Multiple keratomas in an equine foot

Abstract

A 19-year-old, Arabian gelding was presented for left hindlimb lameness and deviation of the hoof wall. Radiographs revealed a semicircular, radiolucent area in the lateral edge of the distal phalanx. Three distinct keratomas were successfully removed following hoof wall resection.

A 19-year-old, Arabian gelding was presented to the WesternCollege of Veterinary Medicine (WCVM) for Grade IV/V intermittent and progressive lameness of the left hindlimb. A traumatic injury of the left hind coronary band had occurred 15 y ago. A left hind solar puncture and abscess developed 2 y ago, followed by multiple recurrent abscesses. The horse had been examined 3 mo prior, when there was no evidence of a solar abscess. A lateral abaxial nerve block had resolved most of the lameness and localized the pain.

On physical examination, a dorsal deviation of the left lateral coronary band and a bulge extending from the coronary band to the distal aspect of the hoof wall were observed. A small draining tract at the white line on the lateral side was identified and pared out to 3 cm up the hoof wall. The tract was packed with a povidoneiodine and sugar poultice and the hoof was bandaged and placed in a protective boot. The horse was treated with phenylbutazone (Butequine Paste; Bioniche, Belleville, Ontario), 1 g, IV, q24h, for pain control. Radiographs revealed a well-circumscribed, oval, nonsclerotic radiolucent area in the lateral aspect of the distal phalanx, midway between the apex of the extensor process and the distal border of the 3rd phalanx. A presumptive diagnosis of keratoma was made, based on the history of recurrent progressive lameness and radiographic images that were typical of, but not pathognomonic for, keratoma (1–3). Differential diagnoses included fibroma, mast cell tumor, squamous cell carcinoma, intraosseous epidermoid cyst, melanoma, bone cyst, calcified hematoma, seroma, or abscess (4).

A complete resection of the hoof wall was performed under general anesthesia. Anesthesia was induced with ketamine (Vetalar; Bioniche), 2 mg/kg bodyweight (BW), IV, and diazepam (Diazepam; Sandoz Canada, Boucherville, Quebec), 0.1 mg/kg BW, IV, and maintained with xylazine (Rompun, Bayer Health Care, Toronto, Ontario), 500 mg; ketamine (Vetalar; Bioniche), 1000 mg; guaifenesin 5% (Guaifenesin; Wiler Fine Chemicals, London, Ontario), 1.0 L in 5% dextrose (Dextrose; Hospira Health Care Corporation, Montreal, Quebec); administered IV at a maintenance rate of 2.2 mL/kg BW/h. A tourniquet was placed at the midmetacarpal level to assist in hemostasis. A 4-cm wide window was created by removing hoof wall with a dremel tool burr 5 mm distal to the coronary band. When this section of hoof wall was separated from the underlying sensitive laminae, a 5-cm × 4-cm, well-demarcated, oval-shaped, yellow-white, space-occupying mass was observed (Figure 1). The mass was easily disengaged from the laminae with a bone curette. The surrounding bone and soft tissue were debrided to reduce the likelihood of recurrence. During debridement, 2 additional oval masses of similar appearance, but smaller size (4 cm × 3 cm, 4 cm × 2 cm), were encountered under the hoof wall on either side of the resected window. These masses were also easily shelled-out and the surrounding tissue was debrided. The hoof wall overlying the smaller masses was left in place in order to not further jeopardize the hoof structure. The surgical area was copiously lavaged with sterile saline, packed with povidoneiodine-soaked sterile gauze, and covered with a sterile bandage.

Figure 1.

Three keratomas, (5 cm × 4 cm, 4 cm × 3 cm, 4 cm × 2 cm) removed from the left hind foot of a 19-year-old Arabian gelding.

The horse recovered well and showed marked improvement in lameness immediately after surgery (Grade II/V). The horse was maintained on phenylbutazone (Butequine Paste; Bioniche), 1g, IV or PO, q24h for 10 d, and trimethoprimsulfa (Nu-Cotrimox; Nu-Pharm, Richmond Hill, Ontario), 21 mg/kg BW, PO, q12h for 10 d. The bandage was changed q12h for 2 d, then q48h. The horse was discharged 4 d after the surgery, following the placement of an eggbar shoe with toe clips to provide support and reduce relative motion at the hoof wall defect (Figure 2). Two weeks later, an examination revealed that the horse was moving well and that healthy pink granulation tissue had begun to fill the defect.

Figure 2.

Complete hoof wall resection to remove multiple keratomas in a 19-year-old Arabian gelding. An eggbar shoe with clips was placed 4 d postsurgery to support the compromised hoof wall. Note the dorsal deviation of the coronary band, proximal to the resected area.

Keratoma is an uncommon benign hyperplasia of keratinocytes, most commonly found between the sensitive laminae and hoof wall in simple- and cloven-hoofed animals (3–10). Keratoma is somewhat of a misnomer, as it is not recognized as a neoplastic growth, rather a hyperplastic process (6). Hyperplasia of the keratin producing cells of the stratum germinativum at the coronary band or the secondary epidermal laminae of the interior hoof wall or sole results in a space-occupying mass (5–7). Keratomas can occur in any age, breed, or sex, and they have no typical signalment (3,5,6). Although the exact etiology is unknown, trauma or chronic irritation are often suspected as being the cause. A large proportion of horses (22/26) with keratomas have been reported to have experienced 1 or more previously diagnosed purulent draining tracts and suspected hoof abscesses (3,8). Whether these draining tracts or abscesses are primary or secondary to the keratoma is unknown (3,9). Repeated concussive impacts, such as would occur when the medial and lateral sides of the hoof wall are of uneven length, have also been implicated as a source of significant chronic irritation (2). Intermittent or chronic, moderate to marked (III–IV/V) lameness of variable duration is a common history (2,3,10). Lameness develops secondary to increased pressure below the hoof wall, similar to that of the common hoof abscess (3). Most reported cases involve only 1 hoof and there are no reported cases of multiple keratoma masses in the same foot (3,5,9). Depending on location of the keratoma, mass effect may cause dorsal deviation of the coronary band, bulging of the hoof wall, or thickening and displacement of the white line towards the center of the foot (3,5,8).

Radiographs often reveal a crescent-shaped, well-circumscribed, radiolucent osseous defect of the 3rd phalanx, with occasional surrounding sclerosis (23% of 26 horses) (5,8). Radiographic evidence may be lacking in cases where the keratoma has not progressed sufficiently to cause abnormalities in the distal phalanx (9).

Ultrasonography can be a useful diagnostic tool to localize and examine suspect deviations of the coronary band (2). A hypoechoic soft tissue mass can be seen on transverse and longitudinal views between the distal phalanx and hoof wall with a 7.5 MHz transducer and standoff pad.

Treatment involves partial or complete hoof wall resection, surgical removal of the mass, and curettage of surrounding necrotic tissues (8–10). The defect in the hoof is packed with betadine-soaked gauze and the hoof is bandaged. These bandages are changed regularly until healthy granulation tissue has filled the defect. The horse is usually treated with systemic antimicrobial drugs to reduce postoperative infections and nonsteroidal anti-inflammatories to reduce pain and inflammation. An eggbar or heartbar shoe will reduce motion within the disrupted hoof wall and encourage healing (3,6,8–10). Partial resolution of the lameness is usually immediate following the reduction of pressure caused by the mass (3,6,8–10).

Complications include osteomyelitis of the distal phalanx, permanent defect in the hoof wall, and recurrence. Recurrence is not common if the area around the keratoma has been surgically debrided aggressively (3,9). Hoof wall defects and postoperative pain can be minimized by removal of as little of the hoof wall as possible, postoperative stabilization with an eggbar or heartbar shoe, and stall rest until the hoof wall defect begins to keratinize (3,6,8).

The diagnosis is based on a history of intermittent lameness; dorsal deviation of the hoof wall; crescent-shaped, nonsclerotic radiographic lesions; and typical gross appearance of the masses. The diagnosis is confirmed with light microscopic examination (2,3,5,6,8–10).

This case is unique, as multiple keratomas were located in the same hoof. In addition, the hoof had evidence of coronary band injury at the level of the keratoma lesions. Few reported cases have documentation of traumatic injury, although this is commonly cited as a predisposing risk (2,3,5,6,9). As well, most reported cases involve a front foot rather than a hind foot (1).