A scan anu101 updated

A SCAN BIOMETRY
PRESENTED BY:-
ANURAG SHUKLA
MODERATOR:
SAPTARSHI MUKHERJEE

BIOMETRY
BIO
(LIFE)
METRY
(MEASURE)
BIOMETRY
(MEASUREMENT OF
TISSUE)
Biometr
y
OCULAR
ULTRASOUN
D
OPTICAL
BIOMETR
Y
(PCI)

OCULAR
BIOMETRY
OCULAR
ULTRASOUND
A-SCAN B-SCAN
M-SCAN
(C-SCAN)
PCI
IOL MASTER LENSTAR

SOUND WAVE
 LONGITUDINAL WAVE
 ALTERNATING COMPRESSIONS AND
RAREFACTIONS OF MOLECULES
 WAVE LENGTH = DISTANCE BETWEEN
BANDS OF COMPRESSION OR
RAREFACTION

“ULTRA”…….SOUND?
AUDIBLE RANGE IS 20 TO 20,000 CYCLES PER SECOND
ULTRASOUND HAS FREQUENCY GREATER THAN 20,000 CYCLES PER SECOND

HISTORY
 FIRST SUCCESSFUL APPLICATION – SONAR IN WORLD WAR 2 (SOUND NAVIGATION
AND RANGING)
 IN 1956, FIRST TIME: MUNDT AND HUGHES, AMERICAN OPH.
A-SCAN (TIME AMPLITUDE ) TO DEMONSTRATE VARIOUS OCULAR DISEASE
 IN 1958, BAUM AND GREENWOOD DEVELOPED THE FIRST TWO-
DIMENSIONAL(IMMERSION) (B-SCAN)
 IN THE EARLY 1960S, JANSSON AND ASSOCIATES, IN SWEDEN
 USED MEASURE THE DISTANCES BETWEEN STRUCTURES IN THE EYE

DIAGNOSTIC OPHTHALMIC ULTRASOUND
• FREQUENCY 8-10 MHZ FOR A SCAN (SANDRA FRAZER ET. AL)
• 8-25 MHZ FOR POSTERIOR SEGMENT & ORBIT (JAGER’S DUANE OPH.)
• 50 MHZ FOR IMAGING ANTERIOR SEGMENT (JAGER’S DUANE OPH.)
• HIGH FREQUENCY=SHORT WAVELENGTH (<0.2MM)=GOOD RESOLUTION=SHORT
OR SLOW PENETRATION

BASIC OF SOUND WAVE
• THE SOUND WAVE IS CHARACTERIZED BY:
• FREQUENCY (HZ): NUMBER OF COMPLETE CYCLES PER UNIT OF TIME.
• VELOCITY (M/S): THE SPEED OF PROPAGATION OF THE WAVE
• WAVELENGTH (M): THE DISTANCE TRAVELED BY ONE CYCLE
VELOCITY = WAVELENGTH X FREQUENCY

FREQUENCY OF SOUND
FREQUENCY IS DEPEND ON SOURCE
• LOWER FREQUENCY
• HIGHER THE PENETRATION AND
• LOWER THE RESOLUTION
• HIGHER FREQUENCY
• LOWER THE PENETRATION AND
• HIGHER THE RESOLUTION

VELOCITY OF SOUND
• VELOCITY AND WAVELENGTH DEPEND ON MEDIUM PROPERTIES
• SOUND TRAVELL FASTER IN SOLID > LIQUID > GAS

FACTOR INFLUENCING THE REFLECTION (ECHO)
• 1. ANGLE OF THE SOUND BEAM
• 2. INTERFACE
• 3. SIZE AND SHAPE OF INTERFACES

REFLECTION OF SOUND WAVE
• REFLECTED SOUND WAVES ARE PRODUCED BY ACOUSTIC INTERFACES THAT HAVE
DIFFERENT ACOUSTIC IMPEDANCES.
• ACOUSTIC IMPEDANCE OF A MATERIAL IS THE OPPOSITION TO DISPLACEMENT OF
ITS PARTICLES BY SOUND AND OCCURS IN MANY EQUATIONS:-
•
Z = ACOUSTIC IMPEDANCE
C = MATERIAL SOUND VELOCITY
R = MATERIAL DENSITY
• THE BOUNDARY BETWEEN TWO MATERIALS OF DIFFERENT ACOUSTIC IMPEDANCES IS
CALLED AN ACOUSTIC INTERFACE
Z=PC

REFLECTIVITY OR ECHO
• WHEN SOUND TRAVELS FROM ONE MEDIUM TO ANOTHER MEDIUM OF
DIFFERENT DENSITY, PART OF THE SOUND IS BACK INTO THE PROBE
• THIS IS KNOWN AS AN ECHO.
• ECHOES: ECHOES ARE PRODUCED BY ACOUSTIC INTERFACES CREATED AT THE
JUNCTION OF TWO MEDIA OF DIFFERENT ACOUSTIC IMPEDANCES.
• ECHO AFFECTED BY- ACOUSTIC INTERFACE, ANGLE OF INCIDENCE,
ABSORPTION. SCATTERING, REFLECTION

1. ANGLE OF INCIDENCE
• WHEN THE SOUND BEAM IS DIRECTED
PERPENDICULARLY TO
A STRUCTURE
– MAXIMUM AMOUNT OF SOUND WILL BE REFLECTED
BACK TO THE PROBE.
• THE FARTHER AWAY FROM THE IDEAL ANGLE , THE
LOWER THE
AMPLITUDE.

2. INTERFACE
• DEPENDS UPON THE DIFFERENCE BETWEEN ACOUSTIC
IMPEDANCE
– GREATER THE DIFF. AI STRONGER THE REFLECTED
ECHOES
EXAMPLE
• – ANTERIOR LENS SURFACE PRODUCE STRONG ECHO
WHEN BORDERED BY AQUEOUS THAN BY BLOOD
• INTERFACE BETWEEN VITREOUS AND FRESH BLOOD IS
VERY SLIGHT RESULTING IN SMALL ECHO.
• THE DIFFERENCE BETWEEN A DETACHED RETINA AND
THE VITREOUS IS GREAT PRODUCING A LARGE ECHO

3. SHAPE AND SIZE OF INTERFACE
• A) SMOOTH SURFACE LIKE RETINA WILL GIVE
STRONG
REFLECTION.
• B) SMOOTH AND ROUNDED SURFACE SCATTER THE
BEAM.
• C) COARSE SURFACE LIKE CILIARY BODY OR
MEMBRANE WITH FOLDS TEND TO SCATTER THE
BEAM WITHOUT ANY SINGLE STRONG REFLECTION.
• D) SMALL INTERFACE PRODUCES SCATTERING OF
REFLECTION.

PENETRATION
• BENDING OF WAVES AS THEY PASS FROM ONE
MEDIUM TO OTHER
• THE CHANGE IN WAVELENGTH AND DIRECTION
OF PROPAGATION OF SOUND OCCURS, BUT
FREQUENCY REMAINS CONSTANT
• ARTIFACTS DUE TO REFRACTION ARE
– LOSS OF RESOLUTION OF IMAGE
– SPATIAL DISTORTION

ABSORPTION
• ULTRASOUND IS ABSORBED BY EVERY MEDIUM THROUGH WHICH IT PASSES
• THE MORE DENSE THE MEDIUM, THE GREATER THE AMOUNT OF ABSORPTION
• WHEN PERFORMING AN USG THROUGH A DENSE CATARACT,
• - MORE OF THE SOUND IS ABSORBED BY THE DENSE CATARACTOUS LENS
• - LESS IS ABLE TO PASS THROUGH TO THE NEXT MEDIUM
• - RESULTING IN WEAKER ECHOES AND IMAGES ON BOTH A-SCAN AND B-SCAN

TRANSDUCER
• TRANSDUCER
• CONVERTS ONE FORM OF ENERGY TO
OTHER.
• THE HEART OF THE TRANSDUCER IS A
PIEZOELECTRIC CRYSTAL.
• ELECTRICAL ENERGY MECHANICAL
ENERGY
• BASIC COMPONENTS –
• PIEZOELECTRIC PLATE
• BACKING LAYER
• ACOUSTIC MATCHING LAYER

TRANSDUCER
 IN A TRANSDUCER THE PIEZOELECTRIC CRYSTAL IS PLACED BETWEEN TWO
ELECTRODES
WHICH BEHAVE AS CAPACITORS.
 THE VOLTAGE BETWEEN THEM PRODUCES AN ELECTRIC FIELD WHICH CAUSES
CHANGE IN SHAPE OF PIEZOELECTRIC CRYSTAL.
 IF THE VOLTAGE IS APPLIED IN MULTIPLE SHORT BURSTS, THE CRYSTAL VIBRATES
AND
GENERATES SOUND WAVES.
 THE BACKING BLOCK DAMPENS THE SOUND WAVES IMMEDIATELY IN ORDER TO
PRIME THE CRYSTAL FOR THE RETURNING ECHOES FROM PATIENTS BODY.

• BACKING LAYER (DAMPING MATERIAL: METAL POWDER WITH PLASTIC OR EPOXY)
• LOCATED BEHIND THE PIEZOELECTRIC ELEMENT
• DAMPENS EXCESSIVE VIBRATIONS FROM PROBE
• IMPROVES AXIAL RESOLUTION
• ACOUSTIC MATCHING LAYER
• LOCATED IN FRONT OF PIEZOELECTRIC ELEMENT
• REDUCES THE REFLECTIONS FROM ACOUSTIC IMPEDANCE BETWEEN PROBE AND
OBJECT
• IMPROVES TRANSMISSION

PIEZOELECTRIC ELEMENTS
• “ PIEZO” ( PRESSURE ) + ELECTRIC ( PRODUCE ULTRASOUND WAVE)
• THE “PIEZOELECTRIC EFFECT” WAS DESCRIBED 1880 PIERRE AND
JACQUES CURIE
• A ELEMENTS WHICH CONVERT THE FORM OF ENERGY
• SOUND (PRESSURE) ENERGY ELECTRIC ENERGY
1.Electrical Energy
converted to Sound
waves
2. The Sound
waves
are reflected by
tissues
3.Reflected
sound wave
converted to
electric energy

CHARACTERISTICS OF ULTRASOUND
BEAM
 INTENSITY OF THE ULTRASOUND BEAM VARIES ALONG THE LENGTH OF THE
BEAM.
 THE BEAM HAS A NATURAL TENDENCY TO DIVERGE.
 THE PARALLEL COMPONENT IS CALLED THE NEAR ZONE OR ‘FRESNEL ZONE’.
 DIVERGING PORTION OF THE BEAM IS CALLED FAR ZONE OR ‘FRAUNHOFFER
ZONE’.
 FRESNEL ZONE IS LONGER WITH
• 1. LARGER TRANSDUCERS AND
• 2. HIGH FREQUENCY SOUND.

PRINCIPLE
PULSE- ECHO SYSTEM
• EMISSION OF MULTIPLE SHORT PULSES OF ULTRASOUND WAVES WITH BRIEF
INTERVAL TO DETECT, PROCESS AND DISPLAY THE TURNING ECHOES
Electric current
Transducer
Surfac
e
US wave

SIGNAL PROCESS IN ULTRASOUND DEVICE
OUTPU
T
DEVICE
Signal
Processo
r
AMPFIRE
(amplify
the weak
signal)
Pulse
Generat
or
Receiver
(very low
frequency
)
TRANSDUCE
R

AMPLIFICATION
• AMPLIFICATION CAN BE OF 3 DIFFERENT TYPES: LINEAR,
LOGARITHMIC, OR S-SHAPED.
• THE RANGE OF ECHO INTENSITIES CAN BE DESCRIBED IN UNITS
OF DECIBELS.
• A SMALL DYNAMIC RANGE, CHARACTERISTIC OF LINEAR
AMPLIFIERS, CAN DISPLAY MINOR DIFFERENCES IN ECHO
STRENGTH BETWEEN ECHO SOURCES, BUT THE RANGE IS VERY
LIMITED.
• A LARGE DYNAMIC RANGE, CHARACTERISTIC OF LOGARITHMIC
AMPLIFIERS DISPLAY A WIDE RANGE OF ECHO INTENSITIES BUT
SHOW SLIGHT DIFFERENCES BETWEEN ECHO SIGNALS.
• THE S-SHAPED AMPLIFIERS IS THE COMBINATION OF THE
LOGARITHMIC AMPLIFIER AND THE LINEAR AMPLIFIER.

MODE OF ULTRASOUND
• A-MODE- AMPLITUDE MODE.
• B-MODE- BRIGHTNESS MODE.
• M-MODE ( C- MODE)- MOTION MODE

A MODE ( A SCAN )
• AMPLITUDE-MODE
• A TYPE OF ULTRASONOGRAPHY
 ONE DIMENSIONAL MEASURING SYSTEM
 ECHOES REPRESENTED AS VERTICAL SPIKES FROM BASELINE
• USE- TO JUDGE THE DEPTH OF AN ORGAN.
TO CALIBRATION OF OTHER MODE

B MODE
• BRIGHTNESS-MODE
• TWO DIMENSIONAL MEASURING SYSTEM
• HIGH REFLECTIVITY (SOLID AREA) APPEAR WHITE
• LOW REFLECTIVITY (FLUID AREA) APPEARS BLACK
GRAY
SCALE

M MODE
• MOTION-MODE
• ALSO CALLED TIME-MOTION MODE
• USE TO ANALYSE MOVING PART
TO SEE ACCOMMODATIVE PROCESS

INTRODUCTION
• MEASURING VARIOUS DIMENSIONS OF THE EYE, ITS COMPONENTS AND THEIR
INTERRELATIONSHIPS, AND USING THIS DATA TO DETERMINE THE IDEAL
INTRAOCULAR LENS POWER.
• ESSENTIALLY CONSISTS OF A KERATOMETRY READING TOGETHER WITH AN
ULTRASONIC MEASUREMENT OF AXIAL LENGTH OF THE EYE.

BRIEF HISTORY OF BIOMETRY
• THE FIRST IOL WAS IMPLANTED BY SIR HAROLD RIDLEY ON NOVEMBER 29, 1949.
• THE FIRST BIOMETRY A SCAN MACHINES WERE INTRODUCED IN 1970. HENCE
IOL IMPLANTATION PREDATES BIOMETRY AND A SCAN MACHINES

A SCAN DISPLAY
• THE DISPLAY MAY BE IN ONE OF THE TWO MODES
• A) A MODE( AMPLITUDE)- 1D DISPLAY
• TIME AMPLITUDE DISPLAY
• ECHOES REPRESENTED AS VERTICAL SPIKES
• SPIKES REPRESENTS REFLECTIVITY, LOCATION AND
SIZE OF ANATOMIC STRUCTURE
• X AXIS SHOWS TIME ELAPSED( FUNCTION OF
TISSUE DEPTH)
• Y AXIS- REFLECTIVITY IN DECIBELS

TERMINOLOGY
• GAIN :- SETTING OF ADJUSTING THE AMPLIFICATION OF THE ECHO SIGNAL
• THIS IS SIMILAR TO TURNING THE VOLUME UP OR DOWN.
• MEASURED IN DECIBELS
• THE HIGHER THE GAIN LEVEL, THE GREATER THE SENSITIVITY IN DISPLAYING WEAKER
ECHOES AND DECREASES THE AXIAL AND LATERAL RESOLUTION AND VICE VERSA
GAIN
CHANGE
NO CHANGE IN AMOUNT
OF EMITTED ENERGY
INTENSITY OF
RETURNING ECHO
CHANGE

• HIGHER THE GAIN, BETTER THE SENSITIVITY, BUT THE RESOLUTION GETS
COMPROMISED
• AT LOW GAINS THE SENSITIVITY IS LESS, BUT THE RESOLUTION IS GOOD.
• USE OF GAIN IN DIFFICULT SITUATIONS- GAIN REFERS TO ELECTRONIC
AMPLIFICATION OF THE SOUND WAVES RECEIVED BY THE TRANSDUCER.
• INCREASE IN GAIN IS REQUIRED WHEN HEIGHT OF ECHOES ACHIEVED IS
INADEQUATE AS IN DENSE CATARACTS.
• DECREASE IN GAIN IS REQUIRED WHEN ARTEFACTS ARE SEEN NEAR THE RETINAL
ECHOES AS IN SILICONE FILLED EYES, PSEUDOPHAKIA EYES.

VELOCITIES OF SOUND THROUGH OCULAR PARTS
MEDIUM VELOCITY(M/SEC)
AIR 331 M/SEC
CORNEA 1,641 M/SEC
AQUEOUS & VITREOUS 1,532 M/SEC
PHAKIC EYE 1,641 M/SEC
APHAKIC EYE 1,532 M/SEC
SILICON OIL 987 M/SEC
PSEUDOPHAKIC EYE 1,532 M/SEC + CORRECTION FACTOR FOR IOL LENS
 DEPENDS ON THE DENSITY AND PROPERTIES OF MEDIUM.
 TAKES 33 MICRO SEC TO COME BACK FROM POSTERIOR POLE TO TRANSDUCER

IOL MATERIAL AND THEIR VELOCITY
MATERIAL VELOCITY
PMMA 2,713 m/s (2,780 m/sec in 35*c eye temp.)
ACRYLIC 2,078 m/s (2,180 m/sec in 35*c eye temp.)
1ST GENERATION SILICON IOL 990 m/s (980 m/sec in 35*c eye temp.)
2ND GENERATION SILICON IOL 1,090 m/s (1080 m/sec in 35*c eye temp.)
HYDROGEL 2,000 m/s
HEMA 2,120 m/s
COLLAMER 1,740 m/s

AXIAL LENGTH MEASUREMENT
• • 2 TYPES
1. ULTRASOUND
2. OPTICAL

ULTRASOUND BASED A-SCAN
• PRINCIPLE- THE ULTRASOUND PROBE HAS A PIEZOELECTRIC CRYSTAL THAT
ELECTRICALLY EMITS AND RECEIVE HIGH FREQUENCY (10MZ) SOUND WAVES.
• MEASUREMENT IS FROM ANTERIOR CORNEAL SURFACE TO INTERNAL LIMITING
MEMBRANE.
• 1 MM ERROR LEADS TO 2.5D ERROR IN POSTOPERATIVE REFRACTION.
• ERRORS OF 2.00D OR MORE ARE ALMOST ALWAYS A-SCAN RELATED.

INSTRUMENTATION OF A-SCAN:
THERE ARE TWO TECHNIQUES FOR MEASURING THE AXIAL EYE LENGTH WITH A-
SCAN ULTRASOUND:
 CONTACT TECHNIQUE
 IMMERSION TECHNIQUE .
• IN BOTH TECHNIQUES THE SOUND BEAM MUST BE DIRECTED ALONG THE OPTICAL
AXIS OF THE EYE, PERPENDICULAR TO THE MACULA

CONTACT TECHNIQUE
1.. PROBE IS PLACED DIRECTLY ON CORNEA.
2. I, INITIAL SPIKE CORRESPONDING TO PROBE TIP ON
CORNEA
3. A, ANTERIOR LENS CAPSULE
4. P, POSTERIOR LENS CAPSULE
5. R, RETINA
6. S, SCLERA

FOR IMMERSION METHODS
1.PROBE IS PLACED ON FLUID WITHIN IMMERSION SHELL
(NOT TOUCHING CORNEA)
2.I, INITIAL SPIKE CORRESPONDING TO TIP OF PROBE IN
FLUID
3.C, CORNEA
4.A, ANTERIOR LENS CAPSULE
5.P, POSTERIOR LENS CAPSULE
6.R, RETINA
7.S, SCLERA

CONT….
• CONTACT PROBES: 1ST GENERATION BIOMETERS USED
WATER-FILLED PROBES WITH A SOFT MEMBRANOUS TIP
TO MINIMIZE CORNEAL INDENTATION.
• IT NEEDS TO BE FILLED FREQUENTLY WITH DISTILLED
WATER AND THUS SMALL AIR BUBBLES TRAPPED IN WATER
CHAMBER THUS ERRONEOUS AXIAL LENGTH READING
• THE MORE CURRENT BIOMETERS USE A SOLID PROBES
AVOIDS THESE PROBLEMS BUT IT CAN EASILY INDENT
CORNEA

INSTRUMENT SETTINGS:
1.GAIN: FIRST, NEARLY MAXIMAL GAIN TO DISPLAY HIGHLY
REFLECTIVE SPIKES AS THE SOUND BEAM IS DIRECTED
PERPENDICULAR TO VARIOUS INTERFACES ALONG THE
OPTICAL AXIS. THEN REDUCE GAIN TO CLEARLY VISUALIZE
THE PEAKS OF THE SPIKES.
2.MEASUREMENT MODE: IT CAN BE AUTOMATIC OR MANUAL
MODE. IN AUTOMATIC MODE, THE “PATTERN RECOGNITION”
MODE INDICATE THE COMPLETION OF MEASUREMENT OF
THE AXIAL LENGTH IN AUDIBLE TONE. IN MANUAL MODE,
THE EXAMINER CAN TAKE ORE TIME TO ALIGN THE SOUND
BEAM ALONG THE OPTICAL AXIS OF THE EYE IN ORDER TO
SELECT THE BEST SCAN FOR MEASUREMENT
3.SOUND VELOCITY: THE USE OF APPROPRIATE SOUND
VELOCITIES IS IMPORTANT FOR ACCURATE AXIAL LENGTH
MEASUREMENTS.

CONT…
• GATES
MODE
AUTO
MANUA
L
GATE SYSTEM
TWO
GATE
FOUR
GATE
1. CORNE
A
2. RETINA
1. CORNEA
2. AL
SURFACE
3. PL
SURFACE
4. RETINA

PROCEDURES OF PERFORMING A-SCAN
• INCASE OF THE USE FOR DETERMINING IOL POWER, THE KERATOMETRY READING
IS PREFERABLY DONE FIRST TO AVOID MILD DISTORTION OF THE CORNEAL
MIRES.
• WHEN USING WATER FILLED PROBE, IT SHOULD BE CHECKED TO ENSURE THAT
AIR BUBBLES ARE NOT PRESENT
• THE ROOM LIGHTING SHOULD BE DIMMED
• INSTILLED ANAESTHETIC EYEDROP IN BOTH EYES OF THE PATIENT JUST PRIOR
TO BEGINNING THE EXAMINATION.
• BOTH THE EXAMINER AND THE PATIENT SHOULD BE POSITIONED COMFORTABLY
AND THE INSTRUMENT CHECKED FOR PROPER SETTINGS ((GAIN, MEASUREMENT
MODE, SOUND VELOCITY, AND GATE POSITION WHILE KEEPING IN MIND
WHETHER THE EYE IS PHAKIC, APHAKIC, OR PSEUDOPHAKIC

CONTACT TECHNIQUE
o THE PROBE IS DIRECTLY PLACED ON THE CORNEA
AND THE SOUND BEAM IS DIRECTED ALONG THE
OPTICAL AXIS.
o THIS CAN BE DONE BY HAND-HELD OR BY
APPLANATION.
o WITH HAND-HELD, THE PATIENT IS USUALLY
INCLINED.
o WITH APPLANATION METHOD, THE PATIENT IS
NORMALLY SEATED IN AN UPRIGHT POSITION.
o IN APPLANATION METHOD, THE PROBE IS MOUNTED
IN A PRESSURE-SENSITIVE, SPRING-LOADED SLEEVE
ON A SLIT LAMP OR SIMILAR APPARATUS.

CONT….
o THE PATIENT IS INSTRUCTED TO FIXATE IN PRIMARY GAZE
OR GIVEN A FIXATION LIGHT WITHIN THE CENTER OF THE
PROBE.
o INITIALLY, THE JOY STICK IS FULLY RETRACTED WITH
PROBE AWAY FROM EYE AND THEN THE JOYSTICK IS
SLOWLY ADVANCED UNTIL THE PROBE JUST TOUCHES THE
CENTER OF THE CORNEA AND THEN REMOVED AFTER THE
MEASUREMENT IS OBTAINED.
o THE PROCEDURE IS REPEATED SEVERAL TIMES UNTIL
THREE TO FIVE HIGH-QUALITY, CONSISTENT
MEASUREMENTS (GENERALLY WITHIN 0.3 STANDARD
DEVIATION) IS OBTAINED.

CONTACT TECHNIQUE
• : PHAKIA
• FOUR SPIKES APPEAR IN THE ECHOGRAM.
• THE INITIAL SPIKE REPRESENTS THE CORNEAL SURFACE.
• THE FIRST SPIKE REPRESENTS THE ANTERIOR LENS SURFACE.
• THE SECOND SPIKE REPRESENTS THE POSTERIOR LENS SURFACE.
• THE THIRD SPIKE REPRESENTS THE RETINAL SURFACE
• THE FOURTH SPIKE REPRESENTS THE SCLERA
• A VERY DENSE CATARACT MAY HAVE ONE OR MORE SPIKES WITHIN
THE LENS OR MULTIPLE SIGNALS OF IRREGULARLY SPACED AFTER THE
POSTERIOR SURFACE OF THE LENS.
• VITREOUS OPACITIES MAY ALSO PRODUCE HIGHLY REFLECTIVE SPIKES
ALONG THE BASELINE.
• IN ORDER TO DISTINGUISH MULTIPLE SIGNALS OR VITREOUS OPACITIES
FROM THE RETINAL SPIKE, THE GAIN SHOULD BE REDUCED. THE
RETINAL SPIKES REMAINS WHILE THE SPIKES ALONG THE VITREOUS
BASELINE DISAPPEAR

APHAKIA
DENSE
CATARACT
WHEN INCREASE THE
GAIN

SOURCES OF ERROR
• – CORNEAL COMPRESSION
(SHORTER AXIAL LENGTH)
• • 1MM ERROR IN AXIAL LENGTH –
2.5 TO 3.0 DS ERROR IN IOL POWER
• – MISALIGNMENT OF SOUND BEAM
• – FLUID MENISCUS TRAPPED
• ERRONEOUSLY LONG AL

IMMERSION TECHNIQUE
• • CAN USE IN THE SAME INSTRUMENT
• – REQUIRES SCLERAL CUP
• • COUPLING AGENT – METHYLCELLULOSE
• • PROBE IS NOT DIRECTLY PLACED ON THE CORNEA
– IMMERSED INTO THE FLUID
• ERROR
• – SMALL AIR BUBBLES IN THE FLUID
• GIVES FALSELY LONG AL MEASUREMENT
Prager Scleral
Shell

IMMERSION TECHNIQUE
• THIS METHOD EMPLOYS A SMALL WATER BATH SO THAT THE PROBE IS NOT PLACED
DIRECTLY ON THE CORNEA ALLOWING THE DISPLAY OF A SEPARATE CORNEAL SPIKE
THAT IS NOT SEEN WITH THE CONTACT METHOD.
• THE PATIENT IS TYPICALLY RECLINED, WITH THE EYE TO BE MEASURED POSITIONED
CLOSE TO THE SCREEN.
• INSTILL THE TOPICAL ANAESTHETIC DROP IN BOTH EYE.
• SCLERAL SHELL IS INSERTED BETWEEN THE LIDS AND IS FILLED ABOUT TWO-THIRDS
FULL OF METHYLCELLULOSE.
• THE PATIENT FIXATES ON A TARGET IN PRIMARY GAZE OR ON THE FIXATING TARGET
IN THE PROBE, AND THE PROBE IS IMMERSED INTO THE FLUID.
• BEGINNING AT A HIGH GAIN SETTING, THE EXAMINER DIRECTS THE SOUND BEAM
PERPENDICULAR TO THE CORNEA, THE ANTERIOR AND POSTERIOR LENS SURFACES,
THE RETINA, AND THE SCLERA.

IMMERSION TECHNIQUE
• ONCE STEEPLY RISING, HIGHLY REFLECTIVE SPIKES ARE DISPLAYED FROM THESE
INTERFACES AT A HIGH GAIN SETTING, THE DECIBEL LEVEL IS REDUCED FOR
IMPROVED RESOLUTION.
• AS THE GAIN IS TURNED DOWN, THE DISPLAYED SPIKES DECREASE IN HEIGHT,
TO ENSURE THAT ALL SPIKES REMAIN AS HIGH AND DISTINCT AS POSSIBLE
INDICATING PERPENDICULARITY.
• AT LEAST 3 HIGH QUALITY ECHOGRAMS WITH A STANDARD DEVIATION OF 0.3
MM SHOULD BE OBTAINED.
• ADVANTAGES: NO CORNEAL COMPRESSION OCCURS, THE PROBLEM OF FLUID
MENISCUS BETWEEN THE PROBE TIP AND CORNEA IS AVOIDED. SEPARATE
CORNEAL SPIKES MAKES IT EASIER TO DETERMINE WHEN THE SOUND BEAM IS
PROPERLY ALIGNED ALONG THE OPTICAL AXIS.

IMMERSION TECHNIQUE
Initial spike (IS), the anterior (C1) and posterior (C2) corneal
surfaces,
the anterior (L1) and posterior (L2) lens surfaces, the retina
(R), sclera
(S), and orbital tissues (O).

APPLANATION AND IMMERSION TECHNIQUE
APPLANATION IMMERSION
ADVANTAGES DISADVANTAGES ADVANTAGES DISADVANTAGES
1. Convenient
2. Portable
3. Accurate with trained
examiner
4. Less expensive
1. Risk of excess pressure resulting
in falsely long or shorter reading
2. Risk to abrade the cornea
3. Less accurate in very short AL
eyes
4. Problems due to misalignment
5. Fluid meniscus between probe tip
and the cornea
1. No risk of inaccurate
reading from excess
pressure applied
2. No risk for corneal
abrasion
1. require separate area of
clinical space
2. Patient must be in supine
3. methylcellulose blurs
vision
4. More time require to learn
and to obtain a good
reading

SELECTION OF EYE STATUS MODE
• SELECTTHEAPPROPRIATEEYESTATUS
• VITREOUSCAVITYSTATUS
DENSE
/
LONG
APHAKI
A
PSEUDOPHAKI
A
SILICON
PHAKIC &
CATARACT
APHAKIC
CONDITION
IOL IMPLANTED
CONDITION
NORMA
L THE VELOCITY IS 1532 m/sec
TWO TYPE SILICON OIL
1000 CS & 5000 CS
1000 cs => 980 m/sec
5000 cs => 1080 m/sec
Change the velocity according the
Lens material

INTRAOCULAR LENS POWER CALCULATIONS
• CHOOSING THE APPROPRIATE IOL POWER IS A MAJOR DETERMINANT OF PATIENT
SATISFACTION WITH CATARACT SURGERY
• 3 MAIN FACTOR
ACCURATE MEASUREMENTS (BIOMETRY)
I. SELECTING CALCULATIONS FORMULAS
II. AND ASSESSING THE PATIENT’S NEEDS TO DETERMINE POSTOPERATIVE
REFRACTIVE TARGET

FORMULA
• DEPENDING UPON THE BASIS OF THEIR DERIVATION:
• • THEORETICAL &
• • REGRESSION FORMULAE
• • GROUPED INTO VARIOUS GENERATION

GENERATIONS OF FORMULA
3RD GENERATION 4TH GENERATION1ST GENERATION
• BRINKHROST
2ND
GENERATION
• CLAYMAN
• SRK- I
MODIFIED
BRINKHROST
SRK-II
SRK/T
HOFFER
Q
HOLLADAY
HOLLADAY- II

 THEORETICAL FORMULAE
 DERIVED FROM THE GEOMETRIC OPTICS AS APPLIED TO THE
SCHEMATIC EYES, USING THEORETICAL CONSTANTS.
 BASED ON 3 VARIABLES – AL, K READING AND ESTIMATED
POSTOPERATIVE ACD.
 REGRESSION FORMULAE
BASED ON REGRESSION ANALYSIS OF THE ACTUAL POSTOP RESULTS OF
IMPLANT POWER AS A FUNCTION OF THE VARIABLES OF CORNEAL
POWER AND AL

USG A SCAN IOL CALCULATION
FORMULAS

FIRST GENERATION
BINKHORST FORMULA (1972)
• P= 1.336(4R-A)/(A-D)(4R-D)
• WHERE P - POWER OR IOL
• R – CORNEAL RADIUS
• A – AL
• D – ASSUMED POSTOP ACD PLUS CT
THEORITICAL FORMULA

 CLAYMAN’S FORMULA (1973)
ASSUME, EMMETROPIZING IOL=18D
 EMMETROPIC AL=24MM
KERATOMETERY READING = 42 D
 IF IOL POWER>21D, DEDUCT 0.25 FOR EVERY DIOPTRE > 18
THEORITICAL FARMULA

DRAWBACK OF THEORITICAL FORMULA
I. RELIABLE FOR EYES WITH AL BETWEEN 22 AND 24.5MM
II. TEND TO PREDICT TOO LARGE VALUE IN SHORT EYES
(<22MM) AND TOO SMALL VALUE IN LONG EYES(>24.5MM)
III. ASSUMPTION ABOUT THE OPTICS OF THE EYE
IV. STILL REQUIRES A GUESS ABOUT AC DEPTH

SRK FORMULA
REGRESSION FORMULA
• EMPIRIC FORMULAS GENERATED BY AVERAGING LARGE NUMBERS OF POST-
OPERATIVE CLINICAL RESULTS
• INTRODUCED BY SANDERS, RETZLAFF AND KRAFF
• 1980S; POPULAR BECAUSE IT WAS SIMPLE TO USE

A CONSTANT
• A THEORITICAL VALUE RELATED TO
• LENS POWER TO AXIAL LENGTH AND KERATOMETRY
• SPECIFIC TO DESIGN OF IOL AND LOCATION AND ORIENTATION IN EYE
• SPECIFIED BY THE IOL MANUFACTURER
SRK FORMULA - Suitable to use on axial length range : 22mm- 24.5mm

SRK FORMULA
• P= IOL POWER TO BE USED (D)
• A = IOL SPECIFIC A CONSTANT
• K = AVERAGE CORNEAL REFRACTIVE POWER (D)
• L = AXIAL LENGTH OF THE EYE (MM)
P = A – 2.5 L – 0.9
K

SECOND GENERATION
 BASED ON REGRESSION ANALYSIS
 2ND GENERATION OF SRK FORMULA
 OPTIMIZED A CONSTANT BASED ON AXIAL LENGTH OF THE EYE
• INCREASE THE A CONSTANT FOR SHORTER EYE
• DECREASE THE A CONSTANT FOR LONGER EYE
 THE NEW SRK II FORMULA WAS MORE ACCURATE THAN THE ORIGINAL SRK AND
BINKHORST II FORMULAE
SRK –II FORMULA

SRK- II FORMULA
• P= IOL POWER TO BE USED (D)
• A = IOL SPECIFIC A CONSTANT
• K = AVERAGE CORNEAL REFRACTIVE POWER (D)
• L = AXIAL LENGTH OF THE EYE (MM)
P = A – 2.5 L – 0.9
K

• A CONSTANT
• OPTIMIZED BASED ON AXIAL LENGTH
A1 = (A – 0.5) for axial length greater than
24.5mm
A1 = (A) for axial length between 22 and
24.5mm
A1 = (A + 1) for axial length between 21 and
22mm
A1 = (A + 2) for axial length between 20 and
21mm
A1 = (A + 3) for axial length less than 20mm

3RD GENERATION FORMULA :
HOFFER Q, HOLLADAY 1, SRK/T

Merger of the linear regression methods with theoretical
eye models
3RD
GENERATION
HOFFER Q
(pseudophakic
ACD)
HOLLADAY
(surgeon factor)
SRK/T
(A constant)

3RD GENERATION FORMULA
 IMPROVED ACCURACY
 BETTER RESULT AND SIMPLE
 TAKE INTO ACCOUNT OF
AXIAL LENGTH
K-READING
 OPTIMIZATION OF FORMULA
to predict the effective lens position ELP

ASSUMPTIONS IN ELP
• ERRORS IN PREDICTING THE ELP CAUSED: REFRACTIVE SURPRISE
• SHALLOW AC SITTING MORE ANTERIOR LOWER IOL POWER
• SHORT EYE
• FLAT K
• LONGER EYE
• STEEPER K
SHALLOW
ELP
DEEPER ELP

HOFFER Q
• INTRODUCED BY DR KENNETH HOFFER IN 1993
• WAS DEVELOPED TO PREDICT THE PSEUDOPHAKIC ANTERIOR CHAMBER DEPTH
(ACD)
• IT RELIES ON A PERSONALIZED ACD , AXIAL LENGTH AND CORNEAL CURVATURE
• PERSONALISED ACD-CONSTANT = 0.58357
• PERSONALISED A-CONSTANT – 63.896

USE OF HOFFER Q
• HYPEROPES (AL < 22 MM) (KENNETH HOFFER)
• MOST ACCURATE IN SHORT EYES < 22.0MM, CONFIRMED IN LARGE STUDY OF
830 SHORT EYES
• HAD THE LOWEST MEAN ABSOLUTE ERROR (MAE) FOR AL 20.0MM TO 20.99MM
• HOFFER Q AND HOLLADAY 1 HAD LOWER MAE THAN SRK/T FOR AL 21.0MM TO
21.49MM
• IN POST CORNEAL REFRACTIVE SURGERY

IOL POWER ERROR IN POST CORNEAL REFRACTIVE
SURGERY
• CONTRIBUTION OF IOL POWER ERRORS:
I. INACCURATE MEASUREMENT/CALCULATION OF ANTERIOR CORNEAL POWER
(ESPECIALLY IN THOSE REMOVE CORNEAL TISSUE I.E PRK)
II. INCORRECT ESTIMATION OF ELP
Flat central corneal power after LASIK, the formula assumes that the
AC is shallow
Myopic-LASIK: underestimation of the IOL power
Hyperopic-LASIK: overestimation of the IOL power

IOL POWER ADJUSTMENT IN LASIK
ARAMBERRI DOUBLE K MODIFICATION
DOUBLE K FORMULA
• K-READING BEFORE REFRACTIVE SURGERY IS USED TO ESTIMATE THE ELP
• K-READING AFTER REFRACTIVE SURGERY IS USED TO CALCULATE THE IOL
POWER
• TRADITION METHOD: SINGLE K FORMULA
• K-READING IS USED FOR BOTH CALCULATIONS
• TENDS TO UNDERESTIMATE THE IOL POWER IN MYOPIC LASIK EYES

SINGLE K FORMULA
• NUMBERS IN EACH ROW REPRESENT
THE AMOUNT (D) THAT MUST BE
ADDED TO THE CALCULATED IOL
POWER
• NUMBERS IN EACH ROW REPRESENT THE
AMOUNT (D) THAT MUST BE
SUBTRACTED TO THE CALCULATED IOL
POWER
MYOPIC CORRECTION HYPEROPIC CORRECTION

HOLLADAY -I
• PRODUCED BY JACK HOLLADAY IN 1988
• USED AXIAL LENGTH AND KERATOMETRY TO DETERMINE ELP
• WORK BEST FOR EYES BETWEEN 24.5 TO 26 MM (MEDIUM LONG)
• TAKES INTO ACCOUNT AC DEPTH, LENS THICKNESS AND CORNEAL RADIUS
• USEFUL FOR AXIAL MYOPIA
• CALCULATES PREDICTED DISTANCE FROM CORNEA TO IRIS PLANE + DISTANCE
FROM IRIS PLANE TO IOL
• USES SURGEON FACTOR FOR OPTIMIZATION OF FORMULA (SPECIFIC FOR EACH
LENS)

SURGEON FACTOR
• DISTANCE BETWEEN IRIS PLANE & IOL OPTIC PLANE
• SF SHOULD BE PERSONALIZED
• A CHANGE IN THE TRUE POST-OPERATIVE AC DEPTH WILL AFFECT THE REFRACTIVE
STATUS OF THE EYE.
• A CHANGE IN 1 MM CAUSES A 1.5 D CHANGE IN THE FINAL REFRACTION
• SF CONSTANTS MUST BE PERSONALIZED TO ACCOMMODATE ANY CONSISTENT SHIFT
THAT MIGHT AFFECT IOL POWER CALCULATION
• EACH CONSTANT HAS TO BE BACK CALCULATED FOR AT LEAST 20 CASES, WITH
CARE TO ENSURE THAT THE SAME PERSON TAKES THE MEASUREMENTS.

SRK/T
• 1983, OVER THE YEARS, SURGEONS DISCOVERED THAT THE SRK FORMULA IS BEST
USED IN EYES WITH AVERAGE AL, BETWEEN 22.00 AND 24.50 MM.
• 1989, A SUBSEQUENT FORMULA, THE SRK II, WAS DEVELOPED FOR USE IN LONG AND
SHORT EYES.
• 1990, EVEN MORE CUSTOMIZED FORMULAS ARE REQUIRED TODAY TO CALCULATE
ANTERIOR CHAMBER DEPTH (ACD)
• BASED ON AL AND CORNEAL CURVATURE. THE SRK/T (T FOR THEORETICAL) IS ONE
SUCH FORMULA

SRK/T
• RECOMMENDED FORMULA USAGE : BEST FOR EYES LONGER THAN 26.00 MM.
• IT CAN BE CALCULATED USING THE SAME A CONSTANTS USED WITH THE ORIGINAL
SRK FORMULA OR WITH ACD ESTIMATES
• “A-CONSTANT” IS ADJUSTABLE & DEPENDS ON MULTIPLE VARIABLES INCLUDING IOL
MANUFACTURER, STYLE AND PLACEMENT WITHIN THE EYE.
• DIFFERENT MODEL OF IOL , HAS DIFFERENT A-CONSTANT.
• EG ~
• IOL BRAND NO. 1 : A-CONSTANT OF 118.4 = +21.0 D
• IOL BRAND NO. 2 : A-CONSTANT OF 118.9 = +21.5 D
• TO GET THE SAME PLANO POSTOP REFRACTION

CONCLUSION OF 3RD GENERATION IOL FORMULA
• HOFFER Q < 22MM
• HOLLADAY 1 24-26MM
• SRK/T >26MM

4TH GENERATION :
HOLLADAY - II

HAIGIS FORMULA
• DEVELOPED BY WOLFGANG HAIGIS.
• BY REGRESSION ANALYSIS, THE 3
CONSTANTS (A0, A1, A2) ARE
CALCULATED INDIVIDUALLY TO
CLOSELY REPRODUCE OBSERVED
RESULTS OVER A WIDE RANGE OF
AXIAL LENGTHS AND ANTERIOR
CHAMBER DEPTHS.

HOLLAYDAY-II HISTORY
• IOL POWER CALCULATIONS WERE FIRST DEVELOPED OVER 100 YEARS AGO.
• FIRST GENERATION: “SINGLE VARIABLE” FORMULAS
• MEASUREMENT OF AXIAL LENGTH
• AN ASSUMED ANTERIOR CHAMBER DEPTH (ACD) OF 4.5 MM
• THIRD GENERATION:
• 1988-HOLLADAY 1 FORMULA ADDED KERATOMETRY TO OFFER THE FIRST “TWO
VARIABLE” FORMULA, WHICH HELPED IMPROVE ACCURACY IN SHORT AND LONG
EYES.
HOLLADAY 1, HOFFER Q, SRK-T :
• ASSUMED ANTERIOR SEGMENT SIZE WAS DIRECTLY RELATED TO AXIAL LENGTH
RESULTED IN “SURPRISE” OUTCOMES SPECIALLY IN SMALL EYE

HOLLADAY –II IOL POWER CALCULATION FORMULA
• HOLLADAY 2 FORMULA DETERMINES EFFECTIVE LENS POSITION (ELP) USING 7
PARAMETERS
• HOLLADAY 2 FORMULA HAS BEEN CONSIDERED AS ONE OF THE MOST ACCURATE IOL
FORMULA TODAY
AXIAL
LENGTH
K READING
WHITE TO
WHITE
PRE-OP
RX.
ACD
LENS
THICKNES
S
PATIENT
AGE

HOLLADAY -II
• THIS FORMULA HAS BEEN FOUND TO BE HIGHLY ACCURATE FOR A LARGE
• VARIETY OF PATIENT EYES.

RANGE OF AXIAL LENGTH & PREFERRED FORMULA
AXIAL LENGTH (MM)
FORMULA
< 20 MM
HOLLADAY-II
20-22 MM HOFFER Q
22 – 24.5 MM SRK/T / Hoffer
Q/Holladay (average)
> 24.5-26 MM
HOLLADAY-I
>26 MM SRK/T

ACHIEVING ACCURATE PSEUDOPHAKIC A-SCAN
• A BETTER WAY TO PERFORM PSEUDOPHAKIC IMMERSION IN APHAKIC MODE
• IN THE AMERICAN JOURNAL OF OPHTHALMOLOGY HOLLADAY AND PRAGER
HAVE DESCRIBED THIS ELEGANT METHOD FOR MEASURING PSEUDOPHAKIC EYES
AS FOLLOWS:
• TAL= TRUE AXIAL LENGTH
• AAL 1532 = IOL POWER
• CF = CONVERSION FACTOR
• T = THICKNESS OF IOL
TAL = AAL 1532 + ( cf x t )

• CONVERSION FACTOR- DEPENDING ON THE STYLE AND MANUFACTURER)
• CF = 1 – (VE/VIOL)
• VE- VELOCITY USED (APHAKIC= 1532 M/S )
• VIOL- VELOCITY OF IOL MATERIAL
• CALF = CF × T
CALF – CALCULATING AXIAL LENGTH FACTOR
T- THICKNESS OF IOL
TO AVOID ERRORS, OBTAIN THE EXACT THICKNESS OF THE LENS DIRECTLY FROM THE
MANUFACTURER.
CF For an acrylic lens
CF = 1-(1532/2180)
= 1- 0.70
=0.30
CF For an PMMA
lens
CF =1-
(1532/2780)
=1- 0.55
= 0.45
CF For an NEW silicon lens
CF = 1- (1532/1,090 )
= 1- 1.40
= -0.60
NOTE – THE CF FOR SILICON LENS IS A NEGATIVE (-) VALUE

PSEUDPHAKIC EYE TRUE AXIAL LENGTH CALCULATION
EXAMPLE -
if at an ultrasound velocity of 1,532 m/sec, a pseudophakic eye with a 6.0 mm diameter +22.00 D
acrylic intraocular lens ( thickness = 0.86 mm ) shows an apparent axial length of 24.00
mm, the true axial length would be:
EXAMPLE -
IF AT AN ULTRASOUND VELOCITY OF 1,532 M/SEC, A PSEUDOPHAKIC EYE WITH A 6.0 MM DIAMETER
+21.00 D SILICONE INTRAOCULAR LENS IMPLANTED IN 1999 ( VELOCITY = 1,090 M/SEC, THICKNESS =
0.92 MM ) SHOWS AN APPARENT AXIAL LENGTH OF 24.00 MM, THE TRUE AXIAL LENGTH WOULD BE:
TAL = 24.00 + ( 0.30 x 0.86 ) = 24.26 mm.
TAL = 24.00 + ( - 0.41 x 0.92 ) = 23.62 mm.

OPTICAL BIOMETRY
IOL POWER CALCULATION

PARTIAL COHERENCE
INTERFEROMETRY ( PCI
)
 BASED ON LASER
INTERFEROMETRY
 MEASURES SOLELY AXL
 LIGHT SOURCE :-
MMLD 780nm
 ARRANGEMENT
DUAL BEAM SETUP,
WHEREBY THE
REFRECTION FROM THE
CORNEA AND
REFELECTION FROM
RETINA ARE ASSESED
PARALLEL.
OPTICAL LOW
COHERENCE
REFLECTROMETERY
(OLCR)
 AXL MEASUREMENT
OF INTIRE RETINA
 LIGHT SOURCE SLD
(820nm)
 BASED ON
MICHELSON
INTERFEROMETERY
OPTICAL
BIOMETERY

OPTICAL FORMULAS
• HOLLADAY-II :- W-K AXIAL LENGTH ADJUSTMENT
• SRKT :- W-K AXIAL LENGTH ADJUSTMENT
• OLSEN:- REQUIRES ACD AND LT BY OPTICAL BIOMETRY
• BARRETT II :- EXCELLENT FOR MINUS POWER MENISCUS DESIGN IOL
• HILL-RBF :- USE ONLY WITH AN “IN-BOUNDS” INDICATION

SRK/T FORMULA
IT CAN BE CALCULATED USING THE SAME A CONSTANTS USED WITH THE
ORIGINAL SRK FORMULA OR WITH ACD ESTIMATES.
 SRK/T FORMULA OPTIMIZES THE PREDICTION OF POSTOPERATIVE ACD, RETINAL
THICKNESS AL CORRECTION, AND CORNEAL REFRACTIVE INDEX.
RECOMMENDED FORMULA USAGE : BEST FOR EYES LONGER THAN 26.00 MM.

SRK/T
EFFECT OF A-CONSTANT ON IOL POWER
THE TERM “A-CONSTANT” SEEMS MISLEADING BECAUSE, IT VARIES AMONG IOL
MODELS AND EVEN AMONG SURGEONS.
“A-CONSTANT” IS ADJUSTABLE & DEPENDS ON MULTIPLE VARIABLES INCLUDING
IOL MANUFACTURER, STYLE AND PLACEMENT WITHIN THE EYE.
DIFFERENT MODEL OF IOL , HAS DIFFERENT A-CONSTANT.

SRK/T FORMULA
1:1 RELATIONSHIP WITH THE A CONSTANTS:
IF A DECREASES BY 1 DIOPTRE ,
IOL POWER DECREASES BY 1 DIOPTER.

OLSAN
• DEVELOPED BY THOMAS OLSEN 1980
• THE OLSEN FORMULA USES PARAXIAL & EXACT RAY TRACING BASED ON PHYSICAL
DATA TO AVOID THE ERRORS OF THE ‘THIN LENS’ FORMULA.
• THE TRUE NET POWER OF THE CORNEA IS CALCULATED AND IT IS NOT NECESSARY
TO FUDGE THE EFFECTIVE LENS PLANE (ELP)
• USE THE INFORMATION OF THE EXACT IOL POSITION FROM C-CONSTANT DIRECTLY
IN THE FORMULA.
• C CONSTANT - AS FUNCTION OF LT AND ACD

OLSAN FORMULA POST OP ACD ASSUMPTION
Uses ray tracing to get the pre op lens thickness
And ACD to derive C, which can be thought of as
A fraction of the preop lens thickness.
C constant is then used to determine where the IO
will come to the rest in the eye.
SRK/T formula and the Holladay – use corneal
height (H), which is calculated from the corneal
curvature and diameter.

‘C’ CONSTANT
DEFINES THE POSITION OF THE IOL AS A FRACTION
OF CAPSULAR BAG SIZE.
PREDICT THE FINAL IOL POSITION FROM THE
PREOPERATIVE ACD AND LENS THICKNESS.
PRODUCE BETTER RESULTS OF ACCURATE
PREDICTIONS FOR BOTH SHORT AND LONG EYES
COMPARED TO HAIGIS.
IT WORKS IN ANY TYPE OF EYE INCLUDING POST-
LASIK EYES

NEEDED PARAMETERS
• THE OLSEN FORMULA ADDRESSES 4 AREA OF CONCERN
K AL
ACD IOL
CALCULATION OF
CORNEAL POWER
ACD PREDICTION
MEASUREMENT OF
THE AXIAL LENGTH
IOL OPTICS

(I) CALCULATION OF CORNEAL POWER
METHOD CONVENTIONAL
KERATOMETERY
GULLSTRAND BINKHORST
CURVETURE Only measure front
curvature
Assume P
proportional to A
surface
(6.8/7.7 =0.883)
Use volume 4/3
PHYSIOLOGICAL n Use fictitious n 1.376 --------
EQUIVALENT n 1.3375 1.3315 1.333
The difference in calculated power
almost 1D might introduce a prior
error of IOL calculation

CONVENTIONAL THICK LENS FORMULA
D1 = DIOPTRIC POWER OF FRONT SURFACE
OF CORNEA
D2 = DIOPTRIC POWER OF THE BACK SURFACE
OF CORNEA
D12 =TOTAL DIOPTRIC POWER OF THICK LENS ( CORNEA)
Apply a total dioptric power from thick lens
formula, it results the refractive index as
fallows

(II) MEASUREMENT OF THE AXIAL LENGTH
 THE AL MEASURED BY ULTRASOUND ≠ TRUE AL
 “RETINAL” SPIKE ORIGINATE FROM VR INTERFACE
 COMPRESSION OF THE CORNEA (CONTACT TECHNIQUE)
 SO, THE TERM ‘RETINAL THICKNESS’ WAS INTRODUCED AS A CORRECTIVE TERM IN ORDER TO
ELIMINATE ERROR.
 PREVIOUSLY, LARGE ERROR RAISED IN EXTREME SHORT & LONG EYE DUE TO VELOCITY
ASSUMPTION.
 THE AVG VELOCITY FROM CORNEA TO RETINA IS 1550 M/S
 AVG VELOCITY IN EXTREME MYOPIA (INCREASE) & HYPEROPIA CHANGE
 TO CORRECT AL ACC TO SHIFT OF VELOCITY, THE AL CAN BE CORRECTED WITH EQUATION:
Real AxL = AxL/Mean Vel – L thick / Lens Vel) x Aqueous Vel +

(III) THE ACD PREDICTION
ACD PREDICTION PLAYS SIGNIFICANT ROLE IN THE IOL POWER CALCULATION.
PREVIOUSLY, LACK OF EMPIRICAL DATA ON POSTOP POSITION OF THE IMPLANT
(POSTOP ACD) – TEND TO RESULT MYOPIC ERROR (OVEREST IOL POWER) IN
SHORT EYE.
THE METHOD TO PREDICT THE POSTOP ACD IN A GIVEN EYE BASED ON THE
ACTUAL PREOP MEASUREMENTS OF THE EYE.

 OLSEN PROPOSED HIS REGRESSION FORMULA FOR THE PREDICTED POSTOP ACD AS
FOLLOWS:
 ACDPOST = EXPECTED POSTOP ACD OF THE IOL (IN MM)
 ACDMEAN = AVERAGE POSTOP ACD OF THE IOL (IN MM)
 H = HEIGHT OF CORNEA SEG BASED ON KERATOMETRY AND CORNEAL
DIAMETER
 ACDPRE = PREOP ACD(MM)
 T’ = LENS THICKNESS (MM)
 L = AXIAL LENGTH (MM
ACDpost = ACDmean +0.12H + 0.33 ACDpre +
0.3T’ + 0.1L – 5.18

(IV) THE IOL OPTIC
IN ORDER TO CALCULATE THE POWER ACCORDING TO GAUSSIAN OPTICS, IT IS
NECESSARY TO KNOW THE POSITION OF THE PRINCIPAL PLANE OF THE IOL
OPTIC.
THIS POSITION IS IMPORTANT IN DETERMINING THE EFFECTIVE POWER OF THE
LENS WITHIN THE EYE.
ALL THE DIOPTRIC POWER OF A PLANOCONVEX LENS IS ON ONE SURFACE AND
THUS THAT SURFACE REPRESENTS THE EFFECTIVE LENS PLANE.
 WITH A BICONVEX LENS, THE EFFECTIVE LENS PLANE IS ‘INSIDE’ THE LENS.

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