Introduction to Navicular Disease
Lameness specialists and researchers of navicular disease have replaced the term "disease" with the term "syndrome". A syndrome is a commonly recurring group of symptoms of unknown cause. We are not sure why navicular syndrome (NS) occurs. The diagnosis of NS involves trying to rule out some of these better understood causes of lameness. The proximity of these structures and that they lie under a thick horn makes direct examination difficult, but diagnosis is still possible.
Current Theories
The navicular region is a complicated mix of joints, bones, ligaments, tendons, and synovial bursas. All of these structures can succumb to the known causes of lameness referable to this area. Some of the more common causes of lameness in this area are
- subsolar bruising in the frog region
- synovitis of the coffin/navicular joint area
- degeneration of the navicular bone and associated deep digital flexor tendinitis
- fractured navicular bone
This list however can usually be accurately diagnosed with a good exam and radiography. Occasionally you get to the bottom of your exam and radiographs and all you can find is two lame front feet that refers to the navicular region. These are the horses with NS.
One current theory focuses on ischemia as a cause of NS. Ischemia means low oxygen levels in the tissues. The navicular bone
(below, blue)
is a very metabolically active bone. The bone is constantly
remodeling to adjust to changing work loads. This remodeling requires
many nutrients, including oxygen. If the bone cannot get enough
oxygen, ischemia results. One of the results of ischemic tissues is
increased sensitivity to painful stimuli. The navicular bone hurts
because it cannot get enough oxygen. It is hypothesized, that if this
goes on long enough, degenerative changes occur, resulting in
permanently lame horses.
The results of recent work using special dyes and tagging techniques have yielded a second possible mechanism. The deep digital flexor tendon (yellow) and the navicular impar ligament (orange) insert together (red) on the back of the coffin bone. At maximal dorsoflexion of the foot during locomotion, just prior to the foot coming off the ground, shear stress is placed at this intersection of the tendon and ligament.
- deep digital flexor tendon = yellow
- navicular bone = blue
- impar ligament = orange
- intersection of ligament and tendon = red
Improper conditioning, certain anatomic features, and excessive weight may result in damage to the area. In this area there is a rich bed of vessels and nerves along with some of the peptides associated with pain (substance p). In a small group of horses with navicular syndrome each had changes at this intersection consistent with this theory.
Diagnosis
The diagnosis rests on history and physical signs.
Since there is no proven cause nor diagnostic radiographic changes, a diagnosis is based on:
- verifying the navicular region as the area that is painful
- and ruling out known causes of lameness there.
History
A history of a slow onset of front limb lameness that initially comes and goes is a
frequent finding. As the disease worsens the horse develops a short,
choppy gait that may have no or only a very mild head bob at the
trot.
Physical Exam Findings
The horse is much worse over hard, irregular footing like
gravel. Frequently, in the turns at a trot, the horse will develop a
head bob associated with the inside foot: He is lame on one foot
going one way, and the other foot going the other way. Lameness of
both forefeet is an important diagnostic feature. Occasionally, one
limb is significantly more lame than the other and nerve blocks are
required to show the bilateral nature of NS.
Diagnostic tests for NS center around stressing the navicular
region and seeing if this aggravates the lameness. Hoof testers over
the center third of the frog frequently elicits pain. Elevating the
toe by having the horse stand with his toe on the handle of a hoof
knife and the heel on the ground, creates stress in the deep digital flexor, navicular bursa, and
navicular bone and should exacerbate the lameness. Still another test
done is by having a horse stand on the handle of a hoof knife
arranged to where the knife puts pressure over the central and back
portion of the frog. The opposite foot is lifted up to increase
pressure on the frog. Again, temporary increase in lameness is
supportive of this disease. Many authors say that lack of response
(exacerbated lameness) to these stress tests does not mean NS is not
present. I think lack of response to the hoof testers over the frog
makes it very risky to say that the lameness is coming from this
area without further evidence. If there is no response to hoof testers then nerve and articular blocks become critical to the localization.
Nerve and Articular Blocks
A critical test for NS is the posterior digital nerve block. All
of the nerves that run to the navicular region branch off the
posterior digital nerve (see graphic below). Blocking the nerve at the level of the
lateral cartilages should eradicate lameness if from the navicular
area. Note well that all of the structures in the navicular region
and the heel will be blocked. This makes a thorough exam, including
hoof testers, before the block extremely important. Not only does the
PDN block help in localizing the lameness, but another important
piece of information is obtained: with the obviously lame foot
blocked, it allows you to see if lameness is present in the opposite
foot. It should be investigated in the same manner to assure that the
soreness originates from the navicular region.
Recently It has been verified that the nerve supply to the navicular bone is derived from the proximal suspensory ligament of the n. bone and the distal impar ligament. The significance of this is that lameness caused by navicular disease would be alleviated by intra-articular analgesia of the distal interphalangeal joint (coffin joint). This makes further characterization of this disease possible by combining posterior digital nerve blocks(PDNb) and coffin joint blocks:
- Sound on PDN but not coffin joint block .....foot pain that is not coffin joint or navicular related
- Sound on PDN and coffin joint block......foot pain referable to the coffin joint and/or navicular area
- No or little response to low PDN but sound on coffin joint block.....coffin joint pain. (Caution: The PDN block may diffuse up the nerve somewhat resulting in significant loss of sensation in the coffin joint. Interpret in light of the location, amount required, and rapidness of response.)
- No response to PDN or coffin jt block......pain is higher than the hoof.
Horses that are not bilaterally lame with soreness and do not
completely block out with a PDN block should not be diagnosed with
NS.
Traditionally, radiographs were the cornerstone of diagnosing NS.
In 1994 at the American Association of Equine Practitioners it was
stated by lameness researcher Tracy Turner, DVM, that there is little
correlation between radiograph changes of the navicular bone and NS.
This has been the belief in Europe for years and confirms my personal
experience. I have seen horses with radiographically terrible
navicular bones with no history of lameness and several cases that
fit all the signs discussed above but have perfect navicular bones on
radiographs. Radiographs are important, not to diagnose NS, but to
rule out other problems of the foot. addendum: In 1995 a study by Dr.
David Ramey, DVM, of 85 horses showed no correlation between
radiographs and navicular syndrome.
Treatment and Prognosis
There is no treatment that is always successful. Generally a
combination of proper shoeing, phenylbutazone, and isoxsuprine will
allow approximately 2/3 of the horses to return to use if the disease
is treated early. Another important finding is the localization of pain mediating neuropeptides. Not only do these chemicals mediate pain, promote vasodilation, and even the inflammatory process. That is these chemicals may cause the pain and inflammation. It would seem possible that if we can develop ways to reduce or block these peptides we might be able to effectively treat navicular syndrome.
Trimming and Shoeing
Many arguments surround what is good trimming for this problem. As
a rule it makes the most sense to correct any imbalances in the foot
and trim the foot to properly align the three bones making up the toe
and pastern, then shorten the toe as much as possible. Toe and quarter rockers make sense also.
Many navicular horses will respond to good shoeing. Some stay usably sound following the farriers visit for four weeks, only
to have the gait begin to shorten until retrimmed and shod. If this
fails, try elevating the heels 3 degrees, rocker and roll the toe, and a set
of egg bar shoes that fit full particularly in the heel. Another
option is to try pads and bar shoes that protect the middle third of
the frog.
Isoxsuprine
In one study isoxsuprine was shown to be helpful in the treatment of early NS. This has not been my experience and more recent work is having trouble finding a pharmacological response at published doses. On the other hand the medication is safe and low cost. Along with
proper shoeing, isoxsuprine has an overall success rate of over 60%.
Proper shoeing alone has a success rate of only 30%. This drug works
by increasing the circulation to the bone, so that it may repair and
remodel its shape to adapt to changing stress. Dosage should start at
500 mg./1000 lbs. twice daily. The dosage is then adjusted every two
weeks depending on response. If sound, the horse is reduced to once
daily dosing and then weaned off the drug over the next 45 days.
Remember that isoxsuprine is a forbidden substance by the American
Horse Shows Association.
Neurectomy
Though the last resort of horses with NS is neurectomy, neurectomy is very effective and has a low complication rate when done in a hospital setting under general anesthesia . The nerves (yellow) to the navicular region are severed at the level of the heavy black line.
As large a segment as is possible is removed as are all obvious branches. Though complication rate is low some are serious and often the nerves regrow in two or three years.
[digital nerve (yellow), d. artery (red), d. vein (violet), ligaments orange]
Recent improvements in surgical
procedures may increase the time before regrowth. It has been recommended that two incisions be made, above and below the extensor ligament (orange). The nerve is transected in both locations, heavy and light line, and a section removed.
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