This document discusses intraocular lens (IOL) power calculation methods for eyes that have and have not undergone refractive surgery. It covers: 1. The evolution of IOL power calculation formulas from first to fourth generation formulas, which have improved accuracy by incorporating additional eye measurements and constants. 2. Sources of error in IOL power calculations from incorrect keratometry and axial length measurements. 3. Challenges in calculating IOL power for eyes that had refractive surgery due to changes in corneal shape and index of refraction. 4. Methods to estimate pre-operative keratometry and account for post-operative keratometry changes, including double-K, theoretical K, and tomography approaches.

1. IOL POWER
CALCULATION
NORMAL & POST
LASIK EYES
INDOREDRISHTI.WORDPRESS.COM

2. DR DINESH MITTAL DR SONALEE MITTAL
DRISHTI EYE HOSP VIJAYNAGAR INDORE

8. 1 Cataract surgery has evolved into a
refractive procedure with the goal of
eliminating or significantly reducing the
need for spectacle dependence.
•2. One must consider not only the
astigmatism induced by the cataract
incision itself, but also the correction of
preexisting astigmatism.
•3. Incision length, depth, and distance
from visual axis all affect astigmatism.

9. •4. LRIs are commonly used to correct
preexisting astigmatism, and
•published nomograms are helpful in
tailoring a surgical approach.
•5. Toric IOLs and excimer laser ablation
are alternative approaches to correcting
astigmatism in the patient for refractive
cataract surgery.
•No single approach is best suited for all
patients.

10. INTRAOCULAR LENS CALCULATIONS
•Choosing the appropriate IOL power is a
major determinant of patient
satisfaction with cataract surgery.
•accurate measurements ,
•selecting calculations ),
• and assessing the patient’s needs to
determine postoperative refractive
target

12. BIOMETRY
•At minimum, 2 measurements reqd
to calculate implant power:
&
( )
•Precise measurements critical
•error of 0.3 mm in axial length will
result in a 1-D error in IOL power.

13. Axial Length
•Axial length has been obtained utilizing
.
•This measurement is determined by
;
• an ultrasound pulse is applied and the transit
time through the eye is measured. Using
estimated velocities of ultrasound waves
through various media (ie, cornea, aqueous,
lens, and vitreous) the distance travelled
through the eye is calculated.

14. • The instrument should have an screen
to differentiate a good measurement from a poor
one. or spikes should be
observed when the probe is aligned properly
• These include the following: a tall peak for the
cornea, tall peaks for the anterior and posterior
lens capsule, tall peak for the retina, mod-erate
peak for the sclera, and moderate-to-low peaks for
orbital fat. If these spikes are not well seen, then
the probe may be misaligned.
A SCAN ULTRASOUND

15. •The must
be performed carefully, as com-
pressing the cornea will result in a
shorter-than-expected measurement
and should
be taken and averaged.
•If several are taken and differ by a
significant amount, they should be
readings
can be obtained.

16. •It is also prudent to
for comparison
•the machine should be
, checking measurements
against an eye of known axial length.

18. Immersion technique
The immersion technique may more accurately
represent the true axial length because there is
.
In this technique, the patient lies in the supine
position and a scleral shell is placed on the eye
and filled with Goniosol. The ultrasound probe is
placed in this solution and the beam is aligned
with the macula by having the patient look at the
probe tip fixation light.
Although the immersion method may be strongly
advocated by some users, applanation A-scan is
the more commonly used method

19. IMMERSION
TECHNIQUE
SHOWING
PROBE IN
OSSOINIG
SHELL

20. Optical biometry: IOLMaster
• In the last decade, the technique of
optical coherence biometry was in-
troduced by Haigis, which utilizes light
rather than ultrasound to measure the
length of the eye. The first device
introduced was the IOLMaster (Zeiss),
based on the principle of partial
coherence interferometry using a 780-
nm multimode laser diode.

22. •Measurements taken without contact
to eye, thus eliminating variability due
to an examiner technique.
•The distance measured lies between
the anterior surface of tear film &
retinal pigmented pigment epithelium
, which may
be more physiologically accurate

23. patient asked to focus on a small red
fixation light, & examiner maneuvers
focusing spot within the measurement
reticule, sampling areas until the best
peak pattern is obtained.
5 to 20 measurements obtained until the
readings differ by less than 0.1 mm.
Maximal axial length measured 40 mm.
IOLMASTER USE

24. IOL MASTER DISADVANTAGE
The primary disadvantage of this
optical device is that
, such as a corneal scar,
dense posterior subcapsular plaque,
darkly brunescent cataract, or vitreous
hemorrhage, will reduce the signal-to-
noise ratio (SNR) to the point that
.

25. A-scans should be
under the following conditions
• 1. Axial length is less than 22 mm or
greater than 25 mm in either eye.
• 2. The difference between the 2 eyes is
greater than 0.3 mm.
• 3. The measurements do not correlate
with the patient’s refraction (ie,
hyperopes should have shorter eyes,
and myopes should have longer eyes).

26. Lens power
calculation
formulas
evolved over
past 30 yrs,
since first
theoretical
formula for
iris-supported iol
published by
Fyodorov in
1967.

28. First Generation
• Initial formulas were ,
but with the availability of posterior chamber
implants, consideration had to be given to the
distance from the cornea to the implant (anterior
chamber depth). By studying large numbers of
cases, linear regression techniques were used to
determine a formula for predicting emmetropic
implant power. The most widely used regression
formula was developed by Sanders, Retzlaff , and
Kraff in 1980 and is known as the .

29. First Generation
• where is the lens implant power
is the axial length
is average keratometry
• and is the constant a theoretical
value that relates the lens power to axial length
and keratometry. It is specific to the design of the
IOL and its intended position inside the eye. This
number is specified by the IOL manufacturer.

30. Second Generation
• The SRK formula is a linear equation derived
by fitting collected data to a straight line.
However, the optical system is nonlinear and
begins to produce significant error with short
or long eyes. To improve accuracy, the formula
was modified, taking into consideration
variation in axial length.
• This en-hancement is known as the
formula in which the
:

31. Second Generation SRK II
•A = A + 3 (AL < 20 mm)
•A = A + 2 (20 mm < AL < 21 mm)
•A = A + 1 (21 mm < AL < 22 mm)
• A = A (22 mm < AL < 24.5 mm)
•A = A – 0.5 (24.5 mm < AL)

32. Third Generation
• Holladay further refined the
theoretical formulas by proposing a
.
•Instead of factoring in anterior
chamber depth, formula would
calculate the distance from the
cornea to iris plane and & distance
from the iris plane to the IOL.

34. Third Generation
• . This second variable was termed the
, was specific to each lens,
and had the ability to be personalized and
adjust for any consistent bias in the
surgeon’s results. Hoffer achieved the
same effect via another approach in his
. Retzlaff followed suit to
take into consideration not only the
position of the implant, but also
incorporated a correction for the retinal
thickness, thus developing the
formula in 1990.

35. Fourth Generation
• Haigis presented the notion that
model should be
considered in determining which formula
to use. The geometry for a particular IOL
model is not the same at all powers;
therefore, the Haigis formula utilizes 3
lens constants to address it
constant moves the power prediction
curve up or down constant is tied to
the anterior chamber depth constant
is tied to the measured axial length

36. • Optimization of the Haigis formula requires
collecting pre- and postop-erative data from over
200 patients in order to allow surgeon-specific
optimization, which is available online.
• In the late 1990s, the Holladay 2 IOL consultant
software was introduced to improve upon
predictability by incorporating additional data
points. It requires 7 measurements including
. Th is formula may be more precise in
unusual eyes such as those that have undergone
refractive surgery.

37. •One or more of the
are generally programmed
into A-scan biometers sold today.
•The optical biometers are now
incorporating the
•Over time, some trends have
emerged regarding which formulas
to use in general categories of
patients:

39. FORMULAE BASED
ON MATHEMATICAL PRINCIPLES
REVOLVING AROUND THE
SCHEMATIC EYE
FORMULAE WORKING
BACKWARDS ON POSTOPERATIVE
OUTCOMES
GENERATION
MIX OF BOTH

42. IOL CALCULATION POST
REFRACTIVE SURGERY

43. INSTRUMENT ERROR
INSTRUMENT
ERROR IS
BECAUSE
KERATOMETERS
READ AT 3.2
MM AND NOT
AT THE
CENTRE

44. INDEX OF REFRACTION ERROR
THIS K READING
ERROR OCCURS
BECAUSE OF
CHANGED RATIO OF
RADIUS OF
CURVATURE OF
ANTERIOR AND
POSTERIOR SURFACE
OF CORNEA
SECONDARY TO
KERATO REFRACTIVE
PROCEDURE

45. UNOPERATED EYES IN
UNOPERATED
EYES THE
RATIO
BETWEEN
ANTERIOR AND
POSTERIOR
SURFACE OF
CORNEA IS
1.21

46. MYOPIC OPERATED EYES
IN OPERATED
EYE THIS
RATIO
CHANGES

47. POST MYOPIA TREATED EYES
KERATOMETRY OVER ESTIMATED

48. POST HYPERMETROPIA
TREATED EYES

49. POST HYPEROPIA TREATED EYES
KERATOMETRY UNDER ESTIMATED

50. KERATOMETRY DEPENDENT &
INDEPENDENT FORMULA

51. HOW TO CALCULATE IOL POST
KERATO REFRACTIVE SURGERY

52. ARAMBERRI DOUBLE K METHOD

53. PRE OP K READING FOR ELP

54. HOW TO DETERMINE PRE OP K

55. PRE OP K READING

56. PRE OP K READING

57. TRUE POST OP K READING FOR
IOL POWER CALCULATION

58. MEASUREMENT OF POST OP K READING

59. POST OP K READING HISTORY DEPENDENT

60. POST OP K READING HISTORY
INDEPENDENT

61. K CORRECTED METHODS

62. K CORRECTED METHODS SHAMMAS

63. K CORRECTED METHODS THEORETICAL

64. K CORRECTED KOCH & WANG

65. POST OP K READING
TOMOGRAPHY METHODS

66. TOMOGRAPHY METHODS

67. HEIDELBERG RAYTRACING STUDY
FOUR
FORMULAS
ARE BEST
HAIGIS
HOFFER Q
OLSEN 2
RAYTRACING

68. IOL CALCULATION BEST
IN UNOPERATED EYES

69. IOL CALCULATION BEST POST
KERATOREFRACTIVE SURGERY

70. ASCRS SITE FOR IOL POWER
CALCULATION

71. THANK YOU
DR DINESH
DR SONALEE