The flank area, or regio lateralis, is the soft region on the side of the body between the bottom of the ribs and the top of the hip. Medically, this area is often referred to as the lumbar region. The flank serves as a protective yet flexible wall for several vital internal organs. Understanding the flank involves examining its surface boundaries, the underlying structures it contains, and the common causes of pain originating there.
Anatomical Definition and Boundaries
The flank is defined by bony and muscular landmarks, forming a rectangular area on each side of the torso. The superior boundary is the lower margin of the rib cage, specifically the tenth through twelfth ribs and the costal margin. The inferior boundary is the iliac crest, the curved bone at the top of the pelvis.
The flank region extends from the front to the back of the body. Its anterior limitation is the mid-clavicular line, which runs down from the collarbone. The posterior limitation is the lateral edge of the erector spinae muscles, the large muscle groups running alongside the spine.
The flank wall structurally consists of three layers of muscle: the external oblique, internal oblique, and transversus abdominis. The quadratus lumborum muscle forms a significant portion of the posterior wall, connecting the iliac crest to the lowest rib and the lumbar vertebrae.
Key Internal Structures
The internal anatomy of the flank region is dominated by structures lying behind the abdominal cavity lining, known as the retroperitoneal space. The most well-known organs here are the two kidneys, which sit high up on the posterior abdominal wall. The kidneys are cushioned by fat and protected by the lower ribs and thick flank muscles, making them a frequent source of pain in this region.
The flank also contains parts of the large intestine, specifically the ascending colon on the right and the descending colon on the left. The lower poles of the liver (on the right) and the spleen (on the left) may extend into the upper flank region. Furthermore, the ureters, which carry urine from the kidneys to the bladder, pass through this area close to the posterior wall.
Understanding Flank Pain
Pain localized to the flank can arise from the skin, muscles, or deep internal organs, but kidney and ureter pathology is the most frequent cause. Urolithiasis, commonly known as kidney stones, causes a distinctive discomfort called renal colic. This pain is acute and comes in severe waves, often radiating from the flank toward the groin or lower abdomen as the stone moves down the ureter.
A kidney infection, or pyelonephritis, presents with a different character of flank pain, usually a constant, dull ache that is less severe than colic. This infectious cause is accompanied by systemic symptoms like fever, chills, and nausea, differentiating it from stone passage. Tenderness upon tapping the area over the kidney, known as costovertebral angle tenderness, is a common sign associated with pyelonephritis.
A prevalent, non-organ-related cause is musculoskeletal strain, involving flank muscles like the obliques or the quadratus lumborum. This pain is a dull soreness that worsens with specific movements, such as twisting, lifting, or bending. Unlike pain originating from the kidney, musculoskeletal discomfort can often be reproduced or intensified by directly pressing on the affected muscle. Persistent or severe pain in this area warrants professional medical evaluation to accurately determine its source.