Which parts of the colon are fixed and which are mobile?

Fixed and Mobile Segments of the Colon

Traditional Anatomical Teaching

The traditional teaching states that the ascending and descending colon are fixed retroperitoneally, while the transverse and sigmoid colon are mobile with mesenteric attachments—however, modern cadaveric studies reveal significant anatomical variation that challenges this simplified model. 1

Fixed Segments (Retroperitoneal)

The following segments are typically fixed to the posterior abdominal wall:

• Ascending colon: Traditionally considered fixed and retroperitoneal, though anatomical variation is common (see below) 1
• Descending colon: Traditionally considered fixed and retroperitoneal, though approximately one-third of individuals have a mobile descending colon 1
• Rectum: Fixed in the pelvis below the peritoneal reflection 2

Mobile Segments (Intraperitoneal with Mesentery)

The following segments have mesenteric attachments allowing mobility:

• Transverse colon: Mobile with a transverse mesocolon averaging 7.4 cm in height (SD: 3.6 cm) 1
• Sigmoid colon: Mobile with a sigmoid mesocolon averaging 6.3 cm in height (SD: 2.6 cm), making it the most mobile segment and prone to volvulus 2, 1
• Cecum: Variable mobility depending on degree of peritoneal fixation 1

Critical Anatomical Variations

Two-thirds of individuals have a mobile portion of the ascending colon, and nearly one-third have a mobile descending colon—this is significantly more common in females and contradicts traditional anatomical teaching. 1

Specific Variations Include:

• Mobile ascending colon: Present in 66% of cadaveric specimens, significantly more common in females, which may explain why colonoscopy is more challenging in female patients 1
• Mobile descending colon: Present in approximately 30% of individuals 1
• Jackson's membrane: A congenital peritoneal band of the right colon present in 66% of individuals, affecting colonic mobility 1
• Intraperitoneal ascending or descending colon: Results from incomplete embryologic fixation occurring between the 12th-17th week of intrauterine life 3, 4

Clinical Implications

For Surgical Planning:

• Preoperative imaging is essential to identify anatomical variations before laparoscopic or open colorectal procedures, as unexpected mobility or fixation patterns can significantly alter surgical approach 2, 5
• Sigmoid volvulus management requires understanding that the sigmoid's excessive mobility (due to elongated mesocolon) predisposes to torsion, and fixation procedures (sigmoidopexy) have high recurrence rates of 29-36% 2
• Descending colon volvulus is extremely rare precisely because this segment is typically fixed retroperitoneally, but can occur when anatomical variations create abnormal mobility 6, 3

For Endoscopic Procedures:

• Colonoscopy difficulty correlates with mobile ascending colon segments, particularly in females where this variation is more prevalent 1
• CT colonography reporting should document anatomical landmarks using standard terminology: rectum, sigmoid colon, descending colon, transverse colon, ascending colon, and cecum 2

Common Pitfall:

Do not assume the ascending and descending colon are always fixed—failure to recognize mobile segments preoperatively can lead to unexpected intraoperative findings and complications during laparoscopic mobilization. 1, 5