The issue of whether intraocular lens design or lens material is more effective in preventing posterior capsular opacification seems to have been resolved. Research results from laboratories in the United States and abroad are consistent in their findings that a square-edge IOL optic is far more effective than a round one in preventing lens epithelial cells from engulfing the posterior capsule. To get an idea how the design of your next IOL may affect your PCO rate, read on.

Research Findings
David Spalton, MD, an ophthalmologist with St. Thomas Hospital, London, has investigated the effect of square-edge IOL profile on lens epithelial cell migration. He and his co-workers developed a mathematical model that correlated the force between an IOL and the posterior capsule. Their results suggest that a square-edge IOL exerts 60 to 70 percent more pressure on the posterior capsule at the optic edge than a round-edge IOL.

Dr. Spalton found that the physical pressure of the IOL against the posterior capsule is a major reason behind the low incidence of lens epithelial cell migration and PCO found with square-edge IOLs.

AMO’s Sensar OptiEdge has been the subject of several studies on the effects of edge design.

Another of Dr. Spalton’s studies examined why square-edge IOLs act as a barrier to lens epithelial cell migration. He and his group developed an in vitro model of human lens capsule culture using post mortem eyes. Lens epithelial cell migration characteristics were defined in 110 normal specimens. Square- or round-edge IOLs of similar design were implanted in the model and the effect of these implants on cell migration was measured using time-lapse photography. Within days of culture, lens epithelial cells reached the IOL optic and then covered it. Progress to cell confluence was delayed two to three days in models with square-edge IOLs compared to those with round-edge IOLs. From the data, Dr. Spalton observes that square-edge IOLs provide a physical barrier to LEC migration.

Austrian researcher Rupert Menapace, MD has also studied the effect of design on PCO. He and other investigators conducted a prospective, randomized study of 53 patients with bilateral age-related cataract using two AMO lenses, the Sensar OptiEdge AR40e lens with a sharp-edge design and the Sensar AR40 lens with a round-edge. Each patient received one Sensar AR40 in one eye and one Sensar OptiEdge AR40e lens in the other.

Follow-up visits were scheduled for one week, one month, six months and a year. Posterior capsular opacification was assessed at each visit. Forty-six patients were examined at the one-year follow-up visit, at which time digital slit-lamp and retroillumination images were taken bilaterally. At this visit, the amount of PCO was assessed subjectively at the slit lamp and objectively by automated image analysis software.

The sharp edge design showed significantly less regenerative and fibrotic material on the posterior capsule after one year. The AR40e scored 0.78 as a mean PCO score and 1.59 for peripheral fibrotic material and wrinkling of the posterior capsule, notes Dr. Menapace. He found that, compared to the round-edge IOL, the modified sharp-edge design leads to significantly less PCO one year postop.

Another study conducted by Tucson, Ariz. cataract and refractive surgeon William Fishkind, MD, confirms these findings. He implanted 39 eyes of 39 subjects with the Sensar AR40 lens, with a mean follow up of three years, and implanted the Sensar OptiEdge in 75 eyes of 50 patients with a mean follow up of 18 months. At two years, Nd:YAG capsulotomy was performed on six of the 39 eyes implanted with the round-edge optic (15.4 percent). According to Dr. Fishkind, the Nd:YAG rate for the OptiEdge was 2.7 percent at the most recent postop visit.

Other Factors
The most important thing that’s been studied is lens design. What’s been clearly shown is that the sharp posterior edge that touches the posterior capsule is important, says Nick Mamalis, MD, a professor at the Moran Eye Center at the University of Utah. A sharp posterior edge provides an effective barrier that stops lens epithelial cells from proliferating across the posterior lens capsule.

The Sensar AR-40 (above left) and the square-edge design of the OptiEdge. Besides a square edge, overlap of the anterior capsule is also important.

But the sharp-edge design is not the only factor involved in halting PCO’s progression, Dr. Mamalis continues. There are a few other things that are necessary to make the barrier effect work. For example, the implant must be within the capsular bag and there must be a 360-degree overlap of the anterior capsule on the IOL optic. This creates a taut, stretch-wrap effect of the posterior capsule against the posterior surface of the IOL, he explains.

The importance of this 360-degree overlap can’t be overstated, according to Dr. Mamalis. It’s likely that further research from the Moran Eye Center and other labs will show that this completely circular barrier is critical. If you have an area where there is no sharp edge, say, for example, a broad haptic that could potentially keep the barrier from forming, you may have a possibility of PCO occurring there, he adds.

Different Designs
For now, most researchers agree that the best IOL is one that has a sharp edge for the entire 360 degrees of the posterior surface of its optic. “You really need a complete sharp edge,” Dr. Mamalis stresses.

Haptics do play a role in that they must center the implant well within the capsular bag. At this point, there is no data to suggest a difference between, for example, a three-piece or a one-piece lens. Nor does the shape of haptics seem to matter much in PCO proliferation. Most modern domestic implants are variations on the modified C haptic design. Dr. Mamalis says, “They all center quite well. The thinking is that centration is less important than having the 360-degree barrier effect posteriorly.”

The effects of the anterior capsule on the posterior capsule have often been overlooked. That’s beginning to change, however, as researchers increasingly turn their attention to this relationship.

The sloping side design and rounded anterior edge in this design serve to improve optical performance.

“The more we look at the effects of the anterior capsule on PCO the more we see that some results are different from what we previously thought,” Dr. Mamalis explains. He recalls that surgeons often tried to vigorously remove all the anterior lens epithelial cells in an attempt to reduce anterior capsule opacification. The thinking was that the least amount of anterior capsular opacification the better.

“It’s interesting,” he continues. “Different groups have found that vigorous removal of the anterior lens epithelial cells and a [resultant] decrease in anterior capsular opacification cause a slight increase in PCO.”

The logical reason for this, he postulates, is that a little anterior capsular opacification is necessary to get the lens to seal off and push back against the posterior capsule. This is what causes the shrink-wrap effect. When all the anterior lens epithelial cells are removed and anterior capsular opacification is taken out of the equation, Dr. Mamalis says that tight contraction of the capsule against the implant posteriorly doesn’t occur.

Although most research indicates that lens design is the key factor in PCO, another theory holds that the IOL material itself may play a significant role. The sandwich theory, Dr. Mamalis explains, describes the proliferation of PCO, which begins peripherally in the fornix of the bag and then expands as the lens epithelial cells scaffold across the posterior capsule. The theory asserts that an acrylic IOL material might be important in terms of its interactions with fibronectins and other biomaterials that can be absorbed through the surface of the lens and coat it. This theory states that, when fibronectin forms, it encourages a monolayer of lens epithelial cells to grow between the implant and capsule. This layer, the thinking goes, stops further growth of the cells. Dr. Mamalis calls this a small connection, one that he doubts is nearly as important as lens design. “In multiple animal models,” he says, “ have been able to show a barrier effect with posterior chamber IOLs composed of PMMA, silicone, acrylic, even hydrogel when those IOLs have sharp-edge posterior surfaces. We found that the barrier effect is a very strong one, independent of material.”

CIBA Vision is currently conducting a post-market data collection of its CV232 SRE lens, which also features a square-edge design. According to the study protocol, posterior capsular epithelial cell development/progression is a secondary efficacy variable of the study. It will be another 10-16 months before the trial is completed and the data analyzed.