Anal Triangle

The anal triangle is the posterior portion of the perineum, containing the anal canal, surrounding muscles, fascia, and neurovascular structures. It is one of the two subdivisions of the perineum (the other being the urogenital triangle), and it plays an essential role in fecal continence and defecation. The triangle is defined by bony and ligamentous landmarks and contains the ischioanal fossae, which provide space for the expansion of the anal canal during defecation.

Structure

The anal triangle is a fat-filled, wedge-shaped region containing the terminal part of the gastrointestinal tract and supporting soft tissues. It is primarily composed of the anal canal, the external anal sphincter, ischioanal fossae, and associated vessels and nerves.

Boundaries

• Anterior: Line between the ischial tuberosities (separating it from the urogenital triangle)
• Posterior: Tip of the coccyx
• Lateral: Sacrotuberous ligaments covered by gluteus maximus

Contents

• Anal canal
• External anal sphincter muscle
• Ischioanal fossae (right and left)
• Inferior rectal arteries and veins
• Inferior rectal nerves (branches of pudendal nerve)
• Perineal body (anterior border)

Ischioanal Fossa

The ischioanal fossae are paired, wedge-shaped fat-filled spaces that lie on either side of the anal canal. They allow for expansion of the canal during defecation and are traversed by neurovascular structures supplying the anal region.

Location

The anal triangle lies posteriorly in the perineum and is positioned between the thighs and buttocks, below the pelvic diaphragm. It is best visualized with the patient in the lithotomy or prone position.

Function

• Defecation: Houses the anal canal and muscles responsible for regulating the passage of feces
• Fecal Continence: External anal sphincter and levator ani muscles maintain closure of the anal canal
• Structural Support: Ischioanal fossae provide space for canal expansion during bowel movements
• Neurovascular Conduit: Transmits inferior rectal vessels and nerves to the anal region

Clinical Significance

• Abscess Formation: Common in the ischioanal fossa; may spread to contralateral side forming a horseshoe abscess
• Fistula-in-Ano: Chronic infection may form a tract from anal canal to perianal skin
• Surgical Landmark: Important region during procedures like hemorrhoidectomy or fistulotomy