SOUND BITES
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.
REFERENCES
1
Wiggs RB, Lobprise HB. Oral anatomy and physiology, In: Wiggs RB, Lobprise HB. Veterinary Dentistry:
Principles & Practice. Philadelphia: Lippincott-Raven1997; 55-86.
2
Eubanks DL. Overview of embryological development of the canine oral cavity. J Vet Dent 2008:25: 213-215.
3
Wiggs RB, Lobprise HB. Clinical oral pathology, In: Veterinary Dentistry: Principles & Practice. Philadelphia:
Lippincott-Raven1997; 104-139.
4
Regezi JA, Sciubba JJ, Jordan RCK. Cysts of the jaws and neck, In: Regezi JA, Sciubba JJ, Jordan RCK. Oral
Pathology: Clinical Pathologic Correlations 4th Edition. St. Louis: Saunders 2003; 241-265.
5
Au AY, Au RY, Al-Talib TK, Eves B, Frondoza CG. Consil® bioactive glass particles enhance osteoblast
proliferation and maintain extracellular matrix production in vitro. J Biomed Mat Res 2007: 86: 678-684.
.
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