MERRY CYST-NESS (AND HAPPY NEW YEAR!)
HOW THE JAW CAN BE A HOLEY SITE
Eric M. Davis, DVM, FAVD, Dipl. AVDC
Even though the country is in the middle of a recession and there is a barrage of dismal economic news being reported daily, people still seem to be purchasing or adopting new pets. Yet most veterinary practice managers predict that overall, fewer patients will come through our doors in 2009. Therefore, practitioners are reminded to take advantage of every opportunity to carefully examine the oral cavity of each and every patient. One of the best times to get a good look is while the patient is under general anesthesia for surgery to remove the reproductive organs. For ease of discussion, I will assume that the majority of veterinarians in this area remain “traditional”, and continue to perform neutering surgery when the pet is approximately six months old. By that age, all of the adult dental crowns in both dogs and cats should be erupted except perhaps the mandibular third molar teeth of small-breed dogs which usually erupt by seven months of age. Every practitioner should familiarize themselves with the anatomically expected number of teeth that should be seen in their patients, and thereby recognize if there are more teeth, or fewer teeth than there ought to be.
In both dogs and cats, there should be four canine teeth and 12 incisor teeth…that part is easy. In dogs, there should be four maxillary and four mandibular premolars on each side, with two maxillary molar teeth and three mandibular molar teeth on each side, for a total number of 42 teeth. Cats, who delight in not following rules, have only one maxillary and mandibular molar tooth on each side, and they have three maxillary premolar teeth and two mandibular premolar teeth on each side, for a total number of thirty teeth. If you discover that a tooth crown is inexplicably absent when it should be present, you should ask yourself, “Where the heck is it?” The only way to find out is….TA DA…a dental radiograph! If the crown of a tooth is not visualized when it should be present, either the tooth is truly absent (this needs to be documented in the record but no treatment is necessary) OR it is really there but you just cannot see it. If the tooth is there but it is not erupted, it is considered to be either an impacted or an embedded tooth.
If the eruption pathway is obstructed, the tooth is said to be impacted. For example, if a tooth has developed normally but is positioned at an abnormal angle in the bone, the eruption pathway of that tooth may become obstructed by bone, dense connective tissue, or an adjacent tooth. A tooth that has not erupted and has no obvious obstruction within the eruptive pathway is said to be an embedded tooth. Sometimes developmentally deformed teeth may fail to erupt and remain as embedded teeth.
As an educator and a specialist, I have been dismayed to discover that the majority of veterinarians are unaware of the potentially damaging consequences that can result from the failure of a tooth to erupt. Early recognition and intervention can spare animals from progressive bone destruction and the loss of many teeth. Now, because you are reading this newsletter article, the following developmental problem should be more frequently recognized.
During tooth development within the confines of the incisive, maxillary, and mandibular bones, a protective membrane is formed around the enamel of each tooth which is referred to as the primary enamel cuticle or Nasmyth’s membrane.1 This membrane acts as a protective layer for the erupting tooth and remains in place for weeks or months following eruption until it eventually wears away.2 If a tooth fails to erupt, a fluid filled sac begins to develop between the crown and the protective membrane which is attached to the tooth at the cementoenamel junction. It has been theorized that an osmotic gradient develops within the cyst lumen so that fluid begins to be drawn into the cyst from the surrounding connective tissues. As fluid accumulates and increases within the cyst, steady expansion leads to progressive resorption of surrounding bone. A distinct radiolucent space surrounding an unerupted tooth is pathognomonic for a dentigerous cyst.3 A dentigerous cyst is considered to be a benign lesion of odontogenic origin, but because of its expansile nature, it destroys bone, causes attachment loss of adjacent teeth, and occasionally results in external root resorption. Untreated, the epithelial lining of a dentigerous cyst can even undergo neoplastic transformation.4 If the problem of an unerupted tooth is discovered early enough and appropriate surgical intervention is performed, extensive damage can be prevented.
While only rarely reported in cats, dentigerous cysts are rather commonly discovered in dogs, particularly associated with an unerupted mandibular first premolar tooth, often in brachycephalic breeds (Figures 1-4). Remember that there should normally be four premolar teeth between the canine tooth and the mandibular first molar tooth in the dog. Another little pearl of knowledge is that the adult first premolar teeth do not have a deciduous predecessor and so are among the earliest of the adult teeth to erupt, roughly about the same time as the third incisor teeth (approximately four months of age). If you do not see that crown, you better be thinking to yourself, “hmmm”. Of course if you don’t take a radiograph, you won’t know if the first premolar tooth never developed (agenesis) or if the tooth is actually there but did not erupt. Not every unerupted tooth goes on to produce a dentigerous cyst, and occasionally an unerupted tooth may be discovered radiographically in a geriatric patient. If a dentigerous cyst is going to develop, the process usually starts soon after the tooth would ordinarily erupt through the gingiva. Therefore, if an unerupted tooth is discovered in an older animal and it is not associated with a dentigerous cyst, a cyst would not be expected to suddenly develop later in life, and the area can be monitored radiographically. The exact diagnosis of what type of cyst might be present depends upon the clinical and radiographic appearance, as well as the histopathological diagnosis of the cells that comprise the cyst lining. The differential diagnoses of radiolucent cystic structures in the oral cavity of young animal patients include primordial cysts, which are characterized as lucent areas within the alveolar bone in the place of where a tooth should be. An eruption cyst is a fluid-filled cyst that may occasionally develop within the gingiva, coronal to a tooth that is about to erupt. No treatment is necessary because the tooth erupts through the lesion and the cyst disappears. Neoplasms of young animals including ameloblastoma, fibro-odontoma, and squamous cell carcinoma may also have a cystic appearance radiographically that may interfere with tooth eruption (Figures 5 and 6).
Treatment of a dentigerous cyst involves removal of the unerupted tooth, removing the cyst lining for submission for histopathological examination, and if the remaining bone is significantly weakened, consideration may be given to filling the enucleated defect with a cancellous bone graft or synthetic oseoconductive material.5 Please do not miss a dentigerous cyst…count the teeth, and radiograph the area or refer the patient if a cyst is suspected. Early intervention will limit the expansile destruction.
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