Subsolar bruising is a common hoof problem encountered in equine practice that causes various degrees of lameness. Sometimes it is barely observable, presenting as a performance failure, other times it presents as intermittent, severe, unilateral or bilateral lameness. As a result, it can remain undiagnosed or be misdiagnosed as navicular syndrome or other problems.

WEIGHT BEARING

Many farriers and horse owners mistakenly believe that during locomotion the horses weight passes directly from the coffin bone down through the sole to the ground. Actually, the horse's principal area of weight bearing lies at the border of the sole and the hoof wall. Nature designed the hoof as a flexible structure that yields and dissipates concussion when it is under pressure from ground impact. The heels and frog normally strike the ground first followed by the quarters and toe. With weight bearing the frog expands and the bars spread while impact is transmitted down through the digital cushion. The sole of the foot alone is simply not equipped to accept continual impact of energy. Studies performed by Dr. Christopher Pollit, DVM, have demonstrated that the impact of a horse's weight against the ground surface during weight bearing is transmitted from the quarters and toe sole border to the hoof wall which compress in the direction of its horn tubules. The ground impact then passes from inside the hoof wall through the laminae to the periosteum that covers the coffin bone. The heel is spread by distortion of the wall, frog and digital cushion. The collateral ligaments and the deep flexor tendon stretch, causing the navicular bone to compress downward, which allows the coffin joint to yield under pressure exerted upon it by the short pastern bone. Because of the coffin joints ability to yield, P3 does not cause excessive pressure on the sole; thus, the sole supports little weight.

Why does sole bruising occur if we know that the horse's weight does not pass directly from P3 down to the sole? The structure of the sole is similar to the hoof wall: it is composed of an outer insensitive layer of horn and an inner layer of sensitive tissue. This sensitive layer called the corium or dermis (popularly called the "quick"), underlies the sole. The corium contains lamellae that interdigitate with the lamellae of the inner hoof wall layer (the stratum internum) of the wall and bars. The corium is dependent upon arteries that supply the dorsal surface of P3 and course medially under its margin for its blood supply. Most of the hoof's venous drainage transverses the solar corium in the soft tissue existing between the bottom of P3 and the horny sole. This area is so vascular that ischemic necrosis (tissue death from a lack of oxygenated blood) can result if too much pressure is applied directly to the sole. This rich area is subject to bleeding and bruising from trauma or other causes.

As a rule of thumb, one can generalize that the depth or thickness of a horse's sole is equal to the width of the tissue that is visible between the white line and the outer wall. Thus, thin walled horses tend to be thin soled.

A bruise is defined as tissue discoloration resulting from the rupture of blood vessels (in this case beneath the sole in the corium). These blood vessels rupture when excessive external pressure is placed on the sole-from mild repetitive trauma or from a single severe event. In rare cases, a hematoma may form beneath the sole with an accumulation of a large quantity of serum that is detectable on x-ray.

CAUSES OF SUBSOLAR BRUISING

Let's examine some traumatic, congenital and pathological conditions that cause sole bruising in horses. Generally, subsolar bruises will result when there is abnormal focal weight bearing on a certain portion of the sole. Single traumatic events such as the sudden landing on a sharp, hard object or continual exposure to hard surfaces such as asphalt, excessively dry or frozen ground, and some racetrack surfaces are sources of injury.

Congenital conditions can also contribute to subsolar bruising. Horses with excessively flat feet with thin soles are subject to bruising because of both solar concavity and less protection against ground strike, especially if they run unshod over hard terrain. Club-footed horses develop toe bruising due to a contracture or shortening of the deep flexor tendon, which causes the heels to elevate abnormally, impacting weight upon the toe.

Poor farrier trimming and shoeing practices are, unfortunately, another cause of sole bruising. Overzealous trimming of the quarters and heel along with excessive paring of the sole with a hoof knife can be the culprit. A horse trimmed out of medial/lateral balance may bruise on the higher hoof side that initially contacts the ground. A long toe- low heel configuration, where abnormally low hoof angles cause the horse to land toe first, can cause toe bruising from constant focal concussion to that area. The use of heel caulks can concentrate impact to a small caudal portion of the sole or at the angles of the hoof which can bruise. Toe grabs designed to gain traction can also cause bruising of the toe. Shoes that are fit too small and that do not extend properly under the heels reduce the total ground surface and place the impact further forward upon landing, once again resulting in toe bruising.

The poor practice of deliberate axial redirection of the inside branch of a horseshoe by a farrier who hopes to prevent shoe pulling can cause corn formation in the angles of the sole, especially the medial heel. Loose, displaced or improperly positioned shoes can cause excessive pressure on any area of the sole (rather than the wall) because the web of the shoe exerts too much pressure inside the white line.

Certain pathologic conditions such as laminitis cause tissue tearing without any external trauma. In this instance the coffin bone tip which has rotated downward places pressure on the circumflex artery lying beneath it, resulting in extensive toe bruising-a classic symptom of this condition. Chronic sole bruising can gradually develop into a condition called primary nonseptic pedal osteitis. Radiographic changes in this condition show bone demineralization occurring around the toe margin and irregular bone formation along the solar margins and dorsal surface of P3. Bone reabsorption seen in pedal osteitis is permanent unless the condition worsens from further unresolved bruising.

SYMPTOMS

Sole bruising in any location can result in varying degrees of lameness which is exacerbated when the horse is taken from a soft surface and exercised on a harder one. Digital pulse may be increased immediately following exercise. Unilateral and bilateral palmar digital nerve blocks will improve the gait of a lame sole-bruised horse. Hoof tester examination will elicit focal pain responses medially, laterally, in both sole angles or the toe. Pain is due to local inflammation and the accumulation of serum which increases pressure in the surrounding tissue. Discoloration of the sole can and often remains unapparent until several weeks following the injury when hemorrhages spread into the epidermis and finally reach the visible weight-bearing surface because the hoof has grown distally. Hemorrhage within the horn has a stippled pattern, reflecting the tubular and intertubular horn structure. Bruising that occurs from an injury to the coronary band differs from injury to the sole because it appears immediately as a linear wall hemorrhage that is perpendicular to the hoof growth direction. Acute sole bruises are normally not evident radiographically, but in rare cases when serum accumulation forms between P3 and the inner face of the external sole it will present as a fluid line on X-ray.

TREATMENT

Successful treatment of subsolar bruising is dependent upon identifying and eliminating the causative factor in combination with corrective shoeing and proper trimming. The majority of sole bruises will resolve spontaneously once the source of trauma is removed and the horse is rested to reduce weight bearing on the bruised site. Removing heel caulks and toe grabs may be necessary if they are contributing factors. Transferring weight bearing off the sole and onto the hoof wall can be employed by using rim pads or a deeply concaved shoe. Full pads with fillers such as Equithane or the use of Thoro'Bred Company's Honeycomb Pad (which contains air pockets) can be used to provide adequate protection for thin soles when needed. The use of silicone as a filler should be used with caution since too much silicone can cause sole bruising by over filling under the pad or uneven distribution if it is not spread correctly. Forging should not be prevented by using too small a shoe or bending the medial branch inward. Instead, a lighter front shoe combined with a heavier hind shoe that is squared at the toe or has a slightly extended heel will prevent overreaching and eliminate forging. Low-heeled horses may require lifts and backing up the toe from the dorsal wall (which moves the support plane in a palmar direction) and rockering or rolling the toe to enhance breakover. The "short- shod" horse needs its correct shoe size with a proper fit that extends the shoe branches under the buttress of the heels. Often proper trimming techniques alone that achieve medial/lateral balance, proper hoof angles, and level hooves are all that is needed. Light paring over the bruise will relieve pressure, but deep pairing is never indicated. Supportive bruises are treated like subsolar abscess and are drained and bandaged until a firm, dry layer of keratinized tissue has developed. Along with rest from exercising, phenylbutazone (bute) can be administered by the veterinarian to reduce inflammation and pain.

Horses presenting with long-term sole bruising or that have a history of chronic lameness or laminitis should be examined by x-ray in order to rule out pedal osteitis. Thin- soled, barefoot horses may benefit from being shod or trimmed so that the hoof wall protrudes 1/4" distal to the sole. For years, it has been a common practice to soak the feet of sole-bruised horses in epsom salts and warm water to decrease inflammation. Stephen O'Grady, DVM, of the Northern Virginia Equine Center in The Plains, Virginia, questions the therapeutic practice of foot soaking. He explains that "excessive moisture damages the hoof wall by softening it to the point where it can deteriorate and separate causing it to bend outward as the wall expands. The white line width increases, the sole drops, moving closer to the ground and further adding discomfort to a horse that is sole bruised." He explains that "In reality, chronic soaking may make the problem worse by softening the foot to the point where the sole provides even less protection to the sensitive corium above. Foot bruising is a common problem in show horses during the warmer months when they are frequently bathed and their feet are continually in water." Dr. O'Grady advocates correcting the cause of the bruising and systematically relieving inflammation with Butazolidin and vasodilators if necessary. He recommends placing the horse in a stall containing sawdust or shavings in an attempt to dry out the hoof. "Treatment of foot bruises should be directed toward toughening the hoof capsule," he explains. Topical applications of iodine and formalin in a 1:1 solution can be applied to the sole to toughen it.

Subsolar bruising is a common cause of lameness that should not be overlooked. Simple sole bruising carries an excellent prognosis once the cause is removed. Proper shoeing and trimming methods can prevent and eliminate a large percentage of sole bruising. It is important that farriers help the horseowner understand that numerous causes may need to be ruled out and that conformational, environmental and pathologic conditions may need to be treated.

References

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• DeBowes, R.M. Yovich J.V.: Penetrating Wounds, Abscesses, Gravel and Bruising. Vet. Clinic North. Am. Equine Practice 1989, 5:179-194.
• Lambert F., Jr. (1971). Some Observable Physical Principles of Shock Diffusion in the Horse's Hoof. Vet. Med. Animal Clinic. 66:601-604.
• Leach, D.H. (1980). The Structure and Function of the Equine Hoof Wall. Phd. thesis. University of Saskatchewan, Saskatoon, Saskatchewan.
• Reeves M.J., Yovich J.V., Turner A.S.: Miscellaneous Conditions of the Equine Foot. Vet. Clinic North Am. Equine Pract. 1989:5(1):221-242.
• Pratt, G.W., and O'Connor J.T.(1976). Force Plate Studies of Equine Biomechanics. Am. J. Vet. Res. 37:1251-1255.