Overview & Anatomy of Peripheral Nerves
Peripheral nerve injuries are common in orthopaedic trauma and represent a spectrum from transient conduction block to complete nerve division with poor prognosis. Understanding the anatomical organisation of the peripheral nerve — and the relationship between the degree of structural disruption and the capacity for recovery — is the foundation of nerve injury classification and management. The Sunderland classification (1951) refines Seddon`s three-grade system (1943) into five grades based on the specific anatomical layer disrupted, providing greater prognostic precision.
- Peripheral nerve anatomy (from innermost to outermost): individual axons are surrounded by the endoneurium (innermost connective tissue layer — surrounds each individual axon and its myelin sheath); groups of axons are bundled into fascicles, each fascicle surrounded by the perineurium (a strong, tight-junctioned layer that maintains the endoneurial microenvironment and forms the blood-nerve barrier); multiple fascicles are bundled together within the epineurium (the outermost connective tissue sheath of the entire nerve trunk); understanding which of these layers is disrupted determines the Sunderland grade and the potential for spontaneous recovery
- Wallerian degeneration: after axonal disruption (Sunderland 2 or above), the axon segment distal to the injury undergoes Wallerian degeneration — the axon and myelin sheath disintegrate (over 3–5 days); Schwann cells proliferate and phagocytose the debris; the remaining empty endoneurial tubes (Bands of Büngner) provide a scaffold for axonal regeneration; proximal to the injury, the cell body undergoes chromatolysis (metabolic switch to regenerative mode); axon regeneration proceeds at approximately 1 mm/day (or 1 inch/month) from the injury site distally
Sunderland Classification
| Sunderland Grade | Seddon Equivalent | Structure Disrupted | Pathology | Prognosis |
|---|---|---|---|---|
| Grade 1 | Neuropraxia | Myelin sheath only; axon structurally intact; all connective tissue layers intact (endoneurium, perineurium, epineurium all intact) | Local demyelination at the injury site; axonal continuity is preserved; conduction block — the nerve cannot transmit impulses across the demyelinated segment but the axon is alive and connected distally; no Wallerian degeneration | Complete recovery — weeks to months (2–12 weeks typical); recovery is by remyelination of the intact axon; no axonal regeneration required; excellent prognosis; the most favourable nerve injury |
| Grade 2 | Axonotmesis | Axon disrupted; endoneurium intact; perineurium and epineurium intact | The axon is physically severed but the endoneurial tube (which normally guides the axon distally) remains intact; Wallerian degeneration of the distal axon occurs; the intact endoneurial tube provides perfect guidance for regenerating axon back to the original target organ | Complete recovery — but requires axonal regeneration at ~1 mm/day; recovery time depends on distance from injury to target organ; recovery is COMPLETE because the endoneurial guidance tubes are intact; the regenerating axon follows the exact original path to the correct motor or sensory end-organ |
| Grade 3 | Axonotmesis | Axon + endoneurium disrupted; perineurium intact; epineurium intact | The axon and its surrounding endoneurial tube are disrupted; the perineurium (fascicular boundary) remains intact so the fascicular structure is preserved; regenerating axons may end up in the wrong endoneurial tube within the fascicle — misdirection occurs; intraneural fibrosis within the damaged endoneurium impedes regeneration | Incomplete recovery — misdirected axonal regeneration leads to imperfect functional recovery; the degree of recovery depends on the extent of intraneural fibrosis and the degree of misdirection; motor fibres innervating wrong muscle targets, sensory fibres reaching wrong areas; spontaneous recovery possible but incomplete |
| Grade 4 | Axonotmesis / Neurotmesis (border) | Axon + endoneurium + perineurium disrupted; epineurium intact | The fascicular architecture is destroyed — there is no longer any internal guidance for regenerating axons; the epineurium remains intact, so the nerve trunk appears continuous macroscopically (`in continuity`) but internally is a disorganised mass of axons, fibroblasts, and scar tissue; regenerating axons face a chaotic intrafascicular environment and cannot find their correct targets | Very poor spontaneous recovery — the nerve is in continuity but functionally equivalent to a complete division; surgical intervention (internal neurolysis, nerve grafting) is usually required for functional recovery; spontaneous recovery is minimal; surgery should be considered at 3–6 months if no improvement |
| Grade 5 | Neurotmesis | Complete transection of all layers — axon + endoneurium + perineurium + epineurium all divided; the nerve trunk is physically cut in two | Complete structural discontinuity; both ends retract; a traumatic neuroma forms at the proximal end (disorganised axonal sprouting into scar tissue); the distal stump undergoes complete Wallerian degeneration; no spontaneous recovery is possible across a complete anatomical gap | No spontaneous recovery; requires surgical repair (primary end-to-end neurorrhaphy if possible without tension; nerve grafting using sural nerve or other donor if a gap exists); outcomes of surgical repair are variable and depend on the level of injury, the gap size, the time to repair, and the age of the patient |
Seddon Classification — Simplified Three-Grade System
| Seddon Grade | Sunderland Equivalent | Key Features |
|---|---|---|
| Neuropraxia | Sunderland Grade 1 | Conduction block; axon intact; no Wallerian degeneration; complete recovery in weeks; the mildest injury |
| Axonotmesis | Sunderland Grades 2, 3, 4 | Axon disrupted; Wallerian degeneration; nerve macroscopically in continuity; recovery variable (Grade 2 = complete; Grade 3 = partial; Grade 4 = very poor); the broadest Seddon category — hence Sunderland`s subdivision |
| Neurotmesis | Sunderland Grade 5 | Complete anatomical transection; no spontaneous recovery; requires surgical repair; worst prognosis |
Clinical Assessment & Investigations
- Tinel`s sign: tapping over the nerve at the site of injury or along the course of the regenerating nerve produces a tingling sensation (paraesthesia) in the distribution of that nerve; a Tinel`s sign that is advancing distally over serial examinations (at approximately 1 mm/day) indicates that axonal regeneration is progressing; a Tinel`s sign that is stationary suggests stalled regeneration (neuroma formation, Grade 4 injury); the presence of an advancing Tinel`s sign is one of the most clinically useful signs of nerve recovery
- Nerve conduction studies (NCS) and electromyography (EMG): performed 3–6 weeks after injury (earlier studies are difficult to interpret as Wallerian degeneration takes 3–5 days and denervation potentials take 3–4 weeks to develop); NCS assesses axonal conduction velocity and amplitude; EMG assesses the electrical activity of muscle (denervation = fibrillation potentials and positive sharp waves; reinnervation = nascent motor units with polyphasic morphology); EMG can detect early reinnervation before clinical motor recovery is apparent — positive reinnervation potentials on EMG at 3–6 months without clinical recovery suggests regeneration is occurring but has not yet reached the muscle; wait further before surgery
- MRI neurography: high-resolution MRI of peripheral nerves using specialised sequences (3D-NERVE, DWIBS, DTI — diffusion tensor imaging); can directly visualise nerve disruption, neuroma formation, nerve compression, and the fascicular anatomy at the injury site; increasingly used pre-operatively to plan nerve repair
Management Principles
- Conservative management (Sunderland 1–3): Grade 1 (neuropraxia) and Grade 2 (axonotmesis with intact endoneurium) recover spontaneously; conservative management = physiotherapy (prevent joint contractures and muscle atrophy in denervated limbs); splinting (prevent deformity — e.g., cock-up wrist splint for radial nerve palsy; AFO for peroneal nerve palsy); electrical stimulation of denervated muscle (delays muscle atrophy); protective measures (insensate skin is at risk of undetected burns and pressure sores — patient education essential); serial Tinel`s assessment and EMG to monitor recovery
- Surgical management indications: (1) open nerve injury (sharp transection) — primary repair within 72 hours if possible; (2) no clinical or EMG evidence of recovery at 3–6 months after closed injury — exploration and repair; (3) Grade 4 injury confirmed on intraoperative nerve action potential (NAP) recording — negative NAP across a segment = no functional axons = resect and graft; (4) painful neuroma causing intractable pain; (5) nerve injury associated with vascular repair — explore at time of vascular surgery
- Nerve repair options: (1) Primary neurorrhaphy — tension-free end-to-end repair with 8/0 or 9/0 epineurial sutures; only if nerve ends can be approximated WITHOUT tension (tension = failure); (2) Nerve grafting — sural nerve (most common donor; harvested from the lateral leg and ankle; 30–40 cm available; sensory nerve only; minor donor site deficit); other donors: medial cutaneous nerve of forearm, lateral femoral cutaneous nerve; (3) Nerve conduits (tubes) — for small gaps (<3 cm) in sensory nerves; bioabsorbable or collagen tubes guide regeneration across the gap; (4) Nerve transfers — for proximal nerve injuries where the distance to the target muscle is too great for regeneration to occur before irreversible muscle fibrosis (approximately 12–18 months); a nearby functioning donor nerve is transferred to the distal stump of the injured nerve, providing a close source of axons
Common Nerve Injuries in Orthopaedics
| Nerve | Associated Injury | Motor Loss | Sensory Loss | Prognosis |
|---|---|---|---|---|
| Radial nerve | Holstein-Lewis fracture (distal 1/3 humeral shaft); anterior shoulder dislocation; Saturday night palsy (axilla compression) | Wrist drop (extensor carpi radialis, ECRB, ECRL); finger and thumb drop (EDC, EPL, EPB, EIP); inability to extend wrist and fingers | Dorsal first web space (PIN branch sensory); small patch over dorsum thumb | Mostly neuropraxia (Grade 1); 90%+ spontaneous recovery; observe for 3–4 months before exploration |
| Anterior interosseous nerve (AIN) | Supracondylar fracture; pronator syndrome | Flexor pollicis longus + flexor digitorum profundus to index/middle fingers; pinch deformity (cannot make `OK sign`) | NONE (pure motor nerve) | Usually recovers spontaneously; observe 3–6 months; rare surgical exploration |
| Axillary nerve | Shoulder dislocation (anterior); proximal humerus fracture; surgical neck fracture | Deltoid weakness (shoulder abduction); teres minor | Regimental patch (lateral shoulder — `sergeant`s badge`) | Usually neuropraxia; good recovery; EMG at 6–12 weeks; surgery if no recovery at 3–6 months |
| Common peroneal nerve (CPN) | Knee dislocation; fibular neck fracture; cast pressure at fibular head | Foot drop (tibialis anterior, EHL, EDB); eversion weakness (peroneus longus/brevis) | Dorsum of foot; first web space | Variable; mostly neuropraxia/axonotmesis; 30–50% complete recovery; AFO for foot drop; surgical exploration at 3–6 months if no recovery |
Exam Pearls
- Sunderland: 1 = myelin only (neuropraxia — complete recovery, remyelination); 2 = axon disrupted, endoneurium intact (axonotmesis — complete recovery, perfect guidance); 3 = axon + endoneurium disrupted, perineurium intact (incomplete recovery — misdirection); 4 = axon + endoneurium + perineurium disrupted, epineurium intact (very poor spontaneous recovery — surgery); 5 = complete transection (neurotmesis — no recovery without surgery)
- Key anatomical principle: the endoneurium guides the axon to its correct target; if the endoneurium is intact (Grades 1–2), recovery is complete; if the endoneurium is disrupted (Grades 3–5), misdirection and incomplete recovery occur
- Axon regeneration rate: 1 mm/day (1 inch/month); use this to calculate expected time to recovery; injury at the axilla with target muscle in the hand (30 cm) = approximately 10 months to recovery; if no recovery by expected time + 3 months → explore
- Tinel`s sign: advancing distally at 1 mm/day = active regeneration; stationary = stalled (neuroma or Grade 4 injury); document the level of Tinel`s at serial examinations
- Radial nerve palsy at the Holstein-Lewis fracture (spiral groove): mostly neuropraxia (90%+ recovery); observe 3–4 months; wrist drop + loss of finger/thumb extension; dorsal first web space sensory loss; cock-up wrist splint for function during recovery
- Sural nerve graft: most common donor for nerve grafting; sensory nerve; harvested from lateral leg and ankle; 30–40 cm available; minimal donor deficit (small patch of lateral foot anaesthesia)
- Nerve transfer: used when proximal injury prevents regeneration reaching the target muscle before irreversible muscle fibrosis (~12–18 months); nearby functioning donor nerve transferred to distal stump; e.g., intercostal nerve transfer to musculocutaneous nerve in brachial plexus avulsion
- EMG timing: perform 3–6 weeks post-injury (not earlier — Wallerian degeneration and denervation changes take time to develop); positive reinnervation potentials without clinical recovery = wait further; no reinnervation at 3–6 months = explore