Morphology of dynamic graciloplasty compared with the anal sphincter.

Dynamic graciloplasty for fecal incontinence includes gracilis muscle transposition around the anal canal as a new sphincter and subsequent electrical stimulation. The aim of electrical stimulation is to transform the gracilis fast-twitch, "fatigue-prone" fibers into slow-twitch, "fatigue-resistant" fibers to achieve a sustained tonic contraction. The latter is considered essential for sphincter function. Therefore, the following features of transposed gracilis muscle morphology were studied in nine patients before and after electrical stimulation: 1) the percentage of Type I fibers, 2) the lesser diameter of these fibers, and 3) the positive collagen staining area. Furthermore, the external and sphincter and gracilis muscle histology was investigated in six autopsy cases. The mean percentage of Type I, slow-twitch, fatigue-resistant fibers in transposed gracilis muscle increased from 46 percent before electrical stimulation to 64 percent (P < 0.01, paired Student's t-test) after electrical stimulation. The mean lesser diameter of these fibers did not change significantly (from 32 to 29 microns), and the mean percentage of collagen increased from 4 percent before electrical stimulation to 7 percent (P < 0.01) afterward. The external sphincter in cadavers demonstrated a predominance of Type I fibers (80 percent) with a lesser diameter of 23 microns and a high percentage (12 percent) of collagen. Gracilis muscle histology was uniform at six different sample sites in these cadaver dissections. We conclude that electrical stimulation induces histologic changes in transposed gracilis muscle, allowing this muscle to function as an external anal sphincter.