Abdominal wall and para-spinal structures anatomy

ABDOMINAL WALL AND PARASPINAL STRUCTURES

Definitions

• Abdomen: Region between diaphragm and pelvis

GROSS ANATOMY

Anatomic Boundaries

• Anterior abdominal wall is bounded superiorly by xiphoid process and costal cartilages of 7th-10th ribs

• Anterior wall is bounded inferiorly by iliac crest, iliac spine, inguinal ligament, and pubis

• Inguinal ligament is inferior edge of aponeurosis of external oblique muscle

Muscles of Anterior Abdominal Wall

• Consist of 3 flat muscles (external oblique, internal oblique, and transverse abdominal), and 1 strap-like muscle (rectus)

• Combination of muscles and aponeuroses (sheet like tendons) act as a corset to confine and protect abdominal viscera

• Linea alba is a fibrous raphe stretching from xiphoid to pubis

◦ Forms central anterior attachment for abdominal wall muscles

◦ Formed by interlacing fibers of aponeuroses of oblique and transverse abdominal muscles

◦ Rectus sheath is also formed by these aponeuroses as they surround rectus muscle

• External oblique muscle

◦ Largest and most superficial of 3 flat abdominal muscles

◦ Origin: External surfaces of ribs 5-12

◦ Insertion: Linea alba, iliac crest, pubis via a broad aponeurosis

• Internal oblique muscle

◦ Middle of 3 flat abdominal muscles

◦ Runs at right angles to external oblique

◦ Origin: Posterior layer of thoracolumbar fascia, iliac crest, and inguinal ligament

◦ Insertion: Ribs 10-12 posteriorly, linea alba via a broad aponeurosis, pubis

• Transverse abdominal muscle

◦ Innermost of 3 flat abdominal muscles

◦ Origin: Lowest 6 costal cartilages, thoracolumbar fascia, iliac crest, inguinal ligament

◦ Insertion: Linea alba via broad aponeurosis, pubis

• Rectus abdominis muscle

◦ Origin: Pubic symphysis and pubic crest

◦ Insertion: Xiphoid process and costal cartilages 5-7

◦ Rectus sheath: Strong fibrous compartment that envelops each rectus muscle

▪ Contains superior and inferior epigastric vessels

◦ 3 tendinous intersections divide rectus sheath into distinct muscle bellies

▪ This gives rise to '6-pack' appearance of well developed rectus abdominus musculature

◦ Arcuate line lies about midway between umbilicus and pubic crest

▪ Above arcuate line, internal oblique aponeurosis splits to enclose rectus muscle

▪ Below arcuate line, internal oblique passes wholly in front of rectus muscle so that rectus muscle lies directly on transversalis fascia

• Actions of anterior abdominal wall muscles

◦ Support and protect abdominal viscera

◦ Help flex and twist trunk, maintain posture

◦ Increase intraabdominal pressure for defecation, micturition, and childbirth

◦ Stabilize pelvis during walking, sitting up

• Transversalis fascia

◦ Lies deep to abdominal wall muscles and lines entire abdominal wall

◦ Separated from parietal peritoneum by layer of extraperitoneal fat

Muscles of Posterior Abdominal Wall

• Consist of psoas (major and minor), iliacus, and quadratus lumborum

• Psoas: Long, thick, fusiform muscle lying lateral to vertebral column

◦ Origin: Transverse processes and bodies of vertebrae T12-L5

◦ Insertion: Lesser trochanter of femur (passing behind inguinal ligament)

◦ Action: Flexes thigh at hip joint; bends vertebral column laterally

• Iliacus: Large triangular sheet of muscle lying along lateral side of psoas

◦ Origin: Superior part of iliac fossa

◦ Insertion: Lesser trochanter of femur (after joining with psoas tendon)

◦ Action: "Iliopsoas muscle" flexes thigh

• Quadratus lumborum: Thick sheet of muscle lying adjacent to transverse processes of lumbar vertebrae

◦ Invested by lumbodorsal fascia

◦ Origin: Iliac crest and transverse processes of lumbar vertebrae

◦ Insertion: 12th rib

◦ Actions: Stabilizes position of thorax and pelvis during respiration, walking

▪ Bends trunk to side

Paraspinal Muscles

• Also called erector spinae muscles

◦ Invested by lumbodorsal fascia

• Composed of 3 columns

◦ Iliocostalis: Lateral

◦ Longissimus: Intermediate

◦ Spinalis: Medial

• Origins: Sacrum, ilium, and spines of lumbar and 11th-12th thoracic vertebrae

• Insertions: Ribs and vertebrae with additional muscle slips joining columns at successively higher levels

• Action: Extends vertebral column

ANATOMY IMAGING ISSUES

Imaging Recommendations

• High-frequency (7-10 MHz) linear transducer for anterior abdominal wall and paraspinal muscles

◦ 3-5 MHz for posterior abdominal wall muscles

• Patient may be asked to perform Valsalva maneuver to increase abdominal pressure and exaggerate abdominal wall hernia for better visualization

• Comparison with contralateral side to check for Symmetry

Gallery and Imaging

ANTERIOR ABDOMINAL WALL

Graphic shows the aponeuroses of the internal and external oblique and transverse abdominal muscles are 2-layered and interweave with each other, covering the rectus muscle and constituting the rectus sheath and linea alba. About midway between the umbilicus and symphysis, at the arcuate line, the posterior rectus sheath ends. The transversalis fascia is the only structure between the rectus muscle and parietal peritoneum below the arcuate line. The arcuate line is also the location where the inferior epigastric artery enters the rectus sheath.

POSTERIOR ABDOMINAL WALL

Graphic shows the lumbar vertebrae are covered and attached by the anterior longitudinal ligament, and the diaphragmatic crura are closely attached to it, as are the origins of the psoas muscles which also arise from the transverse processes. The iliacus muscle arises from the iliac fossa of the pelvis and inserts into the tendon of the psoas major, constituting the iliopsoas tendon, which inserts into the lesser trochanter. The quadratus lumborum arises from the iliac crest and inserts into the 12th rib and transverse processes of the lumbar vertebrae. Diaphragmatic and transverse abdominal fibers interlace. The psoas and quadratus lumborum pass behind the diaphragm under the medial and lateral arcuate ligaments.

 MUSCLES OF BACK IN SITU

Graphic shows the paraspinal muscles and muscles of the back. The latissimus dorsi muscles are not included. The erector spinae have thick tendinous origins from the sacral and iliac crests and the lumbar and 11th-12th thoracic spinous processes. Superiorly, the muscle becomes fleshy and subdivides in the upper lumbar region to become the iliocostalis, longissimus, and spinalis muscles (from lateral to medial), tapering as they insert into the vertebrae and ribs. The erector muscles flank the spinous processes and span the length of the posterior thorax and abdomen. They are responsible for extension of the vertebral column.

ANTERIOR ABDOMINAL WALL

Transverse extended field of view grayscale ultrasound of the midline anterior abdominal wall shows the paired rectus abdominis muscles separated by the linea alba. The rectus abdominis muscles are comparable in echogenicity and thickness. The surrounding rectus sheath is seen as a fine, thin, echogenic structure around the muscles.

Transverse color Doppler ultrasound of a rectus abdominis muscle shows arteries within the muscle. At the upper abdomen, these are branches of the superior epigastric artery, and at the lower abdomen comprise branches of the inferior epigastric arteries. These anastomose at the umbilicus.

Longitudinal extended field of view (EFOV) grayscale ultrasound shows the distal rectus abdominis muscle and its tendinous insertion into the symphysis pubis. Note how the rectus abdominis muscle tapers distally.

 

ANTEROLATERAL ABDOMINAL WALL

Transverse extended field of view grayscale ultrasound shows the relationship of the medially located rectus abdominis and the laterally located oblique and transverse abdominal muscles. Medially, the external and internal oblique and transversus abdominal muscles form aponeuroses that compose the rectus sheath.

Transverse grayscale ultrasound at the right anterolateral abdominal wall shows the relationship of the lateral abdominal wall muscles in better detail. Note the oblique and transverse abdominal muscles taper medially as they become aponeuroses.

correlative axial contrast-enhanced CT illustrates the muscles of the abdominal wall. The rectus abdominis muscle in the anterior abdominal wall and the oblique and transverse abdominal muscles in the anterolateral abdominal wall and their aponeuroses are shown.

 POSTERIOR ABDOMINAL WALL

Coronal oblique grayscale ultrasound using the liver and right kidney as acoustic windows shows the right psoas muscle that originates from the lumbar spine and inserts into the proximal femur.

Transverse grayscale ultrasound shows the right upper abdomen using the liver and kidney as acoustic windows. The kidney is identified lateral to the psoas and anterior to the quadratus lumborum. The psoas is identified along the paravertebral region in its entire abdominal course. The quadratus lumborum originates from the iliolumbar ligament and iliac crest to insert into the last rib and lumbar transverse processes. It is easily identified as the muscle on which the kidney rests.

Transverse grayscale ultrasound shows the right upper abdomen. Continuing the scan inferiorly, the relationship of the posterior abdominal wall musculature would be maintained.

 POSTERIOR ABDOMINAL WALL, CT CORRELATION

Correlative coronal CECT shows the paralumbar location of the psoas muscles and their medial location relative to the kidneys. The psoas muscles originate from the lumbar and 12th thoracic vertebral bodies and their transverse processes, running past the pelvic brim where they course inferolaterally to be joined by the iliacus muscle.

Correlative axial CECT better illustrates the anatomic relationships of the kidney with the posterior abdominal wall muscles. The kidney is lateral to the psoas muscle and rests upon the quadratus lumborum muscle. The erector spinae muscles are immediately posterior to the quadratus lumborum, and the 2 muscles are invested by lumbodorsal fascia.

Correlative axial CECT shows the level of the inferior pole of the right kidney. The psoas muscle and quadratus lumborum muscles, seen in their midsections, are now thicker.

 POSTERIOR ABDOMINAL WALL

Transverse grayscale ultrasound in the lower abdominal region shows the right psoas muscle, composed of the psoas minor resting upon the psoas major muscle. The 2 muscles cannot be separated clearly on ultrasound. Owing to their depth, the paraspinal muscles cannot be demonstrated in detail.

Transverse grayscale ultrasound of the same patient shows the right lower abdomen. The distal psoas muscle has diminished in size. It rests on the medial portion of the iliacus muscle. The iliacus muscle is a flat muscle that fills the iliac fossa. Both the psoas and iliacus muscles continue inferiorly together.

Distally, the fibers from the iliacus muscle converge and insert into the lateral side of the psoas muscle to form the iliopsoas muscle.

 POSTERIOR ABDOMINAL WALL, CT CORRELATION

Correlative axial CECT shows the level of the inferior pole of the kidney. The quadratus lumborum muscle is more laterally located, and the psoas muscle is directly anterior to the erector spinae muscle.

Correlative axial CECT shows that the psoas muscle has begun its dorsolateral course and is now anterior to the iliacus muscle. The iliacus muscle is easily identified as a flat muscle filling the iliac fossa, arising from the upper 2/3 of the iliac fossa, inner lip of the iliac crest, anterior sacroiliac and the iliolumbar ligaments, and base of the sacrum.

Axial correlative CECT shows the psoas and iliacus muscles have converged and are now indistinguishable. The resultant iliopsoas muscle passes beneath the inguinal ligament and becomes tendinous as it inserts into the lesser trochanter of the femur.

PARASPINAL MUSCLES

Transverse extended field of view grayscale US of the back (with patient prone) shows the erector spinae muscles flanking the spinous process. They are invested by lumbodorsal fascia, which also invests the anteriorly located quadratus lumborum muscle. The kidneys are partially demonstrated.

Transverse EFOV grayscale US shows the right erector spinae muscle (with patient prone). The 3 columns (iliocostalis, longissimus, and spinalis muscles, from lateral to medial) composing the erector spinae are not clearly separated on ultrasound. They are identified collectively as a thick fleshy muscle lateral to the spinous process.

Correlative axial CECT shows the paraspinal muscles at the level of the kidneys. The erector spinae muscles originate from a broad and thick tendon that is attached to the sacrum and iliac crest distally as well as the lumbar and lower 2 thoracic spinous processes more proximally.

 PATHOLOGY

Transverse grayscale ultrasound shows a small hyperechoic nodule in the subcutaneous fat of the anterior abdominal wall. Mild intrinsic vascularity on color Doppler imaging was present (not shown). Histology following surgical excision revealed an angiolipoma. No deep extension is present.

Transverse grayscale ultrasound shows a well-defined hypoechoic mass in the anterior abdominal wall. There is a small fascial defect of the linea alba.

Transverse ultrasound of the same patient as the previous image shows widening of the fascial defect and increased herniation of omental fat with Valsalva maneuver. The intraperitoneal communication is much more conspicuous with Valsalva maneuver. Fatty hernias of the linea alba usually occur in the epigastrium and are typically small and painful. The predilection for the epigastrium may be related to extra tension on the linea alba at this location due to pull from the diaphragm

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