Biomechanics of Nerve

Spinal Peripheral Nerves

Nerve fibers Connective tissueVascular structures

Mechanical Properties

Resist- compression- tension

Characteristics

Small sensory fibers – pain etc.

Large motor fibers

Intraneural Connective Tissue

Endoneuriumsurround each fiber - collagen & fibroblasts

Perineurium provides mechanical strength &

biomechanical barrier

Epineurium - outermost- protect fascicles, vascular supply

Intraneural Connective Tissue

Spinal Nerve Roots

devoid of:- epineurium - perineurium

Endoneurial Fluid

Interstitial fluidTrauma fluid pressure circulation conduction

Injuries

Tension 20 trauma / child birth

Compression 20 to trauma / insidious

Mechanisms vs. Tension Injuries

dependent on perineurium for elasticity

dependent primarily on epineurium

Rabbit Tibial Nerve

Elongation

Load

Rabbit Tibial Nerve

Initially: low load significant elongation

Elastic/linear region

disruption of endoneurial tubes & perineurium rupture

Peripheral Nerves

Maximal elongation at elastic limit – 20%

Complete structural failure at 30%

Injury stiffness elasticity

Tensile Injuries to Brachial Plexus

Tensile Injuries to Brachial Plexus

high speed MVAfall from heightdirect hit to superior shoulder or

lateral head (stinger)child birth

Nerve Regeneration

Nerve rupture before endoneurial tubes & perineurium

pathway for regeneration

1 mm/day

Compression Injuries

numbness, pain, muscle weakness

ischemia may be more important than actual compression

duration is important

Mechanisms of Compression Injuries

intermittent or constant low pressure intraneural edema fibrotic scar (common in spinal nerves)

proximal compression axonal transport distal nerve more susceptible to compression injuries (carpal tunnel)

Mechanisms of Compression Injuries

Large nerves (motor) affected more

Small nerves (sensory) spared

Factors influencing damage

Direct vs. indirect

Magnitude

Duration

Regeneration

Intact Schwann Cells

completely severed no guiding structures preserved no direction of regeneration loss of sensory and motor function

Regeneration

Effects of Aging

Diminished:Vibratory sensation2 point discriminationDTRsNCVs (50-70 m/s in young, 10 m/s

in 30-40)Touch sensation

Effects of Aging

Slightly pain threshold

motor function

Functional recovery is less satisfactory