Pediatric cochlear implantation: strategies to accommodate for head growth

Accommodation for head growth presents one of several challenges unique to pediatric cochlear implantation. Given contemporary cochlear implant device designs, an electrode cable implanted at the age of 2 years must extend 2 to 3 cm as the head grows during childhood. In an initial study we found that model lead wires with redundant loops extended effectively when they were maintained within air-containing spaces such as the mastoid cavity or middle ear space. However, when looped leads traversed soft tissues overlying the parietal bone, they became embedded in fibrous tissue and did not extend. The present study evaluated three different configurations of expansile devices that were enclosed in polytetrafluoroethylene (PTFE) envelopes to deter fibrous ingrowth. This simple strategy was designed to ensure effective cable extension over cable pathlengths by protecting the redundant leads from any mechanically significant connective tissue ingrowth. Twelve such devices were implanted across the calvaria of four newly weaned piglets. Skull growth and changes in electrode dimensions were documented by sequential computed tomographic scans. At 3 months of age, cranial circumferences had increased substantially. Animals were then killed, the model cable extension appliances examined physically, and their implantation sites examined histologically. For all experimental devices, extension of redundant lead wires was satisfactory, and there was no mechanically significant invasion of fibrous connective tissue into the PTFE envelope. This indicates that enclosure of excess lead wire within a PTFE envelope may be an effective means of inhibiting fibrous ingrowth. This strategy should prove useful for ensuring effective electrode cable extension in cochlear implants applied in young children.