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