This article is from January 2007 and may contain outdated material.
Timing is everything. Multifocal intraocular lenses received FDA approval just as millions of Baby Boomers hit their late 50s and early 60s, giving a generation of patients the promise of distance, intermediate and near vision after cataract surgery. Yet the advent of multifocal IOLs has also raised an unanticipated issue: the importance of proper centration.
“Optics with traditional IOLs were straightforward, and mild decentration did not dramatically affect vision. However, with the introduction of newer multifocal IOLs such as ReZoom and ReStor, decentration is emerging as a key issue, as centration is critical to their performance,” noted Ike K. Ahmed, MD, assistant professor of ophthalmology, University of Toronto.
The Problem With Decentration
Kenneth J. Rosenthal, MD, associate professor of ophthalmology, University of Utah, explained the issue in terms of the three IOL types: traditional, prolate and multifocal. Mild decentration in traditional IOLs is not a significant problem and does not affect the function of the lens. With prolate IOLs, mild decentration can cause mild vision problems. In contrast to the traditional IOLs, however, prolate IOLs do not exert their complete benefits unless they are well-centered. And with multifocal lenses, centration is even more crucial. “These lenses are required to be perfectly centered,” Dr. Rosenthal said. “Otherwise, they not only lose their multifocality but cause issues with spherical aberration.”
Is off-center on the rise? Theoretically, advances in surgical procedures should result in fewer decentration issues for patients. While this is partly the case, noted Dr. Ahmed, the more liberal use of foldable lenses in nonroutine situations, including pseudoexfoliation syndrome, has increased the incidence of decentration. “In contrast to using the large PMMA rigid lenses in these cases, some surgeons are choosing foldable lenses, which aren’t the ideal material when there is a complication like a rupture,” Dr. Ahmed said.
Dr. Rosenthal gave several other causes of decentration, including improper placement of the lens, lack of meticulous centration of the implant, irregular capsulorhexis and irregular capsular contraction due to weak zonules. “Another phenomenon found in the acute type of decentration is when the capsule contracts, pushing the lens out of the capsular bag—a phenomenon known as peapodding,” he said.
Dr. Rosenthal divided decentration cases into two types: chronic decentration, which occurs after a long period of time, perhaps even years later, and perioperative decentration, which can happen anytime from the day after surgery to a few months later.
Progressive zonular deterioration is one of the leading causes of chronic decentration, Dr. Ahmed added.
The Symptoms of Decentration
Dr. Ahmed explained that symptoms of IOL decentration can include fluctuation in vision, flickering of vision, light-related visual aberrations such as halos or crescents around lights, and, in severe cases, frank vision loss with complete dislocation of the lens. Other patients may experience refraction changes, with lenses moving out of position, as well as associated complications of decentration, including glaucoma, uveitis and cystoid macular edema. Some patients may be asymptomatic, with evidence of decentration only apparent through postoperative slit-lamp observation.
Think before you YAG. Dr. Rosenthal issued a particular warning about mistaking IOL decentration for posterior capsular opacification as the cause of a decline in vision. “Both can occur concurrently,” he noted. “If you proceed to do a YAG capsulotomy on the assumption that the patient is experiencing posterior capsular opacification, this approach limits the options for repositioning and/or exchanging the lens should there be an IOL problem.”
In cases where there may be a question about whether the patient is experiencing posterior capsular opacification or IOL decentration, Dr. Rosenthal may bypass the YAG capsulotomy and instead return to the OR to reposition the lens and meticulously polish the capsule to remove the lens epithelial cells. “While this involves returning to the operating room and making a 1.2 mm incision, with today’s antibiotics and topical anesthesia this approach represents a relatively low risk to patients, and you can save the capsule,” Dr. Rosenthal said. “This approach actually illustrates a paradigm shift today that leans toward reoperation. The smaller incision and reduced surgical risks can outweigh the possibility that the patient will never achieve the quality of vision he or she desires given the limiting nature of the YAG capsulotomy.”
Ring Around the Capsule
Prevention is the optimal strategy to address decentration. “For example,” noted Dr. Ahmed, “in scenarios where there is a capsular rupture or tear, be careful putting the lens in the bag, because those IOLs can sublux.” Dr. Rosenthal suggested that when operating on an unstable capsule with missing zonules, “the capsular tension ring is a great way to stabilize the capsule.”
Dr. Rosenthal, who participated in the trials of both the Morcher and Ophtec capsular tension rings, noted that these devices make it possible to implant prolate optic lenses or multifocal lenses, even in the presence of zonular disease. Tension rings can be inserted into the capsular bag anytime after the capsulorhexis, generally at the first sign of zonular insufficiency. “In cases where the pseudoexfoliation is more diffuse, even in the absence of profound laxity, I am more inclined to use the capsular tension rings to prevent problems down the line,” he explained.
In most instances, the capsular tension ring provides sufficient stability, but in severe cases the capsule may remain loose or decentered. The Cionni Modified Capsular Tension Ring (MCTR), designed by Robert J. Cionni, MD, provides a fixation hook that can be sutured to the scleral wall without piercing the capsular bag.
Dr. Ahmed has also modified the capsular tension ring, addressing one of the problems with capsular tension rings, in which insertion, especially before phacoemulsification, can create significant torque and stress that may further weaken zonules. The Ahmed Capsular Tension Segment (CTS) is a partial ring with a fixation hook that can be placed following anterior capsulotomy and fixated using an iris retractor. It can also be permanently fixated with a suture.
Dr. Rosenthal has recently developed another technique to address intraoperative centration. It is a modification of the IOL optic capture technique in which the capsular tension ring is deployed and followed by the optic capture. He says that by using the ring, the optic stays perfectly centered in the capsular bag, and then the ring can be sutured to the iris. “If there is an issue with zonular weakness during surgery,” he said, “this approach obviates the need for the MCTR or the CTS. The ring redistributes the forces, the capture in the bag will ensure centration and fixation of the optic, and suturing the haptic will ensure that the optic won’t tilt and become unstable.”
For that late-in-the-day decentration. Dr. Ahmed’s surgical approach for patients who underwent primary IOL implantation several years earlier is determined by the anatomical position of the lens. For example, is it in the bag or out of the bag? Does the case warrant an IOL exchange, or would the patient benefit by repositioning the existing lens?
“In most cases, I tend toward repositioning the existing lens, and do this with a minimally invasive approach,” Dr. Ahmed said. “The exception is in cases such as a subluxed anterior chamber IOL, where I couldn’t reposition it. In this situation, I would suture the posterior chamber IOL away from the anterior chamber.” When Dr. Ahmed repositions by suture, the vast majority of IOLs that are not in the bag are sutured to the iris because “it is technically easier, less risk of tilt or erosion of the suture, and decreased risk of infection,” he said. “When the lens is in the bag, then I make a judgment call—sclera or iris. While one could suture the entire bag to the iris, it is easier to suture just the free mobile lens to the iris.”
Zeroing in on Perfection
Both Drs. Rosenthal and Ahmed stressed the importance of vigilance in preventing decentered IOLs from occurring in the first place, especially when working with the newer generations of IOLs.
Dr. Rosenthal pointed out that the patient who chooses multifocal lenses is the one who is determined to see without glasses after cataract surgery. “In this era of multifocal lenses, it is imperative to take a few seconds to ensure the lens is perfectly centered. I take a moment after IOL placement, and repressurizing the eye, to have the patient fixate on the center light of the microscope. I then check the first primary and second primary Perkinje images, from the anterior corneal and anterior IOL respectively, to see that they are aligned with one another. This ensures that the IOL is well centered, and is a better method than relying on an estimate of the center of the cornea or of the pupil, both of which may be skewed away from the true visual axis.”
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Dr. Ahmed has no related financial interests. Dr. Rosenthal has received research support from AMO and Ophtec