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

HOFFER(USA)		KENNETH J
FORMULA	%	AXIAL LENGTH
SRK II	72%	22mm-24.5mm
Holladay I	15%	24.5mm-26mm
SRK/T	05%	26mm>
Hoffer Q	08%	<22mm

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- 1532/1541)=.55x.066423=.04mm

Lens= TLX(1- 1532/1541)=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

NORMAL AXIAL LENGTH VALUES
Age	AL mm
New born	17.02
10-45 days	17.22
46-75 days	18.77
76-120 days	19.43
5-9 months	20.09
10-18 months	20.14
19-36 months	22.01
4-5yrs	22.78
6-7yrs	22.56
8-10yrs	23.12

CURRENT APPROACH

Age	3	4	5	6	7	8	10	13
Goal	+5	+4	+3	+2.25	+1.5	+1.0	+.50	Plano

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.

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

TYPES OF IMMERSION 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

the highlighted IOL calculation power will be the power to be implanted. Am I right?

Yes, that is the calculated power

I am asking this question as ultrasound probe remains 5-10mm from the  cornea. I am facing some problem to keep BBS in the shell. Very often it leaks through the limbus, then I have to push BSS again from the syringe.

I do not have this problem. I apply slight pressure on the eye with the patient looking straight forward and there is no leakage. It will take about five patients for you to become skilled with the procedure.

I will be glad if you have any comment on Hansen shell.

The Hansen shell is an open cylinder and you have to add Goniosol. It is difficult to use because you need to be at a certain distance from the cornea for the RxP to acquire the signal and you also must be perpendicular to the fovea. The Prager shell does this for you and is a one-handed procedure. Using the Hansen shell is a two-handed procedure that takes a long time to learn

sincerely,
tom prager PhD

NEXT DISCUSSION

IOL POWER CALCULATIONS IN POST LASIK PATIENTS

IOL POWER CALCULATION AFTER KERATOPLASTY

IOL POWER WITH CORNEAL DYSTROPHY

IOL POWER CALCULATIONS IN CENTRAL CORNEAL SCAR/PTERYGIUM/ASTEROID HYALOSIS/CORNEAL LACERATION

IOL POWER CALCULATIONS WITH SILICON OIL

IOL POWER CALCULATIONS AFTER MACULAR HOLE SURGERY

IOL POWER CALCULATIONS WITH EXTREME MYOPIA 27mm>

COMMENT

ANY CONSTRUCTITIVE COMMENT IS MOST WELCOME. I WILL BE MOST HAPPY IF ANYBODY LEARNS A SINGLE WORD FROM THIS CD & MORE HAPPY IF HE/SHE TRANSMITS THIS TO HIS/HER FELLOW.

FURTHER READINGS

ULTRASOUND of the EYE and ORBIT SANDRA FRAZIER BYRNE & RONALDL.GREEN SECOND EDITION 2002

INTRAOCULAR LENS POWER CALCULATIONS H.JOHN SHAMMAS 2004

MASTERING THE TECHNIQUES OF IOL POWER CALCLATIONNS Ashok Garg,Jaior E Hoyos, Dimitrii Dementiev First edition 2005

THANK YOU ALL