Pseudophakic A-scans

Achieving Accurate Pseudophakic A-scans

Accurate A-scan biometry for the pseudophakic eye is best achieved when approached in a specific manner.

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 IOLMaster, these measurements can be easily made using optical coherence tomography (OCT) or optical coherence biometry. However, if you do not have access to an IOLMaster, A-scan ultrasonography can render highly satisfactory results if done correctly.

A more accurate method for performing pseudophakic immersion A-scan is to set all gates to 1,532 m/s (the aphakic velocity) and then apply 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 – the true axial length
AAL 1532 – the apparent axial length when a pseudophakic eye is measured at a sound velocity of 1,532 m/sec
CF – the material-specific conversion factor
T – 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 varies depending on 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/s. Because this is faster than 1,532 m/s, 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 a speed of 1,532 m/s. 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 (such as the SI-26NB) at eye temperature (35 °C) is known to be 980 m/s. 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 a speed of 1,532 m/s. 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/s, 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 SI-26NB 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 SI-40NB 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.

Due to the differences in silicone intraocular lens implant materials, it is crucial 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/s. 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 a speed of 1,532 m/s. 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 )

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.

Holladay JT: Standardizing constants for ultrasonic biometry, keratometry, and intraocular lens power calculations. J Cataract Refract Surg 1997; 23: 1356 – 1370

Sanders DR, Retzlaff JA, Kraff, MC: A-scan biometry and IOL implant power calculations. In Focal Points, Clinical Modules for Ophthalmologists 1995; 13(10): 1 – 14

Holladay JT, Praeger TC: Accurate ultrasonic biometry in pseudophakia. (letter) Am J Ophthalmol 1993; 115(4): 536 – 537

Holladay JT, Praeger TC: Accurate ultrasonic biometery in pseudophakia. (letter) Am J Ophthalmology: 1989; 107(2):189