Enchondromas are benign cartilaginous neoplasms that are usually solitary lesions in intramedullary bone. The primary significant factors of enchondromas are related to their complications, most notably pathologic fracture, and a small incidence of malignant transformation, which may be associated with pathologic fracture. 
The lesions replace normal bone with mineralized or unmineralized hyaline cartilage, thereby generating a lytic pattern on radiographs or, more commonly, a lytic area containing rings and arcs of chondroid calcifications. The lesions likely arise from cartilaginous rests that are displaced from the growth plate. See the images below.
When multiple enchondromas coexist, the diagnosis of enchondromatosis should be considered.
Multiple enchondromas may occur in 3 distinct disorders:
Metachondromatosis consists of multiple enchondromas and osteochondromas. Of the 3 disorders, metachondromatosis is the only one that is hereditary, which is by autosomal dominant transmission.
Initially, radiography is the imaging modality of choice. If further characterization is necessary, magnetic resonance imaging (MRI) or computed tomography (CT) scanning is performed. If uncertainty remains, bone scanning may be helpful, but some lesions require biopsy.
Radiographic findings may not demonstrate or adequately characterize subtle calcifications to the extent that CT scans do. When calcifications are not present on either examination, MRI may be performed, which should reveal the classic appearance of noncalcified hyaline cartilage. Dystrophic calcifications in bone infarcts may be difficult to differentiate from chondroid matrix with ionizing radiation, and MRI may be helpful in these cases. When a clearly serpiginous rind of sclerosis encapsulates the lesion, the diagnosis of bone infarct is straightforward; however, when this finding is not present, MRI may be useful in differentiating the 2 entities. CT scans may also demonstrate the presence of calcification not depicted on radiographs. [9, 10, 11, 12, 13, 14, 15, 16, 17]
Chondrosarcoma—in particular, low-grade chondrosarcoma—may be indistinguishable from enchondroma. Certain imaging features may be helpful in distinguishing enchondroma from chondrosarcoma. However, even a lesion with no imaging features suggestive of malignancy and an imaging appearance compatible with an enchondroma should be resected when it is associated with pain. [15, 17]
When the lesion has calcifications, the primary differential diagnoses are bone infarct and chondrosarcoma. When the lesion is purely lytic, as shown on radiographs, the differential diagnosis consists of benign lytic lesions such as nonossifying fibroma, simple bone cyst, fibrous dysplasia, eosinophilic granuloma, and clear cell chondrosarcoma (which tends to involve the end of the bone—in particular, the proximal humerus).
In a study from 1998 to 2005, the authors concluded from study findings that for accurate diagnosis, conventional radiographic examination and, if necessary, contrast-enhanced MRI should be performed; in addition, histologic investigation was determined to be compulsory because of the risk of malignancy. 
A classic pattern of calcifications, described as rings and arcs, is pathognomonic when it is seen in the hands. In the long bones, calcifications may be difficult to distinguish from the dystrophic calcifications seen in bone infarction. In addition, radiographs may not depict the rings and arcs of calcifications, particularly in the hands and feet.
See the radiographic images below.
On radiographs, the degree of confidence in the diagnosis of an enchondroma depends on the clinical situation and the appearance of the lesion. In the presence of pain, almost regardless of appearance, further investigation is warranted. Lesions that are predominantly lucent or are nonuniformly mineralized often require further investigation as well.
In most cases, enchondroma and a low-grade chondrosarcoma cannot be reliably distinguished on the basis of radiographic findings alone. A calcified enchondroma can also mimic the appearance of a calcified marrow infarct. However, in most cases, chondrosarcoma has certain imaging features that are indicative of its aggressive behavior. Cortical breakthrough, soft-tissue mass, and deep endosteal scalloping of the cortex are 3 features that are described more frequently in chondrosarcoma. Deep endosteal scalloping with consequent pathologic fracture in the small bones of the hands and feet does not imply malignancy, however, because enchondromas are more cellular and expansile in these locations.
In Ollier disease, enchondromas often appear to be larger than they do in other conditions. Because enchondromas occur in young patients and can be large, growth of the affected limbs may be adversely affected, and pathologic fractures may occur. Enchondromatosis can occasionally have the appearance of linear lucencies, in which the chondrocytes appear to line up in a vertical orientation along the length of the bone.
In Maffucci syndrome, associated soft-tissue hemangiomas are seen. Soft-tissue hemangiomas typically have numerous rounded calcifications with central lucencies, which are consistent with phleboliths on plain radiograph. T2-weighted MRIs of these lesions show high-signal-intensity tubular structures, which are consistent with slow flow in vascular channels.
Metachondromatosis has associated osteochondromas, which differ from conventional osteochondromas in that they point toward rather than away from the joint.
The features of enchondroma on CT scan are the same as those depicted on radiographs (see the image below). Enchondromas are endosteal lesions with a lobular morphology and variable mineralization. Often, the mineralization is in the form of rings and arcs, which correspond to calcification around lobules of cartilage. A pathologic fracture may be present. Sometimes, endosteal scalloping is present, but this feature may be suggestive of degeneration of the enchondroma to a chondrosarcoma. Enchondromas should not penetrate the cortex or extend into the soft tissues. If a densely mineralized or uniformly mineralized lesion has a region that is lucent, degeneration of the enchondroma to a chondrosarcoma is suggested, and biopsy is likely necessary.
As with radiographs, the degree of confidence in the diagnosis of an enchondroma with CT scanning depends on the clinical situation and the appearance of the lesion. The absence of pain, uniform or dense mineralization, and endosteal scalloping or cortical penetration are all indications of an enchondroma. However, if the patient lacks any of these features, further investigation may be necessary. Distinguishing an enchondroma from a low-grade chondrosarcoma is often problematic; sometimes, the diagnosis cannot be resolved without a biopsy. Because cartilage lesions are often not uniform at histologic examination, the entire lesion may need to be removed at biopsy.
Magnetic Resonance Imaging
MRI findings may be useful in distinguishing enchondromas in the long bones from bone infarcts (see the images below). Typically, bone infarcts are encapsulated by a serpiginous rind of decreased signal intensity on T1- and T2-weighted images. Frequently, this rind is subtended by a high-signal-intensity line. This finding has been called the double-line sign, and it consists of a high-signal-intensity rind circumscribed by and immediately adjacent to a low-signal-intensity rind on T2-weighted images.
Conversely, enchondromas tend to have lobulated borders with a cluster of numerous tiny locules of high-signal-intensity foci on T2-weighted images that appear to coalesce with one another and reflect the high fluid content of hyaline cartilage. On T1-weighted images, enchondromas demonstrate low-to-intermediate signal intensity.
When a calcific chondroid matrix is observed on radiographs, decreased signal intensity is expected to occur in those areas on images obtained with all MRI pulse sequences. These areas may become particularly pronounced on gradient-echo images.
When combined with a typical appearance on radiographs and an absence of pain, the diagnosis of enchondroma with MRI findings can be made with a high degree of confidence. Enchondromas may also be incidental findings at routine examination, such as MRI of the knee. If the lesions have a typical appearance, a confident diagnosis may be made; however, the acquisition of correlative radiographic findings is often prudent.
Occasionally, enchondromas can be difficult to distinguish from other intraosseous benign and malignant lesions on the basis of MRI findings alone. Correlation with other results, particularly radiographic results, may help. The presence of bone pain always warrants further investigation.
With technetium-99m (99mTc) radionuclide bone scanning, findings are typically negative in uncomplicated enchondromas, but the presence of pathologic fracture results in intense activity at the fracture site. In patients with multiple enchondromas or patients in whom an enchondroma is actively calcifying, radionuclide bone scans may show increased activity, but the activity is typically less intense than that of intramedullary chondrosarcomas.
Positron emission tomography (PET) using 18-fluorodeoxyglucose (18-FDG) has been applied increasingly to the evaluation of tumors. Using PET to evaluate enchondromas—particularly to distinguish benign enchondromas from low-grade chondrosarcomas—has been reported in small series.
Feldman et al recommended 18-FDG PET as an adjunct for the evaluation of aggressive cartilage lesions. In their study, 11 enchondromas were studied by PET with 18-FDG; the maximum standard uptake values (SUVs) ranged from 0.8 to 1.8. The authors established a maximum SUV of 2.0 as the cutoff between benign and malignant; 10 of the 11 chondrosarcomas in their study had maximum SUVs ranging from 2.4 to 12.4. One lesion was considered a borderline chondrosarcoma, having a maximum SUV of 1.4, which was below the value of 2.0 that they used as the cutoff between benign and malignant. 
Radionuclide bone scanning is one method used to assess lesions depicted on radiographs or MRIs that are presumed to be enchondromas. If the bone scan results are negative, the possibility of a malignancy (eg, chondrosarcoma) is extremely remote. If the scan results are positive, biopsy is typically necessary. Radionuclide bone scan findings are not reliable in differentiating enchondroma from calcified marrow infarct.
Radionuclide bone scan findings are nonspecific; therefore, one cannot reliably diagnose a focal bone lesion on the basis of these findings alone. However, the physiologic information provided by a bone scan, when combined with information from other imaging modalities, can be helpful.
Angiography is not used in the diagnosis or evaluation of solitary enchondroma. In Maffucci syndrome, angiography may play a secondary role in identifying the extent and origin of the vascular malformation.