More "Zebras": Mandibular Swellings

Dr. John Lewis shares zebra diagnoses, those symptoms that can be the sign of something common and severe.


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My November and December columns focused on zebras. Not the actual animals, but rather the zebra diagnosis, that rare clinical presentation that may mimic a more common abnormality.

I finished my November column by asking readers to share with me any zebras they have encountered.

Linda Molesworth, VMD, of Huntingtown, Md., sees a zebra more frequently than most of us, because she is the proud owner of Zena (see photo above). Let’s continue to discuss dental/orofacial zebra diagnoses, in the context of mandibular swellings in dogs.

Signalment—age, breed and sex—may be a very important component of the information gathered when creating a list of differential diagnoses. Take the example of a patient that presents for a ventral mandibular swelling.

Many a mandibular swelling has been caused by a nonvital tooth with endodontic infection, often as a result of a tooth fracture, or extension of periodontal disease to the level where it can invade the endodontic system of a tooth. However, numerous other possible causes exist.

If unilateral mandibular swelling is seen in a 14-year-old dog, the nonzebra diagnosis of neoplasia would come to mind.

However, if nonpainful swellings were occurring in both mandibles of a similarly aged dog with a history of severe chronic renal failure, fibrous osteodystrophy due to secondary renal hyperparathyroidism might come to mind.


What if the patient presenting with a mandibular swelling is a juvenile? There are many possibilities that could cause such a swelling, including bone fracture, callus formation, cellulitis, soft tissue abscess, tooth root abscess, dentigerous cyst, developmental abnormalities, fibrous osteodystrophy, odontoma, osteomyelitis, benign neoplasia and malignant neoplasia.

If the patient with a mandibular swelling is a 9-month-old West Highland white terrier, craniomandibular osteopathy (CMO) would come to mind as a top differential, particularly if the swelling was occurring on both mandibles, and if there was pain and limited range of motion when opening the mouth.

If a mandibular swelling arises in a 4-month-old Labrador retriever, the cause might be periostitis ossificans, a newly described entity in dogs.

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Large Breeds

A case series of five large-breed dogs affected by periostitis ossificans was published in the Journal of Veterinary Dentistry in 2010.1 The five dogs identified in this retrospective study ranged from 3 to 5 months of age, with a median age of 4.1 months. All were large breed dogs: two Labradors, one Dogue de Bordeaux, one great Dane and one great Pyrenees. All were male.

The swellings appeared clinically to be unilateral in all five puppies, and pain on palpation was not detected when patients were assessed for pain. The swellings were centered at the level of the mandibular first molar tooth in all five cases.

Character of the swellings in those dogs who were palpated was consistent: firm ventrally and fluctuant on the lateral intraoral component of the swelling. Dogs whose rectal temperatures were checked on presentation were within normal limits.

Radiographically, each patient showed a consistent finding on the lateral intraoral radiographic view of the affected caudal mandible: a double cortex appearance at the ventral aspect of the mandible.

Histopathological examination in each case tested revealed periosteal new bone formation with no inflammatory or neoplastic disease. Samples obtained from the core of the fluid-filled swelling in one dog revealed acute inflammation, fibrin deposition and small pieces of necrotic bone embedded within inflammatory foci surrounded by marked proliferation of immature granulation tissue and new blood vessels.

Treatment Stuff

Treatment of these patients varied, but the take-home point of periostitis ossificans is this: Regardless of treatment or lack of treatment, all five cases appeared to be self-resolving.

This is in contrast to CMO, which can be a life-threatening disease due to progressive production of bone in the caudal mandible, TMJ area and calvarium.

Long-term steroids are our treatment of choice for CMO due to their catabolic effects on bone.


Compare the radiographic appearance of CMO in a 9-month-old West Highland white terrier (Figure 1) with the radiographic appearance of periostitis ossificans in a 4.5-month-old Labrador retriever (Figure 2).

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Figure 1: The radiographic appearance of craniomandibular osteopathy. John Lewis, University of Pennsylvania

Clinical differences between these two diseases include level of discomfort, likelihood of bilateral changes and decreased range of jaw motion seen in CMO due to hypermineralization in the TMJ area, the calvarium and the tentorum cerebelli.

Figure 2: Periostitis ossificans. The asterisk shows the space between the double ventral cortices. Journal of Veterinary Dentistry with persmission.

Radiographically, when comparing changes of the mandibular body, the periosteal reaction of CMO tends to be more severe with no evidence of the "double cortex” seen with periostitis ossificans.

This list of differentials could help with your next case of mandibular swelling. Odds are good that the next mandibular swelling you see will be due to the common tooth-root abscess, but keep an eye out for the zebra diagnoses so they won’t catch you off guard.

If you make an interesting craniofacial zebra diagnosis, tell me about it at jlewis@northstarvets.com

Dr. Lewis, FAVD, Dipl. AVDC, practices veterinary dentistry and oral surgery at NorthStar Veterinary Emergency Trauma & Specialty Center  in Robbinsville, N.J.

1. Blazejewski SW 3rd, Lewis JR, Gracis M, Woodward TM, LeVan LM, Ross DL, Reiter AM. Mandibular periostitis ossificans in immature large breed dogs: 5 cases (1999-2006). J Vet Dent. 2010;27(3):148-59.


One thought on “More "Zebras": Mandibular Swellings

  1. I took my 10 year old lab to the vet today again after the first two rounds of antibiotics did nothing for his bladder problem. The vet took him back for testing and came out and told me she believes he has a rare condition called zebra bone. Plus some other long term for it. His front legs have doubled In size and she showed me the same fungus growth on his ribs and other parts of his body. She is going to do more research and contact me with what to do next.

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