a scan ultra sound biometry calculation for intra ocular lens

1. A SCAN BIOMETRY
Mahantesh.B HOD of optometry

2. A SCAN Biometry
 A-scan ultrasound biometry, commonly
referred to as an A-scan, is routine type of
diagnostic test used in optometry or
ophthalmology.
 The A-scan provides data on the length of the
eye, which is a major determinant in common
sight disorders

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 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.

4. 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.

5. Ultrasound Principles
 Sound is defined as a vibratory disturbance
within a solid or liquid that travels in a wave
pattern
 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
manufacturer

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 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
both

7. 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.

8. Keratometer
 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
of astigmatism.

9. Keratometery
 Keratometry by Manual
 Topography
 Autokeratometer
 IOL master/ Lenstar 900

10. Source of keratometry errors
 Unfocused eye piece
 Failure to calibrate unit
 Poor patient fixation
 Dry eye
 Drooping eye lids
 Irregular cornea
 Contact lens user

11. Repeat Keratometery
 If Corneal curvature more than 47D or less than
40D.
 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.
 Difficult Situations
 Post Refractive Surgery
 Corneal Transplantation
 Corneal Scar
 Keratoconus etc.

12. REMEMBER
 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

13. Method
 Contact –
Applanation
Method
Hand-Held Method
 Immersion

14. Applanation Method
 By the Applanation biometry method, an
ultrasound probe is placed directly on the
cornea, with attached slit lamp

15. Hand-Held Method

16. PROCEDURE
 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.

17. 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.

18. Corneal Compression
 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
3.75 D

19. Immersion
 Immersion A-scan Biometry • The immersion
technique requires the use of a Prager Scleral
Shell

20. 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

21. formulae
 Theoretical formulae
 Regression formulae

22. Theoretical formulae
 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)

23. Binkhorest formula
P=1336(4rd)/(a-d)(4rd-d)
P = IOL power in dioptrs
r = corneal radius in mm
a = axial length in mm
d = assumed post operative anterior chamber
depth+ corneal thickness

24. Regression formulae
 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
modification
 It was introduce by Sanders, Retzlaff and
Kraft
 Its based on the regression analysis
taking into account the retrospective
computer analysis of a large number of
post operative refraction

25. SRKl
P= A-(2.5L-0.9K)
P=IOL power
A= constant specific for each lenses
L= axial length
K= average keratometry in dioptrs

26. SRK ll
P= A-(2.5L-0.9K)
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

27. SRKT FORMULA
 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
eyes

28. THANK YOU