Clinical stability of a new method for fixation and explanation of epiretinal implants

Editor, Since the first electrical stimulation of the retina in blind patients (Humayun et al. 1996), epiretinal prostheses to restore vision have been developed (Velikay-Parel et al. 2006). Epiretinal implants are commonly fixed to the posterior pole with a retinal tack (Majji et al. 1999; Walter et al. 1999). In order to provide the option for a later exchange with subsequent generations of implant, the removal of the implant should not cause substantial intraocular trauma. We therefore developed a new method for epiretinal implant fixation which should provide for firm fixation and allow an atraumatic explantation. The clinical stability of the new method and the feasibility of the implant explantation were tested in an experimental study in 11 Göttinger minipigs. This animal trial was approved by the Austrian legal authorities and the Medical University of Graz and the animals were treated according to the guidelines of the Association for Research in Vision and Ophthalmology. Surgical procedures for the implantation and explantation of epiretinal implant dummies (Fig. 1) were performed in sterile conditions under general anaesthesia. After combined vitrectomy with lensectomy, a retinal titanium tack was inserted at the posterior pole above the upper vessel arcade using a retinal forceps (Fig. 2) (Geuder AG, Heidelberg, Germany). Then, a limbus-based scleral flap was prepared on the upper temporal globe and the polyimide strip was introduced into the vitreous cavity through a 5-mm incision, 2 mm from the limbus. Intraocularly, the polyimide strip was grasped with an endoforceps and docked onto the tack (Fig. 3A). The retainer tool with the fixed silicone tube was docked onto the tack (Fig. 3B). The silicone tube was then released on the tack by squeezing the handle. Next, the extraocular portion was sutured to the sclera at the upper temporal quadrant and the scleral flap, vitrectomy ports and conjunctiva were sutured. Special care was taken to make a watertight wound closure. Immediately after the tack insertion a minor subretinal haemorrhage was observed in three of 11 eyes. Retinal implant dummies consist of a calotte (18 × 14 mm) and a flexible polyimide strip (length 32 mm) with parylene coating. The preformed hole on the intraocular portion allows the implant to be positioned on the tack. The holes on the extraocular portion are used for suturing the implant to the sclera. The retainer tool consists of two tubes with an overall diameter of 1 mm; the aperture of the inner tube has a diameter of 0.5 mm. The outer tube is fixed and the inner tube protrudes 2 mm out of the outer tube (upper right). By squeezing the handle, the inner tube can be retracted (lower right). Preoperatively, the silicone tube is slipped over the inner tube of the retainer tool. Intraoperatively, the outer tube is pushed over the inner tube by pressing the release at the squeeze handle. (A) The previously inserted tack served as docking station for the implant. (B, C) Definite implant fixation is provided by the silicone tube. (D) Optical coherence tomography reveals close attachment of the intraocular portion to the inner retina. After observation periods of 3–8 weeks, the eyes were clinically examined and the implants subsequently explanted. In all eyes the implants had remained in place with no dislocation of the silicone tube. Pupillary membranes and vitreous strands were observed in all animals. In three of 11 animals, the retinal architecture remained unchanged, whereas in four animals vitreous strands caused localized traction detachments at the posterior pole. In four animals a total retinal detachment had occurred. During the explantation procedure vitreous membranes were dissected and removed with the vitreous cutter. The silicone tube was then dissected with the Microvitreoretinal (MVR) blade (Fig. 4) and removed with an endoforceps. The implant was grasped with the forceps distal to the tack, slipped off the tack and withdrawn from the posterior pole. The scleral flap was reopened and the extraocular part unfixed from the sclera after incision of the fibrotic capsule in which the extraocular part was embedded. The complete implant was then withdrawn from the eye. Dissection of the silicone tube with the Microvitreoretinal blade. This new approach to epiretinal prosthesis implantation proves to be surgically feasible, ensures a close attachment of the implant to the inner retina and provides an option for explantation. The occurrence of pupillary membranes is a known problem in pigs after lensectomy (Mahmoud et al. 2003). The fixation, however, remained stable despite this condition, even in total retinal detachment. Explantation of the implant is an uneventful and relatively easy surgical procedure. The remaining tack can be reused for the fixation of new epiretinal devices with upcoming technologies.