Trypan blue may affect capsule strength, study finds

By helping make the capsule more rigid, trypan blue makes it easier to tear.

Trypan blue may facilitate cataract surgery beyond just being visually helpful, according to a recent study.
The use of trypan blue (VisionBlue, Dutch Ophthalmic Research Center International, B.V., Zuidland, The Netherlands)) under the right conditions could increase the elastic stiffness of the anterior lens capsule, according to the study’s lead author, Gregor Wollensak, M.D., Department of Ophthalmology, Technical University of Dresden, Germany.
The study, published in the July 2004 issue of the Journal of Cataract & Refractive Surgery, noted that light irradiation of the capsule for at least one minute combined with trypan blue dye led to an increase in elastic stiffness at 25% strain by up to 50% and a decrease of the tear point by up to 13%.
In other words, the combination of light irradiation and dye would decrease the capsule’s extensibility, making the capsulorhexis easier, said Dr. Wollensak.
“Capsulorhexis is facilitated from a biomechanical standpoint because the lens capsule — which is made up of collagen type IV — can be torn more easily because it becomes less elastic and also the tear point level is reduced,” Dr. Wollensak said.

Leading up to a surprise

Material tester with a strip of trypan
blue-stained porcine lens capsule
between the clamps.
Source: Gregor Wollensak, M.D.

In his study, Dr. Wollensak used 55 anterior lens capsules from porcine cadaver eyes. After trypan blue staining for various time intervals combined with exposure to white light or with absence of light, Dr. Wollensak performed biomechanical stress-strain measurements. He also used untreated and 0.1 % glutaraldehyde-treated specimens as controls.
In lens capsules treated with trypan blue and light for one minute, the stress at 25% strain was 459.5 ± 117.4 x 103 Pa compared with 337.3 ± 120.3 x 103 Pa in the non-treated specimens.
Therefore, there was a 36.2% increase in elastic stiffness for the group treated with trypan blue and light for one minute compared with the stiffness of the non-treated group. That was statistically significant, Dr. Wollensak said. In addition, there was a decrease in ultimate strain by 11%.
Treating the lens capsule with trypan blue and light for one minute “is a realistic time scheme also for a real cataract operation, especially after subcapsular injection of the dye,” Dr. Wollensak wrote in his study.
The elastic stiffness increased even more (a 70.1% increase) for the specimens treated with trypan blue and light for 30 minutes compared with that of the non-treated group, he said.
“Remarkably, there was no such effect after trypan blue staining in the darkness and after trypan blue staining and light exposure for only 30 seconds, indicating a light dependent process as is typically the case with photosensitized collagen crosslinking,” Dr. Wollensak wrote. “If the incubation with the dye is shorter than one minute, the biomechanical effect might be absent or weaker, which might be the case for some surgeons.”
Trypan blue is known to be a photosensitizer from other studies, suggesting that it can make the lens capsule sensitive to light and cause it to stiffen, Dr. Wollensak said. The first step of the photodynamic reaction is absorption of light by the trypan blue sensitizer, he said. The excited sensitizer can then react directly with the substrate yielding free oxygen radicals or singlet oxygen, he said.
“The so-called reactive oxygen species (ROS) lead to physical crosslinking of collagen,” Dr. Wollensak wrote. That collagen crosslinking causes a stiffening effect, similar to the UV-induced crosslinking of the crystalline proteins of the lens that leads to cataract formation, he said.
Glutaraldehyde is an excellent chemical crosslinker of collagen, and so it was used as a control for comparison, Dr. Wollensak said.
Indeed, in lens capsules treated with 0.1% glutaraldehyde for 30 minutes, the stress at 25% strain was 1418.3 ± 257.32 x 103 Pa compared to 336.4 ± 130.8 x 103 Pa in the non-treated specimens, signifying a 321.6 % increase, he said. In addition, there was a decrease in ultimate strain by 47.6%.

Try this at home

The difference in elasticity between an anterior lens capsule treated with trypan blue and light and one not treated with either can be visualized using a few household items, Dr. Wollensak said.
“It could be compared to the difference between aluminum foil and a transparent household foil,” Dr. Wollensak said. “You can try this yourself in your home.”
A tear can be better controlled in aluminum foil (representing the treated capsule) than transparent household foil (representing a non-treated capsule), Dr. Wollensak said. One also would need less force to make a tear in aluminum foil as with a treated capsule, he said.
“But if you make a mistake during the capsulorhexis (on a treated capsule), the negative effect like a radial tear might also be greater,” he said.

Editors’ note: Dr. Wollensak has no financial interests related to his comments.

Contact Information
Wollensak: gwollens@hotmail.com