ULIB
User Group for Laser Interference Biometry

Relations between IOL constants for optical and acoustical biometry:

The following rules of thumb may be applied as a starting point to estimate new IOL constants for optical biometry with the Zeiss IOLMaster:


(1) AIOLMaster = AUltrasound + 3 * (ALIOLMaster - ALUltrasound)
    
where A IOLMaster :(optimized) A-constant for optical biometry (with the IOLMaster)
A Ultrasound:(optimized) A-constant for acoustical biometry (ultrasound a-scan and keratometer)
ALIOLMaster :mean of axial length as measured with the IOLMaster
ALUltrasound:mean of axial length as measured by ultrasound
    
e.g. 119.6 =119.3 + 3 * (22.98 - 22.88)

For formula (1) it is assumed that keratometry results with the IOLMaster and the hitherto used separate keratometer are identical. If this is not the case, i.e. if there is a difference in the mean corneal radii measured with the two instruments, formula (2) applies:

(2) AIOLMaster = AUltrasound + 3 * (ALIOLMaster - ALUltrasound) - 6 * (RIOLMaster - RKeratometer)
    
where A IOLMaster :(optimized) A-constant for optical biometry (with the IOLMaster)
A Ultrasound:(optimized) A-constant for acoustical biometry (ultrasound a-scan and keratometer)
ALIOLMaster :mean of axial length as measured with the IOLMaster
ALUltrasound:mean of axial length as measured by ultrasound
RIOLMaster :mean of corneal radii as measured with the IOLMaster
RKeratometer:mean of corneal radii as measured by separate keratometer
    
e.g. 119.6 =119.6 + 3 * (22.98 - 22.88) - 6 * (7.80-7.75)

Please note::
Formulas (1) and (2) are merely rules of thumb: the constants (+3 and -6) in these formulas are averages. In fact, each IOL formula has its own constants ranging from +2.5 to +3 (axial length factor) and -4.7 to -6.4 (corneal radius factor). The resulting lens constants from formulas (1) and (2) can serve as starting points. The final IOLMaster constants may be expected to lie in between +/- 0.3 D of these values.
More details on this subject were dealt with in a talk at the 2001 ASCRS meeting in San Diego, USA: HAIGIS W, STROBEL J: Influence of measurement conditions on IOL constants. A summary is given
here
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Last Revision: Jul 17, 2009 WH