Biometry Instruments &

Equipment

Dr Devdutta NayakFellow Ant. Segment

Biratnagar Eye Hospital

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.

Keratometery

• Keratometry by - Manual Topography Autokeratometer IOL

master/Lenstar 900

Hand-held Autorefractometer and Keratometer

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, with one eye typically within 1D of each other.

Difficult Situations

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

A-Scan biometry/laser interferometry

• A-Scan ultrasound by applanation

method by immersion

method• Laser interferometry IOL Master

(Zeiss) Lenstar LS 900

(Haag-Streit)

• A scan: Amplitude Scan; utilizes ultrasound waves of 10 - 12 MHz frequency.

• 2 Principles: Piezoelectric Phenomenon

Acoustic Impedence.

• Pulsed-echo system.• Components: Transducer Amplifier Display

Monitor

• 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.

• 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).

• The gain setting on biometers 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.

• 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.

• Ultrasound biometry machines use the formula

Distance = Velocity x Time.

• Sound velocity through different media:Phakic – 1550 m/sAphakic – 1532 m/sPseudophakic – 1532 + Correction factor

for IOL

•  Velocity through PMMA is 2718 m/s, through acrylic is 2120 m/s, and through silicone is 980-1107 m/s.

• Correction factor is +0.4 mm for PMMA, +0.2 mm for acrylic, and -0.4 mm to -0.8 mm for silicone

• 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.

A-scan facts

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

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

• All A-scan unit make mistake in echo interpretation.

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, or by holding the ultrasound probe by hand.

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

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

localization 3.  Limited resolution. 4.  Incorrect assumptions regarding

sound velocity. 5.  Potential for incorrect

measurement distance.

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. 6 spikes instead of 5.

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

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

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.

NON CONTACT • The Zeiss IOLMaster

(1999)- non- contact optical device that measures the distance from corneal vertex to the RPE by dual beam partial coherence laser interferometry.

• Uses 780 nm infrared light

& Michelson Interferometer.

• The IOL Master is consistently accurate to within ±0.02 mm or better.

• Haag-Streit launched similar device named Lenstar LS 900 in 2009.

• IOL Master provides following measurements:

AC depth Lens thickness Axial Length Keratometry White to white distance

• In-built IOL power calculation by diff. formula: SRK II, SRK – T, Holladay II, Hoffer Q, Haigis L.

• 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.

•  IOLMaster produces a primary maxima (narrow, well-defined, centered peak identified by a circle above it), secondary maxima (discrete lower peaks, sometimes disappearing into the baseline), and a baseline (which is low and even, but may become high and uneven with decreasing signal-to-noise ratio (SNR)).

• SNR is a measure of accuracy and decreases with increasing cataract density.

• SNR > 2.0 is valid and good if

repeatable, SNR between 1.6-2.0 is borderline but usable if repeatable, and SNR < 1.6 is not usable.

• However, a proper waveform is more important than the SNR value.

Advantages of IOLMaster

• Easy & technician independent• Noncontact• No water bath is needed• Can measure through glasses• Accurate for silicone oil filled eyes and

posterior staphyloma.• Accurate (Holladay II)• Haigis L formula incorporated for post-

LVC pts.• For Piggyback IOLs

• Lenstar LS 900 measures CCT, ACD, Lens thickness, Retinal thickness, AL, Keratometry, White to white distance, Pupillometry & eccentricity of optical axis.

• Lenstar measures keratometry & ACD more accurately than IOL Master.

Accuracy of axial length by different machine

Applanation A -scan

Immersion A-scan

IOL Master

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

Do not throw away old ultrasound machine

Immersion ultrasound

IOL master

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

DifficultDifficultVariable•Yes•Yes•Yes•Yes

•Yes•Yes•YesNoNoNoNo

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

IOL FORMULA 2nd generation

• SRK formula – works well for average

eyes. less accurate for long,

short eyes.• SRK II formula modification of SRK works on ELP 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

IOL FORMULA 3rd generation

• Third generation formulas-

• SRK/T -very long eyes >26mm

• Holladay I -long eyes 24-26 mm

• HofferQ -Short eyes<22mm

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 optimizing

a set of surgeon- and IOL-specific outcomes for a wide

range 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"

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

IOL calculation after Refractive surgery

• Clinical History Method• Shammas Equation corrected K = 1.14 (average K) -

6.8• Topography Method (Wang et al) corrected K = 1.114K – 6.1• Corneal Bypass Method (Wake

Forest Univ.)• Masket Formula • Online Calculators (doctor-hill.com,

ASCRS)

Summary

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

• Use fourth generation IOL formulas.

• Examine and reevaluate your result periodically.

THANK YOU