Anatomical Differentiation: Flank (Lumbar) vs. Periumbilical Regions
The flank region and periumbilical region are completely distinct anatomical areas: the flank (also called lumbar region) refers to the lateral abdominal wall extending from the lower ribs to the pelvis, while the periumbilical region is the central area immediately surrounding the umbilicus. 1
Flank (Lumbar) Region
The flank is the lateral portion of the abdominal wall, NOT the periumbilical area. This terminology confusion needs immediate clarification:
Anatomical Boundaries
- Superior border: Inferior margin of the rib cage 1
- Inferior border: Iliac crest/pelvic brim 2
- Medial border: Extends to the mid-axillary lines laterally 1
- Location: Posterior lateral aspect of the trunk, confined by costal margin, spine, and pelvic brim 2
Clinical Context
- The flank region is specifically identified as a separate injection site distinct from the abdomen in insulin delivery guidelines, described as "posterior lateral aspect of both upper buttocks and flanks" 1
- In body region classifications, the lower back/buttocks region (which includes flanks) has distinct borders: superior at the inferior rib cage margin, inferior at the gluteal fold, with lateral borders at mid-axillary lines 1
Periumbilical (Umbilical) Region
The periumbilical region is the central abdominal area immediately surrounding the umbilicus:
Anatomical Boundaries
- Location: Central abdomen, directly around the umbilicus 1
- Exclusion zone: Insulin injection guidelines specify staying at least 1 cm away from the umbilicus when injecting in the abdomen 1
- Broader abdominal context: The abdomen proper extends from 1 cm above the symphysis pubis to 1 cm below the lowest rib, with the periumbilical area being the central portion 1
Key Distinction in Practice
- When describing abdominal injection sites, guidelines explicitly state "2 adult fingerbreadths away from the umbilicus" for the abdomen, while flanks are listed as an entirely separate anatomical region 1
- The abdomen/genital region is bounded laterally by mid-axillary lines, with flanks being posterior and lateral to this 1
Critical Clinical Pitfall
Never use "flank" and "periumbilical" interchangeably—they represent opposite sides of the torso. The flank is lateral/posterior, while periumbilical is anterior/central. 1, 2 This distinction is essential for accurate anatomical localization, surgical approaches, and clinical documentation.