Peripheral Nerve Injury is an injury to peripheral nerves which may result in loss of motor function, sensory function, or both.
Anatomy of Peripheral Nerve
Epineurium: Connective tissue layers of the peripheral nerve which both muscle and runs between fascicles
Functions: Nourish and protects fascicles
Within and through epineurium:- Lie neutral fasciclesàeach surrounded by the peripheral sheath
Perineural layer:- Major contributor to nerve tensile strength
Endoneurium:- Innermost loose collagen matrix within the fascicles
Axon runs through the endoneurium and is protected and nourished by this layer.
In nerve repair, fascicular matching is critical to the outcome
Blood Supply:
2 Major blood supply
- 1st lies superficial to nerve
- 2nd lies in interfascicular epineurium
1 Longitudinal system within endoneurium/perineurium
Segmental vascular supply is number of nutrient arteries that enter the nerve at the regular intervals
Endoneurium capillaries: functions as an extension of Blood-Brain Barriers (BBB)
Injury classification
Physiology of nerve degeneration
Axonal transaction
Cell body
- Swells
- Chromatolysis
- Nissl granules(basophilic neurotransmitters): Disperse and become eosinophilic
- Change of priority form
- Production of neurotransmitters to production of structural material for nerve regeneration (mRNAs, lipids, actins, tubulins, GPs)
Axons
Proximal Axons:
- Traumatic degenerations at the zone of injury ( up to node of Ranvier) + Death of cell body can also occur
Distal Axons:
- Wallerian degenerations initiated (48 -96 hr)
- Myelin deterioration starts
- Axon-disorganized
- Schwann cell will proliferate and Phagocytes myelin +Axonal debris
Physiology of nerve regeneration
After Wallerian degeneration, Schwann cell of basal lamina persist creating a column of cells–>Bungner Band
Bungners band provides a supportive and growth-promoting environment
Tip of regenerating axons: Filopodia (finger-like projections) release protease (path to clear way to target organs and explore microenvironment)
Growth cone response to
- Neurotrophic factors
- Neurite-promoting factors
- Matrix forming precursors
- Metabolic and other factors
Distal Reinnervations:
- Bone develop osteoporosis
- Joint and soft will become fibrotic and stiff
- Muscle ( atrophy / interstitial fibrosis) which is viable for 2 years
Neurorrhaphy
Historically wait for 3 weeks (conclusion of Wallerian degeneration)
Recently: Primary repair is associated with better functional results
Pre-requisites for primary repair
- Clean wound
- Good Vascular Supply
- No crush components
- Adequate soft tissue coverage
Skeletal Stability with minimal tension in the nerve
Techniques
- Group Fascicular repair
- Disadvantages of scarring and damage to blood supply due to additional dissection
- Epi-neural Repair
Both functional outcomes are similar
Monofilament—Nylon (8-0) is used
Identification ( difference between motor and sensory)
- Blue-SAb staining
- Carbonic Anhydrase Staning
- Cholinesterase staining
The sensory nerve is identified
Stain persists 35 days in the proximal stump, 9 days in the distal stump
According to time
- Primary Repair: within 6 – 8 hr of injury
- Delayed Primary Repair: between – 18 days of injury
- Secondary Repair: After 18 days
Nerve grafting
Autograft (standard)
Cable:- multiple small caliper nerve grafts aligned in parallel to span a gap between the fascicular gap
Trunk:- Mixed motor-sensory whole nerve graft (poor functional result)
Vascular Nerve Graft
Common source:- Sural nerve
For most cases cable grafting ( easily obtainable, appropriate diameter, relatively dispensable)
Other Nerves:
Medial antebrachial cutaneous nerve
Lateral femoral cutaneous nerve
Superficial radial sensory nerve
Principle of treatment
Closed low energy injuries: Recovered Spontaneously
If
1. Nerve is seen to be divided and needs repair
2. Sharp cut type injury
3. Delayed in recovery and diagnosis in doubt
↓
NERVE EXPLORATION
The divided nerve should be repaired as soon as possible (nerve ends are not retracted much, relative rotations are undisturbed, no fibrosis)
↓
PRIMARY REPAIR
If closed injury left alone shows no sign of recovery
Patients present late or missed diagnosis
Failure of primary repair
↓
DELAYED PRIMARY REPAIR
- Nerve ends slightly thickened/ soft: No resection
- Neuroma with no conduction on stimulation: Should be resected (nerve must be sutured without tension)
↓
Tension-free gaps:
- 2 cm in the median nerve
- 4-5 cm in ulnar nerve
- 6-8 cm in sciatic nerve
- 1-2 cm in other nerves
Can be kept in limb in relaxing position
↓
If not:
NERVE GUIDES
Collagen tubes, nano-engineered tubes, veins, silicon
↓
If needs to bridge larger gaps
↓
NERVE GRAFTING
Sural neve can be used to bridge up to 40 cm
In smaller diameter: cable grafting
↓
In very proximal injury, root avulsions, distance for graft in long
↓
NERVE TRANSFER
↓
TENDON TRANSFER
↓
BONY RECONSTRUCTION
Principle of Nerve Transfer:
- Sharply transecting the injured nerve ends to excise the zone of injury
- Nerve ends should display a good fascicular pattern
- The defect is measured: Appropriate length of graft is harvested for reconstruction with/out tension
- The graft is matched to corresponding fascicles and sutured to the injured nerve with an epineural suture
- If the injured Nerve has a large diameter relative to the nerve graft, several cable grafts are placed in parallel to reconstruct the nerve
- Fibrin glues can be used for cable graft (minimizing scar/hematoma)
- Placement of graft in reverse orientation in repair site (↓es chances of axonal dispersion through distal nerve branches)
- Well Vascularized Bed
- The graft should be 10%-20% longer than the gap to be filled, as it shortened with connective tissue fibrosis
- Limb immobilized (4 wks):- Graft repair site and graft itself regain some tensile strength.
See also: Nerve Injury Short Cases
See also: Brachial Plexus Injuries
