Healing response associated with balloon-dilated ePTFE

Evaluation of an aqueous drainage glaucoma device constructed of ePTFE

Aqueous drainage devices for the treatment of glaucoma are subject to the same limitations as most polymeric implants, namely a healing response comprised of chronic inflammation and fibrosis. The most widely used devices are currently made of silicone or polypropylene, materials that exhibit biocompatibility difficulties when they are implanted on the sclera underneath the conjunctiva of the eye. Decreased outflow of aqueous fluid to the conjunctival space caused by the development of a fibrous capsule around the device accounts for at least 20% of aqueous shunts failures. Clearly, the need exists to improve the healing response to aqueous drainage devices, and one approach is to develop new polymers or polymer modifications. Improved devices would elicit a limited fibrotic response while increasing neovascularization around the implant. Previous studies have indicated that denucleation markedly improves the healing characteristics and biocompatibility of expanded polytetrafluoroethylene (ePTFE). We reasoned that altering the design of drainage devices to allow the use of denucleated ePTFE in vivo might minimize fibrosis, thereby improving shunt function. We found that after 8 weeks in vivo, experimental shunt function was equivalent to the Baerveldt shunt, while there was less scarring with increased neovascularizatin. These findings suggest that ePTFE has potential as an improved, long-term alternative material for use in constructing glaucoma shunts.

Denucleation promotes neovascularization of ePTFE in vivo

Expanded polytetrafluoroethylene (ePTFE) implants are being increasingly used as vascular prostheses and other devices. However the tissue response associated with this and other polymer implants continues to limit any long-term function. One of the major approaches currently being investigated to improve biocompatibility involves surface modification of the base polymer. In this report, we attempted to alter the healing characteristics of ePTFE by denucleation, a process which removes air trapped within the interstices of the material. Additionally, adsorption of extracellular proteins on the denucleated polymer was also tested. After 5 weeks implanted in subcutaneous and epididymal fat sites of rats, the material was explanted and the healing around the implant evaluated histologically. We found that in skin implants, denucleation alone resulted in a substantial reduction in the fibrous capsule which has been previously reported for untreated ePTFE, and an increase in blood vessel development around and within the polymer. Absorption of extracellular matrix proteins prior to implantation resulted in a reduced vascularity of the implants compared with denucleation-only implants. Implants in fat tissue, regardless of treatment, showed very little tissue reaction, either in the number of inflammatory cells, development of a fibrous capsule or neovascularization. These results suggest that the presence of air nuclei within porous material may contribute to the inappropriate healing response associated with these polymers. In addition, they confirm earlier reports that healing around implanted polymers is tissue-specific.