01Core principlesThe concepts and mechanisms needed to understand the subject.
Upper-limb surgical anatomy is organised by osteofascial compartments and transition points. Axillary nerve passes through quadrangular space around surgical neck; radial nerve enters posterior arm with profunda brachii, crosses the humeral groove and comes anterior near lateral epicondyle. Median nerve follows brachial artery and enters cubital fossa before pronator region. Ulnar nerve passes behind medial epicondyle and between flexor carpi ulnaris heads. Each vulnerable point produces a characteristic but not perfectly isolated deficit.
At the wrist, carpal tunnel contains median nerve and nine flexor tendons beneath flexor retinaculum; flexor carpi radialis has a separate compartment and ulnar nerve/artery pass superficial in Guyon canal. Dorsal extensor compartments and superficial radial sensory branches matter for approaches. In the hand, recurrent motor and digital branches vary, so a safe conceptual map distinguishes tendon sheath, neurovascular bundle and fascial space rather than relying on one crease.
In the lower limb, femoral triangle boundaries are inguinal ligament, sartorius and adductor longus. Femoral artery continues to adductor canal then through adductor hiatus to become popliteal. Femoral nerve branches outside the sheath. Popliteal fossa is bounded by hamstrings and gastrocnemius; tibial nerve is superficial to vein and artery, while common fibular nerve follows biceps femoris toward fibular neck. Great and small saphenous systems relate to cutaneous nerves at different levels.
Leg compartments are bounded by tibia, fibula, interosseous membrane and septa. Anterior compartment contains deep fibular nerve and anterior tibial vessels and produces dorsiflexion/toe extension. Lateral compartment contains superficial fibular nerve and evertors. Deep posterior compartment contains tibial nerve and posterior tibial/peroneal vessels with flexors. A compartment syndrome is a clinical emergency, but anatomy alone should not be converted into a treatment algorithm in this knowledge chapter.
Key points
- The radial nerve lies against the posterior humeral shaft in the radial groove before piercing the lateral intermuscular septum; shaft injury can therefore combine wrist/finger-extension weakness with a variable sensory deficit.
- The axillary nerve winds around the surgical neck of humerus with posterior circumflex humeral vessels, while the ulnar nerve becomes superficial behind the medial epicondyle.
- In the femoral triangle, structures run lateral to medial as femoral nerve, artery, vein and lymphatic/femoral canal; the nerve lies outside the femoral sheath.
- The common fibular nerve follows the biceps femoris region and wraps superficially around the fibular neck before dividing, explaining vulnerability to trauma, compression and positioning.
- From superficial to deep in the popliteal fossa, tibial nerve, popliteal vein and popliteal artery form the central neurovascular stack, with the artery closest to the femur/capsule.
- Compartment knowledge predicts which muscles, nerves and vessels share pressure or an approach, but supplied examination findings must be checked because lesions near plexus or root can mimic a single peripheral nerve injury.
02Mechanisms and patternsImportant relationships and how to distinguish them.
Radial nerve and profunda brachii occupy the posterior humeral groove; the nerve’s branch pattern means triceps may be partly spared in a more distal shaft lesion while wrist and finger extension fail.
Axillary nerve and posterior circumflex humeral vessels pass around the surgical neck; deltoid weakness and regimental-badge sensory change fit this region.
From lateral to medial lie femoral nerve, artery, vein and femoral canal/lymphatics; the sheath encloses artery, vein and canal but not the nerve.
The common fibular nerve is subcutaneous as it wraps around fibular neck, then divides into superficial and deep branches, so a proximal lesion can impair both eversion and dorsiflexion.
Tibial nerve is most superficial, vein intermediate and artery deepest against capsule/femur; the common fibular nerve runs laterally along biceps femoris.
Weakness, sensory change and vascular findings should be grouped by shared compartment and nerve, then compared with root and plexus alternatives before localisation is final.
03Interpreting evidenceInformation, measurements and their limitations.
Consider the information, its meaning and its limitations before deciding what follows.
- 01
Motor-sensory pairing - Why
- Localise a supplied deficit to a peripheral nerve segment.
- Interpretation and limitations
- Choose movements that isolate branches above and below the suspected site, then add an autonomous sensory zone; one weak effort is insufficient.
- 02
Bone-nerve relation - Why
- Predict which nerve is most exposed at a named fracture or operative corridor.
- Interpretation and limitations
- Trace the nerve on the bone rather than matching a memorised pair; proximal/distal level changes which branches remain intact.
- 03
Compartment inventory - Why
- Identify muscles, nerve and vessels sharing a closed osteofascial space.
- Interpretation and limitations
- Name the boundaries and contents, then predict functions lost together; do not diagnose raised pressure from anatomy alone.
- 04
Neurovascular stack - Why
- Orient a cross-section through femoral or popliteal region.
- Interpretation and limitations
- Use fixed boundaries and depth: femoral lateral-to-medial order differs from popliteal superficial-to-deep order.
- 05
Collateral route check - Why
- Explain potential perfusion around shoulder, elbow, hip or knee after interruption of a branch.
- Interpretation and limitations
- Name actual arterial contributors and verify continuity; a collateral network does not guarantee sufficient flow in a clinical case.
04Applied reasoningWorked examples connecting principles to decisions.
01Worked caseLocalise weakness after a humeral shaft lesionA supplied trauma diagram shows a mid-shaft humeral fracture at the radial groove; examination inputs are weak wrist and finger extension, reduced dorsal first-web-space sensation and preserved shoulder abduction.+
- 1Anchor the lesion to the posterior humeral shaft and trace the radial nerve with profunda brachii through the groove.
- 2Group weak wrist/finger extension and first-web-space sensory loss as radial motor and sensory territory distal to the supplied level.
- 3Use preserved shoulder abduction to avoid an axillary-nerve localisation and check triceps because proximal radial branches may have arisen before the groove.
- 4Conclude a radial-nerve lesion at the humeral shaft is the coherent anatomical localisation, without inferring transection or prognosis.
- 5Verify with branch-by-branch motor testing, an autonomous sensory site and serial neurovascular documentation around the injury.
02Applied anatomyOrient the femoral triangle for a model approachA supplied ultrasound diagram immediately below the inguinal ligament shows a nerve lateral to a pulsatile artery and a compressible vein medial to the artery.+
- 1Use pulsatility and compressibility only as supplied labels, not invented patient findings.
- 2Apply lateral-to-medial nerve, artery and vein order.
- 3Place the nerve outside the femoral sheath and the canal medial to vein.
- 4Verify identity by continuity and trained real-time imaging before any procedure.
03Spatial reasoningPredict a fibular-neck deficitA model compression site is immediately distal to the fibular head before the common fibular nerve divides.+
- 1Locate the common trunk in its superficial fibular-neck course.
- 2Predict deep-branch dorsiflexion/toe-extension and superficial-branch eversion deficits together.
- 3Expect sensory change over anterolateral leg/dorsum of foot with branch-dependent sparing.
- 4Check inversion and plantarflexion to separate more proximal sciatic or root involvement.
05Checking understandingVerify the reasoning, revisit uncertainties and apply feedback.
- Draw each named nerve from plexus to terminal branch and mark bone contact, fascial tunnel and motor branch points.
- Practise femoral lateral-to-medial order and popliteal superficial-to-deep order as separate maps to avoid transferring the wrong sequence.
- For any fracture question, state which functions are preserved because their branches left proximal to the lesion.
- Use one motor action and one autonomous sensory region to test a localisation, then add reflex or proximal movement where relevant.
- Keep procedural application conditional on direct imaging, positioning and local competence; textbook landmarks do not constitute a safe procedure.
06Special situationsVariants, exceptions and circumstances that change the usual approach.
Radial deficit varies by level
A very proximal lesion may weaken triceps, whereas many groove-level lesions preserve substantial elbow extension because branches to triceps arise earlier.
Femoral nerve lies outside sheath
The familiar nerve-artery-vein order should not be misread as all three sharing the femoral sheath; this distinction matters in cross-section and regional procedures.
Ulnar nerve is superficial at elbow
Its course behind medial epicondyle explains sensitivity to compression, but hand weakness patterns depend on lesion level and branching above or below forearm muscles.
Popliteal artery is deepest
The vessel lies closest to distal femur and knee capsule; posterior approaches meet nerve and vein before artery in the common superficial-to-deep sequence.
Fibular injury crosses compartments
A lesion before common fibular division affects both anterior and lateral compartment functions, unlike an isolated deep or superficial branch lesion.
07Common pitfallsFrequent interpretation and management errors.
- 01
Calling every wrist drop a radial-groove lesion without checking triceps, sensory territory and more proximal plexus/root signs.
- 02
Putting the femoral nerve inside the femoral sheath because it lies beside the artery and vein.
- 03
Reversing popliteal depth by placing artery superficial to tibial nerve.
- 04
Predicting isolated deep-fibular weakness from a lesion at the common fibular trunk before division.
- 05
Using a bone landmark as permission for a real invasive procedure without image guidance, training and variant recognition.