2. Posterior Staphyloma
Posterior staphylomas often causes
an irregular shape of the ocular wall
resulting in an inability to display a
distinct , high retinal spike , leading
to a significant error in A-scan
Deepest portion of the staphyloma
may be located eccentric to macula
thus te measurement may be longer
than true AL along the visual axis .
B-scan can be used to demonstrate
the shape of posterior ocular wall
and the relationship of macula to
the staphyloma .
A second person should confirm measurements prior to A-scan
ultrasonography if: The corneal power is less than 40.0 diopters, or
greater than 47.0 diopters.
If there has been prior keratorefractive surgery. In this case the corneal
power will need to be estimated by either the historical, or the contact lens
The average corneal power difference between the two eyes is greater
than 1.00 diopter.
The patient cannot fixate, as seen with a mature cataract, or macular hole.
The amount of corneal astigmatism by keratometry, or
topography, correlates poorly with the amount of astigmatism on the most
recent manifest refraction.
The corneal diameter is less than 11.00 mm.
There is any problem with patient cooperation, or understanding.
Immersion A-scan Ultrasonography
A second person should re-measure both eyes if: The axial length is less
than 22.00 mm, or greater than 25.00 mm in either eye.
The axial length is greater than 26.0 mm, and there is a poor retinal
spike, or wide variability in the readings.
There is a difference in axial length between the two eyes of greater than
0.33 mm that cannot be correlated with the patient's oldest refraction.
Axial length measurements do not correlate with the patient's refractive
error. In general, myopes should have eyes longer than 24.0 mm and
hyperopes should have eyes shorter than 24.0 mm. Exceptions to this rule
involve steep, or flat corneas. Be sure to use the oldest refractive data.
There is difficulty obtaining correctly positioned, high, steeply rising
echoes, or wide variability in individual axial length readings for either
There is a difference in axial length between the two eyes of greater
than 0.33 mm that cannot be correlated with the patient's oldest refraction.
Axial length measurements do not correlate with the patient's refractive error.
In general, myopes should have eyes longer than 24.0 mm and hyperopes
should have eyes shorter than 24.0 mm. Exceptions to this rule involve steep,
or flat corneas. Be sure to use the oldest refractive data.
There is difficulty obtaining correctly positioned, high, steeply rising echoes,
or wide variability in individual axial length readings for either eye.
Intraocular Lens Power
A second person should repeat the axial length measurements, keratometry
readings and re-run the IOL power calculations for both eyes if: The IOL
power for emmetropia is greater than 3.00 diopters different than
There is a difference in IOL power of greater than 1.00 diopter between the
If the patient has had prior keratorefractive surgery and the calculated IOL
power for standard phacoemulsification is less than +20.0 D or greater than
IOL Power Formula
These are derived from
Actual postop refractive results
of many lens implantations are
used to predict IOL power
These formulas contain many
assumptions including values of
postop ACD , refractive index of
cornea and ocular humors , retinal
These formulas are reliable for
average AL , but overestimates
in short eyes and underestimates
in long eyes
IOL power calculation without
determination of axial length
P = A – 2.5L – 0.9K
It generally undercorrects short eyes
and overcorrects long eyes
A1 = A + 3 AL < 20mm
A1 = A + 2 AL 20-21
A1 = A + 1 AL 21-22
A1 = A AL 22-24.5
A1 = A – 0.5 AL >24.5
It is a nonlinear theoretical optical
formula empirically optimized for
postop ACD , retinal thickness ,
corneal refractive index .
It combines advantages of theoretical
and regression formulas .
There are currently three IOL constants in use: The
SRK/T formula uses an "A-constant."
The Holladay 1 formula uses a "Surgeon Factor."
The Holladay 2 formula, and the Hoffer Q formula,
both use an "Anterior Chamber Depth." aka: ACD.
d = the effective lens position, where ...
d = a0 + (a1 * ACD) + (a2 * AL)
* The a0 constant basically moves the curve up,
or down, in much the same way that the A-
constant, Surgeon Factor, or ACD does for the
Holladay 1, Holladay 2, Hoffer Q and SRK/T
* The a1 constant is tied to the measured
anterior chamber depth.
* The a2 constant is tied to the measured axial
length. The way the a0, a1 and a2 constants are
derived is by generating a set of surgeon, and
AL < 19 mm (<0.1%)
AL 19-22 mm (8%)
Holladay 2 , Hoffer-Q
AL 22-24.5 mm (72%)
SRK II , Hoffer-Q ,Holladay 1
AL 24.5-26 mm (15%)
Holladay 1 , Hoffer-Q
AL > 26 mm ( 15%)
Axial Length in mm Haigis
Hoffer Q Holladay 1 Holladay 2 SRK/T
20.00 to 21.99 0.25 D 0.25 D 0.25 - 0.50 D 0.25 D 0.51 - 1.0 D
22.00 to 24.49 0.25 D 0.25 D 0.25 D 0.25 D 0.25 D
24.50 to 25.99 0.25 D 0.25 D 0.25 D 0.25 D 0.25 D
26.00 to 28.00 0.25 - 0.50 D 0.25 - 0.50 D 0.25 D 0.25 D 0.25 D
28.00 to 30.00 0.25 - 0.50 D 0.25 - 0.50 D 0.25 D 0.25 D 0.25 - 0.50 D
Minus power IOLs 0.51 - 1.0 D 0.51 - 1.0 D 0.25 - 0.50 D 0.25 D 0.25 - 0.50 D
may be appropriate for all
ranges of axial lengths