Achieving Accurate Pseudophakic Ascans.
Accurate
Ascan biometry for the pseudophakic eye is best accomplished if
approached in a specific way.
Having the ultrasound machine set to "pseudophakic mode" is
not necessarily the best idea, due to the widely different ultrasound
velocities of silicone, acrylic, and PMMA.
Since the introduction of the Zeiss IOL Master, these measurements
can easily be made using optical coherence biometry. However, if
you do not have access to an IOL Master, Ascan ultrasonography
can render highly satisfactory results, if done properly.
A better way to perform pseudophakic
immersion Ascan is by setting all gates to 1,532 m/sec (the
aphakic velocity) and then add a correction factor
for the velocity of the intraocular lens. Performing pseudophakic
Ascans in this way has the advantage of making measurement
independent of subtle velocity errors, caused by
differences in axial length. Remember to lower the
gain to avoid reduplication echoes.
In the American Journal of Ophthalmology Holladay and
Prager(reference 3.,4.) have
described this elegant method for measuring pseudophakic eyes as
follows:
TAL = AAL 1532 + ( cf x t )
Where TAL is the true axial length, AAL 1532 is the apparent axial length when a pseudophakic eye is measured at a sound velocity of 1,532 m/sec, CF is the material specific conversion factor and T is the center thickness of the intraocular lens. For PMMA, the conversion factor is +0.45; for silicone it is either 0.56 or 0.41 (depending on the style and manufacturer); and for acrylic it is +0.30. The IOL center lens thickness will vary with the material, manufacturer, power, diameter and design. To avoid errors, obtain the exact thickness of the lens directly from the manufacturer.
Acrylic Intraocular Lenses
The ultrasound velocity through an acrylic intraocular lens at
eye temperature (35° C) is known to be 2,180 m/sec. Because
this is faster than 1,532 m/sec, the apparent axial length displayed
by the ultrasound machine will be falsely short. However, by
knowing the ultrasound velocity through the acrylic material
and the center thickness of the intraocular lens, it is possible
to estimate its contribution to the overall axial length measurement.
First, we must calculate a conversion factor when the axial length
measurement is carried out at 1,532 m/sec. For an acrylic lens,
this is:
1 ( 1,532 / 2,180 ) = 0.2972, or 0.30.
Therefore, measured at an ultrasound velocity of 1,532 m/sec, the true axial length of an eye with an acrylic intraocular lens implant is determined by adding the apparent axial length at 1,532 m/sec to the conversion factor multiplied by the intraocular lens thickness. TAL
Acrylic
= AAL
1532
+ ( 0.30 x T )
For example, if at an ultrasound velocity of 1,532 m/sec, a pseudophakic eye with a 6.0 mm diameter +22.00 D acrylic intraocular lens ( thickness = 0.86 mm ) shows an apparent axial length of 24.00 mm, the true axial length would be: TAL = 24.00 + ( 0.30 x 0.86 ) = 24.26 mm.
Silicone Intraocular Lenses
The ultrasound velocity through first
generation silicone intraocular lenses (like the SI26NB) at eye temperature
( 35° C ) is known to be 980 m/sec. However, some of the newer silicone
IOLs (like the Allergan SI40NB) have an ultrasound velocity of 1,090 m/sec.
Because both of these ultrasound velocities are slower than 1,532 m/sec, the
apparent axial length displayed by the ultrasound machine will be falsely long.
Again, by knowing the ultrasound velocity through the silicone material, and
the center thickness of the intraocular lens, it is possible to estimate its
contribution to the overall axial length measurement.
First, we must calculate a conversion factor when the axial length measurement
is carried out at 1,532 m/sec. For older silicone lenses, this would be:
1 ( 1,532 / 980 ) = 0.5633, or 0.56. For some of the newer silicone lenses, this would be: 1 ( 1,532 / 1085 ) = 0.4120, or 0.41.
Note: these are negative numbers.
Therefore, measured at an ultrasound velocity of 1,532 m/sec, the true axial length of an eye with a silicone intraocular lens implant is determined by adding the apparent axial length to the conversion factor ( remember that this is a negative number ) multiplied by the intraocular lens thickness.
TALSilicone= AAL1532 + ( 0.56 x T ) For older silicone lenses
TALSilicone= AAL1532 + ( 0.41 x T ) For a few newer silicone lenses
Example 1 If at an ultrasound velocity of 1,532
m/sec, a pseudophakic eye with a 6.0 mm diameter +22.00 D Allergan SI26NB intraocular
lens implanted in 1990 ( velocity = 980 m/sec, thickness = 1.10 mm ) shows an
apparent axial length of 24.00 mm, the true axial length would be:
TAL = 24.00 + ( 0.56 x 1.10 ) = 23.38 mm.
Example 2 If at an ultrasound velocity of 1,532
m/sec, a pseudophakic eye with a 6.0 mm diameter +21.00 D Allergan SI40NB silicone
intraocular lens implanted in 1999 ( velocity = 1,090 m/sec, thickness = 0.92
mm ) shows an apparent axial length of 24.00 mm, the true axial length would
be:
TAL = 24.00 + ( 0.41 x 0.92 ) = 23.62 mm.
Because of the differences in silicone intraocular lens implant materials, it
is important to obtain information regarding the material specific ultrasound
velocity, and power specific center thickness, by contacting the manufacturer
directly.
PMMA Intraocular Lenses
The ultrasound velocity through a PMMA intraocular lens at eye temperature
( 35° C ) is known to be 2,780 m/sec. Because this is faster than 1,532
m/sec, the apparent axial length displayed by the ultrasound machine will be
falsely short. Once again, by knowing the ultrasound velocity through the PMMA
material, and the center thickness of the intraocular lens, it is possible
to estimate its contribution to the overall axial length measurement.
First,
we must calculate a conversion factor when the axial length measurement is
carried out at 1,532 m/sec. For a PMMA lens, this would be:
1 ( 1,532 / 2,780 ) = 0.4489, or 0.45.
Therefore, measured at an ultrasound velocity of 1,532 m/sec, the true axial length of an eye with a PMMA intraocular lens implant is determined by adding the apparent axial length to the conversion factor multiplied by the intraocular lens thickness. TAL
PMMA
= AAL
1532
+ ( 0.45 x T )
For example, if at an ultrasound velocity of 1,532 m/sec, a pseudophakic eye with a single piece 6.0 mm diameter +22.00 D PMMA intraocular lens ( thickness = 0.86 mm ) shows an apparent axial length of 24.00 mm, the true axial length would be: TAL = 24.00 + ( 0.45 x 0.86 ) = 24.39 mm.
Note:
If you are unable to locate center thickness information from the manufacturer, or do not have exact IOL information learn about Pseudophakic Correction Factors that can be added to the axial length measured at 1,532m/sec. The correction factors for acrylic and PMMA lenses will be positive and the correction factors for silicone lenses will be negative.
For those NOT interested in working through the math by hand for every pseudophakic
patient, the Holladay
IOL Consultant can
automatically calculate the corrected axial length.
