Compression neuropathy of the lateral femoral cutaneous nerve (LFCN), a pure sensory nerve, producing burning pain, numbness and paraesthesia over the anterolateral thigh. The classic entrapment point is where the nerve passes under or through the inguinal ligament near the anterior superior iliac spine.
Causes Overview
Critical Must-Knows
- The LFCN is purely SENSORY - any weakness or reflex loss means you have the wrong diagnosis (think L2-L3 radiculopathy or femoral neuropathy instead)
- The sensory patch is sharply bounded on the anterolateral thigh, stopping at the knee - mapping it is the single most useful bedside test
- The pelvic compression test is highly suggestive: sustained downward pressure inferomedial to the ASIS for 45 seconds RELIEVES the symptoms
- A diagnostic LFCN block at the ASIS that abolishes the pain both confirms the diagnosis and predicts surgical success
- Always exclude a retroperitoneal or pelvic mass and an upper lumbar radiculopathy before labelling the case idiopathic
Clinical Pearls
- "Conservative first: removing the compressor and weight loss resolve most cases, and spontaneous recovery is well documented
- "Neurectomy gives the highest complete pain relief but leaves PERMANENT thigh numbness; neurolysis preserves sensation but is revised more often
- "No randomised trial exists - all guidance is observational, so escalate stepwise and share the surgical decision
Clinical Imaging
Lateral Femoral Cutaneous Nerve - Surgical Anatomy
FRACS Examiner Red Flags
Memory Aids
MERALGIACauses and Key Features
| M | Mechanical compression At the inguinal ligament near the ASIS - the core lesion |
| E | Excess weight and belts Obesity, tight belts and clothing raise local pressure |
| R | Retroperitoneal mass Pelvic/retroperitoneal tumour - must exclude in atypical cases |
| A | Anatomical variation Variable course of the nerve at the ASIS (Aszmann-Dellon types) |
| L | L2-L3 origin Pure sensory nerve - no motor, no reflex involvement |
| G | Gravid uterus Pregnancy raises intra-abdominal pressure |
| I | Iatrogenic Iliac crest graft, pelvic/spine surgery, abdominoplasty |
| A | Anterolateral thigh patch Sharply bounded territory that stops at the knee |
| M | Mechanical compression At the inguinal ligament near the ASIS - the core lesion | A | Anatomical variation Variable course of the nerve at the ASIS (Aszmann-Dellon types) | I | Iatrogenic Iliac crest graft, pelvic/spine surgery, abdominoplasty |
| E | Excess weight and belts Obesity, tight belts and clothing raise local pressure | L | L2-L3 origin Pure sensory nerve - no motor, no reflex involvement | A | Anterolateral thigh patch Sharply bounded territory that stops at the knee |
| R | Retroperitoneal mass Pelvic/retroperitoneal tumour - must exclude in atypical cases | G | Gravid uterus Pregnancy raises intra-abdominal pressure |
Hook:MERALGIA literally means thigh pain (Greek meros = thigh, algos = pain) - the name tells you the territory.
SENSORYWhy It Cannot Be a Radiculopathy
| S | Sensory only Pain, tingling and numbness - never weakness |
| E | Ends at the knee The patch never extends below the knee |
| N | No reflex change Knee jerk is preserved and symmetrical |
| S | Sharply demarcated Well-bounded patch that does not cross the midline |
| O | One peripheral nerve Follows the LFCN territory, not a dermatome |
| R | Retroperitoneal cause excluded Image to rule out a mass in atypical cases |
| Y | Yields to a block Local anaesthetic at the ASIS abolishes the pain |
| S | Sensory only Pain, tingling and numbness - never weakness | S | Sharply demarcated Well-bounded patch that does not cross the midline | Y | Yields to a block Local anaesthetic at the ASIS abolishes the pain |
| E | Ends at the knee The patch never extends below the knee | O | One peripheral nerve Follows the LFCN territory, not a dermatome | ||
| N | No reflex change Knee jerk is preserved and symmetrical | R | Retroperitoneal cause excluded Image to rule out a mass in atypical cases |
Hook:If you find weakness or a lost knee jerk, leave meralgia behind and think L2-L3 root or femoral nerve.
Overview
Meralgia paraesthetica is a compression (entrapment) mononeuropathy of the lateral femoral cutaneous nerve (LFCN), a purely sensory nerve. It produces burning pain, tingling, numbness and hypersensitivity over a sharply bounded patch on the anterolateral thigh. Because the nerve carries no motor fibres, there is never weakness and never a reflex change - a point examiners use repeatedly to separate it from an upper lumbar radiculopathy or a femoral neuropathy.
According to PubMed, the disorder is more common in middle-aged adults, with male patients in one hospital-based series peaking at 41-50 years and a strong association with higher body mass index and certain occupations; a recognisable risk factor was identified in only about 58% of cases, so many remain idiopathic (Weng et al., J Clin Neurosci 2017; DOI).
The condition matters in orthopaedics for two reasons. First, it is a classic iatrogenic complication of anterior pelvic surgery, iliac crest bone-graft harvest, the anterior (Smith-Petersen) approach to the hip, and prone spine surgery. Second, it is a frequent clinical mimic that must be separated from lumbar nerve-root pathology.
Anatomy
Nerve Anatomy
LFCN Origin and Course
From root to thigh:
- Arises from the dorsal divisions of the L2 and L3 ventral rami
- Emerges at the lateral border of psoas major
- Crosses the iliacus muscle obliquely (deep to the iliac fascia) toward the ASIS
- Passes under, through, or just medial to the inguinal ligament near the anterior superior iliac spine (ASIS)
- Enters the thigh and divides into anterior and posterior branches
- Supplies sensation to the anterolateral thigh only
Why It Gets Trapped
Vulnerable points:
- Sharp angulation as it turns from the pelvis into the thigh under the inguinal ligament
- Tethering as it pierces fascia near the ASIS
- Marked anatomical variation in where it crosses relative to the ASIS (it can lie several centimetres medial or even over the iliac crest)
- Superficial position making it easy to injure during anterior surgery
- Compression from intra-abdominal pressure (obesity, pregnancy, ascites, tight garments)
According to PubMed, this anatomical variability is clinically important: a preoperative ultrasound study found the nerve most often followed an Aszmann-Dellon type B course (about 79% of patients), with several more medial variants, and ultrasound predicted the intraoperative position with 100% correlation, allowing the nerve to be identified in every case (de Ruiter et al., World Neurosurg 2021; DOI).
Pathophysiology
The LFCN is susceptible to focal compression and traction at the inguinal ligament. Chronic compression produces, in sequence, focal demyelination (early, reversible), then axonal injury with potential for recovery after the compression is relieved, and rarely chronic axon loss if the cause persists. Increased intra-abdominal pressure, sharp hip flexion-extension cycling, direct external pressure (belts, tool-belts, body armour, seat belts) and tethering scar all raise the local pressure on the nerve.
FAT BELTReversible Compressors of the LFCN
| F | Fat Obesity and raised intra-abdominal pressure |
| A | Ascites Abdominal distension increasing pressure on the nerve |
| T | Trousers and belts Tight waistbands compressing at the inguinal ligament |
| B | Body armour / tool-belt Heavy occupational loads at the waist |
| E | Expanding uterus Pregnancy raising intra-abdominal pressure |
| L | Lower-limb posture change Altered hip mechanics increasing nerve traction |
| T | Tension from scar Tethering after pelvic or iliac surgery |
| F | Fat Obesity and raised intra-abdominal pressure | B | Body armour / tool-belt Heavy occupational loads at the waist | T | Tension from scar Tethering after pelvic or iliac surgery |
| A | Ascites Abdominal distension increasing pressure on the nerve | E | Expanding uterus Pregnancy raising intra-abdominal pressure | ||
| T | Trousers and belts Tight waistbands compressing at the inguinal ligament | L | Lower-limb posture change Altered hip mechanics increasing nerve traction |
Hook:Loosen the FAT BELT first - removing the external compressor is often curative before any drug or operation.
Classification
The most useful classification for the surgeon is by cause and by the anatomical variant of the nerve at the ASIS.
Classification by Cause
Clinical Presentation
History
Classic Presentation
Typical features in the history:
- Burning, tingling, numbness or hypersensitivity over the anterolateral thigh
- A patch that is sharply demarcated, does not cross the midline, and does not extend below the knee
- Worse with standing, walking and hip extension; often relieved by sitting and hip flexion (which slackens the inguinal ligament)
- A history of weight gain, tight clothing, pregnancy, recent pelvic/iliac surgery or graft harvest
- No back pain, no leg weakness, no bowel or bladder change
Examination
What You Find
Positive findings:
- Altered sensation (numbness, hyperalgesia, allodynia) in the LFCN patch
- Positive pelvic compression test (see below)
- Tinel sign - tapping just medial and inferior to the ASIS reproduces the paraesthesia
- Symptoms reproduced by passive hip extension
- Relief after a diagnostic LFCN block at the ASIS
What You Must NOT Find
Findings that exclude the diagnosis:
- Any motor weakness (quadriceps, hip flexors) - think L2-L3 root or femoral nerve
- Lost or asymmetric knee jerk - the LFCN is sensory only
- Sensory loss crossing the midline or extending below the knee
- Back pain with a positive femoral stretch and root signs
- Bilateral progressive symptoms suggesting a central or systemic cause
The Pelvic Compression Test
Investigations
Meralgia paraesthetica is fundamentally a clinical diagnosis. Investigations are used to confirm, to localise the nerve, and above all to exclude dangerous mimics.
Local anaesthetic block of the LFCN at the ASIS is the single most useful confirmatory test. Abolition of the pain after the block both confirms the LFCN as the pain source and predicts a good response to surgery. A negative block may simply mean the needle missed an anatomically variant nerve, which is why ultrasound guidance improves reliability.
Do Not Miss a Sinister Cause
Management
The guiding principle is a stepwise escalation: remove the cause, then treat the pain, then block, and only then operate. According to PubMed, the evidence base is weak - a Cochrane review found no randomised trials and noted that even spontaneous improvement occurred in 69% of an untreated natural-history cohort, while injection and surgery both gave high improvement rates in observational data (Khalil et al., Cochrane Database Syst Rev 2012; DOI).
- Remove the compressor: weight loss, loosen or remove belts and tight clothing, modify the tool-belt or body armour, treat ascites
- Reassurance that many cases (including most iatrogenic ones) settle spontaneously
- Neuropathic analgesia: gabapentin or pregabalin, tricyclics, or topical agents for burning pain
- Physiotherapy and activity modification to reduce repetitive hip extension
- Time - a several-month trial is reasonable given the high spontaneous recovery rate
STEP DOWNTreatment Ladder
| S | Stop the compressor Belt, weight, garment - first and often curative |
| T | Time and reassurance Spontaneous recovery is common |
| E | Education Activity modification, reduce repetitive hip extension |
| P | Pharmacology Neuropathic analgesia (gabapentin, pregabalin, tricyclics) |
| D | Diagnostic block Ultrasound-guided LFCN block - diagnostic and therapeutic |
| O | Operation if refractory Only after a fair trial and a positive block |
| W | Weigh neurolysis Preserves sensation but is revised more often |
| N | Neurectomy Best pain relief but permanent numbness |
| S | Stop the compressor Belt, weight, garment - first and often curative | P | Pharmacology Neuropathic analgesia (gabapentin, pregabalin, tricyclics) | W | Weigh neurolysis Preserves sensation but is revised more often |
| T | Time and reassurance Spontaneous recovery is common | D | Diagnostic block Ultrasound-guided LFCN block - diagnostic and therapeutic | N | Neurectomy Best pain relief but permanent numbness |
| E | Education Activity modification, reduce repetitive hip extension | O | Operation if refractory Only after a fair trial and a positive block |
Hook:STEP DOWN the ladder one rung at a time - never jump straight from history to scalpel.
Complications
- Persistent or recurrent pain after neurolysis if decompression is incomplete or the nerve re-tethers in scar
- Permanent anterolateral thigh numbness after neurectomy (counsel and consent for this explicitly)
- Painful neuroma at the cut end after neurectomy
- Iatrogenic LFCN injury during pelvic, hip, abdominal or spine surgery - prevented by respecting the unsafe zone at the ASIS
- Failed diagnostic block due to anatomical variation, leading to diagnostic uncertainty
- Missed sinister cause (mass, haematoma) if imaging is omitted in atypical cases
Evidence Base
Treatment Outcomes Meta-analysis - Injection vs Neurolysis vs Neurectomy
- Complete pain relief: 85% neurectomy vs 63% neurolysis vs 22% injection
- Revision rate: 0% neurectomy vs 12% neurolysis vs 81% injection
- Complication rates comparable and low (0-5%) across all three
Cochrane Review - Weak Evidence Base and High Spontaneous Recovery
- No randomised or quasi-randomised trials of any treatment exist
- Spontaneous improvement in 20 of 29 (69%) untreated cases
- High observational improvement for injection (83%), decompression (88%) and neurectomy (94%)
Sonographic Diagnosis of LFCN Entrapment
- Ultrasound positive in 82 of 86 (95.3%) clinically suspected patients
- Signs: abrupt calibre change, indistinct perineurium, intraneural vascularity
- Mean LFCN cross-sectional area 4.47 mm squared; diagnostic cut-off 2.65 mm squared
Preoperative Ultrasound Detects Anatomical Variants
- Type B course most frequent (79%); no type A encountered
- 100% correlation between preoperative ultrasound and intraoperative findings
- Nerve identified intraoperatively in every case regardless of variant
Clinical Relevance
For the orthopaedic surgeon, meralgia paraesthetica is most often encountered either as an iatrogenic complication or as a diagnostic mimic. The unsafe zone lies just inferomedial to the ASIS along the inguinal ligament - the same area entered during iliac crest bone-graft harvest (use a more posterior or inner-table harvest and stay at least 2-3 cm posterior to the ASIS), the anterior approach to the hip, pelvic osteotomy and abdominoplasty. Prone positioning for spine surgery can also compress the nerve against the bolster. Knowing this anatomy is a recurring viva theme.
As a mimic, the key discriminator is the pure sensory nature of the nerve. A patient with thigh numbness plus quadriceps weakness or a depressed knee jerk does not have meralgia paraesthetica; they have an L2-L3 radiculopathy or a femoral neuropathy until proven otherwise.
Viva Scenarios
Use these scenarios to practise clinical reasoning and management decisions
"A 52-year-old obese man presents with 5 months of burning numbness over the outer aspect of his right thigh. It is worse on standing and after wearing his work belt all day, and eases when he sits and loosens the belt. There is no back pain and no weakness. How would you assess and manage this patient?"
"A patient with disabling meralgia paraesthetica has failed 9 months of weight loss, activity modification, gabapentin and two ultrasound-guided steroid blocks that each gave only a few weeks of relief. They want a definitive operation. How do you counsel them and what would you offer?"
MCQ Practice Points
Clinical Pearl
Q: From which nerve roots does the lateral femoral cutaneous nerve arise, and what does it supply?
A: The LFCN arises from the dorsal divisions of L2 and L3 ventral rami. It is a purely sensory nerve supplying the anterolateral thigh down to the knee. It carries no motor fibres, so weakness or reflex loss is never part of meralgia paraesthetica.
Clinical Pearl
Q: What is the most common site of compression in meralgia paraesthetica?
A: Where the nerve passes under, through, or just medial to the inguinal ligament near the anterior superior iliac spine (ASIS). Sharp angulation as the nerve turns from the pelvis into the thigh, combined with anatomical variation in its course, makes this the vulnerable point.
Clinical Pearl
Q: How do you distinguish meralgia paraesthetica from an L2-L3 radiculopathy?
A: Meralgia paraesthetica is sensory only with a sharply bounded anterolateral thigh patch that stops at the knee, a normal motor exam and knee jerk, no back pain, and relief with a local block at the ASIS. L2-L3 radiculopathy may have back pain, quadriceps or hip-flexor weakness, an altered knee jerk, and a positive femoral stretch test, and warrants lumbar MRI.
Clinical Pearl
Q: Which surgical option gives the highest rate of complete pain relief, and what is its main drawback?
A: Neurectomy (transection) gives the highest complete pain relief (about 85% in pooled data) with a very low revision rate, but its main drawback is a permanent patch of numbness on the anterolateral thigh and a small risk of a painful neuroma. Neurolysis preserves sensation but relieves pain less reliably and is revised more often.
Meralgia Paraesthetica - Exam Day Summary
Clinical summary
Definition
- •Compression neuropathy of the lateral femoral cutaneous nerve (LFCN)
- •Purely SENSORY nerve - no motor, no reflex involvement
- •Burning numbness and paraesthesia over the anterolateral thigh
Anatomy
- •LFCN from dorsal divisions of L2 and L3
- •Passes under/through/medial to inguinal ligament near the ASIS
- •Supplies anterolateral thigh down to the knee only
- •Marked anatomical variation in course relative to ASIS (Aszmann-Dellon types)
Causes
- •Idiopathic: obesity, tight belts, pregnancy, diabetes
- •Iatrogenic: iliac crest graft, anterior hip approach, pelvic/abdominal/spine surgery
- •Compressive: retroperitoneal or pelvic mass, iliacus haematoma (must exclude)
Clinical Diagnosis
- •Sharply demarcated anterolateral thigh patch, stops at the knee, no midline crossing
- •Worse with standing and hip extension, eased by sitting and hip flexion
- •Positive Tinel medial-inferior to ASIS
- •Pelvic compression test RELIEVES symptoms (sustained pressure ~45 s)
- •NO weakness and NO reflex change
Investigations
- •Clinical diagnosis; LFCN diagnostic block confirms and predicts surgical success
- •Ultrasound: enlarged nerve (cut-off ~2.65 mm squared), maps variants, ~95% positive
- •Nerve conduction: reduced/absent LFCN sensory response; hard in the obese
- •Lumbar/pelvic imaging to exclude radiculopathy or a mass in atypical cases
Management
- •First line conservative: remove compressor, weight loss, neuropathic analgesia, reassurance
- •Spontaneous recovery common (~69% in natural-history data)
- •Second line: ultrasound-guided LFCN block (diagnostic and therapeutic)
- •Surgery if refractory and block positive
Surgery and Outcomes
- •Neurectomy: best complete pain relief (~85%), low revision, but PERMANENT numbness and neuroma risk
- •Neurolysis: preserves sensation, relief ~63%, revision ~12%
- •No randomised evidence proves one superior - shared decision
Guidelines, Registries & Global Practice
Global epidemiology. Meralgia paraesthetica is a relatively common entrapment neuropathy of middle-aged adults. According to PubMed, a hospital-based series found a male predominance with peak onset at 41-50 years in men and an even age distribution in women, a clear association with higher body mass index and certain occupations, and a recognisable cause in only 58% of patients (Weng et al., J Clin Neurosci 2017; DOI).
Evidence and guidance, side by side. There are no randomised trials, so all major reviews converge on a conservative-first, escalate-as-needed approach rather than a formal society guideline:
- Cochrane (UK/international): no RCTs exist; high spontaneous recovery (69%) and comparable observational success for injection, decompression and neurectomy support starting conservatively (Khalil et al., 2012; DOI).
- European pain-medicine guidance (Patijn et al., Pain Pract 2011): conservative treatment and treating the underlying cause are first choice; interventional pulsed radiofrequency should remain within research only (DOI).
- North American surgical literature: when surgery is required, neurectomy offers the most reliable complete pain relief at the cost of numbness, while a systematic review stresses that the evidence is insufficient to mandate one operation (Lu et al., 2021, DOI; Payne et al., 2017, DOI).
Global practice variation. Increasing worldwide adoption of high-resolution ultrasound is shifting practice toward image-guided diagnosis and blocks and preoperative mapping of the nerve's variant course, improving the reliability of both diagnosis and surgery (Shi et al., 2021, DOI; de Ruiter et al., 2021, DOI).