Penetration Wounds of the Equine Foot
Stephen E. O’Grady, DVM, MRCVS
 

Penetration wounds of the equine foot can be classified as indirect or direct. Indirect penetration initially involves the sensitive laminae and solar corium. Most indirect penetration occurs through the sole-wall junction, commonly known as the white line (Figure 1). This vital area of defense lies between the sole and the hoof wall, and its stability depends upon adequate strength and thickness to protect it from trauma. Once the structure of the white line has been disrupted, bacteria enters and migrates to the subsolar tissues causing a localized abscess commonly known as a gravel. Debris (gravel) penetrates the white line and mechanically introduces bacteria into the circulatory system of the foot.

Direct wounds, on the other hand, are caused by various objects such as nails, sharp rocks, glass, etc. that puncture or are forced through the protective horn (sole or frog) to the underlying sensitive tissue. The site of penetration and vital underlying structures involved become major concerns when evaluating the severity of the injury. It is the opinion of the author that all injuries of this type be considered as potentially career or life threatening until proven otherwise.

Indirect penetration wounds
Conditions that cause mechanical breaks or weakness in the continuity of the white line are improper trimming leading to hoof imbalance (long toe-underrun heel syndrome, excessive toe length, heels too high)h oof wall separations (white line disease, seedy toe), aggressive removal of sole and chronic laminitis. Excessive moisture or dryness may also contribute to weakness in the white line. If left untreated, the subsolar abscess will follow the path of least resistance up the hoof wall and will form a draining tract at the coronet. Once a hoof abscess is present, the animal will show marked lameness, increased digital pulse, heat and one may see soft tissue swelling in the lower limb on the side of the infection. After the foot is cleaned and trimmed properly, visual examination will generally show that the white line is widened and contains small discreet fissures. These fissures are the result of the white line, being weakened due to trauma and becoming filled with debris and bacteria. The exact area of pain can be localized using hoof testers.

Establishing drainage is the most important aspect of therapy. Preferably, this is done at the onset of lameness before the gravel ruptures at the coronet. The offending fissure is opened on the hoof wall side of the white line using a 2 mm bone curette or other suitable probe. A small opening is sufficient to obtain proper drainage and care must be taken to avoid exposing solar corium, as it will invariably prolapse through the opening and create an ongoing source of pain. Drainage is enhanced by the application of an Animalintex® poultice for the first 48 hours. In most cases, this eliminates the need for continued foot soaking.

The hoof is kept bandaged with a suitable antiseptic until all drainage has ceased and the wound has closed. At this point, a small gauze plug is used to fill the opening and is held in place with super glue. This keeps the affected area clean and prevents the accumulation of debris within the wound. The shoe is then replaced.

Many times the painful tract can be located but drainage cannot be established at the white line. In this case, the infection has migrated under the sole away from the white line.

Under no circumstances should an opening be created in the adjacent sole. This only leads to a persistent, non-healing wound and increased susceptibility to bone infection. Instead, a small channel should be made on the hoof wall side of the white line in a vertical direction following the tract to the point where it courses inward. Drainage can be established here in a horizontal plane.

Delay in treatment promotes excessive soft tissue involvement and migration of the infective process through the coronary band. Coronary band rupture leaves a permanent scar. This scarring becomes significant for high speed performance animals and for breeding stock that are allowed to develop “dished” horn capsules due to long toes or any other hoof imbalance. The mechanical stress on the scar derived from speed or bent horn tubules may result in a full thickness toe crack or quarter crack.

Pain that persists after coronary band rupture or when drainage is established requires immediate veterinary attention with radiographs to rule out early bone infection. Most horses with subsolar abscess spontaneously recover within a few days of treatment; a few develop complications such as osteomyelitis which can become life threatening.

Prevention

Prevention is achieved through proper hoof care and centers around promoting a strong, solid white line which resists penetration by debris. Excessive toe length increases the bending force exerted on the toe, leading to a widening and weakening of the white line. This, along with toe cracks and hoof wall separations, is the most common cause of foot abscesses.

To prevent gravels it is important that the foot be trimmed in a manner that accentuates a strong healthy foot. A few basic principles can be used when trimming to create a strong foot and strengthen the white line. First, the bars of the foot are left untouched and the heels are trimmed back to the widest part of the frog, or as far back as possible. This allows a large amount of weight bearing to occur in the posterior portion of the foot and not the toe area. Sole is only removed adjacent to the white line to identify excess hoof wall to be removed. It is not necessary to concave the sole as this occurs naturally. The toe is then backed up from the dorsal surface (front) of the hoof wall to where the pigment change is seen (inner stratum medium). This assures that there is no excessive toe length. A good rule of thumb to use when trimming the foot is to leave the last few rubs on the bottom of the foot. When applying shoes, fitting the shoes hot may be helpful to seal the sole wall junction. The use of hoof hardeners (Keratix®) and bedding the horse on shavings or sawdust may be useful to harden the feet during extremely wet weather or when the horse is being washed frequently such as during horse shows. During dry weather, a hoof dressing such as a combination of cod liver oil and pine tar (mixed in a ratio of 3:1) painted on the entire foot may help to contain moisture.

Preventing indirect penetration is therefore dependent on providing adequate protection to the underlying sensitive structures. The hoof capsule has a natural ability to provide such protection and it is imperative that we strive to enhance these strong features through proper trimming. Excessive removal of protective horn is a common practice, as emphasis is often placed on eye appeal instead of functional strength.

Direct penetration wounds

Subsolar abscessation resulting from direct puncture wounds to the bottom of the foot by a sharp object is a common cause of acute lameness. Lameness varies with the degree of involvement, pain and complications. The final outcome is determined by the depth of penetration, size of the penetrating object, location of penetration, tissues involved and the duration of time before treatment is instituted. Any direct puncture wound requires immediate veterinary assistance. When a foreign object such as a nail, piece of wire or glass is identified, its location, depth and direction of penetration must be evaluated with the aid of radiographs. If a foreign body—especially a nail—is found in the foot, a radiograph should be taken with the object in place (Figure 2). This is especially important when dealing with puncture wounds to the frog because after the object is removed, the elastic nature of the frog seals up the wound much the same as a piece of rubber. Punctures along the sulcus of the frog are also well hidden and often go undetected.

It is commonly believed that a wound will not become infected if it bleeds after a foreign object is removed and antiseptic is applied. This is certainly not the case. Bacteria is carried into the deeper structures with the foreign body, seeds the area and when the wound seals upon removal of the object, drainage is prevented and abscessation occurs.

Puncture wounds in the area of the sole can lead to infections or fractures of the coffin bone, while puncture wounds that penetrate the frog or bar have a high probability of affecting other vital structures such as the deep digital flexor tendon and its sheath, the navicular bone, the navicular bursa and the coffin joint. For these reasons, the veterinarian must make a determination of the depth of injury within the first 48 hours. Conservative medical therapy consisting of soaking the foot and systemic antibiotics will invariably fail if these structures are involved unless sufficient surgical debridement is performed early to allow drainage of these tissues. Any delay in appropriate treatment only decreases the prognosis.

If the coffin bone is involved, any fractured or abnormal bone should be removed and drainage established. When vital structures over the frog area are involved, a surgical procedure known as a “street nail” surgery is performed. In this procedure, a window is cut around the puncture wound, all devitalized tissue is removed and drainage is established.

Most deep puncture wounds in or adjacent to the frog carry a grave prognosis. Successful treatment requires an early and accurate diagnosis coupled with aggressive therapy.

Figure 1:Ground surface of hoof

Figure 2: Lateral x-ray which reveals nail extending to navicular bursa