Lens Constants

Because there is no corneal contact, optical biometry lens constants will always be higher than those optimized for applanation A-scans, in which a variable amount of corneal compression occurs.

Optical biometry measures from the corneal vertex to the retinal pigment epithelium, but the axial length displayed by optical biometry is adjusted by an internal algorithm to mirror immersion A-scan results with the exquisitely accurate Grieshaber Biometric System. In essence, the axial length displayed by optical biometry is the equivalent of an upright, non-contact, ultra-high resolution immersion A-scan. For this reason, optical biometry lens constants will be very close to those optimized for immersion A-scan ultrasonography, but consistently higher than those optimized for applanation A-scans. If you already have optimized immersion A-scan lens constants, you can use them as a starting point.

We offer specific lens constants for the following optical biometers:

Dr. Wolfgang Haigis at the University of Würzburg has recommended the following approach for calculating the initial IOL Master SRK/T A-constant:

For example, if your previously optimized applanation A-scan ultrasound constant for a specific IOL was 118.00, and for a specific patient, the applanation A-scan axial length was 23.46 mm, and the optical biometry axial length was measured at 23.74 mm. The SRK/T A-constant for optical biometry would be:

118.00 + (3 * (23.74 – 23.56)) = 118.54

Suppose you are transitioning from previously optimized applanation A-scan lens constants to optical biometry. In that case, the following is a general rule of thumb that provides a reasonable starting point for one of several popular IOL power calculation formulas.

It is recommended that the results of every 10 to 20 patients be reviewed, and the lens constant for the formula being used be adjusted up or down by no more than 0.10. This will prevent wide oscillations within a relatively small sample size.