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Not too long ago one of the great figures of ophthalmology, Sir Harold Ridley, was ostracized by his colleagues for placing a foreign object into the eye. His innovation, the intraocular lens (IOL), eventually became the standard in cataract surgery resulting in vastly improved vision compared to surgery without use of an IOL. In retrospect, we have reason to be grateful to his insight and willingness to buck the established beliefs of his time.
Any material that is implanted in the eye (or elsewhere in the body) should be expected to last without issue for decades.
Nonetheless, Sir Harold Ridley’s early detractors were justified in their concern. Any material that is implanted in the eye (or elsewhere in the body) should be expected to last without issue for decades. We have seen with IOLs that not all material choices are long-lasting or that materials that appear to work fine for years may eventually degrade. For example, one of the most commonly implanted IOLs in both the USA and Canada, the Alcon AcrySof, is now recognized to suffer from the development of “sub-surface nanoglistenings” that may degrade the quality of vision over time. Let’s hope that for most people with this IOL these nanoglistenings are not going to become visually significant or eye surgeons will be very busy someday swapping out these popular IOLs.
All of the above is meant to frame my concern with another Alcon product: the CyPass® suprachoroidal stent. Although I’m intrigued by the concept of creating a controlled cleft allowing aqueous flow into the suprachoroidal space, I’m very concerned about the choice of material for this stent: polyimide.
I’m very concerned about the choice of material for this stent: polyimide.
Polyimide is known to become brittle with time when exposed to heat and humidity. In the aviation industry this material has been used to insulate electrical wiring. Because polyimide degrades in warm, moist environments, this choice has become a major headache for both the air force and commercial airlines. Additionally, it is recognized among the few anterior segment surgeons who regularly perform intraocular lens (IOL) exchanges years after IOL implantation that haptics made out of polyimide can shatter like glass when handled by forceps.
Far from being cold and dry, the suprachoroidal space in which the CyPass® is implanted is both warm and humid. Indeed, all living human tissue can be described as both warm and humid. What, then, can we expect of this material when implanted just under the surface of the eye in an area that is not protected from minor trauma such as from eye rubbing?
Imagine that your eye itches. Go ahead and rub your eye now. Where did you reflexively rub? Most likely it was near the nose where the inflammatory debris collects. Given that the CyPass® is meant to be implanted in the nasal angle (a very common location for eye rubbing) this implant is going to be exposed to repeated minor trauma over many years.
We already know what happens when a relatively smooth haptic rubs against uveal tissue: an often difficult to treat syndrome aptly termed UGH (Uveitis Glaucoma Hyphema) syndrome. Now, what do you think will happen if minor trauma, combined with the increasing brittleness of the polyimide material over decades, results in the shattering of this material into microscopic shards in (of all places) the uveal tissue?
Could anything be done to effectively treat chronic inflammation that could result from the presence of sharp microscopic debris against uveal tissue? Unlike removal of a sulcus-placed haptic causing UGH, complete excision of a brittle, shattered tube positioned in the suprachoroidal space is going to be challenging, if even possible.
it would seem prudent to reflect on whether this is really the best glaucoma surgical implant to use.
Because of the increased safety of Minimally Invasive Glaucoma Surgery (MIGS), there is a growing trend among eye surgeons to recommend MIGS in younger patients. With all surgical implants it is important for both the patient and surgeon to reflect upon the lifetime commitment being made. When, in the case of the CyPass® and implant is made of a material known to break down in a warm, moist environment (such as the suprachoroidal space) and the tissue in contact with the implant is known to become inflamed with microtrauma (such as with chaffing from sharp objects), it would seem prudent to reflect on whether this is really the best glaucoma surgical implant to use.
Of course, I cannot be certain that just because it is made of a material known to break down over time in moist, warm environments that the CyPass® implant will, indeed, be susceptible to shattering inside the eye leading to complications such as the dreaded Uveitis Glaucoma Hyphema syndrome (UGH). However, in the spirit of “primum non nocere” (above all, do not harm) I have decided that I will not personally be implanting the CyPass® in any patient with a life expectancy of much greater than 10 years. Even then, I will only do so if (1) no other options are available that would fit the individual patient’s needs and limitations, and (2) after personally discussing my concerns with the patient.
- Matsushima H, Nagata M, Katsuki Y, et al. Decreased visual acuity resulting from glistening and sub-surface nano-glistening formation in intraocular lenses: A retrospective analysis of 5 cases. Saudi Journal of Ophthalmology. 2015;29(4):259-263. doi:10.1016/j.sjopt.2015.07.001.
- Murray, Steven & Hillman, Craig & Pecht, Michael & Page, Murray. (2004). Environmental Aging and Deadhesion of Polyimide Dielectric Films. Journal of Electronic Packaging. 126. . 10.1115/1.1773853.
- Stallings S, Werner L, Chayet A, et at. Intraocular polyimide intraocular lens haptic breakage long-term postoperatively. J Cataract Refract Surg. 2014 Feb;40(2):323-6. doi: 10.1016/j.jcrs.2013.11.024.
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David Richardson, MD
Medical Director, San Marino Eye
David Richardson, M.D. David Richardson, M.D. is recognized as one of the top cataract and glaucoma surgeons in the US and is among an elite group of glaucoma surgeons in the country performing the highly specialized canaloplasty procedure. Morever, Dr. Richardson is one of only a few surgeons in the greater Los Angeles area that performs Micropulse(r) P3 "Cyclophotocoagulation" (MP3) glaucoma laser surgery. Dr. Richardson graduated Magna Cum Laude from the University of Southern California and earned his Medical Degree from Harvard Medical School. He completed his ophthalmology residency at the LAC+USC Medical Center/ Doheny Eye Institute. Dr. David Richardson is also an Adjunct Assistant Professor of Clinical Ophthalmology at the Keck School of Medicine of USC. Twice weekly, he treats veterans at the VA Greater Los Angeles Veterans Healthcare System.
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