Solar keratoma: An atypical case

This case report shows that keratomas can occur in both hind feet of equine patients and should be considered as a diagnosis for long-standing, intermittent lameness localised to the hooves. A Thoroughbred racehorse presented with long-standing abscessation of the right hind hoof. Owing to the long-standing nature, the abscess draining tract was surgically explored. A focal mass was found within the solar horn. Histopathology revealed the mass to be a keratoma. A similar mass was removed from the left hind hoof a few months later after similar presenting signs. This case shows that keratomas can occur in more than one hoof within a short period and should be considered a differential diagnosis for long-standing lameness localised to the hoof.


Introduction
This case report describes that keratomas can occur in both hind feet of equine patients and should be considered as a diagnosis for long-standing, intermittent lameness localised to the hooves.
In a normal hoof, the keratinised epidermal laminae interlock with the dermis originating from the coronary band (Bowker 2003;Hamir et al. 1992) and, once formed, migrate downwards almost unchanged (Bowker 2003). The sole also contains keratinised horn (Bowker 2003;O'Grady & Horne 2001). Hoof horn normally reacts to loading stress by producing an increased amount of keratinised horn as a protective measure (Back et al. 2007) and adapts accordingly by changing its structure when formed (Bowker 2003;Stashak & Hill 1996).
Two typical forms of keratomas have been described, namely a cylindrical and a more discrete, spherical form (Cullimore & Booth 2010;McDiarmid 2007). In the front hoof, the cylindrical form usually occurs as a solitary lesion at the toe or quarter (Boys Smith et al. 2006;Mair & Linnenkohl 2012;McDiarmid 2007), whereas multiple locations have been reported in a hind hoof (Christman 2008).
Radiographically, a keratoma presents as a distinct semilunar notch with a smooth sclerotic rim on the mid or distal dorsal phalanx 3 (P3), which distinguishes it from an infectious process (Boys Smith et al. 2006;Butler et al. 2000;Farrow 2006;Honnas 1997 suggested that surgical exploration and histopathology are the only methods available for accurate diagnosis (Hamir et al. 1992;Lloyd et al. 1988;Moyer 2008;Pickersgill 2000), as radiography may be inconclusive because of the soft tissue detail limitations (Boys Smith et al. 2006;Honnas 1997;Schulze et al. 2006), the location of the keratoma or the extent of growth (Chan & Munroe 1997). Differential diagnoses include squamous cell carcinoma, malignant melanoma, mast cell tumour, glomus tumour and fibroma (Brounts et al. 2008;Durham & Walmsley 1997;Moyer 2008;Valentine et al. 2000), as well as septic and nonseptic pedal osteitis (Cauvin & Munroe 1998;Chaffin et al. 1989;Chan & Munroe 1997).
Keratomas may lead to recurrent submural or subsolar abscesses (Ange 2010;Boys Smith et al. 2006) and are often associated with local secondary infection (Boys Smith et al. 2006;Butler et al. 2000;Lloyd et al. 1988). However, it has also been suggested that keratomas rather mimic an abscess (Cullimore & Booth 2010;Wagner et al. 1986). Keratomas commonly consist of poor-quality horn that easily decays, which allows bacteria and fungi to enter the hoof (Furst & Lischer 2006), resulting in a necrotic core (Honnas et al. 2003).

Management and outcome History
A four-year-old Thoroughbred gelding racehorse presented with mild, intermittent lameness (grade 1, as defined by the American Association of Equine Practitioners scale [Anon 1991]) in the right hind limb. Hoof testers placed on the lateral quarter of the hoof induced discomfort and evoked a reaction. The initial diagnosis was that of a subsolar abscess or sole bruising. The hoof was treated topically with a poultice of acriflavine, glycerine and magnesium sulphate and systemic anti-inflammatories (phenylbutazone: 2.2 mg/kg body weight) were administered per os twice a day (Virbac RSA, Halfway House). The solar abscess and associated lameness recurred repeatedly over a four-month period. As part of the diagnostic work-up, a regional (lateral sesamoid) nerve block of the affected limb was performed, resulting in resolution of lameness. The horse had normal hoof conformation and was shod with 1° heel wedges on its hind hooves.

Physical examination
When examined, the horse was shod with steel shoes with 1° heel wedges. The hoof was considered within normal limits with regard to size, shape and angle to the ground and pastern. The structure of the hoof appeared normal, with no deviations of the white line. The persistent cycle of assumed abscessation and resolution in the lateral quarter, and the positive reaction to hoof testers in the same location, suggested a sequestrum of unknown origin or possible foreign body. At the time of examination, there was a draining tract in the lateral quarter. Surgical exploration of the tract was performed. Radiographs were not taken initially.

Procedure
The procedure was performed with the horse in a standing position. Sedation was achieved using a combination of detomidine (0.01 mg/kg body weight; Domosedan, Pfizer, South Africa) and butorphanol (0.05 mg/kg body weight; Torbugesic, Fort Dodge Animal Health, Fort Dodge) administered intravenously. The hoof was desensitised with an abaxial plantar nerve block at the level of the sesamoid bones. Standard and looped hoof knives were used to pare away solar horn along the draining tract in the lateral quarter of the right hind hoof. As the tract enlarged, an irregular spherical mass was observed within the cavity. The mass was loosely attached to the surrounding tissue and was easily removed. The wound cavity was curetted to remove all necrotic tissue, packed with gauze swabs soaked in an astringent (copper sulphate) and then bandaged with a waterresistant bandage. The mass was preserved in 10% buffered formalin and submitted for histopathological examination.

Aftercare
The horse received a course of phenylbutazone (Virbac RSA, Halfway House) for 5 days post procedure (2.2 mg/kg body weight per os once a day). The bandage was replaced every 3 days for 2 weeks and then weekly for a further 2 weeks, repeating the initial packing with copper-sulphate-soaked gauze each time. Granulation tissue was evident after the second bandage change and the tissue keratinised, becoming horn like within a few weeks. At 4 weeks post-removal, a bar shoe was fitted and a light dressing was maintained on the cavity and changed every second day. Systemic antimicrobials were not administered. Owing to the histopathological findings, lateral, dorsopalmar, dorsoproximal-palmarodistal oblique and palmaroproximal-palmarodistal oblique radiographic views were taken of the hoof to determine whether P3 was affected and confirm resolution. No radiographic abnormalities were detected on P3.

Histopathology
The irregular spherical mass of 3 cm × 2 cm × 1 cm (Figure 2) was submitted for histopathology in 10% buffered formalin. The mass consisted of concentric laminated keratin embedded in solar keratin. The final diagnosis confirmed suspicions of a solar keratoma.

Discussion
The horse returned to ridden exercise 1 month after the procedure was performed. It then developed a similar subsolar abscess in the medial quarter of the left hind hoof. The abscess recurred repeatedly over a period of 2 months, with associated intermittent lameness (grade 1, [Anon 1991]). Based on the similar nature of the clinical signs, the abscess was investigated surgically. A mass of similar appearance (2 cm × 1 cm × 1 cm) was found in the hoof. It was more firmly attached to the underlying hoof tissue than the one in the right hind hoof. The mass was removed and submitted for histopathology. However, histopathological findings were inconclusive, possibly owing to the mass having been at an early stage of formation, before maturation could occur. The mass was suspected to be a keratoma.
The horse returned to ridden exercise 1 month after removal of the mass. Five months after returning to ridden exercise, the horse placed second in its second racing start and subsequently won its third start.
This case is unusual because a keratoma was diagnosed in one hind foot and highly suspected in the other hind foot in a fairly short time period. Keratomas are not commonly reported in more than one hoof (Mair & Linnenkohl 2012). There was no evidence of previous trauma to either hoof. As shown in this case, radiographs were not useful as a diagnostic tool, possibly owing to the specific location of the mass. As the solar horn is not as rigid as the dorsal wall, the growing keratoma likely resulted in less pressure necrosis on P3 (Gasiorowski & Richardson 2011). Modalities such as nuclear scintigraphy or MRI may have been more helpful (Floyd & Mansmann 2007), as focal thinning of the sole may be seen (O'Grady & Horne 2001) and may be more readily visible on MRI than on radiographs (Mair & Linnenkohl 2012). MRI would also allow earlier detection, prompt treatment and good surgical planning (Barrett & Subrod 2008;Dyson 2003;Mair & Linnenkohl 2012), although the costs are prohibitive (Schulze et al. 2006). Ultrasound would have been a possible diagnostic aid if the lesion had been above the coronary band (Floyd & Mansmann 2007;Honnas et al. 2003).
Solar keratomas are difficult to diagnose and should be considered when long-standing abscessation or chronic lowgrade lameness attributed to the hoof are the presenting complaints. As shown in this case, it is possible that keratomas can occur in both hind hooves of a horse. This suggests that keratomas can occur on any hoof and can potentially occur in multiple hooves of a single horse.