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IOL power calculation special situations

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IOL power calculation special situations

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IOL power calculation special situations

  1. 1. IOL Power Calculation In Special Situations Dr Ravish Vaishnav Moderator- Dr Vijay Shetty
  2. 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. 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. 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. 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. 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. 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. 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. 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. 10. Axial Length Measurement  1 mm error leads to 2.5D error in postoperative refraction  2 types 1. Optical 2. Ultrasound
  11. 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. 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. 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. 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. 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. 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. 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. 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. 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. 20. Biometry in specific situations
  21. 21. Aphakic eyes
  22. 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. 23. Keratoconus eyes
  24. 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. 25. Eyes with silicone oil
  26. 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. 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. 28. High myopic eyes
  29. 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. 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. 31. Pediatric eyes
  32. 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. 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. 34. Piggyback IOL
  35. 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. 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. 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. 38. Corneal refractive surgery
  39. 39. Common Keratorefractive surgeries which pose a challenge for Cataract surgery MYOPIC  LASIK  PRK  RK WITH OR WITHOUT AK HYPEROPIC  LASIK  PRK
  40. 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. 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. 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. 43. Web Help  http://www.doctor-hill.com/iol-main/keratorefractive- calculator.html  www.ascrs.org, http://iol.ascrs.org/wbfrmCalculator.aspx
  44. 44. 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.
  45. 45. 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
  46. 46. 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
  47. 47. 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]
  48. 48. 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.
  49. 49. Thank you!!

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