biometry

1. OCULAR
BIOMETRY
Yonas Tilahun, MD

2. Objectives
 A-Scan Principles
 Steps in Biometry
 Source of Errors
 Minimizing Errors-good biometrist
 IOL calculation formula

3. Pre Test
 The usual frequency of A-scan biometry
probe is;
 A. 10Mhz
 B. 15Mhz
 C. 25Mhz

4. Pre test
2. Which technique of biometry has a
higher tendency of corneal compression
 A. Contact Biometry
 B. Immersion biometry
 C. Optical biometry

5. Pre Test
3. Which of the following information can
not be expected from A-scan biometry
 A. lens thickness
 B. Axial length.
 C. Keratometry
 D. AC depth

6. Pre Test
4. The junction between any two ocular
media of different densities and
velocities is called
 A. Gate
 B. Gain
 C. Interface
 D. Frequency

7. Pre test
5. How many gates do you expect in a
routine a scan measurement?
 A. 1
 B. 2
 C. 3
 D. 4

8. Pre Test
6. Which one of the following
measurement is considered the most
important in Iol power determination?
 A. Keratometer
 B. AC depth
 C. Axial length
 D. Lens thickness

9. Pre Test
7. Which one of the following uses optical
interferometry for measuring intraocular
distances?
 A. A-scan E. A&B
 B, B-scan F. C&D
 C. Lenstar
 D. IOL master

10. Pre Test
8. Ultrasound travels faster in lens than
in aqueous or vitreous
A. True
B. B. False

11. Pre Test
9. The IOL master is better than U/S for
measuring axial length for eyes with
dense cataract or media opacity
 A. True
 B. False

12. Pre Test
10. Optical Biometry measures axial
length from apex of the cornea to the
level of
 A. Internal limiting membrane
 B. Retinal nerve fiber level
 C. Photoreceptors
 D. Retinal Piegment Epithelium

13. Pre Test
11. Which one of the following is a more
reliable IOL calculation formula ?
 A. SRK T
 B. Holladay II
 C. Haigis
 D. Hoffer Q

14. Pre Test
12. The most common error in contact
biometry is
 A. Corneal Compression
 B. Misallignment
 C. Wrong IOL formula
 D. Wrong label

15. BIOMETRY
 Is a clinical procedure used to
 Meassure axial length for IOL
powercalculation
 Monitor congenital glaucoma, myopia,
nanophthalmos
 Meassure intraocular parameters like:
 AC depth
 Lens thickness

16. A-Scan Techniques

17. A-Scan Ultrasound
(PRINCIPLES)
 A-Scan what does A stand for?
 Sound wave-a vibration that propagates
as acoustic waves through Gas, liquid or
solid.
AMPLITUDE

18. A-Scan Ultrasound
(PRINCIPLES)
 Sound wave frequency ranges 20-
20,000hz
 Ultrasound (in audible sound) >20k hz
 A-Scan –Biometry uses ultrasound
(10mhz) to measure distances between
ocular structures using echoes of u/s

19. A-Scan parts
 1. Pulser
 2. Receiver
 3. Display system

20. Pulser and Receiver
 Comes in a probe
 Piezoelectric
Substance that
Generates US when stimulated by burst of
electricity.
The crystal converts the electric energy to
sound wave and Mechanical vibration from
echoes are converted to electrical energy and
plotted as spikes

21. BIOMETRY
 There is no machine brighter than a
good operator

22. A-Scan principle
 In A-scan biometry, one thin, parallel sound beam is emitted
from the probe tip at its given frequency of approximately 10
MHz, 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, which, in the eye, include the anterior
corneal surface, the aqueous/anterior lens surface, the
posterior lens capsule/anterior vitreous, the posterior
vitreous/retinal surface, and the choroid/anterior scleral
surface.
 The echoes received back into the probe from each of these
interfaces are converted by the biometer to spikes arising from
baseline.

23. U/S Velocity

24. A-Scan Typical spikes

25. A-Scan typical spikes

26. Gates
 Electronic calipers on the
display..Biometers are programmed to
place..check correctness
 4 typical gaits..3 sections to be
meassured
 A. corneal spike
 B. ant lens surface spike
 C. Post lens surface spike
 D. Retinal surface spike

27. Measurement formula principles
 Summation of gates
 Cornea to ant lens surface (AC depth)
 Velocity through aqueous 1532m/s (D=VxT/2)
 Ant lens surface to Post lens surface (lens
thickness)
 Velocity 1641m/s
 Ant vit surface to ant retinal surface
 Velocity 1532m/s

28. Modes
 Phakic—3 gates displayed as above select
cataract, dens cataract etc to adjust
velocity)
 Phakic average..takes average spped of
1550m/s and 2 gates (cornea/retina) –gross
 Aphakic -2 gates (Cornea/Retina) V=1532
 Pseudophakic –lens options/ if not consider it
as PMMA

29. BIOMETRY

30. Gain
 Electrical amplification of signals
(Intensity)
 Gain knob
 Too high..picks signal fast and increases
amplitude of spikes but results in poor
resolution and poor accuracy
 Too low ..difficult to get spikes.
….Measurement
 Recommended gain 50-70

31. Source of Errors
 A 0.1 mm error in an average length eye will result in
about a 0.25 diopter (D) postoperative refractive
error.
 A 0.5 mm will result in approximately 1.25 D and an
error of 1.0 mm will result in approximately 2.50 D
 Longer eyes are more forgiving, with a 1.0 mm error in
an eye of 30 mm length result in 1.75 D.
 Small eyes are the least forgiving, an error of 1.0 mm
in an eye that is 22.0 mm long will result in a post-
operative error of about 3.75 D.

32. Source of errors
 Corneal compression-myopic shift
 Check for ac depth
 Misallignment (not perpendicular)- The
angle of incidence, which is determined
by the probe orientation to the visual
axis… hyperopic shift
 low Ant/post lens surface spikes
 Absent scleral spike

33. Source of Errors

34. Compression/Misallignments

37. WRONG

38. Source of Errors
 The shape and smoothness of each interface also
affects spike quality. Lubrication, osd Rx
 Macular pathology could adversely affect spike
quality. A perfect high, steeply rising retinal spike
may be impossible when macular pathology is present
(eg, macular edema, macular degeneration, epiretinal
membranes, posterior staphylomas).

39. Source of Errors

40. Source of errors
 Gates position..
 Not properly placed (adjust or repeat)
 Poor spikes repeat
 Dry eye, OSD
 Corneal opacity
 Squint
 AMD
 Poor patient and eye position

41. Pseudophakic biometry
 To check fellow eye power
 Iol exchange
 Type of iol pmma/foldable –
Reverberation artifact

42. Reverberation artifact
The longer chain of artifact spikes from
polymethyl methacrylate implants. The
image on the right demonstrates the
shorter chain of artifact in the vitreous

43. Steps in Biometry
 Calibrate and clean probe
 Patient should be seated looking straight ahead or at
the probe light (if could fix)
 Stand at the side of the patient and screen should be
placed where you can easily see it
 Apply anesthetic drop
 Align the probe to the optical axis and applanate at
the cornea apex
 Check variation in ACD, and select one with max value
 SD should be less than 0.3mm (ideally 0.06)

44. A Good Biometrist .must be
smarter than the machine!
 Must be able to recognize
 when readings appear abnormal
 standard dimensions of the eye.
 The average axial eye length is 23.5 mm, with a range of 22.0-24.5 mm.
 A patient can be myopic because of steep corneal curvature rather than long axial
length, and a patient can be hyperopic because of flat corneal curvature rather than
short axial length.
 Compare axial length to the precataract refractive error of the patient to ensure that
the readings appear accurate.
 The reference range of AL between the right eye and the left eye of the same patient
is within 0.3 mm, unless evidence suggests the contrary (eg, previous scleral buckling,
anisometropia, corneal transplantation, keratoconus, refractive surgery, hypotony).
 The average anterior chamber depth is 3.24 mm but varies greatly.
 The average lens thickness is 4.63 mm but this also varies, and, with cataractous
changes, the lens will increase in thickness to as much as 7.0 mm in extremely dense
cases.

45. A Good Biometrist
 Should realize;
 The average keratometry (K) reading is 43.0-44.0 D, with one
eye typically within a diopter of each other.
 If one eye is found to differ from the other by more than 1 D,
immediately begin researching the cause and alert the physician.
( refractive surgery, corneal transplantation, an injury with a
resultant corneal scar, or has keratoconus)
 If any of the above eye measurements is found to be unusual,
another technician should recheck the measurements and
immediately alert the physician.

46. Reviewing measurements
 SD of AL with in 0.06mm delete extremes
 Check corneal compression by variation of AC depth
 Ant and Post lens spikes should be nearly equal (post
slightly shorter)
 Retina spike straight and high
 Scleral spike should be seen separately from that of
the retina
 Do both eyes and if there is a difference of >0.3mm
in AL… review

47. IOL calculation formula
 2 variable formula ( AL and
keratometry)
 Using the correct IOL calculation formula is important for good
surgical outcomes.
 SRK Formula: P=A-2.5L-0.9K
 Current 2-variable formulas that are considered the most
accurate include the Hoffer Q, SRK/T, and Holladay I.
 Multivariable formulas have proven to be the most accurate due
to more of the eye anatomy being considered

48. IOL Calculation Formula
 The Haigis formula is a 3-variable equation, using not
only axial length and corneal curvature but also the
anterior chamber depth of the eye.
 The Holladay II formula is a 7-variable equation
widely thought to be the most accurate formula; it
takes into account axial length, corneal curvature,
horizontal white-to-white, anterior chamber depth,
lens thickness, precataract refractive error, and age
of the patient.

49. IOL Calculation Formula
 Predicting lens position is one of the most common
causes of a postoperative surprise; by taking more of
the eye anatomy into account, this can be more
accurately predicted. For average-length eyes with
average Ks, these formulas give almost identical
calculations. [3] However, when the eye is small,
formula selection is more critical. In eyes that are
less than 22 mm in length, the Hoffer Q and the
Holladay II IOL Consultant formulas are the most
accurate. For long eyes, the SRK/T and the Holladay
II IOL Consultant formulas are the most accurate.

50. Simple formula recommendation
Axial Length <22mm 22-24mm >24mm
Formula HofferQ SRKT,HofferQ,
HolladayII
Holladay II,
SRKII

51. VELOCITY CONVERSION
 Intra op You found that the patient is
aphakic hile iol was calculated with
phakic mode
 Velocity (correct)/Velocity (measured) X
Apparent Length = True Length
 E.g. 1532/1550 X 24.1 = 23.82 mm = true eye
length.
 Intraop you found pt has silicon oil
 980/1532 X Apparent Vitreous Length =
True Vitreous Length

52. Optical Biometers
 Current method for highly accurate axial length measurements
does not use ultrasound at all, but rather optical coherent light.
In this method, optical coherent light passes through the visual
axis and reflects back from the retinal pigment
epithelium.(internal limiting membrane as with
ultrasound/0.1mm)
 However, this method cannot be used in the event of significant
media opacity (eg, dense cataracts or corneal or vitreal opacity)
due to absorption of the light

53. Other Biometers
 Optical/Laser (near infra red..partial
coherence laser)
 IOL MASTER (Carl Zeiss)
 Lens star (Hagstreit)
 low coherence interferometry
 Alladin (Topcon)

54. Optical