Techniques for Processing Eyes Implanted with a Retinal Prosthesis for Localized Histopathological Analysis: Part 2 Epiretinal Implants with Retinal Tacks

Abstract

Retinal prostheses for the treatment of certain forms of blindness are gaining traction in clinical trials around the world with commercial devices currently entering the market. In order to evaluate the safety of these devices, in preclinical studies, reliable techniques are needed. However, the hard metal components utilised in some retinal implants are not compatible with traditional histological processes, particularly in consideration for the delicate nature of the surrounding tissue. Here we describe techniques for assessing the health of the eye directly adjacent to a retinal implant secured epiretinally with a metal tack.Retinal prostheses feature electrode arrays in contact with eye tissue. The most commonly used location for implantation is the epiretinal location (posterior chamber of the eye), where the implant is secured to the retina with a metal tack that penetrates all the layers of the eye. Previous methods have not been able to assess the proximal ocular tissue with the tack in situ, due to the inability of traditional histological techniques to cut metal objects. Consequently, it has been difficult to assess localized damage, if present, caused by tack insertion.Therefore, we developed a technique for visualizing the tissue around a retinal tack and implant. We have modified an established technique, used for processing and visualizing hard bony tissue around a cochlear implant, for the soft delicate tissues of the eye. We orientated and embedded the fixed eye tissue, including the implant and retinal tack, in epoxy resin, to stabilise and protect the structure of the sample. Embedded samples were then ground, polished, stained, and imaged under various magnifications at incremental depths through the sample. This technique allowed the reliable assessment of eye tissue integrity and cytoarchitecture adjacent to the metal tack.

In vivo feasibility of epiretinal stimulation using ultrananocrystalline diamond electrodes

“…Once the device was seen to gently touch the retina, the tack was inserted using the same forceps. The tack length was chosen such that the head would penetrate the retina, choroid and sclera [26]. If a magnetic attachment was used, the device was simply advanced towards the retina with the surgeon aligning the epiretinal magnet visually with the approximate position of the suprachoroidal magnet.…”

The development of neural stimulators: a review of preclinical safety and efficacy studies

“…Recent immunohistochemical techniques describe the evaluation of the tissue response with microelectrodes in situ (228, 229). Techniques are also available for the in situ inspection of macroelectrodes, although the approach is more limited in terms of resolution and tissue staining (230).…”

Wireless induction coils embedded in diamond for power transfer in medical implants

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