Osteochondromas are the most common benign bone tumors.

Complications of osteochondroma are typically related to mass effect, including vascular compromise, muscular or bursal irritation, or neurologic sequelae [14]. However, malignant degeneration may also occur, with transformation into chondrosarcoma.

The majority are located about the knee or proximal humerus.

Osteochondromas are exophytic, typically metaphyseal lesions and may be sessile (broad-based) or pedunculated. They demonstrate continuity of the periosteum, cortex, and marrow with the underlying bone with an overlying cartilage cap (Fig. 10.4).

MRI demonstrates the continuity of the osteochondroma with the underlying bone as well as the cartilaginous cap.

Complications related to mass effect are well appreciated on MRI [15].

Transformation to chondrosarcoma is a well-documented complication, and this risk correlates with a cartilage cap thickness greater than 2 cm. This is reproducibly measured on MRI. Cartilage cap thickness is measured from the junction of the T2 low-signal intensity mineralization with the near-fluid signal intensity cartilage to the surface of the cartilage, ignoring the undulations in the interface between them [16].

Osteochondromas do not typically yield cellular aspirates because they are predominantly made up of mature hyaline cartilage and bone. Aspirates may contain bone marrow elements and fragments of cartilaginous or bony matrix.

Findings in core biopsy depend on which portion of the lesion is sampled. The underlying bony stalk is made up of trabecular bone and typically contains normal bone marrow elements.

The cartilaginous cap is composed of mature hyaline cartilage that may display some degree of myxoid degeneration. The interface between the cartilaginous cap and the bony stalk is characterized by active enchondral ossification.

Enchondromas present as well-marginated medullary lucencies. The most commonly affected locations include the hands and feet as well as the femur, humerus, and tibia.

Lesions usually arise near an epiphysis and cause osseous expansion with endosteal scalloping and cortical thinning (Fig. 10.5).

These benign lesions often demonstrate a typical internal chondroid matrix superimposed on a background of lucency. This may involve minimal stippling to larger areas of heterogeneous density [17].

Enchondromas demonstrate typical lobulated margins and an internal chondroid matrix.

Lesion size can be accurately assessed by CT.

CT helps to separate enchondromas from chondrosarcomas by better demonstrating other aggressive features, such as cortical destruction or periosteal reaction. Enchondromas typically measure less than 5 cm in greatest dimension [7].

Aspirates contain a metachromatic fibrillary chondroid matrix with clear round lacunae with bland round chondrocytes

Lesions are characterized by hyaline cartilage containing bland chondrocytes in the lacunae.

Myxoid change, increased cellularity, and cytologic atypia suggest malignancy but may be pronounced in enchondromas involving the small bones of the hands and feet [12].

Enchondromas are almost invariably circumscribed lesions without permeation of the trabeculae.

RF ablation has been described as an alternative to surgery for some cases of chondroblastoma , depending on the risk of mechanical collapse and lesion size [18].

Chondroblastomas demonstrate geographic bone destruction with well-defined margins [19]^.

Lesions are centered at the epiphysis in tubular long bones.

Chondroblastomas demonstrate low to intermediate T1 signal and T2 heterogeneity [20].

Postcontrast images demonstrate mild tumoral enhancement.

Adjacent soft-tissue and bone marrow edema may also be present.

Chondroblastomas typically display sheets of polygonal mononuclear cells with relatively well-defined cell borders and uniform round nuclei that may be grooved and somewhat hyperchromatic (Fig. 10.6). Scattered throughout are multinucleated osteoclast-like giant cells. Interstitial “chicken wire” calcifications and a chondroid matrix may be present and allow distinction from other giant cell rich lesions such as aneurysmal bone cyst and giant cell tumor of bone.

Chondrosarcomas most frequently involve long bones and occur in older individuals. Although these tumors may grow large and be locally advanced, metastatic spread is less common than in other sarcomas.

Secondary chondrosarcoma may occur in the settings of Ollier and Mafucci syndromes, synovial chondromatosis, and preexisting osteochondroma.

While there is occasionally overlap with features seen in enchondromas, chondrosarcomas typically demonstrate aggressive features that include cortical destruction, periosteal thickening, and endosteal scalloping (Fig. 10.7) [22].

Lesions may include mixed lytic and sclerotic regions, with periosteal reactions ranging from thick, unilaminar, more benign-appearing periosteal reaction to a spiculated, aggressive appearance.

Classicly, a chondroid matrix of a chondrosarcoma appears as alternating high T2 signal intensity lobules with heterogeneous areas of low T2 signal intensity. The low T2 signal intensity areas represent areas of calcification within the matrix.

The extent of soft-tissue involvement is much better delineated on MRI.

Larger lesion size correlates with a higher likelihood of malignancy.