A 28-year-old woman presents with severe perianal pain during defecation. Examination reveals a linear ulcer in the posterior midline of the anal canal. Which anatomical factor is primarily responsible for the location of this fissure?

• A Increased density of anal glands in the posterior commissure
• B Hypertonia of the external anal sphincter in the posterior midline
• C Direct trauma from hard stool impacting the posterior anal wall
• D Relative ischemia of the posterior midline due to branching of the superior rectal artery ✓

Explanation

The posterior midline is the most common site for anal fissures because the superior rectal artery supplies the anal canal via branches that predominantly supply the lateral and anterior walls, leaving the posterior commissure relatively ischemic. This relative ischemia, combined with the greatest mechanical stress during defecation at this site, predisposes to fissure formation. While sphincter hypertonia is involved in chronicity, the primary anatomical reason for the location is the vascular anatomy.

Reference: Bailey and Love's Short Practice of Surgery, 27th ed.

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Written and medically reviewed by the StethoPrep medical team.