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The Haigis Formula.


The  Haigis Formula for IOL 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 eyes with a normal anterior segment. However, for eyes where the anterior segment may be abnormal in some way (influencing the effective lens position), consistently accurate IOL power calculations have remained elusive.

IOL Constants and IOL Power Prediction.

For most formulas, the shape of its IOL power prediction curve is fixed: for a given lens constants and pair of axial length and central corneal power measurements, the IOL power predicted will always be the same. Another equally inflexible feature is that the larger the IOL constant, the more IOL power each formula will recommend for the same set of measurements; the smaller the IOL constant, the less IOL power the same formula will recommend for the same set of measurements. 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 for emmetropia. 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:

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SRK/T formula — uses "A-constant"

Holladay 1 formula — uses "Surgeon Factor"

Holladay 2 formula — uses "Anterior Chamber Depth"

Hoffer Q formula — uses "Pseudophakic Anterior Chamber Depth"

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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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East Valley Ophthalmology, Arizona's premier eye specialists, provides this on-line information for educational and communication purposes only and it should not be construed as personal medical advice. Information published on this website is not intended to replace, supplant, or augment a consultation with an eye care professional regarding the viewer/user's own medical care. East Valley Ophthalmology's disclaims any and all liability for injury or other damages that could result from use of the information obtained from this site. Please read our full Disclaimer