How to convert the rabbit gracilis muscle into a neoanal sphincter

For reconstruction of anal function in fecally incontinent patients, transposition of the gracilis muscle around the anal canal, with electrical stimulation to maintain contraction, seems practical. But the fast-twitch gracilis muscle is incapable of prolonged contraction without fatigue. Furthermore, the muscle must keep some tension between each stimulus. When the pressure decreases even transiently between stimuli, the neoanus cannot maintain continence. To fulfill these criteria, the neoanal sphincter must be stimulated at a frequency that can induce sufficient summation. We demonstrated that conditioning with long-term electrical stimulation will induce such summation at low frequency. The nerve to the gracilis muscle of rabbits was continuously stimulated at 5 Hz and 10 Hz for 4 to 8 weeks. This conditioning reduced the frequency necessary for summation, and conditioning at 10 Hz for 6 or 8 weeks induced sufficient summation (fusion index [FI] > 90%) at 20 Hz. The muscles conditioned at 10 Hz for 8 weeks showed sufficient summation (FI = 81%) at 15 Hz, a frequency that produced muscle contraction without fatigue, since even unconditioned muscle will remain contracted at 10 Hz without fatigue for 8 weeks. It is concluded that conditioning at 10 Hz for longer than 6 weeks produces enough summation at a low frequency stimulation to permit prolonged contraction.