Lecture 5 BIOMECHANICS OF PERIPHERAL NERVE AND SPINAL NERVE ROOTS.ppt

2
BIOMECHANICS OF
PERIPHERAL NERVE
AND SPINAL NERVE
ROOTS

Nervous System
• The nervous system --- control center and
communications network
• Three broad roles:
 It senses changes in the body and in the external
environment
It interprets these changes
Responds to this interpretation by initiating action in the
form of muscle contraction or gland secretion

Division of Nervous
System
• Central nervous system:???
• Peripheral nervous system
• 12 pairs of cranial nerves
• 31 pairs of Spinal nerves
• Posterior (dorsal) root and an Anterior root, which unite to
form the spinal nerve in intervertebral foramen

• After exit from foramine:
• Dorsal rami, which innervate the muscle and skin
of the head, neck, and back.
• Generally larger and more important Ventral rami,
innervate the ventral and lateral parts of structures
as well as the upper and lower extremities

Biomechanical Behavior of Peripheral
Nerves
• External trauma to the extremities and nerve entrapment
• There may be changes in nerve structure and function if
mechanical trauma exceeds a certain degree.
• Common modes of nerve injury are stretching and
compression caused by rapid extension and crushing.

Stretching (Tensile) Injuries Of
Peripheral Nerves
• The maximal load that call be Sustained by the median and
ulnar nerves is in the range of 70 to 220 Newton (N) and 60 to
150 N, respectively.
• Initially: low load  significant elongation
• Elastic/linear region
• Disruption of endoneurial tubes & perineurium  rupture

• Elongation of the nerve under a very small load is followed by an
interval in which stress and elongation show a linear relationship
• As the limit of the linear region is approached, the nerve fibers
start to rupture inside the endoneurial tubes and inside the intact
perineurium.
• The perineurial sheaths rupture at approximately 25 to 30%
elongation
• Stretching, or tensile, injuries of peripheral nerves are usually
associated with severe accidents
• Partial or total functional loss of some or all of the nerves in the
upper extremity, and the consequent functional deficits

• The outcome depends on which tissue components of the nerves
are damaged as well as on the extent of the tissue injury.
• High-energy plexus injuries represent an extreme type of
stretching lesion caused by sudden violent trauma
• Suturing of the two ends under moderate
tension………………….. The moderate, gradual tension
applied to the nerve in these cases may stretch and angulate local
feeding vessels.
• Complete cessation of all blood flow in the nerve usually occurs
at approximately 15% elongation

Schematic representation of a peripheral nerve and its
blood supply at three stages during stretching.
Stage III: 15% elongation

Compression Injuries Of
Peripheral Nerves
• Compression of a nerve can induce symptoms such as
numbness, pain, and muscle weakness.
• Pressure level and mode of compression

Critical Pressure Levels
• At 30 mm Hg of local compression, functional changes may
occur in the nerve, and its viability may be jeopardized during
prolonged compression (4 to 6 hours) at this pressure level..
(impaired blood flow.)
• Corresponding pressure levels (approximately 32 mm Hg) were
recorded close to the median nerve in the carpal tunnel in
patients with carpal tunnel syndrome

• Changes in the axonal transport systems, and long-standing
compression may thus lead to depletion of axonally transported
proteins distal to the compression site
• Such blockage cause axon More susceptible to additional
compression distally the so-called double crush syndrome.
• Slightly higher pressure (80 mm Hg, for example) causes
complete cessation of Intraneural blood flow; the nerve in the
locally compressed segment becomes completely ischemic.
• Magnitude of the applied pressure and the severity of the induced
compression lesion appear to be correlated.

Mode of Pressure Application
• Mode of pressure application is also of major significance
• Direct compression of a nerve at 400 mm Hg by means of a
small inflatable cuff around the nerve induces a more severe
nerve injury than does indirect compression of the nerve at 1000
mm Hg via a tourniquet applied around the extremity.

Mechanical Aspects of Nerve
Compression
• Experiments on baboon's nerve by tourniqet compression.
• “Edge effect” that is a specific lesion was induced in the nerve
fibers at both edges of the compressed nerve segment
• Nodes of Ranvier were displaced toward the non compressed parts
of the nerve.

• Nerve fibers in center not effected.
• Large diameter nerve fibers were usually affected, but the thinner
fibers were spared.
• Compression lesion of a nerve first affects the large fibers carrying
motor function while the thin fibers carrying pain sensation) are
often preserved
• Intraneural blood vessels have also been shown to be injured at the
edges of the compressed segment

• Consequences of the pressure gradient= higher at the edges
• Effect of a given pressure depends on the way in which it is applied,
its magnitude and duration.
• Two basic types of pressure applications
• Uniform pressure applied around the entire circumference of a
longitudinal segment or a nerve or extremity.
• Radial pressure that is applied by the common pneumatic tourniquet.
Example : carpal tunnel syndrome

• 2nd
experiment= Nerve is compressed laterally
• When a nerve or extremity is placed between two parallel flat rigid
surfaces that moved toward each other squeezing the nerve or
extremity
• Example: sudden blow by a rigid object squeezes a nerve against
the surface of an underlying bone

Duration of Pressure Versus
Pressure Level
• Mechanical factors are relatively more important at higher than at
lower pressures.
• Ischemia plays a dominant role in longer duration compression.
• Time factors most imp………………….
• Compression at 400 mm Hg causes a much more severe nerve
injury after 2 hours than after 15 minutes. Indicate-------------
High pressure has to act for a certain period of time for injury
to occur.

Biomechanical Behavior of
Spinal Nerve Roots
• The nerve roots in the thecal sac lack epineurium and
perineurium, but under tensile loading they exhibit both elasticity
and tensile strength.
• Ultimate load for ventral spinal nerve roots from the thecal sac is
between 2 and 22 N
• For dorsal root is 5-33 N

• The values of ultimate load are approximately five times higher
for the foraminal segment of the spinal nerve roots than for the
intrathecal portion of the same nerve roots under tensile loading.
• Nerve roots in the spine are not static structures
• Capacity to glide
• Disc herniation and/or Foraminal stenosis. can thus impair the
gliding capacity of the nerve roots.

• Repeated "microstretching" injuries of the nerve roots even
during normal spinal movements
• Further tissue irritation in the nerve root components
• Cadaver experiments=It was found that straight leg raising moved
the nerve roots at the level of the Intervertebral foramina
approximately 2 to 5 mm.

• Symptoms induced by nerve root deformation in association with
disc herniation & spinal stenosis and resulting in radiating pain.
• Disc herniation, only one nerve root is usually compressed
• Contact pressure of approximately 400 mm Hg
• Sciatic pain is relieved after chemonucleolysis disc degeneration
progresses over time and the disc height thereby decreases

• Central spinal stenosis, the mechanics or nerve root
compression are completely different.
• The pressure is applied circumferentially
around the nerve roots in the Cauda Equina at a slow,
gradual rate
• Nerve roots centrally within the Cauda Equina differ
completely from the nerve roots located more laterally, close
to the discs

Lecture 5 BIOMECHANICS OF PERIPHERAL NERVE AND SPINAL NERVE ROOTS.ppt

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