BIOMETRY & PEDIATRICS IOL CALCULATION

BY DR RAKESH JAISWAL

BIOMETRY

Biometry is the process of measuring thepower of the cornea (keratometry) and theAxial length of the eye, by using this data todetermine the ideal intraocular lens power

Measurement of axial length(AL)• A-scan • IOL master

A-scan Biometry• A scan: Amplitude Scan;

utilizes ultrasound waves of 10 - 12 MHz frequency.

• 2 Principles: Piezoelectric Phenomenon

Acoustic Impedence.

• Components: Transducer Amplifier

Display Monitor • Ultrasound biometry

machines use the formula Distance = Velocity x Time

• In A-scan, thin, parallel sound beam is emitted from the probe tip, with an echo bouncing back into the probe tip as the sound beam strikes each interface.

• An interface is the junction between any two media of different densities and velocities.

anterior corneal surface aqueous/anterior lens surface posterior lens capsule/anterior vitreous posterior vitreous/retinal surface choroid/anterior scleral surface.

• The echoes received back into the probe from these interfaces are converted by the biometer to spikes arising from baseline.

• The greater the difference in the two media at each interface, the stronger the echo and the higher the spike.

• Average Axial Length of Normal Eye 23.06 mm

•Majority 22.0 to 24.5 mm

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

•Difference in AL measurement Between both eyes + 0.3 mm

• Contact - Applanation Method - Hand-Held Method

• Immersion

Applanation A-scan Biometry

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

Applanation A-scan Biometry.

• a: Initial spike (probe tip and cornea)b: Anterior lens capsulec: Posterior lens capsuled: Retinae: Scleraf: Orbital fat

Applanation A-scan Biometry

• When echoes b through d are high and steeply rising, the ultrasound beam is most likely on visual axis.

• If no scleral or orbital fat echoes visible, then ultrasound beam is most likely aligned with optic nerve.

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

1. Variable corneal compression. 2. Broad sound beam without precise localization3. Limited resolution. 4. Incorrect assumptions regarding sound velocity

5. Potential for incorrect measurement distance.

PROCEDURE: Hand-Held Method A probe is placed on the patient’s cornea.The probe is attached to a device that delivers adjustable sound waves.The measurements are displayed as spikes on the screen of an oscilloscope (Visual monitor).The appearance of the spikes and the distance between them can be correlated to structures within the eye and the distance between them.

Probe positioning: The probe lightly touches the cornea and is positioned, such that the barrel of the probe is aligned with the optical axis or visual axis of the eye.The operator aims the probe towards the macula of the eye.Alignment with the optical axis will be indicated by high lens spikes and a high retina spike on the scan graph.

• Spike height is affected by the difference in density & by the angle of incidence, which is determined by the probe orientation to the visual axis.

• If the probe is held nonparallel, part of the echo is diverted at an angle away from the probe tip, and is not received by the machine.

• A perfect high, steeply rising retinal spike may be impossible when macular pathology is present (eg, macular edema, macular degeneration, epiretinal membranes, posterior staphylomas).

Corneal Compression • If pressure is applied on the cornea, the axial

length measurment may be falsely too short.• It can be monitored by observing the anterior

chamber depth, read out by an instrument.• Most eyes will have an ACD readings between

2.5 to 4.0mm.• The corneal compression error factor can be

avoided by using the immersion technique

Error caused by 1 mm Corneal Compression

Average eye 2.5 DLong eye 1.75 DShort eye 3.75 D

Immersion A-scan Biometry

• The immersion technique is accomplished by placing a small scleral shell between the patient's lids, filling it with saline, and immersing the probe into the fluid, being careful to avoid contact with the cornea.

• More accurate than contact method because corneal

compression is avoided.

• Eyes measured with the immersion method are, on average, 0.1-0.3 mm longer.

Immersion A-scan Biometry

• The immersion technique requires the use of a Prager Scleral Shell .

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.

Immersion A-scan Biometry

• When the ultrasound beam is properly aligned with the center of the macula, all five spikes will be steeply rising and of maximum height.

• Both the peaks of corneal spike should be equal in

height ideally.

• Other advantage: Easier, better repeatability.

• The gain setting on A-scan is measured in decibels and affects amplification and resolution of spikes.

• When on highest gain, spike height and sensitivity of display screen are maximized, enabling visualization of weaker signals, but resolution is affected adversely.

• When gain is lowered, the spike amplitude and sensitivity are decreased, which eliminates the weaker signals but improves resolution.

• Error can occur when the gain is set too high or too low .

• Very high gain short reading• Very low gain long reading

• Resolution: ability to display two interfaces that lie in close proximity, one directly behind the other, as separate echoes or spikes.

• The more dense the cataract, the higher the necessary gain.

• Gain setting may vary not only from patient to patient but from one eye to the next in the same patient, depending on cataract density.

• Gates are electronic calipers on the display screen that measure distance between two points.

• Proper gate placement is on the ascending edge of each appropriate spike.

• If the biometer does not allow for movement of gates, scans must be repeated until they automatically align properly.

The scan of an aphakic eye• It will either have no lens spikes,

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

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

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

• .• Its required for scondary IOL

• Immersion techique is choice

• Biometry of pseudophakic eye performed:- To compare to the fellow phakic eye for accuracy- IOL exchange- Checking an unwanted postoperative refractive error.

• A scan of pseudophakic eye → multiple reverberation echoes in the vitreous cavity that tend to decrease in amplitude from left to right.

• Decreasing the gain in pseudophakic eye is helpful.

Factors to Consider for IOL material The type of material is important because the

velocity of sound is a function of the material that the sound is passing through.

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

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

PMMA protocol : (Polymethylmethacrylate)

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

•If a pseudophakic mode is not available,you can measure the eye in the aphakic mode and add a standard compensating factor of 0.4mm to the resultant axial length.

Silicon protocol:

•Silicon IOL’s are foldable. Sound travels much slower through 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 refractive error.

•If your biometer does not have a pseudophakic or silicon mode, use the aphakic mode and subtract a compensation factor of 0.8mm.

Acrylic protocol:

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

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

1 Inadequate Patient Fixation Low Vision Nystagmus Blepharospasm Strabismus2. Posterior StaphylomaPosterior staphylomas often causeresulting in an inability to display adistinct , high retinal spike , leadingto a significant error in A-scan measurement . 3. High Hyperopia Immersion technique is preferable4. Macular Lesions 5 Vitreous Lesions6 Dense Cataract

IOL MASTER

WORKING PRINCIPAL• IOL Master (Zeiss Humphrey system) is a

combined biometric instrument that measures quickly and preciesely parameters of human eye needed for IOL power calculation by non-contact technique.

• It also incorporates the softwere to calculate IOL power from various formulae

WORKING RINCIPALIt is a non contact optical device that measures the

various parameters based on the following principal1. AL measurement is based on patented interference

optical method known as Partial Coherence Interferometry (PCI).This technique relies on a laser Doppler technique to measure the echo delay and intensity of infrared light reflected back from the tissue interface-cornea and retinal pigment epithelium .

2. Corneal curvature(K) is determined by measuring the distance between reflected light images as in conventional keratometry

Anterior chamber depth (ACD) is determined as the distance between the optical section of the crystalline lens and cornea produced by lateral slit illumination

White-to-White is determine from the image of the iris

Calculation of IOL power by softwere incorporating internationally accepted calculation formulae

Advantages of the IOL master• Patient comfort as the technique involves non

contact measurements• User friendly • Single instrument for measuring AL,corneal

curvature and ACD• Cross-infection risk is not there,as technique is

non-contact• More accurate AL measurement as compared

to A-scan (five times) with AL ranging b/w 14 to 40mm

• Specially useful in certain ocular conditions like small corneal scar,ant cortical spokes,extream myopia, post staphyloma

• Incorporates IOL power

• This method cannot be used in significant media opacity (eg. dense cataracts or corneal or vitreal opacity) due to absorption of light or inability of the patient to fixate on target.

Accuracy of axial length by different machine

Applanation A -scan

Immersion A-scan

IOL Master

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

Immersion ultrasound

IOL master

Posterior staphylomaSilicone oilPseudophakia4++brunescent lensCentral PSC plaqueVitreous hemorrhageCentral corneal scar

DifficultDifficultVariable•Yes•Yes•Yes•Yes

•Yes•Yes•YesNoNoNoNo

Introduction• Keratometry .1. Keratometry is the measurement of a patients corneal

curvature .2. It provides on objective,quantitative measurement of

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

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

4. It is use for measurement the corneal dioptric power.5. The measurement of the curvature of the anterior

corneal surface by using the first Purkinje image.

Keratometer:• Determines corneal curvature by measuring the size

of a reflected “mire”. n - 1

• Surface power formula: D = ......... R

• D = the dioptric power of the cornea n = the refractive index of the cornea used (1.3375) R = the radius of curvature of the cornea in meters

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

Principle of Keratometry • Cornea is a convex refracting surface

• In order to find the refracting power of the cornea, we need to reflect an object of a known size at a known distance off the corneal surface.

• Then determine the size of the reflecting image with measuring telescope and calculate the refractive power of the cornea based on the refractive index of n= 1.3375

Doubling Principle Helps us to measure the size of something that moves

around since the small movements of the eye makes the measurement impossible

Keratometry 43

Bausch & Lomb Keratometer

• One position keratometer: it measures two meridians at the same time

• It contains two prisms

Principle of Keratometry

• The object used is an illuminated circle with plus and minus rings .

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

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

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

Parts of keratometer

Parts of keratometer1. Telescope2. Eye piece at one end (near to examiner)3. Objective (near to patient)4. Knobs for adjustment of vertical and horizontal

curvature on the telescope.5. Knobs adjusting height of telescope.6. Chinrest-adjusting knobs.7. Bulb for illumination.8. Knob for focusing of mires.9. Model cornea with occluder.

Performing Keratometry1. Looking the through the eye piece of the

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

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

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

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

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

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

Range of keratometer

• 36-52D• Its lower limit can be extented up to 30D by

interposing -1.0D in front of objective of telescope

• Its upper limit up to 61D by interposing a lens of +1.25D in front of objective of telescope

Source of keratometry errors

• Unfocused eye piece• Failure to calibrate unit• Poor patient fixation• Dry eye• Drooping eye lids• Irregular cornea

Repeat Keratometery If

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

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

• The average keratometry (K) → 43.0-44.0D,

Difficult Situations

• Post Refractive Surgery• Corneal Transplantation• Corneal Scar• Keratoconus etc.

Formula forIOL Power Calculation

Depending upon the basis of their deviation ,the various formulae for calculating IOL power have been grouped into

•Theoretical formulae –these is based on mathematical principal revolving around the schematic eye

•Regression formulae-these were arrived at by looking at postoperative outcomes retrospectively

IOL Formulae

• First generation

• Second generation

• Third generation

• Fourth generation

First generation

Theoretical

1. Binkhorst 2. Colenbrander-

Hoffer3. Gilll’s

4. Clayman 5. Fyodorov

Regression

SRK - I

IOL FORMULA Ist generation• Most are based on regression formula developed by

Sander ,Retzlaff & Kraff• Known as SRK formula.

• P = A - 2.5(L) - 0.9(K)

P=lens implant power for emetropiaL= Axial length (mm)K=average keratometric reading (diaopters)A= lens constant

Second generation

Theoretical

Modified Binkhorst

Regression

SRK - II

IOL FORMULA 2nd generation• SRK formula – works well for average eyes. less accurate for long, short eyes.• SRK II formula Modification of SRK A-constant is modified on the basis of AL P = A1 – 2.5L – 0.9K

A1 = A + 3 AL < 20mmA1 = A + 2 AL 20-21A1 = A + 1 AL 21-22A1 = A AL 22-24.5A1 = A – 0.5 AL >24.5

Third generation

• Holladay I• SRK/T• Hoffer Q

IOL FORMULA 3rd generation

• Third generation formulas-

• SRK/T -very long eyes >26mm• Holladay I -long eyes 24-26 mm• HofferQ -Short eyes<22mm

Fourth generation

• Holladay - II

IOL FORMULA 4th generation

• Holladay II• Haigis formula- d = a0 + (a1 * ACD) + (a2 * AL)ACD is the measured anterior chamber depthAL is the axial length of the eye

The a0, a1 and a2 constants are set by optimizinga set of surgeon- and IOL-specific outcomes for a widerange of ALs and ACDs.

• SRK/T formula — uses "A-constant“

• Holladay 1 formula — uses "Surgeon Factor“

• Holladay 2 formula — uses "Anterior Chamber Depth“

• Hoffer Q formula — uses "Anterior Chamber Depth"

AL < 19 mm (<0.1%) Holladay 2AL 19-22 mm (8%) Hoffer-Q

AL 22-24.5 mm (72%) SRK II

AL 24.5-26 mm (15%) Holladay 1 , AL > 26 mm ( 15%) SRK/T

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

• >=28.5 D Decrease by 1.5 D• +17 To 28 D Decrease by 1.0 D • +9 To 17 D Decrease by 0.5 D• <+ 9 D No change

Pediatric biometry• Pediatric eye is not a miniaturized adult eye. • It has shorter axial length, steeper cornea with

higher keratometry value and smaller anterior chamber depth.

• Errors in axial length measurement affect IOL power calculation the most, it increases to 3.75 D per mm in children.

• no formula has been proven to have an advantage over others.

• newer theoretic formulas such as Holladay II that take anterior segment measurements into account, may be recommended for pediatric IOL power calculation or Hoffer Q

• 20% undercorrection if the child is less than age 2 years 10% undercorrection for age 2–8 years .

A-SCAN:A-SCAN We measure AL with an immersion technique, often under sedation. KERATOMETRY For the K measurement, we use a handheld keratometer. Alcon autokeratometer produces accurate measurements of curvature of the cornea in pediatric eyes.

• Measurements should be taken without the use of an eyelid speculum• To avoid the problems associated with corneal dryness, measurements should be taken as soon as possible (following IOP measurement) after induction of anesthesia. •Balanced salt solution should be instilled as necessary to maintain a smooth corneal surface

Using hand –held instruments , reliable keratometry & axial length measurement. For each individual case the IOL power should be customized based on many characteristics including the age , one eye or both , amblyopia status , likely compliance with glasses & family history of myopia.

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