A routine eye treatment is raising new concerns for glaucoma patients

This is the first study to combine clinical observations with experimental evidence to clearly link petrolatum-based eye ointments to structural damage in this type of implant.

Glaucoma and the Role of the MicroShunt

Glaucoma is a chronic eye disease that damages the optic nerve and can result in permanent vision loss. The condition is often caused by elevated pressure inside the eye when fluid drainage becomes blocked. Researchers estimate that glaucoma affects approximately 76 million people worldwide.

One treatment option is the MicroShunt, a tiny filtration device surgically implanted in the eye to help excess fluid drain more effectively. Compared with traditional glaucoma surgeries, the MicroShunt is associated with fewer post-operative complications and often reduces the need for ongoing medication.

Why the Implant Material Can Be Affected

The MicroShunt is manufactured from a styrenic thermoplastic elastomer made from a polystyrene-block-polyisobutylene-block-polystyrene (SIBS) block polymer. This material is designed to be flexible, highly biocompatible, and less likely to cause inflammation or scarring inside the eye.

At the same time, the material is sensitive to contact with hydrocarbon- and oil-based substances. Because it has a strong affinity for oils, petrolatum-based eye ointments can penetrate the implant. When oil components enter the material, the device may swell and experience changes in its shape and mechanical strength.

Manufacturer Warnings Often Overlooked

The MicroShunt manufacturer specifically cautions against this type of exposure. According to the instructions, "the MicroShunt should not be subjected to direct contact with petrolatum-based (i.e., petrolatum jelly) materials, such as ointments and dispersions." Despite this guidance, the warning is not always widely recognized or consistently followed in clinical settings.

"Swollen MicroShunts can be structurally fragile," said ophthalmologist and Assistant Professor Ryo Tomita of Nagoya University Graduate School of Medicine, the study's first author. "During surgery, I observed a rupture in a swollen MicroShunt. If more clinicians are aware of this risk, they will be able to prevent similar problems."

Collaboration Between Medicine and Engineering

To investigate the issue more closely, Tomita worked with Assistant Professor Taiga Inooka and Associate Professor Kenya Yuki from Nagoya University Hospital and the Graduate School of Medicine. They collaborated with Dr. Takato Kajita and Junior Associate Professor Atsushi Noro from the Graduate School of Engineering to study how the MicroShunt changes after contact with petrolatum-based eye ointments.

The medical team analyzed patient cases, while the engineering researchers carried out laboratory experiments. The results were published in Graefe's Archive for Clinical and Experimental Ophthalmology.

Clinical Evidence From Patient Cases

The clinical analysis involved seven glaucoma patients whose MicroShunt implants were later removed for various reasons. A clear pattern emerged based on whether the implant had been exposed to petrolatum-based ointment.

In three cases, the MicroShunt was exposed outside the conjunctiva and patients were treated with a petrolatum-based eye ointment. All three devices showed noticeable swelling, and two of them had ruptured.

In another three cases, the MicroShunt remained covered by the conjunctiva and no ointment was used. These implants maintained their original structure.

One additional case was particularly revealing. Although the MicroShunt was exposed outside the conjunctiva, no ointment was applied. In this case, the implant did not swell. This finding indicates that direct contact with the ointment, rather than conjunctival exposure alone, is the primary cause of swelling.

Laboratory Tests Confirm the Mechanism

Laboratory experiments reinforced the clinical findings. Researchers immersed unused MicroShunts in petrolatum-based eye ointment to recreate the changes observed in patients.

Microscopic measurements revealed rapid expansion. After 24 hours of exposure, the outer diameter of the MicroShunt increased to 1.44 times its original size. The fin-like portion of the device widened to 1.29 times its initial dimension.

Chemical testing explained why these changes occurred. After 24 hours of immersion, oil components accounted for about 45% of the MicroShunt's total weight. After three months, oil content increased to 73%.

These results confirmed that swelling is driven by the absorption of oil-based ointment components into the implant material.

Implications for Glaucoma Treatment

Based on their findings, the researchers advise clinicians to avoid using petrolatum-based eye ointments in patients with MicroShunt implants, especially when the device is exposed outside the conjunctiva. They recommend considering alternative post-operative treatments and note that further studies are needed to determine whether swelling affects implant performance even when rupture does not occur.

"Our study found that commonly used medical materials can cause unexpected complications if their chemical properties and usage environments are not fully understood," Noro stated. "From both medical and engineering perspectives, we emphasize the importance of understanding the chemical properties of medical materials and appropriately managing their usage environments."