Figure 1: Diffuse swelling of the left rostral mandible in a 4-year-old coonhound. Back in the day, when I was a resident at the University of Pennsylvania, Tanya, a second-year veterinary student, brought her black and tan coonhound to my office. Chester was 4 years old, and Tanya was concerned that she found an oral tumor in his mouth. I examined Chester’s mouth and saw a large, diffuse swelling associated with the left rostral mandible just distal to the left mandibular canine tooth (tooth 304). The swelling was approximately 3 centimeters in diameter, hard in some areas, and fluctuant in other areas. It was not ulcerated and was the same color as the surrounding mucosa. The left mandibular first premolar tooth (305) was missing, and the left mandibular second premolar tooth (306) was displaced distally (Figure 1). I told Tanya that I would need to obtain dental radiographs under anesthesia to be sure, but I didn’t think this was a tumor. In fact, I was pretty sure what it was. What comes to your mind as the most likely diagnosis? Dentigerous cyst discovered After placing Chester under anesthesia, I obtained dental radiographs that revealed significant bone loss and an unerupted tooth 305 at the center of the bone loss. A rim of sclerotic bone bordered the area of bone loss. The bone loss extended mesially to the distal mid-root of tooth 304 and distally to the distal root of tooth 306. Tooth 306’s roots showed evidence of resorption as seen in Figure 2. Figure 2: Radiographic appearance of the left rostral mandibular swelling. Have you seen a dentigerous cyst in your practice? If not, one probably has seen you. If you look for them, unerupted teeth are pretty common. The dentigerous cyst is a specific type of odontogenic cyst associated with an unerupted tooth. In a retrospective study that spanned four years at a dental specialty practice, 136 dogs ranging in age from 3 months to 17 years were diagnosed with 213 unerupted teeth.1 Brachycephalic breeds are predisposed; boxers, pugs, Shih Tzu, and Boston terriers are at the top the breed predilection list.1 Dentigerous cysts arise due to a retained epithelial lining that surrounds the crown. This epithelial lining is normally shed when a tooth erupts. When a tooth does not erupt, the epithelial lining either produces fluid or draws in fluid osmotically, which results in expansion of the cyst over time and resultant bone loss. The teeth most likely to be unerupted are the mandibular first premolar teeth in dogs. Other sites include mandibular third molar, maxillary and mandibular incisor teeth, and mandibular canine teeth. Figure 3: Squamous cell carcinoma that may have been caused by a prior dentigerous cyst in a 10-year-old boxer. Not every unerupted tooth results in formation of a dentigerous cyst. Of those 213 unerupted teeth radiographed in one study, cystic lesions were identified in 29 percent of these unerupted teeth.1 I’ve seen a 14-year-old dog with an incidental finding of an unerupted tooth with no radiographic evidence of a dentigerous cyst, and I’ve seen an 8-month-old dog with such a large dentigerous cyst that it was compromising the bony integrity of the mandible. There’s really no way to predict which dogs’ unerupted teeth will develop cysts, so I follow the general guideline to remove any unerupted tooth if the owner allows it. Another reason not to “wait and watch” unerupted teeth is the fact that dentigerous cysts have been reported to occasionally malignantly transform into carcinomas (Figure 3).2 Cyst treatment In Chester’s case, I incised over the swelling and made releasing incisions rostral and caudal to the swelling to allow for adequate visualization. Removal of a thin shell of cortical bone dorsally allowed me to identify and retrieve the unerupted tooth 305 and remove the cystic lining in its entirety (Figure 4). The cyst usually is removed in unrewarding thin pieces of epithelium, but occasionally you can retrieve a large piece of the cyst wall. It is important to take the time to meticulously curette the lining of the cyst away from the sclerotic bony wall of the defect. Removal of the cyst lining is referred to as enucleation. I usually use a small bone curette or a feline periosteal elevator (EX-9) to remove the lining. Always look for cyst lining on the inside of the raised flap, as some of the lining may be adherent to the underside of the flap. Depending on how the cyst has affected the adjacent teeth, further extractions may be necessary. Chester’s mid-root of tooth 304 was visible when I examined the surgical site, but apex of 304 was nicely embedded in bone. Figure 4: Intraoperative photograph showing unerupted tooth 305. In many of these cases, I have found alveolar bone loss on the distal surface of the mandibular canine resulting in mid-root exposure. If I see this, I don’t necessarily extract the mandibular canine. It seems these teeth are able to maintain their vitality if the apex is still submerged in bone and the gingival attachments around the tooth are intact. Chester’s tooth 306 was partially resorbed and displaced into an abnormal position, as the cyst acted like an orthodontic device to move the tooth to a different location. Therefore, tooth 306 was extracted. It’s a judgment call as to whether or not to place bone graft material in the defect. Considerations include degree of bone loss and whether there is concern for the graft to act as a nidus for infection. Some bone grafts contain bone morphogenetic protein, which is contraindicated in areas of suspected tumors. I’ll discuss bone grafts further in a future column. Many thanks to John Heidgerd for the recommendation for this month’s topic—it’s an important one for sure. If you have a future topic recommendation, please email me at jlewis@northstarvets.com. l Dr. John Lewis practices veterinary dentistry and oral surgery at NorthStar VETS in Robbinsville, N.J. References 1 Babbitt SG, Krakowski Volker M, Luskin IR. “Incidence of radiographic cystic lesions associated with unerupted teeth in dogs.” J Vet Dent. Dec. 2016; 33(4):226-233. 2 Colbert S, Brennan PA, Theaker J, Evans B. “Squamous cell carcinoma arising in dentigerous cysts.” J Craniomaxillofac Surg. 2012; 40(8):e355-357.
