A-scan BiometryA-scan Biometry

Introduction:- Introduction:-

Ophthalmic ultrasound uses the reflection of high Ophthalmic ultrasound uses the reflection of high frequency sound waves to define the outlines of ocular and orbital frequency sound waves to define the outlines of ocular and orbital structures and to measure the distance between them. structures and to measure the distance between them.

A-A-Scan biometry is to determine the power of the intra ocular Scan biometry is to determine the power of the intra ocular lens, that replaces the natural lens during cataract surgery.lens, that replaces the natural lens during cataract surgery.

A-Scan biometry is also called as axial length measurement scan. A-Scan biometry is also called as axial length measurement scan. This measurement is combined with Keratometric readings to This measurement is combined with Keratometric readings to obtain the IOL power. obtain the IOL power.

Error of 0.4mm in the measurement of axial length may result in Error of 0.4mm in the measurement of axial length may result in a one diopter change in calculated IOLa one diopter change in calculated IOL power.power.

Physical principlesPhysical principles The A-scan probe contains a ultrasonic transducer that projects a The A-scan probe contains a ultrasonic transducer that projects a

thin sound beam that travels through liquid or tissue.thin sound beam that travels through liquid or tissue. Ultrasound waves do not travel through air.Ultrasound waves do not travel through air. The frequencies most often employed for diagnostic work are The frequencies most often employed for diagnostic work are

between 2.5 MHZ and 20 MHZ. Higher the frequency greater the between 2.5 MHZ and 20 MHZ. Higher the frequency greater the resolution.resolution.

Although increasing the frequency increases the resolution,it Although increasing the frequency increases the resolution,it simultaneously decreases the depth of penetration of the sound.simultaneously decreases the depth of penetration of the sound.

When the sound beam encounters the interface of a substance that When the sound beam encounters the interface of a substance that is dissimilar from the substance it is traveling through,part of the is dissimilar from the substance it is traveling through,part of the sound beam energy is reflected , and part of the sound energy sound beam energy is reflected , and part of the sound energy projects through the new substance. projects through the new substance.

PROCEDURE: PROCEDURE: A probe is placed on the patient’s cornea.A probe is placed on the patient’s cornea. The probe is attached to a device that delivers The probe is attached to a device that delivers

adjustable sound waves.adjustable sound waves. The measurements are displayed as spikes on the The measurements are displayed as spikes on the

screen of an oscilloscope (Visual monitor).screen of an oscilloscope (Visual monitor). The appearance of the spikes and the distance The appearance of the spikes and the distance

between them can be correlated to structures between them can be correlated to structures within the eye and the distance between them. within the eye and the distance between them.

Probe positioningProbe positioning: : The probe lightly touches the The probe lightly touches the

cornea and is positioned, such cornea and is positioned, such that the barrel of the probe is that the barrel of the probe is aligned with the optical axis or aligned with the optical axis or visual axis of the eye.visual axis of the eye.

The operator aims the probe The operator aims the probe towards the macula of the eye.towards the macula of the eye.

Alignment with the optical axis Alignment with the optical axis will be indicated by high lens will be indicated by high lens spikes and a high retina spike spikes and a high retina spike on the scan graph.on the scan graph.

In the manual mode the operator will have to freeze the In the manual mode the operator will have to freeze the scan with a foot pedal when an acceptable scan is obtained.scan with a foot pedal when an acceptable scan is obtained.

The gain should be adjusted high enough such that the The gain should be adjusted high enough such that the spikes can be maintained above the threshold level needed spikes can be maintained above the threshold level needed for an automated acquisition of the scan if this feature is for an automated acquisition of the scan if this feature is used.used.

The gain should be low enough to allow the operator to The gain should be low enough to allow the operator to visually maximize the spike height during probe visually maximize the spike height during probe alignment. alignment.

The scan of phakic eyeThe scan of phakic eye1.1. It shows anterior lens spike(B),the It shows anterior lens spike(B),the

posterior lens spike(C),and the retina posterior lens spike(C),and the retina spike (E), the probe tip / cornea spike spike (E), the probe tip / cornea spike is represented by(A).is represented by(A).

2.2. These spikes should be tall and These spikes should be tall and steeply rising.The retina spike should steeply rising.The retina spike should not have smaller spikes immediately not have smaller spikes immediately in front of it.(D)in front of it.(D)

3.3. The retina spike should be followed The retina spike should be followed by tall scleral spike (F) and spikes by tall scleral spike (F) and spikes from the orbital fat layer of the from the orbital fat layer of the orbit(G).A scan without orbital fat orbit(G).A scan without orbital fat spikes may indicate that the beam is spikes may indicate that the beam is striking the optic nerve instead of the striking the optic nerve instead of the macula. macula.

The scan of an aphakic eyeThe scan of an aphakic eye It will either have no lens spikes,(or) It will either have no lens spikes,(or)

it will have one lens spike (A) that it will have one lens spike (A) that represents an intact posterior lens represents an intact posterior lens capsule. ( C ) capsule. ( C )

Be sure to use the aphakic mode of Be sure to use the aphakic mode of the A-scan instrument.the A-scan instrument.

The velocity of sound will be The velocity of sound will be different because the beam is not different because the beam is not passing through the lens.passing through the lens.

A velocity of sound of 1532m/s is A velocity of sound of 1532m/s is typically used for aphakic typically used for aphakic measurements.measurements.

Pseudophakic scanPseudophakic scan If one is pseudophakic, A-scan and K-readings should be If one is pseudophakic, A-scan and K-readings should be

done for both the eyes before calculating the IOL power for done for both the eyes before calculating the IOL power for the eye required.the eye required.

Since both eyes have similar measurements in most people, Since both eyes have similar measurements in most people, this provides a double check of the measurement.this provides a double check of the measurement.

It is some times necessary to replace an IOL that was It is some times necessary to replace an IOL that was inserted many months or years ago. Even if you have IOL inserted many months or years ago. Even if you have IOL specifications and measurement information from the specifications and measurement information from the previous surgery, its nice to have the confirmation of a previous surgery, its nice to have the confirmation of a current measurement. current measurement.

Maintenance of an Instrument Maintenance of an Instrument

It is done by,It is done by,

General inspection General inspection Calibration checkCalibration checkSensitivity test Sensitivity test Cleaning Cleaning Storage.Storage.

Measuring GatesMeasuring Gates A-scan instrument calculate measurements by A-scan instrument calculate measurements by

means of electronic measuring ‘gates or lights’.means of electronic measuring ‘gates or lights’.This instruments has two gates,This instruments has two gates, One for the cornea spike. One for the cornea spike. One, that the retina spike must come within.One, that the retina spike must come within. The instrument to the right has measuring lights The instrument to the right has measuring lights

that attach to the cornea, lens, and the retina spikes.that attach to the cornea, lens, and the retina spikes. Errors may occur if cataract formation results in Errors may occur if cataract formation results in

irregular inter faces within the lens that create extra irregular inter faces within the lens that create extra lens spikes.lens spikes.

This will result in a very short measurement This will result in a very short measurement because the total length is figured from the first because the total length is figured from the first light to the last light. light to the last light.

Corneal Compression Corneal Compression Since ultra sound does not travel through air the Since ultra sound does not travel through air the

A-scan probe must come into contact with the A-scan probe must come into contact with the cornea either directly or through a liquid.cornea either directly or through a liquid.

If undue pressure is applied on the cornea, the If undue pressure is applied on the cornea, the axial length measurment may be falsely too short.axial length measurment may be falsely too short.

It can be monitored by observing the anterior It can be monitored by observing the anterior chamber depth, read out by an instrument.chamber depth, read out by an instrument.

Most eyes will have an ACD readings between Most eyes will have an ACD readings between 2.5 to 4.0mm.2.5 to 4.0mm.

The corneal compression error factor can be The corneal compression error factor can be avoided by using the immersion technique. avoided by using the immersion technique.

Corneal CompressionCorneal Compression

Reproducibility Reproducibility Reproducibility is an indicator Reproducibility is an indicator

of accuracy:of accuracy: This means being able to take This means being able to take

four or five consecutive four or five consecutive measurements with consistent measurements with consistent appearance, ACD and total appearance, ACD and total length results.length results.

A commonly accepted level of A commonly accepted level of reproducibility in terms of total reproducibility in terms of total length is to have several length is to have several measurements within 0.20mm measurements within 0.20mm of each other.of each other.

IOL Power calculationIOL Power calculation

Introduction:-Introduction:- The importance of accurate IOL power implant The importance of accurate IOL power implant

power calculation has increased with the power calculation has increased with the development of improved astigmatism control, development of improved astigmatism control, and surgeon expectations. and surgeon expectations.

Precise biometric measurements of preoperative Precise biometric measurements of preoperative axial length,corneal curvature are the corner axial length,corneal curvature are the corner stones of this system.stones of this system.

CLASSIFICATION OF IOL POWER CLASSIFICATION OF IOL POWER

CALCULATIONCALCULATION Methods based on basic refraction.Methods based on basic refraction. Methods based on measurements like axial Methods based on measurements like axial

length,corneal curvature. length,corneal curvature. Theoretical formulas.Theoretical formulas. Regression or empirical formulas.Regression or empirical formulas. Combination of both.Combination of both.

A-SCAN METHODSA-SCAN METHODS

There are two types, There are two types,

Hand held Method:- Hand held Method:- Performed by using hand held ultra sound transducer probe. Performed by using hand held ultra sound transducer probe.

Disadvantage is, Disadvantage is, Compression of corneaCompression of cornea In probe alignment of the probe attribute to accurate measurement of In probe alignment of the probe attribute to accurate measurement of

the eye.the eye.

Stand held Method:-Stand held Method:-Probe is fitted in place of Tonometer prism in Tonometer slit lamp Probe is fitted in place of Tonometer prism in Tonometer slit lamp models.models.

Immersion Technique:-Immersion Technique:-Also called water bath method, the patients is supine and ultrasound Also called water bath method, the patients is supine and ultrasound probe is suspended in fluid filled scleral cup placed over the eye. probe is suspended in fluid filled scleral cup placed over the eye.

Regression Formula Regression Formula

SRK Formula P = A – 2.5 L – 0.9K.SRK Formula P = A – 2.5 L – 0.9K.

P = Implant power to produce emmetropia (diopter).P = Implant power to produce emmetropia (diopter).

L = Axial length.L = Axial length.

K = Average Keratometry Readings. K = Average Keratometry Readings.

A = Specific Constant for each lens types. A = Specific Constant for each lens types.

AMMETROPIC CALCULATIONAMMETROPIC CALCULATION The SRK emmetropia calculation uses the refraction factor (RF) The SRK emmetropia calculation uses the refraction factor (RF)

of 1.5Dsph. But when targeting for emmetropia of greater than of 1.5Dsph. But when targeting for emmetropia of greater than 1.0Ds use the following RF’S.1.0Ds use the following RF’S.

RF of 1.25 if emmetropia power >14D.RF of 1.25 if emmetropia power >14D. RF of 1.0 if emmetropia power <=14DRF of 1.0 if emmetropia power <=14D

Formula used for this,Formula used for this,

IOL = P – ( RT + RF )IOL = P – ( RT + RF )

IOL = Power of the needed to produce ammetropia.IOL = Power of the needed to produce ammetropia.

P = Power from SRK formula.P = Power from SRK formula.

RT = Post operative refraction needed.RT = Post operative refraction needed.

RF = Refraction Factor. RF = Refraction Factor.

Factors Affecting IOL Power CalculationFactors Affecting IOL Power Calculation 1. Axial Length measurement:-1. Axial Length measurement:-

Accurate measurement of distance from the corneal cortex Accurate measurement of distance from the corneal cortex to vitreo – retinal interface along the visual axis is essential for the to vitreo – retinal interface along the visual axis is essential for the accurate IOL power calculation. accurate IOL power calculation.

2. Ultrasound Velocity:-2. Ultrasound Velocity:- Since the axial length measurement depends on time it takes for Since the axial length measurement depends on time it takes for

the emitted sound beam to reach the vitreo retinal interface and the emitted sound beam to reach the vitreo retinal interface and return, the velocity of sound through various structure like return, the velocity of sound through various structure like aqueous, vitreous, & lens is important.aqueous, vitreous, & lens is important.

The accepted average velocity of sound is 1532m / sec in The accepted average velocity of sound is 1532m / sec in aphakic eyes and 1550m / sec for cataract eyes.aphakic eyes and 1550m / sec for cataract eyes.

But in pseudophakic eyes the velocity depends on velocity in But in pseudophakic eyes the velocity depends on velocity in PMMA, central thickness of IOL in situ and velocity in aqueous PMMA, central thickness of IOL in situ and velocity in aqueous & vitreous.& vitreous.

3.3. Axial length correction for retinal thickness:- Axial length correction for retinal thickness:- The axial length measured doesn’t take into account the The axial length measured doesn’t take into account the

distance from the vitreo retinal interface to the visual cell distance from the vitreo retinal interface to the visual cell layer, that is estimated to be from 0.15 to 0.5mm.layer, that is estimated to be from 0.15 to 0.5mm.

4.4. Keratometry :-Keratometry :- Failure to calibrate the eye piece leads to error , this can Failure to calibrate the eye piece leads to error , this can

translate to 0.90D error in IOL power calculation.translate to 0.90D error in IOL power calculation.

5. Estimated PO ACD :- (5. Estimated PO ACD :- (Post operative Anterior chamber depth)Post operative Anterior chamber depth)

ACD = AL / 23.45 X ACD (3.42m). ACD = AL / 23.45 X ACD (3.42m).

6. Surgical Techniques:- 6. Surgical Techniques:- Measurement of lens in bag places lens further back Measurement of lens in bag places lens further back

decreasing the effective power of lens,there is usually a loss of 0.5D decreasing the effective power of lens,there is usually a loss of 0.5D to 1.5D. to 1.5D.

Errors IOL Power Calculation Errors IOL Power Calculation

It can be categorized into 3 groups,It can be categorized into 3 groups,

Measurement errors : 43 – 67 % of errors.Measurement errors : 43 – 67 % of errors. Errors from insufficiencies of the formula.Errors from insufficiencies of the formula. Errors in prediction of surgical effect, post Errors in prediction of surgical effect, post

operative astigmatism and site of lens.operative astigmatism and site of lens.

Calculation of Individual A Calculation of Individual A ConstantConstant

A 1A 1 = 1+ ( RA x RF ) + 2.5 x L + 0.9 x K – C= 1+ ( RA x RF ) + 2.5 x L + 0.9 x K – C

A1 A1 = Individual A Constant. = Individual A Constant.

L L = Axial Length. = Axial Length.

K K = Average Keratometry. ( D ) = Average Keratometry. ( D )

I I = Power of lens implanted. ( D ) = Power of lens implanted. ( D )

RA RA = Actual Post operative spherical equivalent. ( D ) = Actual Post operative spherical equivalent. ( D )

RF RF = Refraction factor. = Refraction factor.

C C = Short & long eye Correction. = Short & long eye Correction.

Measurement errorsMeasurement errors IIt can be prevented by A-scan data. The following t can be prevented by A-scan data. The following

guidelines can be followed.guidelines can be followed.1. Always measure both eyes.1. Always measure both eyes.2. Indications for repeat scans2. Indications for repeat scans• Axial Length < 22.0mm or >25.0mm.Axial Length < 22.0mm or >25.0mm.• Average corneal power < 40 and > 47D. Average corneal power < 40 and > 47D. • Calculated implant power is more than 3D from the average for Calculated implant power is more than 3D from the average for

the specific IOL style used.the specific IOL style used.• Difference between the eyes.Difference between the eyes.

Average correct power > 1 D.Average correct power > 1 D. Axial Length > 0.3mm. Axial Length > 0.3mm. Emmetropia implant power > 1 D.Emmetropia implant power > 1 D.

3. Check the position of the measuring gates light on the display.3. Check the position of the measuring gates light on the display.

Characteristics of quality A –Scan TechniqueCharacteristics of quality A –Scan Technique 1.1. 5 Principle echo spikes are present,5 Principle echo spikes are present,

a)a) Corneal.Corneal.

b)b) Anterior chamber.Anterior chamber.

c)c) Posterior camber.Posterior camber.

d)d) Retina.Retina.

e)e) Sclera & orbital flat.Sclera & orbital flat.

2.2. Echo heights are adequate if anterior lens echo is 90% or more Echo heights are adequate if anterior lens echo is 90% or more of maximum height. Posterior lens echo is between 50% & 75% of maximum height. Posterior lens echo is between 50% & 75% of maximum.of maximum.

3.3. Retinal echo is 75% or more of maximum echo rise angle must Retinal echo is 75% or more of maximum echo rise angle must be clear. The take off of the retinal spike must be clean and be clear. The take off of the retinal spike must be clean and forms a 90forms a 90°° angle from the lease line. angle from the lease line.

IOL Power calculation in special IOL Power calculation in special situationsituation

IOL calculation following Refractive surgery.IOL calculation following Refractive surgery.

IOL in silicon oil filled eyes.IOL in silicon oil filled eyes.

1. 1. It can be done by two methods depending on the amount of It can be done by two methods depending on the amount of refractive error.refractive error.

Mild to moderate Myopia:-Mild to moderate Myopia:- Corrected by Radial Keratometry ,postoperative Corrected by Radial Keratometry ,postoperative

Keratometric and axial length data can be used Keratometric and axial length data can be used directly.directly.

High Refractive error :-High Refractive error :- Corrected by Radial Keratometry or other methods of refractive Corrected by Radial Keratometry or other methods of refractive

surgery may cause an error in IOL calculations by providing surgery may cause an error in IOL calculations by providing inaccurate Keratometry data.inaccurate Keratometry data.

In this case IOL power is calculated using original refraction & In this case IOL power is calculated using original refraction & Keratometry data. The difference between original refraction and Keratometry data. The difference between original refraction and the current refraction is determined. the current refraction is determined.

If the patient was myopic this difference in refraction was added If the patient was myopic this difference in refraction was added to previously calculated IOL power.to previously calculated IOL power.

If Patient was Hypermetropic subtract this difference. If Patient was Hypermetropic subtract this difference.

2.2. Biometry in silicon oil filled Eye.Biometry in silicon oil filled Eye.

The echographically measured axial length of a eye is greater The echographically measured axial length of a eye is greater in the presence of silicon oil.in the presence of silicon oil.

This is because speed of sound in silicon oil ( 987m / s in This is because speed of sound in silicon oil ( 987m / s in silicon oil of viscosity 1000 centistokes) is slower than in silicon oil of viscosity 1000 centistokes) is slower than in vitreous humour ( 1532 m/s).vitreous humour ( 1532 m/s).

Theoretically this speed of sound in silicon oil (viscosity 1000 Theoretically this speed of sound in silicon oil (viscosity 1000 centistokes) compared with the speed of sound in vitreous centistokes) compared with the speed of sound in vitreous humour decreases by a factor of 0.64mm. humour decreases by a factor of 0.64mm.

It is therefore possible to calculate the true depth of the It is therefore possible to calculate the true depth of the

vitreous cavity vitreous cavity ( VCD oil x 0.64) = True Axial length( VCD oil x 0.64) = True Axial length..

• AL without oil = ACD oil +LT oil+ [VCD oil x (sound AL without oil = ACD oil +LT oil+ [VCD oil x (sound vel oil /sound vel in vitreous) ].vel oil /sound vel in vitreous) ].

• AL With out oil = Axial Length of vitrectomised eye (tissue AL)AL With out oil = Axial Length of vitrectomised eye (tissue AL)• Al oil = Measured Axial Length in presence of silicon oil.Al oil = Measured Axial Length in presence of silicon oil.• ACD oil = Anterior chamber depth in presence of silicon oil.ACD oil = Anterior chamber depth in presence of silicon oil.• LT oil = Lens thickness in presence of silicon oil.LT oil = Lens thickness in presence of silicon oil.• Sound velocity in oil = Velocity of sound in silicon oil (1000 cs) Sound velocity in oil = Velocity of sound in silicon oil (1000 cs)

987m/s. 987m/s. • Sound vel vit = Velocity of sound in vitreous (1532m/s).Sound vel vit = Velocity of sound in vitreous (1532m/s).• SRK/T – Formula was used to compare the measured power of SRK/T – Formula was used to compare the measured power of

IOL and the estimated power of IOL.IOL and the estimated power of IOL.Conversion factor of 0.71 is used when ACD,LT,and VCD are Conversion factor of 0.71 is used when ACD,LT,and VCD are not known. This Conversion factor can be used for silicon oil not known. This Conversion factor can be used for silicon oil of viscosity 1300 centistokes. of viscosity 1300 centistokes.

Selection of IOL PowerSelection of IOL Power 1. 1. Monocular Hypermetropia:-Monocular Hypermetropia:-

Patient alternates this one can ignore anisometropia and aniesokonia, Patient alternates this one can ignore anisometropia and aniesokonia, & reduce the pre existing hypermetropia.& reduce the pre existing hypermetropia.

2. Bilateral strong Hypermetropia :-2. Bilateral strong Hypermetropia :- Pseudophakic eye should not be made emmetropic because one might Pseudophakic eye should not be made emmetropic because one might

introduce greater degree of anisometropia .introduce greater degree of anisometropia . In this case standard lens should be implanted and stronger convex In this case standard lens should be implanted and stronger convex

lens placed in front of operated eye will magnify the retinal image by lens placed in front of operated eye will magnify the retinal image by 4% & decrease anisometropia.4% & decrease anisometropia.

3. Bilateral emmetropia and unilateral implantation and implant standard 3. Bilateral emmetropia and unilateral implantation and implant standard lens creating a mild myopia , thus patients can use the other eye for lens creating a mild myopia , thus patients can use the other eye for distance and the operated eye for near.distance and the operated eye for near.

4. If primary refraction of one eye is mild to moderate myopia around 4. If primary refraction of one eye is mild to moderate myopia around – – 2.0 D and other eye is emmetropic. It is advantage and ideal lens 2.0 D and other eye is emmetropic. It is advantage and ideal lens

should be implanted.should be implanted.

5. If b5. If both eyes are myopic with monocular cataract, implanting oth eyes are myopic with monocular cataract, implanting emmetropic lens will induce greater anisometropia.Hence some emmetropic lens will induce greater anisometropia.Hence some degree of residual myopia is desired.degree of residual myopia is desired.

6. 6. In binocular cataract aim for postoperative emmetropia.In binocular cataract aim for postoperative emmetropia.

In case of BSV keep one eye slightly myopic & other In case of BSV keep one eye slightly myopic & other emmetropic. emmetropic.

7. In Primary myopia of more than - 7.0D, eye can be made 7. In Primary myopia of more than - 7.0D, eye can be made aphakic. aphakic. So visualization of peripheral fundus become difficult during So visualization of peripheral fundus become difficult during

prophylactic photocoagulation and RD surgeries due to prophylactic photocoagulation and RD surgeries due to refraction from IOL Hindrance of pupillary dilatation may refraction from IOL Hindrance of pupillary dilatation may occur due to synechiae formation.occur due to synechiae formation.

Correction of high myopia with low power IOL have shown Correction of high myopia with low power IOL have shown good results.good results.

If the postoperative refraction is highly different, lens If the postoperative refraction is highly different, lens exchange should be done.exchange should be done.

If the IOL power is correct, consider posterior If the IOL power is correct, consider posterior staphyloma or IOL dislocation. staphyloma or IOL dislocation.

The determination of power of implant in situ can be The determination of power of implant in situ can be done as last resort.done as last resort.

Analyzing and correcting power Analyzing and correcting power calculation mistakescalculation mistakes

Factors to Consider for IOL materialFactors to Consider for IOL material

The type of material is important because the The type of material is important because the velocity of sound is a function of the material that the velocity of sound is a function of the material that the sound is passing through.sound is passing through.

A-Scan does not actually measure length, they A-Scan does not actually measure length, they measure how long it takes a sound beam to bounce measure how long it takes a sound beam to bounce off an object ( Anterior lens, Posterior lens,and off an object ( Anterior lens, Posterior lens,and Retina) & return to the probe.Retina) & return to the probe.

The instrument is pre programmed with the velocity The instrument is pre programmed with the velocity of sound factors for the aqueous, the lens material, of sound factors for the aqueous, the lens material, and the vitreous.and the vitreous.

PMMA protocol : PMMA protocol : (Polymethylmethacrylate)(Polymethylmethacrylate)• Sound travels faster through PMMA than it does through the Sound travels faster through PMMA than it does through the

natural lens.natural lens.• If a pseudophakic mode is not available,you can measure the If a pseudophakic mode is not available,you can measure the

eye in the aphakic mode and add a standard compensating eye in the aphakic mode and add a standard compensating factor of 0.4mm to the resultant axial length.factor of 0.4mm to the resultant axial length.

Silicon protocol:Silicon protocol:• Silicon IOL’s are foldable. Sound travels much slower through Silicon IOL’s are foldable. Sound travels much slower through

a silicon lens than it does through the natural lens.a silicon lens than it does through the natural lens.• If not taking this in to account could result in a –3.0D post - op If not taking this in to account could result in a –3.0D post - op

refractive error. refractive error. • If your biometer does not have a pseudophakic or silicon mode, If your biometer does not have a pseudophakic or silicon mode,

use the aphakic mode and subtract a compensation factor of use the aphakic mode and subtract a compensation factor of 0.8mm.0.8mm.

Acrylic protocol:Acrylic protocol:

• Sound travels faster through acrylic than it Sound travels faster through acrylic than it does through the natural lens.does through the natural lens.

• If biometer does not have an acrylic If biometer does not have an acrylic mode,use the aphakic mode and add a mode,use the aphakic mode and add a compensation factor of 0.2mm.compensation factor of 0.2mm.

IntroductionIntroduction Keratometry (or) ophthalmometerKeratometry (or) ophthalmometer . .1.1. Keratometry is the measurement of a patients Keratometry is the measurement of a patients

corneal curvature .corneal curvature .2.2. It provides on objective,quantitative measurement It provides on objective,quantitative measurement

of corneal astigmatism,measuring the curvature in of corneal astigmatism,measuring the curvature in each meridian as well as the axis.each meridian as well as the axis.

3.3. Keratometry is also helpful in determining the Keratometry is also helpful in determining the appropriate fit of contact lens .appropriate fit of contact lens .

4.4. To measurement the corneal dioptric power.To measurement the corneal dioptric power.5.5. The measurement of the curvature of the anterior The measurement of the curvature of the anterior

corneal surface by using the first Purkinje image.corneal surface by using the first Purkinje image.

Keratometer:Keratometer: Determines corneal curvature by measuring the Determines corneal curvature by measuring the

size of a reflected “mire”.size of a reflected “mire”. Doubling of image avoids problems from eye Doubling of image avoids problems from eye

movements.movements. Radius scale is exact. Diopter scale is derived Radius scale is exact. Diopter scale is derived

from the radius using the formula for surface from the radius using the formula for surface power D= (n-l)/r,where n=1.3375,an empirically power D= (n-l)/r,where n=1.3375,an empirically derived “standardized”refractive index for the derived “standardized”refractive index for the cornea.cornea.

Keratometer measures only the central 3mm of Keratometer measures only the central 3mm of the corneal diameter.the corneal diameter.

Principle of KeratometryPrinciple of Keratometry

Principle of KeratometryPrinciple of Keratometry It measure the size of image reflected from corneal It measure the size of image reflected from corneal

surface, because cornea acts as convex mirror.surface, because cornea acts as convex mirror.

When an object is in front of the cornea a virtual When an object is in front of the cornea a virtual image is seen inside the convex mirror (cornea). The image is seen inside the convex mirror (cornea). The size of the image depends on, size of the image depends on,

1.1. The distance of the object and The distance of the object and 2.2. The curvature of the cornea for a fixed distance The curvature of the cornea for a fixed distance

of the object the size of the image depends on of the object the size of the image depends on the curvature of the cornea.the curvature of the cornea.

Similarly for a given size of the image distance of the Similarly for a given size of the image distance of the object is different depending on the curvature of the object is different depending on the curvature of the cornea. cornea.

The object used is an illuminated circle with plus and The object used is an illuminated circle with plus and minus rings as shown in figure.minus rings as shown in figure.

The two prisms inside the instrument give two additional The two prisms inside the instrument give two additional one displaced horizontally and another displaced one displaced horizontally and another displaced vertically. Three images are seen as in figure. vertically. Three images are seen as in figure.

While taking the reading the pluses and minuses coincide. While taking the reading the pluses and minuses coincide. This is achieved by moving the keratometer with the This is achieved by moving the keratometer with the object forward or backward in front of the eye.object forward or backward in front of the eye.

When coincidence takes place the size of the images of When coincidence takes place the size of the images of fixed value. The distance of the object is different for fixed value. The distance of the object is different for different curvatures. The instrument is calibrated. As the different curvatures. The instrument is calibrated. As the drum rotate the distance varies. drum rotate the distance varies.

Parts of keratometerParts of keratometer1.1. TelescopeTelescope2.2. Eye piece at one end (near to examiner)Eye piece at one end (near to examiner)3.3. Objective (near to patient)Objective (near to patient)4.4. Knobs for adjustment of vertical and horizontal Knobs for adjustment of vertical and horizontal

curvature on the telescope.curvature on the telescope.5.5. Knobs adjusting height of telescope.Knobs adjusting height of telescope.6.6. Chinrest-adjusting knobs.Chinrest-adjusting knobs.7.7. Bulb for illumination.Bulb for illumination.8.8. Knob for focusing of mires.Knob for focusing of mires.9.9. Model cornea with occluder.Model cornea with occluder.

Parts of keratometerParts of keratometer

Performing KeratometryPerforming Keratometry1.1. Looking the through the eye piece of the Looking the through the eye piece of the

keratometer,use the eye piece to focus the reticule keratometer,use the eye piece to focus the reticule (cross hair) in the same way as for the lens (cross hair) in the same way as for the lens meter.meter.

2.2. The patient can comfortably put the chin and The patient can comfortably put the chin and forehead on the appropriate rests.forehead on the appropriate rests.

3.3. Use the occluder attached to the keratometer to Use the occluder attached to the keratometer to cover the eye not being measured.cover the eye not being measured.

4.4. Then use the height adjustment knob of the Then use the height adjustment knob of the keratometer to position the light reflections at the keratometer to position the light reflections at the level of the cornea.level of the cornea.

1.1. To obtain proper focus, rotate the focus knob until the To obtain proper focus, rotate the focus knob until the bottom-right circles converge to form a fused image.bottom-right circles converge to form a fused image.(fig)(fig)

1.1. To locate the proper axis, rotate the keratometer until the To locate the proper axis, rotate the keratometer until the pluses between the two bottom circles are in the same pluses between the two bottom circles are in the same plane.(fig) plane.(fig)