The posterior abdominal wall is a complex region of anatomy, which is formed by the lumbar vertebrae, pelvic girdle, posterior abdominal muscles and their associated fascia. This region is home to a host of important organs, vessels, and nerves which can be found just beneath its inner surface. In this article, we will look at the anatomy of this region, including its muscles, fascial layers, and some clinical correlations.
The posterior abdominal wall is populated by a total of five muscles — the iliacus, psoas major, psoas minor, quadratus lumborum, and the diaphragm. We shall now look more closely at the attachments, actions, and innervation of these muscles.
The quadratus lumborum muscle is located laterally in the posterior abdominal wall. It is a thick sheet of muscle with a quadrilateral shape, positioned superficially within the psoas major. Its attachments originate from the iliac crest and iliolumbar ligament, with its fibres traveling up and laterally towards the transverse processes of L1-L4, and then towards the inferior border of the 12th rib. Regarding its action, it helps to extend and laterally flex the vertebral column, while also performing the vital task of fixing the 12th rib during inspiration, to make the contraction of the diaphragm more efficient. It is innervated by the anterior rami of T12-L4 nerves.
The psoas major is located near the midline of the posterior abdominal wall, which is immediately lateral to the lumbar vertebrae. Its attachments originate from the transverse processes and vertebral bodies of T12-L5, then running inferiorly and laterally, deep within the inguinal ligament, and finally attaching to the lesser trochanter of the femur. Its actions involve flexion of the thigh at the hip and lateral flexion of the vertebral column, and it is innervated by the anterior rami of L1-L3 nerves.
The psoas minor muscle is only present in 60% of the population, located anteriorly within the psoas major. Its attachments include the vertebral bodies of T12 and L1, attaching to a ridge on the superior ramus of the pubic bone, known as the pectineal line. Its action is the flexion of the vertebral column, and it is innervated by the anterior rami of L1 spinal nerve.
The iliacus muscle is a fan-shaped muscle which is situated inferiorly in the posterior abdominal wall. Combining with the psoas major, it forms the iliopsoas, otherwise known as the major flexor of the thigh. Its attachments originate from the surface of the iliac fossa and anterior inferior iliac spine, and its fibres combine with the tendon of the psoas major in order to insert into the lesser trochanter of the femur. The action of this muscle is the flexion and lateral rotation of the thigh at the hip joint, and it is innervated by the femoral nerve (L2-L4).
The posterior aspect of the diaphragm is considered to be part of the posterior abdominal wall. Regarding clinical relevance, the psoas sign is a medical sign which indicates irritation to the iliopsoas muscle group. This is elicited by flexion of the thigh at the hip, with a positive sign resulting in lower abdominal pain. A right sided psoas sign is an indication of appendicitis, as the iliopsoas contracts in contact with the inflamed appendix, producing pain.
Between the parietal peritoneum and the posterior abdominal wall muscles lies a layer of fascia, which is a sheet of connective tissue. This fascia is continuous with the transversalis fascia of the anterolateral abdominal wall, and while anatomically correct to name the fascia according to the structure which it overlies, it remains one continuous sheet.
The psoas fascia covers the psoas major muscle. Its attachments include being attached to the lumbar vertebrae medially, continuing with the thoracolumbar fascia laterally, and then connecting to the iliac fascia inferiorly.
The thoracolumbar fascia includes three main layers — the posterior, middle, and anterior.
The posterior abdominal wall is composed of five muscles- the iliacus, psoas major, psoas minor, quadratus lumborum and the diaphragm. This article will explore each of these muscles in greater detail, discussing their attachments, actions, and innervation.
The quadratus lumborum is a thick, quadrilateral-shaped sheet of muscle located laterally in the posterior abdominal wall, located superficially to the psoas major.
The psoas major is located near the midline of the posterior abdominal wall, immediately lateral to the lumbar vertebrae.
The psoas minor muscle is only present in about 60% of the population. It is located anterior to the psoas major.
The iliacus muscle is a fan-shaped muscle situated inferiorly on the posterior abdominal wall. It combines with the psoas major to form the iliopsoas- the major flexor of the thigh.
The posterior aspect of the diaphragm is considered to be part of the posterior abdominal wall, and is explored in greater detail in another article.
The psoas sign is a medical sign indicating irritation to the iliopsoas group of muscles. The test is performed by flexing the thigh at the hip and is positive if the patient reports lower abdominal pain. A right-sided psoas sign is an indication of appendicitis, as the iliopsoas contracts and comes into contact with the inflamed appendix, producing the pain.
A layer of fascia (a sheet of connective tissue) lies between the parietal peritoneum and the muscles of the posterior abdominal wall. This fascia is continuous with the transversalis fascia of the anterolateral abdominal wall, and while it is one continuous sheet, it is named according to the structure it overlies.
The psoas fascia covers the psoas major muscle, and is attached to the lumbar vertebrae medially, to the thoracolumbar fascia laterally, and to the iliac fascia inferiorly.
The thoracolumbar fascia consists of three layers- posterior, middle and anterior. Muscles are enclosed between these layers, with the quadratus lumborum located between the anterior and middle layers and the deep back muscles located between the middle and posterior layers.
The posterior layer extends between the 12th rib and the iliac crest posteriorly, while laterally it meets the internal oblique and transversus abdominis muscles, though not the external oblique. As it forms these attachments it covers the latissimus dorsi. The anterior layer attaches to the anterior aspect of the transverse processes of the lumbar vertebrae, the 12th rib and the iliac crest, while laterally it is continuous with the aponeurotic origin of the transversus abdominis muscle. Superiorly, the fascia thickens to become the lateral arcuate ligament, which joins the iliolumbar ligaments inferiorly.
