be
adjusted to amplify the signals.
Error can occur when the gain is set too high or too low
oVery high gain short reading
oVery low gain long reading

42. Important Terms …
Gates are electronic calipers on the display screen that measure distance
between two points.
Proper gate placement is on the ascending edge of each appropriate spike.
Error can occur when the gates are not appropriately placed.

43. POTENTIAL SOURCES OF ERROR IN AL
MEASURES
SHORT MEASUREMENT
Corneal compression in contact A-Scan
Corneal gate to right of corneal spike
Retinal Gate incorrectly positioned at
spike in the vitreous cavity
Gain set too high
Macular swelling
Retinal detachment
Misalignment of sound beam
Lens measured too thin
LONG MEASUREMENT
Air bubble in fluid bath (immersion method)
Fluid bridge contact method
Retinal gate at right of spike
Gain set too low
Posterior staphyloma eccentric to macular
Misalignment of sound beam
Lens measured too thick

44. OPTICAL BIOMETERS
Optical biometers are noncontact instruments that use infrared laser light (780 nm) and partial
coherence interferometry to measure multiple parameters, such as; AL, corneal curvature,
anterior chamber depth, lens thickness, and horizontal white-to-white distance (corneal
diameter).
These devices require the patient to fixate on a target, which gives an AL along the visual axis.

45. IOL MASTER
Principle of functioning:
Based on partial coherence interferometry (PCI)’.
Diode laser (780nm) measures echo delay and intensity of infrared light reflected
back from tissue interfaces– cornea & RPE

46. LENSTAR
PRINCIPLE OF LENSTAR
Based on ‘low coherence optical reflectometry (LCOR)’. Superluminescent Diode
laser of 820nm is used.

47. IOL MASTER 700
PRINCIPLE OF FUNCTIONING
Based on swept source OCT technology. It provides an image-based measurement, allowing to
view the complete longitudinal section of eyeball.

48. LENSTAR/IOL MASTER
LENSTAR IOL MASTER 500
NO OF POINTS TESTED – 32 POINTS IN TWO CIRCLES (16
EACH)
NO OF POINTS TESTED – 6 POINTS IN HEXAGONAL
PATTERN
ZONE OF CORNEA TESTED – INNER CIRCLE DIAMETER –
1.65MM
OUTER CIRCLE DIAMETER – 2.3MM
ZONE OF CORNEA TESTED – DIAMETER OF 2.3MM
BETTER IN TERMS OF MEASURING TRUE CENTRAL
CORNEAL POWER(1.65mm)
MEASURES 2.3mm
MEASURES ACD USING OPTICAL BIOMETRY MEASURES ACD USING SLIT IMAGERY

49. KERATOMETRY

50. Measurement of Cornea Curvature
Used to measure the corneal curvature
It is use for measurement the corneal dioptric power D
It provides an objective, quantitative measurement of corneal
astigmatism, measuring the curvature in each meridian as well as the axis.
The average keratometry value (K) → 43.0 -44.0D

51. Formula for corneal power
Keratometer:
Determines corneal curvature by measuring the size of a reflected Image
Surface power formula: D = ......... n - 1
R
D = the dioptric power of the cornea
n = the refractive index of the cornea used (1.3375)
R = the radius of curvature of the cornea in meters
Keratometer measures only the central 3mm of the corneal diameter.

52. Types of keratometer:
Manual keratometer
Auto keratometer, keratometers incorporated in IOL master and lenstar
Topography – placido disc based or elevation based topography

53. Manual Keratometry
Two Types:
Bausch And Lomb
Javal Schiotz
Principle of Functioning:
In order to find the refracting power of the cornea, we need to reflect an object of a
known size at a known distance off the corneal surface.
Then determine the size of the reflecting image with measuring telescope and
calculate the refractive power of the cornea.

54. AUTOMATED KERATOMETER
Principle- Focuses the reflected corneal image on to an electronic photosensitive
device, which instantly records the size and computes the radius of curvature.
Zone of measurement- central 3mm zone

55. Source of keratometry errors
• Unfocused eye piece
• Failure to calibrate unit
• Poor patient fixation
• Dry eye
• Drooping eye lids
• Irregular cornea
NB: Repeat Keratometery If
• Corneal curvature more than 47D or less than 40D.
• The difference in corneal cylinder is more than one diopter
between eyes.

56. Difficult Situations
• Post Refractive Surgery
• Corneal Transplantation
• Corneal Scar
• Keratoconus etc.

57. FORMULA FOR
IOL POWER CALCULATION

58. Formulae
On the basis of their derivation ,the various formulae for calculating IOL power have been grouped into
THEORETICAL FORMULAE
Derived from the geometric optics as applied to the schematic eyes, using theoretical constants.
Based on 3 variables – AL, K reading and estimated postoperative ACD.
REGRESSION FORMULAE
Based on regression analysis of the actual postop results of implant power as a function of the variables
of corneal power and AL
Grouped into various generation
1st ,2nd ,3rd and 4th

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

60. FIRST GENERATION
Most are based on regression formula developed by Sander ,Retzlaff & Kraff
Known as SRK formula.
P = A - 2.5(L) - 0.9(K)
P=lens implant power for emetropia
L= Axial length (mm)
K=average keratometric reading (diaopters)
A= lens constant
Tends to predict too small value in short eyes and too large value in long eyes.

61. SECOND GENERATION
THEORETICAL REGRESSION
MODIFIED BINKHORST SRK-II

62. SECOND GENERATION FORMULA
SRK formula –works well for average eyes.
less accurate for long, short eyes.
SRK II formula Modification of SRK
A-constant is modified on the basis of AL
P = A1 – 2.5L – 0.9K A1 = A + 3 AL < 20mm
A1 = A + 2 AL 20-21
A1 = A + 1 AL 21-22
A1 = A AL 22-24.5
A1 = A – 0.5 AL >24.5

63. THIRD GENERATION
THIRD
GENERATION
HOLLADAY I SRK/T HOFFER Q

64. THIRD GENERATION FORMULA
SRK/T -very long eyes >26mm
Holladay I -long eyes 24-26 mm
HofferQ -Short eyes<22mm

65. FOURTH GENERATION
HOLLADAY - II

66. STATE OF AL/CHOICE OF FORMULA
CIRCUMSTANCE OF AL CHOICE OF FORMULA
AL < 20 MM HOLLADAY II/ HOFFER Q
20- 22 MM HOFFER Q
22- 24.5 MM SRK/ T; HOLLADAY
24.5-26 MM HOLLADAY I
> 26 MM SRK / T ; HOLLADAY I

67. NEWER GENERATION: ONLINE CALCULATORS
FOR TORIC IOL:
◦ http://eyecryltoriccalculator.com/
◦ http://www.ascrs.org/barrett-toric-calculator
◦ https://www.myalcon-iolcalc.com/#/calculator
FOR ERV(Extended Range of Vision) IOL:
◦ https://www.amoeasy.com/toric2(bd1lbizjpta1ma==)/toric.h tm

68. CONSIDERATIONS IF IMPLANTING IN THE SULCUS
IOL POWER POWER CONSIDERATION
>=28.5 D Decrease by 1.5 D
+17 To 28 D Decrease by 1.0 D
+9 To 17 D Decrease by 0.5 D
<+ 9 D No change

69. Biometry In Pediatric Cases
It has shorter axial length, steeper cornea with higher keratometry value and
smaller anterior chamber depth.
The formular to use for IOL calculation should be carefully picked( Holladay II or
Hoffer Q)
Errors in axial length measurement affect IOL power calculation the most, it
increases to 3.75 D per mm in children thus AL should be measured by
immersion technique.
Keratometry: hand held keratometer should be used.

70. Considerations in IOL Power
As myopia increases rapidly in pediatric age group, the goal should be
under correction, The younger the age, more is the under correction.
20% undercorrection if the child is less than 2 years 10% undercorrection for age 2–8
years.
Example:
Age :4
calculated IOL power: +20.0 D
Power to implant -(10%) of calculated power: +18.0 D

72. POTENTIAL ERRORS IN BIOMETRY
•Error caused by Corneal Compression
•Difference in AL measurement between both eyes more than 0.3 mm
(anisometropia)
•Difference Power of lens in both eyes more than 1.00 D always check for refraction if
available.
•K reading differences should not be more than 1.50 D if not check for any corneal
pathology (dystrophy, Keratoconus, pterygium…etc )

73. Possible causes of Bad outcomes Post Surgery
•Inaccuracy of formula used
•Inaccuracy AL measurment
•Inaccuracy Keratometry measurment
•Wrong A constant of IOL chosen
•Predisposal Eye condition (Staphyloma, Retinal Detachment, Choroidal Detachment…)
•Wrong IOL brought in OR
•Mixing right eye value to left eye

74. Consequences of Bad Biometry
•Bad visual acuity post operatively
•Need of spectacles after surgery for far vision
•Leaving the patient aphakia
•May need re-surgery for secondary implantation or IOL removal
•Bad reputation of the hospital

75. Benefits of good Biometry
•Good visual acuity post operatively
•Meet the patient’s need
• (Mostly emmetropic target)
• (Slightly myopic to give a comfortable near vision also)
• (Far and near vision if doing mono vision but anisometropia must be kept below 3.00
D)
•Patient’s satisfaction
•Medical staff satisfaction
•Good reputation of the hospital

76. Clear vision begins with Precise
Ocular Biometry
The Key to unlocking a world of
visual clarity