IOL power calculation is challenging in eyes with prior refractive surgery or other special situations. In eyes with prior radial keratotomy, standard keratometry overestimates corneal power due to flattening outside the central optical zone. Multiple methods of IOL power calculation should be used, including topography to measure the flattest central corneal power. A study comparing methods in eyes with prior RK found IOL power calculation using topographic keratometry was least accurate compared to formulas from the ESCRS calculator. No single method provided reliable results, highlighting the difficulty in IOL power calculation for eyes with prior refractive surgery.

1. IOL Power Calculation In
Special Situations
Dr Ravish Vaishnav
Moderator- Dr Vijay Shetty

2. Introduction
 Precise IOL power calculation is essential for optimal benefits.
IOL Power Calculation Errors can arise from
 Keratometry – 1.0D = 0.9D error in IOL power
 Axial length – 1mm = 2.5D error in IOL power
 IOL Formula

3. Keratometry
 Used to measure the corneal curvature.
 Done before axial length measurement.
 Types of Keratometer
1. Manual Keratometer
2. Auto Keratometer, Keratometers incorporated in IOL Master
and Lenstar
3. Topography – Placido disc based or Elevation based
topography

4. Keratometry
 Principle
1. Fixed object and variable image size
2. Fixed image and variable object size
 Two types
1. Bausch and Lomb
2. Javel Schiotz

5. Manual Keratometer – Bausch
and Lomb
 Measures radius of curvature of anterior corneal surface and
converts into total corneal power (anterior + posterior) by
using standardized refractive index of 1.3375 instead of true
corneal refractive index 1.36.
 Measures a larger zone away from the centre - 3.5mm zone
and calibrated to give central corneal power, Works well in
virgin regular corneas.
 Limitation – In post RK and post keratorefractive corneas, the
central corneal power measurement is inaccurate.

6. 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.
 Slightly better than manual K in terms of the zone
measured.

7. Keratometry of Optical Biometer
 IOL MASTER –
1. No of points tested – 6 points in hexagonal
pattern
2. Zone of cornea tested – Diameter of 2.3mm
 LENSTAR –
1. No of points tested – 32 points in two circles (16 each)
2. Zone of cornea tested – Inner circle
diameter – 1.65mm
Outer circle diameter – 2.3mm
BETTER IN TERMS OF MEASURING TRUE
CENTRAL CORNEAL POWER

8. Topography
 Sim K – It is determined from the power of placido mires 7,8 and 9
of videokeratoscope for 128 equally spaced meridians within 3mm
zone.
 EKR – EKR is determined by adding 0.7D to total corneal refractive
power (TCRP) measured by Pentacam rotating Scheimpflug
camera. This conversion factor is applicable after myopic
refractive surgery because changes in TCRP are equal to changes in
refraction in the 4mm zone.
 Flattest central K on topo – Standard keratometry tends to
overestimate the corneal curvature and we expect a hyperopic
shift post-RK. So flattest central K is used for IOL power calculation
in post RK. Seo K, Young C et al. New equivalent keratometry reading calculation with a rotating Scheimpflug
camera for intraocular lens power calculation after myopic corneal surgery. JCRS 2014. Vol 40. 1834-42.
Savini G, Hoffer K. Pentacam equivalent K-reading. JRS 2010. Vol 26. 388-89.

9. Topography
 Posterior corneal measurement in Oculyzer – Selecting toric
IOL based on anterior corneal measurement can lead to
overcorrection in eyes with WTR astigmatism and
undercorrection in eyes with ATR astigmatism.
 Oculus Pentacam AXL – Utilise the anterior + posterior
corneal astigmatism and axial length on the same machine to
calculate toric IOL power.
Koch D, Ali S et al. Contribution of posterior corneal astigmatism to total corneal astigmatism.
J Cataract Refract Surg. 2013;39(12):1803-1809.

10. Axial Length Measurement
 1 mm error leads to 2.5D error in postoperative refraction
 2 types
1. Optical
2. Ultrasound

11. A-scan
 PRINCIPLE- The ultrasound probe has a piezoelectric crystal
that electrically emits and receive high frequency sound
waves.
 Measurement is from anterior corneal surface to internal
limiting membrane.
 WAVES- One thin parallel sound beam is emitted from the
probe tip at a frequency of 10MHz, with an echo bouncing into
the probe tip as the sound beam strikes each interface.

12. A-scan
 USE OF GAIN IN DIFFICULT SITUATIONS- Gain refers to
electronic amplification of the sound waves received by the
transducer. Increase in gain is required when height of echoes
achieved is inadequate as in dense cataracts. Decrease in gain
is required when artefacts are seen near the retinal echoes as
in silicone filled eyes, pseudophakic eyes.
 IMMERSION ADVANTAGE- It is more accurate than contact
method as the corneal compression is avoided.
 LIMITATIONS- Does not give accurate readings in silicone filled
eyes, posterior staphyloma.

13. Optical Biometer
 PRINCIPLE OF IOL MASTER – Based on
‘Partial Coherence Interferometry (PCI)’. Diode laser (780nm)
measures echo delay and intensity of infrared light reflected
back from tissue interfaces– Cornea & RPE.
 PRINCIPLE OF LENSTAR – Based on ‘Low coherence
optical reflectometry (LCOR)’. Superluminescent
diode laser of 820nm is used.

14. Optical Biometer
 PRINCIPLE OF IOL MASTER 700 – Based on swept source OCT
technology. It provides an image-based measurement,
allowing to view the complete longitudinal section of eyeball.
Akman A, Asena L. Evaluation and comparison of the new swept source OCT-based IOLMaster 700
with the IOLMaster500. Br J Ophthalmol . 2015-307779.

15. Optical Biometer
 ADVANTAGES OF LENSTAR OVER IOL MASTER V5 –
- Lenstar measures ACD using optical biometry as compared to
IOLMaster which measures ACD using slit imagery.
- Measures ACD from posterior surface of cornea which is
important in short anterior chambers and thicker corneas.
- Takes two sets of readings so covers more central area.
- Pachymetry, Pupillometry, Retinal thickness.
- Lens thickness is measured which is one of the factors in latest
- formulas like Holladay II.
Hill W, Angeles R, Otani T. Evaluation of a new IOLMaster algorithm to measure axial length.
J Cataract Refract Surg 2008;34:6:920-4.

16. Optical Biometer
 ADVANTAGES OF IOL MASTER 700 OVER LENSTAR-
- In PSC cataracts, opacity are located nearer to the nodal point
of the lens, so more light rays are affected and thus AL
measurement becomes difficult with LENSTAR. IOL MASTER
700 overcomes this problem.
- Unusual eye geometries like tilt or decentration of lens can be
detected.

17. IOL Formulae
 3RD and 4th generation formulae are superior to earlier formulae.
 These are a merger of linear regression methods with theoretical
eye models.
 They predict better ELP.
 Haigis uses multiple A-constant.
 Holladay uses multiple parameters including lens thickness
measured by LENSTAR.

18. IOL formula depending on AL
Circumstance 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

19. IOL power in specific situation
Myopic refractive surgery Haigis L
Nanophthalmos Hoffer Q ; Haigis
Post refractive surgery Topography with true net k and
flattest k with formula according to
the AL

20. Biometry in specific situations

21. Aphakic eyes

22. Aphakic eyes
 Two lens spikes are replaced by a single spike - Anterior
vitreous face and Posterior lens capsule.
 Immersion technique is the method of choice.
 In present Biometers, options are available for aphakic mode
where in the calculation compensate for the change in speed
of the sound waves in cataract lens or aqueous or vitreous.
 In ACIOL or Scleral fixated IOL, the appropriate A constant is
used.

23. Keratoconus eyes

24. Keratoconus eyes
 As cornea with keratoconus is steep, using K reading of such eyes
will yield an overestimated reading due to ELP calculation error.
 Formulas which consider only axial length and not Keratometry to
calculate ELP gives better prediction of true IOL power.
 K reading has less of this effect in the Hoffer Q formula.
 Also, overestimation is not a factor with the Haigis formula as it
does not use the K reading in estimating the lens power.

25. Eyes with silicone oil

26. Eyes with silicone filled vitreous/
vitrectomized eyes
 The refractive index of the oil is much less than that of the
vitreous.
 Usage of a standard sound velocity can give an error of upto 8
mm.
 Difficulty of measuring the AL can be overcome by increasing
the ‘system gain’.

27. Eyes with silicone filled vitreous/
vitrectomized eyes
 Error in AL measurement occurs as ultrasound travels slower in
silicon oil compared to vitreous and thus taking a longer time to
reach the probe which is interpretated as longer wavelength.
 Usually a factor of (0.72) gives a rough estimate of the power.
 TAL = 1133/1550 × AAL.
 As silicone oil alters the optics of the eye due to its refractive index,
further adjustments in IOL power are required. However it is less
affected in optical biometer
 Usually, IOL required is 2 – 3 D stronger than indicated by standard
power calculation.

28. High myopic eyes

29. Eyes with high myopia
 Accurate axial measurement is critical for IOL power
calculation
 Paraxial measurement in ultrasonic measurement due to
posterior staphyloma is likely to give a refractive surprise
 It is partly overcome by optical biometer using a fixation
target
 Most of it is taken care in IOL master 700 by directly visualising
fovea on the OCT image during measurement

30. Eyes with high myopia
 IOL formula needs to be chosen accordingly
 Minus IOL powers have to be chosen carefully by
reducing amount of minus power
 The surgeon should aim for a -0.50 D to -1.00 D
postoperative refraction as most elderly will prefer
being near-sighted

31. Pediatric eyes

32. Pediatric biometry
 As myopia increases rapidly in pediatric age group, goal should
be undercorrection.
 Undercorrection of 60-75% is recommended depending upon
the child’s age.
 Younger the age, more is the undercorrection.

33. Pediatric biometry
 Simple rule of thumb- Target refraction = 7 – age in years
 A greater undercorrection can lead to anisometropia and
difficulty in amblyopia correction. At times managing
amblyopia is more difficult than future IOL exchange.
 Therefore undercorrection has to be done with care especially
in unilateral cases.

34. Piggyback IOL

35. Primary piggyback
 Haigis or Hoffer Q
 Ideally 1 acrylic and 1 silicon IOL to avoid interlenticular
opacification
 Usually single piece in the bag and 3 piece in the sulcus
 Divide the power between the IOL and reduce 1 D for sulcus
placed IOL

36. Secondary piggyback IOL for
pseudophakia
 Patients with refractive error following the primary IOL
implantation.
 Calculated based on refractive error.
 Holladay’s refractive formula.
 No knowledge of primary implant or the AL is required.

37. Secondary piggyback IOL for
pseudophakia
 The IOL power can also be calculated by one of these formulae:
Myopic correction: P = 1.0 × Error
Hypermetropic correction: P = 1.5 × Error
 Rayner sulcoflex IOL power is calculated by giving the residual
refraction while ordering.

38. Corneal refractive surgery

39. Common Keratorefractive surgeries which
pose a challenge for Cataract surgery
MYOPIC
 LASIK
 PRK
 RK WITH OR WITHOUT AK
HYPEROPIC
 LASIK
 PRK

40. Keratometric error in RK
 Radial keratotomy (RK) flattens both the
anterior and posterior corneal surfaces, but only in a small
central optical zone.
 The effective optical zone diameter can be significantly smaller
than the measurement zone of standard keratometry.
 Therefore standard keratometry tends to overestimate the true
corneal power. Furthermore, there can be central flattening after
cataract surgery due to corneal edema. Most of the flattening
effect resolves over several months.

41. Sources of error in post refractive
cases (LASIK/PRK)
 Radius measurement error/ Keratometer error
 Keratometry index error (Altered Gullstrand ratio)
 Formula Algorithm error (ELP)
 These 3 errors cause hyperopic error in myopic LASIK and
myopic error in hyperopic LASIK.

42. Methods of estimating true postoperative
corneal power
1. Clinical history method
K=K PRE + R PRE – R PO
2. Contact lens method
K= B CL + P CL + R CL – R NO CL
3. Maloney” central K method
K= 1.1141 * TK PO-CTRI
4. Shammas method
K = 1.14 * KMANUAL PO – 6.8
5. Haigis method
K = -5.1625 * K IOL MASTER + 82.2603 - 0.35

43. Web Help
 http://www.doctor-hill.com/iol-main/keratorefractive-
calculator.html
 www.ascrs.org, http://iol.ascrs.org/wbfrmCalculator.aspx

46. Choice of IOL
 IOL with negative spherical aberration like Technis or Acrysof
IQ would correct spherical aberration induced by myopic
LASIK.
 Refractive multifocal IOLs further reduce contrast sensitivity
and so not recommended.
 Diffractive multifocal IOL is not preferred as diffractive ring
adds to already multifocal cornea and so not recommended.
 Following RK, the overall lack of precision of IOL power
calculations, a diurnal fluctuation in refractive power and
increase in spherical aberration precludes use of refractive
and diffractive multifocal IOL.

47. Rule of 2s
 Inspite of our best efforts, if the final refractive objective
remains elusive, plans for an IOL exchange, or a secondary
piggyback IOL, should not be made until at least 2 months
have passed and two refractions are stable at two consecutive
visits.
Warren Hill Cataract Surgery after keratorefractive surgery

48. Our study
Purpose:
 To compare the various methods for IOL calculation post
radial keratotomy (RK).
 To predict the most accurate method.
Study Type:
 Retrospective analysis
Sample size:
 27 eyes post RK included in the analysis

49. Our study
Methods:
 IOL Power was calculated by 3 methods:
 IOL Master: K used were the one derived from IOL master
 Topography: K values used were the flattest K on corneal
topography
 ESCRS calculator: Average value of IOL power to be
implanted
Results:
 IOL power calculation using Topo-K gave a significantly higher
proportion of patients in whom the difference between
predicted and actual SE was more than 0.75, compared with
ESCRS method. [p=0.04]

50. Our study
Conclusion:
 No single reliable method for accurate calculation of IOL
power in post RK patients.
 Multiple methods of IOL power calculation using IOL-M K,
flattest central K on topography and ESCRS calculator with
final fine tuning based on surgeon’s experience helps in
achieving near accurate post op refraction.

51. Thank you!!