Achieving Accurate Pseudophakic A-scans.

Accurate A-scan 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, A-scan ultrasonography can render highly satisfactory results, if done properly.

A better way to perform pseudophakic immersion A-scan 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 A-scans 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.

TAL Acrylic = AAL 1532 + ( 0.30 x T )

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 SI-26NB) at eye temperature ( 35° C ) is known to be 980 m/sec. However, some of the newer silicone IOLs (like the Allergan SI-40NB) 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.

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

TAL = 24.00 + ( -0.56 x 1.10 ) = 23.38 mm.

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.

TAL PMMA = AAL 1532 + ( 0.45 x T )

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.