This document discusses various biometry techniques and considerations for accurate intraocular lens (IOL) power calculation in complex cases. It describes tools and methods for measuring ocular parameters in conditions like mature cataracts, posterior staphylomas, and post-refractive surgery eyes. Formulas for calculating IOL power for different IOL types and surgical procedures are presented. Intraoperative techniques like retinoscopy and aberrometry are also covered to optimize refractive outcomes.

1. Biometry: Not Only A Mere
Measurement
Ajit K Thakur
M. Optom

2. Objective
• Biometry and its importance
• Some difficult biometry conditions
– Very Matured Cataract
– IOL power calculation in Toric
– Lens after PCR
– Posterior Staphyloma
– Micro cornea or nanophthalmos
– Post vitrectomized eye
– Cataract with Retinal Detachment
– Toric Calculator
– Post PK
– Post LASIK
– Phakic IOL

3. Cataract Surgery
•Refractive Surgery
•Quality Vision
•Spectacle free world
•Intracapsular cataract
surgery to Femto-
second
•Multifocal IOL
•Toric IOL
•Implantable Contact
Lens (ICL)

4. Biometry
• The statistical analysis of biological
observations and phenomena
• Ocular Parameters Measurement
• Play Maker of successful Cataract
Surgery

5. Source of Refractive Surprise
• Measurement Error
•Axial Length: 54%
•K reading: 8%
• Formula Error (Olsen JCRS 1992)
• ELP (estimated lens position) error-38%
• Lack of Personalization
• IOL insertion Error
• IOL mislabeling
•improper IOL power insertion at time of surgery

6. Tool for Biometry
• Keratometer/Topographer and A scan
• Optical Biometry
• Retinoscope and trial lenses

7. Instrumentation
• A Scan
•Principle
•Benefits
•Way to improvise the
Precision
• Repeatability Vs Reliability
• SD????
•Source of Error
• Compression
• Misalignment
• Air trap
•Contact Vs Immersion
• Immersion mandatory
• High myopia
• Ectatic Corneal cases
• Paediatric Cases
• Refractive Upset with your contact technique

8. Marketed Instrument
• Nidek A Scan
• Sonomed A Scan
• DGH - 5000e A-scan
• Quantel Medical - Axis II PR
• Accutome - A-scan

9. Perfect and Imperfect Scan
A Scan Contact A Scan Immersion

10. Perfect and Imperfect Scan
A Scan Contact

11. Instrumentation
• Cause of Error
Myopic Shift (making eye
short)
Hyperopic Shift (Making eye
Large)
Compressing cornea Air Bubble
Sound velocity slow Sound Velocity high
Gain set too high Gain too Low

13. Optical Biometry
• Uses Light source instead of Sound Waves
• Pentacam AXL
• Al Scan
• IOL Master 500
• Lenstar
• Alladin
• IOL Master 700
• Eyestar 900 (Haag-Streit)
• Advantages of Optical Biometry
• a) Increased precision with minimal training- ±25μm
• b) Consistency between testers- variability 21μm between 5 examiners
• c) Superior for Staphylomas, Pseudophakic eyes, Silicone Oil eyes
• Takes the K reading as well as AXL in One shot
• Disadvantages
• Dense cataract or media opacities
• Fixation error
•Cost

14. Difference

17. ALop = ALus - 0.117 ( (for eye AXL, 19–29 mm)
Alop: Optical
Alus: Ultrasound

18. Biometry by retinoscopy
• Doing Retinoscopy during OT
•F= P/1-dF where
•F= New Power at Lenticular Plane
•P= Power at spectacle plane
• d= Distance between spectacle plane and
Lenticular plane (vd+acd)

21. • Chief Author : Dr. Hardik A. Shroff, Co-Authors : Dr. Ashok P. Shroff,
Dr. Dishita H. Shroff
• IOL power was selected by addition of 11D (surgeon’s factor) in the
refraction value.
• From our experience, we calculated IOL power from intraoperative
retinoscopy and we found that it was matching better with
postoperative BCVA rather than with IOL power calculated from
biometry.

22. Recommendation for Measurement
• Data screening criteria (Knox Cartwright Eye 2010)
•Calibrate your machine preferably at the start of
the day and with change in the observer
•Repeat measurements with second observer if:
• (1) Axial Length < 21.30 or >26.60mm
• (2) Avg Corneal Power < 41.00 or >47.00D and cylinder >2.50D
• (3) Between eyes: asymmetry of AL >0.70 mm
• (4) Between eyes: mean K >0.90D

23. IOL calculating Formulae

24. Some Formulae

25. IOL Calculating Formulae
• 1st Generation Formulae
•Theoretical Formula:Hoffer, Binkhorst
•Regression Formulae:SRK
• Second generation Formulae
•SRK II formula, Hoffer, Binkhorst II
• 3rd Generation Formulae
•SRK/T, Hoffer Q, Holladay
• 4th Generation Formula
•Holladay II Formula, Haigis Formula, Barrette
Formula, Olsen formula, Hill RBF formula

26. IOL Formula
• SRK formula
• Calculated IOL: A-2.5L-0.9 K
• A: A constant (ELP)
• L: Measured Axial Length
• K: Average K reading
• Mathematical Formulae (Fyodorov and its
modifications)
• P = (1336/[AL- ELP]) (1336/[1336/{1000/([1000/DPostRx] - V) + K} - ELP])
• K: Net corneal power
• AL: Axial length
• P: IOL power
• ELP: Effective lens position
• DPostRx : Desired refraction
• V: Vertex distance

27. A constant??
•Not A constant……
•Effective lens position
•A constant optimization
•How to do Optimization??
•Optimization on the basis of AXL and
specific Doctor, Lens type

28. A constant Optimization
AXL: <20.00 mm, 20-23 mm, 23mm-26 mm, 26-
29 mm, >29 mm

29. Constant Optimization
hill@doctor-hill.com

30. Which is best?
Barrette universal formula and Holladay II

31. Online IOL calculating Assist
• https://ascrs.org/tools/rbf-calculator (BARRETT UNIVERSAL II FORMULA)
• https://ascrs.org/tools/rbf-calculator (Hill RBF)

34. Cause of Refractive Surprise

35. Cases Discussion

36. Very Matured Cataract
• Use A scan rather Optical Biometry
• Poor Vision and Fixation
•Light source as a target
• Gain
•Make gain lower (less then 90 DB)
• Mode
•Dense and long AXL
• K Reading
•Assure the fixation and the K reading is taken in
front of the pupil

37. Lens after PCR
• Sulcus Fixation/ ACIOL/ Scleral Fixation/ Iris Claw Lens
• 0.5 to 1 D less then the PCIOL

38. Sulcus vs Bag
Power at
Capsular Bag
Power at
Ciliary Sulcus
Subtract from
Bag Power
+30.00 D +28.55 D -1.50 D
+29.50 D +28.09 D -1.50 D
+29.00 D +27.61 D -1.50 D
+28.50 D +27.14 D -1.50 D
+28.00 D +26.67 D -1.00 D
+27.50 D +26.20 D -1.00 D
+27.00 D +25.73 D -1.00 D
+26.50 D +25.26 D -1.00 D
+26.00 D +24.79 D -1.00 D
+25.50 D +24.31 D -1.00 D
+25.00 D +23.84 D -1.00 D
+24.50 D +23.36 D -1.00 D
+24.00 D +22.89 D -1.00 D
+23.50 D +22.42 D -1.00 D
+23.00 D +21.94 D -1.00 D
+22.50 D +21.47 D -1.00 D
+22.00 D +21.00 D -1.00 D
+21.50 D +20.53 D -1.00 D
+21.00 D +20.05 D -1.00 D
+20.50 D +19.58 D -1.00 D
+20.00 D +19.11 D -1.00 D
+19.50 D +18.63 D -1.00 D
+19.00 D +18.16 D -1.00 D
+18.50 D +17.69 D -1.00 D
+18.00 D +17.21 D -1.00 D
+17.50 D +16.73 D -1.00 D
+17.00 D +16.26 D -0.50 D

39. Sulcus vs Bag
+9.00 D +8.63 D No Change
+8.50 D +8.16 D No Change
+8.00 D +7.68 D No Change
+7.50 D +7.20 D No Change
+7.00 D +6.72 D No Change
+6.50 D +6.24 D No Change
+6.00 D +5.76 D No Change
+5.50 D +5.28 D No Change
+5.00 D +4.81 D No Change
+16.50 D +15.78 D -0.50 D
+16.00 D +15.31 D -0.50 D
+15.50 D +14.83 D -0.50 D
+15.00 D +14.35 D -0.50 D
+14.50 D +13.88 D -0.50 D
+14.00 D +13.40 D -0.50 D
+13.50 D +12.93 D -0.50 D
+13.00 D +12.45 D -0.50 D
+12.50 D +11.97 D -0.50 D
+12.00 D +11.49 D -0.50 D
+11.50 D +11.02 D -0.50 D
+11.00 D +10.54 D -0.50 D
+10.50 D +10.07 D -0.50 D
+10.00 D +9.58 D -0.50 D
+9.50 D +9.11 D -0.50 D

40. IOL power calculation in Toric

42. IOL power calculation in Toric
• Toric calculator
•https://ascrs.org/tools/barrett-toric-calculator/

43. BARRETT TORIC CALCULATOR

44. SIA Estimation
• https://sia-calculator.com/

45. Verion (Alcon)

46. Rotation??
https://www.astigmatismfix.com/

47. Posterior Staphyloma
• In posterior Staphyloma, optical biometry is better suited
Use SRK/T formulae or barette II

48. Micro cornea or nanophthalmos
• Nystagmus and difficult in doing Keratometery
• Locate Null Gaze
• Low gain
• Normal ACD or shallower ACD
• Formulae???
•Haigis and Hoffer Q
•Barrette II
• Retinoscopy during time of surgery
•0.7 D Spectacle Plane: 1 D Lenticular plane

49. Post vitrectomized eye
• Preoperative IOL calculation is preferred
• Use of optical biometry
• Silicon Oil Filled Globe mode
•1000 stock/ 5000 stock
•Freeze at low gain
• Do at seating position
• OVERCORRECTION??
•Planoconvex vs biconvex

50. Post vitrectomized eye
• Posterior Segment filled with Gas or perfluorocarbon
•ultrasound echoes are blocked
•Optical biometry
•Retinoscopy per operatively
•CT-scan imagecan be used to measure axial
length in eyes with incomplete silicone oil fill and

51. Cataract with Retinal Detachment
• The retinal spike should be viewed cautiously as the echo can reflect
from the detached surface.
• May cause Myopic shift of 0.5-0.75 D.
• Optimize at your clinic

52. Post LASIK/Post RK
• At least 30 methods
• Confusing for the clinicians
• The best ones

53. Post LVC
Post RK

54. Post LASIK
• When preoperative Keratometery and/or refractive change are
available

55. Post PK
• Clinical History Method
•K=KPRE-RCC
• K: calculated corneal power
• KPRE: corneal power before refractive surgery
• RCC: change in manifest refraction at the corneal plane
•

56. Post PK
• Contact Lens Method
•Suitable for Post LASIK and Post RK
• K=BCL+PCL+RCL-RNoCL
• BCL: contact lens base curve
• PCL: contact lens power
• RCL: contact lens over-refraction
• RNoCL: spherical equivalent of the manifest refraction without a contact lens
• The accuracy of this method worsens with poorer best corrected visual
acuity (BCVA). Therefore it is not suitable for cases of dense cataracts.

57. Post PK
• Topography-Based Post-LASIK Adjusted Keratometry
•Koch and Wang Formula
• K=1.1141×TK -6.1
• K: calculated corneal power.
• TK: post-LASIK corneal topography central Ks
•Shammas Formula
• K=1.14×TK -6.8
• K: calculated corneal power.
• TK: post-LASIK corneal topography central Ks
•

58. Post PK
• Net corneal power measurement
• Orbscan
• Pentacam
• Optical coherence tomography (OCT)
• Formulae Used
•Double K method
•Haigis-L Formula
•Barret

60. Intraoperative refraction
• Aphakic refraction
•IOL power (D) = Aphakic refraction × 1.75 (For
ACIOL)
•IOL power (D) =0.07x 2 + 1.27x2 + 1.22, where x =
aphakic refraction (PCIOL)
• Intraoperative aberrometry
•Optiwave Refractive Analysis (ORA)

61. Masket’s formula (for previously myopic and
hyperopic eyes
• The IOL power is calculated as if the eye had not undergone previous
LASIK or PRK.
• The IOL power obtained either by Single-K SRK/T (in the case of
myopia) or Single-K Hoffer Q (in the case of hyperopia) is then
• IOL power adjustment : SIRC *(−0:326) + 0:101
•SIRC = surgical induced refractive change.
• The value thus obtained is added from the standard IOL power
calculation in patients with previous myopic laser correction and
subtracted in patients with previous hyperopic laser correction.

62. • Corneal Bypass method (Walter 2005)
•Keith Walter et al proposed preop LASIK
refraction spherical equivalent and preop LASIK
Keratometry readings into the IOL calculation
formulas.He presented a small case series of
patient with myopic LASIK and found it to be very
accurate when you enter the preoperative LASIK
refraction as your target refraction or
postoperative target into your IOL calculation
formulas.
• ****WARNING- NOT TESTED IN LARGE SERIES****

64. • Barrett true-K no history formula
• This version of Barrett’s formula has been developed to
• work without historical data and can be accessed via the
• same websites reported for the “historical” version (see
• 2.1.4). The formula has not been published, but the results
• are good [33].

65. • IOL power ¼ aphakic refraction ðspherical equivalentÞ
• 2:01449 ðIanchulevÞ
• IOL power ¼ aphakic refraction ðspherical equivalentÞ
• 1:75 ðMackoolÞ

68. IOL calculation for Phakic IOL
• The first pIOLs were placed in the anterior chamber angle as early as
1953 by Dr. Strampelli
• Phakic IOL
•ACIOL
•PCIOL
• Parameters
•Size of the lens: WTW distance and ACD
•Power

70. Post PK
• https://ascrs.org/tools/iol-calculator