What is the knuckle of the small bowel?

What is a Knuckle of Small Bowel?

A "knuckle" of small bowel refers to a loop or segment of small intestine that appears folded or bent upon itself, creating a U-shaped or hairpin configuration. This is primarily a descriptive anatomical and radiological term used to characterize the appearance of bowel loops on imaging or during surgical inspection.

Anatomical Context

• The small bowel naturally forms multiple loops and curves within the abdominal cavity due to its considerable length (275-850 cm) relative to the available space 1
• A knuckle represents a single loop or fold of bowel, typically appearing as a curved segment where the bowel doubles back on itself 1
• This configuration is a normal anatomical arrangement in most cases, though the term becomes clinically significant when associated with pathology 2

Clinical Significance

Normal Anatomy

• Small bowel knuckles are routinely visualized on cross-sectional imaging (CT, MRI) and represent the normal folded configuration of jejunal and ileal loops 3
• The jejunum typically shows 4.6 folds per 2.5 cm, while the terminal ileum shows approximately 1.5 folds per 2.5 cm 3

Pathological Conditions

When a knuckle becomes clinically relevant, it typically indicates one of several pathological processes:

• Small bowel obstruction: A knuckle may represent the site of mechanical obstruction, with proximal dilation (>3 cm) and distal collapse 1, 4
• Volvulus: A twisted knuckle of bowel around its mesenteric axis, creating a surgical emergency with risk of ischemia and perforation 2
• Intussusception: One segment of bowel telescoping into an adjacent segment, creating a characteristic folded appearance 2
• Adhesive bands: Post-surgical adhesions may cause a knuckle of bowel to become kinked or angulated, leading to partial or complete obstruction 2, 4

Imaging Characteristics

CT and MRI Findings

• Normal small bowel diameter ranges from 24.8 mm in the duodenum to 18.7 mm in the terminal ileum 3
• Bowel wall thickness should measure 1.5 ± 0.5 mm throughout the small bowel 3
• Pathological knuckles demonstrate: wall thickening >3 mm, luminal dilation >3 cm, upstream dilation with distal collapse, or abnormal enhancement patterns 1, 4

The "Small Bowel Faeces Sign"

• When a dilated knuckle contains gas bubbles mixed with particulate matter resembling feces, this indicates significant pathology 5
• In the presence of moderate-to-severe bowel dilation (>3 cm) and distal collapse, this sign has 82% association with mechanical small bowel obstruction 5, 6
• However, when seen with normal or mildly dilated bowel (<2.9 cm), the majority of patients (90%) do not have obstruction 6

Surgical and Endoscopic Context

• During laparotomy or laparoscopy, surgeons routinely examine individual knuckles of bowel to assess for inflammation, ischemia, perforation, or obstruction 1
• In Crohn's disease, affected knuckles may show transmural inflammation, "fat wrapping" (mesenteric fat extending toward the antimesenteric surface), fistulas, or strictures 1
• Surgical resection often involves removing diseased knuckles with primary anastomosis of healthy bowel segments 1

Common Pitfalls

• Do not assume all visualized knuckles are pathological: The small bowel naturally forms multiple loops, and identifying a knuckle on imaging does not inherently indicate disease 3
• Distinguish between fixed narrowing and transient spasm: A single imaging study cannot always differentiate between a strictured knuckle and temporary spasm; serial imaging or fluoroscopic observation may be needed 1
• Recognize that "knuckle" is descriptive, not diagnostic: The term describes anatomy or appearance but requires clinical correlation and additional imaging features (dilation, wall thickening, enhancement) to determine significance 4, 3