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Ab interno collagen stent enhances drainage to subconjunctival space
In human eyes, the implant was mobile in the subconjunctival space for 6 months, with no adhesion, fibrosis or vascularization.
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A minimally invasive ab interno collagen stent may be a viable alternative to other glaucoma drainage methods, according to preclinical testing results.
In a review published in the Journal of Cataract and Refractive Surgery, Richard A. Lewis, MD, described extensive preclinical and animal testing of the Xen gel stent (AqueSys). The device is undergoing human clinical trials in the U.S.
The implant was designed to regulate aqueous drainage from the eye.
Richard A. Lewis
Determining the optimum size of the device lumen to avoid hypotony and maximize long-term aqueous drainage was critical in the design phase, according to Lewis, an OSN Glaucoma Board Member.
“One of the big concerns in all of our glaucoma implants was how much flow you want out because if you have too much flow, you end up with hypotony and complications. So, regulating outflow was a critical component of this,” Lewis said.
Material and design
The Xen gel stent is a hydrophilic tube made of porcine or bovine gelatin cross-linked with glutaraldehyde. Both versions of the material appeared to be stable, Lewis said.
“There are advantages to each in terms of certain areas of the world where you can’t use a porcine material,” he said.
Three 6-mm implants were designed: Xen 140, Xen 63 and Xen 45. Approximate internal diameters are 140 µm, 63 µm and 45 µm, respectively.
Poiseuille’s law, used to describe flow rate, time and velocity, was applied when designing outflow control parameters for the device, Lewis told Ocular Surgery News.
“It can be for any kind of device that you place,” he said. “It is regulated by the production of aqueous as well as the diameter of the tube.”
The implant material was originally designed to be absorbed by the ocular tissue within a few months or years; however, this approach was changed to use a cross-linked permanent version of the gelatin tube, according to AqueSys.
The implant is hard when dry but soft and flexible when hydrated. After being implanted in the eye, it becomes soft in 1 to 2 minutes. In its natural hydrated state, the implant is straight, but it adapts to the tissue shape.
“The stiffness of the material was really critical. It couldn’t be too stiff because it would erode the conjunctiva, yet we needed some flexibility to be able to maneuver it through the eye and out in the subconjunctival space,” Lewis said.
Preclinical and clinical studies
In preclinical animal testing with rabbits, primates and dogs, implanted cross-linked times and inflammatory reaction were evaluated to assess biocompatibility. An ab externo approach showed cell proliferation and scarring from surgical trauma to the conjunctiva. An ab interno approach showed excellent biocompatibility and a longer duration of drainage than with trabeculectomy in rabbits.
In primates, a study showed a biologic response similar to that seen in rabbits, with a conjunctival bleb that formed immediately and drainage vessels that formed in the conjunctiva.
“Hydrolytic stability shows no evidence of hydrolytic degradation. The biocompatibility properties of gelatin are well established and do not cause a foreign-body reaction,” Lewis said in the review.
Testing also showed that the implant did not occlude inside the lumen. Outflow was blocked by the formation of fibroblast tissue around the exit of the implant.
In human eye testing, the implant material remained mobile in the subconjunctival space for 6 months, with no adhesion, fibrosis or vascularization, he said in the review.
“Clinical trials are ongoing. There have been trials all over the world. The trials are proceeding along nicely,” Lewis said. “They’re in the second U.S. trial right now. We’re getting very favorable clinical results from it.”
Lewis cited ongoing concern in the U.S. about combining the Xen gel stent with an antifibrotic agent.
“Up to 2 years ago, there was no antifibrotic that was approved,” Lewis said. “When it got approved, it was approved in a way that isn’t ideally how we wanted it designed for this particular product.”
The implant is being used in Europe and Canada with good results, he said. – by Matt Hasson
Reference:
Lewis RA. J Cataract Refract Surg. 2014;doi:10.1016/j.jcrs.2014.01.032.
For more information:
Richard A. Lewis, MD, can be reached at 1515 River Park Drive, Sacramento, CA 95815; 916-649-1515; email: rlewiseyemd@yahoo.com.
Disclosure: Lewis is a consultant for AqueSys.