BIOMETRY

1. BIOMETRY Deepak Vayalambrone FRCOphth

2. it can be a bit overwhelming

3. NOVEMBER 29 1949 St. Thomas Hospital London

4. First ever IOL implant • In a 49 y.o. nurse • IOL as thick as natural lens • Power of 24 D in water • Resultant myopia of -18 D • Due to higher R I of Perspex CQ. • Biometry? ‘Ridley met John Pike, an optical scientist with Rayner in a parked car and ‘discussed’ the Implant’

5. Stewart Duke-Elder ‘’an ophthalmic surgeon's job is to take material out of the eye, not to put it in’’ Harold Ridley "I am the only man to have invented his own operation" 

6. Basic refraction based formula • IOL power = 18 + 1.25 x refraction. • i.e. Refraction = 0  IOL power 18 D. • Binkhorst RD. Ophthalmic Surg. 1976 Fall;7(3):69-82 ‘’The rule is simple......careful history and an examination of old glasses and past refraction ...... When the basic refraction cannot be ascertained, capital errors may occur.’’

7. Lens power tables ‘’Ophthalmologists who do not own a personal computer will find the lens power table a valuable adjunct to their present repertoire of analytical skills’’. Estimates of primary implant power using an intraocular lens table. J Cataract Refract Surg. 1986 Jul;12(4):401-7

8. The human eye • Geometric thin lens optics not applicable • Lens does not have a uniform RI • Variable lens position • Interplay of AL, K, Lens Power, RI of lens.

9. The human eye 42 D 24 mm 18 D

10. A few words on terminology • Axial length • Keratometry • Anterior Chamber Depth • Effective Lens Position • Position of principal plane of IOL behind cornea • 1st gen- constant of 4 mm • 2nd gen- AL as a scaling factor Binkhorst • 3rd gen- AL and K • 4th gen- AL, K, pre-op ACD and LT

11. Standardised pseudophakic eye

12. Theoretical formulae IOL POWER = Required vergence in the plane of the IOL minus Effective power of the cornea in the same plane Problems Prediction of the Lens position Theoretical ‘thin lens’ Inaccuracy of Keratometry

13. Theoretical formulae • Thijssen • Colenbrader • Fyodorov • Van der Heijde • Binkhorst

14. Regression formulae • Post-op refractive results analysed • Correction factors applied • SRK formula • IOL power= A- 2.5L – 0.9K • A- constant reflecting and affected by • position of lens • angulation of haptics • Lens shape • L- axial length • K- Corneal power

15. II generation formulae • AL and IOL position related • SRK-2 and Binkhorst. • Correction for longer and shorter eyes. Axial lengths < 20mm Al = A + 3 20mm to 21 mm Al = A + 2 21 mm to 22 mm Al = A + 1 22 mm to 24.5 mm Al = A >24.5 mm Al = A - 0.5

16. Koch suggested classification of IOL formulae

17. 9 types of eyes*

18. Current state of play • SRK-T is the most widely used (RCOphth recommended) • Hoffer –Q for eyes shorter than 22 mm • Barrett Universal 2 • Good agreement with SRK- T when 3 variables used • Changes when 5 variables are used • Hill RBK • Wang-Koch correction for high axial lengths • Haigis-L for post LASIK eyes • 85% of eyes expected to be within 1 D SE

19. Axial length measurement • Ultrasonography • Cornea to ILM- anatomic AL • Partial coherence interferometry • Cornea to photoreceptor- optical AL • Swept source OCT • Optical low coherence reflectometry

20. Keratometry • Measure the anterior cornea • Keratometers have a ‘central scotoma’ of 3.2 mm • Post refractive surgery patients need correction • If entering manually in the IOL Master- • Check RI that the Keratometer uses. • Contact lenses • Discontinue RGP for 3 weeks • Discontinue soft lenses for 1 week • 1 D error  nearly 1 D error in post op ref

21. It’s a bit like choosing a cereal….. • IOLMaster 500 • IOLMaster 700 • AL scan • Lenstar LS900 • Aladdin/Aladdin LT • Argos (Movu) • Galilei G6

22. IOL MASTER 500 • Gold standard of biometry • Use 780 nm semiconductor diode laser- PCI • Interference at tear film- cornea interface • Interference at RPE • AL accurately to within ±0.02 mm • Inability to measure through dense cataract • Tear film abnormalities can cause problems

23. IOL master 700 • SS OCT - 2000 scans/s. • Can identify unusual ocular geometry • (e.g., crystalline lens tilt/decentration) • More accurate than IOLmaster 500 • Fixation check using the foveal pit • The fixation check helps identify macular pathologies • Telecentric K and distance-independent • Restless patients, pupil size independent. • Can perform biometry even through dense cataracts

24. Lenstar • OLCR • Superluminescent diode laser (820 nm) • Keratometry- dual zone 1.63mm and 2.3 mm • T cone – topography using Placido disc • measured at 32 points • Directly measures ACD • Measures retinal thickness • Hill RBF included • Eye tracking • Cannot measure through dense cataract • AL range only upto 32 mm

25. Special circumstances Every other eye!

26. Refractive surgery-1 • Standard ‘K’ • 3.2 mm ‘central scotoma’ • Measures outside central ablation zone • Corneal standardised index of refraction (n=1.3375) invalid 1 - 2 r • RK flattens both post and ant curves • Ablative flattens ant curve

27. Refractive surgery-2Historical ‘K’ • Assuming a vertex distance of 12 mm. • Pre LASIK refraction at corneal plane= −9.91 D • Post LASIK refraction at corneal plane= −2.19 D • Amount of correction • −2.19 − (− 9.91) = −2.19 + 9.91= 7.72 D • Corrected K 42.0 – 7.72 = 34.28 D. • Source: Intraocular Lens Power Calculations 2003. H. John Shammas

28. Refractive surgery-3Historical Refraction • Assuming a vertex distance of 12 mm. • Pre LASIK refraction at corneal plane= −9.91 D • Post LASIK refraction at corneal plane= −2.19 D • Amount of correction • −2.19 − (− 9.91) = −2.19 + 9.91= 7.72 D • Corrected K • Post LASIK K − (0.23 x Correction at cornea) =36.0 – (0.23 x 7.72)= 34.22 D • Source: Intraocular Lens Power Calculations 2003. H. John Shammas

29. Refractive surgery-4Clinically derived method • Regression formula • Shammas-2, Haigis-L K = (1.14 x KpostPRK) – 6.8 • Used when historical data not available • Disadvantage • Does not relate to amount of correction by refractive surgery

30. Refractive surgery-5Contact lens method • Refract eye • Fit plano Rigid CL with known BC e.g. 41D • Refract e.g. −8.75 D • Difference added algebraically to BC 41+ [−8.75 D-(-2.25)]= 34.50D • Source: Intraocular Lens Power Calculations 2003. H. John Shammas

31. Refractive surgery-6Other possible options • Nomogram based assessment * 63% within 0.5D 84% within 1D 100% within 1.5D Better than ‘Historical method’ but..... Data from 19 eyes • Lenstar • Probably the best currently • IOL master 700 • Pentacam • With correcting software • Orbscan II • Measures central1.5 mm

32. Penetrating keratoplasty • Triple procedure not ideal. • Keratoplasty + Cataract extraction • Planned Aphakic IOL later.

33. PIGGY BACK IOL • MYOPIC ERROR • Implant power same as error • HYPERMETROPIC ERROR • Implant power 1.5 x error

34. Post vitrectomy • Convex- plano PMMA lenses better? • Biconvex IOLs could lose 1/3- ½ refractive effect • *117 eyes • 85% within 1 D • Retro silicone space important if AL in supine position • Myopic shift (Not OIL filled eyes) • Ranges -0.50 to -0.86 from predicted refraction *Graefe's Archive for Clinical and Experimental Ophthalmology Vol 243;10 / 2005

35. Post Vitrectomy eyes • Silicone oil • Change in AL on USG due to change in velocity • Not an issue with LI techniques • Hypermetropic shift with retained oil • Degree of shift varies depending on IOL design • Additional IOL power • (Ns - Nv) / (AL - ACD) x 1,000 • Ns – RI of Silicone oil.(14034) • Nv- RI of vitreous. (1.336) • 3.0 – 3.5 D additional power usually

36. Royal college guidelines • SRK/T and Holladay recommended • SRK-1/ SRK-2 not to be used • Post refractive surgery • Corrected ‘K’ by one of the methods • Use highest IOL power obtained

37. And finally • Check the glasses! • Do focimetry on both lenses • Surgeons need to customise constants • For each IOL that they use • Standard surgical technique • Rhexis • less than IOL size • round • Well centred

38. Useful material • http://www.doctor-hill.com • Intraocular lens power calculations (Slack) • H J Shammas. • Royal College guidelines

39. Thank you