INTRODUCTION

APART FROM THE EXCELLENT SURGICAL TECHNIQUES PRE-OPERATTIVE BIOMETRY IS VERY IMPORTANT IN PREVENTING REFRACTIVE SURPRISES POSTOPERTIVELY.

SIR HAROLD RIDLEY
DID HIS FIRST IOL SURGERY THE POST-OPERATIVE REFRACTION WAS +24DS/6.00*30

PERFECTION NEEDS

• CONSISTENCY & ACCURACY OF IOL POWER CALCULATIONS
• ACCURATE MEASUREMENT AL & KERATOMETRY
• WHICH FORMULA WE SHOULD FOLLOW

ALL MEASUREMENT

BY ULTRASOND

• Applanation (Contact) A-Scan
• Immersion A-Scan (To get accurate AL/like IOL master)
• Immersion A/B Scan

BY SHORT WAVE LENGTH LIGHT OPTICAL COHERENCE BIOMETRY

• IOL Master, best but (Disadvantage axial opacities &Very expensive too!)

KERATOMETRY

• MANUAL KERATOMETRY(CHEAP)
• AUTOMATIC KERATOMETRY(COSTLY)
• SECOND MOST IMPORTANT AFTER AL

KERATOMETRY

• 1 D error in Keratometric reading would lead 0.9 D error in the calculation of IOL power.
• Gonioscopy & Tonometry before keratometry should be avoided as it distorts the mires.
• Putting artificial tear before Keratometry
• Neutralise (+) mire first than (–) mire
• Correct eye piece setting before Keratometry
• Diopteric power between 40D> to <50D
• Frequently check calibration of the Keratometer

FORMULA

• BEFORE 1975 IOL=18+(1.25XRefrcation)
• Error exceeding 1D occurred over 50%> some errors large as 9D refraction surprise. Now a number of formulas for IOL calculations Published since 1980

Based on accurate measurement of

• Corneal power
• Axial length

Now modern formulas include

• Corneal power
• Axial length
• Estimated lens position
• Anterior Chamber (ELP) depth

WHICH FORMULA TO CHOOSE

• Every Surgeon Should Personalize his
• A-Constant,SF,aO,a1&a 2
• SRK II works well AL between 22 to 24.5mm
• SRK T & Hoffer Q works well AL < 21mm
• Holladay 2 very good in high myopia

Haigis for all axial lengths

PERSONALIZATION/OPTIMIZING (To make perfect)

• The concept of personalizing a formula based on a surgeon's past experience and data was introduced by Retzlaff using the A constant to refine the formula.
• Holladay personalize his backsolving SF
• Hoffer q backsolved his personalized ACD
• Haigis personalized his a0(A- Con),a1(ACD),a2(AL)

Optimizing constants a0,a1,a2 for HAIGIS (hill@doctor-hill.com/w.haigis@augenklinik.uni-wuerzburg)

• If you send the following data to Dr.Hill or Dr.Haigis he will send your own a0,a1,a2 at least 50 to 100 patients.
• IOL Manufacturer ALCON, Model SA60AT,ACD 5.20
• Pt.ID K 1 K 2 ACD AL SPH CYL IOL 1 42.25 43 3.58 24 +.25 00 19 100

FORMULAS

First IOL power formula was published by FYODOROV(1967)

FIRST GENERATION
SRK (SANDERS RETZLAFF-KRAFF)

SECOND GENERATION
SRK II, BINKHORST(1988)

THIRD GENERATION
SRK-T, HOLLADAY 1 and HOFFER Q

FOURTH GENERATION
HOLLADY 2 (1996) ,HAIGIS (LATEST)

SEVERAL VALUES NEEDED IN IOL POWER

• Accurate Corneal power
• Actual axial length
• Accurate prediction of estimated lens position(1/2 mm shift may cause dramatic change on final vision)
• A good understanding of various IOL power formulas is also required.
• It is very important to discuss with patients their postoperative refractive expectations.

MEASURING AL IS THE KEY FOR IOL CALCULATIONS

• 1-mm error in measurement of axial length will give you 3 D error post operative refraction.
• Between two eyes it varies .1mm normally but should not exceed >.3mm
• Majority of AL 22mm-24.5mm(18D-22D) 72%

BASED ON AL

ROLE OF A-SCAN SPIKE

• ACCEPT ONLY A STEEPLY RISING RETINAL SPIKE
• ACCEPT A STRONG SCLERAL SPIKE ABOUT 1.5mm to 2mm POSTERIOR TO RETINAL SPIKE
• ACCEPT A SERIES OF ORBITAL FAT SPIKES IN DESCENDING AMPLITUDE

SPIKES OF RxP

FORMULAS

• SRK/T&SRKII FORMULA – USES A-CONSTANT
• HOLLADAY 1 FORMULA –USES SURGEON FACTOR(SF=118.4x.5663 - 65.6=1.45) Alcon SA60AT
• HOLLADAY 2 FORMULA USES ACD
• HOFFER Q USES ACD(5.2 in Alcon's SA60AT)
• Haigis uses three constants a0,a1&a2
• a0=1.513,a1=.4,a2=.1(Alcon SA60AT)

A-CONSTANT

A Constant includes multiple variables that include the

• Implant manufacturer
• Implant style
• Surgeon's technique
• Implant placement within the eye
• measuring equipment

H.John Shammas Page 18

New Terminology ELP & ALP

• ELP effective lens position by Holladay or ALP actual lens position by FDA
• ELP=aACD+ SF distance between the corneal vertex and IOL's optical center
• aACD =anatomical AC distance between corneal vertex & iris
• SF (Surgeon factor) distance between iris plane & IOL's optical center

PSEUDOPHAKIC EYE

WHY HAGIS IS LATEST FOR ALL LENGTH OF EYES !

• Effective lens position is d d=a0+(a1*ACD)+(a2*AL)
• Haigis uses three constant in his formula
• a0=tied to A constant =118.4x.62467- 72.434=1.513(ALCON SA60AT)
• a1=.4 constant is tied to the measured anterior chamber depth.
• a 2=.1constant is tied to the measured axial length

PROBLEMS OF IOL CALCULATION

• Uses of an older, outdated IOL formula
• Incorrect measurement of AXIAL LENGTH
• Incorrect keratometry readings
• Mistake in entering IOL calculation program
• Incorrect labeling of IOL by manufacturer
• Mix-up with an IOL for another patient

ULTRASOUND

• Ultrasound machine does not measure the AL or distance directly.
• Instead it measures the time (T) it takes the sound to traverse the eye and converts it to a linear value using the velocity formula D=VxT (24mm=1532m/sec.X.01566)
• So, appropriate sound velocity settings is essential for obtaining accurate AL

Velocity of sound

1,532m/sec A-Scan Biometry (CALF)

• Some US machine use single sound velocity 1555,1553,1550,or 1548m/s for phakic eyes.
• Above values does not work well for all AL lengths.
• For 29mm eye 1550m/s & 20mm 1560m/s
• This is the reason why Long & Short eyes tend to be less accurate, in spite of very best technique.

Corrected axial length factor (CALF, +.32)

• To avoid this error in AL Holladay took all eyes at a velocity of 1532m/s aphakic eye as bag of water
• US machine sets at 1532m/s to measure the AL
• Here true AL length is obtained by adding (.04mm cornea+.28mm Lens)= +.32mm(CALF) to the displayed axial length.(22+.32=22.32)
• This new(22.32mm) number is then used for IOL calculation.

Velocity conversion equation

• Cornea= T cX(1- 1532/1641)=.55x.066423=.04mm
• Lens= TLX(1- 1532/1641)=4.21x.066423=.28mm
• So, CLAF=.04mm+.28mm=+.32mm

ERRONEOUSLY SHORT EYE

• CORNEAL COMPRESSION (CONTCACT )
• SOUND VELOCITY TOO SLOW
• RETINAL GATE IN VITREOUS CAVITY
• GAIN SET TOO HIGH
• LENS MEASURED TO THIN
• MACULAR THICKNENING/R.DETACHMENT
• MISALIGNMENT OF THE SOUND BEAM

ERRONEOUSLY LONG EYE

• AIR BUBBLE IN FLUID BATH (IMMERSION)
• FLUID BIRDGE(CONTACT METHOD)
• SOUND VELOCITY TOO FAST
• GAIN SET TOO LOW
• LENS MEASURED TOO THICK
• POSTERIOR STAPHYLOMA
• MISALIGNMENT OF SOUND BEAM

BIOMETRY IN CHILD

CURRENT APPROACH

PREVENTION OF COMMON ERRORS

• Ensure well trained, experienced person performing biometry and keratometry on a daily basis.
• Check calibration of A-sacn & Keratometry on a daily basis.
• Perform biometry in both eyes, for comparison
• Between the two eyes there cannot be more than .3mm difference in AL (Unless anisometropia)
• Use third generation Holladay, Hoffer Q, SRK T, Haigis
• Immersion A-scan/IOL master, for AL
• Use CALF for AL measurement

PREVENTION OF COMMON ERRORS

• After calculating the scan, double check numbers to ensure the proper numbers were inserted into the program
• Patient's pre cataract refraction
• Good biometry machine (Latest formula)
• Lens should be in the operating room before sedation/anesthesia

ADVANCEMENT

• AROUND 1990,WITH THE FORMULAS AVAILABLE AT THAT TIME,BEING WITHIN ± 1.00 D OF TARGET REFRACTION WAS CONSIDERED A HIGH STANDARD
• TODAY BY PAYING CAREFUL ATTENTION TO DETAIL, IT IS POSSIBLE TO BE WITHIN ± 0.50 D IN 95% OF SURGERIES OR BETTER.

RxP
S-15

RxP

FEW TIPS

• LONG H/O EMMETROPIA NEED 18-22 D
• LONG TIME MYOPE NEED <18 D
• HYPEROPE NEED >22 D
• ANY CONFUSION REPEAT BEFORE SURGERY
• IMMERSION TECHNIQUE IS THE CHOICE

ERROR WITH CONTACT METHOD

• CORNEAL COMPRESSION
• A FLUID MENISCUS BETWEEN PROBE AND CORNEA
• MISALIGNMENT OF SOUND BEAM

ADVANTAGE OF IMMERSION

• OBVIOUS ADVANTAGE is CORNEA CANNOT BE COMPRESSED
• PRAGER SHELL & PROBE WORKS AS A SINGLE UNIT
• THUS GOOD ALIGNMENT OF SOUND BEAM ALONG VISUAL AXIS
• WISE TO MEASURE AL IN DIFFICULT CASES STRABISMUS,NYSTAGMUS, BLEPHAROSPASM, POSTERIOR STAPHYLOMA

IMMERSION TECHNIQUE(IT)

• OSSINIG shown IT more accurate than standard applanation & equal to IOL master over last 15yrs.
• Argument against IT are that time-consuming, expensive, messy and requires the patient to be supine.
• But new shell immersion technique are becoming popular for giving accurate IOL power.

Types of Immersion Technique

• The Kohn Shell
• Hansen Shell
• Prager Shell

HANSEN SHELL

PRAGER SHELL

Prager Shell

• Developed by Thomas C.Prager, Phd 1982
• The accuracy of Prager Shell immersion biometry = optical biometry (IOL master)
• Immersion US not limited by media density
• Accurate insertion of US probe
• Easy side tube connection
• Clear shell for improved visibility
• Uesd by 3,000 opthalmologist worldwide

CONCLUSION

• VERIFYING MEASUREMENTS WHEN NECESSARY
• RELY EITHER IMMERSION US OR OPTICAL COHERENCE BIOMETRY
• OPTIMIZING LENS CONSTANTS
• GOOD CAPSULORHEXIS SLIGHTLY SMALLER THAN IOL OPTIC

PC RUPTURE WITH VITREOUS LOSS/SULCUS FIXATION

PC Ruptures

	IOL in Sulcus Approx 0.5 to 0.75mm more anteriorly
Each mm Change in ACD affects 1.5D Now to avoid PO Myopia give 21 D IOL in the SULCUS. This difference is less long Myopic eyes if target IOL 10 D Emmetropia 9.50 D IMPLANT This difference is more in short hypermetropic eyes If a 28 D planned for emmetropia Implant 26.50D
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