High Precision High Precision BiometryBiometry

Dr Vibha Dr ShekharDr Vibha Dr Shekhar

Several values are required to calculate IOL Power

• · Accurate Corneal power• · Actual axial length• · Accurate prediction of estimated lens

position• (half a mm shift in lens position can have

a• dramatic effect on final vision)• · Desired post op refraction• · A good understanding of the various IOL• Power calculation formulas is also

required.• .

KeratometeryKeratometery

• Keratometery by—Manual Keratometery by—Manual

TopographyTopography• AutokeratometerAutokeratometer• IOL master IOL master

Source of keratometry Source of keratometry errorserrors

• Unfocused eye pieceUnfocused eye piece• Failure to calibrate unitFailure to calibrate unit• Poor patient fixationPoor patient fixation• Dry eyeDry eye• Drooping eye lidsDrooping eye lids• Irregular corneaIrregular cornea

Repeat Keratometery IfRepeat Keratometery If

• Corneal curvature more than 47D or Corneal curvature more than 47D or less than 40Dless than 40D

• The difference in corneal cylinder is The difference in corneal cylinder is more than one diopter between eyes.more than one diopter between eyes.

A-Scan biometery/laser A-Scan biometery/laser inferometeryinferometery

• A-Scan ultrasoundA-Scan ultrasound• by applanation by applanation

methodmethod

by immersion by immersion methodmethod

• Laser inferometeryLaser inferometery• IOL Master IOL Master

A-scan factsA-scan facts

• 50% of a surgeon post operative 50% of a surgeon post operative surprises are A-scan errors (olsen).surprises are A-scan errors (olsen).

• Error of 2.0D or more are always A Error of 2.0D or more are always A scan related (Holladay).scan related (Holladay).

• All A-scan unit make mistake in eco All A-scan unit make mistake in eco

interpretation.interpretation.

Applanation A-scan BiometryApplanation A-scan Biometry

• A-scan biometry by applanation requires A-scan biometry by applanation requires that the ultrasound probe be placed that the ultrasound probe be placed directly on the corneal surface. This can directly on the corneal surface. This can either be done at the slit lamp, or by either be done at the slit lamp, or by holding the ultrasound probe by hand. holding the ultrasound probe by hand.

• Even in the most experienced hands, Even in the most experienced hands, some compression of the cornea is some compression of the cornea is unavoidable; this typically being 0.14 mm unavoidable; this typically being 0.14 mm - 0.28 mm. - 0.28 mm.

Applanation A-scan Biometry.Applanation A-scan Biometry.

• a:a: Initial spike Initial spike (probe tip and (probe tip and cornea)cornea)b:b: Anterior lens Anterior lens capsulecapsulec:c: Posterior lens Posterior lens capsulecapsuled:d: Retina Retinae:e: Sclera Scleraf:f: Orbital fat Orbital fat

Applanation A-scan Applanation A-scan BiometryBiometry

•   When echoesWhen echoes b b through through d d are high and are high and steeply rising, the ultrasound beam is steeply rising, the ultrasound beam is most likely on axis. The scleral echo most likely on axis. The scleral echo should easily be identified and the should easily be identified and the orbital fat echoes should descend orbital fat echoes should descend quickly and at a steep angle. If there are quickly and at a steep angle. If there are no scleral or orbital fat echoes visible, no scleral or orbital fat echoes visible, the ultrasound beam is most likely the ultrasound beam is most likely aligned with the optic nerve rather than aligned with the optic nerve rather than the macula. the macula.

The five basic limitations of The five basic limitations of applanation A-scan biometry applanation A-scan biometry

are: are: • 1.  Variable corneal compression. 1.  Variable corneal compression. • 2.  Broad sound beam without precise 2.  Broad sound beam without precise

localization localization • 3.  Limited resolution. 3.  Limited resolution. • 4.  Incorrect assumptions regarding 4.  Incorrect assumptions regarding

sound velocity. sound velocity. • 5.  Potential for incorrect 5.  Potential for incorrect

measurement distance. measurement distance.

Immersion A-scan Immersion A-scan Biometry.Biometry.

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

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

• c:c: Anterior lens capsule. Anterior lens capsule.• d:d: Posterior lens capsule. Posterior lens capsule.• e:e: Retina. This echo needs to Retina. This echo needs to

have sharp 90 degree take-have sharp 90 degree take-off from the baseline.off from the baseline.

• f:f: Sclera. Sclera.• g:g: Orbital fat. Orbital fat.

Immersion A-scan BiometryImmersion A-scan Biometry

• The immersion The immersion technique requires technique requires the use of a the use of a PragerPrager ScleralScleral Shell Shell . .

Immersion A-scan BiometryImmersion A-scan Biometry

• When the ultrasound beam is When the ultrasound beam is properly aligned with the center of properly aligned with the center of the macula, all five spikes (cornea, the macula, all five spikes (cornea, anterior and posterior lens capsule, anterior and posterior lens capsule, retina and sclera) will be steeply retina and sclera) will be steeply rising and of maximum height. rising and of maximum height.

NON CONTACT NON CONTACT • The The ZeissZeiss IOLMasterIOLMaster . A . A

non- contact optical device non- contact optical device that measures the distance that measures the distance from the corneal vertex to from the corneal vertex to the retinal pigment the retinal pigment epithelium by partial epithelium by partial coherence interferometry, coherence interferometry, the the IOL MasterIOL Master is is consistently accurate to consistently accurate to within ±0.02 mm or within ±0.02 mm or better. better.

Accuracy of axial length by Accuracy of axial length by different machinedifferent machine

Applanation A Applanation A -scan-scan

Immersion A-Immersion A-scanscan

IOL MasterIOL Master

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

Do not throw away old Do not throw away old ultrasound machineultrasound machine

Immersion Immersion ultrasoundultrasound

IOL IOL mastermaster

Posterior staphylomaPosterior staphyloma

Silicone oilSilicone oil

PseudophakiaPseudophakia

4++brunescent lens4++brunescent lens

Central PSC plaqueCentral PSC plaque

Vitreous hemorrhageVitreous hemorrhage

Central corneal scarCentral corneal scar

DifficultDifficult

DifficultDifficult

VariableVariable

•YesYes•YesYes•YesYes•YesYes

•YesYes•YesYes•YesYes

NoNo

NoNo

NoNo

NoNo

IOL FORMULA Ist IOL FORMULA Ist generationgeneration

• Most are based on regression formula Most are based on regression formula developed by Sander ,Retzlaff & Kraffdeveloped by Sander ,Retzlaff & Kraff

• Known as SRK formula.Known as SRK formula.• P--A-2.5(L)-0.9(K)P--A-2.5(L)-0.9(K)• P=lens implant power for emetropiaP=lens implant power for emetropia• L= Axial length (mm)L= Axial length (mm)• K=average keratometric reading K=average keratometric reading

(diaopters)(diaopters)• A= lens constant A= lens constant

IOL FORMULA 2IOL FORMULA 2ndnd generationgeneration

• SRK formula –SRK formula –• work well for average work well for average

eyes.eyes.• less accurate for long, less accurate for long,

short eyesshort eyes• SRK II formulaSRK II formula• modification of SRKmodification of SRK• work on ELPwork on ELP

IOL FORMULA 3IOL FORMULA 3rdrd generationgeneration

• Third generation formulas-Third generation formulas-

• SRK/T -very long eyes SRK/T -very long eyes >26mm>26mm

• Holladay -long eyes 24-26 mmHolladay -long eyes 24-26 mm• hofferQ -Short eyes<22mmhofferQ -Short eyes<22mm

IOL FORMULA 4IOL FORMULA 4thth generationgeneration

• Holladay2Holladay2• Haigis formula-Haigis formula-• d = a0 + (a1 * ACD) + (a2 * AL)• ACD is the measured anterior chamber depth• AL is the axial length of the eye• The a0, a1 and a2 constants are set by

optimizing• a set of surgeon- and IOL-specific outcomes

for a wide• range of ALs and ACDs.

• SRK/T formula — uses "A-SRK/T formula — uses "A-constant"constant"

• Holladay 1 formula — uses Holladay 1 formula — uses "Surgeon Factor""Surgeon Factor"

• Holladay 2 formula — uses Holladay 2 formula — uses "Anterior Chamber Depth""Anterior Chamber Depth"

• Hoffer Q formula — uses Hoffer Q formula — uses "Anterior Chamber Depth""Anterior Chamber Depth"

When capsular tear does When capsular tear does not allow bag placement of not allow bag placement of the lens change IOL power the lens change IOL power

for sulcus placementfor sulcus placement• >=28.5 D Decrease by 1.5 >=28.5 D Decrease by 1.5

DD• +17 To 28 D Decrease by +17 To 28 D Decrease by

1.0 D 1.0 D • +9 To 17 D Decrease by +9 To 17 D Decrease by

0.5 D0.5 D• <+ 9 D No change<+ 9 D No change

SummarySummary

• Use IOL master or immersion Use IOL master or immersion ultrasound for most accurate axial ultrasound for most accurate axial length measurementlength measurement

• Use fourth generation IOL formulasUse fourth generation IOL formulas• Examine and reevaluate your result Examine and reevaluate your result

periodicallyperiodically

THANK YOUTHANK YOU