| The palmar digital nerve block desensitizes
the palmar one-third to one-half of the foot. Lameness
localized to this region accounts for more than one-third of
all chronic lameness in the horse. It must be understood that
a palmar digital nerve block simply localizes the source of
the pain the horse perceives to the back of the foot. It is
important to identify as specifically as possible, the
pathological and clinical findings. This in turn will help the
clinician make their best assessment of the problem, and
recommend the most specific treatment.
There are numerous
causes of pain in the palmar aspect of the foot of the horse.
These causes can be arbitrarily divided into conditions of the
hoof wall and horn producing tissues, conditions of the third
phalanx, and conditions of the podotrochlear region. Hoof
problems would include hoof wall defects, such as cracks or
clefts that involve the sensitive tissue; any laminar tearing,
separation or inflammation; contusions of the hoof causing
bruising or corn formation; abscess formation; and
pododermatitis (thrush or canker). Third phalanx lameness'
blocked out by palmar digital anesthesia would include wing
fractures, marginal fractures, solar fractures, or deep
digital flexor insertional tenopathy. Conditions of the
podotrochlear region have been reported to include distal
interphalangeal synovitis, deep digital flexor tendonitis,
desmitis of the impar (distal navicular ligament) or
collateral sesamoidean ligaments, navicular osteitis or
osteopathy, and vascular disease. The common denominator of
all these conditions is that they are characterized by pain
that can be localized to the caudal aspect of the hoof.
The first step in developing a logical approach to the
treatment of palmar hoof pain is accurate assessment of the
pain and careful evaluation of hoof structure that may
predispose to or cause the pain. Four diagnostic tests should
be performed: hoof tester examination, distal limb flexion,
hoof extension wedge test, and palmar hoof wedge test. A
positive response to any of these tests is important but a
negative response is equivocal and does not rule out any
problem. Hoof tester examination should begin with systematic
evaluation of the sole and then to the distal sesamoid (navicular)
region, which includes the collateral sulci to opposite hoof
wall, central sulcus to toe, and across the heels. A positive
response should be repeatable, and in the distal sesamoid
region the pain response should be uniform over those areas
and must be evaluated in relation to examination of the
remaining foot. That is, a positive response in the heels and
quarters of the sole would also be expected to cause a
positive response across the distal sesamoid region in the
same area of the foot. Percussion utilizing a small hammer can
also provide important information regarding pain in the hoof
wall or sole.
Distal limb flexion test may exacerbate lameness if any of
the three distal joints of the leg are affected by synovitis
or osteoarthritis. A positive response could also be expected
by any condition that causes induration of the tissues of the
foot. This has been shown to be positive in over 95% of horses
with navicular disease.
The hoof extension test is performed by elevating the toe
with a block while holding the opposite limb off the ground,
and then trotting the horse away after 60 seconds. The palmar
hoof wedge test is performed by placing the block under the
palmar two-thirds of the frog, and then forcing the horse to
stand on that foot. The horse is trotted away after 60
seconds. The test can be further modified so that the wedge
can be placed under either heel to determine if the pressure
there causes exacerbation of the lameness.
Typically, all these before mentioned lameness' will be
improved by at least 90% after perineural anesthesia of the
palmar digital nerves but it does not help differentiate these
lameness'. Anesthesia of the distal interphalangeal (DIP)
joint or the podotrochlear bursa are additional procedures
that provide information about palmar hoof pain. In a study
reported by Dyson, in 95% of the horses examined using DIP and
bursa anesthesia, significant new information about the pain
the horse exhibited was realized. The pain relief by
anesthesia of any of these three regions has been shown to
overlap. The DIP joint and podotrochlear bursa do not
communicate, and yet the results of injecting anesthetic into
these synovial cavities is similar. Both cavities have in
common the navicular bone, the impar ligament, and the
collateral sesamoidean ligament (proximal suspensory ligament
of the navicular bone). The neuroreceptors for the navicular
bone are in those 2 ligaments and they can be anesthetized
from either synovial cavity. Further, Bowker has showed that
the palmar digital nerve is in very close proximity to the
medial and lateral limits of the bursa and the nerve may be
anesthetized at this level whenever the bursa is injected.
Palmar heel pain can be divided into 2 groups, those horse
with navicular region pain and those with other sources of
Navicular region pain is diagnosed by each of the following
blocks having an equal effect; palmar digital nerve block, DIP
joint block, and navicular bursa block. It has also been noted
recently that injection of the podotrochlear bursa can be very
difficult and that it is quite easy to inject the DIP joint
instead. We have found that not only is radiographic control
necessary to successfully perform this block but that adding
contrast media to the anesthetic to prove the limits of the
block is also necessary.
This has lead to a new method of assessing navicular
pathology; by evaluating the cartilage of the flexor surface
of the navicular bone by contrast arthrography. In several
cases we have been able to conclusively prove the presence of
adhesions between the navicular bone and bursa. Radiographic
examination is the imaging method most often used to assess
osseous changes in the distal sesamoid bone and third phalanx.
These changes with the exception of fractures are usually not
pathognomonic but do provide insight into damage that has
occurred to the foot. Fractures also may not be
radiographically visible until 10 to 14 days after the injury
Scintigraphy is a technique that measures gamma ray
emission from a radioactive nuclide injected into the animal.
The technique provides information on relative vascularity and
rate of tissue metabolism. This is particularly useful in
studying bone pathology and can help differentiate sites of
injury in the foot.
Thermography provides information regarding skin
temperature. It has been shown to be useful in assessing the
relative blood flow to a region. This information is of
particular interest when pre- and post exercise temperatures
are determined. Exercise will normally cause a 0.5oC rise in
skin temperature. Whenever, the skin temperature does not
rise, poor blood flow should be considered a factor in the
disease being assessed.
Ultrasonography can be used to examine the podotrochlea. The
superficial horn must be pared from the frog to expose soft,
spongy frog tissue. Next, sonographic gel is liberally applied
to the frog. The ultrasound transducer is then applied to the
frog. Images of the podotrochlea are apparent from the center
of the frog to the apex. A 7.5 MHZ probe provides the best
image. Generally, at the center third of the frog, the flexor
surface of the navicular bone is readily noticeable as a hyper
echoic line. The bursa is seen as a hypo echoic (fluid filled)
region juxtaposed to the navicular bone. The deep flexor
tendon fibers can be seen curving around the bone. As the
probe is moved toward the apex of the frog, the distal aspect
of the navicular bone can be identified as can the
intersection between the deep flexor tendon and the impar
ligament. As the probe reaches the apex of the frog, the deep
flexor's insertion on the third phalanx becomes apparent.
Ultrasound is an excellent means to visualize soft tissue
structures. However, examination of the foot has been limited
to the pastern because the hoof capsule served as a barrier to
examination of the hoof. The proximal regions of the navicular
bone could be examined if one had a special probe that would
fit between the bulbs of the horse's heels. However, this gave
no information as to what may be occurring further distally.
The frog however, because of its high water content can serve
as the hoof's standoff. By removing the hard, outer layers
this exposes tissue that can transmit sound waves allowing the
examiner to see this distal tissues. To date, we have
performed only enough cases to have an appreciation for the
anatomy. Many more cases will need to be examined to determine
how useful this technique might be.
In addition to the distal podotrochlea, the collateral
ligaments of the distal interphalangeal joint can also be
evaluated. This is done by going through the coronary band
just dorsal to the collateral cartilages. The collateral
cartilages and portions of the digital cushion can also be
evaluated sonographically. A thorough examination of the horse
affected by pain in the palmar region of the foot can allow a
more precise diagnosis to be made, whether the diagnosis
reflects injury to the hoof capsule, third phalanx, or
podotrochlear region. Treatment then should be based on the
type of injury.
There are differences in the clinical presentation of
navicular region pain (NRP) and palmar heel pain (PHP). The
University of Minnesota has had an ongoing prospective study
of these findings. So far approximately 54% of the cases seen
are affected by NRP and 46% by other sources of PHP. Clinical
signs for these two groups have shown interesting differences.
Distal limb flexion has been positive in 100% of the NRP and
only 88% positive in horses in the PHP group. Hoof tester
examination, which is considered a cardinal sign of navicular
problems, was positive in only 54% of the horses with NRP as
compared to 65% for those with PHP. The frog wedge was
positive in 79% of the NRP as compared to 70% of the PHP
horses; whereas, the toe wedge was positive 64% in NRP and
only 43% in PHP. Circulatory testing indicated that only 26%
of the NRP horses had poor circulation as a component to their
disease, compared to 53% of the PHP horses having compromised
circulation. Scintigraphy was positive in only 62% of the NRP
cases indicating that pain can be present without
scintigraphic changes. Also 20% of the PHP horses have a
positive bone scan indicating that the navicular bone may be
involved in a complex problem of heel pain.
1. Turner TA: Diagnosis and treatment of navicular syndrome in
horses. Vet Clin NA Equine Pract. 5: 131-144, 1989.
2. Bowker RM, Rockershouser SJ, Linder K, et al: A
silver-impregnation and Immunocytochemical study of
innervation of the distal sesamoid bone and its suspensory
ligaments in the horse. Equine Vet J. 26: 212-219, 1994.
3. Turner TA: The use of hoof measurements for the objective
assessment of hoof balance. Proceedings of Am Assoc Eq Practnr,.
38: 389-396, 1992.
4. Stashak TS: Adams lameness in horses. 4th Ed. Philadelphia,
Lea & Febiger, 1987, pp 499-514.