Answer: Enchondroma

Histology: This lesion is an enchondroma. The principle feature of distinguishing an enchondroma from a chondrosarcoma is whether the lesion is growing. Enchondromas in adults do not grow. Therefore, pain is an important clinical finding that suggests chondrosarcoma. This patient had knee pain secondary to trauma and the lesion was found as an incidental finding. It was not painful. Radiographically, this lesion shows cartilage-type calcifications which are extremely well-marginated and are doing nothing to the cortex. Histologically, the lesion shows well-defined islands of bland hyaline cartilage. Although there are occasional binucleated chondrocytes, these histologic features are of less importance than the radiographic picture and the fact that the patient was asymptomatic.

Discussion: This lesion is not a chondrosarcoma because there is no evidence that the lesion is growing. The radiographic pattern is that of a benign cartilage lesion and the patient was asymptomatic relative to the bone lesion. Histologically, the lesion shows no evidence of a permeation of the marrow spaces by the cartilage, a feature that indicates growth. In making this distinction between enchondroma and low-grade chondrosarcoma, the features which are the most helpful are those which are obvious before a biopsy is taken.

This lesion is not a chondromyxoid fibroma. The typical histology of this lesion is myxoid and fibrous tissue and these are not present among cartilagenous areas. Typically, chondromyxoid fibromas do not show mineralization radiographically.