Nerve Adaptation in Response to
Mechanical Loading
Marko Bodor MD
Physical Medicine and Rehabilitation,
Spine and Sports Medicine
Napa, California
Voluntary Assistant Professor
Departments of Neurological Surgery
University of California San Francisco
Physical Medicine and Rehabilitation
University of California Davis
Disclosure
No disclosures relative to the current
presentation.
Skin Skin adapts to mechanical loading by
getting thicker.
Muscle Muscles adapt to mechanical loading by
getting larger.
Bone
From Ireland A et al
Med Sci Sports 2013
Bones adapt to mechanical loading by
increasing in thickness and diameter.
Nerve
Do nerves adapt to mechanical loading?
Tires
How is it that a 2000 kg car has tires that
need only 2.5 ATM pressure while those of
an 80 kg bicycle/rider require 5 ATM?
Larger tires allow lower tire pressures. 1) 2.5 ATM ≈ 2.5 kg force/cm2.
2) Contact area = 20 x 25 cm
= 500 cm2.
3) 2.5kg/cm2 x 500 cm2 =
1250 kg.
4) 4 tires x 1250 kg/tire = 5000
kg support for the car.
1) 5 ATM ≈ 5 kg force/cm2
2) Contact area = 3 x 5 cm =
15 cm2.
3) 5 kg/cm2 x 15 cm2 = 75
kg.
4) 2 tires x 75 kg/tire = 150
kg support for bike & rider
Tire‐Nerve Analogy
Larger nerves will have lower intraneural
pressure for the same externally applied load.
100 gm load on a 5 mm2 nerve = 20 gm/mm2 ≈ 200 kPa
100 gm load on a 10 mm2 nerve = 10 gm/mm2 ≈ 100 kPa
Evidence for Nerve Enlargement #1
Gitlin G Concerning the gangliform enlargement
(pseudoganglion) on the nerve to the teres minor
muscle. J Anat 1957.
Nerve enlargement observed in 26/36
normal cadavers at quadrangular space
Gitlin G Concerning the gangliform enlargement
(pseudoganglion) on the nerve to the teres minor
muscle. J Anat 1957.
Evidence for Nerve Enlargement #2
Ulnar nerve
enlargement
seen at medial
epicondyle (ME)
in 12/12
cadavers:
P6
ME
D4
Neary D, Ochoa J, Gilliat RW. Subclinical
entrapment neuropathy in man. J Neurol Sci 1975
Evidence for Nerve Enlargement #2 7/12 – no
pathologic
changes
5/12 – bulbous
swellings and
intercalated
segments.
P6
ME
D4
Neary D, Ochoa J, Gilliat RW. Subclinical
entrapment neuropathy in man. J Neurol Sci 1975
Evidence for Nerve Enlargement #3
Median nerve
enlargement
seen at flexor
retinaculum in
12/12 cadavers:
RB
P6
RB
RB
ME
Renaut bodies (RB)
seen in all nerves at
level of retinaculum
D4
Neary D, Ochoa J, Gilliat RW. Subclinical
entrapment neuropathy in man. J Neurol Sci 1975
Evidence for Nerve Enlargement #3
7/12 - no
pathologic
changes
RB
P6
RB
5/12 – bulbous
swellings and
intercalated
segments
RB
ME
D4
Neary D, Ochoa J, Gilliat RW. Subclinical
entrapment neuropathy in man. J Neurol Sci 1975
Evidence for Nerve Enlargement #4 Wrist-forearm ratio:
1.6 in 89 controls
2.4 in 161 patients
with CTS
P6
ME
D4
Visser LH, Smidt MH, Lee ML. Diagnostic value of
wrist-forearm ratio in carpal tunnel syndrome
Evidence for Nerve Enlargement #4 Therefore:
It is normal for
median nerves to be
enlarged up to 1.6x
at the wrist.
P6
ME
D4
Visser LH, Smidt MH, Lee ML. Diagnostic value of
wrist-forearm ratio in carpal tunnel syndrome
Evidence for Nerve Enlargement #5 Median Palmar Cutaneous Nerve enlargement
between the wrist and palm - Case #1:
P6
D4
Wrist - 0.6 mm2
Palm – 1.8 mm2
Bodor M, McAuliffe M, Derrington S, Uribe Y. – preliminary
unpublished data
Evidence for Nerve Enlargement #5 Median Palmar Cutaneous Nerve enlargement
between the wrist and palm - Case #2:
P6
Wrist - 0.9 mm2
D4
Palm – 2.4 mm2
Bodor M, McAuliffe M, Derrington S, Uribe Y. Preliminary
unpublished data
Evidence for Nerve Enlargement #5 Limitations:
-
P6
Two cases shown were weight-lifters
- predisposing
to MPCN enlargement at the palm.
Presence of enlargement has been inconsistent
among other subjects.
MPC has a variable course and may enlarge slightly
next to FCR tendon in wrist.
D4
Bodor M, McAuliffe M, Derrington S, Uribe Y. Preliminary
unpublished data.
P6
Is there a point at which nerve enlargement becomes maladaptive?
D4
Median Nerve DML vs CSA at Carpal Tunnel among Controls (□) and Patients (○)
P6
ME
Adapted from Fujimoto K et al. Diagnosis of severe
carpal tunnel syndrome using electrodiagnosis and
sonography. Ultrasound Med Biol 2015 Oct.
Ulnar Nerve NCV vs. Diameter among UNE Patients
P6
D4
Adapted from Beekman R et al. Clinical, electrodiagnostic
and sonographic studies at the elbow. Muscle & Nerve
2004.
Which graph shows a better correlation between Nerve Size and Velocity?
P6
ME
Median (Fujimoto 2015)
Ulnar (Beekman 2004)
Note Difference in Population and Parameters!
CSA and DML for Median, Diameter and NCV for Ulnar.
Which graph shows a better correlation between Nerve Size and Velocity?
1/NCV
P6
ME
Median (Fujimoto 2015)
Ulnar (Beekman 2004)
Note Difference in Population and Parameters!
CSA and DML for Median, Diameter and NCV for Ulnar.
Conclusions:
Nerve enlargement can occur in the absence
disease at locations of mechanical stress
susceptible to compression neuropathies.
Nerve enlargement can be a normal adaptive
response which reduces intraneural pressure.
Nerve enlargement within the confines of a bony
space such as the carpal tunnel or spinal canal
becomes maladaptive after a certain point.
Disclaimer: Not all enlargement is adaptive - it
can be secondary to disease.
Nerve Adaptation - Flattening
6
N
6
N
Nerve Adaptation - Flattening
6
N
3
2
6
N
3
2
3
2
3
1.5
2
1.5
1.5
1.5
2
2
Lateral Femoral Cutaneous
Nerve
Sciatic Nerve
Median compression at proximal retinaculum with severe flattening of nerve.
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