What is the relevant anatomy in anal fissure?

Updated: Jul 24, 2020
  • Author: Lisa Susan Poritz, MD; Chief Editor: John Geibel, MD, MSc, DSc, AGAF  more...
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A thorough knowledge of the anatomy of the anal canal is vital for effective surgical treatment of an anal fissure.

The anal canal may be defined in two ways, as follows:

  • Functional (or surgical) anal canal
  • Anatomic anal canal

These two terms are often used interchangeably, even though they do not mean the same thing. The surgical anal canal is approximately 4 cm long and extends from the anal verge or intersphincteric groove distally to the anorectal ring, proximally. The anatomic anal canal is only approximately 2 cm long and extends from the anal verge distally to the dentate line proximally.

The dentate line is the junction of the ectoderm and endoderm in the anal canal. The anal verge is an anocutaneous line approximately 2 cm distal to the dentate line. The anal verge marks the beginning of the anal canal.

The internal anal sphincter is a smooth muscle that is the most distal extension of the inner circular smooth muscle of the colon and the rectum. It is 2.5-4 cm long and is normally 2-3 mm thick. The internal sphincter is not under voluntary control and is continuously contracted to prevent unplanned loss of stool.

The external anal sphincter is striated muscle that forms a circular tube around the anal canal. Proximally, it merges with the puborectalis and the levator ani to form a single complex. Control of the external anal sphincter is voluntary.

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