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Optical Biometry Lens Constants.


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

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 always higher than those optimized for applanation A-scans.

If you already have optimized immersion A-scan lens constants, you can begin with these as a starting point.

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

Optical Biometry = A Ultrasound + 3 * (AL Optical Biometry - AL Ultrasound )
A Optical Biometry = Optimized A-constant for Optical Biometry
A Ultrasound = Optimized A-constant for ultrasonography
AL Optical Biometry = Average Optical Biometry axial length
AL Ultrasound = Average ultrasound axial length

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 optical biometry axial length was measured at 23.74 mm, then the SRK/T A-constant for optical biometry would be:

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

If you are making the transition from previously optimized applanation A-scan lens constants to optical biometry, below is a general rule of thumb that will give you a reasonable starting point with one of several popular IOL power calculation formulas.

Formula Add to Applanation
Lens Constant
Holladay 1 0.16
Holladay 2 0.16
Hoffer Q 0.16
SRK/T 0.34
SRK II 0.36

It is recommended that every 10 to 20 patients, the results 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.

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