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IOL power calculation formulae

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IOL power calculation formulae

  1. 1. IOL power calculation FORMULAE
  2. 2. Generations! • First – SRK 1 & Binkhorst formula • Second – SRK2 • Third – SRK T , Hoffer Q , Holladay • Fourth – Holladay 2 , Haigis
  3. 3. • Theoretical formulae – based on mathematical principles revolving around the schematic eye • Regression formulae – working backwards on post operative outcomes • 3rd and 4th generation – mix of both.
  4. 4. A closer look 1. SRK formula • P = IOL Power • K = avg central corneal power in Diopters • L = axial length in mm. P = A – 0.9K – 2.5L Range - 22mm – 24.5mm
  5. 5. 2. SRK 2 formula – An attempt to OPTIMIZATION A1 =(A–0.5) for axial lengths greater than 24.5 A1 =A for axial lengths between 22 and 24.5 A1 =(A+1) for axial lengths between 21 and 22 A1 =(A+2) for axial lengths between 20 and 21 A1 =(A+3) for axial lengths less than 20
  6. 6. Why Optimize??
  7. 7. • All formulae work well within the range of 22.5-26 mm AL • A constant computed based on Avg AL 23.5mm • Assumption Reason?? AL α ACD K α ACD Dr Holladay
  8. 8. • SRK/T formula — uses "A-constant" • Holladay 1 formula — uses "Surgeon Factor" • Holladay 2 formula — uses "Anterior Chamber Depth" • Hoffer Q formula — uses "Pseudophakic Anterior Chamber Depth"
  9. 9. • Hoffer-Q formula • Dr Kenneth Hoffer – 1993 • P = f (A,K,Rx,pACD) • A = AL • K = avg corneal ref power • Rx = previous refraction
  10. 10. • pACD = The personalized ACD (pACD) is set equal to the manufacturer's ACD-constant, if the calculation was selected to be based on the ACD-constant. In case the A-constant was chosen, pACD is derived from the A-constant [HOFFER, 1998] according to [HOLLADAY et al, 1988] pACD = ACD-const = 0.58357 * A-const - 63.896 .
  11. 11. Haigis formula • Gernet, Ostholt, Werner -1970 (GOW70 formula) • D = a0 + a1 ACD+ a2 AL • a0 constant moves the power prediction curve up or down • a1 constant is tied to the measured anterior chamber depth • a2 constant is tied to the measured axial length
  12. 12. Holladay 2 • Currently – most sophisticated formula ACCURACY PREDICTABILITY
  13. 13. 2 reasons for success
  14. 14. • 7 PARAMETERS Axial length White to white AC depth Previous refraction Age of pt Lens thickness Central corneal power
  15. 15. Effective lens position • Human eye – a dual lens system In any dual lens system, if the primary lens and the distance of the screen are fixed, then the effective power of the system will depend on the power and position of the second lens
  16. 16. Factors affecting ELPo • Anatomical factors 1. K value 2. AL 3. Limbal white to white dist 4. Pre-op AC depth 5. Lens thickness • IOL related factors 1. Shape 2. Length 3. Flexibility 4. Anterior angulation (if any) 5. Material of haptic 6. Shape, design material of optic
  17. 17. • Surgeon’s technique 1. CCC 2. Inadequate visco removal 3. Capsular fibrosis Bag to Sulcus shift
  18. 18. • AL = 22.5-26mm Almost any modern • K = 41D-46D formula Outside this range ???? Haigis formula Holladay 2
  19. 19. Special cases
  20. 20. Post ref sx IOL calculation
  21. 21. • Require pre ref sx data 1. Clinical History method (manifest refraction, K values) 2. Feiz–Mannis method (manifest refraction, K values) 3. Topographic method based on adjusting the measured effective refractive power (EffRP) (manifest refraction)
  22. 22. • Do not require 1. CL overrefraction (adjusting corneal power using a correcting factor) 2. Orbscan topography 3. Maloney method
  23. 23. MESA-GUTIÉRREZ JC1, RUIZ-LAPUENTE C2,INTRAOCULAR LENS POWER CALCULATION AFTER CORNEAL PHOTOREFRACTIVE SURGERY; LITERATURE REVIEW. ARCH SOC ESP OFTALMOL 2009; 84: 283-292
  24. 24. Clinical history method Postop K = (K before ref sx) – (change in refraction at corneal plane induced by the sx) Corr-Kpost = Kpre - SEpre + SEpost *SE = spherical equivalent
  25. 25. Feiz Mannis method • Best used when good historical data is present • Least likely to cause post op hyperopia IOL pre – (∆D / 7) = IOL post where…… IOL pre = the power of IOL using pre LASIK keratometry ∆D = the stable refractive change aftr LASIK at the spectacle plane then… IOL post = the estimated power of the IOL to be implanted following LASIK
  26. 26. • It is helpful to keep in mind that the sign convention for the change in refractive error (ΔD) following myopic LASIK is a negative number. Using the above formula, the new calculated IOL power will always result in a larger number.
  27. 27. The Koch-Wang Method • We perform a corneal topography and take the value of EffRp (effective refractive power). Corr-Kpost = EffRp (ΔD x 0.19)
  28. 28. The Hammed Method • We also take the EffRp value and calculate the correction as follows: Corr-Kpost = EffRPadj = EffRp-(ΔD/0.15)
  29. 29. The Shammas Formula • Kpost(Shammas)=1.14 K-6.8. • Kpost(Shammas)=1.14*44.25-6.8=50.45-6.8 • Kshammas=43.65D
  30. 30. The contact lens Method • Subjective refraction → place rigid PMMA CL → refraction • If refraction same…cornea has same power as CL • If refraction is myopic… CL has more power than cornea • Opposite in hypermetropia
  31. 31. The contact lens Method Rpost=Refraction in post-op eyeglasses = -1D Rlc= Correction with the contact lens = +1D. Base Curve =CB=40 DR (Difference in Refraction)=Rlc-Rpost=+1-(-1)=+2 Corr-Kpost=CB+DR=39.25+2=41.25 D.
  32. 32. Silicon oil • There are presently two viscosities of silicone oil in use: • 1,000 mPa.s. silicone oil (Silikon, Alcon Laboratories, Ft. Worth, Texas) slows sound waves to a little more than half the speed (980 m/sec) of normal vitreous and can attenuate the returning sound wave during ultrasonography so much that a good echoes are difficult, if not impossible, to obtain. • 5,000 mPa.s. silicone oil (ADATO SIL-ol 5000, Bausch & Lomb Surgical, San Dimas, California) has a somewhat higher density, and slows sound waves to approximately 1,040 m/sec. Typically, when ultrasound measurements are made through silicone oil, hugely erroneous axial lengths (such as 35 mm) are displayed.
  33. 33. • Without IOL master • each component of the eye had to be individually measured (usually at 1,532 m/sec) and the true axial length calculated using the velocity conversion equation (TAL = Vc / Vm x AAL) for the lens thickness and the vitreous cavity.
  34. 34. Holladay et al Biconvex IOL PMMA plano convex lens with plano side towards vitreous…….over an intact capsule Silicone lens
  35. 35. • The additional power that must be added to the original IOL calculation for a convex-plano IOL is determined by the following relationship, as described in 1995 by Patel and confirmed by Meldrum: • Ns = refractive index of silicone oil (1.4034). • Nv = refractive index of vitreous (1.336). • AL = axial length in mm. • ACD = anterior chamber depth in mm. • Additional IOL power (diopters) = ((Ns - Nv) / (AL - ACD)) x 1,000
  36. 36. Average eye with silicon oil - +3 to +3.5D
  37. 37. Posterior staphyloma • 70% eyes with AL >33.5mm • Pathological myopia Difficulty in obtaining posterior retinal spike ???????? Posterior staphyloma
  38. 38. • Anatomic AL ≠ refractive AL • The simplest method • Optical biometry using either the 1. Haag-Streit Lenstar or the 2. Zeiss IOL Master. • If the patient's visual acuity is good enough, have him or her look directly at the red fixation light, and the axial length measurement will typically be to the center of the macula.

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