ELECTRICAL STIMULATION FOR CONVERTING GRACILIS MUSCLE TO A FATIGUE-RESISTANT POWER SOURCE AND RECONSTRUCTION OF ANAL FUNCTION

For reconstruction of anal function in fecally incontinent patients, it may be feasible to transpose the gracilis muscle around the anal canal, using electrical stimulation to trigger contraction. However, because the fast-twitching gracilis muscle is incapable of prolonged contraction without fatigue, it is necessary to convert it to a slow-twitching, fatigue-resitant muscle. When the muscle contraction decreases even transiently between stimuli, the neoanus cannot maintain continence. Thus, the muscle must retain some tension between each stimulus. To fulfill these criteria, the neoanal sphincter must be stimulated at a frequency that can include sufficient summation. We demonstrated this conversion and contractile summation by long-term electrical pulse stimulation (0.2 msec pulse width) at low frequencies using a rabbit model. The nerve to the gracilis muscle of rabbits was continuously stimulated at 2Hz, 5Hz and 10Hz for 2 to 8 weeks. This conditioning reduced the frequency necessary for summation; the muscles conditioned at 10 Hz for 6 or 8 weeks induced sufficient summation (fusion index>90%) at 20Hz, and also showed sufficient summation (fusion index=80%) at 15Hz, a frequency that produced muscle contraction without fatigue. The conditioning also prolonged twitch contraction time and half relaxation time for one-shot pulse stimulation, but the twitch contraction speed was reduced with conditioning at a frequency greater than 5Hz. In the 6-week conditioning group, the percentage of type I fibers, identified by ATPase staining method, increased as the conditioning frequency became higher and the conditioning period became longer. After assessment of the conversion of gracilis muscle to a fatigue-resitant muscle, transposition of the muscle around the rectum was performed in Japanese white male rabbits. The muscles conditoned at 10Hz for 6 weeks were stimulated at 15Hz, a low frequency to permit prolonged contraction, and the neoanal pressure increased maximally to more than 100cmH_2O. The pressure suitable for fecal continence maintained in a period longer than 10 weeks (maximally 38 weeks) without any trouble, while the non-conditioning group showed decay of neoanal pressure in a short period. The conditioning made it possible for the rabbit's gracilis muscle to create anal pressure with a sufficient rise in the basal pressure at a frequency permitting prolonged contraction. Thus, we concluded that conditioning at 10Hz for 6 weeks can convert rabbit gracilis muscle to a slow-twitching, fatigue-resitant muscle and produce enough summation at a low frequency stimulation to permit prolonged contraction suitable for a power source to reconstruct a neoanal sphincter.