WHILE Navicular disease is usually diagnosed only with obvious lameness, a number of clinical signs are evident to the careful observer 18 to 24 months before the actual onset of lameness. This is called “pre-navicular syndrome.”
The majority of horses in Great Britain that exhibit navicular syndrome signs share one factor, anterior-posterior hoof-pastern imbalance (broken-back axis), which may be complicated by medial-lateral hoof imbalance.
Under such conditions, greater weight is borne by the caudal parts of the foot rather than distributed over the entire ground surface of the wall and bars. The heels collapse under excessive pressure and become underrun, failing to support the foot. Once this collapse has begun, the angle of weight bearing becomes greater and greater.
The structures within the caudal aspect of the foot—lateral cartilages, digital cushion, navicular bone, deep flexor tendon, blood vessels and nerves—are all subjected to abnormal stresses and pressures. Passive venous congestion occurs within the foot, nutrition to the bone is impaired and if left unchecked, this condition will lead to arterial sclerosis and thrombosis with necrosis. Spur formation on the navicular bone may be noted in radiographs (see Figure 1).
Once the disease is producing structural changes in the bone, navicular suspensory ligaments or deep flexor tendon, it must be considered incurable.
The clinical evidence of 10 diagnosed cases of navicular disease in the mid 1980s was catalogued, along with the horses’ owners’ answers to questions on changes in the horses’ disposition and performance levels, to establish a control group.
The clinical evidence from group A was compared to the clinical evidence collected from a second group of 10 horses, whose symptoms were similar to that of group A. However, they could not be diagnosed as having navicular disease because of a lack of radiographic evidence.
Since the symptoms apparent in both groups could not be attributed to any other pathological condition, it seemed reasonable to assume the group B horses were in a pre-navicular condition.
Both groups were treated with corrective shoeing to restore correct foot and limb balance. The horses were also treated with drug therapy prescribed by the veterinarian in charge of the case.
The causes of pre-navicular syndrome fall into three areas: farriery, management and environment, and conformation.
1. Failure to recognize, achieve and maintain a correct hoof-pastern axis causes the angle of weight bearing to increase and leads to heel collapse.
2. The failure to achieve correct medial-lateral hoof balance may lead to coronary band shunting and undue strain on medial or lateral aspects of the navicular joint and navicular suspensory ligaments.
3. Application of the old-fashioned “frog pressure theory” meant lowering the heels to gain frog contact with the ground to maintain hoof expansion. In practice, the quarters over-expand and separate because of laminal shearing and the heels then contract.
4. Narrow-webbed concave steel sections fail to support the hoof wall and encourage fine nailing, which splits and weakens the wall while causing the foot to collapse.
5. British hunter-style close-fitted shoes with tapered heels are often fit too short, offering inadequate heel support and leading to collapse.
All of these factors lead to increased or uneven stresses within the caudal part of the hoof.
Management, Environment Causes
1. Excessive moisture through climatic conditions or poor management causes saturated, weak feet that squash under a horse’s weight.
2. Complications of sheared heels and general discomfort in the heel area may be aggravated by thrush.
3. Shoeing beyond the recommended 4 to 6 week program lets the toe grow too long. The heels of the shoe become buried in the foot, collapsing the heels and giving a lack of support to the caudal area of the foot.
4. Irregular exercise, such as sudden hard work alternated with periods of inactivity, disrupts lower limb circulation. Long periods of standing in stables causes passive venous congestion.
5. In obese horses, the feet carry more weight than they were designed to carry, especially young animals being overfed to satisfy show judges.
1. Long, sloping pasterns cause the line of weight distribution down the limb to fall behind the heels.
2. Medial-lateral imbalance strains the medial or lateral aspects of the navicular joint. This can cause sheared heels.
3. Disproportionate body weight to bone size and foot size, especially in Hanoverian and Thoroughbred crosses.
4. Shoulder lengths and angles that do not correspond with pastern angles reduce proper flexion and extension of the limb and increase the negative effects of concussion.
Diagnostic Clinical Signs
There are many signs or clues in a horse’s behavior, gait, feet and shoe wear. If interpreted correctly, these clues aid in a positive diagnosis of pre-navicular syndrome before lameness becomes apparent. With experience, a characteristic pattern can be detected.
1. The horse may become sour and nappy. It is in constant pain.
2. May show reluctance to work on one diagonal.
3. May be reluctant to work in small circles.
4. May be reluctant to canter on one lead.
5. May be reluctant to extend the stride.
6. May flatten over jumps or refuse to jump.
7. May be uncooperative while being shod.
8. May develop changes in manners not associated with front foot pain (such as head shaking).
Many of these symptoms are often interpreted as schooling problems and are treated as such.
The psychology of each horse has a very important influence on the progress of the disease and treatment. Working horses (such as hunters and polo ponies) adjust better to discomfort. They may come out of the stable lame but warm up with relative soundness, depending upon each individual’s courage.
Pet horses often seem to play on lameness and exploit it when encouraged by the owner. They come out of the stable lame and are immediately taken out of work and rested.
This is a vital factor, as treatment response is directly related to keeping the horse in regular daily work.
It is important to appreciate that many pre-navicular horses that are not “lame” are uncomfortable in both front feet and some degree of bilateral forelimb lameness is present.
When this is suspected, it can be tested by nerve blocking one foot, in which case lameness may become apparent in the opposite foot. Corrective shoeing of one foot may have the same result.
There is also much to learn from watching the horse while it’s standing or being shod.
Diagnosis Signs In Motion
1. Disinclination to extend into the anterior phase of the stride.
2. Characteristic mild lameness disappears as the horse warms up.
3. Unmistakable “punching” of the toe into the ground as the foot lands.
4. Dragging the front toe (squaring the toe of the shoe) or dragging all four feet.
5. Spidery action if the front limbs seem to follow no distinct flight pattern. This often gives the appearance of throwing all the weight back onto the hindquarters.
6. Hind feet advanced further under the body to relieve the front feet.
7. A reluctant attitude to work.
8. Shortening of the stride.
9. Adduction or abduction of one or both forelimbs in flight.
10. Brushing or interfering.
Diagnostic Signs At Rest
1. Constant shifting of weight from side to side (rocking).
2. Pointing one or both feet alternatively.
3. Reluctance to bear weight on one foot when the other is held.
4. Reluctance and obvious discomfort when one forelimb is held in an extended position, such as clinching when the weight is
borne on the heel area of the other foot.
5. Hind limbs advanced further under the body than normal and spread wider to enlarge the base of support.
6. Standing up off the heel area. The pastern becomes vertical (although the heels may not leave the ground) to relieve the caudal parts of the foot.
The farrier is in an ideal position to observe changes in a horse’s feet, far more so than the owner or veterinarian. Dramatic change can occur in the shape of a hoof in a 6-week interval while other changes can be long-term.
The front feet of navicular and pre-navicular horses fall into three distinct categories: flat, squashed feet (type A); upright, boxy feet (type B); and one foot of each type (type C).
Diagnostic evidence of front foot pain is often also apparent in the hind feet.
Type “A” Feet
1. Hoof-pastern axis is broken back, often to an extreme degree.
2. Hoof measures longer from quarter to quarter than from toe to heel.
3. Heels are collapsed, folded under and contracted.
4. Frog is bulbous and prominent, sometimes bruised.
5. Sole is flat, weak and easily bruised.
6. Toe is long and at a low angle.
7. White line at the toe is stretched and may exhibit signs of hemorrhage.
8. There may be signs of bruising in the heel area and seat of corn. On a white foot, this may be visible on the outer side of the hoof wall.
9. The bulbs of the heels become squashed out behind the foot. The distance from the last bearing point of the heel to a line dropped behind the bulbs may measure as much as 2 inches.
10. The heels may shear. Each half of the foot moves independently, because of a lack of support to the caudal parts of the foot.
11. Horn quality and rate of growth are poor.
12. The bars of the foot are non-existent, weak or distorted.
Type “B” Feet
1. The hoof to pastern axis is broken back, although this may only be evident in radiographs as the hoof angle is more upright than in type A feet.
2. The hoof contracts from the toe and quarter back.
3. The heels are spiky and contracted.
4. The frog is narrow, pinched and subject to thrush and atrophy.
5. The sole is concave, even vaulted.
6. Horn quality and growth appear to be normal (especially at the heels).
Type “C” Feet
Horses in this category develop this front foot disparity (one foot of type A and type B) over months or years. If discomfort is felt in one foot, the horse will favor it, throwing a greater portion of its weight into the opposite foot.
The supporting foot will be crushed under the added load while the favored foot adjusts to pain in the heel area by landing toe-first, keeping weight off the heels and creating a more upright hoof with deeper heels and concave sole.
Compensating for pain in the affected foot by developing an accommodating foot shape and redistributing its weight load leaves the horse apparently sound. This is termed compensatory lameness.
If the weight-bearing foot is not adequately supported, the heels will collapse, blood circulation to the caudal parts of the foot will be impaired and radiographic examination may indicate evidence of a more advanced navicular condition than in the lame foot.
If discomfort is felt in both front feet, the hind limbs will be advanced further than usual under the body (both at rest and in motion) to participate in weight bearing and to relieve the front feet.
Under this abnormal load, the hind feet appear to be bursting out of the shoes. The nails will break up the wall and you may have trouble nailing safely and keeping shoes tight. Under these conditions, toe-dragging of the hind feet may be significant.
If only one foot is affected as in type C horses, a definite diagonal factor will be noticeable. The contralateral hind foot is invariably longer and broader than the other hind foot. A difference of up to 1 inch in shoe size is common.
The sound diagonal carries the greater proportion of the body weight. The hind limb of the lame diagonal deviates inward and forward under the body and is effectively used as a prop or crutch to the painful front foot. As a result, medial-lateral imbalance of both hind feet and abnormal shoe wear will be evident.
The diagonal factor accounts for many symptoms attributed to schooling problems, proving the horse will adapt to pre-navicular discomfort by altering its balance and movement.
The interpretation of different patterns of wear on shoes can reveal a great deal about developing conditions in individual feet or
imbalance of the whole horse. The following are significant to the diagnosis of a pre-navicular or navicular condition.
Front Shoe Wear
1. Heavy wear at the toe portion while the heel portion looks new. The heads of the heel nails remain prominent.
2. Toe-dragging is caused by a shortened anterior phase of the stride which squares off the toe of the shoe. This is characteristic of navicular pain (see Figure 3).
3. Grooves work into the quarters of heels, caused by the overexpansion of a flat, collapsed foot.
Hind Shoe Wear
1. Heavier wear on the medial branch than the lateral branch, either on both hind feet or the hind foot of the weight-bearing diagonal (see Figure 3).
2. Excessively heavy wear of the hind shoes when accompanied by front shoes worn as in “Front Shoe Wear” symptom 1.
3. Toe-dragging (squaring the toe of the shoe) usually accompanies front shoe wear as in “Front Shoe Wear” symptoms 1 and 2. It’s significant in these cases, but should not be confused with concurrent hind limb lameness.
Successful treatment of navicular syndrome is dependent on suitable mechanical correction of the foot. Three main priorities in all cases must be restoration of correct hoof-pastern axis, restoration of correct medial-lateral balance and full support of the caudal parts of the foot through shoeing.
Trimming The Foot
The toe of the foot should be trimmed back to create a hoof angle consistent with the horse’s conformation, even to the extent of rasping away the toe and the stretched white line with very low-angled, collapsed feet.
Where the heels form an acute angle with the ground and are long, weak and folding under (or likely to collapse), they should be trimmed to provide a strong base of support when a correctly-fit shoe is applied. Shoeing overly weak heels will only crush them further, delaying the objective of growing a new, solid heel at a more upright angle.
The angle through the center of the pastern will be determined by the heel dressing. The pastern will take on a less upright conformation as the base of the support is lengthened.
Do not thin the sole on feet that require severe correction. For practical purposes, the hoof must not be lowered too far to provide as much protection as possible and to keep the horse comfortable at work.
The hoof and its internal parts will restructure to accommodate the new directions of stress and weight bearing once the point of breakover has been brought back and correct conformation regained.
The equine foot is more malleable than you think. Just as it will collapse and stretch under unfavorable conditions, it can be reshaped and restored to normal functions.
Once the feet have been trimmed to correct the balance and angulation of the lower limb and foot conformation, fit a flat, fullered concave shoe with generous cover and full length at the heels.
Heel length should extend back to where the frog is at its widest, generally about 1/8 inch longer than the last bearing point of the wall and boxed-back to keep the shoe from being pulled off.
From the quarters back, the shoe is fit slightly wider than the wall. The heels are supported on the center of the web of the shoe toencourage natural function of the caudal parts of the foot. Where contraction has occurred, fit the shoe to where the hoof should be to encourage the foot to regain its natural shape.
The heels of the shoe should be upright, as opposed to the tapered “hunter heel,” for maximum support.
When the wall at the toe has been drastically reshaped to influence the hoof-pastern axis, some modification should be made. The toe of the shoe should be thinned to facilitate breakover and the shoe fit unclipped (or with side clips) to avoid pressure on the laminae of the toe. Nail holes must be placed safely back in the quarters.
In extreme cases of collapsed feet (type A), it may be necessary to use an egg bar shoe fit directly under the bulbs of the heels (see Figure 4). Other types of bar shoes are useful in cases of sheared heels to stabilize the caudal parts of the foot and to restrict independent movement of the medial and lateral parts of the foot. In certain cases, a bar shoe is the only way to cover the heels and offer enough width to promote expansion.
A graduated heel “navicular shoe” can be useful when it’s used to mechanically restore the correct hoof-pastern angle and relieve acute discomfort in the navicular area. This principle can be combined in a bar shoe by selecting a thicker section, fire-welding the toe and graduating the height from the quarters.
It is imperative to extend the length of the shoe back directly under the line of weight bearing on the limb. Raised-heel shoes (specifically with type A feet) can compress and further weaken the heels.
Wedge pads temporarily alleviate the symptoms of pain in the heel area. But they eventually bury themselves in the heels and frog and can do great damage. Consequently, they have no real place in this type of corrective shoeing.
In group A (horses with diagnosed navicular disease), only one of the 10 horses failed to show any initial improvement with drug therapy and corrective shoeing.
Six horses were sound at the end of the trial. Five out of the six showed significant improvement in foot balance following corrective shoeing and one course of drug therapy. The sixth horse regained complete soundness after a second treatment with Isoxsuprine, but failed to show improvement in foot shape or balance.
In group B (those identified with symptoms of pre-navicular syndrome), only one horse did not improve after corrective shoeing. One horse improved dramatically following corrective shoeing but had to be destroyed because of colic. At the end of the trial, the remaining seven horses had shown improvement in performance and hoof balance.
With four horses in group A, the lameness reappeared when the shoeing program was extended past 5 or 6 weeks, or if the horses were stall-rested for any length of time.
Thrush was a problem with six of the 20 horses, producing pain in the heel area similar to navicular symptoms and complicating cases where the heels were sheared.
The results of this trial bear out the theory that egg bar shoes achieve good results in cases of type A feet. Flat or raised-heel shoes used in combination with Isoxsuprine produced better results with Anglo-Arab, Arab and pony types, which are not predisposed to the flat-footed type of conformation.
Although the size of the trial was limited, the results indicate the restoration of correct foot and limb balance by shoeing has a positive effect on the long-term prognosis of both pre-navicular syndrome and diagnosed navicular disease.