A 75-year-old man with a remote history of fistulotomy and radiation therapy for prostate cancer presents for evaluation of his worsening fecal incontinence. He has poor sphincter tone and a palpable muscular defect. Anal manometry confirms diminished resting and squeeze pressures that do not improve after pelvic floor physical therapy with biofeedback. His symptoms persist despite bowel habit optimization and sacral nerve stimulator implantation. The best next option for ths patient is