Average Length of the Rectum
The rectum extends approximately 12-15 cm from the anal verge, as measured by rigid sigmoidoscopy, which is the clinical standard for defining rectal anatomy. 1
Clinical Definition and Measurement Standards
Tumors with distal extension to ≤15 cm from the anal margin (measured by rigid sigmoidoscopy) are classified as rectal, while more proximal tumors are classified as colonic. 1
The NCCN provides an alternative anatomical definition: the rectum is the area below the line connecting the sacral promontory to the upper edge of the pubic symphysis on mid-sagittal MRI. 1, 2
Intraoperative measurements in Korean populations showed the sacral promontory averaged 16.5 ± 2.2 cm in males and 16.1 ± 2.2 cm in females from the anal verge, with no statistically significant gender differences. 3
Anatomical Subdivisions for Clinical Management
Rectal cancers are categorized by distance from the anal verge to guide surgical approach: 1
- Low rectum: up to 5 cm from the anal margin
- Middle rectum: >5 to 10 cm from the anal margin
- High rectum: >10 to 15 cm from the anal margin
Important Anatomical Boundaries
The anorectal ring (palpable upper border of the anal sphincter and puborectalis muscles) marks the inferior boundary of the rectum, located approximately 3-5 cm from the anal verge. 2, 4
The surgical anal canal (anorectal ring to anal verge) averages 4.2 cm in length (range 3.0-5.3 cm), with men averaging 4.4 cm and women 4.0 cm. 4
Peritoneal Reflection Landmarks
The peritoneal reflection varies by location around the rectum: 3
- Anterior peritoneal reflection: 8.8 ± 2.2 cm in males, 8.1 ± 1.7 cm in females
- Lateral peritoneal reflection: 10.8 ± 2.7 cm in males, 11.4 ± 1.9 cm in females
- Posterior peritoneal reflection: 13.8 ± 2.5 cm in males, 14.0 ± 1.9 cm in females
Clinical Pitfalls to Avoid
Always use rigid sigmoidoscopy rather than flexible endoscopy for precise measurement, as rigid sigmoidoscopy is the gold standard for determining tumor location and surgical planning. 1
The relationship of rectal tumors to the anterior peritoneal reflection is clinically critical, as it predicts risk of both local and peritoneal recurrence. 2
Height correlates with the length of the sacral promontory (p=0.015 in males, p=0.018 in females), but age and gender do not significantly affect overall rectal length measurements. 3
Document the distance between rectal lesions and the puborectalis muscle, and specify the involved quadrant, particularly the anterior 1/4 quadrant (10 o'clock to 2 o'clock position), as this affects surgical margins and outcomes. 1, 2