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 Innovation Spotlight - Lens Material

Lens material and design do affect visual outcome

By Lynda Charters
Reviewed by Randall J. Olson, MD

Salt Lake City-Not all IOL materials are created equal. A number of factors contribute to the inequality among lenses, according to Randall J. Olson, MD.

"Previously, it was believed that there was no difference between foldable lenses and polymethylmethacrylate (PMMA) over the long term," Dr. Olson said.

However, a number of recently published studies are contradicting this belief, said Dr. Olson, the John A. Moran professor, chair, and director, department of ophthalmology, John A. Moran Eye Center, University of Utah School of Medicine, Salt Lake City.

In a prospective randomized study conducted with 110 patients undergoing phacoemulsification with a 3.2-mm incision, he and colleague Alan Crandall, MD, determined that after 3 years of follow-up, there were very distinct differences between the visual results obtained with a 5.5-mm PMMA lens and a silicone IOL (model SI30, Allergan Surgical).

In addition to significantly less "against-the-rule" astigmatism with the silicone lens (p < 0.001), they also found better uncorrected and, surprisingly, corrected visual acuity, indicating the foldable IOLs and the use of smaller incisions are clinically important in the long term.

In the 84 patients examined at 3 years, they also determined that objective and subjective measurements of posterior chamber opacification (PCO) were significantly lower with silicone IOLs compared with PMMA IOLs (objective, p = 0.02; subjective, p = 0.0001).

These results were confirmed by other authors in studies of PMMA lenses versus silicone and acrylic lenses (AcrySof, Alcon). In a study published by Hayashi and associates in Archives of Ophthalmology (1998;116:1579-1582), 2-year follow-up indicated that both AcrySof and SI30 were better than PMMA in objective capsular measurements and the incidence of YAG capsulotomies were similar to each other. Spalton and colleagues later reported (Ophthalmology 1999;106: 49-55) that AcrySof had a distinct advantage over PMMA and first-generation silicone lenses.

Not all silicone lenses alike

"An important point is that we cannot lump silicone lenses together. The first and second generations of the material are not the same," Dr. Olson said. "The lower PCO rates apply to the second-generation (PHACOFLEX II SLM2, Allergan Surgical) silicone lenses.

"We now have clear evidence that there may not be a biomaterials effect that results in lower PCO rates, but rather the edge of the IOLs may be the most significant factor," he pointed out.

A study of PCO in a rabbit model (Nishi et al. J Cataract Refract Surg 1999;25:521-526) compared the effects of AcrySof and PMMA IOLs that both had sharp edges and found that epithelial cells seemed to be prevented from migrating onto the posterior capsule and proliferating.

IOL design matters

"The study concluded that the PCO advantage found with AcrySof is not dependent on the material, but rather the flattened edge digs into the capsule and creates a mechanical barrier that the lens epithelial cells cannot get beneath," he said. "Therefore, the IOL design is the significant factor that affects PCO formation.

"However, having a flattened edge is not necessarily an advantage," he said. "There are some disadvantages because of the presence of unwanted visual images.

"In all nine patients that we studied, the images were greatly diminished following IOL exchange with rounded-edge IOLs," he said. "In a study that is examining explanted foldable lenses nationwide, the AcrySof lens has a very low explantation rate; however, the primary reason is because of unwanted images. AcrySof IOLs also cause a reflective glint in pupils.

Also noteworthy, however, is that at the 1999 meeting of the European Society of Cataract and Refractive Surgery, Nishi and colleagues reported that AcrySof loses its PCO advantage when the edges of the lenses are rounded. The biomaterials effect seems to be limited to the second-generation silicone IOLs (SLM2), because they seem to be able to inhibit PCO without having a sharp edge, Dr. Olson explained.

"The biomaterials effect occurs only with the SLM2 IOLs, but no one knows why. Some property of the material is responsible. Part of the explanation may be the material's hydrophobic ability," he said. "However, that is not the sole factor because all silicones are hydrophobic, but the first-generation silicones did not inhibit PCO. There is another factor that has yet to be determined."

Silicone (SI30) and PMMA IOLS fared much better when compared with Hydroview IOLs (Bausch & Lomb), which had significantly more lens epithelial cell growth on the anterior surface (Hollick et al. Arch Ophthalmol 1999;117:872-878). However, the hydrogel IOLs had significantly fewer inflammatory cells (small cells) than the other two materials.

"These authors reported that hydrophilicity increases the PCO rate," he said. "Hydrophilic acrylic likely will not offer a PCO advantage over PMMA, unless a lens also has a flat edge and a biomechanical advantage."

Giant cells and glistenings

In the same study published by Hollick and associates (Hollick et al. Arch Ophthalmol 1999;117:872-878), 30% of PMMA IOLs had giant cell deposits compared with no deposits on the SI30 and Hydroview after 90 days.

Unpublished data from Samuelson et al. indicated that 15% of AcrySof, 4.3% of SI30, 5.8% of SI40 second-generation silicone, and 33% of first-generation silicone lenses had giant cells 6 months postoperatively.

Another area of concern, according to Dr. Olson, is highly visible water vacuoles (glistenings) that can develop within AcrySof IOLs.

"We showed that there was a contrast sensitivity loss associated with severe glistenings in early lenses and the visual acuity was not affected, however, the lens was voluntarily removed from the market," he said.

"These acute glistenings only associated with AcuPak packaging did get better with time," he added.

"In 1999, a study by Mitooka reported at the American Society of Cataract and Refractive Surgery meeting found that there was a 27.1% incidence of level 3 glistenings in patients with diabetes and an 11.8% incidence in patients who did not have diabetes," Dr. Olson said.

"They also showed a contrast sensitivity loss associated with heavy glistenings," he added.

"After these results, we conducted a study in which we found that in 42 patients 100% had glistenings (most trace); four had level 3 to 4 glistenings," he stressed. The visual acuity loss in those patients with level 2+ or greater was significant (p = 0.01).

Wound size also appears to be an issue associated with success postoperatively. In a study of 400 patients, Dr. Olson and colleagues compared the AcrySof MA60 (3.9-mm incision) with the PhacoFlex II SI40 NB (3-mm or smaller incision).

The incidence of wound events (instability, leakage) with AcrySof was 13 in 200 patients compared with three of 200 who received the SI40 IOL (p = 0.01).

"Enlarging the wound is obviously a clinical issue," he said.

Foldable IOLs are the future

"Foldable IOLs have numerous advantages and are the future. At this time, AcrySof and second-generation PhacoFlex II silicone lenses are the PCO leaders. It appears that the hydrophilicity probably causes higher PCO rates. Unwanted images can really have an impact on patient satisfaction and are the result of flattening the edge of the IOL. Glistenings are still a concern and need to be addressed more than they are now. Wound size does matter," Dr. Olson concluded. "Finally, all of these factors are providing clues for the future about what we can do to obtain the optimal IOL and the best patient outcomes."

He has no proprietary interest in any product mentioned, but is a paid consultant to Allergan.

Ophthalmology Times / APRIL 15, 2000


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