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Health Tips & Writeups
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HIGH PRECISION BIOMETRY

DR. REZAUL MURSHED
M.B.B.S(D.M.C)D.O

CONSULTANT
AHMAD MEDICAL CENTER
E mail: amc@bol-online.com

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- 1532/1641)=.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

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

UNDER CORRECTION

  1. 20% .............1-2yrs
  2. 15% .............2-4yrs
  3. 10% .............4-8yrs
  4. After 10yrs PLANO

Ashok Garg Jaypee

1yr. Old if IOL 28D Under correct by 20% 22.5 D To be implanted with +5 Spec

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.

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


TYPES OF IMMERSION SHELL



KOHN SHELL BSS



HANSEN WITH METHYL CELLULOSE 1%


THE US PROBE IS IMMERSED IN SOLUTION , KEEPING IT 5 TO 10 mm FROM CORNEA

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
  Target IOL 22 D
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

H.John Shammas Page 206

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