Module 1: Biometry

Achieving emmetropia is certainly a challenge for any surgeon. Selecting the optimal IOL is a complex process that can involve multiple measurements with multiple devices, data analysis and interpretation, familiarity with new formulas, patient counseling and so on. It is amazing that surgeons achieve the good outcomes that they do, but many times surgeons still end up with refractive surprises. Below, Douglas D. Koch, MD, assures surgeons that fear of off-target outcomes should not be a deterrent to embracing refractive cataract surgery.

Q. Describe how you became one of ophthalmology’s go-to experts on biometry.

Douglas D. Koch, MD: Nearly everything in my research has emanated from problems I have encountered in practice. The challenge in IOL calculations, of course, is growing patient expectations. I understand my patients’ disappointment when I do not reach our mutually agreed-upon refractive target, especially with their increasing desire for great uncorrected vision at all distances. Therefore, I have tried to solve some IOL calculation challenges that prevent surgeons from achieving accurate refractive outcomes after cataract surgery. The answers lie several areas: biometry, formulas, topography, experience and teamwork. I collaborate with wonderfully gifted colleagues such as Li Wang, MD, PhD, Warren E. Hill, MD, FACS, Mitchell P. Weikert, MD, and Adi Abulafia, MD, and our goal has been to improve IOL calculation accuracy for both the easy and the very challenging eyes.

Q. How close to emmetropia can refractive cataract surgeons get with current biometric technology?

Koch: Some studies — especially those using the radial basis function (RBF) developed by Dr. Hill — report more than 90% accuracy in patients within ± 0.5 D of target refraction. This is a spectacular result by current standards, albeit in study patients mainly with “normal” eyes.¹ Data from Dr. Hill, however, suggest that most clinicians achieve, at best, up to 75% accuracy within ± 0.5 D of the intended target.^2,3

We still have a long way to go, but newer IOL power calculation formulas, including the Hill RBF, Barrett toric calculator, Johnson & Johnson Vision (formerly Abbott Medical Optics) IOL calculators, AcrySof toric calculators (Alcon Laboratories, Inc.) and, of course, the American Society of Cataract and Refractive Surgery (ASCRS) post-refractive calculator, have helped surgeons along the path to emmetropia, and attention to outliers has improved our outcomes.

Q. What is your preferred method of obtaining biometry readings, and what do you recommend fellow surgeons use to obtain the most accurate, consistent data?

Koch: Despite some limitations in current technologies, clinicians can still achieve great refractive outcomes with steps that include using up-to-date biometers, obtaining at least two corneal measurements from separate devices to check for consistency, using the best IOL power calculation formulas and always being attentive to the integrity of the data.

Every new patient with cataracts presenting to my office undergoes corneal topography using the Galilei combined Placido-dual Scheimpflug (Ziemer) optical biometer. This enables me to detect from the outset any issues that might impair corneal measurement accuracy. For biometry, I then use both the IOLMaster 700 (Carl Zeiss Meditec) and Lenstar (Haag-Streit) data, often adding the Cassini device (i-Optics) as well. In my experience:

• Biometry readings should differ minimally, rarely by more than 0.5 D or 10° between instruments for eyes with more than 0.5 D of astigmatism. Anything above that requires careful examination of the tear film and corneal surface, as well as repeat measurements. Also, I use validation criteria. According to Dr. Hill, standard deviations for the Lenstar should not exceed 0.3 D or 3.5° for the steep and flat meridians. I also perform direct inspection of the LED reflections for both the IOLMaster 700 and Lenstar. If a surgeon has only one device, then two sets of measurements will be helpful.
• Biometric and topographic measurements tend to differ by a larger amount because of the difference in the data acquired and the software used for data processing. For these, any difference over 0.7 D and 10° will often prompt me to conduct repeat measurements and another examination.
• The refraction often gives clues to posterior corneal astigmatism. Again, large differences in magnitude and axis of astigmatism between glasses and corneal measurements prompt greater scrutiny.

Q. Do you suggest surgeons use manual keratometers as well?

Koch: This is a matter of personal preference. I do not use manual keratometers anymore because I have confidence in the reliable, consistent measurements and greater corneal coverage that the IOLMaster 700 and Lenstar provide. Other surgeons prefer manual keratometers as a more hands-on approach to taking measurements, but they have limitations: They measure anterior corneal curvature over only a small annular zone, are more susceptible to human error and are unable to detect all but large amounts of irregular astigmatism or other significant aberrations.

Q. How can surgeons accurately measure the posterior corneal surface, and how does it contribute to total corneal power?

Koch: Despite researchers’ best efforts, surgeons live in a world in which they can consistently measure the anterior corneal surface, but only make assumptions about the posterior corneal surface, which is difficult to measure because it has a refractive index similar to the aqueous. This creates two problems:

• Astigmatic errors. Generally, posterior corneal astigmatism is steep vertically, and, because it is a minus lens, it creates net refractive plus power along the horizontal meridian. In addition, the amount varies according to the orientation of the anterior corneal astigmatism. In patients with increasing amounts of with-the-rule anterior corneal astigmatism, the posterior cornea becomes progressively steeper, thereby progressively counteracting the anterior corneal astigmatism and reducing total corneal astigmatism. For example, +4 D of astigmatism anteriorly and –1 D posteriorly will net +3 D of total corneal astigmatism. For against-the-rule astigmatism, posterior corneal astigmatism is fairly consistent at approximately 0.2 D to 0.3 D. These are only average values, however, and outliers occur. Clearly, there is a huge need for technology that reliably measures posterior corneal power.
• Spherical power errors. Surgeons must assume a fixed ratio between the anterior and posterior curvatures to estimate total corneal power. In a normal eye, that assumption rarely causes significant errors, but I have seen assumed measurements that resulted in 0.5 D of error. In “abnormal” eyes, such as those in patients who have undergone prior corneal refractive surgery or any form of therapeutic keratoplasty, this assumption is no longer valid and introduces errors that can exceed 2 D.

Q. More than ever before, patients who have undergone prior refractive surgery are presenting with cataracts. How do you suggest surgeons specifically address spherical errors?

Koch: Because measurement inconsistency is magnified in post-refractive eyes, and the problem is further compounded in formulas that use corneal power to predict effective lens position (ELP), many regression formulas are available to compensate for spherical power errors. In fact, Drs. Wang, Hill and I have created a spreadsheet, available on the ASCRS website, in which surgeons can access a variety of regression formulas. Importantly, much effort is being put into directly measuring total corneal power, which requires, of course, technology to measure posterior corneal curvature. OCT, Scheimpflug and direct reflection technologies are being evaluated for this use. Progress is occurring, but outcomes with these technologies are still not where we want them to be. I do not know of any outcomes that show accuracy of 80% or higher within 0.5 D of target. Intraoperative aberromety is another option, but again, published results are no better than the best online formulas.

Q. How do IOL constants influence patient outcomes, and how do you recommend surgeons improve upon them?

Koch: Each IOL comes with its own recommended optical biometry constant based on data collected from numerous clinicians over the years. However, this is merely a starting point, and every surgeon must periodically optimize his or her own IOL constant to help ensure good refractive outcomes.⁴ Fortunately, Dr. Hill has dedicated countless hours of his time to help surgeons do just this with his online surgically induced astigmatism calculator. Another reasonable option is to use constants that have been developed from many surgeons and hundreds of cases, which are located on Dr. Hill and Wolfgang Haigis, PhD’s website.

Because refraction is key in determining the A-constant, it is critical to have either a surgeon or topnotch refractionist perform the refractions used for optimization. In addition, lane length — the distance from the patient to the eye chart — affects IOL constants. The standard lane length for IOL calculations is 6 m; a shorter lane will cause overestimation of the A constant. A simple formula can help determine lane length dioptrically to even further optimize a surgeon’s A-constant.⁵

Q. Do long and short eyes affect IOL power calculations?

Koch: Unfortunately, hyperopic refractive errors have been the norm for long eyes, and Dr. Wang and I believe the problem lies in the way axial length is measured in these types of eyes. Optical biometers are calibrated to match ultrasonic measurements and, in our opinion, ultrasound tends to overestimate axial length because they use an incorrect refractive index value for a long vitreous cavity. In other words, long eyes are not quite a long as we think they are. Dr. Wang and I evaluated the option of optimizing IOL constants for long eyes, but the standard deviations were too high. She then devised the concept of optimizing axial length.

Dr. Wang and I, along with other colleagues, published a paper introducing the Wang-Koch axial length modification that alters axial length in eyes longer than 25 mm.⁶ This adjustment virtually eliminates hyperopic surprises and greatly reduces the standard deviation. Other formulas have followed suit, including the Barrett formula, Hill RBF and the PhacoOptics system. For standard formulas such as Holladay 1 and 2, SKR/T and Haigis, surgeons must manually add the Wang-Koch adjustment.

Short eyes, on the other hand, remain problematic because we cannot reliably predict ELP in these cases and, with high-power IOLs, it does not take much deviation in IOL position from the calculated ELP to create excessive refractive errors. Even with best IOL calculation formulas, we have not found outcomes that exceed 75% within 0.5 D of target. Surprisingly, one of the best in our study is the Holladay 1 formula.

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