Anatomical Segments and Length of the Rectum
Definition and Length
The rectum extends approximately 12-15 cm from the anal verge to the rectosigmoid junction, though in a distended lumen during endoscopy, 16-20 cm of endoscope insertion may be required to reach the rectosigmoid junction. 1
The rectum is formally defined as tumors with distal extension to 15 cm from the anal margin as measured by rigid sigmoidoscopy, with more proximal tumors classified as colonic. 1
Three Anatomical Segments
The rectum is divided into three distinct segments based on distance from the anal verge 1:
- Lower rectum: Up to 5 cm from the anal verge 1
- Middle rectum: Greater than 5 cm to 10 cm from the anal verge 1
- High (upper) rectum: Greater than 10 cm up to 15 cm from the anal verge 1
Key Anatomical Boundaries
Superior Boundary
The rectosigmoid junction is defined by the National Comprehensive Cancer Network as the line connecting the sacral promontory to the upper edge of the pubic symphysis on mid-sagittal MRI. 2
Inferior Boundary
The anorectal ring marks the inferior boundary, formed by the palpable upper border of the anal sphincter and puborectalis muscles, located approximately 3-5 cm from the anal verge. 2
Peritoneal Coverage Varies by Segment
The peritoneal coverage of the rectum differs significantly by anatomical level 2:
- Upper third: The rectum is covered by peritoneum anteriorly and laterally 1
- Middle third: Peritoneal coverage exists only on the anterior wall 1
- Lower third: No peritoneal covering (serosa), which facilitates easier transmural spread of rectal adenocarcinoma 1
Clinical Significance of Segmentation
Surgical Implications by Segment
Upper third tumors typically require low anterior resection (LAR) extended 4-5 cm below the distal tumor edge using total mesorectal excision (TME), followed by colorectal anastomosis. 3
Middle third tumors usually permit sphincter-preserving surgery through anterior resection or coloanal anastomosis. 3
Lower third tumors that directly involve the anal sphincter or levator muscles require abdominoperineal resection (APR) with TME. 3
Critical Measurement Considerations
Location must be documented in relation to specific anatomical landmarks including the ileocecal valve, previous anastomosis, rectosigmoid junction, and dentate line. 1
The distance should be measured using rigid sigmoidoscopy rather than flexible endoscopy, as flexible endoscopy can overestimate the true distance due to bowel redundancy. 1
Common Pitfall
Always annotate the distance between rectal tumors and the puborectal muscle, and specify the involved quadrant, particularly the anterior quadrant, as this affects surgical planning and prognosis. 2