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Understanding The Haigis Formula.
As an original innovation, the Haigis Formula holds out the promise of a
new level of mathematical flexibility for increasing the accuracy of all IOL
power calculations.
Dr. Wolfgang Haigis  
One of the final frontiers in ophthalmology is the consistently
accurate calculation of intraocular lens power for all eyes.
When properly
"personalized," any of the modern IOL power calculation formulas
will do a good job for
normal axial lengths and normal central corneal powers. However, for very
long or short eyes, or for eyes with very flat or very steep corneal powers,
consistently accurate IOL power calculation has remained elusive.

IOL constants and IOL power prediction
The present system of IOL constants works by simply moving the position of an IOL
power prediction curve for the utilized formula up or down.
For each formula, the shape of this power prediction curve is mostly fixed. The larger the IOL constant, the more IOL power each
formula will recommend for the same set of measurements. And the
smaller the IOL constant, the less IOL power the same formula will recommend for the same set of measurements.
It is essential to note that the shape of this curve
remains the same. Other than the lens constant, these formulas treat all IOLs
as if they were the exactly same and make similar assumptions for all eyes
regardless of individual differences.
In reality, two eyes with the exact same axial length and the same
keratometry may require completely different IOL powers. This is due to
two additional variables: the actual (not assumed) distance of the lens
from the cornea (known as the effective lens position) and the
individual geometry of each lens model. Commonly used
lens constants simply do not take this into account.
These include:
SRK/T formula — uses "Aconstant"
Holladay 1 formula — uses "Surgeon
Factor"
Holladay 2 formula —
uses "Anterior Chamber Depth"
Hoffer Q formula —
uses "Anterior Chamber Depth"
These standard IOL constants are mostly interchangeable. Knowing one,
it is possible to calculate another. In this way, surgeons can move
from one formula to another for the same intraocular lens implant. 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 creation 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 practicespecific variables.
Also consider that 3rd generation 2variable formulas (SRK/T, Hoffer Q and
Holladay 1) assume that the distance from the principal plane of the cornea
to the thin lens equivalent of the IOL is in part related to the axial
length. That is to say, short eyes will have more shallow anterior chambers
and long eyes will always have deeper anterior chambers.
We
now know that this is not necessarily so. In reality, short eyes most
commonly have perfectly normal anterior chamber anatomy in the
pseudophakic state.
What these eyes do have is large lenses. Take out
the lens and the anterior chamber dimensions, 80% of the time, are not
all that different from an eye of normal axial length.
Think about when we do phaco for a patient with a short axial length and
prior angle closure — what does the resultant anatomy look like? It looks
just like a normal eye; and that is why all 3rd generation 2variable formulas
have a limited axial length range of accuracy. The Holladay 1, for example,
works well for eyes of normal to moderately long axial lengths, while the Hoffer
Q has been reported to work better for shorter axial lengths.
A recent exception to all of this is the Haigis Formula, which here in
North America comes as part of the IOL Master software package.
Rather than moving a fixed formulaspecific 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.
d = the effective lens position, where ...


d = a0 + (a1 * ACD) + (a2 * AL) 





ACD is the measured anterior chamber depth of the eye (corneal vertex
to the anterior lens capsule), and ... 





AL is the axial length of the eye; the distance from the cornea vertex,
to the vitreoretinal interface. 




*

The a0 constant basically moves the power prediction curve up, or
down, in much the same way that the Aconstant, Surgeon Factor, or
ACD does for the Holladay 1, Holladay 2, Hoffer Q and SRK/T formulas. 




* 
The a1 constant is tied to the measured anterior chamber depth. 




* 
The a2 constant is tied to the measured axial length. 
In this way, the value for d is determined by a function, rather than a
single number.
The a0, a1 and a2 constants are derived by multivariable regression
analysis from a large sample of surgeon and IOLspecific outcomes for a
wide range of axial lengths and anterior chamber depths. The resulting a0,
a1 and a2 constants are such that they closely match actual observed results
for a specific surgeon and the individual geometry of an intraocular lens implant.
This means that a portion of the mathematics of the Haigis Formula is individually
adjusted for each surgeon/IOL combination. Dr. Wolfgang Haigis gets high marks
for this innovative approach.
The Haigis Formula IOL constants will appear different than what we are
normally used to seeing, as they interact with the ACD and the AL.
Recall that 3rd generation 2variable formula lens constants all
basically represent the same thing, which is an attempt to predict the
distance from the principal plane of the cornea to the thin lens
equivalent of the IOL. In the parley of IOL mathematics, this is known
as "d." The Haigis constants, when viewed all together, also determine
this distance, but calculate it in a new and more flexible manner.
"d" for the five formulas commonly in use are:
SRK/T d = Aconstant
Hoffer Q d = pACD
Holladay 1 d = Surgeon Factor
Holladay 2 d = ACD
Haigis d = a0 + (a1 * ACD) + (a2 * AL)
The key to highly accurate IOL power calculations is being able to
correctly predict "d" for any given patient and IOL.
One way is to measure the ACD, lens thickness and axial length, and
then force the formula to make adjustments based on previous
observations from some large research data set. This is probably what
the Holladay 2 formula does, adding or subtracting power from a
Holladay 1type IOL power prediction based on prior observations of
ACD, AL, LT, Rx, corneal diameter, etc.
The calculation data base for
the Holladay 2 formula is obviously substantial, as the Holladay 2 formula
works exceptionally well. We've used it for eyes as short as 16 mm and
as long as 38 mm. Dr. Holladay deserves high marks for what must have been
painstaking research and excellent science.
Another way is to look at actual observed outcomes and adjust "d" for
measured axial lengths and anterior chamber depths. This can be done by multivariable
regression analysis.
Now we're back to:
d = a0 + (a1 * ACD) + (a2 * AL)
The following example uses two different sets of actual regression
analysis derived Haigis constants for two intraocular lenses with the same
SRK/T Aconstant of 118.40.
Lens #1 is a single piece acrylic IOL with a
positive shape factor and lens #2 is a biconvex 3piece PMMA IOL with
10° per mm of posterior haptic angulation. At first glance (as we're
used to looking at an Aconstant, SF, or ACD) these two sets of Haigis
constants look completely different. However, they simply represent a
similar power prediction curves with a slightly different shape that
takes into account the differences in lens geometry between these two
IOLs.
