K. A. Ahmed, BDS, MSc,*
F. Al-Ashgar, BDS, MSc**
* Department of Biomedical Dental Sciences, King Saud University, College of Dentistry, P.O.Box 60169, Riyadh 11545, Saudi Arabia.
** Dental Department,  King Fahad National Guard  Hospital, Riyadh, Saudi Arabia.

Solitary bone cyst (SBC), a relatively uncommon lesion of the jawbones is a type of the so-called "pseudocysts" of the jaws since the bone cavity is not lined with proper cystic membrane. The numerous synonyms coined with this lesion make it obvious that there has been much disagree-ment on the pathogenesis of the lesion. The SBC presents usually in the posterior region of the mandible and rarely in the maxilla. A case of anterior maxiilary SBC has been presented in a patient with hypothyroidism. The lesion presented itself as an asymptomatic buccal expansion above the upper central incisors with no history of previous local trauma. This article suggests possible rote of disturbed calcium metabolism as an etiologic factor for SBC in hypothyroidism patients and to present a review of the literature of the cyst.

The solitary bone cyst which was first described by Lucus1 in 1926 is no longer considered as a rare lesion of the jaws, although it is more common in long bones, namely humerus2. In the head and neck region, the solitary bone cyst (SBC) presents commonly in the mandible,3 occasionally in the maxilla,24 and rarely in the zygoma.5 Mandibular lesions predominate in the posterior region,2,6,7 however, lesions in the anterior mandible were reported8 and found by Beasley9 and Kuroi10 to
have an incidence of 0.13 and 0.24, respectively. In the maxilla, the lesion is relatively uncom­
mon4,9,11-16 with few reports in the literature all involving only one or two cases, with the exception of Hansen4 who reported 21 maxillary cases in his series. The clinical presentation of the lesion may be asymptomatic and discovered on routine radiog­ raphic examination. The existence of presenting symptoms was reported and found to range from 3% to 35% of the cases,6,17 however. These
symptoms ranged from osseous expansion,6,17 pain,17 paraesthesia of the affected sides,18,19 pathological fracture of the mandible,19,20 multiplicity of the lesion,9,20-24 and the presentation of SBC in the mandible and the humerus.21 In addition, the lesion can be associated with other pathological conditions such as necrotic pulp,4,22, facial cel­ lulitis,22 impacted third molar,23 and odontogenic keratocyst.25 The SBC was believed to have an age and sex predilection since it shows commonly between the second and third decades of life, and the lesion affects males more than the females in a ratio of 3:2.2,4 Kougars and Cale,26 on reviewing 161 cases of SBCs, found that there is no gender predilection for the lesion, however. The diagnosis of SBC is mainly made on clinical basis, i.e., by surgical intervention. The exploration procedure reve­als a relatively empty bone cavity with little tissues, connective tissue,2 with either littie fluid content or gas in the cavity. The fluid content of the cyst was said to have a higher bilirubin level27 and/or acid phosphatase level28 as compared to other cyst fluids. Histologically, the SBC is characterized by the absence of an epithelial fining with only loose connective tissue covering the bony wall2,6,29 con­ taining congested capillary vessel, extra-vasated blood cells, hemosiderin and multinucleated giant cells.30'31 Radiographically, the SBC commonly presents as a single, unilocular, well demarcated, radiolucency of variable size. In the premolar region, the lesions may demonstrate a scalloped appearance due to the projection of bony cavities into the intra-radicular septa.31 The involved teeth may be displaced, but seldomly resorbed and often reposition themselves after resolution. Atypical radiographic representation of the SBC has been reported by Mitchell and Ward-Booth32 as a multiloculated radiolucency, however.
Various treatment modalities were adopted for SBC: (1) keeping the case under observation and waitingfor spontaneous regression;2,4,6,7,33 (2) aspi­ ration of the contents of the SBC;34 (3) surgical exp­ loration and curettage to stimulate bleeding, heal­ ing, and initially to confirm the diagnosis;16 (4) packing with gel foam saturated with thrombin and penicillin;35 (5) endodonticintervention;36,37 (6) injection of methyl prednisolone acetate (MPA) sol­ ution for treatment of long bone cases;38 (7) injection of autogenous blood to stimulate the osteogenic activity;39 and (8) bone grafting.40 Recurrence of the lesion is not commonly encountered, however, it is being postulated that existence of another cyst within the bone cavity, which may not have been enucleated, is a cause for recurrence.32,40,41

Case Report

On April 6, 1986, a 33-year-old Syrian woman was referred to the Oral Surgery Unit, College of Dentistry, King Saud University in Riyadh, Saudi Arabia for evaluation of an asymptomatic unilocular radiolucency related to the apices of the maxillary central incisors, which was discovered on routine radiographic screening.
The patient gave a history of hypothyroidism in the last 10 years and under treatment with Eltroxin 200 mg tab. once a day. In addition, there was a history of calcium malabsorption especially during pregnancy. No history of local trauma was reported.
On examination, the teeth were vital and they responded normally to both thermal and electrical pulp testing. There was no periodontitis in the area and the teeth were not mobile or tender to percus­ sion. On palpation, there was a non-tender osse­ous expansion of the labial cortical plate of bone about 2 cm in diameter above the maxillary central incisors. The overlying mucosa was intact and of normal appearance. Clinically, no draining sinuses or lymph node involvement were observed [Fig. 1]. Radiographic examination showed a 1.5 x 0.5 cm. radiolucent area with thin sclerotic border superior to the apices of the maxillary central incisors. The apical third of the roots of the central incisors were involved in the radiolucency. The lamina dura of the teeth was intact [Fig. 2].
Complete skeletal survey of the patient revealed an irregular contour of the iliac crest. Hematologi­
cal study showed slight poikilocytosis and polyc-hromasia of the RBCs and an elevated alkaline phosphatase level at 314 U.I. Serum calcium and inorganic phosphorus were within normal limits. The differential diagnoses included SBCs, nasopalatine duct cyst, primordial cyst, central giant cell lesion, early stage of fibro-osseous lesion, odontogenic tumor and central hemangioma.
On April 13, 1986, the decision was made to explore the area under local anaesthesia using two carpules of 1.8 ml of 2% plain xylocaine solution after initial failure of aspiration biopsy.
A semilunar mucoperiosteal flap was raised from the right to the left lateral incisors exposing the bony expansion labially. A window was opened in the expanded labial cortical plate using the postage stamp bur technique. This revealed a bony cavity with very little blood-tinged fluid and some fine fri­ able soft tissue [Fig. 3]. Curettage of the cavity was performed and the flap was sutured back with black silk suture. The removed piecesof bone, with the minimal amount of soft tissue and fluid, were sent for histopathological study. The wound healed uneventfully and the stitches were removed after seven days of the operation. Two weeks after the surgery, endodontic treatment of the maxillary right central incisor was started after the tooth showed diminished vitality response.
Histopathological report confirmed the clinical diagnosis of SBC made from clinical examination and surgical intervention.

The numerous synonyms coined with the SBC which varies from progressive bone cyst, traumatic bone cyst, traumatic hemorrhagic cyst, hemorrhagic cyst, extravasation cyst, hemorrhagic extravasation cyst, unicameral cyst, simple bone cyst reveals a considerable disagreement regarding the pathogenesis of the lesion. The pathogenesis and the etiology of SBC still remains uncertain, with the most widely accepted theory of an intra-medullary hemorrhage as a result of trauma, which fails to organize, and the subsequent degeneration of the clot producing an empty cavity within the bone.2- 4,13,15,16 On the other hand Kougars and Cale26 found that in 161 reviewed cases, the history of prior trauma among these patients was equivalent to that described for the general population. Other theories for the pathogenesis of SBC2,9,17 included : (1) infection of bone marrow; (2) loss of blood supply to a hemangioma or lymphoma; (3) cystic degeneration of existing bone tumor; (4) changes and reduction in the osteogenic activity; (5) faulty calcium metabolism as a result of systemic disease, such as parathyroid diseases; (6) ischemic necrosis of the fatty bone marrow; (7) low grade chronic infection; (8) imbalance between the osteoclastic and osteoblastic activity due to trauma; (9) developmental defect; (10) failure of mesenchymal tissue to form bone and cartilage, and instead becomes immature as multiple bursa-like synovial cavities.41
The present case suggests the possible implica­ tion of hypothyroidism which induced disturbed calcium metabolism in the pathogenesis of SBC without denying the role of trauma in some cases as an initiating factor for their lesion. There is also the remote possibility of drug-induced (Eltroxin) etiol­ ogy in this case.
Hosseini41 suggested that according to existing theories of pathogenesis, the SBC should appear with the same frequency in the maxilla as in the mandible. Hansen4 and Kaffe et al43 assumed that the low incidence of maxillary SBC is due to the superimposition of various anatomical radiolucen-cies in the maxilla which obscures the SBC. This report further supports the latter view specially for anterior maxillary SBC, where suspect of superim­ position of the naris can cause some cases of SBC to be missed specially if the lesion is asymptomatic.
The diagnosis of SBC prior to surgical interven­ tion, constitutes in most instances a great difficulty to general dental practitioners as the lesion can have different presentation in nearly every region of the jaws. For these reasons, Rushton29 adopted the following criteria for establishing diagnosis : (1) a single lesion, (2) no epithelial lining, (3) no infec­ tion, (4) no perforation of the bony walls, and (5) fluids in the lesion. Later on, Hansen4 modified these criteria as follows : (1) upon surgery, the lesion is essentially empty and, occasionally, the cavity contain some fluid and/or small amount of tissues; (2) other findings (clinical, radiographic, historic, histopathologic, etc.) do not exclude the diagnosis of SBC. Compared to both Rushton29 and Hansen4, the reported case was found to meet all the described criteria.