Extraction forces and tissue changes during explant of CWRU-type intramuscular electrodes from rat gastrocnemius

Intramuscular electrodes are currently in use for clinically implementing several electrotherapeutic and neuroprosthetic protocols. A decrease in motor recruitment is often reported in these systems due to movement of the electrode tip from the initial implant site. In the study reported here, multistrand intramuscular electrodes of the CWRU design were implanted aseptically in the gastrocnemii of adult rats under anesthesia. These electrodes were explanted immediately after implant in one group and after periods of 1 and 4 hr; 1, 3, and 5 days; 1 week; 10 days; and 2 and 4 weeks in others. Force as a function of displacement was recorded during explantation. Analysis of the results showed that there was a significant increase in the force required to dislodge the electrode tip between 5 and 7 days of implant. Electrodes seemed to be vulnerable to movement in the first 5 days when the barb provided the only fixation. Histology of muscles from which electrodes had been explanted did not show any increase in the area of tissue changes, compared with control muscles in which the electrode remained in situ. These results indicated that electrode removal occurred within the encapsulation tissues, and the surrounding muscle was mainly unaffected by the explant process.