Background: An ideal reinforcing neo-sphincter should be innervated by the pudendal nerve to work in coordination with the external anal sphincter. The aim of this study was to create a skeletal muscle innervated by the pudendal nerve without inducing external anal sphincter morbidity.
Methods: Seven dogs were used. On the right side of each dog, the distal end of the transected nerve innervating the biceps femoris muscle was anastomosed to the perineural window of the pudendal nerve, where the epineurium was excised over a small area without injuring the funiculus. Reinnervation was studied 5 months after the operations. The left side of the dogs was used as a control.
Results: For all 7 dogs, there was no macroscopic difference detected in the external anal sphincter. For 6 dogs, the biceps femoris muscle was preserved after end-to-side pudendal nerve anastomosis. After the pudendal nerve central to the anastomosis site was electrically stimulated, the external anal sphincter contracted in all dogs. After end-to-side pudendal nerve anastomosis, the biceps femoris muscle contracted with the evoked potential in 5 dogs (71%) and demonstrated electric activity at rest in 3 dogs (43%), but there was no reflex activity after anal stimulation. The ratio of type 1 to type 2 muscle fiber in the biceps femoris muscle after end-to-side pudendal nerve anastomosis significantly changed and became the same as that in the external anal sphincter. The diameter of type 2 muscle fibers in the biceps femoris muscle significantly decreased after surgery. In addition, regenerated myelinated axons were observed in a cross section of the anastomosed nerve in 6 dogs.
Conclusions: End-to-side pudendal nerve anastomosis is a promising technique for the creation of an anal sphincter in patients who have fecal incontinence. The technique preserved the original external anal sphincter and created a skeletal muscle that was innervated by the pudendal nerve in 71% of the dogs studied. This newly innervated skeletal muscle was capable of contracting in coordination with the original external anal sphincter on electric stimulation and also demonstrated characteristics closely similar to those of the external anal sphincter.