Biometry

keratometry, a scan, iol calculation

1. BIOMETRY Dr Binesh tyagi

2. Introduction Measuring various dimensions of the eye, its components and their interrelationships, and using this data to determine the ideal intraocular lens power. Essentially consists of a keratometric reading together with an ultrasonic measurement of axial length of the eye. 2

3. Claims relating to biometry errors/wrong intraocular lens power were the second most frequent cause of claims and result in payment of damages in 62% of closed cases ! ! ! ! * Causes of cataract surgery malpractice claims in England 1995–2008. Nadeem Ali , Brian C Little Br J Ophthalmol doi:10.1136/bjo.2010.182774

4. Keratometry Used to measure the corneal curvature. Done before axial length measurement. Types of keratometer : Manual keratometer Auto keratometer , keratometers incorporated in IOL master and lenstar Topography – placido disc based or elevation based topography

5. Manual Keratometry Principle Fixed object and variable image size Fixed image and variable object size Two types Bausch and Lomb Javal Schiotz

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.

7. Axial Length Measurement 2 types Optical Ultrasound

8. Ultrasound based A-scan PRINCIPLE - the ultrasound probe has a piezoelectric crystal that electrically emits and receive high frequency (10Mz) sound waves. Measurement is from anterior corneal surface to internal limiting membrane. 1 mm error leads to 2.5D error in postoperative refraction 50% of a surgeon’s post operative surprises are A-scan errors (Olsen) Errors of 2.00D or more are almost always A-scan related (Holladay)

9. Tips for accurate measurement of axial length (using applanation): Ensure the machine is calibrated and set for the correct velocity setting The echoes from cornea, anterior lens, posterior lens, and retina should be present and of good amplitude Average the 5–10 most consistent results giving the lowest standard deviation (ideally < 0.06 mm) Misalignment along the optic nerve is recognized by an absent scleral spike. The gain should be set at the lowest level at which a good reading is obtained 9

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11. High quality contact A-scan of the phakic eye. 5 high-amplitude spikes Steeply rising retinal spike, as well as the good resolution of the separate retinal and scleral spikes. Little or no “stair steps” in leading edge of retinal echo.

12. Higher the gain, better the sensitivity, but the resolution gets compromised At low gains the sensitivity is less, but the resolution is good. 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.

13. Immersion Technique Time consuming, expensive, messy and requires the patient to be supine. The immersion technique minimizes technician variables leading to more reproducible results Can be performed in the standard ophthalmic chair reclined at 45° Ossoinig shell / prager shell

14. Immersion scan of the phakic eye. The corneal spike demonstrates 2 peaks, representing the epithelium and endothelium.

15. Limitations of immersion After a single immersion biometry measurement, 18 of 34 samples (53%) grew organisms from either the probe/shell or the tubing. Positive cultures were found in 32% of the immersion shell/probes (11 of 34) and in 31% of the infusion tubing samples (10 of 32).* Recommendations: The shell and probe should be soaked in alcohol or hydrogen peroxide for at least 5 minutes. The immersion shell should be allowed to dry completely and flushed with balanced saline solution (BSS). *Velazquez- Estades LJ, Wanger A, Kellaway J, et al. Microbial contamination of immersion biometry ultrasound equipment. Ophthalmology 2005;112: e13–8.

16. A scan Biometry in Special Cases

17. Inadequate Patient Fixation Low Vision Nystagmus Blepharospasm Strabismus

18. Causes an irregular shape of the ocular wall Inability to display a distinct , high retinal spike Deepest portion of the staphyloma may be located eccentric to macula, thus the measurement may be longer than true AL along the visual axis. 2. Posterior Staphyloma

19. RD Edema Tumor 3. Macular Lesions An elevated macular lesion may prevent the display of a distinct retinal spike and often causes a shortened AL measurement.

20. Asteroid Hyalosis Vitreous Hemorrhage Gas Bubble 4. Vitreous Lesions

21. 5. Dense Cataract Strong sound attenuation can significantly impair the ability to display spikes from the various interfaces. Maximum gain setting may be required to obtain spikes of sufficient height from the posterior lens capsule and retina.

22. 6. Silicone Oil Sound velocity in silicone oil 1040 m/s 5000 cs 980 m/s 1000 cs This low sound velocity can result in pronounced sound attenuation and difficulty in identifying the retinal spikes .

23. 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.

24. 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

25. ADVANTAGES OF LENSTAR OVER IOL MASTER – 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. 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.

26. 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. IOLMaster 700 overcomes this problem. Unusual eye geometries like tilt or decentration of lens can be detected.

27. 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.

28. Operating principle Reflected into the eye by mirrors Two equal coaxial beams by beam splitter . Both coaxial beams enter the eye, where reflections take place at the corneal and retinal interfaces. On leaving the eye, the difference in frequency detected by a photodetector. 32

29. A spatial resolution of 0.01 mm for axial length measurement Measures the axial length of the eye in approximately 0.4 seconds Designed to measure the axial length along the visual axis It also uses lateral slit illumination of the crystalline lens and the cornea to determine anterior chamber depth and autokeratometry to estimate corneal curvature. 33

30. limitations Positioning patients with mobility problems on the IOLmaster machine may occasionally be a problem. Dense ocular media—that is, corneal scarring, mature or posterior subcapsular cataracts, prevent acquisition of Optical AL measurements. It may be inaccurate for patients with axial or dense cataracts or gross astigmatism. It is also expensive. 34

31. IOL FORMULAE

32. Formulae Depending upon the basis of their derivation: Theoretical & regression formulae Grouped into various generation 36

33. 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 37

34. Third generation Holladay I SRK/T Hoffer Q Fourth generation Holladay - II

35. First generation Earliest formulae Binkhorst formula P= 1336(4r-a)/(a-d)(4r-d) Where P - power or IOL R – corneal radius A – AL D – assumed postop ACD plus CT 41

36. Colenbrander - Hoffer formula P = 1336/a-d-0.05 – 1336/(1336/k –d-0.05) K- Average keratometry in dioptres 42

37. Gills formula P= 129.40+(-108k) = +(-2.79xLeye) = + (0.26x LCL) = + (-0.38xref) LCL – Distance of apex of anterior corneal surface to apex of IOL in mm Ref- desired postop refraction 43

38. Clayman’s formula Assume, emmetropizing IOL=18D Emmetropic AL=24mm Keratometer reading = 42 D If IOL power>21d, deduct 0.25 for every dioptre > 18 D 44

39. Fyodorov formula P = 1336- LK/(L-C) – CK/1336 C- Estimated postop ACD 45

40. These seemingly different formulae are in fact identical Can be transformed algebraically P = N/L-C - NK/N-KC P= implant power N=aqueous and vitreous refractive index C= estimated postop AC depth 46

41. drawbacks Reliable for eyes with AL between 22 and 24.5mm Tend to predict too large value in short eyes (<22mm) and too small value in long eyes(>24.5mm) Assumption about the optics of the eye Still requires a guess about AC depth 47

42. Regression formulae SRK – I formula Introduced by Sanders, Retzlaff and Kraff P = A- 2.5L-0.9K Tends to predict too small value in short eyes and too large value in long eyes 48

43. SRK –II formula P = A-2.5L-0.9K A constant is modified on the basis of axial length as follows: If L is < 20mm : A+3.0 If L is 20-20.99 : A + 2.0 If L is 21-21.99 : A+1.0 If L is 22-24.5 : A If L is > 24.5 : A-0.5 49

44. Modified SRK II formula Based on axial length, A constant is modified as If L is < 20mm : A+ 1.5 If L is 20-21 : A + 1.0 If L is 21-22 : A+0.5 If L is 22-24.5 : A If L is 24.5 – 26 :a-1.0 If L is >26mm :A-1.5 50

45. Other formulae SRK/T formula Nonlinear theoretical optical formula empirically optimized for postop AC depth, retinal thickness, and corneal refractive index Significantly more accurate for extremely long eyes(>28mm) 51

46. Holladay formula Third generation formula Its enhanced ability to predict the position of the implants Various constant and equations used in this formula 52

47. Hoffer’s formula Third generation theoretical formula Optimized with regression techniques for ACD Performs best for short eyes P= 1336/A-C-0.05 – 1.336/{(1.336/K+R) – (C+0.05/1000)} 53

48. Haigis formula This formula was published as early as 1970 by GERNET, OSTHOLT and WERNER DL = n/L-D - n/(n/z-d Z = DC + ref/1-refdBC DC = nC-1/RC D : refractive power of IOL DC : refractive corneal power RC : corneal radius NC : (fictitious) refractive index of cornea 54

49. Ref : desired refraction DBC : vertex distance between cornea and glasses D : optical ACD L : axial length N : refractive index of aequeous and vitreous (1.336) 55

50. 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

51. 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

52. 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 IOL: https://www.amoeasy.com/toric2(bd1lbizjpta1ma==)/toric.htm

53. Biometry in specific situations

54. aphakic eyes Aphakic eyes – sound travel at slower speed, 1532m/s Two lens spike – replaced by a single spike of variable height – obtained from anterior vitreous face or posterior lens capsule Immersion technique/ modern biometers is the method of choice. 60

55. Keratoconus eyes 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 . Overestimation is not a factor with the haigis formula as it does not use the k reading in estimating the lens power.

56. Eyes with silicone filled 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’. Usually a factor of (0.72) gives a rough estimate of the power. Tal = 1133/1550 × al.

57. high myopia Accurate axial measurement is critical for IOL power calculation Paraxial measurement in ultrasonic measurement 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 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

58. 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. Undercorrection has to be done with care especially in unilateral cases, as amblyopia risk is high.

59. 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

60. Secondary piggyback IOL Patients with refractive error following the primary IOL implantation. Calculated based on refractive error. Holladay’s refractive formula .

61. 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.

62. 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 * K MANUAL PO – 6.8 5. Haigis method K = -5.1625 * K IOL MASTER + 82.2603 - 0.35

63. Choice of IOL IOL with negative spherical aberration like technis or acrysof IQ would correct spherical aberration induced by myopic LASIK. Refractive multifocal IOLs not recommended. Diffractive multifocal IOL is not preferred.

64. How to Avoid Common Errors Keratometry Use a single instrument for all measurements Javal- schiotz keratometer preferred Autokeratometry needs multiple readings to be reliable Use topography when indicated.

65. How to Avoid Common Errors IOL calculation formulas Most 2-variable formulas assume that short eyes produce a shallower effective lens position(ELP) and longer eyes will result in a deeper effective lens position. Also assume that flat k readings will result in a more shallow effective lens position and steeper Ks will result in a deeper effective lens position This is the main reason why the accuracy of 2- variable formulas decreases at the extremes of axial length and corneal power, especially in the setting of axial hyperopia .

66. How to Avoid Common Errors IOL calculation formulas Holladay 2 formula is the best “off-the shelf” tool for improving the accuracy of IOL power calculations for all axial lengths. SRK-t formula for long eyes Hoffer-q / haigis formula for extremely short eyes

67. How to Avoid Common Errors IOL Constant Optimization After maintaining a record of your own biometries and carefully calibrating with the refractory outcomes one should develop his as her own personal a-constant to minimize the surgeon’s factor.

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