IOL Calculations: When Millimeters Count

This article is from June 2008 and may contain outdated material.

As cataract and refractive surgeons undoubtedly know, a relatively small axial length miscalculation during intraocular lens calculations can result in poor vision—and a very unhappy patient. And in an era when refractive considerations have crept into every aspect of IOL surgery, even 80-year-old cataract patients expect outstanding outcomes, said Parag A. Majmudar, MD, associate professor of ophthalmology at Rush University in Chicago and in private practice at Chicago Cornea Consultants. “Meeting these expectations starts with correctly calculating IOL power. Of course, proper surgical technique is always crucial, but even in patients who have never had prior ocular surgery, the calculations are a very important part of IOL im-plantation surgery.”

Spot-On Biometry

William B. Trattler, MD, in private practice in Miami, said the most important step in calculating IOL power is accurate biometry. This can prove difficult in the eyes of patients who have undergone prior ocular surgery. And it is a challenge that will continue to intensify with the increasing number of individuals needing cataract surgery who have undergone previous RK, PRK and LASIK procedures.

Measuring axial length. Dr. Trattler noted that technology to measure axial length has dramatically improved with the advent of the IOLMaster (Carl Zeiss Meditec), a device that measures the shape and axial length of the eye to help the surgeon fine-tune the power. Its latest version features new axial length algorithms and an advanced keratometry mode.

“Only about five years ago, we were using A-scan ultrasound biometry,” Dr. Majmudar noted. “This technology was dependent on the person taking the measurements, producing variable results. The newer technology is more standardized, and more reproducible—leading to better results.”

Assessing corneal shape. While the ability to accurately measure axial length has improved, measuring corneal topography is more complex. Dr. Majmudar explained that keratometers measure the curvature of the anterior surface about 3.2 mm from the center of the cornea. “In patients who have not undergone previous surgery, the value at the center of the cornea is roughly the same as the value at 3.2 millimeters,” he said. “However, patients who have undergone LASIK or PRK can have altered corneas, and the value at the central cornea, which is the goal of measurement, may be very different from that at 3.2 millimeters. Consequently, if you just rely on the topography, you may be off, and for every 1 diopter you are off in measuring the corneal curvature, a roughly 1 diopter miscalculation will result for the patient’s refractive outcome.”

Working with (or without) preop records. Steven I. Rosenfeld, MD, associate clinical professor of ophthalmology at the Bascom Palmer Eye Institute and in private practice in Delray Beach, Fla., also pointed to the challenges of ascertaining accurate corneal topography. “There are ways to get around these limitations, and the first one is having historical data to help you calculate the lens implant power,” Dr. Rosenfeld said. “If you are lucky enough to have the patient’s preoperative information before they underwent PRK or LASIK, and you know their postoperative results, that can help guide you and give you a more accurate reading. You can plug that into your formulas.”

However, obtaining these preoperative data may be easier said than done, especially in places such as Boca Raton, which attract retirees who may have undergone a refractive procedure in one area of the country and decided to have cataract surgery in their new retirement city. “Patients rarely carry this information with them,” Dr. Rosenfeld said. “And many ophthalmologists purge their old records after seven years. Consequently, if a patient had LASIK 10 years ago and now needs cataract sur-gery, there is a good chance that the physician may not even have these rec-ords. We have experienced many situations in which we send a record release to an ophthalmologist up north just to learn that the records no longer exist.”

Corneal measurement: backup tools. Dr. Rosenfeld said that without previous records, the next alternative is a con-tact lens overrefraction. This involves inserting a contact lens of known base curve and power on the eye in question, and doing an overrefraction to predict the corneal power. It represents an indirect way to obtain information about the shape of the cornea, which can then be plugged into an IOL calculation formula. “Yet even this approach has its drawbacks because ophthalmologists have varying levels of confidence in mak-ing these estimations,” he said. “Recently, several studies have demonstrated the value of using the central corneal measurements from the Humphrey Atlas corneal topographer, the Orbscan II and the Pentacam, to more accurately calculate the correct IOL power.”

Both Drs. Trattler and Majmudar also noted that the Pentacam can image the front and rear surfaces of the cornea in patients who have previously undergone RK, PRK or LASIK, and it can provide a keratometry value that may be put into the IOLMaster. With this equivalent K reading, they said, the surgeon may not need previous LASIK records.

Refractive Rogues Gallery

Previous refractive surgery can complicate IOL calculations unexpectedly.

RK weaklings. Patients who had RK can experience fluctuations in the shape of the cornea in the course of a single day, let alone week to week, making it extremely difficult to obtain an accurate IOL calculation. “Some patients had 16 of these RK cuts, which can permanently weaken the cornea,” Dr. Rosenfeld pointed out. “I will be operating on two patients within the next month who presented with this challenge. One is a gentleman with a four-cut RK, and he now needs cataract surgery, and another has a 16-cut RK. Both their vision and corneal curvature fluctuate during the day, and thus it will be virtually impossible to achieve a perfect IOL calculation. Instead, we do the best we can given these anatomic limitations.”

PRK suspects. Dr. Rosenfeld noted that Baby Boomers seeking cataract surgery who have had LASIK and previous RK are easy to detect on clinical examination. However, this is not the case with PRK. “If the patient doesn’t tell you he or she had previous surgery, the surgeon can easily make a calculation mistake. Obviously, a thorough history is an important component of accurate IOL calculations,” he said.

Contact corruption. Even patients who have worn contact lenses most of their lives, especially the hard, gas permeable or extended-wear lenses, may present challenges. “Contact lenses are not benign,” Dr. Rosenfeld said. “They can cause corneal stress, which can be manifested in many ways. The cornea can become swollen. In addition, contact lenses can change the shape of the endothelial cells and alter the normal mosaic pattern, and some individuals have experienced loss of endothelial cells after decades of use.” Corneal warpage is also a real condition in these patients.

Dr. Rosenfeld recommends that these patients stay out of contact lenses for weeks or even months prior to surgery, so that the ophthalmologist can obtain accurate keratometry for the IOL calculation. This also holds true for patients undergoing refractive surgery, as the cornea needs to resume its natural shape before a procedure can be done accurately. “We don’t proceed with refractive surgery until the patient has two visits where the corneal topography is the same,” he said.

Accuracy Is Everything

Dr. Trattler noted that the introduction of and increasing demand for presbyopia correcting IOLs necessitates even more accurate biometry, as these implants need to be right on target. “If you end up a quarter- or a half-diopter off with the ReStor or ReZoom multifocal lenses, patients will be unhappy,” he said. “This also holds true for the Crystalens accommodative IOL. Presbyopic patients tend to be extremely sensitive to any miscalculation.”

He said that in his presbyopic population, he experiences a 10 percent to 12 percent enhancement rate. “In these patients, it is important that you can offer solutions such as laser vision correction or limbal relaxation surgery,” Dr. Trattler said. “The more accurate you can be with your calculations, the lower the enhancement rate.”

Anatomy is money. Dr. Trattler stressed that the “unhappiness factor” associated with miscalculating IOL strength results in added costs for the practice, and thus, he said, “there is a financial incentive to getting back on target.” He also stressed the importance of informed consent with any cataract or refractive lens exchange patient. “Managing expectations is of vital importance when working with increasingly challenging patient anatomy and lens technology.”

Dr. Majmudar reports interests in Alcon, Allergan, AMO and Inspire Pharmaceuticals; Dr. Rosenfeld reports interests in Allergan; Dr. Trattler reports interests in Allergan, AMO, Bausch & Lomb, Inspire Pharmaceuticals and Vistakon.