2. Bone Tumors of the Foot Unicameral bone cyst Unicameral bone cyst is a benign lesion of uncertain origin. Several etiologies have been proposed, including expansion of synovial tissue trapped in the bone during development, local failure of ossification, or obstruction of the venous outflow of the bone. In the foot, UBC occurs almost exclusively in the calcaneus, and presents in teenagers or young adults as an incidental finding or with mild aching pain during sports or running. The location and appearance is characteristic and biopsy may not be needed to confirm the diagnosis. The location is very specific. The lesion is in the lateral portion of the calcaneus sub-adjacent to the middle facet. The apex of the lesion is towards the forefoot. The margin of the lesion is sharply defined, sometimes with a sclerotic rim. There is no matrix mineralization or periosteal reaction. On MRI the lesion has high signal intensity with minimal loculations or heterogeneity consistent with a fluid filled cavity. Differentiation of UBC from other similar lesions should be possible based on the MRI findings. Intraosseouslipomatyopically has signal intensity consistent with adipose tissue (high signal intensity on T1 and T2 alike) and aneurysmal bone cyst has a multiloculated appearance with fluid-fluid levels with each loculation. On CT the lesion has no matrix, and fills the bone to a variable degree. There is no periosteal reaction. The cortex is often quite thin. Bone scan is not recommended for these lesions.Displaced fractures through UBC in the calcaneus are uncommon. Treatment for asymptomatic cases consists of observation and follow-up radiographs to insure the lesion is not growing or changing. Painful cysts can be treated with a wide variety of more or less invasive techniques and there is a lack of consensus as to the optimum choice of treatment. Minimally invasive techniques including aspiration and injection with methylprednisolone, bone marrow, autogenous or allogenous bone graft or bone graft substitutes, or a combination of these, or curettage without grafting, creating multiple drill holes, or percutaneous screw decompression. All of these techniques have been shown to be effective for some cysts. Virtually all the reports in the literature, however, have a relatively low level of evidence (level III, IV, V). New reports from a randomized, multi-center trial are due out soon, which apparently show no significant benefit for bone matrix or bone marrow preparations over simple injections of methylprednisolone. However, the biological mode of action of the injections remains unknown. Various technical factors have been proposed that may increase the success rate of steroid injections, including wide spacing of the needles to ensure complete exposure of the lesion, and using radiological dye to insure complete filling of the lesion. These techniques may decrease the need for multiple injections. The author of this site recommends that simple treatments with minimal morbidity be tried first, and that more invasive, expensive, and risky interventions be reserved for difficult cases. Click on an image to enlarge(All images © bonetumor.org)