Ameloblastoma is a rare, benign tumor of odontogenic epithelium much more
commonly appearing in the lower jaw than the upper jaw. It was recognized in 1827
by Cusack. This type of odontogenic neoplasm was designated as an
adamantinoma in 1885 by the French physician Louis-Charles Malassez. It was
finally renamed to the modern name ameloblastoma in 1930 by Ivey and
Churchill. While these tumors are rarely malignant
or metastatic, and progress slowly, the resulting lesions can cause severe
abnormalities of the face and jaw. Additionally, because abnormal cell
growth easily infiltrates and destroys surrounding bony tissues, wide surgical
excision is required to treat this disorder. If an aggressive tumor is left
untreated, it can obstruct the nasal and oral airways making it impossible to
breathe without oropharyngeal intervention.
Subtypes There are three main clinical subtypes
of ameloblastoma: unicystic, multicystic, peripheral. The peripheral
subtype composes 2% of all ameloblastomas. Of all ameloblastomas in
younger patients, unicystic ameloblastomas represent 6% of the
cases. A fourth subtype, malignant, has been considered by some oncologic
specialists, however, this form of the tumor is rare and may be simply a
manifestation of one of the three main subtypes. Ameloblastoma also occurs in
long bones, and another variant is Craniopharyngioma
Clinical features Ameloblastomas are often associated with
the presence of unerupted teeth. Symptoms include painless swelling,
facial deformity if severe enough, pain if the swelling impinges on other
structures, loose teeth, ulcers, and periodontal disease. Lesions will occur
in the mandible and maxilla,although 75% occur in the ascending ramus area and
will result in extensive and grotesque deformitites of the mandible and
maxilla. In the maxilla it can extend into the maxillary sinus and floor of
the nose. The lesion has a tendency to expand the bony cortices because slow
growth rate of the lesion allows time for periosteum to develop thin shell of
bone ahead of the expanding lesion. This shell of bone cracks when palpated and
this phenomenon is referred to as "Egg Shell Cracking" or crepitus, an
important diagnostic feature. Ameloblastoma is tentatively diagnosed
through radiographic examination and must be confirmed by histological
examination. Radiographically, it appears as a lucency in the bone of
varying size and features—sometimes it is a single, well-demarcated lesion
whereas it often demonstrates as a multiloculated "soap bubble" appearance.
Resorption of roots of involved teeth can be seen in some cases, but is not
unique to ameloblastoma. The disease is most often found in the posterior body
and angle of the mandible, but can occur anywhere in either the maxilla or
mandible. Ameloblastoma is often associated with
bony-impacted wisdom teeth—one of the many reasons some dentists recommend
having them extracted. Histopathology
Histopathology will show cells that have the tendency to move the nucleus away
from the basement membrane. This process is referred to as "Reverse
Polarization". The follicular type will have outer arrangement of columnar or
palisaded ameloblast like cells and inner zone of triangular shaped cells
resembling stellate reticulum in bell stage. The central cells sometimes
degenerate to form central microcysts. The plexiform type has epithelium that
proliferates in a "Fish Net Pattern". The plexiform ameloblastoma shows
epithelium proliferating in a 'cord like fashion', hence the name 'plexiform'.
There are layers of cells in between the proliferating epithelium with a
well-formed desmosomal junctions, simulating spindle cell layers.
Variants The six different histopathological
variants of ameloblastoma are desmoplastic, granular cell, basal cell,
plexiform, follicular, and acanthomatous.
The acanthomatous variant is extremely rare.
One-third of ameloblastomas are plexiform, one-third are follicular.
Other variants such as acanthomatous occur in older patients. In one center,
desmoplastic ameloblastomas represented about 9% of all ameloblastomas
encountered. Treatment
While chemotherapy, radiation therapy, curettage and liquid nitrogen have been
effective in some cases of ameloblastoma, surgical resection or
enucleation remains the most definitive treatment for this condition. In a
detailed study of 345 patients, chemotherapy and radiation therapy
seemed to be contraindicated for the treatment of ameloblastomas. Thus,
surgery is the most common treatment of this tumor. Because of the invasive
nature of the growth, excision of normal tissue near the tumor margin is often
required. Some have likened the disease to basal cell carcinoma in its tendency
to spread to adjacent bony and sometimes soft tissues without metastasizing.
While not a cancer that actually invades adjacent tissues, ameloblastoma is
suspected to spread to adjacent areas of the jaw bone via marrow space. Thus,
wide surgical margins that are clear of disease are required for a good
prognosis. This is very much like surgical treatment of cancer. Often,
treatment requires excision of entire portions of the jaw.
Radiation is ineffective in many cases of ameloblastoma. There have also been
reports of sarcoma being induced as the result of using radiation to treat
ameloblastoma. Chemotherapy is also often ineffective. However, there is
some controversy regarding this and some indication that some ameloblastomas
might be more responsive to radiation that previously thought.
= Molecular biology= There is evidence that suppression of
matrix metalloproteinase-2 may inhibit the local invasiveness of ameloblastoma,
however, this was only demonstrated in vitro. There is also some research
suggesting that α5β1 integrin may participate in the local invasiveness of
ameloblastomas. A recent study discovered a high
frequency of BRAF V600E mutations in solid/multicystic ameloblastoma. These
data suggests drugs targeting mutant BRAF as potential novel therapies for
ameloblastoma. = Recurrence=
Recurrence is common, although the recurrence rates for block resection
followed by bone graft are lower than those of enucleation and curettage.
Follicular variants appear to recur more than plexiform variants. Unicystic
tumors recur less frequently than "non-unicystic" tumors. Persistent
follow-up examination is essential for managing ameloblastoma. Follow up should
occur at regular intervals for at least 10 years. Follow up is important,
because 50% of all recurrences occur within 5 years postoperatively.
Recurrence within a bone graft does occur, but is less common. Seeding to
the bone graft is suspected as a cause of recurrence. The recurrences in these
cases seem to stem from the soft tissues, especially the adjacent
periosteum. Recurrence has been reported to occur as many as 36 years after
treatment. To reduce the likelihood of recurrence
within grafted bone, meticulous surgery with attention to the adjacent soft
tissues is required. Epidemiology
The annual incidence rates per million for ameloblastomas are 1.96, 1.20, 0.18
and 0.44 for black males, black females, white males and white females
respectively. Ameloblastomas account for about one percent of all oral tumors and
about 18% of odontogenic tumors. Men and women tend to be equally affected,
although women tend to be 4 years younger than men when tumors first occur
and tumors appear to be larger in females.
See also Ameloblastic fibroma
Bone grafting Epithelial cell rests of Malassez
List of cutaneous conditions Matrix Metalloproteinase-2
Tooth development and Odontogenesis References
External links Radiology and Pathology MedPix Image
Database