Goodsall's rule

One of the most commonly cited principles to assist in the surgical management of an anal fistula is Goodsall's rule, which states that all fistula tracks with external openings within 3 cm of the anal verge and posterior to a line drawn through the ischial spines travel in a curvilinear fashion to the posterior midline, and all tracks with external openings anterior to this line enter the anal canal in a radial fashion. The line of demarcation is depicted in this diagram as the "transverse anal line." Although Goodsall's rule is often quoted, it may not always be accurate. In particular, fistula tracks longer than 3 cm from the anal verge do not necessarily follow Goodsall's rule; they often have an internal opening in the posterior midline regardless of the location of the external opening.