[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Anatomy](https://durgical.com/en/anatomy/) › [Abdomen](https://durgical.com/en/anatomy/region/abdomen/) › Lumbar plexus [Nervous system](https://durgical.com/en/anatomy/system/nervous-system/) · [Abdomen](https://durgical.com/en/anatomy/region/abdomen/) # Lumbar plexus The lumbar plexus is a network of nerves from the lower [spine](https://durgical.com/en/anatomy/region/spine/), inside the psoas muscle; it supplies the lower belly wall, the groin and the front of the thigh. [See in 3D →](https://durgical.app/?from=durgipedia&lang=en#region=abdomen&select=lumbar_plexus&side=r) Opens in the Durgical app, no sign-up needed. ## What it is A network of nerves (L1-L4) inside the psoas muscle. ## What it does Gives the nerves to the lower abdominal wall, groin and thigh. See the spine model for details. ## Key points - Branches leave psoas major at fixed borders: lateral (iliohypogastric, ilioinguinal, lateral femoral cutaneous, femoral), front (genitofemoral), medial (obturator and the lumbosacral trunk) - A psoas abscess or hematoma can compress the [femoral nerve](https://durgical.com/en/anatomy/femoral-nerve/); spinal tuberculosis is a classic cause of psoas abscess - The lumbar plexus block (psoas compartment block) numbs the femoral, lateral femoral cutaneous and obturator nerves together for hip and knee surgery ## Key facts **Roots** Ventral rami of L1–L4 (often a T12 contribution); forms within psoas major **Moves** Femoral nerve (L2–L4): iliacus, quadriceps; [obturator nerve](https://durgical.com/en/anatomy/obturator-nerve/) (L2–L4): thigh adductors; direct branches to psoas and quadratus lumborum **Feeling from** Iliohypogastric, ilioinguinal, genitofemoral, lateral femoral cutaneous (L2–L3), femoral and obturator: groin, anterior, lateral and medial thigh ## Relations **Course** The ventral rami leave the intervertebral foramina and enter [psoas major](https://durgical.com/en/anatomy/psoas-major/) between its attachments to the vertebral bodies and transverse processes; the branches emerge from the muscle and cross [quadratus lumborum](https://durgical.com/en/anatomy/quadratus-lumborum-muscle/) and [iliacus](https://durgical.com/en/anatomy/iliacus-muscle/) toward the [inguinal ligament](https://durgical.com/en/anatomy/inguinal-ligament/) and obturator foramen **In front** The bulk of psoas major; in front of the muscle, the [kidney](https://durgical.com/en/anatomy/kidney/), [ureter](https://durgical.com/en/anatomy/ureter/) and gonadal vessels **Behind** Lumbar transverse processes and quadratus lumborum **Medial** Lumbar vertebral bodies, with the lumbar [sympathetic trunk](https://durgical.com/en/anatomy/sympathetic-trunk/) along the medial edge of psoas major **Below** The lumbosacral trunk (L4–L5) descends over the ala of the [sacrum](https://durgical.com/en/anatomy/sacrum/), medial to psoas major, to join the sacral plexus ## How to find it It can't be felt. It lies in the back part of psoas major, in front of the lumbar transverse processes; for a block it is found from the back with ultrasound or nerve stimulation at about L3–L4, a few centimetres from the midline and deep to the transverse processes. ## Examination - Patellar reflex: reduced in femoral nerve or L3–L4 root lesions - Knee extension and hip flexion strength: weak with a femoral nerve lesion; hip adduction weak with an obturator nerve lesion - Cremasteric reflex: stroking the upper inner thigh lifts the [testis](https://durgical.com/en/anatomy/testis/) on that side; lost with [genitofemoral nerve](https://durgical.com/en/anatomy/genitofemoral-nerve/) or L1–L2 lesions - Femoral nerve stretch test (prone, knee bent, hip extended): pain in the front of the thigh suggests irritation of the L2–L4 roots - [Skin](https://durgical.com/en/anatomy/skin/) map: iliohypogastric over the iliac crest and above the pubis; ilioinguinal to the upper inner thigh and front of the [scrotum](https://durgical.com/en/anatomy/scrotum/) or labium; lateral femoral cutaneous to the outer thigh (meralgia paresthetica) ## On imaging Not seen on radiographs. It forms within psoas, so MRI is the best study: the roots and the femoral, obturator and lateral femoral cutaneous nerves can be traced in and around the muscle. CT or MRI shows what damages it, such as a psoas abscess or hematoma, a retroperitoneal tumor or enlarged nodes. Ultrasound guides blocks of the plexus at the lumbar level or of its branches in the groin. ## Clinical notes - The lateral transpsoas approach to the lumbar spine passes through the muscle and can injure the genitofemoral or femoral nerve, causing thigh numbness or weakness - The [iliohypogastric and ilioinguinal nerves](https://durgical.com/en/anatomy/iliohypogastric-nerve/) can be cut or trapped in scar after lower abdominal incisions (appendectomy, Pfannenstiel), causing neuropathic groin pain - Diabetic lumbosacral radiculoplexus neuropathy (diabetic amyotrophy): pain followed by wasting and weakness of the front of the thigh - Retroperitoneal bleeding, tumours or enlarged nodes along the psoas can press on the plexus and present as groin or thigh pain ## Anatomical variants - Prefixed (more T12) or postfixed (more L5) plexus, shifting the root levels of its branches - Accessory obturator nerve (L3–L4) in a minority of people (figures vary) - The genitofemoral nerve may split into its two branches high, inside the muscle ## Related conditions - [Chronic inflammatory demyelinating polyneuropathy (CIDP)](https://durgical.com/en/conditions/chronic-inflammatory-demyelinating-polyneuropathy/) ## Test yourself Think of the answer, then tap the question to see it. ## What are the roots of the lumbar plexus, and where does it form? The ventral rami of L1–L4, often with a T12 contribution; it forms within psoas major. ## Which lumbar plexus branches leave psoas major at its lateral, front and medial borders? Lateral: iliohypogastric, ilioinguinal, lateral femoral cutaneous and femoral. Front: genitofemoral. Medial: obturator nerve and lumbosacral trunk. ## Which muscles do the femoral and obturator nerves supply? Femoral (L2–L4): iliacus and quadriceps. Obturator (L2–L4): the thigh adductors. ## Which lumbar plexus nerve can a psoas abscess compress, and what is a classic cause? The femoral nerve; spinal tuberculosis is a classic cause of psoas abscess. ## What does loss of the cremasteric reflex suggest? A lesion of the genitofemoral nerve or the L1–L2 roots; normally stroking the upper inner thigh lifts the testis on that side. [Add to my cards (5)](https://durgical.app/estudiar/?add=/en/anatomy/lumbar-plexus/) Educational information, not medical advice. For any symptom, see a health professional. ## Its names **Official Latin (TA2)** plexus lumbalis TA2 6517 **Official Spanish (DeCS)** Plexo lumbar DeCS D008160 **English (TA2)** Lumbar plexus Sources: Terminologia Anatomica (TA2); [DeCS](https://decs.bvsalud.org/es/ths/resource/?id=8335&filter=ths_termall&q=Plexo%20Lumbosacro); [Anatomy, Back, Lumbar Plexus](https://www.ncbi.nlm.nih.gov/books/NBK545137/). ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/anatomy/lumbar-plexus/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/anatomy/lumbar-plexus/