J Orthop Sci (2003) 8:599–601 DOI 10.1007/s00776-003-0680-4 Unusually large solitary unicameral bone cyst: case report Sandeep Singh, Ish Kumar Dhammi, Anil Arora, and Sudhir Kumar Department of Orthopaedics, University College of Medical Sciences, Guru Teg Bahadur Hospital, Shahdara, Delhi 110 095, India is nothing in the literature about this subject. The differential diagnosis includes histiocytosis X, fibrous dysplasia, and aneurysmal bone cyst. The withdraw- ing of typical straw-colored serous fluid on needle aspiration usually is considered specific enough to make the diagnosis without histological sections. 1 Various methods have been described to treat a unicameral bone cyst, with varying results. We present a huge UBC involving the proximal half of the tibia that was treated successfully with curettage and allogeneic bone grafting twice. To the best of our knowledge, a solitary unicameral bone cyst of such large dimensions has never been reported in the English literature. Case report A 15-year-old boy presented at the orthopedic out- patient department with dull aching pain in his right leg that had been present for 3–4 months; it was aggravated by walking and relieved by resting. Examination revealed swelling with tenderness over the upper two-thirds of the tibia and a mildly elevated local temperature. The swelling was firm to hard in consistency with distinct margins, and it was continuous with the shin. The knee, hip, and ankle were within normal limits. There was no limb-length discrepancy. A hemogram with an erythrocyle sedimentation rate (ESR) was within normal limits. A radiograph of the leg revealed a large lytic lesion measuring 14 cm in length in the proximal half of the tibia. There was no periosteal reaction or cortical thickening, although there was endosteal scalloping and thinning of the cortices. The distal medullary canal appeared to be blocked by a sclerotic line (Fig. 1). The differential diagnosis included a simple bone cyst, fibrous dysplasia, and a nonossifying fibroma. His skeletal survey revealed no such swelling elsewhere in the body. Abstract A 15-year-old boy presented with complaints of pain and swelling of the right leg. Radiography revealed a large lytic lesion involving the proximal half of the tibia. The patient was taken to surgery, where the cavity was curetted and packed with allogeneic bone graft. Tissue was sent for histopathological evaluation. The patient was kept in an above-knee cast for 4 months, after which partial weight- bearing was allowed. The histopathological diagnosis was a unicameral bone cyst. A radiograph at 1 year showed incor- poration of the graft, but a small part of the cavity was still visible and there was a persistent discharge. The patient was again subjected to allogeneic bone grafting in the remaining cavity. At 3.5 years of follow-up now, the patient is walking unsupported, there is no pain, and the radiograph shows complete obliteration of the cavity. Unicameral bone cysts are usually a few centimeters in size. To the best of our knowledge, a unicameral bone cyst of such a large size has never been reported in the literature. Key words Unicameral bone cyst · Simple bone cyst · Allogeneic bone grafting Introduction Unicameral bone cysts (UBCs) account for 3% of all bone tumors and usually involve the metaphysis of long bones. 7 These cysts have a distinct predilection for the proximal humerus and the proximal femur, with as many as 80% of these cysts occurring in these regions. 1 The incidence of UBCs involving the tibia has not been clearly defined in the literature. Kaelin and MacEwen reported an incidence of 3.6% in their series. 5 Most of the patients with UBCs present with a pathological fracture or a complaint of mild pain in the affected region. 3,8 UBCs vary in size and volume, although there Offprint requests to: I.K. Dhammi Received: May 27, 2002 / Accepted: April 17, 2003