LinkedIn emplea cookies para mejorar la funcionalidad y el rendimiento de nuestro sitio web, así como para ofrecer publicidad relevante. Si continúas navegando por ese sitio web, aceptas el uso de cookies. Consulta nuestras Condiciones de uso y nuestra Política de privacidad para más información.
LinkedIn emplea cookies para mejorar la funcionalidad y el rendimiento de nuestro sitio web, así como para ofrecer publicidad relevante. Si continúas navegando por ese sitio web, aceptas el uso de cookies. Consulta nuestra Política de privacidad y nuestras Condiciones de uso para más información.
A SCAN Biometry
A-scan ultrasound biometry, commonly
referred to as an A-scan, is routine type of
diagnostic test used in optometry or
The A-scan provides data on the length of the
eye, which is a major determinant in common
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
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.
calculation of IOL
the process of measuring the power of the
cornea ( keratometry ) and the length of the
eye, and using this data to determine the ideal
intraocular lens power.
Sound is defined as a vibratory disturbance
within a solid or liquid that travels in a wave
When the sound frequency is between 20
hertz (Hz) and 20,000 Hz, the sound is audible
to the human ear.
In ophthalmology, most A-scan and B-scan
probes use a frequency of approximately 10
million Hz (10 MHz) that is predesigned by the
This meets unique needs, because at times,
the probe is placed directly on the organ to be
examined, and its structures are quite small,
requiring excellent resolution.
The velocity of sound is determined
completely by the density of the medium
through which it passes.
Sound travels faster through solids than
through liquids, an important principle to
understand because the eye is composed of
calculate IOL Power required
Accurate Corneal power (keratometry)
Actual axial length
Accurate estimated lens position (half a mm
shift in lens position can have a dramatic effect
on final vision)
A good understanding of the various IOL
power calculation formulas is also required.
A keratometer, also known as an
ophthalmometer, is a diagnostic instrument for
measuring the curvature of the anterior surface
of the cornea, particularly for assessing the axis
Source of keratometry errors
Unfocused eye piece
Failure to calibrate unit
Poor patient fixation
Drooping eye lids
Contact lens user
If Corneal curvature more than 47D or less than
The difference in corneal cylinder is more than
one dioptre between eyes.
The average keratometry (K1) 43.0(K2) 44.0D,
with one eye typically within 1D of each other.
Post Refractive Surgery
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 Dioptre change in
calculated IOL power
Difference in AL measurement Between both
eyes +/- 0.3 mm
Explain the procedure
Use topical Anaesthesia
Clean the probe
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 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.
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.
If pressure is applied on the cornea, the axial
length measurement 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 D Long eye 1.75 D Short eye
Immersion A-scan Biometry • The immersion
technique requires the use of a Prager Scleral
Immersion A-scan Biometry
The immersion technique is accomplished by
placing a small scleral shell between the
patient's lids, filling it with saline,
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
Various formulae derived from the geometric
optics using schematic eye
These formulae are based on 3 variables
1Axial length of the eye ball(AL)
2 keratometry reading(K)
3 estimated post operative AC depth(ACD)
P = IOL power in dioptrs
r = corneal radius in mm
a = axial length in mm
d = assumed post operative anterior chamber
depth+ corneal thickness
This formulae is based on regression
analysis of the post operative results of
implant power using variable of corneal
power and AL
The commonly used SRK Formula and its
It was introduce by Sanders, Retzlaff and
Its based on the regression analysis
taking into account the retrospective
computer analysis of a large number of
post operative refraction
A= constant specific for each lenses
L= axial length
K= average keratometry in dioptrs
But A is modified on the basis of the axial length
If L is <20 mm then A+3.0
If L is 20-20.99 mm then A+2.0
If L is the 21-21.99 mm then A+1.0
If L is the 22-24.0 mm then A
If l is >24.50 = A-0.50
It is regression formula empirically optimized
for post refractive ACD Retinal thickness and
corneal refractive index
This combines the advantages of both the
theoretical and empirical analysis
Significantly more accurate for extremely long