Simple bone cyst is a benign cystic lesion found commonly in skeletally immature patients in the metadiaphyseal region [1, 2]. Most common to the femur and humerus, hand is an extremely rare area for cysts to be present. So far, only a few reports have described unicameral bone cysts (UBCs) of the phalanges, metacarpals, and carpals. Treatments have mainly consisted of curettage and bone grafting with fixation as required [2, 3, 4, 5, 6, 7].

A 42-year-old manual laborer male (right hand dominant) gave history of trivial trauma to right hand thumb with hammer 2 days before presentation. He complained of mild swelling and pain since trauma. The pain was constant dull aching in nature and with mild swelling. The patient did not have any complaints of the part before the injury. There was no history of previous trauma or infection of the said area previously. His family and personal history were insignificant. On examination, there was minimal swelling of the dorsum of thenar side with no redness or rise in temperature locally. The patient had moderate tenderness and was able to do movement of the area with difficult. No sensorimotor or vascular deficit was found distally and no obvious deformity was visible.
On imaging the right hand (Fig. 1), an expansive lytic lesion was seen involving the entire metaphyseo-diahyseal region of the first metacarpal. There was thinning of the cortex with break in cortex continuity visible at mid diaphyseal region. There was absence of any soft-tissue involvement or periosteal reaction. Proximal and distal joints were normal and rest all the bones appeared to be normal. Magnetic resonance imaging (MRI) was performed with T1, T2, and short tau inversion recovery (STIR) imaging in multiple planes (Fig. 2, 3). It showed a T2/STIR hyperintense and T1 isointense benign osteolytic lesion involving entire of the 1st metacarpal with incomplete cortical fractures involving the shaft. The periosteum, surrounding soft tissue, and articular surfaces were normal. With the above report in mind, we went for a biopsy of the lesion. On aspiration, a reddish tinged fluid was found. All routine blood investigations including coagulation profile were normal. A closed fixation was performed with the help of an intramedullary k-wire (Fig. 4). The patient had minimal discomfort post-operative and a 2-week follow-up radiograph of the patient showed a well healing lesion (Fig. 5). K-wires were removed at 3 weeks and a 4 weeks follow-up X-ray (Fig. 6) showed a healing fracture with good amount of callus formation, decrease in soft-tissue swelling and intramedullary calcification. The patient was able to mobilize his thumb with almost no pain. There were no signs of infection and the k-wire wounds healed well.

Differential diagnosis of a benign osteolytic lesion of hand in a skeletally mature male is as follows [8, 9, 10, 11, 12, 13]:
• Enchondroma
Benign cartilage forming lesions of the hand, present as lytic lesion in metadiaphysis, well circumscribed, histologically show cartilage cells
• Aneurysmal bone cyst
An expansile eccentric lesion with multiple air fluid levels (on MRI) with blood filled cavities
• UBC [14, 15]
Expansile lesion in metadiaphyseal region. Single cavity and X-rays show single cavity with classical “fallen leaf” appearance in some cases with fracture
• Chondroblastoma (exceedingly rare)
• Chondromyxoid fibroma (exceedingly rare)
• Giant cell reparative granuloma
• Intraosseous ganglion cyst
Benign expansile lesion, histologically contains myxoid material. From the above based on MRI and aspiration findings, our diagnosis pointed toward either a UBC or an aneurysmal bone cyst. This was associated with a pathological fracture. Various modes of treatment have been considered for a UBC such as intralesional steroid injection, curettage, and bone grafting and intramedullary nailing [16, 17]. However, cyst recurrence is a problem common to all treatment methods. UBCs have been proposed to occur as a result of trauma, intraosseous synovial cyst, and venous occlusion leading to raised intramedullary pressures. We broke the septae intraoperatively and fixation was done by a Kirschner wire appropriately. The patient had minimal pain postoperatively and was able to go home immediately on the next day. Fifteen days follow-up is awaited.

The end result at 4 weeks follow-up was a well healing calcification of the pathological fracture of UBC in the 1st metacarpal. The patient had almost no pain and was able to do active movements of his thumb easily. There were no signs of risk of infection or further progress of the bone cyst. Soft-tissue edema had subsided and bone girth had almost returned to normal.

Simple bone cysts, although extremely rare in the hand bones, need to be considered as a differential in such cases and treated appropriately.

UBCs of the hand bones can be a challenging diagnosis and difficult to treat considering the size of bones and the surrounding tissues. K-wire fixation offers an easy and effective technique for the same. However, further studies into this area can boost better and faster diagnosis and treatment.