Considering the radiologic and histopathologic features, the location and the age of the patient, this lesion was most consistent with the diagnosis of a juxtacortical chondroma (periosteal chondroma).
Juxtacortical chondroma of the leftproximal medial metaphyseal tibia
In this young, female patient with left-knee pain associated with a cartilaginous lesion of benign appearance, the relatively rare juxtacortical location, with associated cortical break, raises concern for a low-grade cartilaginous malignancy, such as a juxtacortical chondrosarcoma as opposed to the benign juxtacortical chondroma.
A juxtacortical chondroma may extend into the soft tissues without infiltration.
Clinically, presentation of a juxtacortical chondroma may be with a firm, nonmobile, slowly growing, palpable mass that may have pain that increases with lesion size.
Radiographically, a juxtacortical chondroma usually appears as a small, cortically based, radiolucent oval/oblong bony defect, longer than wide with little or no calcification.
A juxtacortical chondroma must be differentiated from a juxtacortical (or periosteal) chondrosarcoma, particularly as a juxtacortical chondroma often demonstrates high cellularity, binucleated cells, hyperchromatism and a myxoid appearance.