Optical vs U/S
All IOLs in the Program database have U/S Lens Constants on the package insert as required by the FDA in the past. Optical (IOL Master and LenStar) are not available, so people like Warren Hill and Wolfgang Haigis provide them for several IOLs. These values are only a starting point and only correct for the surgeon who did the study. Your surgeons should be using Personalized Lens Constants as soon as possible by entering post-op refractions.
The lens constant (Aconstant and ACD) for most IOLs are 0.3 D and 0.2 mm higher respectively than the U/S Lens constant, so these are good starting points. If the Optical (IOL Master/LenStar) Manufacturer’s Optical Lens Properties is BLANK (no information provided by manufacturer or UIL) then the PROGRAM automatically adds 0.2 mm to the ULTRASONIC ACD (which is 0.3 D to the Aconstant) and uses this value for the optical MDG ACD. This is the average difference between U/S and Optical.
Regarding the 0.2 difference...It is actually due to the difference in size of the keratometric ring from manual to IOL Master (Optical Keratometry).
The IOL Master and LenStar were calibrated to immersion Ascan, so the average axial length would be the same. However, the IOL Master’s keratometer is 2.5 mm and the manual keratometer that was used with immersion Ascan was 3.2 mm. The difference in the reading is 0.3 D, which is equivalent to a ELP (effective lens position) difference of 0.2 mm.
So, if one has trouble getting an AXIAL LENGTH on the IOL Master and goes to immersion Ascan, but uses the Keratometry from the IOL Master, then you should still use the Optical Constant. The IMMERSION ASCAN CONSTANT (U/S) should only be used if the KERATOMETRY is taken on the MANUAL KERATOMETER.
Effect of Ring Diameter differences
Question....I entered 118.4 as the MFG A-constant bit was changed to to 118.74 in the Lens Calc print-out. Why?
Here is what is happening with the A-constant.
The manufacturer’s supplied ACD’s and A-constants are from many years ago, when U/S and manual keratometers were used. The OPTICAL BIOMETER (IOL Master) was calibrated to Immersion Ascan (U/S), so it correlates pretty well up to about 26 mm, then it gets a longer axial length than U/S which is the reason why the Holladay NON LINEAR LONG EYE AXIAL LENGTH ADJUSTMENT box that should be checked.
The IOL Master keratometer uses a 2.5 mm diameter RING rather than a 3.2 mm diameter ring used by the manual keratometer. The smaller ring resulted in K-readings that were 0.34 D higher than the manual keratometer, resulting in a need to increase the A-constant by 0.34 D (or ACD by 0.21 mm). So when Optical Biometry (IOL Master/LenStar) is used, the U/S A-constant is increased by 0.34 D to compensate for the Ring Diameter used (it has nothing to do with U/S vs OCT).
ULIB and some manufacturers have actually begun to determine their ACD and Aconstants from data. When that occurs they will usually report both U/S and Optical. When that occurs, we put BOTH IN THE DATABASE. When this occurs we leave OPTICAL BLANK and increase the VALUE by 0.34 D. The USER can go to ULIB or somewhere else and then ENTER THEIR own value and the software no longer uses the DEFAULT DIFFERENCE.
Differences from conversions
IF there is a slight difference in the other converted Constants (ACONSTANT, ACD & SF) it is because they have not used the STANDARDIZED conversions as described in
article #61. Holladay JT. Standardizing constants for ultrasonic biometry, keratometry, and intraocular lens power calculations. J Cataract Refract Surg. 1997; 23: 1356-70),
HICSOAP automatically implements this conversion between the constants and there is no way to change this functionality. When they are different use the value that gives the largest numbers. When they are that different, the older ACD value is usually for the sulcus, not the bag where IOLs were placed for the original ACDs.