Localization: Chondroma occurs only in bones that are preformed in cartilage. Most cases are observed in tubular bones of the hand. Enchondroma is the most common bone tumor of the hands. The other chondromas are mainly distributed in the long bones, with preference for the femur. The real incidence of chondroma outside the hand is underestimated, as most of these lesions are asymptomatic.

Clinical: Because of its slow growth, minimal peritumoral reaction, and avascularity, chondroma is usually painless. In some active chondromas (mainly in young age), or after exercise, or because of pathologic fracture, immediate pain does occur. A mild expansion of the cortex can be seen or palpated in superficial bones, such as tubular bones of the hand or foot, ribs, and fibula where pathologic fractures are also common.

Imaging: The lesion is frequently central, sometimes eccentric or intracortical. It is an osteolysis, with rounded, lobulated, well-defined edges with a thin rind of reactive sclerosis. Usually the lesion contains granular, popcorn, ringlike opacities that represent calcification and ossification at the periphery of the lobules. Chondroma can reach considerable extension in the major long bones but rarely exceeds 6 cm; in small ones (hand, ribs, fibula), the cortex is thinned and the bone slightly to moderately expanded. CT scan demonstrates the lobular or multi-island nature of the lesion, its sharp limits, and its radiodensities, as well as the lack of permeative alterations of the cortex. MRI at best defines longitudinal extension; lesion is low signal in T1, high signal in T2; and calcifications are seen as black signal voids in both T1 and T2. Isotope scan is very hot in most lesions and sometimes reveals asymptomatic chondromas when performed for staging or follow-up of other conditions; it is useful in showing asymptomatic chondromas.

Pathology: Lesion consists of lobules of cartilage. The aspect is typical of hyaline cartilage. Calcified areas appear as white opaque granules. Reactive or enchondral ossifications manifest as white-yellow hard rings and streaks around and between the lobules. The limits of the lesion are often irregular, as lobules of cartilage push toward the cancellous bone and excavate little niches in the cortex but always well defined. The chondrocytes are sparse, with small, round, dense nuclei, of relatively uniform size. Occasionally, isogenous groups of cells can be seen. Double-nucleated cells are present, usually rare, but can be moderate. This does not really distinguish well benign enchondroma from chondrosarcoma. While diagnosis of cartilage tumor is usually easy even on clinico-imaging findings alone, the real problem is to differentiate chondroma from grade 1 chondrosarcoma. Histologically, this distinction is difficult: