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Knowing one of these lens constants, it is possible to approximate another.
In this way, surgeons can move from one formula to another for the same intraocular
lens implant. However, the shape of the power prediction curve generated by
each formula remains the same no matter which IOL is being used. However, variations
in keratometers, ultrasound machine settings and surgical techniques (such
as the configuration of the capsulorrhexis) can all have an impact on the refractive
outcome as independent variables. "Personalizing" the lens constant for a given
IOL and formula can be used to make global adjustments for a variety of practice-specific
variables.
An exception to all of this is the Haigis Formula, which comes as part of the software packages of many biometers. Rather than moving a fixed, formula-specific IOL power prediction curve up (more IOL power recommended) or down (less IOL power recommended), the Haigis Formula instead uses three constants (a0, a1 and a2) to set both the position and the shape of a power prediction curve. Why is this important?
Why 3 Lens Constants?
For the Haigis formula, the a0 constant moves the power prediction curve up,
or down, in much the same way that the SRK/T A-constant. The a1 constant is
tied to the measured anterior chamber depth and the a2 constant is tied to
the measured axial length. Both the a1 and the a2 constants are used to vary
the shape of the power prediction curve, changing the power based on the central
corneal power, anterior chamber depth, axial length and individual lens geometry.
One little appreciated reality is that the geometry of many IOL models may
not be the same for all powers. When this is the case, it would be helpful
if a formula was able to take this information account. It is not uncommon
for some IOLs to have a shift in the lens constant by more than 1.00 D at the
low end of the power range. With three lens constants, the Haigis formula is
able to make adjustments adding or subtracting power when necessary, based
on actual observed results for a specific surgeon and the individual geometry
of an intraocular lens implant. All of this gives the Haigis Formula a new
level of mathematical flexibility not yet before seen in ophthalmology.
Dr. Haigis is the Head of the Biometry Department at the University of Wurzburg
Eye Hospital and the:
Users Group for Laser Interference Biometry (ULIB)
Follow this link for a free Excel spreadsheet you can use to derive your own
set of a0, a1 and a2 Haigis Formula IOL Constants and instructions for submitting
this data to Dr. Hill in North America or Dr. Haigis in Europe.
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