Preferred Lens Calculations

BJÖRN JOHANSSON, MD, PHD

The selection of an IOL type is based on many factors including the performance of the IOL during surgery, its optical properties, its long-term safety, the accuracy and stability of the targeted refraction, and its economic value. When discussing lens power calculations, the first requirement is a good understanding of the IOL’s typical refractive outcomes. This can be achieved only by systematic follow-up of one’s postoperative refractive results and assessing their relation to the targeted refraction. Each surgeon must devise a strategy to monitor his or her results in this way.

Optical biometry is the gold standard in determining axial length because it provides precise measurements; however, because not all cases can be measured optically (due to the presence of cataract), ultrasound A-scan biometry is also beneficial for preoperative assessment in some eyes. The precision of immersion ultrasound A-scans is equal to that of optical biometry and superior to that of contact ultrasound A-scans, in which the probe is in direct contact with the corneal surface. Compared with optical biometry, however, immersion ultrasound is more demanding for the staff and less comfortable for the patient.

Modern IOL power calculation formulas such as Haigis and Olsen should be the surgeon’s first choice, and the formula’s lens constants must be updated from the manufacturer and adjusted as needed after evaluation of the surgeon’s own refractive outcomes. In difficult cases, my underlying principle is to collect as much data as possible for the IOL power calculation; I double-check unusual keratometry (K) values with alternative methods including the old Javal-Schiøtz keratometer, and I complement a questionable optical biometry with ultrasound biombiometry. This does not always remove all uncertainties, but it does provide a better basis for decision-making.

SIMONETTA MORSELLI, MD

I find that the IOLMaster (Carl Zeiss Meditec) is the best way to obtain correct IOL power. An additional benefit is that the certification of the IOLMaster avoids legal problems in case of incorrect IOL power implantation.

THOMAS F. NEUHANN, MD

I implant aspheric IOLs only. Because these lenses are available in the German health care system at no extra cost to the patient, I cannot justify implanting spherical IOLs with their inferior optical properties, especially with regard to spherical aberration. Today, nobody would accept spherical spectacles—even the cheapest ones are sophisticatedly aspheric.

With regard to the degree of asphericity, I select the IOL according to the patient’s corneal spherical aberration as provided by the corneal wavefront map of the Pentacam (Oculus Optikgeräte). I target a spherical aberration as close to zero as the available lens choice will allow.

I disagree with the opinion that more spherical aberration is advantageous because of better depth of focus, as it comes at the cost of reduced modular transfer function (MTF) at all distances; the better the depth, the worse the MTF throughout it.

If a patient does not wish to bear the extra cost for the Pentacam (about €60 for publicly insured patients in Germany), I implant an aspheric IOL with zero spherical aberration, so at least I do not increase the corneal spherical aberration.

With regard to IOL calculations, I consider the Holladay, Hoffer, and Haigis formulas on the IOLMaster. When these are in agreement, I feel encouraged with my power choice; when not, I am inclined to use the formula that has a documented or reported advantage in the refractive range of the patient in question. Additionally, I inform patients about any predictive uncertainty, so that they understand any deviation from target is not due to sloppiness but occurs despite careful evaluation and due to factors not under the surgeon’s control.

TOBIAS H. NEUHANN, MD

Due to the myriad publications on lens calculations, a review of the literature would take many months. Hence, I have opted to use one formula, the Hoffer Q, which I have optimized to suit my needs. I use two optical biometers, the IOLMaster and the AL-Scan (Nidek).

In collaboration with Sheraz M. Daya, MD, FACP, FACS, FRCS(Ed), I have tried to optimize the A-constants of the AL-Scan while monitoring results with the IOLMaster (Figure 1). I feel confident using this approach, as the rate of corrective operations due to adverse target refraction, although not quite zero, has dropped in the past 12 months to less than 1%.

KHIUN F. TJIA, MD

I am not particularly knowledgeable on this topic. We recently switched to the Haigis formula on the Lenstar (Haag-Streit), which is known to be rather accurate throughout the entire IOL diopter range. We will soon also have the Olsen and Holladay 2 formulas available.

ABHAY R. VASAVADA, MS, FRCS

IOL power calculation is one of the most important determinants of making a patient happy postoperatively. Accurate IOL power calculation is based on the foundation of good axial length (AL) measurement, precise K readings, and appropriate use of IOL formulas.

We prefer optical coherence biometry (IOLMaster) for AL measurements, but when this is not possible we use immersion A-scan biometry. For keratometry, we still rely on manual K readings as the gold standard. We also assess the regularity of astigmatism with Placido-based topography, and we compare our manual K readings with those from the IOLMaster and the iTrace Surgical Workstation (Tracey Technologies). Comparing K readings from multiple devices helps us to rule out outlier values. When in doubt, particularly in choosing the axis of toricity in patients undergoing toric IOL implantation, we rely on the manual K readings.

We use the SRK-T formula for all patients with an AL between 21 and 24 mm. For patients with a short AL (less than 21 mm), short anterior segment (relative anterior microphthalmos), or long AL (greater than 24 mm), we use the Holladay 2 formula. We use the American Society of Cataract and Refractive Surgery IOL calculator, the Holladay IOL Consultant & Surgical Outcomes Assessment software (Holladay Consulting), and the Haigis-L formula in patients who have undergone previous corneal refractive surgery.

It is important to emphasize that, irrespective of the IOL formula of choice, it is extremely important to customize the formula to your patient population. This requires each surgeon to develop a personalized A-constant for every type of IOL formula, IOL design, and patient parameter (eg, short eyes, long eyes). Equally important is to calculate one’s surgically induced astigmatism. Both of these parameters can be calculated easily by comparing pre- and postoperative measurements. Various online tools are available; we prefer the website of Warren E. Hill, MD (doctorhill.com). The software is easy to use and is available free of charge.