Anatomy of the Mandible & Maxilla

The mandible consists of the body and ramus, and the bodies are fused rostrally. There are 3 sets of muscles to close the jaws: masseter, temporal, and pterygoid; only the digastricus muscle functions to open it. The masseter inserts on the caudal body and ramus, the temporal inserts on the ramus, and the pterygoid inserts on the angular process. The digastricus inserts on the ventral portion of the body. The maxillary artery supplies the mandible alveolar artery, and an important landmark is where it enters the mandibular foramen medially near the angle. This is an important landmark for complete hemi-mandibulectomy. The mandibular alveolar artery is accompanied by the mandibular nerve, a branch of the 5th cranial nerve, or the trigeminal nerve.

The maxilla is comprised of the maxillary, incisive, and nasal bones. The maxillary artery branches to form the major palatine and infraorbital arteries. Innervation via the infraorbital nerve, a branch of the maxillary nerve, enters the infraorbital foramen with the artery and exits adjacent to the rostral aspect of the maxillary carnassial tooth (maxillary premolar 4), which is another important landmark to remember for surgery.

Oral Neoplasia

Surgery is usually important in treating oral neoplasia. The most common tumors include fibrosarcoma (FSA), melanoma, and squamous cell carcinoma (SCC). Osteosarcoma (OSA) is also reported. Malignant neoplasms require the removal of bone for complete excision, but they vary in their tendency to metastasize. FSA has a 35% rate of metastasis, and SCC and melanoma metastasize 80% of the time. Epulides are technically all benign and may be less (fibromatous, ossifying) or very invasive (acanthomatous). These masses originate in the periodontal ligament, and removal of bone is important to prevent regrowth.

Since the biologic behavior varies significantly from benign to malignant and likely metastasized at the time of diagnosis, any oral mass diagnosed incidentally on physical examination or during a dental prophylaxis should be sampled for histopathologic evaluation. Take a biopsy and submit for analysis and be certain to document the location and extent of the mass if performing excisional biopsy. Photographic documentation will help for future treatment if the mass is excised and mandibulectomy or maxillectomy become part of the treatment plan. Only gingival hyperplasia does not require removal of bone.

Staging of oral tumors depends on the tumor type, 3 view thoracic radiographs are recommended and aspiration of the mandibular lymph nodes may be required for masses with a tendency to metastasize. Distinguishing melanoma from benign change to the lymph nodes can require biopsy, however. The best means of evaluating the extent of local disease is via CT scan or MRI; however, CT scan provides more information about bony involvement, which determines the extent of the excision. The retropharyngeal lymph nodes should also be evaluated for involvement and can best be seen with advanced imaging.

Mandibulectomy

The procedure is generally the same for any portion of the mandible to be removed. Rostral hemimandibulectomy, bilateral rostral mandibulectomy, segmental mandibulectomy, ¾ mandibulectomy, and complete hemi-mandibulectomy have been described. Incise the associated soft tissues first, around the portion to be removed. Excision of a portion of the adjacent lip may be required, depending on the mass and its extent of involvement of the adjacent tissues. Use a periosteal elevator to free the remaining soft tissues from the bone, preparing it for transection with an oscillating saw. Remember that the mandibular artery is ventral in the mandible, so transect that portion last. Use a mosquito hemostatic forceps to grasp the mandibular artery for ligation or cauterization. Bone wax can be used to tamponade the artery, but is not usually required, as the vessel does not retract far into the canal evading ligation or cauterization. Close the defect using local buccal flaps in 2-layers for security. The first layer secures the flap to the mucoperiosteum that remains or the adjacent soft tissue. The second layer apposes the gingival and oral mucosa to the mucosa of the buccal flap. For rostral procedures, avoid the sublingual caruncle to avoid closure of the sublingual openings of the salivary ducts. For exposure of a complete hemimandibulectomy, the commissure is incised through all layers, and extreme care is taken to identify and ligate the mandibular artery caudally. Advance the commissure rostrally during closure to decrease lingual deviation.

Maxillectomy

As with mandibulectomy, many different types of maxillectomy exist and are performed similarly. Hemimaxillectomy, rostral hemi-maxillectomy, segmental maxillectomy, and caudal maxillectomy are commonly performed. The structures passing through the infraorbital foramen may be divided if necessary. As with mandibulectomy, make the incision around the tissue to be removed followed by subperiosteal elevation to expose the bone for transection with an oscillating saw. Be careful when performing the osteotomy and avoid trauma to the nasal turbinates, which results in significant hemorrhage. Use buccal flaps and a 2-layer closure, similar to mandibulectomy. Caudal and large maxillectomy benefits from the dual approach via the oral cavity and a lateral approach to the maxilla (Lascelles BD, et al. J Am Anim Hosp Assoc, 2003). The approach is do-able; save the caudal-most aspect of the bony transection for last, and proceed with caution to not transect too deeply. Careful exposure and transection of the bone caudally allows visualization and ligation of the maxillary blood supply without transection during the osteotomy. Practice on a cadaver so that you completely understand the dual approach prior to use on clinical patients.

Complications

Complications of oral surgery are significant and should be discussed with the owner prior to surgery. Obvious facial changes, or deformity following surgery, should be discussed, the use of images of similar procedures will help the owners to understand the proposed change in appearance of their pet. Tension on the line of closure and trauma to the tissues during or after surgery increase the risk of dehiscence, so tension free closures and use of 2-layer closures whenever possible are important.

Careful tissue handling and avoiding chew toys and rough play is important as is the type of food presented after surgery. Slurries of canned food are not necessary and allow liquefied foreign material to penetrate the suture line. Hard food requires significant mastication for breaking into small particles and saliva mixing with the broken hard food should also be avoided. The easiest material to swallow after oral surgery is a pre-made bolus of canned food. The boluses are often smaller than expected and should be swallowed readily without the need for manipulation by the patient to break them down and reform the bolus themselves. Mandibulectomy and maxillectomy can result in trauma to the remaining soft tissues where intact teeth impact the soft tissues. Dental extractions or endodontic therapy may be required to prevent such trauma. Also, drift of the mandible following surgery can occur, causing grating of the teeth, altered appearance, or trauma due to the intact teeth contacting soft tissue.