Boundaries and division of the lumbar region
The lumbar region (regio lumbalis) has clear anatomical landmarks. Superiorly, it is bounded by the XII ribs, and inferiorly by the iliac crests and the sacrum. The lateral boundaries are the posterior axillary lines.
The posterior median line, running along the spinous processes of the vertebrae, divides this zone into two symmetrical halves. Each of them is subdivided into two subsections:
- Medial lumbar region (regio lumbalis medialis, or regio vertebralis).
- Lateral lumbar region (regio lumbalis lateralis).
The key topographic structures here are the lumbar triangle (trigonum lumbale) and the lumbar quadrangle (tetragonum lumbale). All layers of this region collectively form the posterior wall of the abdominal cavity.
Retroperitoneal space: location and boundaries
The retroperitoneal space (spatium retroperitoneale) is located deep within the abdominal cavity. It significantly exceeds the lumbar region in its extent, as its adipose tissue extends into the hypochondria and iliac fossae. It contains retroperitoneal fat, internal organs, as well as major vessels and nerves.
Boundaries of the space:
- Anteriorly: parietal peritoneum of the posterior abdominal wall.
- Posteriorly and laterally: endoabdominal fascia (fascia endoabdominalis).
- Superiorly: lumbar and costal parts of the diaphragm.
- Inferiorly: conventional boundary running along the pelvic brim (linea terminalis).
Fasciae and ligaments of the posterior abdominal wall
An important role in topography is played by the transversalis fascia (fascia transversalis), which covers the deep surface of the aponeurosis and transverse abdominal muscle, being part of the fascia endoabdominalis. Medially, it thickens and forms isolated sheaths for the muscles:
- Fascia quadrata — covers the quadratus lumborum muscle (m. quadratus lumborum).
- Fascia psoatis — forms a sheath for the psoas major and minor muscles (mm. psoas major et minor).
In the upper section of the region, the thickening of the fascia over these muscles forms tendinous arches:
- Medial arcuate ligament (arcus lumbocostalis medialis): extends from the body of the I (or II) lumbar vertebra to its transverse process.
- Lateral arcuate ligament (arcus lumbocostalis lateralis): runs from the transverse process of the I lumbar vertebra to the XII rib.
Topography of nerves
Nerve trunks in this zone have a strict localization relative to the muscles and fasciae:
- On the anterior surface of m. quadratus lumborum (directly beneath the fascia), the subcostal, iliohypogastric, and ilioinguinal nerves run obliquely from top to bottom and medially to laterally (nn. subcostalis, iliohypogastricus, ilioinguinalis).
- On the anterior surface of m. psoas major lies the genitofemoral nerve (n. genitofemoralis).
Layers of retroperitoneal fat
Retroperitoneal fat is divided into three distinct layers: proper retroperitoneal, perirenal, and paracolic fat.
Perirenal fat (paranephron) has its largest accumulation in the region of the renal hilum and the lower pole of the kidney. This space is reliably isolated due to the fusion of the pre- and retrorenal fasciae (fascia prerenalis and fascia retrorenalis). Inferiorly, these fasciae are connected by connective tissue septa that additionally fix the kidney. Along the ureter, the fat continues as the paraureterium, enclosed between fascia preureterica and fascia retroureterica.
Paracolic fat (paracolon) is the third layer, located behind the ascending and descending parts of the colon. Its boundaries are:
- Anteriorly: retrocolic fascia (fascia retrocolica) and parietal peritoneum of the paracolic gutters.
- Posteriorly: renal and preureteral fasciae.
- Superiorly: root of the transverse mesocolon (mesocolon transversum).
- Inferiorly: on the right — root of the cecal mesentery, on the left — root of the sigmoid mesentery.
- Laterally: line of transition of the parietal peritoneum to the retroperitoneal fascia.
- Medially: root of the mesentery of the small intestine.
Retrocolic fascia
The retrocolic fascia (fascia retrocolica) covers the extraperitoneal sections of the ascending and descending colon. It is formed during ontogenesis due to the loss of the primary mesentery and its fusion with the parietal peritoneum.
Laterally, it is firmly fused with the parietal peritoneum at the sites of its transition to the intestine. Medially on the right, the fascia connects with the fascia of the root of the small intestine mesentery, and on the left, it is lost in the fat layers at the inner edge of the descending colon.
Briefly: answers to topical questions
What muscles and bone structures bound the lumbar triangle (Petit's triangle)?
The lumbar triangle (trigonum lumbale) is formed by the divergence of the edges of the latissimus dorsi and external oblique abdominal muscle against the background of the iliac crest. The boundaries of the area include: laterally — the edges of the latissimus dorsi muscle (m. latissimus dorsi) and the external oblique abdominal muscle (m. obliquus externus abdominis); inferiorly — the iliac crest. The floor of this structure is formed by the internal oblique abdominal muscle, and its clinical significance lies in the potential for lumbar hernias and the spread of purulent accumulations.
Which muscles form the boundaries of the lumbar quadrangle (Lesgaft-Greenfeldt rhombus)?
The lumbar quadrangle (tetragonum lumbale) is a space between the edges of the muscles of the lateral part of the posterior abdominal wall that do not touch each other. The boundaries of the rhombus are formed: superiorly — by the lower edge of the serratus posterior inferior muscle (m. serratus posterior inferior); inferiorly — by the posterior free edge of the internal oblique abdominal muscle (m. obliquus internus abdominis); medially — by the lateral edge of the erector spinae muscle (m. erector spinae); laterally and superiorly — by the XII rib. The floor of the quadrangle is formed by the aponeurosis of the transversus abdominis muscle.
Specifically, which internal organs are located in the retroperitoneal space?
The retroperitoneal space (spatium retroperitoneale) houses retroperitoneal internal organs, which include the kidneys, adrenal glands, ureters, pancreas, and most of the duodenum. These structures lie between the posterior wall of the abdominal cavity, covered by the endoabdominal fascia (fascia endoabdominalis), and the parietal peritoneum of the posterior abdominal wall.
Which major blood vessels pass through the retroperitoneal space?
The retroperitoneal space (spatium retroperitoneale) contains the major blood vessels of the abdominal cavity, including the abdominal aorta (aorta abdominalis) and the inferior vena cava (v. cava inferior). The aorta is located to the left of the midline on the anterior surface of the lumbar spine and is surrounded by the branches of the abdominal aortic plexus, while the inferior vena cava lies to the right of it.
For quick memorization of the three layers of retroperitoneal fat, use the abbreviation RPP: Retroperitoneal, Perirenal, Paracolic.
Class and exam questions
What forms the posterior wall of the abdominal cavity?
It is formed by the layers of the lumbar region, which are covered from the abdominal cavity side by the endoabdominal fascia (fascia endoabdominalis).
Why is the perirenal fat (paranephron) considered an isolated space?
It is closed due to the tight fusion of the fascia prerenalis and fascia retrorenalis, and in the lower part, it is additionally reinforced by connective tissue septa.
Where does the genitofemoral nerve (n. genitofemoralis) run?
This nerve is located on the anterior surface of the psoas major muscle (m. psoas major).
What is the clinical significance of the structure of the retroperitoneal fat?
The specific arrangement of fasciae and cellular spaces is used by surgeons to induce pneumoretroperitoneum and perform perirenal procaine block.
What else to study on this topic
- Topography and clinical significance of the lumbar triangle (trigonum lumbale)
- Boundaries of the lumbar quadrangle (tetragonum lumbale)
- Technique for performing paranephric procaine block
- Features of the transition of retroperitoneal fat into the iliac fossae
- Relationship of the retrocolic fascia with the root of the small intestine mesentery
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