Heel Lameness - Is it Navicular Disease?
by Edward D. Voss, DVM
The term "Navicular Disease" has been a concern of horse owners for many
years. In fact, navicular disease has been determined as a cause of
lameness in horses as early as 1752 when the syndrome was originally
described. The term "Navicular Disease" itself is misleading. A "disease"
implies a specific cause and therefore specific therapy (e.g., Lyme Disease
caused by Borrelia buradorferi and treated with tetracyclines).
Therefore, the term "Navicular Disease" has been replaced by the more
accurate term, "Navicular Syndrome" which implies multiple causes and
therapeutic approaches based on the individual patient.
Navicular syndrome is one of several conditions leading to heel soreness
or lameness. In order to understand this syndrome as well as these other
conditions, a diagrammatical representation of the equine foot is shown
below. Pertinent anatomical features are labeled and the ticked
lines/arrows represent the "heel" region for this discussion.
observation of the horse's gait, application of hoof testers, palpation,
and specific nerve blocks, a diagnosis of heel lameness can be attained.
The nerve block most often used to localize pain to the heel area is the
palmar digital nerve block. This anesthetizes the back 1/3 of the foot
which includes the following structures:
the navicular bone,
the navicular bursa,
the deep digital flexor tendon below the pastern joint
the back of the coffin joint,
the back of the pastern joint,
the corium of bars,
the frog ñ sole,
the digital cushion,
and skin on back of pastern and heel bulbs.
Specific conditions causing heel lameness that can be alleviated by this
nerve block include - puncture wounds (nails, wire sticks) to the back 1/3
of the foot; fractures of the navicular bone, back of coffin bone or side
bones; corns; stone bruises; sheared heels; very low injuries to the deep
digital flexor tendon; navicular syndrome; and less often laminitis or
pedal osteitis. Most of the above conditions are acute causes of heel
lameness with the exception of navicular syndrome.
Navicular syndrome is usually a chronic bilateral foreleg lameness in
horses of all ages. It is most often seen in Quarter horses,
Thoroughbreds, and Warmblood breeds. Horses with this syndrome have a
choppy, shuffling type gait and tend to wear the toes of their feet leaving
the heels to grow longer. This leads to a smaller, upright, contracted
appearing foot. Lameness associated is most evident in the inside leg on
harder concussive surfaces with the horse lunged in a tight circle. The
exact cause for this syndrome is yet to be discovered although several
theories have been put forth. Horses with an upright conformation, small
feet, or that are improperly shod may be at higher risk since they transmit
more of the concussive forces through the navicular region as the navicular
bone bears much of the weight between pastern and coffin bone during weight
bearing. In doing this, the navicular bone is forced back against the deep
digital flexor tendon which is taut during this phase. This repetitive
pressure may result in damage and inflammation to the navicular bone
precipitating chronic lameness. Radiographs (x-rays) are useful to
evaluate the structural changes within the navicular bone but do not always
correlate with navicular syndrome associated lameness. Some horses may be
sound with large structural navicular changes whereas others may be
extremely lame with minimal radiographic changes. The most commonly seen
changes are enlarged blood vessel channels, "lollipop lesions", spurring,
tiny fractures off the navicular edge, cystic or lytic areas within the
bone, and erosion of the contact area between the navicular bone and deep
digital flexor tendon.
Since horses bear between 60 and 65% of their weight on their forelimbs,
foreleg lameness is a common source of unsoundness. Heel lameness can be
caused by several conditions most of which are acute with the exception of
navicular syndrome. Navicular syndrome is a multifactorial condition, not
a specific disease, and as such therapy must individualized to each
patient. Diagnosis is based on clinical findings (characteristic gait, hoof
tester response, specific nerve blocks, and palpation) and radiographic
evaluation supportive of navicular remodeling. Treatment is fourfold. The
mainstay is shoeing which is discussed by Kirk Adkins. Medical therapy
such as anti inflammatory/analgesics (bute) and vasodilators (isoxuprine)
may be used. Management plays a role, these horses need to be exercised
daily as prolonged rest tends to increase lameness. Lastly, surgical
intervention is used for refractory cases (palmar digital neurectomy)
which may allow use of the horse for approximately 1 to 3 years.
At the time this article was published (1994),
Edward Voss was a Large Animal Medicine Resident II at the Veterinary
Medicine Teaching Hospital, University of California, Davis.