Abstracts

The Responsive Neurostimulator System (RNS) is currently undergoing testing under an investigational device exemption from the FDA. The system consists of a cranially implanted pulse generator connected to strip and/or depth electrode arrays and an implantable ferrule for fixating the RNS to the cranium. The RNS is curved to allow for a contoured cranial placement. A cranial bed is required to implant the ferrule and generator unit. This bed is commonly produced by performing a full thickness craniectomy. We report an alternative method which affords certain advantages regarding the support and stability of the unit. After placement of electrode arrays intracranially, a location and orientation for the RNS is determined and a template used to mark the cranium accordingly. Midas drills (M32, AM8) are used to mill the outer table and trabeculated bone to create a cranial bed within the defined site while preserving the inner table. This process requires 20 - 30 minutes and is performed in an iterative manner with the fit checked periodically. The ferrule is imbedded and secured to the cranium with 4 mm screws (Lorenz). The RNS is then attached to the ferrule with an integrated clamp. Any discontinuities between the ferrule, RNS and cranium are transitioined with methylmethacrylate cement to produce a smooth contour. Three patients were implanted using the described approach. All had previously undergone intracranial long-term monitoring through craniotomies in the past. Following the placement of the RNS, patients have been followed 2 - 3 months with good cosmetic results and no complaints of irregularities or auditory sensations resulting from isolated pockets of fluid in the construct. In addition to providing additional structural support for the generator unit, preservation of the inner table allows for a foundation upon which a synthetic cranioplasty may be applied in the event of generator removal. A firm seating of the ferrule and generator unit with the additional application of methylmethacrylate creates a well-contoured surface that is mechanically noiseless.