Ⅰ.INTRODUCTION
Aneurysmal bone cyst (ABC) is a non-neoplastic cystic lesion, but is locally aggressive with high recurrence rate after curettage. Therefore, it was sometimes recommended to perform surgical resection.1,2) The most common presenting features of ABC was swelling and dull pain, and variable radiological features including multilocular or well defined radiolucency with bone expansion and perforation.
It was reported that among 120 cases of ABC, 69 were male (57.5%) and 51 were female (42.5%), the mean age of occurrence was 20.7 +/- 2.5 years (range 4 to 78), and 11 patients (9.2%) were recurred during the follow-up period (1 to 45 years). ABCs were significantly more common in the mandible (P < .05) and posterior areas (P < .05).3) And also in other statistical analysis of ABC incidence there was no gender difference and more frequent in mandible than maxilla. Especially, ABC often occurred in mandibular body and ramus area extending to condyle head.3,4)
About 15% of the ABCs reported were of secondary nature. Amongst the associated lesions, cement-ossifying fibroma and ossifying fibroma were the most common, followed by fibrous dysplasia and central giant cell granuloma.5-7) Histomorphology showed a predominance of the solid type (5/9) and of sinusoidal pseudocystic spaces (4/9). Giant cells, osteoid material, calcified material, blood vessels and hemosiderin deposits were observed in 6/9, 7/9, 8/9, 9/9 and 7/9, respectively.8)
On the other hand, the traumatic bone cyst (TBC) is also an uncommon non-epithelial lined cavity of the jaws with an episode of accidental traumatism. TBC has been reported in the literature under a variety of names: solitary bone cyst, hemorrhagic bone cyst, extravasation cyst and simple bone cyst. The majority of TBCs are located in the mandibular body between the canine and the third molar. Clinically, the lesion is asymptomatic in the majority of cases and is often accidentally discovered on routine radiological examination, usually as a unilocular radiolucent area with a "scalloping effect".9) Conservative treatment in the form of needle aspiration or surgical exploration and curettage is generally considered curative as well as diagnostic for the TBC. Some authors suggest that the TBC is a self-limiting entity, as it is rarely seen in patients older than thirty-five years of age. Because of the ability of the TBC to recur, long clinical and radiographic follow-up is recommended.10) TBC usually shows hemorrhagic spots in bone marrow lesion but no secondary lesions seen in ABC. However, without the information of patient’s traumatic experience the differential diagnosis between ABC and TBC, which are basically similar vascular lesions, is sometimes much difficult.11)
As the term of simple bone cyst or unicameral bone cyst gives no pathogenetic implication for the diagnosis of central pseudocyst,12) the differential diagnosis of ABC/TBC is more agreeable for the surgical treatment. The present study demonstrated a case of secondary type ABC which involved whole periapical area of right anterior maxilla resembling to odontogenic keratocyst. The pathogenetic significance of secondary ABC was discussed with the review of literature.
Ⅱ.CASE REPORT
A 17-year-old boy showed a huge radiolucent lesion at right anterior maxilla, which was accidentally found in routine dental-radiological examination for orthodontic treatment. He had no medical history of systemic disease, and did not remember any traumatic experience on his right anterior maxilla. In the panoramic X-ray view and computed tomography (CT) the radiolucent lesion was ovoid in shape, involved whole periapical area from right central incisor to right first premolar, slightly extending to the right maxillary sinus space (Fig. 1). The radiolucent lesion looked like a cyst demarcated by thin radiopaque line, and its cortical bones of labial, lingual, and sinusoidal sides were partly perforated (Fig. 1 A and C). And CT scan disclosed that the cystic space was filled with relatively dense materials containing fine radiopaque spots dispersed irregularly (Fig. 1 B).
The lesion was primarily diagnosed as odontogenic keratocyst based on the radiological observation. During surgical operation multiple fragments of cystic sac were enucleated with severe hemorrhage, but the bleeding was well controlled by saline gauze compression for a while. The removed specimens were fixed in 10% neutral buffered formalin, processed routinely, and entirely embedded in paraffin. Histologic sections in 4 μm thickness were mounted on glass slides, and routinely stained with hematoxylin and eosin. In order to examine the whole lesion the serial microsections were observed under microscope, and their microscopic images were captured by digital camera (DP-70, Olympus, Japan). The usage of biopsy specimens filed in the Department of Oral Pathology, GWNUDH was approved by our institutional review board (IRB 2016-11).
In the histological observation the thick fibrous sac showed no lining epithelium, and its luminal side disclosed multiple aneurysmal spaces which were shrunken and almost obliterated (Fig. 2 B-D). The fibrous sac itself was hyperplastic with abundant vascular channels, and produced fibromatous thickening associated with ossifying trabecular bones (Fig. 2 A). This fibro-osseous tissue was hamartomatous, which was not directly connected and organized with marrow bone of maxilla in the observation of serial microsections. The fibro-osseous tissue was not neoplastic, and mostly composed of highly cellular and collagenous fibrous tissue and osteoblasts/osteocytes-rich new bones. The new bones generated in the fibrous sac were rudimentary and separated by collagenous fibrous tissue, and rarely anastomosed each other (Fig. 2 E).
For the pathological diagnosis the present case was easily distinguished from odontogenic cyst because of the absence of epithelial element in the serial microsections, but various differential diagnosis was issued with simple bone cyst (ABC or TBC), central fibrous hamartoma, ossifying fibroma, fibrous dysplasia, central angiofibroma, etc. With the histomorphometric analysis of the whole serial microsections, the presence of multiple aneurysmal cystic spaces and the overgrowth of fibro-osseous tissue in the fibrous cystic sac independently from the marrow bone of maxilla indicated the intra-bony cyst formation by the synergistic swelling of ABC and hamartomatous fibro-osseous tissue in the sac. Therefore, this maxillary cystic lesion was finally diagnosed as secondary type ABC with hamartomatous proliferation of fibro-osseous tissue. Its postoperative wound healing was uneventful, and subsequently the patient received endodontic treatment for the involved teeth (#11, #12, #13, #14). Follow-up check was made in six months after operation, and there appeared no sign of recurrence so far.
Ⅲ.DISCUSSION
ABC is a benign, expansile lesion typically affecting the long bones and vertebrae of patients younger than 20 years. Approximately 2% of ABCs occur in the head and neck region, most commonly affecting the mandible.13) Although the most common co-existing lesion associated with ABCs is the giant cell tumor, ABCs can be radiologically confused with telangiectatic osteosarcoma in cases of aggressive behavior and rapid growth.14) In spite of a well-described lesion, much confusion exists in its pathogenesis especially of the associations of this lesion with other lesions.
Approximately, one-third of patients with ABC present simultaneously with another bone pathology.15) Most jaw ABCs are secondary in nature and frequently associated with ossifying fibroma. The patients with jaw ABCs presented with various clinical and radiographic features and therefore often posed a diagnostic dilemma.16,17) The cause of misleading diagnosis is often owing to the rapid growth of the lesion, sometimes associated with painful soft tissue swelling indicating an infectious origin or a malignant tumor.18) The ABC is a curable condition by radical excision, although relapse may occur when only incomplete excision is obtained.19) Therefore, ABC should be considered early when children present with unusual, rapid-growing neo-formations of the extremities or the facial region.20)
The present study investigated a case of ABC which involved whole periapical area of right anterior maxilla resembling to odontogenic keratocyst. Besides the multiple aneurysmal spaces in the luminal side, the cystic sac showed hamartomatous proliferation of fibro-osseous tissue in the absence of the secondary lesions, i.e., central giant cell granuloma, ossifying fibroma, fibrous dysplasia, etc. Although there appeared no feature of reparative granuloma containing multi-nucleated giant cells, the endothelial proliferation represented the reparative reaction to the hemorrhagic granulomatous lesion in the luminal area of aneurysmal space. However, with the plenty of vasculature the stromal fibrous tissue was hyperplastic and thickened, and formed ossifying bony spicules separated far from the marrow bone of maxilla. This ectopic new bones were continuously proliferative in the thick fibrous sac of ABC.
It is assumed that this reactive proliferation of fibro-osseous tissue is only possible in the young people likely 17-year-old boy in the present case. And the aneurysmal vascular dilatation/hemorrhage condition in bone marrow might stimulate the hamartomatous proliferation of fibro-osseous tissue in the cystic sac. In the present case of ABC the fibro-osseous tissue was located at the central portion of ABC, unconnected with the marrow bone of maxilla. The fibro-osseous tissue was well circumscribed by thick collagenous tissue, thereby, it was separated from the maxillary bone and grew independently. Therefore, it is presumed that not only the tumorous growth of central giant cell tumor, ossifying fibroma, and fibrous dysplasia, which had been reported, but also the hamartomatous proliferation of fibro-osseous tissue in ABC could work as swelling pressure to destroy the surrounding bone in the marrow space. Thus, the present cases of ABC may give an implication for the pathogenesis of primary and secondary types of ABCs, which show greatly variable histological features.
