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Keywords:

  • horse;
  • ameloblastoma;
  • cementoma;
  • odontoma;
  • osteosarcoma;
  • ossifying fibroma;
  • fibrous dysplasia

Resection of the rostral mandible, incisive bone and maxilla has previously been described (Auer 2005). Although the equine literature consists primarily of case reports, such procedures are performed routinely in small animal cancer patients (Arzi and Verstraete 2010). With adequate knowledge of the regional anatomy and the prerequisite of adequate margins, resection can be performed with relative ease and good success. Mandibulectomy was recently described in the standing horse (Schumacher 2011). Case reports and this author's personal experience suggest that horses cope remarkably well, showing minimal post operative discomfort and minimal loss of function (with appropriate modifications in feed) following removal of the rostral jaw. In general, cosmetic outcome also seems favourable to good (Richardson et al. 1991) (Fig1).

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Figure 1. Post operative appearance following bilateral rostral maxillectomy for squamous cell carcinoma in an 11-year-old Tennessee Walking Horse (see Fig 6 for preoperative image of tumour).

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In order to avoid confusion, a common (and anatomically correct) classification should be used to describe resection of the jaw bone: mandibulectomy procedures are classified according to the portion of mandible removed. A ‘rim excision’ or partial segmental resection leaves the ventral border of the mandible intact (Brown et al. 2002). The terms unilateral or bilateral mandibulectomy state whether one or both mandibles are removed. Resection may be rostral, central, caudal or total. Maxillectomy can also be performed uni- or bilaterally. Incisivectomy of the upper jaw refers to excision limited to the incisive bone (premaxilla). Rostral maxillectomy removes the rostral aspect of the maxillary bone and the incisive bone (Verstraete 2005). Central, caudal and total maxillectomy are terms that are less likely to be relevant to the equine patient. In the accompanying case report Cousty and Tricaud (2014) describe a unilateral rostral maxillectomy.

A single report has previously described partial maxillectomy for treatment of infection with a dermatiaceous fungi causing phaeohyphomycosis (Schumacher et al. 1996). Tumours are a more common indication for surgery and will be the focus of the remainder of this discussion. Tumours of the equine jaw can be classified as those coming from the dental tissue, of epidermal origin and originating from bone (Knottenbelt 2005).

Tumours originating from dental tissues (odontogenic tumours) are rare. Those most commonly referred to in the literature are ameloblastoma, cementoma and odontoma. Reference to oral adamantinoma can be found in earlier literature (Wahl 1936; Vaughan and Bartels 1968; Hanselka et al. 1974; French et al. 1984). Based on human classification and their odontogenesis, these tumours should possibly be grouped together with the ameloblastoma (Ivey and Churchill 1930). Ameloblastomata show clinical and radiographic signs consistent with other oral tumour types. Differentiation relies on histopathological evaluation of biopsy specimens demonstrating the presence of odontogenic (specifically enamel producing) epithelium. This tumour has previously been described in foals (Roberts et al. 1978; Summers et al. 1979; Jones and Brumbaugh 1991). Reports of early intervention and wide surgical margins suggest that surgery alone can be curative (French et al. 1984; Rosol et al. 1994). This is consistent with the treatment of human patients with ameloblastoma in which no adjunct therapy is recommended (Franca et al. 2012; Tortorici et al. 2012). While metastasis is unlikely, hypercalcaemia as a sign of paraneoplastic syndrome has been described with this tumour type (Rosol et al. 1994). Cementoma is the most common term used to describe ‘hyperplastic cement accumulation’. The terms cementosis and cementicles are also found in the literature. The nomenclature of this group of cement accretions will likely need refining as their aetiology and pathophysiology become clearer (J. Easley, personal communication). The masses are mesenchymal in origin and may be present during tooth development. They are formed of reactive tissue rather than representing a true neoplasm (Freeman 1991). Location of these tumours, at the tooth root, makes early recognition unlikely. Any disease creating periapical inflammation, such as tooth fracture, periodontitis and dental impaction can cause cementoma formation. Their radiographic appearance is very characteristic (Fig2) (Kreutzer et al. 2007; Schaaf et al. 2007). Tooth removal is hampered by expansion in the region of the root through the obstructive presence of the mass. This benign lesion can, however, be resolved with tooth extraction or local resection and currettage alone. Odontomas are hamartomas which contain well differentiated dental tissues. The adjectives complex and compound can be used to further differentiate tumours histologically into those which display relative organisation of the 3 tooth components (compound) and those with a chaotic (complex) structure (Figs3 and 4). There is no apparent age predisposition for this tumour which may present as painless swellings over the root regions of any of the teeth of the jaw and can extend into the paranasal sinuses (Dubielzig et al. 1986; Heufelder et al. 1994; Schumacher et al. 1997; Rubio-Martinez et al. 2011). Depending on location, partial resection of the rostral upper or lower jaw may be indicated. In the past repeat surgeries have been necessary, giving a fair prognosis for resolution of disease (Dillehay and Schoeb 1986; Dubielzig et al. 1986; Brounts et al. 2004; Knottenbelt 2005). Adjunct therapy is not warranted.

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Figure 2. Oblique radiograph of left mandible with evidence of a cementoma (white arrow) adjacent to the caudal root of the second premolar in a 7-year-old Freiberg mare.

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Figure 3. Gross appearance of an odontoma of the maxilla taken from a 12-year-old Thoroughbred mare.

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Figure 4. Cut surface of the mass seen in Figure 3. Disorganised regions of enamel (E) and dentin (D) are visible throughout the mass suggestive of a complex odontoma.

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While osteosarcomas are rarely reported in the horse, most of those recorded have been located in the region of the head (Thorp and Graham 1934; Barber et al. 1983; Livesey and Wilkie 1986; Bush et al. 2007). This malignant mesenchymal tumour causes bone lysis (Fig5) and the haphazard deposition of reactive trabecular new bone by neoplastic cells. A biopsy should be taken and evaluated in order to clearly differentiate it from other tumours (ameloblastoma, ossifying fibroma and fibrous dysplasia). Neoplastic osteoblasts (and a variable number of osteoclasts) show a characteristic high mitotic index. Attempts at surgical excision have been previously described but failed due to inadequate surgical margins (Bush et al. 2007). To the author's knowledge, Cousty and Tricaud (2014) are the first to describe the successful surgical management of an osteosarcoma affecting the equine skull. Through their novel approach of unilateral rostral maxillectomy they were able to minimise morbidity and improve post operative case management by preserving the mare's ability to prehend grass. Surgery and systemic chemotherapy are the current standard of care for osteosarcoma in small animals, with radiation generally only providing tumour control or palliation (Coomer et al. 2009). In man surgical excision remains the mainstay of treatment for osteosarcoma of the jaw, with both chemo- and radiation-therapy being of questionable value (Clark et al. 1983; Kassir et al. 1997; Jasnau et al. 2008). As stated by Cousty and Tricaud (2014), systemic use of chemotherapy is poorly described in the equine literature (Mair and Couto 2006). Local use of platin-based drugs are, however, effective in the management of cutaneous neoplasms (Theon et al. 1999, 2007) and implantation of cisplatin beads might be considered prior to gingival closure (Hewes and Sullins 2006).

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Figure 5. Dorsoventral radiograph of an 18-year-old pony presented for an expansile mass of the right maxillary bone. Lysis and concurrent deposition is evident, suggestive of an osteosarcoma. This was confirmed on histopathology.

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Squamous cell carcinoma can affect any of the structures within the equine oral cavity, pharynx and larynx. As with any mass-lesion, presence and extent of disease must be assessed as early and as thoroughly as possible, ideally through advanced imaging such as computed tomography (Witte and Perkins 2011), prior to any form of intervention. This tumour shows aggressive behaviour which is infiltrative, destructive and ulcerative. Chronic irritation of the mucosa, as seen in chronic periodontitis, has been proposed as a cause of squamous cell carcinoma of the oral cavity (Knottenbelt 2005). While a high index of suspicion is justified based on prevalence and appearance, histopathological confirmation through the identification of the characteristic irregular cords of downward-invading neoplastic keratinocytes and keratin ‘pearls’ should be performed before committing to treatment. Incisivectomy (Orsini et al. 1991) has been reported for the successful management of squamous cell carcinoma. Figures6-8 show this author's experience with bilateral rostral maxillectomy in an 11-year-old Tennessee Walking Horse presented for treatment of a squamous cell carcinoma. Based on the apparent sensitivity of this tumour type to gamma radiation, (Walker et al. 1998) and in order to decrease the likelihood of recurrence, surgical intervention was followed at 6 weeks with weekly radiation. A total dose of 32 Gray was delivered in 4 treatments. On the basis of the potentially beneficial effects of nonsteroidal anti-inflammatory medication in controlling squamous cell carcinoma (Moore et al. 2003; Elce et al. 2007; Thamm et al. 2008) this horse was also discharged on oral piroxicam. Adjunct local chemotherapy (e.g. cisplatin beads) might also be indicated and practical in such cases (Theon et al. 2007; Barabas et al. 2008). The horse in Figure1 showed no signs of local recurrence and was being used for pleasure riding at 6 year follow-up.

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Figure 6. Preoperative image of a horse with squamous cell carcinoma of the rostral maxilla.

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Figure 7. Intraoperative image of horse in Figure 6 showing mucosal closure after bilateral rostral maxillectomy.

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Figure 8. Intraoral dorso-ventral radiograph of the horse seen in Figure 6 following bilateral rostral maxillectomy.

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Ossifying fibroma is a proliferative, fibro-osseus, tumour-like lesion that most commonly develops in the rostral mandible causing distortion of the lip and adjacent teeth (Fig9). Most cases occur in horses less than one year of age (Morse et al. 1988). Dense tissue is seen on radiographs with variable degrees of calcification. Unilateral rostral maxillectomy and bilateral rostral mandibulectomy have been described with adequate margins being the prerequisite for curative surgery (Richardson et al. 1991; Kawcak et al. 1996). Radiation alone has also been reported to be successful (Robbins et al. 1996) and may be helpful as an adjunct therapy in the case of incomplete surgical resection (Orsini et al. 2004).

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Figure 9. Ossifying fibroma of the equine mandible (Image courtesy of Dr J. Easley).

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Fibrous dysplasia presents as a smooth contoured bone deformity arising from loss of bone structure with extensive formation of fibro-osseus matrix (Fig10). A well demarcated radiolucent cavity is visible radiographically (Fig11). The tumour becomes evident through its progressive expansile growth. Most lesions are monostotic (involving only a single bone), asymptomatic and identified incidentally. They can be treated with clinical observation alone. Should the expansile growth impinge upon adjacent structures, surgical excision may be warranted. Use of bisphosphonates, to reduce pain and initiate bone deposition, has been previously described in human patients and could be considered in the equine patient (Wirth 2012).

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Figure 10. Mandibular fibrous dysplasia in a yearling Thoroughbred.

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Figure 11. Lateral radiograph of the horse in Figure 10. Note the large radiolucent cavity and surrounding sclerosis.

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The described mass lesions are those more commonly encountered in the equine oral cavity, affecting the jaw. Additional differential diagnoses include osteomas, exuberant granulation tissue, epulis, myxomatous tumours of the jaw, haemangiosarcoma, lymphosarcoma and aneurismal bone cysts (Knottenbelt 2005). Imaging and biopsy should always be performed prior to undertaking any form of management (Witte and Perkins 2011). Surgical resection, through mandibulectomy or maxillectomy, can be considered in cases of diseases affecting the jaw if adequate surgical margins can be achieved.

Author's declaration of interests

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No conflicts of interest have been declared.

References

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  2. Author's declaration of interests
  3. References