Equine Clinical Case Challenge: A Case of Hoof Abscesses and Limb Lameness
A 21 year- old Thoroughbred mare presented to Tufts Equine Center at Cummings Veterinary Medical Center with a history of lameness and repeated hoof abscessation of her left front foot. The recurrent abscesses were reported to break and drain from the same location of the dorsal coronary band.
Upon presentation, the mare had mild swelling of the left front coronary band but no draining tract was observed. Palpation of her limbs revealed mildly increased digital pulses on her left front, but no sensitivity to hoof testers. Lameness evaluation revealed a moderate left front limb lameness (Grade 3; 0=sound, 5=non-weight-bearing). No improvement of the lameness was achieved with a palmar digital nerve block, while near resolution (>90%) of the lameness was achieved with an abaxial sesamoid nerve block.
Radiographic evaluation of the left front foot revealed an ovoid, partially mineralized soft tissue opacity at the dorsoproximal aspect of the third phalanx. A corresponding curvilinear lucent defect in the dorsoproximal border of the third phalanx was also present.
Based on the clinical history and concurrent radiographic findings, a spherical keratoma underlying the dorsoproximal hoof wall was suspected.
Surgical removal of the keratoma was performed with computed tomographic guidance to map the location and size of the keratoma (Figures 2 and 3). This allowed a targeted partial hoof wall resection on the distal aspect of the keratoma for safe removal without injury to the coffin joint. The defect within the hoof wall was sealed with amikacin-impregnated polymethyl methacrylate (PMMA) and a hoof-pastern cast was applied to protect the surgical site for the next three weeks (Figure 4).
Sagittal CT images of the left front and for comparison the right front third phalanx at the level of the keratoma. Bone weighted images on the left and soft tissue weighted images on the right. Note the round mineralized soft tissue mass (keratoma) as well as the corresponding bone defect in P3 (white arrows)
CT reconstruction image of the left front distal first phalanx, second phalanx, and third phalanx with rounded bone defect and mapping beats placed onto the hoof wall over the keratoma. Note the close proximity to the coffin joint.
Images showing the small targeted partial hoof wall resection to remove the spherical keratoma (small image); the bone cement plaque sealing the hoof wall defect; the hoof-pastern cast applied after surgery, and the acrylic patch over the bone cement plaque to re-enforce the hoof wall defect after cast removal.
After removal of the hoof cast, the bone cement plaque is checked for stability and any amount of drainage. If there are no signs of complications, an acrylic patch is applied over the bone cement plaque and a bar shoe with two clips is used to provide increased stability of the hoof capsule. In cases where the defect is more distal in the hoof capsule a glue-on shoe with a full plate (Sigafoos, Series II, cuff and plate) can be used if a traditional shoe cannot be applied. The hoof wall defect including the bone cement plaque and the acrylic patch will grow down during the following months and can be trimmed as needed until fully grown out. Ridden flatwork can be initiated as soon as the acrylic patch is in place and the horse is sound, which, in an uncomplicated case, is typically around 4 to 6 weeks postoperatively.
A keratoma can be columnar or spherical in shape. As it enlarges over time the resulting pressure on the third phalanx can lead to bone resorption. The underlying cause of keratomas is suspected to be focal trauma or inflammation. Keratomas have also been reported to develop as a consequence of hoof abscesses.
Keratomas can cause lameness due to focal pressure and inflammation of the sensitive lamina and underlying coffin bone. In some cases, they may be incidental findings when obtaining routine foot films. The poor hoof quality which can develop due to the presence of a keratoma will facilitate colonization by bacteria or fungi further weakening the hoof horn. These changes then contribute to recurrent hoof abscesses at the same site. In general, they are relatively rare and therefore may be overlooked.
If a columnar shaped keratoma has grown distally to reach the solar surface of the hoof, it can be recognized by a round area of reduced horn quality at the white line where lamellar horn is replaced by tubular horn and scar tissue. Spherical keratomas can be found at any location of the hoof wall and may not be apparent when inspecting the white line of the sole. Depending on the chronicity, radiographs may show a round bone defect in P3 with a smooth, sclerotic border.
Differential diagnoses that should be considered are septic osteitis of the third phalanx, white line disease, and traditional hoof abscesses.
If radiography is inconclusive, computed tomography or magnetic resonance imaging can provide a definitive diagnosis. Advanced imaging will provide valuable information about the size and location of the keratoma to facilitate surgical planning. This allows for a more targeted approach and use of a partial hoof wall resection as opposed to the originally described more radical debridement. Traditionally, a complete hoof wall resection was the treatment of choice which could result in an unstable hoof capsule and chronic lameness as well as a prolonged postoperative healing period. Mapping the keratoma via computed tomography allows for a targeted partial hoof wall resection while still allowing the complete removal of the keratoma. The more limited partial hoof wall resection will also allow for a shorter convalescent period providing a much earlier return to exercise.
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