BIOMETRY: TARGET

EMMETROPIA

Dr. Mahziba Rahman Chowdhury

BANGLADESH EYE HOSPITAL LTD.

IOL POWER CALCULATION

Keratometry

Axial length

IOL Formula

EYE MEASUREMENTSAverage Range

Axial Eye Length 23.5mm 22.00-24.5

AC Depth 3.24mm Varies with AL

Lens Thickness 4.25mm Up to 6.9

Keratometry 43.0-44.0 D Usually within 1D of each other

KERATOMETRYMEASUREMENT OF RADIUS OF CURVATURE OF ANTERIOR CORNEAL SURFACE

• Manual

• Autokeratometer

• Topography

• IOL master

IOL master

Manual Autokeratometer

Topography

TIPS FOR ACCURATE KERATOMETRY

Calibrate & check the accuracy of the keratometer

Explain the procedure to the patient

Should be done before AL measurement

If high or low results encountered, advisable to have a 2nd person check the measurements

Repeat if the difference in total keratometric power between the eyes exceeds 1.50 D

SOURCE OF KERATOMETRY ERRORS

Poor patient fixation

Dry eye

Drooping eye lids

Irregular cornea

1 D error in measurement will cause

a 0.9 D of refractive error

AXIAL LENGTH MEASUREMENT

A-Scan ultrasound Applanation method Immersion method

Laser interferometery IOL Master

IOL Master

Applanation method

Immersion method

APPLANATION A-SCAN BIOMETRY

a: Initial spike

(probe tip and

cornea)

b: Anterior lens

capsule

c: Posterior lens

capsule

d: Retina

e: Sclera

f: Orbital fat

IMMERSION A-SCAN BIOMETRY.

a: Probe tip. Echo from tip of probe, now moved away from the cornea and has become visible

b: Cornea. Double-peaked echo will show both the anterior and posterior surfaces

c: Anterior lens capsule

d: Posterior lens capsule

e: Retina. This echo needs to have sharp 90 degree take-off from the baseline

f: Sclera

g: Orbital fat

NON CONTACT It measures the distance from

the corneal vertex to the retinal pigment epithelium by

partial coherence interferometry.

Provides following

measurements:

.AL . Keratometry

. ACD . Lens thickness

.White to white distance

Applanation Method

Immersion Method

Non Contact

Accuracy +/- 0.24mm +/- 0.12mm +/- .01mm

Contact +1 mm Corneal Compression , 2.5 D error

Not directly _

Patient compliance

Not good Good Very Good

Result in Media opacity

Good Good Poor

ONE MACHINE IS NOT ALWAYS THE ANSWER

Immersion ultrasound

IOL master

Posterior staphyloma

Difficult Yes

Silicone oil Difficult Yes

Pseudophakia Variable Yes

4++brunescent lens

Yes No

Central PSC plaque Yes No

Vitreous hemorrhage

Yes No

Central corneal scar Yes No

TIPS FOR ACCURATE MEASUREMENT OF AXIAL LENGTH

Ensure the machine is calibrated and set for the correct velocity setting

Echoes from cornea, anterior lens, posterior lens, and retina should be present and of good amplitude

Gain set at a low level at which a good reading is obtained

Don't push too hard – corneal compression commonly causes errors

Must take reading of both eye

Importance of proper alignment

Effect of corneal compression

IOL FORMULA

IOL FORMULA 1ST GENERATION

Most are based on regression formula

developed by Sander , Retzlaff & Kraff

Known as SRK formula

P= A-2.5(AL)-0.9(K)

Now obsolete

IOL FORMULA 2ND GENERATION

SRK II formula-

Modification of SRK

IOL power based mainly on

AL

Not used now a days

IOL FORMULA 3RD GENERATION• SRK/T -Very long eyes >26mm

( High myopes) • Holladay 1 -long eyes 24-26 mm

• Hoffer Q -Short eyes<22mm (Hypermetropes)

AL 22-24.5 mm- Either of the 3 formulas

IOL FORMULA 4TH GENERATION Haigis formula

-Appropriate for all ranges of axial lengths

Holladay 2 - Currently most sophisticated formula - Most accurate - All ranges of axial lengths

- Requires 7 different variables 1.White to white 2. Lens Thickness 3. Corneal diameter 4. ACD 5. AL 6. Preoperative Rx 7. Patient’s Age

CHANGE 0F IOL POWER

Sulcus Decrease by 1.0 D

ACIOL Decrease by 2.0

D

Scleral fixation IOL Increase by 2.0 D

PEDIATRIC BIOMETRY

Axial Length (mm) IOL Power (D)

17 28

18 27

IOL Power Undercorrection

Age (years)

20% 2-4

10% 4-8

No Need >8

SPECIAL CASES Post Refractive Surgery

Silicon filled eye

POST REFRACTIVE SURGERYo Anterior surface is flattened with no change in the

posterior radius

o Important to store pre refractive surgery keratometry and refractive power

o Formula

- Haigis L

- Masket Method

- Clinical History Method

- Contact Lens Method

- Shammas Method

- Double K SRK/T

- Online Calculators (doctor-hill.com, ASCRS)

SILICON FILLED EYE

o Low sound velocity results in sound attenuation & difficulty in identifying retinal spikes

o Proper sound velocity must be selected or else long AL measurement is obtained

o Sound velocity in silicone oil 1040 m/s 5000 cs 980 m/s 1000 cs

RECENT ADVANCES

Barrett Universal Formula- Can be used for all eye types & all lens types

Verion Image guided system

Intraoperative Wavefront Aberrometry