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Aconstant conversion optical - US biometry

If you are doing US biometry and you are migrating to optical biometry then this is how you can convert one to the other based on the keratometry and contact or immersion US biometry you use currently

  1. The below takes into consideration the type of US biometry being done- contact or immersion

  2. Based on the keratometry that you have been doing so long with US biometry.

If you are doing optical biometry and the lens you are implanting has only optical biometry A-constant, and for some reason you have to do a ultrasound biometry for the patient, then you can follow the below suggestive guideline to convert the optical A-constant to ultrasound A-constant. Below takes into account the following for the patient ultrasound biometry :

  1. Type of ultrasound Axial length - contact or immersion

  2. Type of Keratometry - manual or auto

Based on the axial length and keratometry to be used for the patient, here is a suggestive guideline

Aconstant conversion from optical to contact or immersion based ultrasound biometry taking into consideration keratometry device
Aconstant conversion from optical biometry constant to contact or immersion based on Keratometry


Step 1 : Check the optical Aconstant from the manufacturer.

Step 2: With what would you be deriving corneal readings from? - manual or auto ( auto includes optical biometry)

Step 3: With what would you be measuring axial length? - contact or immersion ultrasound.

Step 4: For all the different permutation and combination based on manual and auto keratometry and contact or immersion ultrasound, you can now select your new Aconstant from the optical biometry Aconstant provided by manufacturer.


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