Presenter : Dr Nikhil Agrawal(1st year resident) Moderator : Dr Tarun Gupta Dhir HOSPITAL POSTGRADUATE INSTITUTE OF OPHTHALMOLOGY

1. BIOMETERY
Presenter : Dr Nikhil Agrawal (1st year resident)
Moderator : Dr Tarun Gupta
Dhir HOSPITAL AND POSTGRADUATE INSTITUTE OF OPHTHALMOLOGY

3. BIOMETRY
• Biometry is the method of applying mathematics to biology.
• The refractive power of the eye primarily depends upon the cornea, the lens,
ocular media, and the axial length of the eye.
• Required power of IOL can be calculated if we know these .

4. History
• In 1949, Harold Ridley implanted the first IOL but
his patient had a refractive surprise of nearly 20
diopters.
• Various theoretical vergence formulas were
developed after development of A-
scan(ultrasound) .
• Formulas derived around the same time have
been subjected to minor and major alterations in
the variables since then in order to increase
accuracy. [3]

5. IOL FORMULAS WE USE TODAY
• SRK-T
• OLSEN
• KANE
• HILL-RBF
• BARRETT UNIVERSAL II

7. GENERATIONS OF IOL FORMULAS
• 1ST GENERATION
Hoffer, SRK-1
• 2ND GENERATION
Binkhorst , SRK-II
• 3RD GENERATION
• Holladay, Hoffer-Q, SRK-T
• 4TH GENERATION
• Holladay-2, Hagis
Effective lens position (ELP)
ELP was the term used to denote the
position of the lens in the eye

8. Keratometry
• Changes in K reading; alters the IOL
power in a ratio of nearly 1:1.
• Measured by keratometry or
topography but neither measures the
actual corneal power directly.
• The radius of curvature is determined,
which is then converted to power in
diopter or mm.
• This is especially altered after
refractive surgeries.

9. Axial Length
• It changes the IOL power by nearly 2.5 to 3 times, more so in short eyes .
• Measured using -Ultrasonography (contact or immersion)
-Optical methods
Ultrasonography uses mechanical waves to calculate the time needed for a pulse
to travel from the cornea to the retina.
Average sound velocity of a normal phakic eye is 1555 m/s.
Distance = Time x Velosity

10. Prager scleral shell

11. Optical Methods
• Similar to ultrasonography, optical methods
also measure time needed for infrared light to
travel from cornea to retina.
• As it is a non-contact procedure, no
indentation errors occur.
• IOL Master from Zeiss and the Lenstar from
Haag-Streit.
HAAG-STREIT Lenstar

13. Steps to Analyse a Biometry Report
• A large difference in AL or
K value of both eyes might
be sign of anisometropic
amblyopia .
• USG B scan in dense
cataract
• Measure twice cut once

16. Special Circumstances
Aphakia-
• The two lens spikes in A-scan are absent, being replaced by a single spike of the
anterior vitreous face and posterior capsule.
• ACD value should be reduced by 0.25 mm for sulcus placement. For posterior iris
fixation a further reduction of ACD by 0.25mm is needed.

18. Special Circumstances
Pseudophakia -
• Eyes with IOL have an extremely high spike at the lens followed by an artificial
chain of reduplication echoes which can be confused with retinal spikes.
• Optical Biometry is preferred which offers more accurate correction of the AL by
correction factor (CF) which varies as per the lens type and thickness.

20. Special Circumstances
Paediatric age group
• The IOL power chosen should allow good vision in growing age to prevent
amblyopia and ideally also give emmetropia in adult age.
• Development of the eye necessitates initial under-correction to avoid later
myopic shift.
Piggy back IOLs with one implant
being permanent in the bag and
the other temporary in the ciliary
sulcus has been suggested by Dr.
M. Edward, where in the
temporary one is removed at
adult age.

21. Special Circumstances
Silicone Filled Eyes
• The velocity of sound in silicone is slower than in vitreous.
• Silicone in the eye itself acts as a negative lens hence the IOL power must be
adjusted by 3-5 D.
• Measurement of AL by optical method has been found to much more accurate for
silicone filled eyes.

22. Special Circumstances
After Refractive Surgery
• Corneal Refractive surgeries alter the basic assumptions - the perfectly spherical
nature of cornea.
• The refractive surgeries mainly affect the central cornea, as well as alter the
posterior corneal curvature, which is not routinely measured.
• The relationship between the curvatures is altered in PRK and LASIK . This
overestimates the corneal power by 1D for every 7D of correction of refractive
error.

23. https://iolcalc.ascrs.org/wbfrmCalculator.aspx

24. Special Circumstances
Corneal Transplants
• It is extremely difficult to accurately predict corneal power in transplant patients.
• If a triple procedure is planned it is suggested that K readings of other eye be
used.
• An alternative option is to use the average k readings from a series of previous
transplants.

25. Special Circumstances
Piggy-back IOLs
• In patients with post IOL refractive surprise or in those with large dioptric
requirement, a piggy back IOL in sulcus can be placed along with the primary
implant.
• This method does not require knowledge of the power of the primary implant or
of the axial length.
Myopic correction: P = 1.0 x Error
Hyperopic correction: P = 1.5 x Error
where
P = the needed power in the piggyback lens
Error = the residual refractive error that needs
to be corrected

26. COMMON MISTAKES
(Lenosmeter)

27. Improving Outcome
• Patient counselling and requirements
• Appropriate alterations to biometry according to surgeon .
• AL measurement- Optical methods can be supplemented with ultrasonography
methods to improve outcomes.
 Corneal echo is seen as a tall single spike
 No echoes from Aqueous humour and
Vitreous cavity
 Anterior and Posterior lens capsule produce
tall echoes
 Retina produces tall sharply rising spike with
no staircase at origin
 If the spike from retina is not followed by
multiple small spikes it means one is hitting
optic nerve, so discard that reading.
 Average of 8-10 measurements

28. Improving Outcome
• K measurement- Perform a double check
 No other contact procedure before keratometry
 Calibrate keratometer before scan
 Take an average of 3 readings
 Compare the refraction, especially the axis with the K values
 Repeat if difference is more than 1.0D between the two eyes.

29. Improving Outcome
• Choice of
formulas -No
single formulae
has been found
to be useful in all
circumstances.
• The newer vergence-
based formula, the
Barrett II universal,
utilized 5 variables
and has greatly
increased post
cataract surgery
refractive outcomes
over older formulas.
WangeKoch (WK) adjustment

30. • The Barrett and
Olsen formulas has
the best outcomes
in terms of accuracy
of postoperative
spherical
equivalent .
• They perform well
across a range of
axial lengths and
biometric
dimensions.

32. High Axial Myopia
• Preoperative Fundus Examination and OCT.
• Compare biometry between both eyes .
• Optical biometer is preferred , due to posterior staphyloma.
• Prefer olsen or barrett.
• Aim for residual myopia.

33. REFERENCES
• https://eyewiki.aao.org/Biometry_for_Intra-
Ocular_Lens_(IOL)_Power_Calculation
• https://www.zeiss.com/meditec/int/c/-optical-biometry-/iol-power-calculation-
formulas-explained.html
• https://cybersight.org/portfolio/lecture-biometry-basics-and-practical-review/
• https://www.aaojournal.org/article/s0161-6420(17)31428-8/fulltext

34. Feedbacks:
nikhilagrawalbv@gmail.com