3. Keratometry (or) ophthalmometer .
Keratometry is the measurement of a patients corneal curvature .
1. It provides on objective,quantitative measurement of corneal
astigmatism,measuring the curvature in each meridian as well as the axis.
2. Keratometry is also helpful in determining the appropriate fit of contact
lens .
3. To measurement the corneal dioptric power.
4. The measurement of the curvature of the anterior corneal surface by using
the first Purkinje image.
5. Determines corneal curvature by measuring the size of a reflected “mire”.
6. Doubling of image avoids problems from eye movements.
7. Keratometer measures only the central 3mm of the corneal diameter.
4. Principle of Keratometry
It measure the size of image reflected from corneal surface, because
cornea acts as convex mirror.
When an object is in front of the cornea a virtual image is seen inside the
convex mirror (cornea). The size of the image depends on,
1. The distance of the object and
2. The curvature of the cornea for a fixed distance of the object the
size of the image depends on the curvature of the cornea.
Similarly for a given size of the image distance of the object is different
depending on the curvature of the cornea.
5. The object used is an illuminated circle with plus and minus rings as shown in
figure.
The two prisms inside the instrument give two additional one displaced
horizontally and another displaced vertically. Three images are seen as in
figure.
While taking the reading the pluses and minuses coincide. This is achieved by
moving the keratometer with the object forward or backward in front of the
eye.
When coincidence takes place the size of the images of fixed value. The
distance of the object is different for different curvatures. The instrument is
calibrated. As the drum rotate the distance varies.
8. Optics of Bausch and Lomb keratometer
When the instrument is correctly aligned, the operator sees three images of
the instrument’s mires.
The first is produced by light passing through aperture C and the vertically
displacing prism
The second is produced by light passing through aperture D and the
horizontally displacing prism, and the third by light passing through aperture
A and B
Back and forth movement of the vertically doubling prism results in
movement of the vertically displaced image, while movement of the
horizontally doubling prism results in movement of the horizontally displaced
image
9. The central image formed by the light passing through A and B is unaffected
by movement of either prism
The aperture A and B act like a Schiener disc and double the central image of
the mire when the intermediate image, produced by the objective lens does
not coincide with the focal point of the eyepiece lens
This system is designed to assist the operator in judging when the microscope
is out of focus.
11. The images of the mire as seen through the doubling system of the
keratometer are shown in the figure for the conditions where
1. The vertical doubling is correct and the horizontal doubling is insufficient
2. The vertical doubling is too great and the horizontal doubling is correct
3. The vertical and horizontal degrees of doubling are correct
4. The mires are viewed after reflection by an astigmatic cornea, the axes of
which do not coincide with that of the keratometer
12. A-scan Biometry
Ophthalmic ultrasound uses the reflection of high frequency sound waves to
define the outlines of ocular and orbital structures and to measure the
distance between them.
A-scan biometry is to determine the power of the intra ocular lens, that
replaces the natural lens during cataract surgery.
A-Scan biometry is also called as axial length measurement scan. This
measurement is combined with Keratometric readings to obtain the IOL
power.
Error of 0.4mm in the measurement of axial length may result in a one
diopter change in calculated IOL power.
13. Physical principles
The A-scan probe contains a ultrasonic transducer that projects a thin
sound beam that travels through liquid or tissue.
Ultrasound waves do not travel through air.
The frequencies most often employed for diagnostic work are
between 2.5 MHZ and 20 MHZ. Higher the frequency greater the
resolution.
14. Although increasing the frequency increases the resolution,it
simultaneously decreases the depth of penetration of the sound.
When the sound beam encounters the interface of a substance that is
dissimilar from the substance it is traveling through,part of the sound
beam energy is reflected , and part of the sound energy projects
through the new substance.
15. PROCEDURE:
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 oscilloscope
(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.
16. 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.
17. The gain should be adjusted high enough such that the spikes can be
maintained above the threshold level needed for an automated
acquisition of the scan if this feature is used.
The gain should be low enough to allow the operator to visually
maximize the spike height during probe alignment.
18. Biometry technique
Contact
Applanation method
Hand help method
Immersion
Values are 0.14 to 0.36mm longer with immersion technique than with
contact method.
19. Contact technique
A-scan biometry by applanation requires that the ultrasound probe to
be placed directly on the corneal surface. This can be done at the slit
lamp
A-scan biometry can alo be done holding the ultrasound probe by
hand.
20. Immersion Technique:-
Also called water bath method, the patients is supine and ultrasound
probe is suspended in fluid filled scleral cup placed over the eye.
23. Gates
Gates are electronic markers on the screen that provide
measurement of distance between 2 or more anatomic interfaces .
24. Gain Setting
Initially high gain setting should be used to assess the overall
appearance of the echogram , then gain should be reduced to a
medium level to improve resolution of spikes
25. Error can occur when the gain is set too high or too low.
Very high gain short reading
Very low gain long reading
27. Scan of phakic eye
1. It shows anterior lens spike(B),the
posterior lens spike(C),and the
retina spike (E), the probe tip /
cornea spike is represented by(A).
2. These spikes should be tall and
steeply rising.The retina spike
should not have smaller spikes
immediately in front of it.(D)
3. The retina spike should be followed
by tall scleral spike (F) and spikes
from the orbital fat layer of the
orbit(G).A scan without orbital fat
spikes may indicate that the beam
is striking the optic nerve instead
of the macula.
28. Scan of an aphakic eye
1. It will either have no lens spikes,(or) it will have
one lens spike (A) that represents an intact
posterior lens capsule. ( C )
2. Be sure to use the aphakic mode of the A-scan
instrument.
3. The velocity of sound will be different because
the beam is not passing through the lens.
4. A velocity of sound of 1532m/s is typically used
for aphakic measurements.
29. Scan of an pseudophakic eye
If one is pseudophakic, A-scan and K-readings should be done for both the
eyes before calculating the IOL power for the eye required.
Since both eyes have similar measurements in most people, this provides a
double check of the measurement.
It is some times necessary to replace an IOL that was inserted many months or
years ago. Even if you have IOL specifications and measurement information
from the previous surgery, its nice to have the confirmation of a current
measurement.
30. Characteristics of a good scan
1. Corneal echo is seen as a tall single peak
2. Aqueous chamber should not provide any echo
3. Anterior and posterior lens capsule produces tall echo
4. Vitreous cavity should produce few to no echoes
5. Retina produces tall, sharply rising echoes with no staircase at the origin
6. Oribital fat produces medium to low echoes
31. IOL formula
Depending upon the basis of their derivation:
Theoretical formula
Regression formula
These are grouped into various generations.
32. Theoretical formula
Derived from geometric optics as applied on the schematic eyes, using theoretical constants.
Based on 3 variables:-
AL
K- reading
Estimated post-operative ACD
Regression formula
Based on regression analysis of the actual post-operative results of implant power as a function
of the variables of corneal power and AL.
36. Regression formulae
SRK-I formula (Sanders, Retzlaff and Kraff)
P=A-2.5L-0.9K
Tends to predict too small value in short eyes and too long value in long eyes
37. SRK-II formula
P=A-2.5L-0.9K
A constant is modified on the basis of axial length as follow:-
If L is <20mm :A+3.0
If L is 20-20.99 :A+2.0
If L is 21-21.99 :A+1.0
If L is 22-24.5 :A
If L is >24.5 :A-0.5
38. Modified SRK-II formula
Based on axial length, A constant is modified as
If L is <20mm :A+1.5
If L is 20-21 :A+1.0
If L is 21-22 :A+0.5
If L is 22-24.5 :A
If L is 24.5-26 :A-1.0
If L is >26mm :A-1.5
39. SRK/T formula
Nonlinear theoretical optical formula optimized for post op AC depth, retinal
thickness and corneal refractive index
Significantly more accurate for extremely long eyes (>28mm)
40. Optical biometers
The introduction of optical biometer has significantly improved the accuracy
of AL measurement from 0.12mm in immersion ultrasound to 0.02mm in
optical methods.
commercially available optical biometer include:-
1. IOL master
2. Lens star
41. IOL Master
IOL MasterTM (Zeiss Humphrey System) is a
combined biometric instrument that
measures quickly and precisely parameters
of human eye needed for IOL power
calculation by a noncontact technique.
It also incorporates the software to calculate
IOL power from various formulae.
42. Working principle
it is a noncontact optical device that measures the various parameters based on
following principles:
AL measurements- It is based on a patented interference optical method
known as ‘Partial Coherence Interferometry (PCI)’. This technique relies on a
laser Doppler technique to measure the echo delay and intensity of infrared
light reflected back from the tissue interfaces- cornea and RPE. The
instrument is calibrated against the ultra high resolution of 40MHz. An
internal algorithm approximates the distance to the viteroretinal interphase
for the equipment of an immersion A-scan ultrasonic AL.
Corneal curvature (K)- It is determined by measuring the distance between
reflected light images as in conventional keratometry
43. ACD- it is determined as the distance between the optical sections of the
crystalline lens and the cornea produced by lateral slit illumination.
White-to-white – It is determined from the image of iris.
Calculation of IOL power- it is done by software incorporating internationally
accepted calculation formulae
44. Advantages of the IOL Master
Patient comfort, as the technique involves noncontact measurements.
User-friendly, as the operater can learn the technique very quickly
Single instruments is required for measuring AL, corneal curvature (K), and
ACD.
Cross-infection risk is not there, as the technique is noncontact
It incorporates 5 IOL power calculating formulae in an integrated manner,
these are- Haigis, Hoffer, Holladay,SRK-II, and SRK/T formula.
45. Lens Star
LenStar LS900 (hag-streit diagnostics)
provides highly accurate laser optic
measurements for every section of the
eye and is the first optical biometer that
can measure the thickness of the
crystalline lens.
With its integrated Olsen formula and
the optional Toric Planner, the LenStar
provides user with latest technology in
IOL prediction for any patient.
46. Working principle
It is a noncontact optical device which measures multiple parameters on the
following parameters:-
Central corneal thickness- it uses optical coherence biometry to measure
central corneal thickness (CCT), with stunning reproducibility of ± 2μm.
Keratometry/Topography- LenStar’s unique dual zone keratometry, featuring
32 marker points, provides perfect spherical equivalent, magnitude of
astigmatism and axis position. With the optional T-Cone topography add-on,
LenStar provides full topography maps of the central 6mm optical zone.
White-to-white- Based on high-resolution colour photography of the eye,
every white-to-white measurement can be reviewed and adjusted by the user.
Pupillometry- Measurements of the pupil diameter in ambient light
conditions can be used as a n indicator for the patient’s suitable apodized
premium IOLs, as well as for laser refractive procedures.
47. Lens thickness- Accurate measurement of LT is the key to optimal IOL
prediction accuracy when using the latest IOL calculation formulae, Olsen or
Holladay II.
ACD- It is measured by optical coherence biometry which provides more
precision and reproducibility. This allows ACD to be measured on phakic as
well as on pseudophakic eyes.
Axial length- it uses a superluminescent diode as the laser source which
enables measurements of the AL of the patient’s eye, precisely on the
patient’s visual axis and even in the presence of dense media.
