BIOMETRY BY DR.NIKITA JAISWAL 1ST YEAR RESIDENT IMS & SUM HOSPITAL

Biometry :The analysis of biological data using mathematical & statistical methods.

Glossary

Measurement of axial length

Keratometry{ measurement of k reading}

Iol power calculation

A-SCAN

Axial length-it is the distance between the anterior surface of the cornea and the fovea.

A-scan is used to measure axial length.

It measures the time required for a sound pulse to travel from the cornea to retina.

Normal axial length of an eye is –----22-25mm

1 mm error of IOL = 2.88 D 0r 3.0 D of IOL POWER

Ultrasonography

Crystal oscillates—high frequency sound wave penetrates into eye—sound wave encounters a media interface—part of the sound wave is reflected back to the probe

USG doesn’t measure the distance rather it measures the TIME required for a sound pulse to travel from cornea to retina.

cornea 1620m/s

Anterior chamber

1532m/s

Lens thickness

1641m/s

Vitreous cavity

1532m/s

1555m/s

Types of a scan {methods of measurement}

1- ultrasonic measurement –this comprises of –applanationn method immersion technique

2- optical measurement –this uses partial coherence laser ,the iol master measures time required for infrared light to travel to the retina & this technique doesn’t requires contact with the globe. {iol master}

Applanation

Things required:A scan machineProbeAnaesthetic dropsChairPatient

Procedure:

Anaesthetize the eye Touch the probe “DO NOT PRESS OVER THE CORNEA” NOTE THE READING a=initial spike(probe tip& cornea) b=ant lens capsule c=post lens capsule d=retina e=sclera f=orbital fat

Immersion A scan biometry

The ultrasonic beam is coupled to the eye through fluid. Because of no corneal compression the results display true axial

length. Procedure:pt lies supine looking up at the ceiling scleral shell is placed between the eyelids centered over the cornea.

Scleral shellFilled with 40-60 mixture of goniosol & dacriose & the probe tip into the solutionAlign the ultrasound with macula by asking the pt to look to the fixation light of the probe.

Reading

a-probe tipb- cornea- double peaked echo will show both ant & post surfacesc-anterior lens capsuled-posterior lens capsulee- retina this echo needs to havre sharp 90’ take off from baselinef-sclerag-orbital fat

Essentials :

Calibration

Gain or sensitivity

Sound velocity

Calibration: it is done with the help of model eye.Required time to timeInstructions are specific and the model eye is provided with the manufacturers

Gain/sensitivity:

Gain = electronic amplification of the sound waves received by the transducer …{ is called as a decibel(db)}

• Normal is 70% in most of the biometersNormal

•When the echo height is inadequate.•High myopia,dense cataracts,ocular opacitiesIncrease•When artifacts are seen•Eg: silicone oil, pseudophakic eyedecrease

Features of a good scan:

One dimensional image in which spikes of variable hts are seen:

Cornea: single tall peak Aqueous chamber: does not produce any echo Ant & post lens capsule : produce tall echoes Vitreous: no to few echoes Retina: tall,sharp echoes with staircase at the origin Orbital fat produces : medium to low echoes

Keratometry : It is used to measure the corneal curvature.In the optical area i.e 2-3 mm

History :Helmholtz—javal & schiotz---Bausch & laumb{reichert}{1854} {1881} {1932}

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HELMHOLTZ: This keratometer consist of two plates which displaces the image through half of its length & the total displacement gives the size of image.

Javal schiotz

Javal schiotz- This is on the principle of “variable object sizeconstant image size”

The object

Objective lens & doubling prism

The eyepiece lens

The object: 2 mires A & B mounted on an arcA+B= object thus of variable size.“Stepped mirerectangular mire”--divided horizontally through the center.Image of two mires=object.

Bausch & laumb keratometer (1932)“constant object sizevariable image size”

B & L:- THE OBJECT {CIRCULAR MIRE}The imag eof the circular mires appears on the patients cornea appears diminished & serves as object.

The objective lens : from the image of the mire (new object)

DIAPHRAGM & DOUBLING PRISMS: -A 4 APERTURE DIAPHRAGM -A 2 DOUBLING PRISMS{ONE WITH BASE UP AND ONE WITH BASE OUT} moved independently parallel to the central axis

UNIQUE– Image doubling mechanism is unique image is produced side by side or at 90’ thus also k/as “one position keratometer”

Procedure:Instrument:calibration is done with the a steel ball along with the machine with a known radius of curvature mires formed correctly machine is calibrated.PATIENT: chin on chin rest head on head rest h occlude occludes the non examining eye patients pupil & projective knob at the same level

Focusing of mire:center of corneapatients view

examiners view

Reading:perfectly spherical mires.Oval mires horizontal-with the rule astigVertical –against with the rule

Keratoconus:irregular

Jumping of mires Wavy mires

Uses:

--It helps to assess the radius of curvature of cornea. --To monitor the shape of cornea in keratoglobus & keratoconus. --The K readings have been taken to measure the iol power with axial length by SRK formula.

Limitations: refractive status of very small central area of cornea is measured{3-4 mm}

It loses its accuracy when measuring very flat and steep cornea.

Small corneal irregularities would preclude the use due to irregular astigmatism.

Sources of error:Improper calibration

Position of the patient at fault

Any corneal pathology

Examiners fault

Any lid position abnormalitytearing.

Recent advances :automated keratometer

IOL calculation

Formulaes

Theoretical formulaes:

This measures IOL based on principles from schematic eyes.

Regression formulaes:

These formulaes arrived after postoperative outcomes.

With the age the formulaes changed to -First generations

-Second generations -Third generations

-Fourth generations

First generation

Theoretical formulaesBinkhorst formula:P=1136(4r-a)/(a-d)(4r-d)P =iol powerr=corneal radius in mma=axial length in mmd=assumed post op acd plus corneal thickness

Colenbrander –hoffer:P={1336/a-d-.05}-{1336/1336/k-d-.05}

Gill’s formulaP=129.40+(-108*k)+(-2.79*L eye)+(0.26*LCL)+(-0.38*ref)K= ref power in DL eye=AL in mmLCL=dist of apex of ant corneal surfaceRef=desired post op refraction

FYODOROV:P=1336-LK/(l-c)-CK/1336

CLAYMAN’S FORMULA:ASSUME EMMETROPIZING IOL=18DEMMETROPIC AL=24mmEmm avg kertometr reading=42.0 D

Drawback : - cumbersome - guess work - less accurate - wrong prediction - based on simplistic assumptions about the optics of the eye

Regression formula

SRK 1 (sanders ,retzlaff & kraff) a breakthrough in calculating iol: they analysed the post op results and

found that the theoretical formulaes can be odified They replaced ACD with A constant which was unique for diff types of iols

P=A-2.5L-0.9K P= IOL POWER A=A CONSTANT L=AXIAL LENGTH K=AVERAGE KERATOMETRY IN DIOPTRES

SECOND GENERATION

THEORETICAL FORMULAES BINKHORST IN 1981 IMPROVISED IT

BY USING A SINGL EVARIABLE PREDICTOR the AXL and presented with a formula to predict better ACD

REGRESSION FORMULAE The basic were same A-const was modified

<20mm

A+3.0

20-20.99

A+2.0

21-21.00

A+1.0

22-24.5

A

>24.5 A-0.5

<20 mm A+1.520-21mm A+1.021-22mm A+0.522-24.5mm A24.5-26 mm A-1.0>26 mm A-1.5

MODSRK II

SRK II

A-CONSTANT:The concept was originated for the SRK equation & depends on multiple variables including IOL manufacturer,style & placement within the eye.

-Theoretical value that relates the lens power to

AL.-used directly in SRK II-it is not expressed in

units.Specific to the design of

the iol

THE POWER OF THE LENS VARIES 1:1 relationship with the A-constant If A decreases by 1 D,IOL power

decreases by 1 dioptre also.

THIRD GENERATIONHOLLADAY 1 FORMULAHOLLADAY II FORMULAHOFER Φ FORMMULA

FOURTH GENERATION HOLLADAY II FORMULAHOLLADAY CONST. IOL PROGRAMME

FORMULAES AXIAL LENGTHSRK1 22.0-24.5MMHOFFER Q <24.5MMSRK/ T >26.0MMHOLLADAY I NORMAL as well as AL 24.5-

26.0mm4th generation More universal application

A recent study in 2011 showed this formulaes on

8000 eyes

IOL : THE SECOND CHANCE OF VISION

A secondary IOL back up in the OT. The staff should be aware of the power to be used. Proper labelling of the iol along with patient’s name. ACIOL should be calculated & to be kept ready in any eventful condition.