Anatomical Boundaries of the Lumbar Region, Flank, and Para-umbilical Region
The lumbar region refers to the posterior lower back area overlying the L1-L5 vertebrae, the flank (or lumbar flank) is the lateral region between the lower ribs and iliac crest, and the para-umbilical region is the area immediately surrounding the umbilicus anteriorly.
Lumbar Region
The lumbar region is defined by specific vertebral landmarks and muscular anatomy:
Vertebral boundaries: The lumbar spine consists of L1-L5 vertebrae, with the iliac crests typically aligned with the L4-L5 intervertebral space 1.
Posterior muscular anatomy: The lumbar erector spinae consists of two distinct muscles—iliocostalis lumborum and longissimus thoracis—each with thoracic and lumbar components that arise from the lumbar accessory processes and L1-4 transverse processes 2.
Deep fascial structures: The thoracolumbar fascia (TLF) forms a girdling structure with multiple aponeurotic and fascial layers that separate the paraspinal muscles from the posterior abdominal wall muscles 3.
Lateral extent: The lumbar region extends laterally to where the paraspinal retinacular sheath meets the aponeurosis of the transversus abdominis, forming a lateral raphe that marks the junction between the epaxial (paraspinal) and hypaxial (abdominal) muscle compartments 3.
Flank (Lumbar Flank) Region
The flank represents the lateral aspect of the trunk in the lumbar area:
Anatomical position: The flank is the lateral region of the trunk between the lower ribs superiorly and the iliac crest inferiorly, positioned between the anterior and posterior axillary lines 1.
Muscular boundaries: This region is bounded by the external abdominal oblique muscle anteriorly, which has direct fascial continuity with the posterior layer of the thoracolumbar fascia (measuring approximately 40.7 mm in width along the posterior border) 4.
Clinical relevance: While there is no universally accepted precise definition of the anterior and posterior boundaries, the flank generally corresponds to the lateral sector between the anterior axillary line (approximately 60 degrees from the mid-thoracic point) and the posterior axillary line (approximately 120 degrees from the mid-thoracic point) 1.
Para-umbilical Region
The para-umbilical region is defined by its relationship to the umbilicus:
Location: This is the area immediately surrounding the umbilicus on the anterior abdominal wall, typically within a few centimeters radius of the umbilical center.
Anatomical significance: The para-umbilical region overlies the midline where the rectus abdominis muscles are separated by the linea alba, and represents a common site for hernias and surgical access.
Important Clinical Caveats
Standardization challenges: There is no universally accepted nomenclature for precise anatomical boundaries, particularly for the flank region, where muscle markings do not run in vertical lines and may be difficult to identify on imaging 1.
Imaging considerations: When describing the lumbar region on DXA scans, the lumbar spine ROI should include L1-L4 vertebrae, with vertebrae identified by counting from the bottom up 1.
Lumbosacral plexus anatomy: The lumbar roots (L1-L5) emerge from the psoas major muscle and form the lumbosacral plexus, which is distinct from but adjacent to the lumbar region proper 1.