Use of the C-constant with optical biometry
Let's have a look on the different sources of error:
1. The axial length. If you make a 0.1 mm error in the measurement of the axial length this will translate into about 0.25 D error in the spectacle plane for the average eye (more for a short eye and less for a long eye). At the time of ultrasound this was a major source of error which accounted for more than 50% of the total error in IOL power calculation.1 However, after the introduction of the IOLMaster (Carl Zeiss Meditec, Jena, Germany) more than 10 years ago the biometry error has been brought down to about 20–30 um (SD) between repeated measurements, which is about 10 times better than that of ultrasound.2 Thus, if the axial length were the only source of error we would have an error of less than 0.1 D in the average case. As you know this is not the standard of everyday clinical practice. So, other sources of error must be at hand.
3. The position of the IOL implant after surgery (often called the 'ELP' - Estimated Lens Position). This is the dark horse in every IOL power calculation formula and is the true 'prediction' made by the formula (the remaining are 'just' optics). If you make a 0.1 mm error in the estimation of the ELP this will translate into a 0.14 D error in the spectacle plane for the average eye. The error is higher in a short eye and lower in a long eye. I have previously found a standard deviation of the postoperative IOL position of about 0.35 mm in normal cases following standard capsulorhexis and in-the-bag IOL placement.3 Thus, if you had no other errors in the system the ACD error would account for about 0.50 D error (SD). Compared with the other errors mentioned before it is evident that this source of error must be considered with special interest. In the next section you will find how modern optical biometry can help you to improve the prediction of the ELP as well as the true, physical position of the IOL.
4. Other errors. Other errors such as IOL mislabelling, calibration errors, surgical variation (variation in the IOL placement) and errors associated with the measurement of the refraction itself should also be considered for a comprehensive analysis. However, the above errors (points 1–3) are to be regarded as the minimum errors in any IOL power calculation system.