IOL Master Lens Constants.

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

The IOL Master measures from the corneal vertex to the retinal pigment epithelium, but the axial length displayed by the IOL Master 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 the IOL Master is the equivalent of an upright, non-contact, ultra high resolution immersion A-scan. For this reason, IOL Master 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:

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 IOL Master axial length was measured at 23.74 mm, then the SRK/T A-constant for the IOL Master 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 the IOL Master, below is a general rule of thumb that will give you a reasonable starting point with one of several popular IOL power calculation formulas.

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.

Follow this link to download an Excel spreadsheet, used for calculating Four-Formula Lens Constants (Holladay 1, SRK/T, SRK II and Hoffer Q) for patients whose axial length has been measured with the IOLMaster. Simply enter data from 30 patients and an average lens constant for each of these formulas will be calculated. This can also be used as a starting point for IOL constant optimization.