BIOMETRY Deepak Vayalambrone FRCOphth it can be a

BIOMETRY Deepak Vayalambrone FRCOphth

it can be a bit overwhelming 20/02/2019

NOVEMBER 29 1949 St. Thomas Hospital London 20/02/2019

• 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’ 20/02/2019

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" 20/02/2019

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. ’’ 20/02/2019

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 20/02/2019

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. 20/02/2019

The human eye 42 D 24 mm 18 D 20/02/2019

A few words on terminology – – Axial length Keratometry Anterior Chamber Depth Effective Lens Position – Position of principal plane of IOL behind cornea – – 20/02/2019 1 st gen- constant of 4 mm 2 nd gen- AL as a scaling factor Binkhorst 3 rd gen- AL and K 4 th gen- AL, K, pre-op ACD and LT

Standardised pseudophakic eye 20/02/2019

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 20/02/2019

Theoretical formulae • • • Thijssen Colenbrader Fyodorov Van der Heijde Binkhorst 20/02/2019

Regression formulae • Post-op refractive results analysed • Correction factors applied SRK formula IOL power= A- 2. 5 L – 0. 9 K A- constant reflecting and affected by • position of lens • angulation of haptics • Lens shape L- axial length K- Corneal power 20/02/2019

II generation formulae • AL and IOL position related • SRK-2 and Binkhorst. • Correction for longer and shorter eyes. Axial lengths < 20 mm Al = A + 3 20 mm 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 20/02/2019

Koch suggested classification of IOL formulae 1 Historical Refraction based 2 Regression analysis based 3 Vergence formula based SRK/ SRK II/ BINKHORST 2 variable Holladay 1, SRK-T, Hoffer-Q 3 variable Haigis, Ladas super formula 5 variable Barett Universal 2 7 variable Holladay-2 4 AI based Hill RBF, Clarke neural network 5 Ray tracing Okulix, Phacoptix

9 types of eyes* AXIAL LENGTH SHORT NORMAL LONG Megalocornea + Axial Hypermetropia 0% Megalocornea 2% Large eye Buphthalmos Megalocornea+ axial myopia 10% Axial hypermetropia 80% Normal 96% Axial myopia 90% Small eye Nanophthalmos 20% Microcornea 2% Microcornea+ axial myopia 0% ANT. SEG SIZE LARGE NORMAL SMALL *Essential optics for the cataract and refractive surgeon. I ed 2007. JT Holladay 20/02/2019

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

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

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 20/02/2019

It’s a bit like choosing a cereal…. . • IOLMaster 500 • IOLMaster 700 • AL scan • Lenstar LS 900 • Aladdin/Aladdin LT • Argos (Movu) • Galilei G 6

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

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

Lenstar • OLCR • Superluminescent diode laser (820 nm) • Keratometry- dual zone 1. 63 mm 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

Special circumstances Every other eye!

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 20/02/2019

Refractive surgery-2 Historical ‘K’ • • K Spectacle correction Pre- LASIK 42. 0 − 11. 25 Post- LASIK 36. 0 − 2. 25 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 20/02/2019

Refractive surgery-3 Historical Refraction K Spectacle correction Pre- LASIK NOT AVAILABLE − 11. 25 Post- LASIK 36. 0 − 2. 25 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 20/02/2019

Refractive surgery-4 Clinically derived method • Regression formula • Shammas-2, Haigis-L K = (1. 14 x Kpost. PRK) – 6. 8 • Used when historical data not available • Disadvantage – Does not relate to amount of correction by refractive surgery 20/02/2019

Refractive surgery-5 Contact lens method K Spectacle correction Pre- LASIK NOT AVAILABLE Post- LASIK 36. 0 − 2. 25 • • Refract eye Fit plano Rigid CL with known BC e. g. 41 D Refract e. g. − 8. 75 D Difference added algebraically to BC 41+ [− 8. 75 D-(-2. 25)]= 34. 50 D • Source: Intraocular Lens Power Calculations 2003. H. John Shammas 20/02/2019

Refractive surgery-6 Other possible options • Nomogram based assessment * 63% within 0. 5 D 84% within 1 D 100% within 1. 5 D Better than ‘Historical method’ but. . . Data from 19 eyes • Lenstar – Probably the best currently • IOL master 700 • Pentacam – With correcting software • Orbscan II – Measures central 1. 5 mm 20/02/2019

Penetrating keratoplasty • Triple procedure not ideal. • Keratoplasty + Cataract extraction • Planned Aphakic IOL later. 20/02/2019

PIGGY BACK IOL • MYOPIC ERROR – Implant power same as error • HYPERMETROPIC ERROR – Implant power 1. 5 x error

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 20/02/2019

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 20/02/2019

Royal college guidelines • SRK/T and Holladay recommended • SRK-1/ SRK-2 not to be used Axial length <22 mm • Post refractive surgery Formula Hoffer Q or SRK/T 22 -24. 5 mmone of the methods. SRK/T or Holladay or Haigis – Corrected ‘K’ by >24. 6 IOL mmpower obtained SRK/T – Use highest 20/02/2019

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 20/02/2019

Useful material • http: //www. doctor-hill. com • Intraocular lens power calculations (Slack) – H J Shammas. • Royal College guidelines 20/02/2019

Thank you 20/02/2019