DOS Article

8/2/2019 DOS Article

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REFINEMENT OF IOL POWER CALCULATION

IOL POWER CALCULATION

There are three major components of IOL P calculation-

Biometry- Measuring AL, corneal power, IOL position

Formulas

Clinical variables

Biometry-Measurement of AL

Certain facts - AL is MC source of error

Error of 1 mm causes miscalculation of 2.5 to 3 D

Factors affecting measurement of AL-

1) Technique and machine

2) Setting the ultrasound velocity

3) Retinal thickness

Biometry- A scan

CHARACTERISTICS OF A GOOD A- SCAN-

1) A tall echo from the cornea, one peak with a contact probe, and a double peaked

echo with an immersion probe.

2) Tall echoes from the anterior and posterior lens capsule.

3) Tall sharply rising echo from retina.

4) Medium tall to tall echo from sclera.

5) Medium to low echoes from orbital fat.

Biometry


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Try to get all spikes

If not possible concentrate more on posterior spikes

3 different A scan techniques-

Applanation A scan

Immersion A scan

Immersion Vector A / B scan

Applanation A scan:

Disadvantages- this has the disadvantage of Indentation of cornea which

cause error of 0.3 D to 1D

Immersion A scan

Advantages-

More accurate

Removes error due to corneal indentation

Immersion Vector A / B scan

Advantages-

A scan vector made to pass through center of cornea

direct AL from region of fovea

High myopia with staphyloma

Mature cataract


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Partial coherence interferometry

(IOL Master)

Non contact method

Uses partial coherent beam of light

Optical device cannot be used in media opacity in axial region


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IOL Master

Advantages-

5 times more accurate and reproducible

Simultaneously measures corneal P, ACD, performs IOL P calculation using

modern theoretical formulas saves time

Measurement of AL - Setting the ultrasound velocity

Ultrasound velocity through -

Aqueous and vitreous: 1532 m / s

Cornea and lens : 1641 m /s

Ideally diff. ocular compartments at their specific sound velocity

Avg. speed of sound in phakic eye: 1555 m /s o k for avg. 23.5 mm eye

Recommendation-

Short eyes < 20 mm : 1560 m / s

Long eyes > 30 mm : 1550 m /s

Measurement of AL - Setting the ultrasound velocity

If eye measured with wrong velocity:

Velocity (correct) x measured AL =True AL

Velocity (measured)

CALF (CORRECTED AXIAL LENGTH FACTOR) method-

Change velocity to 1532 m/s (aphakic vel.)

To this add CALF factor.


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Setting the ultrasound velocity

CALF calculation:

CALF = TL x ( 1- 1532)

VL

TL = 4 + AGE.

100

VL = 1659 [(AGE -10)]

2

TL-axial thickness of lens VL- sound vel through lens

CALF VALUE OF 0.32 CAN BE APPLIED FOR ALL AGES

Biometry Measurement of AL Retinal Thickness Factor

Recommendation-

RTF considered to account for additional distance from surface of retina to

level of photoreceptors

Add 0.20 to 0.25 mm to measured AL

Measuring Corneal Power

These errors are rarely of high magnitude.

Considerations for obtaining accurate corneal P-

1) Instrumentation

2) Contact lens wear

3) Previous refractive surgery

4) Corneal transplant eyes


Instrumentation

Manual Keratometer-

Calculates P by assuming RI of 1.3375 D = RI -1


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Recommendation for manual keratometry

= 4 /3

Calibrate on regular schedule

Avg. of 3 readings

switching to a Javal-Schiotzstyle

Keratometer


Corneal Topography- K value calculated is more accurate

Hard Contact Lens-(including gas permeable)-

Removed at least 2 weeks before measuring K


Considerations After Photorefractive Surgery-

1) CLINICAL HISTORY/ CALCULATION METHOD

Mean postoperative K =


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(Mean preoperative K) (change in refraction at corneal plane)

NOTE- postoperative refraction (before myopic shift due to cataract)

Convert the pre and postoperative refraction into spherical equivalent at

spectacle plane (SEQS )

SEQS = sphere + 0 .5 ( cylinder)

Convert SEQS with a given vertex distance (V) in mm into spherical

equivalent at corneal plane

(SEQC).

SEQC = 1000 /[ (1000/ SEQS )- V]

Change in refraction at corneal plane =

Preoperative SEQC Postoperative SEQC

This is subtracted from mean preoperative K to get mean postoperative K

value.

Biometry-Measuring Corneal Power

Considerations After Photorefractive Surgery

CONTAC LENS METHOD-

Spheroequivalent refraction (SER)

for eye calculated

SER calculated after wearing hard contact lens [of plano power and base

curve =Effective power of cornea]


Considerations after Photorefractive Surgery

After wearing contact lens -


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If SER remains same K = base curve of contact lens.

If myopic shift with contact lens base curve of contact lens >than that of

cornea by magnitude equal to amount of shift.

If hyperopic shift with contact lens base curve of contact lens < than that forcornea by amount of shift.



Double K method-

In theoretical formulas, corneal P required for 2 purposes-

Predict position of IOL (ACD / ELP)

Calculate IOL P

Anatomy of ant segment not changed by surgery

Kpreop can be used for ELP

IOL P calculated with Kpostop

Biometry


Other methods-

Shammas no history method K=1.14 x Kpo-6.8

Maloney Corneal topography method:

K= Kt x (376/337.5) - 5.5

Koch modification of Maloney method:

K= Kt x (376/337.5) - 6.1

(Kt central K fromcorneal topography)

Biometry- Prediction of Post Op IOL Position

ACD /ELP/ALP

Distance between the posterior corneal surface (some authors use the

anterior corneal surface) and the anterior surface of the implanted IOL

Methods of measuring ACD:

1) Ultrasonography (applanation & immersion)


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2) Partial coherence interferometry

3) Scanning slit topography (Orbscan)

4) Optical methods (less popular)

Biometry- Prediction Of Post Op IOL Position

ACD /ELP/ALP

Distance between the posterior corneal surface (some authors use the

anterior corneal surface) and the anterior surface of the implanted IOL

Methods of measuring ACD:

1) Ultrasonography (applanation & immersion)

2) Partial coherence interferometry

3) Scanning slit topography(Orbscan)

4) Optical methods (less popular)

FORMULAS

2 major formula categories:

Theoretical formulas-

Ex. Holladay, Hoffer Q, SRK / T

Regression formulas

Ex. SRK

The commonly used Formulas

3rd generation :

use two predictor of the ELP axial length keratometry

Ex.

Holladay 1,SRK / T, Hoffer Q

Holladay 1-

No ACD input


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Calculates predicted distance from cornea to iris plane +distance from iris

plane to IOL (Surgeon Factor :Specific for each lens)

3rd generation formulas:

Hoffer Q:

P is function of-

Axial length

Avg. K reading

Refraction (Rx) { f of AL, K, P, pACD}

PACD (constant) {= manufacturers ACD or derived from A constant)

4th generation formula :

Holladay 2 formula:

Uses seven variables to predict lens position

1) Axial length

2) Keratometry

3) Horizontal white-to-white corneal diameter

4) ACD

5) Lens thickness

6) Refraction

7) Age of the patient

Haigis Formula:

ELP = a0 +[a1 x ACD] +[a2xAL]

a0- same as lens constant

a1 - tied to ACD

a2 - tied to the measured axial length

FORMULAS USAGE

Normal AL(22 24.5mm) : any formula

Avoid using SRK 1 in AL outside this range


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Short eyes (< 22mm):Hoffer Q

Very short eyes (< 19 mm):Holladay 2

Medium long (24.5 to 26mm) :Holladay 1

Very long (> 26mm): SRK / T

Formulas- Personalization

Based on surgeons past experience and data

Increases accuracy

Data collected : Same surgeon, same lens style, same biometry instruments

Can be performed using Hoffer programme, Holladay IOL Consultant

computer programmes

Following parameters noted-

AL (preop)

Corneal power (preop)

IOL P

Postop refractive error (stable)

Clinical variables

Patients needs and desires

FINAL SELECTION OF IMPLANT POWER

1) FELLOW EYE REFRACTION:

If refractive P of opposite eye lies between 2 D and + 2 D then emmetropia

should be aimed.

Else stepwise reduction (if BE have cataract) can be done

Ex - 4D preoperative refraction can be reduced by aiming for 2D under

correction in 1 eye and emmetropia in other.

Clinical variables

Patients needs and desires


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Elderly patient-

reading important

err on myopic side (choosing power higher by about 0.5 D)

Active person- near emmetropia is best

Clinical variables-Paediatric IOL power calculation

3 Major approach

1) Adult power / Initial hyperopia amblyopia

2) Initial emmetropia significant late myopia

3) Customized approach (compromise between these 2 extremes

Ideally ,choose P intermediate between the ones calculated by 1st and 2nd

approach

Clinical variables

Paediatric IOL power calculation

Preferably AL and K : during EUA

Preferably use Theoretical formulas s / as SRK /T, Hoffer Q, Holladay, Haigis

IOL implantation in < 2 years under correct by 20%

Children 2 and 18 years of age-under correct by 10%

If fellow eye is pseudophakic, minimize anisokonia

Dense amblyopia leave less hyperopia (or emmetropia)

If poor compliance to glasses or CL leave least refractive error

Bag vs sulcus IOL

P of IOL intended for capsular bag placement should be decreased by 0.75 to

1.0 D when placing in ciliary sulcus

Clinical variables

Biometry in vitrectomized


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Perform biometry in sitting position

Sound travels more slowly in silicone oil (980 m /sec).

Some newer ultrasound units have adjustable velocities

Formula suggested by Prof. John Shammus-

TAL= 1133/ 1550 AAL

PREVENTION OF COMMON ERRORS

Use immersion A- scan or IOL Master to measure AL

Make sure A scan instrument has oscilloscope screen

Suspect a staphyloma in eyes >25 mm: use IOL master or immersion vector

A/ B scan

Use CALF method : measure eye using 1532 m/s and add +0.32 mm to

result to correct for any error in sound velocity

Regularly calibrate manual keratometer

Keep CL out for 2 weeks prior to keratometry

Silicone oil eye needs IOL master if possible or ultrasound AL times 0.71

Use Hoffer Q IN


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Dr. D K Verma,

Dy. Commandant (Eye Specialist),

Base Hospital, ITBP Force, New Delhi

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