Warren Hill, M.D. Eye Surgeon Information Intraocular Lens Power Calculations Main Page
IOL power calculations have certainly come a long way. As recently as 1977,
the state of the art calculation to achieve emmetropia with a
posterior chamber IOL, was simply adding +19.0 D to the
pre-cataractous refraction.
Around 1990, with the formulas available at that time, being within
±1.00 D of the target refraction was considered a high
standard.
Today, by paying careful attention to detail, it is possible to be
within ±0.50 D in 95 percent of surgeries, or better.
East Valley Ophthalmology specializes in
ocular biometry and intraocular lens power calculations, in normal and
highly unusual clinical settings.
The following list covers some of the
latest developments in those areas:
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A-Scan Biometry using Ultrasound, an important and sometimes challenging preoperative
measurement, can be carried out in several ways. In this section, we will compare
the advantages of the immersion technique over applanation, and how to
improve accuracy.
If your A-scan machine allows for changes in the ultrasound velocity,
there are several ways to increase the accuracy of axial length
measurements. Follow this link for advanced
A-Scan Biometry Techniques
to axial length measurements.
Optical coherence Biometry using the Carl Zeiss Meditec IOL Master is a state of the art technique for measuring axial
length. By partial coherence interferometry, the IOL Master
achieves a level of accuracy and reproducibility never before seen. This has
its own section on our site.
What if the posterior capsule is broken? Follow this link for a chart
that compares the differences between Capsular Bag and Ciliary Sulcus IOL powers over a standard intraocular lens power range.
When the mathematics is done correctly, it yields some surprising
answers.
Intraocular lens power calculations forCorneal
Transplantation, cataract
removal and intraocular lens implantation, as a combined procedure, often result
in unpredictable refractive outcomes if all three procedures are carried out
during the same operative session. Follow this link for some suggestions as to
how to obtain highly accurate post-operative results.
Pseudophakic Correction
Factors: When performing A-scan biometry on
a pseudophakic eye at a velocity of 1,532 m/sec, add the corresponding correction
factor to the axial length displayed to get a close approximation of the
true axial length.
Determining the Effective Lens Position
(previously referred to as the anterior chamber depth) for each IOL
and each surgeon is an effective way to reduce the prediction error to
a minimum. The actual postoperative position of the intraocular lens
implant can be calculated and used to "personalize" this parameter
for your IOL calculation program.
The Haigis Formula:
Rather than simply move 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.
Prior Keratorefractive
Surgery will make direct measurements for
corneal power problematic. Learn how to to estimate the true corneal
power following RK, PRK, and
LASIK.
Tracking outcomes is an essential part of any cataract surgery
practice. We have a recommendation for a state of the art software
program that will do IOL calculations and Outcomes Database
management.
When properly indicated, primary Polypseudophakia makes it possible to implant total IOL powers beyond +40.0
D.
Determining the correct axial length for Pseudophakia
presents special challenges. Having the ultrasound machine set to "pseudophakic mode" is not necessarily the best idea, due to the widely different ultrasound velocities of silicone, acrylic, and PMMA.
For eyes with existing intraocular lenses, or aphakic eyes, it is a
simple matter to calculate the optical power that must be added to, or
subtracted from, using the Refractive Vergence Formula, which is axial length independent.
Silicone Oil is sometimes temporarily placed in the vitreous cavity for recurrent retinal detachments in eyes with proliferative
vitreoretinopathy, proliferative diabetic retinopathy, cytomegalovirus retinitis, giant retinal tears, and following perforating injuries. Axial length measurements by ultrasound of an eye in which the vitreous cavity has been filled with silicone oil is an exercise with many potential pitfalls, especially
if the silicone oil has become emulsified.
Sometimes the center of the macula is not the most posterior structure
in
axial myopia with a Staphyloma. Using B-scan ultrasonography, combined with a
simultaneous vector A-scan, it is possible to measure the axial length
to the center of the macula.
Something not look right? When should you have someone check on your
findings? Follow this link for a listing of helpful axial length and
keratometry Validation Guidelines.
Not sure about Which IOL Formula
to use? If you have not done so already, now is the time to
incorporate one of the newer theoretic formulas.
Here are some interesting and useful Links that
are updated on a regular basis.
Like to know more about these topics? Follow this link to
References for much of the material presented above.
Just about everything on this web site is the result of information
and suggestions from others. Many deserve our Special Thanks for graciously offering their guidance.
Formula
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