Stability of the input-output properties of chronically implanted multiple contact nerve cuff stimulating electrodes

The objective of this investigation was to measure the input-output (EO) properties of chronically implanted nerve cuff electrodes. Silicone rubber spiral nerve cuff electrodes, containing 12 individual platinum electrode contacts, were implanted on the sciatic nerve of 7 adult cats for 28-34 weeks. Measurements of the torque generated at the ankle joint by electrical stimulation of the sciatic nerve were made every 1-2 weeks for the first 6 weeks post-implant and every 3-5 weeks between 6 weeks and 32 weeks post-implant. In 3 implants the percutaneous lead cable was irreparably damaged by the animal within 4 weeks after implant and further testing was not possible. One additional lead cable was irreparably damaged by the animal at 17 weeks post-implant. The 3 remaining implants functioned for 28, 31, and 32 weeks. Input-output curves of ankle joint torque as a function of stimulus current amplitude were repeatable within an experimental session, but there were changes in EO curves between sessions. The degree of variability in I-O properties differed between implants and between different contacts within the same implant. After 8 weeks, the session to session changes in the stimulus amplitude required to generate 50% of the maximum torque (150) were smaller (15/spl plusmn/19%, mean /spl plusmn/s.d.) than the changes in 150 measured between 1 week and 8 weeks post-implant (34/spl plusmn/42%). Furthermore, the I-O properties were more stable across changes in limb position in the late post-implant period than in acutely implanted cuff electrodes. These results suggest that tissue encapsulation acted to stabilize chronically implanted cuff electrodes. Electrode movement relative to the nerve, de- and regeneration of nerve fibers, and the inability to precisely reproduce limb position in the measurement apparatus all may have contributed to the variability in I-O properties.