A benign neoplasm composed of ectopic cartilage rests that have migrated into bones’ metaphysis from the growth plate. Enchondromas are usually singular, but multiple lesions may occur in enchondromatosis syndromes such as Ollier’s, Maffucci’s and metachondromatosis. Enchondromas are a benign finding seen incidentally on plain radiographs MRI, and/or CT while investigating other pathology.

Typically, patients are 20-40 years old when the tumor is identified. The patient with any of the enchondromatosis syndromes is typically younger when the first lesion is identified. There is no gender or racial predilection.

In the United States, enchondromas account for 12-14% of benign bone neoplasms and 3-10% of osseous neoplasms in general. They are most commonly found in the diaphyses of the hands and feet, and less commonly in the femur, humerus, or tibia. Enchondromas present as small (<5 cm) lytic masses which have a pattern of lobulated arcs and rings indicative of their chondroid matrix. When found in the larger bones, they are usually metaphyseal, centrally located and can have a sclerotic margin.

Frequently plain radiographs are adequate to identify and diagnose enchondromas. Occasionally, further imaging such as CT or MRI may be required to offer a more complete picture of the bone surrounding the tumor. Bone scans are usually ineffective in the diagnosis of enchondromas as typical findings of mildly increased uptake are non-specific.

Enchondromas are benign but can cause pathologic fractures or can rarely progress to chondrosarcoma. This malignant transformation is of concern due to the destructive changes which occur in high grade tumors as well as the 5 year survival rate which is as low as 15% in high grade tumors.

Tumors found in the hands and feet have a virtually negligible transformation rate while those seen in the long and flat bones should be monitored more closely for transformation. Transformation to malignancy occurs much more frequently in patients with enchondromatosis.

The following radiographic changes are suggestive of this transformation to chondrosarcoma:
•   Scalloping of the inner surface of the bone
•   Thickening of the bone cortex
•   Reactive bone growth on the outer surface of the bone
•   Erosion surrounded by reactive bone
•   Noticeable growth in the bony erosion
•   MRI may identify cortical extension and extension to the soft tissue mass.

With the exception of enlarged digits when hands and feet are involved, physical exam of the patient with an enchondroma is usually unremarkable. Pain found on exam is attributable to either adjacent but non-related pathology, pathologic fracture resulting from the enchondroma, or should raise the clinical suspicion for progression to chondrosarcoma.

Differential diagnosis varies depending on whether calcifications are present. In cases in which they are, bone infarct which may exhibit a serpiginous lining and low grade chondrosarcoma must be considered. If the lesion is purely lytic, however, non-ossifying fibroma, simple bone cyst, fibrous dysplasia, eosinophilic granuloma, and clear cell chondrosarcoma should be considered.
When considering these diagnoses, the following factors are each used to rule out the diagnosis in question:
•   Non-ossifying fibroma – usually cortically based, enchondromas are medullary based
•   Simple bone cyst – usually seen before age 20
•   Eosinophilic granuloma – also seen commonly in patients before age 20
•   Fibrous dysplasia – has a ground glass appearance on imaging
•   Clear cell chondrosarcoma – epiphyseally based, with extension into the metaphysis usually seen on MR

When asymptomatic, treatment of enchondromas consists of observation. In the patient in whom a pathologic fracture has occurred, the fracture is usually allowed to heal, followed by curettage and filling of the cavity with bone fragments or cement.