An IOL power calculation is only as good as the sum of its parts. In other words, every component of the calculation process must be optimized to ensure selection of the correct lens power. IOL power calculation formulas are based on accurate preoperative measurements including axial length, central corneal power, anterior chamber depth, and the individually optimized formula constant of the IOL.
Three individuals who have developed IOL power calculation formulas recently spoke with CRST Europe about them. All agree that optimization is the key to highly accurate IOL power calculation. Kenneth J. Hoffer, MD, FACS, a Clinical Professor of Ophthalmology at the University of California, Los Angeles, pioneered the field in 1974.
"The whole subject is based on what the formula requires and what data it needs to calculate the power," he said in a telephone interview. "The first and foremost is axial length, followed by corneal power and the position that the IOL is going to sit in."
Effective lens position, as it is called, is predicted by using specific data, including axial length and central corneal power. The first step, however, is making sure that all of the data is accurate.
"IOL power calculations are a dance of many parts," said Warren E. Hill, MD, of Mesa, Arizona, in a telephone interview. "You only need to have one part wrong to produce a refractive surprise."
Refractive surprises are not uncommon and were seen from the time of the first IOL implantation in 1949. If the refractive error is small enough, Dr. Hoffer said, then the patient may not notice it; however, the patient who demands but does not achieve absolute emmetropia will most definitely be an unhappy patient.
"A patient's happiness with the outcome always depends on how much the patient is expecting," Dr. Hoffer said. "If the refractive error is small, the patient is not going to notice the difference."
As new technologies have become available, however, the incidence of refractive surprises has decreased, and surgeons have to worry less about small refractive errors. According to Dr. Hill, the overall accuracy of refractive outcomes continues to improve. Patient selection, surgical technique, proper IOL power formula, accurate keratometry, and method of biometry all contribute to the decreasing incidence of refractive surprises.
OPHTHALMIC BIOMETRY
Currently, ophthalmic biometry may be performed with one of four methods—applanation A-scan, immersion A-scan, immersion A/B-scan, and optical coherence biometry with the IOLMaster (Carl Zeiss Meditec AG, Jena, Germany). Many surgeons agree that the IOLMaster, introduced in 1999, is currently the most accurate means of biometry. Now, surgeons can routinely provide refractive corrections within 0.25 D of the intended refraction.
"The accuracy in refractive outcomes that is possible today, which patients pay for and deserve, cannot be obtained by guesswork," Wolfgang Haigis, MS, PhD, of Würzburg, Germany, told CRST Europe. "That is why the IOLMaster is my first choice for biometry and keratometry. We also use the ACMaster [Carl Zeiss Meditec AG], which measures the anterior segment with laser precision."
Equipped with the latest software, the IOLMaster Version 5 can be used to accurately measure axial length, anterior chamber depth, and the horizontal corneal diameter.
"A common misconception is that if a surgeon buys an IOLMaster, where he can measure a more accurate axial length, that his power calculation will be accurate. But if the keratometry is not correct ? or another part of the measurement process is not done correctly, then a refractive surprise is still possible, even with that perfect axial length," Dr. Hill said. "Again, the paradox is that if one part is perfect, it does not guarantee a perfect outcome. If one part is bad, however, it can guarantee a refractive surprise. Even the most innocuous part, if it is done wrong, can give you a bad outcome."
In approximately 10% to 17% of eyes, the IOLMaster will not produce an acceptable reading, according to Dr. Hoffer. Most of these cases are because of posterior subcapsular cataracts or poor fixation. "In those eyes that you cannot get a reading using the IOLMaster, you can use immersion ultrasound," Dr. Hoffer said, adding that it is important to adjust the sound speed used to measure the axial length of the patient's eye. Alternatively, the surgeon may use the Holladay corrected axial length factor (CALF).
A combined immersion vector A/B-scan is the most sophisticated form of ultrasound-based biometry. During this procedure, the surgeon carries out a simultaneous horizontal immersion B-scan and vector A-scan, which measure from the corneal vertex to the fovea. John Shammas, MD, of Lynwood, California, recommended this method, which is most useful in cases of staphyloma.
Once a surgeon has gathered the patient's preoperative data, the next challenge is carrying out the calculation. At least five formulas are currently available, three of which are third-generation and two variable formulas. Switching to a newer-generation formula may improve the accuracy of the power calculation. The third-generation formulas include two parts: (1) calculating where the IOL will sit in the pseudophakic state after cataract surgery (ie, effective lens position [ELP]) and (2) calculating the actual power of the IOL. In order to achieve an accurate calculation, both components must be correct. If the ELP is incorrect, Dr. Hill explained, then the IOL calculation will also be incorrect.
The difference among the formulas include how the ELP is calculated. Two-variable formulas base the ELP on axial length and the central corneal power.
"Two-variable techniques are not as accurate, because you can have two eyes with the exact same axial length and central corneal power that require a completely different IOL power," Dr. Hill said. "This is because the IOL may sit at a different distance from the cornea, which determines its effective power."
Holladay 1 and 2. Jack T. Holladay, MD, of Houston, created the Holladay 1 formula in 1988. It determines the ELP by using axial length and keratometry to calculate the corneal height in a formula developed by Professor Svyatoslav Fyodorov. In Dr. Holladay's second formula, the Holladay 2, seven variables are used to predict lens position: axial length, keratometry, horizontal corneal diameter, anterior chamber depth, lens thickness, age, and refraction.
Hoffer Q. After the Holladay 1 formula was created, Dr. Hoffer compared the formula to his own. "I found that in a series of cases, the Holladay 1 was more accurate than the Hoffer formula," Dr. Hoffer said. "I was going to go ahead and use the Holladay 1, but Dr. Holladay cajoled me into improving my own formula."
Dr. Hoffer developed the Q formula and created a method that provided a better predictive average for where the IOL would sit. The Hoffer Q formula is best used in short eyes (less than 22 mm). In medium-long eyes (24.5–26 mm), the Holladay 1 formula is the better choice, and the SRK/T formula is best in very long eyes (more than 26 mm). All formulas work similarly in normal eyes (22–24.5 mm). The Holladay 2 formula improved results in short and very long eyes, compared with the Holladay 1, but the results were inferior to the Holladay 1 in normal eyes (22–26 mm), according to Dr. Hoffer.
"Most formula authors think that their formula is the best," Dr. Hoffer said. "But, I recommend that the Hoffer Q be used in short or normal eyes, the Holladay 1 in eyes between 24.5 and 26 mm, and the SRK/T in eyes greater than 26 mm. If you do that, you will get the best performance from all three formulas."
Haigis formula. Whereas the Holladay and Hoffer formulas use only one constant, this formula employs three constants, which Dr. Haigis labels as a0, a1, and a2. The measured anterior chamber depth is represented by a1, and axial length by a2. The advantage of the Haigis formula comes when the formula is triple-optimized.
"What is unique from the other formulas is that the Haigis formula is characterizing a given IOL by three constants instead of just one," Dr. Haigis said. "I use the preoperative anterior chamber depth as an additional predictor for the effective lens position—by this I can make allowance for different IOL geometries. Also, it is possible to obtain a prediction error that is independent from axial length."
Because the principal plane is different depending on the IOL, it is important to make allowances for those varying geometries, Dr. Haigis said. "This can be done by using three constants—like I do—or by using three A-constants, one for short, one for normal, and one for long eyes. With this trick, you get rid of some of the axial length dependence that each formula has."
To personalize all three constants of the Haigis formula, Dr. Hoffer explained, the user would need approximately 500 to 1,000 postoperative eyes from a single IOL style. "That is quite a burden for the average cataract surgeon," he said. "The Haigis formula is a great formula, but in order to use the great benefit of the formula, a large number of postop eyes and careful analysis is required." Dr. Haigis disagreed. "Usually, 200 eyes will do," he said.
Perhaps the best outcomes are obtained if the surgeon selects a formula based on the patient's eye.
"In the old days, a majority of surgeons used the old Binkhorst formula. Today, clinicians have become much more sophisticated and tend to use different formulas in different eyes," Dr. Hoffer said. "I think it is important to look at the length of the eye and vary the formula that you are using. There have been a significant number of papers that have found that to be true."
Regardless of what power calculation is used, surgeons must remember that it is their responsibility to implant an IOL with the correct power. Personalizing the lens constant, whichever formula is chosen, will hopefully allow for the most predictable outcomes, improve postoperative results, and strengthen patient satisfaction.
Wolfgang Haigis, MS, PhD, is a physicist at Universitäts-Augenklinik, Würzburg, Germany. Dr. Haigis states that he is a consultant to Carl Zeiss Meditec AG. He may be reached at +49 931 201 20640; w.haigis@augenklinik.uni-wuerzburg.de.
Warren E. Hill, MD, FACS, is the Medical Director of East Valley Ophthalmology, Mesa, Arizona. Dr. Hill states that he is a consultant to Carl Zeiss Meditec AG. Dr. Hill may be reached at +1 480 981 6111; hill@doctor-hill.com.
Kenneth J. Hoffer, MD, FACS, is Clinical Professor of Ophthalmology at the University of California, Los Angeles. He is the owner of Eyelab, which sells computer software for IOL power calculations, including the Hoffer Programs and the Holladay IOL Consultant. Dr. Hoffer may be reached at +1 310 451 2020; khoffermd@aol.com.
