Three different A-scan biometry techniques are presently in use:
Prager Shell used for Immersion A-scan
Which is better, applanation or Immersion?
Prager Shell is used for immersion A-scan biometry, which has better reproducibility, leading to an overall increase in accuracy. The immersion technique can be performed rapidly and with greater confidence than the applanation method. Making the transition from applanation to immersion is well worth the minor learning curve.
However, all forms of ultrasound-based biometry have two fundamental limitations. First, they use a rather large 10-MHz sound wave to measure a relatively small distance. Second, the area around the center of the macula is not flat, but thinnest at the fovea, with thicker shoulders. To overcome these two accuracy barriers, axial length must be measured using partial coherence interferometry with the Carl Zeiss Meditec IOLMaster.
By the applanation biometry method, an ultrasound probe is placed directly on the cornea, which slightly indents the surface. More desirable, the immersion technique allows the ultrasound probe to remain in direct contact with the coupling fluid, rather than the cornea, thereby preventing compression.
Quantel Aviso Ultrasound
Using a 10-MHz ultrasound transducer, by the immersion method, the typical accuracy for axial length measurements is within 0.12 mm. This results in approximately 0.28 D of post-operative refractive error in an eye of average axial length. This error would be more for a shorter eye and less for a longer eye. However, the total refractive error involves all components of the measurement process. It is closer to 0.36 D, taking into account keratometry, a 2-variable IOL power calculation formula, and the configuration of the capsulorrhexis.
Why is immersion better?
A-scan biometry by immersion will display an axial length somewhat longer than applanation, because there is no corneal compression, and the displayed axial length is closer to the true axial length. The difference between applanation and immersion can range from 0.14 mm to 0.28 mm, depending on the degree of corneal compression.
Upon implementing immersion A-scan ultrasonography, it is best to aim for a -0.75 D target refraction and start with the IOL manufacturer’s recommended ACD, A-constant, or Surgeon Factor, rather than the constants that were “personalized” in your surgeon-specific outcomes database during the course of past applanation A-scans. Failure to target a higher degree of myopia in the beginning may result in unexpected post-operative hyperopia of approximately +0.50 diopters in eyes of normal axial length. This is continued until your new immersion biometry lens constants can be established.
As described by Dr. Holladay, immersion A-scan ultrasonography can also be successfully carried out for the phakic eye with all gates set to the aphakic sound velocity of 1,532 m/sec. Using this advanced A-scan biometry technique, an extra +0.32 mm is added to the displayed axial length to correct for the thickness and different velocities of the lens and cornea. Measuring the axial length in this manner avoids several common problems and has become our preferred method for immersion A-scan biometry.
How to learn more about A-scan techniques:
We highly recommend the book A-scan Axial Length Measurements by Sandra Frazier Byrne. It is an excellent resource that you wouldn’t want to miss.
Also, there is an excellent national certification program in Ophthalmic Biometry available for your technicians:
