Department of Orthopaedics, Fortis Malar Hospital, Gandhinagar, Adyar, Chennai- 600020, India.

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Aneurysmal bone cyst (ABC) is a relatively rare benign, locally expansile, hemorrhagic lesion predominantly in first and second decade of life (95%) [1-3]. It occurs as primary lesion in 80% cases and secondary to other bony lesion in 20% [4]. Overall it accounts to 1% of bony tumors [5,6]. It can involve any bone, but mostly metaphysis of the long bones are affected (65%), pelvis (15%) and spine (15%) [7]. The differential diagnosis associated with this lesion include unicameral or simple bone cyst, giant cell tumor (GCT), Brown’s tumor secondary to hyperparathyroidism, giant cell reparative granuloma (GCRG) [8-10]. Treatment options for ABC are curettage or resection and bone grafting with or without adjuvant like phenol, liquid nitrogen [11,12]. The present reports a case of ABC localized to metatarsal, a relatively rare presentation of which only few cases have been reported in literature till date.

A 5 year old boy was brought by his parents with complaints of on and off pain in his right foot. They noted swelling in his right foot whenever he had pain. It aggravated on activity and relieved after a period of rest. These symptoms have been for the past five months. Initially they met a doctor who diagnosed it as fracture and advised splinting, but they pain persisted with waxing and waning after the splint was removed. He had no other constitutional symptoms associated.

Physical examination revealed a tender, densely indurated, immobile swelling measuring 5 cm in diameter localised to right forefoot. Standard foot x-ray revealed a well demarcated, expansile lesion with thin cortical margin localised to 3rd metatarsal [Fig.1]. The boy underwent surgery where a window was made through the lateral cortex and the lesion was thoroughly curated and sent for histopathology. The void was filled with synthetic bone graft, calcium sulfate (Stimulan).

Intra-operative finding of the lesion appeared bluish red, bulbous with thinned out cortical shell measuring 5×4×2 cm. Microscopic appearance showed large amounts of dilatation and congestion of the associated small blood vessels, osteoblast proliferation, fibrous connective tissue and multinucleated giant cell proliferation, with reactive hyperplasia and trabecular bone tissue. Final diagnosis of ABC was made after collective clinical information. The treatment was successful as no further intervention was required in one year follow-up and the patient remains healthy at present [Fig.2-4]. But due to 20% recurrence in open physis, the child will be followed closely till skeletal maturity.

ABCs account for 1% of all primary bone lesions that are sent for biopsy [11]. Though the precise pathogenesis of ABC is unclear, the most widely accepted hypothesis of ABC involves local circulatory disturbance, which results in an increase in venous pressure and the development of enlarged and dilated vascular components within the affected bone [2]. The differentiation of ABC and other giant cell-containing tumors of the bone, such as GCT, GCRG and Brown tumor, is crucial [11]. GCT is composed of mononuclear and osteoclast-like multinucleated giant cells, which have the potential to be locally aggressive [14,15]. In GCT, the tumor is an epiphyseal lesion always eccentric and exhibits lytic expansion [16]. GCRG is a rare, benign, intra-osseous reactive lesion, characterized pathologically by a predominance of giant and mononuclear cells in areas of hemorrhage [17]. Brown tumors have been reported to occur in 1-2% of patients with chronic renal deficiency and to have a considerably more lobulated architectural growth pattern; but hyperparathyroidism can be ruled out as a differential diagnosis on the basis of serum calcium, parathyroid and phosphorus hormone levels [18,19]. ABC, on the other hand, is known to be histologically composed of blood-filled cystic spaces separated by fibrous septae [20].

Though X-ray finding is classical, computed tomography, MRI and bone scans may be helpful in the diagnosis of ABC, since T2-weighted MRI could detect a deformity in the involved metatarsal bone as a segmented, expansile, multi-septated lesion with a large quantity of fluid present [21] and bone scan shows donut sign. Both curettage and en-bloc surgical removal is considered the optimal treatment option for ABC. The lesion is removed by intra-lesional curettage through a wide cortical window, and allograft or synthetic bone grafting may be used to fill up of bone defects [22]. Embolotherapy has also been successfully used for the treatment of ABCs [23]. However, patients must be informed that ABC has a high recurrence rate [24], so that any recurrence or malignant transformation can be detected as early as possible.

In summary, ABC is a destructive, hemorrhagic and tumor-like lesion occurring predominantly in teenaged patients. Radiographs and MRI scans can often confirm the diagnosis of ABC; however, accurate histological evaluation is imperative for diagnosis. Curettage with bone grafting [11], embolotherapy and replacement of bone defects with a autograft are considered safe procedures with minimal recurrence risk. 

The present study described a rare case of a ABC in the metatarsal and highlighted the importance of radiological and histological examinations for the accuracy of such diagnosis

Contributors: SKS: manuscript writing, patient management; ANV: manuscript editing, patient management; ABG: critical inputs into the manuscript. SKS will act as guarantor. All authors approved the final version of this manuscript.

Funding: None; Competing interests: None stated.