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Biometry

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  • 1. Axial Length Measurement ( Biometry ) Mohammad Reza ZARRIN (optometrist- MsC) Tehran University of Medical Sciences
  • 2. IOL Power Calculation 1. Keratometry 2. A-Scan Biometry 3. IOL Formula
  • 3. A-Scan Biometry Measurement of Axial Eye Length by Ultrasound
  • 4. Average Axial Length of Normal Eye 23.06 mm Majority 22.0 to 24.5 mm
  • 5. Accuracy of AL measurement using A-scan ultrasound is + 0.1 mm
  • 6. Difference in AL measurement Between both eyes + 0.3 mm
  • 7. Instrumentation
  • 8. Examination Procedure 1. History Taking 2. Patient Preparation 3. Biometry Technique
  • 9. Biometry Technique  Contact - Applanation Method - Hand-Held Method  Immersion
  • 10. Values are 0.14 to 0.36 mm longer with immersion technique than with contact method
  • 11. Potential Sources of Error with Contact Method 1.Corneal Compression 2. Fluid Excess 3. Misalignment of Sound Beam 4. Inappropriate Eye type
  • 12. Error caused by 1 mm Corneal Compression Average eye 2.5 D Long eye 1.75 D Short eye 3.75 D
  • 13. Potential Sources of Error with Immersion Method 1. Air bubbles within fluid 2. Inappropriate eye type
  • 14. Instrument Setting 1. Measurement Mode 2. Gates 3. Gain 4. Eye Type
  • 15. Measurement Mode  Automatic  Semiautomatic  Manual
  • 16. Gates Gates are electronic markers on the screen that provide measurement of distance between 2 or more anatomic interfaces .
  • 17. Gain Setting Initially high gain setting should be used to assess the overall appearance of the echogram , then gain should be reduced to a medium level to improve resolution of spikes .
  • 18. Error can occur when the gain is set too high or too low . Very high gain short reading Very low gain long reading
  • 19. Eye Type ( Sound Velocity ) 1. Phakic 2. Aphakic 3. Pseudophakic
  • 20. Use of average sound velocity ,although sufficient in normal phakic eye , may result in slight error when the lens is inordinately thin or thick or when the eye is very short or very long .
  • 21. The use of individual sound velocity may provide more consistent and accurate AL reading .
  • 22. Aphakia & Pseudophakia Manual measurement mode is better to help ensure alignment of sound beam .
  • 23. If an incorrect eye type is used an erroneous measurement will occur . For determination of correct value Velocity Conversion Equation should be used .
  • 24. Velocity Conversion Equation True AL = V c /Vm x Apparent AL
  • 25. Biometry in Special Cases
  • 26. 1.Inadequate Patient Fixation  Low Vision  Nystagmus  Blepharospasm  Strabismus
  • 27. 2. Posterior Staphyloma Posterior staphylomas often causes an irregular shape of the ocular wall resulting in an inability to display a distinct , high retinal spike , leading to a significant error in A-scan measurement .
  • 28. Deepest portion of the staphyloma may be located eccentric to macula thus te measurement may be longer than true AL along the visual axis .
  • 29. B-scan can be used to demonstrate the shape of posterior ocular wall and the relationship of macula to the staphyloma .
  • 30. Probes with fixation light are preferable
  • 31. 3. High Hyperopia Immersion technique is preferable .
  • 32.  Edema  DMS  Tumor 4. Macular Lesions  RD
  • 33. The presence of an elevated macular lesion may prevent the display of a distinct retinal spike and often causes a shortened AL measurement .
  • 34. 5. Vitreous Lesions  Asteroid Hyalosis  Vitreous Hemorrhage  Gas Bubble
  • 35. 6. Dense Cataract Strong sound attenuation produced by a very dense cataract can significantly impair the ability to display spikes from the various interfaces along the visual axis .
  • 36. Maximum gain setting may be required to obtain spikes of sufficient height from the posterior lens capsule and retina .
  • 37. Semiautomatic mode should be used in eyes with dense cataract
  • 38. 7. Silicone Oil Sound velocity in silicone oil 1040 m/s 5000 cs 980 m/s 1000cs
  • 39. This low sound velocity can result in pronounced sound attenuation and difficulty in identifying the retinal spikes .
  • 40. If proper sound velocity are not used , erroneously long AL measurement will be obtained .
  • 41. For accurate AL measurement , various ocular components should be measured separately with appropriate sound velocity .
  • 42. If biometer provides only preset sound velocity , AL measurement can be obtained using velocity conversion equation .
  • 43. The least preferred method is use of average sound velocity Average sound velocity in eyes with average length (23.5 mm) 1,139 m/s phakic eye 1,052 m/s aphakic eye
  • 44. Due to strong sound attenuation AL measurement often can not be obtained from an eye containing emulsified silicone oil .
  • 45. IOL Master
  • 46. Zeiss IOL Master  Axial Length  ACD  Corneal Power  IOL Power Calculation Hoffer-Q , SRK/T ,Holladay 1, Haigis
  • 47. Keratometry A second person should confirm measurements prior to A-scan ultrasonography if: The corneal power is less than 40.0 diopters, or greater than 47.0 diopters. If there has been prior keratorefractive surgery. In this case the corneal power will need to be estimated by either the historical, or the contact lens method. The average corneal power difference between the two eyes is greater than 1.00 diopter. The patient cannot fixate, as seen with a mature cataract, or macular hole. The amount of corneal astigmatism by keratometry, or topography, correlates poorly with the amount of astigmatism on the most recent manifest refraction. The corneal diameter is less than 11.00 mm. There is any problem with patient cooperation, or understanding.
  • 48. Immersion A-scan Ultrasonography A second person should re-measure both eyes if: The axial length is less than 22.00 mm, or greater than 25.00 mm in either eye. The axial length is greater than 26.0 mm, and there is a poor retinal spike, or wide variability in the readings. There is a difference in axial length between the two eyes of greater than 0.33 mm that cannot be correlated with the patient's oldest refraction. Axial length measurements do not correlate with the patient's refractive error. In general, myopes should have eyes longer than 24.0 mm and hyperopes should have eyes shorter than 24.0 mm. Exceptions to this rule involve steep, or flat corneas. Be sure to use the oldest refractive data. There is difficulty obtaining correctly positioned, high, steeply rising echoes, or wide variability in individual axial length readings for either eye.
  • 49. There is a difference in axial length between the two eyes of greater than 0.33 mm that cannot be correlated with the patient's oldest refraction. Axial length measurements do not correlate with the patient's refractive error. In general, myopes should have eyes longer than 24.0 mm and hyperopes should have eyes shorter than 24.0 mm. Exceptions to this rule involve steep, or flat corneas. Be sure to use the oldest refractive data. There is difficulty obtaining correctly positioned, high, steeply rising echoes, or wide variability in individual axial length readings for either eye.
  • 50. Intraocular Lens Power A second person should repeat the axial length measurements, keratometry readings and re-run the IOL power calculations for both eyes if: The IOL power for emmetropia is greater than 3.00 diopters different than anticipated. There is a difference in IOL power of greater than 1.00 diopter between the two eyes. If the patient has had prior keratorefractive surgery and the calculated IOL power for standard phacoemulsification is less than +20.0 D or greater than +23.0 D.
  • 51. Formula for IOL Power Calculation
  • 52. IOL Power Formula  Theoretical  Regression  Refractive
  • 53. Theoretical Formulas These are derived from geometrical optics
  • 54. Regression Formulas Actual postop refractive results of many lens implantations are used to predict IOL power
  • 55. Theoretical Formula These formulas contain many assumptions including values of postop ACD , refractive index of cornea and ocular humors , retinal thickness
  • 56. Theoretical Formula These formulas are reliable for average AL , but overestimates in short eyes and underestimates in long eyes
  • 57. Refractive Formulas IOL power calculation without determination of axial length
  • 58. SRK I (Sanders,Retzlaff,Kraff) P = A – 2.5L – 0.9K It generally undercorrects short eyes and overcorrects long eyes
  • 59. SRK II 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
  • 60. SRK/T It is a nonlinear theoretical optical formula empirically optimized for postop ACD , retinal thickness , corneal refractive index . It combines advantages of theoretical and regression formulas .
  • 61. Generations of IOL Formulas 1st Generation Fyodorov , Colenbrander ,Hoffer , SRK I 2nd Generation Binkhorst , SRK II 3rd Generation Holladay 1 , Hoffer-Q , SRK/T 4th Generation Holladay 2 , Haigis
  • 62. There are currently three IOL constants in use: The SRK/T formula uses an "A-constant." The Holladay 1 formula uses a "Surgeon Factor." The Holladay 2 formula, and the Hoffer Q formula, both use an "Anterior Chamber Depth." aka: ACD.
  • 63. d = the effective lens position, where ... d = a0 + (a1 * ACD) + (a2 * AL) Haigis Formula
  • 64. * The a0 constant basically moves the curve up, or down, in much the same way that the A- constant, Surgeon Factor, or ACD does for the Holladay 1, Holladay 2, Hoffer Q and SRK/T formulas. * The a1 constant is tied to the measured anterior chamber depth. * The a2 constant is tied to the measured axial length. The way the a0, a1 and a2 constants are derived is by generating a set of surgeon, and IOL-specific
  • 65. Formula Choice
  • 66. AL < 19 mm (<0.1%) Holladay 2 AL 19-22 mm (8%) Holladay 2 , Hoffer-Q AL 22-24.5 mm (72%) SRK II , Hoffer-Q ,Holladay 1 AL 24.5-26 mm (15%) Holladay 1 , Hoffer-Q AL > 26 mm ( 15%) SRK/T
  • 67. Axial Length in mm Haigis unoptimized Hoffer Q Holladay 1 Holladay 2 SRK/T 20.00 to 21.99 0.25 D 0.25 D 0.25 - 0.50 D 0.25 D 0.51 - 1.0 D 22.00 to 24.49 0.25 D 0.25 D 0.25 D 0.25 D 0.25 D 24.50 to 25.99 0.25 D 0.25 D 0.25 D 0.25 D 0.25 D 26.00 to 28.00 0.25 - 0.50 D 0.25 - 0.50 D 0.25 D 0.25 D 0.25 D 28.00 to 30.00 0.25 - 0.50 D 0.25 - 0.50 D 0.25 D 0.25 D 0.25 - 0.50 D Minus power IOLs 0.51 - 1.0 D 0.51 - 1.0 D 0.25 - 0.50 D 0.25 D 0.25 - 0.50 D
  • 68. Haigis formula may be appropriate for all ranges of axial lengths

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