What is the definition of the anal canal?

Definition of the Anal Canal

The anal canal is defined functionally by the sphincter muscles, with its superior border at the palpable upper border of the anal sphincter and puborectalis muscles of the anorectal ring, extending approximately 3-5 cm in length to its inferior border at the anal verge. 1

Multiple Definitions of the Anal Canal

There are several ways to define the anal canal, each with specific clinical applications:

Functional/Surgical Definition

• The functional anal canal is defined by the sphincter muscles 1
• Superior border: palpable upper border of the anal sphincter and puborectalis muscles (anorectal ring) 1
• Inferior border: anal verge (lowermost edge of sphincter muscles) 1
• Length: approximately 3-5 cm 1
• This definition is primarily used in radical surgical treatment of anal cancer 1

Anatomic Definition

• Begins at the anorectal ring 1
• Extends to the anal verge (squamous mucocutaneous junction with perianal skin) 1

Histologic Definition

• Based on the mucosal lining characteristics 1
• Superior aspect: 1-2 cm zone between anal and rectal epithelium with rectal, urothelial, and squamous histologic characteristics 1
• Inferior aspect: area at the anal verge where modified squamous epithelium transitions to epidermis-lined anal margin 1
• The anal canal is predominantly lined with squamous epithelium, in contrast to the rectum (glandular epithelium) and anal margin (skin) 1

AJCC Cancer Staging Definition

• Anal canal cancer: tumors that develop from mucosa that cannot be entirely seen when the buttocks are gently pressed 1
• Perianal cancer: tumors that (1) arise within skin distal to or at squamous mucocutaneous junction; (2) can be visualized completely when buttocks are gently pressed; and (3) are within 5 cm of the anus 1

Surrounding Structures and Boundaries

• The anal margin starts at the anal verge and includes perianal skin over a 5-6 cm radius from the squamous mucocutaneous junction 1
• The anal canal is surrounded by the internal and external anal sphincters 2, 3
• The internal sphincter is a continuation of the circular muscle of the rectum 4
• The external sphincter can be divided into three parts: subcutaneous, superficial, and deep 4
• The levator ani muscle attaches directly to the lateral surface of the longitudinal smooth muscle of the rectum 2

Clinical Significance

• The internal sphincter contributes approximately 55% of the maximum anal basal pressure and is essential for maintaining continence 5
• Lymph drainage varies by location: the perianal skin and distal anal canal drain to superficial inguinal nodes, while the proximal anal canal drains to anorectal, perirectal, and paravertebral nodes 1
• Understanding these anatomical distinctions is crucial for proper surgical approach, especially in intersphincteric resection procedures 2
• The functional definition is particularly important in treatment planning for anal cancer 1