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1714 AJVR, Vol 65, No. 12, December 2004
Extracorporeal shock wave (ESW) treatment is a
newly adapted technique used to treat muscu-
loskeletal disorders in horses.
1
In several case series in
which ESWs were used, successful treatment of soft
tissue and bone disorders including proximal suspen-
sory desmitis,
2,3
dorsal metacarpal disease,
4
navicular
disease,
5
and osteoarthritis of the tarsometatarsal and
distal intertarsal joints was reported.
6
Although few
controlled studies have been performed and the specif-
ic mechanism by which shock waves affect tissues still
needs to be elucidated, ESW treatment has become
increasingly popular in equine practice.
Extracorporeal shock waves are pressure gradient
waves that have a rise time of 5 to 10 nanoseconds and
a peak pressure of up to 100 MPa, followed by a rapid
decrease to negative pressure, before a return to base-
line in a total pulse time of approximately 300
nanoseconds.
1,7
Wave energy is released at interfaces of
tissues that have different acoustic impedances and
results in compression and shear loads on the surface
of the material with the greater impedance. These loads
result in a process referred to as cavitation, which is
caused by the development and collapse of microscop-
ic gas bubbles in the interstitial fluid of tissues. When
gas bubbles develop near an acoustic boundary layer,
they invert asymmetrically and a small jet of fluid
impinges on the surface of the material with greater
impedance at a velocity of several hundred meters per
second, thereby generating high localized stresses.
7
Two fundamentally different techniques are used to
generate ESWs. Focused shock wave generators were
originally developed for noninvasive destruction of uri-
nary calculi in humans
8
and were subsequently adapted
to treat a variety of musculoskeletal disorders.
9,10
Focused shock wave generators initiate a pressure wave
within a fluid medium and focus the wave via reflection
within the generator toward a focal point in the
patient.
7
Nonfocused ESWs or radial pressure waves are
generated via mechanical concussion. They are charac-
terized by lower energies than those of focused shock
waves, a slower rise time, and a negative component
that is of the same magnitude as the positive compo-
nent. Their maximum wave energy is found at the
applicator-skin interface; this energy decreases rapidly
in proportion to the distance from the generator.
1
Treatment with focused and nonfocused ESWs can
induce analgesia. This analgesic effect is likely inde-
pendent of any other potential beneficial effects on tis-
Received December 3, 2003.
Accepted February 24, 2004.
From the Equine Health Studies Program, Departments of Veterinary
Clinical Sciences (Bolt, Burba, Hubert, Hosgood), Comparative
Biomedical Sciences (Strain, Henk), and Pathobiological Sciences
(Cho), School of Veterinary Medicine, Louisiana State University,
Baton Rouge, LA 70803.
This manuscript represents a portion of a thesis submitted by the
senior author to the Graduate School of the Louisiana State
University as partial fulfillment of the requirements for a Master of
Science degree.
Supported by the Louisiana State University Equine Health Studies
Program.
Presented in abstract form at the 30th Annual Conference of the
Veterinary Orthopedic Society, Steamboat Springs, Colo, February
2003.
The authors thank Olga Borkhsenious for technical assistance.
Address correspondence to Dr. Bolt.
Determination of functional and morphologic
changes in palmar digital nerves
after nonfocused extracorporeal
shock wave treatment in horses
David M. Bolt, Dr med vet, MS; Daniel J. Burba, DVM; Jeremy D. Hubert, BVSc, MS;
George M. Strain, PhD; Giselle L. Hosgood, BVSc, PhD; William G. Henk, PhD;
Doo-Youn Cho, DVM, PhD
Objective—To determine functional and morphologic
changes in palmar digital nerves after nonfocused
extracorporeal shock wave (ESW) treatment in hors-
es.
Animals—6 horses.
Procedures—The medial and lateral palmar digital
nerves of the left forelimb were treated with nonfo-
cused ESWs. The medial palmar digital nerve of the
right forelimb served as a nontreated control nerve.
At 3, 7, and 35 days after treatment, respectively, 2
horses each were anesthetized and nerves were sur-
gically exposed. Sensory nerve conduction velocities
(SNCVs) of treated and control nerves were recorded,
after which palmar digital neurectomies were per-
formed. Morphologic changes in nerves were
assessed via transmission electron microscopy.
Results—Significantly lower SNCV in treated medial
and lateral nerves, compared with control nerves, was
found 3 and 7 days after treatment. A significantly lower
SNCV was detected in treated medial but not lateral
nerves 35 days after treatment. Transmission electron
microscopy of treated nerves revealed disruption of the
myelin sheath with no evidence of damage to Schwann
cell bodies or axons, 3, 7, and 35 days after treatment.
Conclusions and Clinical Relevance—Nonfocused
ESW treatment of the metacarpophalangeal area result-
ed in lower SNCV in palmar digital nerves. This effect
likely contributes to the post-treatment analgesia
observed in horses and may result in altered peripheral
pain perception. Horses with preexisting lesions may be
at greater risk of sustaining catastrophic injuries when
exercised after treatment. (
Am J Vet Res
2004;65:1714–1718)
sue healing and is observed rapidly after treatment.
Results of studies in humans
9,10
and horses
2,4,6
reveal
abatement of various painful orthopedic conditions;
however, no concurrent radiographic changes are evi-
dent. Analgesia of an injured limb is of concern in an
equine athlete because it disables protective limiting
mechanisms and may place horses with preexisting
lesions at greater risk of sustaining catastrophic injury
when exercised with altered peripheral pain percep-
tion. This problem has been recognized by the equine
industry and has led to the development of regulations
concerning the use of ESW treatment prior to compe-
tition by racing jurisdictions and the Federation
Equine International.
1
Extracorporeal shock wave treatment does not
appear to be used to specifically treat peripheral nerves
in humans. In horses, nerves of the distal portion of the
limb are often treated directly to provide relief in
painful syndromes of the foot such as navicular dis-
ease.
5
To the authors’ knowledge, there are no reports
of the effects of ESW treatment on function and mor-
phologic features of peripheral nerves in horses.
The purposes of the study reported here were to
determine functional changes in palmar digital nerves
after treatment with nonfocused ESWs by use of sen-
sory nerve conduction velocity (SNCV) measure-
ments and describe morphologic changes in ESW-
treated nerve segments via transmission electron
microscopy (TEM). We hypothesized that a single
treatment with nonfocused ESWs applied directly over
a peripheral nerve would result in lower SNCV and
noticeable morphologic changes in the nerve.
Materials and Methods
Horses—The study was approved by the Institutional
Animal Care and Use Committee of Louisiana State
University. Six horses were used in the study. Two Quarter
Horses and 4 Thoroughbreds (4 geldings and 2 mares) with
mean ±SD (range) weight of 514 ±43.6 (464 to 584) kg and
mean ±SD (range) age of 13.8 ±4.47 (5 to 17) years were
evaluated. Prior to inclusion in the study, horses underwent
clinical examinations and were determined to be free of
lameness. Horses were selected from a pool of horses used for
research purposes. Individual housing was provided at a
nearby research facility, and all horses were fed a standard
pelleted diet and had free access to water.
Nonfocused ESW treatment—All horses were treated
with nonfocused ESWs on day 0 of the study. Prior to treat-
ment, horses were sedated with detomidine hydrochloride
(0.02 mg/kg, IV) and butorphanol tartrate (0.02 mg/kg, IV).
The hair on the palmar aspect of the pastern of both fore-
limbs was clipped, and after application of a coupling gel,
2,000 pulses were applied over the medial palmar digital
nerve of the left forelimb by use of a nonfocused shock wave
generator.
a
Shock waves were applied at a frequency of 240
pulses/min, at a machine pressure of 0.25 MPa, and by use of
an applicator head 15 mm in diameter. The same treatment
was then applied to the lateral palmar digital nerve of the left
forelimb. No treatment was applied to the right forelimb.
Horses were returned to their stalls, and daily hand-grazing
was allowed until subsequent experimental procedures.
Sensory nerve conduction velocity measurements—
Three groups of 2 horses each were evaluated 3, 7, and 35
days after ESW treatment, respectively. Horses were sedated
with xylazine hydrochloride (0.5 mg/kg, IV) and butor-
phanol tartrate (0.02 mg/kg, IV), and general anesthesia was
induced with ketamine hydrochloride (2 mg/kg, IV) and
diazepam (0.15 mg/kg, IV). Horses were positioned in right
lateral recumbency, and anesthesia was maintained with
isoflurane and oxygen in a semiclosed system. The metacar-
pophalangeal (pastern) area of both forelimbs were prepared
for surgery. A lateral abaxial incision 3 cm long was made
along the deep digital flexor tendon in the midpastern area of
the left forelimb. The lateral palmar digital nerve was care-
fully isolated. A second lateral abaxial 1-cm-long incision
was made at the level of the lateral proximal sesamoid bone,
and the lateral palmar digital nerve was isolated at this site.
A pair of sterile needle electrodes was placed directly into the
nerve at the distal incision site (stimulating electrodes) and
at the proximal incision site (recording electrodes). A ground
electrode was placed in the skin of the dorsal surface of the
pastern between the stimulating and recording electrodes.
11
The stimulating electrode was activated by a stimulator that
was a component of an electromyography system
b
(Figure 1).
The recording and ground electrodes were connected to the
preamplifier of an electromyographic recorder. The distance
between stimulating and recording electrodes was measured
by use of a sterile measuring tape, and the value was entered
into the system computer. Square wave stimuli at 1 Hz and of
100 microsecond’s duration were generated by the stimulator
at various voltage settings until a visible compound action
potential was recorded by the recording electrodes and dis-
played on the system monitor. Ten responses per nerve were
used to calculate mean values by the system computer, and
measurements in each nerve were performed in duplicate to
evaluate reproducibility. Sensory nerve conduction velocity
was calculated by the system computer, and the mean of
measurements for each nerve was recorded. Subsequently,
the medial palmar digital nerves of the left and right fore-
limbs were exposed in a similar manner through medial
abaxial incisions and SNCV was measured as described.
Palmar digital neurectomy—Immediately after SNCV
measurements, a 4-cm-long segment of each palmar digital
nerve was resected with a scalpel blade. Representative trans-
verse sections were placed in primary fixative (1.25% glu-
taraldehyde and 2% formaldehyde in 0.1M cacodylate buffer)
overnight for TEM. Incisions were closed with simple inter-
rupted skin sutures with 2-0 nylon suture material,
c
and ban-
dages were applied. After recovery from anesthesia, horses
were returned to their stalls. Bandages were applied until
suture removal 2 weeks after surgery. No surgical complica-
tions were observed.
TEM—Transverse sections (area, 3 mm
2
) of excised
nerve segments were washed twice in 0.1 mol/L sodium
cacodylate that contained 5% (wt/vol) sucrose for 15 minutes
AJVR, Vol 65, No. 12, December 2004 1715
Figure 1—Schematic representation of sites of positioning of
electrodes for measurement of sensory nerve conduction
velocity in palmar digital nerves of horses treated with non-
focused extracorporeal shock waves. S = Stimulating elec-
trode. R = Recording electrode. G = Ground electrode. EMG
= Electromyography system. STIM = Electromyographic
stimulator.
1716 AJVR, Vol 65, No. 12, December 2004
and postfixed in 1% OsO
4
in distilled water for 1 hour. After
several washes in distilled water, nerve sections were stained
with 0.5% (wt/vol) uranyl acetate in distilled water
overnight. Nerve sections were dehydrated in graded ethanol
solutions of increasing concentration (30%, 50%, 70%, 95%,
and 100%), infiltrated with embedding medium,
d
and poly-
merized. Polymerized blocks were cut into sections 0.1 µm
thick by use of an ultramicrotome. Sections were stained
with uranyl acetate and lead citrate and examined by use of a
transmission electron microscope.
e
The investigator (WGH)
was unaware of treatment status of each nerve.
Statistical analyses—Sensory nerve conduction veloci-
ties were considered continuous, and the proportional differ-
ence in SNCV for treated lateral and medial nerves, compared
with control nerves, within each horse, was analyzed by use of
a mixed-effect linear model that accounted for random vari-
ance of horse and limb, nested within time points. Ad hoc
comparisons were made within each nerve and across treated
nerves at each time point, maintaining type I error at 0.05. A
software program
f
was used for the analysis.
Results
SNCVs—Velocities in treated
and control nerves were deter-
mined for each horse (Table 1).
Careful surgical exploration and
identification of palmar digital
nerves prior to needle placement
resulted in reliable and repro-
ducible SNCV measurements. All
nerves treated with nonfocused
ESWs had significantly lower
SNCVs, compared with control
nerves, on days 3 and 7.
Significantly lower SNCVs in treat-
ed medial but not lateral nerves,
compared with control nerves,
were found on day 35. There was
no significant difference in SNCV
between treated medial and lateral
nerves on days 3 and 7.
TEM—Transmission electron
photomicrographs of transverse
sections of treated and control
nerve segments were examined
(Figure 2). Medium- to large- (5
to 15 µm) and small- (1 to 5 µm)
diameter myelinated axons were
observed. Nonmyelinated axons
(0.5 to 2 µm in diameter) were
surrounded by Schwann cells and
embedded in loose collagenous
endoneurium. In control nerves,
uniform concentric layers of
myelin in myelin sheaths were evi-
dent in medium- to large- and
small- diameter myelinated axons,
and no traumatic or inflam-
matory changes were observed.
Transmission electron photomi-
crographs of sections of treated
nerves 3, 7, and 35 days after ESW
treatment, revealed extensive sep-
aration and disruption of the lay-
Table 1—Mean (individual values) sensory nerve conduction
velocities (SNCV [m/s]) in palmar digital nerves of horses, 3, 7,
and 35 days after treatment with nonfocused extracorporeal
shock waves.
Day
Nerve 33773355
LL 32.45* 37.0* 51.95
(32.7, 32.2) (39.1, 34.9) (61.7, 42.2)
LM 32.15* 32.26* 37.9*
(34.9, 30.4) (34.9, 30.4) (37.4, 38.4)
RM 48.2 60.25 43.35
(49.9, 46.5) (61.1, 59.4) (44.7, 42.0)
*Significantly lower, compared with control (RM).
LL = Left lateral palmar digital nerve (treated). LM = Left
medial palmar digital nerve (treated). RM = Right medial pal-
mar digital nerve (control).
No significant (
P
< 0.05) differences in SNCV between LL
and LM nerves on days 3 and 7 were found.
Figure 2—Transmission electron photomicrographs of transverse sections of nontreated
palmar digital nerves (A)—and palmar digital nerves treated with nonfocused extracorpo-
real shock waves (B, C, and D) in horses. A—Medium- to large- (black arrowheads) and
small-diameter (white arrowheads) myelinated axons and numerous nonmyelinated axons
(black arrows) are evident. B—Left medial palmar digital nerve 3 days after treatment.
Notice the separation of layers of myelin in medium- to large-diameter myelinated axons.
No changes are evident in small diameter myelinated axons and nonmyelinated axons.
C—Left medial palmar digital nerve 7 days after treatment. Severe myelin sheath disrup-
tion is evident in medium to large diameter myelinated axons. No changes are evident in
small-diameter myelinated and nonmyelinated axons. D—Left medial palmar digital nerve
35 days after treatment. Medium- to large-diameter myelinated axons reveal similar
changes as in (C). No changes are evident in small-diameter myelinated axons and non-
myelinated axons. Uranyl acetate and lead citrate stain; bar = 10 µm.
ers of the myelin sheath in medium- to large- diame-
ter myelinated axons; however, no changes were
observed in small diameter myelinated and nonmyeli-
nated axons. No traumatic or inflammatory changes
were found in treated nerves.
Discussion
Our study revealed that a single treatment with
nonfocused ESWs over palmar digital nerves resulted
in significantly lower SNCVs, compared with control
nerves, and in separation and disruption of the layers
of the myelin sheath in medium- to large- diameter
myelinated axons. A significant proportional difference
in SNCV between ESW-treated nerves and control
nerves was observed 3 and 7 days after treatment;
however, treated medial, but not lateral, nerves had a
significantly lower SNCV, compared with control
nerves, 35 days after treatment. We believe that the
absence of electrophysiologic changes in treated lateral
nerves on day 35 is most likely attributable to lack of
precision during application of ESWs with the hand-
held applicator and that the effects of ESW application
on conduction in peripheral nerves are likely to last
beyond our follow-up period (35 days).
Results of nerve conduction studies closely par-
allel structural abnormalities of nerves and depend
on the type and degree of nerve damage. Partial
demyelination and traumatic damage of the myelin
sheath of myelinated axons result in impaired salta-
tory conduction because of an increase in internodal
capacitance and conductance.
12
More local current is
lost to charge the membrane capacitance and via
leakage through the internodal membrane before
reaching the next node of Ranvier; therefore, axons
with a damaged myelin sheath characteristically have
lower conduction velocity and temporal dispersion.
12
Impaired nerve conduction without actual structural
damage of the axonal cytoplasm is referred to as neu-
ropraxia and represents the mildest form of periph-
eral nerve injury.
12
Therefore, our electron micro-
scopic findings could explain the lower SNCV mea-
sured in ESW-treated nerves. The lack of electron
microscopic changes in nonmyelinated and small-
diameter myelinated axons in treated nerves was
consistent among horses; we speculate that these
structures are either too small or that they provide an
insufficient local change in impedance for ESWs to
exert their effects.
Treatment of lameness is a challenge for the
equine practitioner who wishes to provide pain
relief, reinstitute athletic use of the horse, and mini-
mize economic loss while operating within ethical
and regulatory constraints of modern competition.
This has led to a growing interest in alternative treat-
ments that appear to result in improved and acceler-
ated healing and a shorter convalescence period.
Extracorporeal shock wave treatment represents
such an alternative and appears to be gaining interest
and acceptance among veterinarians, trainers, and
owners as a treatment for selected orthopedic
injuries in horses.
Shock wave-induced stimulation of bone and soft
tissue healing is reported in humans
9,13
and other
species
1,14,15
; however, the exact effects of high- pressure
waves on tissues are not fully understood. Treatment
with focused and nonfocused ESWs also results in a
transient analgesic effect that is apparently indepen-
dent of any other beneficial effect on tissues.
1
The
mechanism of this analgesic effect is not known. One
investigator suggested 3 hypotheses to explain the
mechanism of shock wave-induced analgesia in
humans. The first hypothesis is that shock waves
induce cell damage; therefore, peripheral nociceptors
cannot build up a membrane potential sufficient to
transmit pain signals. The second hypothesis is that
nociceptors are overstimulated by shock waves and
emit high frequency impulses to peripheral nerve
fibers, which are suppressed by a gate control mecha-
nism. The third hypothesis is that shock wave-induced
pericellular free radicals induce the local release of
unknown pain-suppressing substances.
16
In humans, the analgesic effect after ESW treat-
ment appears to be bimodal. An immediate initial
decrease in pain that lasts 3 to 4 days is followed by a
recurrence of pain and then a second gradual decrease
in pain over the ensuing 3 to 4 weeks.
g
The initial effect
is attributed to the direct effects of shock waves on
nociceptors and impaired substance P synthesis,
whereas the second phase of pain relief is believed to
be the result of angiogenesis and tissue matrix remod-
eling associated with tissue healing.
g
A similar bimodal
analgesic response is observed in horses undergoing
shock wave treatment.
1,6
Musculoskeletal activity without full perception of
peripheral pain could potentially place equine athletes
with preexisting lesions at greater risk of sustaining
career-ending or life-threatening injuries that include
complete spiraling condylar fracture of the third
metacarpal or metatarsal bones after sustaining an
incomplete fracture, and breakdown injuries of the
suspensory apparatus. This risk has been recognized
by the equine industry and strict regulations concern-
ing the use of ESW treatment before competitions have
been issued. McClure and Merritt
1
recommend that
large nerves, blood vessels, and active growth plates be
avoided during ESW treatment in horses. Because of
the anatomic proximity of target tissues to these struc-
tures, ESW-induced trauma cannot always be avoided;
with respect to nerves, occasionally trauma is even
desired. In horses, the palmar digital nerves are target-
ed directly when ESWs are used to treat disorders of
the foot, such as navicular disease.
5
To the authors’
knowledge, ESW treatment is not used to specifically
treat peripheral nerves in humans.
We do not propose a definitive explanation for the
analgesic effect observed after ESW treatment in hors-
es; however, our results support the premise that ESWs
can cause damage to peripheral nerves that results in
slower conduction velocities and potentially impaired
perception of peripheral pain.
Morphologic and functional changes associated
with neuropraxia are reversible over time.
17
Repeated
application of ESWs, however, may cause more exten-
sive damage to exposed peripheral nerves and may
result in prolonged or permanent alterations in nerve
conduction. Renal damage after ESW treatment of
AJVR, Vol 65, No. 12, December 2004 1717
1718 AJVR, Vol 65, No. 12, December 2004
nephroliths in dogs is cumulative depending on dose
(voltage and number of shock waves) and frequency
(number of shock waves/s) of shock waves.
18,19
Results
of a study conducted in rabbits also revealed dose-
dependent morphologic damage in the gastrocnemius
tendon after application of ESWs.
20
We speculate that
application of a higher total dose of ESWs would result
in prolonged duration of impaired nerve conduction
and more extensive morphologic damage of treated
peripheral nerves. The duration of morphologic nerve
damage and impaired nerve conduction was not
assessed beyond 35 days in our study. Our results indi-
cate that further research would be necessary to assess
the effects of nonfocused ESWs on peripheral nerves
after a longer time period and after multiple treat-
ments.
Although the small sample size of our study limits
our ability to draw strong conclusions and despite the
fact that our findings do not provide a conclusive
explanation for the analgesic effect observed clinically,
we recommend cautious use of ESW treatment in
equine athletes before training or competition.
a
Swiss DolorClast Vet, EMS Electro Medical Systems, Nyon, Switzerland.
b
Cadwell Sierra EMG/EP, Cadwell Laboratories, Kennewick, Wash.
c
Ethicon, Johnson & Johnson, Somerville, NJ.
d
Poly/Bed 812, Polysciences Inc, Warrington, Pa.
e
Zeiss (LEO) EM-10C, Carl Zeiss GmbH, Oberkochen, Germany.
f
PROC MIXED, SAS version 8.0, SAS Institute Inc, Cary, NC.
g
Ogden JA, Ogden DA. Electrohydraulic SWT: bimodal response
(abstr), in Proceedings. 5th Cong Int Soc Musculoskeletal Shockwave
Ther 2002;21.
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