INTRODUCTION

Lucas and Blum first described solitary bone cysts of the mandible in 1929. Nevertheless, it was not until 1949 that Rushton established the diagnostic criterion of this condition. It was defined as a simple cyst lacking epithelial lining with intact bone wall, filled with fluid and lacking evidence of chronic or acute inflammation.^{1 2}

Donkor and Punnia-Moorthy suggested a possible sub-classification of solitary bone cyst based upon its content: empty cysts could be named idiopathic cysts, cysts with solid filling could be named according to the histological aspect of its content (fibrous or with granulation tissue), cysts with liquid content with a biochemical profile similar to blood plasma could be called extravasation cysts.³

These cysts have been described with different terms such as: solitary bone disease, solitary bone cyst, haemorrhagic cyst, extravasation cyst, single chamber cyst, simple and idiopathic cysts. This diversity in terms only reflects the uncertain origins of this lesion.^{3 4 5}

The World Health Organization's (WHO) International Tumor Histological Classification accepted the term «simple bone cyst» in 1971, and the «term solitary bone cyst» in 1992 so as to differentiate this lesion from other cystic lesions of the jaws.^{6 7} In the 1997 WHO classification, solitary bone cyst is included in the group of bone-related non-neoplastic diseases along with aneurismatic cysts, ossifying fibroma, fibrous dysplasia, bone dysplasia, giant cell central granuloma and cherubism.^{1 4 8}

LITERATURE REVIEW

DIAGNOSIS

Solitary bone cyst diagnosis is determined by the association of clinical examination, radiographic findings, physical exploration and whenever possible, histopathological examination.¹⁶

Solitary bone cysts are fortuitously found in routine X-ray analyses.^{1 26} Definite diagnosis of solitary bone cysts is inevitably reached during surgery, when a cavity lacking epithelial lining, either empty or with content (blood) and/or friable, hemorrhagic, hard-to remove, grayish-yellow tissue is observed.^{11 12 26}

TREATMENT

Recommended treatment for mandibular solitary bone cyst is surgical exploration followed by curettage of bone walls. Hemorrhage within the cavity produces a clot which is eventually replaced by bone. It is believed that in some cases, spontaneous resolution will take place.^{4 12 13 14}

Selected surgical procedures for treatment of this lesion include cavity exploration, fenestration, aspiration and condyle osteotomy (whenever the condyle is affected). Results obtained in Suei et al‘s study indicated that prognosis is more favorable when lesions are treated with fenestration and cavity filling, although no statistically significant differences were recorded.^{18 22}

It has been proposed to use bovine freeze-dried bone, cancellous bone of the iliac crest, as well as introduction of autologous blood with patient's bone or hydroxyapatite.^{4 12 22} Other materials used are absorbable gel sponge and soluble haemostatic gauze.¹⁸

The aforementioned techniques could be particularly useful whenever rehabilitation with bonesupported implants in the affected area is required. Nevertheless, introduction of radio-opaque materials in the cavity could hinder the diagnosis of a possible lesion recurrence.¹

Many authors concur in the theory that teeth with apexes involved within the lesion must not be subjected to endodontic treatment, since they present favorable prognosis and normal healing takes place without any further complications.^{12 19} Some authors recommend not undertaking curettage on the cavity's ceiling in an attempt to preserve vitality of adjacent teeth,¹² while others suggest that de-vitalization of teeth within the lesion site is a factor that prevents solitary bone cyst healing.¹⁸

It is suggested that solitary bone cysts, concurrent or not with other lesions, must continue under radiographic control after surgical intervention, focusing on those solitary bone cysts which appear with concurrent lesions, since it has been reported they exhibit greater rate of recurrence.^{3 5 18}

Suei et al. assume that suitable or adequate bone formation cannot be considered full lesion healing due to the fact that there might be recurrence even after substantial bone formation or regeneration.¹⁸

Sapone and Hansen suggested that periodic observation should be observed until radiographicallydetermined full healing of the solitary bone cyst takes place. Patients must be recalled every four months during the first year after surgery, every six months on the second year and once a year after that, until full healing is observed.^{3 18}

Suei, Tagucho and Tanimoto mention that the aforementioned intervals are too short, since neither healing nor recurrence can be confirmed at early stages of the post-operative period. Excepting those cases with post-operative complications such as pain and infection, it is more sensible to conduct the first radiographic control in a period of 12 to 17 months after surgery, since this is the time in which most observed solved cases are confirmed.^{1 18}

After the first post-operative control, it is sufficient to follow-up the case at 1 to 2 year intervals, since early detection of recurrence is not particularly useful. In most cases, resolution or recurrence will be confirmed at the first or second radiographic control within the first three years after surgery.^{5 13 18}

Life-long follow-up might be necessary (even after healing) in cases of solitary bone cyst with concurring cement-bone dysplasia, since, in those cases, there is high probability of new lesion development. This also applies to cases with multiple solitary bone cyst lesions.¹⁸

CLINICAL CASE

A 17 year old female patient with base diagnosis of Fallot's tetralogy and denied history of mandibular trauma was remitted to the Maxillofacial Surgery service of the Specialties Regional Hospital «Bicentenario de la Independencia » ISSSTE (Institute of Social Security services for Government Employees) presenting a radio-lucid lesion in the left premolar region.

The patient reported painful symptoms and paresthesia in the left area of the chin. Intraoral physical examination did not reveal volume increase, tooth mobility, periodontal pockets, alterations in adjacent mucosa, data of fistula or any type of secretions. In involved teeth, dental vitality test was positive (Figure 1).

Figure 1 Intra-oral photograph showing lesion area without changes in mucosa color. 

Orthopantomography revealed a radio-lucid lesion, with an irregular border and radio-opaque halo. The lesion involved first premolar, second premolar, and lower left canine, no root resorption or displacement was observed. The lesion exhibited a festooned border in the area of involved dental roots (Figures 2 and 3) and did not present expansion of cortical bone, nevertheless it exhibited thinning in lingual and vestibular cortical bone (Figure 4).

Figure 2 Orthopantomography showing lesion location and involved structures. 

Figure 3 Sagittal section showing radio-opacity in lesion's illumination. 

Figure 4 Cross-sectioned cut of the lesion showing thinning of lingual and vestibular cortical bone, without exhibiting expansion. 

A .3 × .3 cm incision biopsy of the lesion at the external cortical bone was undertaken. An empty bone cavity lacking epithelial lining was observed (Figure 5). The sample was sent for pathological examination; results of said study revealed trabecular bone without significant histopathological changes, therefore, a diagnosis of solitary bone cyst of the mandible was emitted. One year later, radiographic control revealed formation of significant trabecular bone (Figure 6).

Figure 5 Osteotomy for biopsy and anatomical-pathological study at the level of external cortical bone of left mandibular body where the lesion is located. 

Figure 6A Pre-operative X-ray of lesion. B. Control X-ray one year after surgery where new trabecular bone formation is observed. 

DISCUSSION

The case of mandibular solitary bone cyst studied in the present article has the peculiarity of occurring in a patient afflicted with Fallot's tetralogy: a congenital heart disease characterized by exhibiting pulmonary artery stenosis, aortic override, intra-ventricular communication and right ventricle hypertrophy. The mechanism whereby growth is affected is probably multi-factorial, including tissue hypoxia, increase of energy expenditure, decreased food intake as well as frequent respiratory infections.^{28 29}

Solitary bone cysts could be related to Fallot's tetralogy due to the alterations in calcium metabolism found in patients afflicted with this condition. This can determine the development of an osteolysis zone. Children afflicted with Fallot's tetralogy are more prone to suffer chromosomal disorders such as Down's syndrome and Di George's syndrome (condition causing heart defects, low calcium levels and immunodeficiency).²⁹ This latter could be important in cases where Fallot's tetralogy were to be present, due to hypocalcemia produced by hypoparathyroidism, along with other calciummetabolism related factors. All the aforementioned would lead us to consider the etiopathogenesis of solitary bone cyst as the product of an hormonal alteration.

One among the multiple theories about the origins of solitary bone cysts is related to alteration in bone growth,⁹ as well as flaws in mesenchymal tissue during the process of cartilage and bone formation.¹⁹ Due to its location, the solitary bone cyst afflicting our patient could be related to this theory since in the mandible, the primary ossification point is located close to the mental foramen, therefore, it is an ideal area for the onset of a solitary bone cyst due to its predominantly close location to cartilage growth plates. In Blum's and Toma's studies, mentioned in Xanthinaki case¹¹ it is concluded that there must be a predisposition of idiosyncratic factors in the pathogenesis of solitary bone cysts, such as peculiarities in the vessel walls or an abnormal blood clotting, related to chronic hypoxemia present in patients with Fallot's tetralogy, which might include altered calcium functions in blood clotting.²⁸

Even though in most cases solitary bone cysts of the mandible are asymptomatic,^{10 11 18} reported pain and paresthesia were the reasons why the patient sought treatment. In this case, X-rays studies did not reveal presence of radio-opaque lesions concomitant with solitary bone cyst, therefore it was concluded that painful symptoms experienced by the patient were due to cyst location.

The lesion extended along the mandibular body without causing bone expansion; lower buccal and lingual cortical bone became thinner (Figure 4). There was no displacement of adjacent teeth towards the lesion; teeth responded positively to vitality tests. These data concur with those reported by Chrcanovic's³⁰ and Xanthinaki's¹¹ clinical case studies. Microscopic study revealed trabecular bone lacking significant histological data, such as mentioned by Harnet et al.⁹

In spite of multiple treatments suggested in scientific literature to treat solitary bone cysts,^{12 13 18} in our case, when the lesion was approached to harvest the biopsy and no epithelium was found, it was decided to perform bone curettage of cavity walls and bone regeneration induction.

Control X-ray examination was conducted one year after surgery, bearing in mind results reported by Suei et al¹⁸ where it was stated that lower frequency of control examinations will result in lesser patient exposition to radiation. In most cases resolution or recurrence are confirmed at the first or second radiographic controls in a 3 year span after surgery since diagnosis of early recurrence is not significant when we consider that this is a progressively growing lesion.

CONCLUSIONS

In solitary bone cysts cases, undoubtedly, the determinant factor is surgical exploration of the lesion, although its clinical characteristics and scientific literature descriptions must be borne in mind as well as the fact that histological and radiographic characteristics are important to sustain accurate diagnosis.

Since etiology of these cysts is unknown to the present date, it can only be intuited according to personal characteristics of each patient. The most accepted theory suggested a traumatic origin for these lesions. Nevertheless, when trauma is discarded as etiologic factor, the most accepted theory would be related to calcium metabolism alterations associated to the overall condition of our patient.

Some authors suggest to preserve the lesion in observation hoping for a possible spontaneous resolution. Nevertheless, we decided to undertake surgical approach due to the painful symptoms reported by the patient and to discard as well the possibility of other lesions. We suggest to undertake surgical approach of any lesion with presumptive diagnosis of solitary bone cyst.

Although it is reported in scientific literature that solitary bone cysts are asymptomatic, we concluded that paresthesia and painful symptoms experienced by our patient were related to the anatomical location of the lesion. Oftentimes it is not possible to establish a diagnosis through histopathological study due to insufficient amounts of sample, which would be the case in empty solitary bone cysts, or cysts with scarce fibrous connective tissue adhered to the cavity walls. Moreover, results can provide non-significant characteristics for differentiation of other pathological lesions, or even in proper diagnosis of solitary bone cysts.

Other treatments could be applied, such as bovine freeze-dried bone graft, iliac crest cancellous bone, introduction of autologous blood with patient's bone or hydroxyapatite, to be achieved in cases of recurrence, in cases of extensive solitary bone cysts, or when cysts are concomitant with other lesions.