Page 142 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

Musculoskeletal system: 1.3 The foot 117

VetBooks.ir To emphasise the seriousness of such injuries, one is highly suggestive of septic navicular bursitis.
Information about the length and direction of the
study found that only 50% of horses with a puncture
wound to the frog became sound eventually and 35%
that vital structures become involved at a depth of
of horses with a puncture wound to the frog were penetrating object is useful as it has been reported
euthanased. In contrast, 95% of horses with solar 15 mm deep to the frog. Swelling between the heel
puncture wounds outside the frog became sound. To bulbs and the ungual cartilages, pain on flexion of
enter the navicular bursa, a penetrating object must the digit and a marked withdrawal response follow-
invariably pass through the DDFT, causing a septic ing application of hoof testers over the frog add fur-
tendinitis to develop concurrently. The penetrating ther to the index of suspicion. Sometimes the entire
foreign body may also directly damage the bone and foot and pastern regions are diffusely swollen up to
fibrocartilage of the flexor surface of the navicular and beyond the level of the fetlock. If a wound is
bone. The elastic nature of the frog usually seals not readily apparent, the foot should be thoroughly
over any puncture wound and there is often no natu- explored. Regional analgesia may localise the lame-
ral drainage from the wound. A buildup of pressure ness and greatly facilitates exploration of the frog.
in the bursa between the DDFT and the navicular Confirmation of bursal sepsis is achieved by centesis
bone, inflammation of the bursal synovium and revealing bursal fluid with an elevated WBC count
the DDFT and septic osteitis of the navicular bone and/or the presence of bacteria. Confirming com-
may all cause the animal to become severely lame. munication of the bursal cavity with the external
The close proximity of the DIP joint and the digi- wound can be achieved by remote injection of the
tal flexor tendon sheath may lead to these structures bursa with saline (fluid exits from the wound) or
becoming infected at the time of the initial injury radiographic contrast techniques.
or, more rarely, secondarily by spread of the infec- Radiography of the foot in two planes at 90° to
tion. The bursa itself may also become secondarily each other with either a solid probe (Fig. 1.203) or
infected from erosion of a septic focus through the radiographic contrast medium (Fig. 1.204) inserted
DDFT following a solar puncture that did not reach into the wound or bursa is very helpful in reaching
the bursa initially. Occasionally, hoof and heel bulb a diagnosis, although care is required to avoid forc-
lacerations/avulsions may extend deeply enough to ing the probe or external contaminants deeper into
involve the navicular bursa. Septic bursitis may also the tissues by entering the tract. Ultrasound, either
be a sequela of intrabursal medication. through the frog or between the ungual cartilages,
can demonstrate increased fluid in the navicular
Clinical presentation bursa and discontinuities in the flexor surface of
The owner may notice the nail puncture when it the navicular bone. MRI has proven to be the most
occurs. Otherwise, the usual presenting symptom is effective way of demonstrating the path of a pene-
severe lameness in which the horse will not put its heel trating injury and the tissues affected by the injury.
down during the stride. Depending on the duration of Susceptibility artefacts in the frog caused by haem-
the infection there may be swelling visible/palpable orrhage are useful to delineate the tract followed by
between the heel bulbs and the ungual cartilages. the nail. Any injury to the DDFT will invariably be
shown as an area of hyperintense signal in the ten-
Differential diagnosis don (Fig. 1.205). Sepsis of the navicular bursa will
Abscess; fracture of the distal phalanx or navicu- result in marked distension of the bursa and diffuse,
lar bone; severe strain or sprain; other deep digital marked bone oedema in the spongiosa of the navicu-
sepsis. lar bone and sometimes of the distal phalanx as well.
Traumatic or septic damage to the flexor surface of
Diagnosis the navicular bone will also be readily visible on MR
A history and/or visible evidence of a puncture images, long before this becomes radiographically
wound to the frog or adjacent sulci in a severely evident. Later in the disease there may be radio-
lame horse that is not putting its heel to the ground graphic evidence of lysis of the flexor cortex of the