Posterior Abdominal Wall Anatomy

Posterior Abdominal Wall

• The posterior abdominal wall is a structural region composed of deep muscles.
• These muscles stabilize, move, and support the spine and abdominal organs.
• They also assist with posture, respiration, and trunk and lower limb movements.
• The primary muscles are the psoas major, quadratus lumborum, iliacus, diaphragm (posterior portion), and transversus abdominis (posterior fibers).

Psoas Major

• A long, fusiform muscle originating from the lumbar spine and extending to the femur.
• It is a major hip flexor and part of the iliopsoas group.
• The origin is the transverse processes of lumbar vertebrae T12-L5.
• It also originates from the vertebral bodies and intervertebral discs of T12 to L5.
• It inserts onto the lesser trochanter of the femur, via a shared tendon with the iliacus.
• Its primary function is to flex the hip joint, enabling movements like walking and lifting the leg.
• During unilateral contraction, it causes lateral bending of the lumbar spine, contributing to spinal mobility.
• During bilateral contraction, it assists in flexing the trunk.
• It assists when transitioning from a supine position to sitting up.
• It is innervated by the anterior rami of lumbar spinal nerves L1-L3.

Psoas Minor

• A small, slender muscle present in about 50% of the population.
• It is considered vestigial in humans.
• It originates from the vertebral bodies and intervertebral discs of T12 and L1 anterior to the psoas major.
• It inserts onto the pectineal line of the pubis and the iliopectineal eminence.
• Its functions include the weak flexion of lumbar spines, helping to slightly bend the lower back forward.
• It aids with stabilization of the pelvis, supporting the iliopubic region during trunk movement.
• It influences the tension of the iliac fascia, assisting in the stabilization of underlying structures.
• It is innervated by the anterior rami of the L1 spinal nerve.

Iliacus

• A large, flat muscle located in the posterior abdominal wall and pelvis.
• It is a part of the iliopsoas group.
• It originates from the iliac fossa and iliac crest of the pelvic bone.
• Some fibers originate from ala (wing) of sacrum and anterior sacroiliac ligaments.
• It inserts onto the lesser trochanter of the femur, forming the iliopsoas tendon with the psoas major.
• It is one of the primary hip flexors, enabling movements like lifting the thigh during walking and running.
• When the legs are stabilized it helps flex the trunk forward.
• It works alongside the psoas major to stabilize the pelvis and lumbar spine, maintaining balance and upright posture.
• It is innervated by the femoral nerve, which arises from the lumbar plexus (L2-L4).

Quadratus Lumborum

• A deep muscle located in the posterior abdominal wall.
• It stabilises the lumbar spine and supports trunk movement.
• It is involved in respiration and posture.
• Originates from the iliac crest and the transverse processes of the lower lumbar vertebrae (L2-L4).
• Inserts onto the 12th rib and transverse processes of the first to fourth lumbar vertebrae (L1-L4).
• During unilateral contraction, it pulls the trunk laterally, essential for side bending and rotational movements.
• It anchors the 12th rib during respiration, supporting the diaphragm's actions.
• When acting bilaterally, it provides stability to the lumbar spine and pelvis, preventing excessive movement.
• It maintains balance and posture by working with other posterior abdominal wall muscles.

Retroperitoneal Organs

• These are located behind the peritoneum.
• They are partially covered by peritoneum on their anterior surface but not enclosed within the peritoneal cavity.
• These are fixed in position and lie against the posterior abdominal wall.
• Primary retroperitoneal organs develop and remain retroperitoneal throughout embryonic development and include the kidneys, suprarenal glands, ureters, aorta & inferior vena cava (IVC), and sympathetic chain.
• Secondary retroperitoneal organs were initially intraperitoneal but became retroperitoneal due to fusion with the posterior abdominal wall and include the pancreas (except the tail), duodenum (except the first part), ascending and descending colon, and rectum (upper part).
• Key retroperitoneal organs are found within the gutters on either side of the vertebral column.
• Kidneys are bean-shaped organs in the paravertebral gutters which filter blood to form urine and regulate electrolytes, fluid balance, and blood pressure.
• Suprarenal (adrenal) glands are located superior to the kidneys which secrete hormones.
• Ureters are muscular tubes which transport urine from kidneys to the bladder.

Abdominal Aorta

• A continuation of the thoracic aorta in the abdominal cavity.
• It is a major arterial structure that supplies oxygenated blood to the abdominal organs, pelvis, and lower limbs.
• It is a retroperitoneal structure, behind the peritoneum of the abdominal cavity.
• It begins at the aortic hiatus of the diaphragm (vertebral level T12).
• Travels inferiorly along the midline of the posterior abdominal wall and remains retroperitoneal.
• It terminates at the vertebral level L4, bifurcating into the right and left common iliac arteries.
• Gives rise to branches in three planes: anterior/midline, lateral, and posterolateral.
• Anterior/midline branches are for supplying the gastrointestinal tract which includes the celiac trunk (T12) supplies the foregut (stomach, liver, spleen), superior mesenteric artery (SMA) (L1) the midgut (small intestine, cecum), and the inferior mesenteric artery (IMA) (L3) the hindgut.
• Lateral branches supply paired visceral organs and includes the renal arteries which supply the kidneys, the suprarenal arteries which supply the adrenal glands, and the gonadal arteries which supply the testes in males and ovaries in females.
• Posterolateral plane supplies the diaphragm and posterior abdominal wall as subcostal arteries which supply the posterior thoracoabdominal wall, inferior phrenic arteries which supply the diaphragm, and the lumbar arteries which supply the muscles and skin of the posterior abdominal wall.
• Clinical significance is that it is clinically significant vulnerability to certain pathological conditions such as abdominal aortic aneurysm (AAA).
• An AAA is a localized dilation of the abdominal aorta, commonly below the level of the renal arteries which may be caused by hypertension, smoking, and advanced age.
• Aortic dissection is when there is a tear in the intimal layer of the aortic wall.

Inferior Vena Cava (IVC)

• The largest vein in the human body, responsible for returning deoxygenated blood from the lower body to the right atrium of the heart.
• It runs parallel to the abdominal aorta but is situated on right side.
• It is formed at the vertebral level L5, where the right and left common iliac veins join.
• Ascends superiorly along the posterior abdominal wall and passes through the caval opening of the diaphragm (T8), drains into the right atrium of the heart.
• Receives tributaries corresponding to the branches of the abdominal aorta, such as the renal veins, gonadal veins, and lumbar veins.
• The IVC collects blood from the common iliac veins that drain the lower limbs and pelvis, renal veins that drain the kidneys, hepatic veins that drain the liver, gonadal veins that drain the testes and ovaries (right gonadal vein drains directly into the IVC, while the left drains into the left renal vein), and the lumbar veins that drains the posterior abdominal wall.

Lumbar Plexus

• A network of interconnected nerves derived from the anterior rami of spinal nerves L1-L5, with T12 contributions via the subcostal nerve.
• Serves as a critical structure for the motor and sensory innervation of lower limbs by providing upper thigh muscles and portions of the abdominal wall and pelvis.
• Essential for movement and sensation of the lower limb as it provides motor innervation to anterior and medial compartments of the thigh, abdominal wall and pelvis.
• It also provides the sensory innervation to the skin of the lower abdomen, thigh, and parts of the lower limb.
• Positioned within the posterior abdominal wall, embedded within the psoas major muscle.
• Branches emerge from different positions relative to the psoas major, with some coursing anteriorly and others laterally or medially.
• The plexus gives rise to several terminal nerves that descend toward the lower limb and the lumbosacral trunk (L4 and L5) extends downward to join the sacral plexus, the innervation network of the pelvis and lower extremity.
• The branches of the lumbar plexus include subcostal Nerve (T12) which travels below the 12th rib and runs anterior to the quadratus lumborum muscle for sensory innervation of the anterior abdominal wall and helps motor innervation of nearby abdominal wall.
• The iliohypogastric Nerve (L1) travels anterior to the quadratus lumborum muscle and pierces the transversus abdominis muscle and provides sensation to the lower abdominal wall and upper lateral gluteal region. and Supplies internal oblique muscle and transversus abdominis muscle.
• The ilioinguinal Nerve (L1) travels along the iliac crest, passing towards the anterior superior iliac spine (ASIS) that provides sensation to the inguinal region and external genitalia and motor input to the abdominal wall muscles.
• The genitofemoral nerve (L1-L2) pierces through the psoas major muscle that divides into genital motor branch to the cremaster muscle in males and femoral branch to the upper anterior thigh.
• The lateral cutaneous nerve of the thigh (L2-L3) emerges from the lateral border of the psoas major, passes near the ASIS, to provide sensory innervation to the skin of the lateral thigh.
• The obturator Nerve (L2-L4) runs deep and medial to the psoas major, passes through the obturator foramen, that supplies the adductor muscles of the thigh in adductor longus muscle and sensory to the skin of medial thigh.
• The femoral nerve (L2-L4) emerges lateral to the psoas major and travels deep to the inguinal ligament, that innervates the quadriceps femoris, sartorius, and pectineus muscles for knee extension and provides sensation to the anterior thigh and the medial leg.
• The lumbosacral trunk (L4-L5) passes over the ala of the sacrum to join the sacral plexus for fibers to nerves that supply the pelvis and lower limbs.

Lumbar Sympathetics

• The lumbar sympathetics are a component of the autonomic nervous system that plays a role in regulating involuntary functions such as blood vessel constriction, abdominal organ activity, and responses to stress.
• The lumbar sympathetic trunk is a continuation of the thoracic sympathetic trunk and consists of four paired lumbar ganglia.
• It lies along the anterior aspect of the lumbar vertebrae, is medial to the psoas major muscle, and anterolateral to the lumbar vertebrae.
• This structure acts as a relay station for sympathetic fibers that supply the lower limbs, abdominal wall, and pelvic organs and connects to other autonomic structures such as the thoracic sympathetic trunk above and the sacral sympathetic trunk below.
• Postganglionic sympathetic fibers originate from the lumbar ganglia that supply the blood vessels of the lower limbs and abdominal wall muscles.
• Preganglionic sympathetic fibers (via lumbar splanchnic nerves) comes from the sympathetic trunk and travel to prevertebral ganglia (celiac, superior mesenteric, and inferior mesenteric ganglia) by giving innervation to abdominal and pelvic viscera, such as the intestines, kidneys, bladder, and reproductive organs.

Pathways of Sympathetic Fibers

• The lumbar sympathetic trunk facilitates the transmission of autonomic signals through two main fiber types which are postganglionic fibers and preganglionic fibers.
• Postganglionic fibers exit the lumbar ganglia and travel via spinal nerves to their target structures that provide sympathetic innervation to the abdominal wall by enhancing muscle tone during stress.
• These fibers also construct blood vessels in the lower limbs for maintaining vascular resistance and blood pressure.
• Preganglionic fibers (via Lumbar Splanchnic Nerves) do not synapse in the lumbar ganglia but instead go directly to prevertebral ganglia to the abdominal aorta.
• After synapsing, the postganglionic fibers innervate pelvic organs such as the bladder and digestive organs.

Clinical Relevance

• The sympathetic overactivity can lead to increased vascular resistance in the lower limbs, hypertension, or peripheral vascular disease.
• Lumbar sympathectomy (removal or interruption of the sympathetic trunk) are done for severe peripheral vascular disease or chronic pain syndromes.
• Autonomic dysregulation is when damage or dysfunction of the lumbar sympathetic trunk cause the bladder dysfunction, intestinal motility issues, or in sexual dysfunction.

Clinical Correlates of the Posterior Abdominal Wall

• Psoas abscess is a collection of pus within the psoas major muscle caused local or systemic infections that spread along the psoas fascia and include tuberculosis, gastrointestinal, and renal infections.
• Psoas sign is a clinical test used when the patient lies on his/her back on a supine position and the examiner passively extends the hip with there is pain in the lower abdomen or back.
• Causes of positive psoas sign are acute appendicitis, psoas abscess, and pelvic infections.

Aortic Aneurysm

• A localized dilation (bulging) of the abdominal aorta due to weakening of the arterial wall.
• Risk factors include atherosclerosis, hypertension, and smoking.
• Clinical features are a palpable pulsatile mass, severe abdominal/back pain, and/or hypotension/shock.
• Diagnosis include an ultrasound or CT scan.
• Treatment is surgical repair by open surgery with endovascular stent grafting.

Structure of the Bony Pelvis and Joints

• The bony pelvis is a structure connecting the spine to the lower limbs while It provides support, stabilises, and protects pelvic organs.
• The bones of the pelvis consists of two hip bones, the sacrum, and the coccyx.
• The two hip bones is formed via the fusion of the ilium which is broad and fan-shaped, the ischium which is weight-bearing when sitting, and the pubis which forms the pubic symphysis.
• The joints of the pelvis have four major joints located between the sacrum and the ilium also having transfers weight over to the lower limbs, the pubic symphysis between the left, right pubic bones with a fibrocartilage which provides shock absorption and allows flexibility via the lumbosacral joint which allows slight movement especially during childbirth.
• The joint allows slight movement to absorbs mechanical forces.
• Ligaments such as the anterior sacroiliac ligament stabilizes the front of the sacroiliac joint, the interosseous sacroiliac ligament which connects deep within the joint and is the strongest, and the posterior sacroiliac ligament which reinforces the back.
• Ligaments supporting the pelvic stability is the sacrotuberous ligament, prevents forward tilting of the sacrum, and form the lesser sciatic foramen.
• And the sacrospinous ligament that connects to the ischial spine for limiting sacral movement.

Formina Formed by Ligaments

• Foramina is formed to allow passageway for nerves and blood vessels to travel.
• Greater Sciatic Foramen is formed Sacrospinous ligament and sciatic and piriformis muscles and the lesser Sacrotuberous ligament to form passage for the pudendal nerve.

Clinical Relevance

• Pelvic fractures is commonly formed through high-impact traumas with possible organ, vessel, or nerve damage and sacroiliac joint dysfunction to cause chronic low back pain.
• Relaxation of pelvic ligaments during pregnancy in order for hormonal flexibility which allows expansion for childbirth.

Innominate Bone (Hip Bone)

• A large, irregularly shaped bone that forms one side of the pelvic girdle.
• The term "innominate" or "nameless" means its complex structure does not resemble long/short/flat bones.
• The bones meet and fuse the acetabulum to the socket for the femoral head (complete by 16-23 of age).
• Notable features is the iliac crest, the spine for upper muscles, the illiac fossa on the interior side for attachments of muscles.

Ischium

• The ischium forms the lower portion of the hip bone with the key feature of the ischial spine along the greater ischial notches to attach tendons of the hamstring muscles.

Pubis

• The pubis is at the anterior portion which consist of the body, superior ramus, and inferior ramus and notable features for serving as joints to bone muscles nearby in the joint ligaments and contributions to the stable walls nearby the area.

Clinical Relevancy Bone

• Bones in the area is often fractured from motor vehicles etc that may lead to complication and/or harvest (illiac-crest) in procedures harvesting and transplantation.

Pelvic Fractures

• Innominate bones often get fractured from motor accidents etc that may create complications (internal bleeding and organ injury).

Sacrum and Coccyx

• The sacrum and coccyx form the posterior portion of the pelvis, providing support and stability via the fusion of several vertebrata and ligaments.
• Key features: sacral foraminas to pass the nerves and auricular surface to articuliate with the illium on the spine.

Joints

• The joints and stability for transferring joints to spine/spinal nerves

Dimorphism

• The sexual dimorphism is the anatomical differences that can arise for carrying fetus such as wider and more oval, sub-pubic angles etc. The greater the features can impact hip or can create dystocia labor to be impacted for fractures, shape in forensics, and affect how women stand or widen feet more of acetabulum orientation and pelvis.

True Pelvic Cavity and Regions

• True pelcis to separate true form, true false via anatomical boundaries.
• False pelvis is a zone between tru, located above the brim (iliac crests) supporting abdominal viscera with boundaries to vertebrea.
• The true pelvic located below via sacrum that houses pelvic bones the urogenital triangle to anal canal and digestive features
• The function is support and structure to divide everything.
• Regions are relationships in continuation of vessels in true cavities with relationship in lower areas, the region has the support and ligaments/structures.

Communication Points

• These regions can interact for access to the structures that provide access using thigh, Gluteal region, and perineum and their pathways to canals and vessels.