A 48-year-old man with a 10-year history of chronic anal fissure (posterior midline) presents for surgical evaluation after failure of medical management with topical diltiazem and stool softeners for 8 weeks. On examination, the fissure has indurated edges, a sentinel pile at the anal verge, and an internal skin tag. Internal anal sphincter pressure is markedly elevated on manometry. He is concerned about postoperative incontinence. What is the most appropriate surgical intervention?