Layers:

• Individual axons surrounded by myelination from Schwann cells​, enabling rapid nerve conduction at gaps between Schwann cells, known as nodes of Ranvier.

• Fascicles - grouped arrangement of axons

• Endoneurium surrounding each axon

• Perineurium - connective tissue surrounding each fascicle

• Epineurium - outer sheath of peripheral nerve

• Mesoneurium - outer adventitial layer of nerve (Both epi- and mesoneurium incorporated into epineural repair)

Blood supply​

• Arteriae nervosum​ (vaso nervosus)

• Extrinsic vessels

• Capillary plexus

Anatomical variations:

• Martin-Gruber anastamosis​ - Motor connection between median and ulnar nerves in forearm, or distally between anterior interosseous nerve and ulnar branches

• Riche-Cannieu anastamosis - Motor connection between median and ulnar nerves in palm

• Froment-Rauber anastamosis - Radial nerve (posterior interosseous nerve or superficial branch) to ulnar motor branches innervating the 1st, 2nd or 3rd dorsal interosseous

• Berretini connection - Sensory connections between the ulnar and median nerves in the palm

Aetiology of injury - compression, tension, laceration​​

Muscle atrophy

• Loss of 1% of motor endplates per week (minimum of 25% required for functional muscle contraction)

• Repaired nerve regenerates at 1mm/day or roughly 1 inch/month

• Best reinnervation within 3 months, but possible up to 1 year after injury

• No reinnervation possible 2-3 years after injury

• Outcome worse with advancd age (>40 years) and disuse

Sensory recovery​

• Native nerve regeneration not required, axonal collateral sprouting from adjacent axons​​

Seddon classification​

• Neuropraxia

  • local transient block of conduction along a nerve​

  • anatomy of nerve is preserved

  • no Wallerian degeneration occurs

  • rapid recovery - several months

  • selective demyelination of fibers may occur

• Axonotmesis​

  • axonal damage within the nerve​

  • anatomic continuity is preserved

  • Wallerian degeneration occurs

  • recovery rate 1mm/day along nerve once healing begins

  • fibrillations present on electromotor testing

  • recovery typically complete eventually (without surgery)

• Neuronotmesis​

  • nerve is transected​

  • Wallerian degeneration occurs

  • some recovery may occur but is never complete

  • surgical repair required for best outcome

Sunderland classification​​

• 1st degree​ -  similar to Seddon neuropraxia​

• 2nd degree​ - similar to Seddon axonotmesis​

• 3rd degree

  • some endoneurial sheaths disrupted with scarring while the perineurium remains intact​

  • incomplete recovery

  • treatment non-operative

• 4th degree​

  • loss of continuity of the perineum​

  • little or no nerve recovery

  • treatment operative

• 5th degree​

  • nerve completely transected​

  • epineurium disrupted

  • no recovery without operative management

Clinical examination

• Tinel's sign​

• Two-point discrimination

• Vibration thresholds

• Semmes-Weinstein monofilament

Check of vascularity and flexor function to ​digit especially if both radial and ulnar digital nerve suspected

Investigations​

• Electromyography (EMG) and nerve conduction velocity (NCV) - used when unable to arrive at accurate diagnosis, not routinely in open injury​

Principles​​

• Loupe magnification/ microscope

• Secondary repair if zone of injury indeterminate

• Sensory and motor re-education

• Primary tension-free repair when possible

Epineural repair with 8-0/9-0 Ethilon interrupted sutures​

Fascicular repair - not shown to be superior

Cut back nerve ends to healthy tissue with 15- blade against rigid background

When motor-sensory topography unclear, use electrical stimulation

Nerve can be safely mobilised 1-2cm to allow increased length and decreased tension

Nerve gap options (without grafting)

• Neurotisation to target muscle​

• Mobilisation

• Transposition

• Bone shortening

Nerve grafting​

• ​Interpositional nerve grafts, nerve allografts, vein conduits or synthetic nerve conduits

Standard wound care

Splinting if required to avoid excessive postural positioning

Sensory re-education to begin as soon as patient able to perceive any type of sensory stimulus

Stages

• Desensitisation​

• Early phase discrimination and localisation

• Late phase discrimination and tactic gnosis