Osteochondromas have a cartilaginous cap with a growth plate at its interface with the underlying bone [4, 5]. Lesions are well-defined and occur in metaphyses and metaphyseal equivalents. Cortex and medullary space are in contiguity with the underlying bone (Fig. 3). Lesions vary from pedunculated to sessile. Growth ceases with skeletal maturity.

Most osteochondromas are asymptomatic. Some may produce symptoms due to encroachment of adjacent structures including vasculature, nerves, muscles and tendons or other bones [6]. Malignant degeneration is extraordinarily rare in childhood. Malignant transformation to chondrosarcoma is suggested with growth after skeletal maturity, new pain, bone destruction or a cartilaginous cap of greater than 1.5–2 cm thickness.

Multiple hereditary osteochondromatosis (MHE) is an autosomal dominant disorder usually presenting before age 10 years. MHE patients may have secondary osseous deformities which limit function and are more prone to axial lesions which may encroach on visceral structures or the spinal canal.

Enchondromas are most common in the small tubular bones of the hands and feet and with the metaphyses/metadiphyses of long bones [4]. It is the most common pediatric bone tumor in the hand. The incidence of enchondroma increases with age, peaking in the third decade.

Enchondromas are well-defined, lucent lesions Overlying cortex may be thinned. Focal, punctate cartilage matrix may be seen, but is better defined by CT. On MRI, lesions follow cartilage in signal appearing isotense to muscle on T1-weighted images and high signal on T2-weighted images. Adjacent bone marrow edema is absent. Lesions enhance with contrast, varying in enhancement pattern from homogeneous to peripheral.

Enchondromatosis (Ollier disease) is non-hereditary mesodermal dysplasia. The hands are most often affected and may be substantially deformed and functionally limited. Lesions in long bones and the pelvis tend to be larger, expansile and striated. Maffucci syndrome is enchondromatosis in conjunction with soft tissue vascular malformations, typically venous malformations. Metachondromatosis is a rare disorder combining osteochondromatosis and enchondromatosis.

Chondroblastomas are most common in the second decade and occur in epiphyses, apophyses, carpal and tarsal bones and the patella [4, 7]. Larger lesions may extend into the adjacent metaphysis. Chondroblastomas are inflammatory and produce pain leading to presentation.

On radiography, lesions are well-defined and lucent, with a mildly sclerotic border. Non-aggressive periosteal new bone is seen on the adjacent metaphysis (Fig. 4). Lesion anatomy, borders and cartilage matrix are better seen with CT. On MRI, lesion signal intensity parallels cartilage; however, a cystic component is common. Adjacent bone marrow and soft tissue edema and joint effusion are best seen on MRI [8].

Simple bone cysts are of uncertain etiology. Lesions are substantially more common in boys than girls. Bone cysts are seen in the metaphysis and metadiaphysis of long bones [9]. The proximal humerus is the most common site followed by the proximal femur. Axial sites are rare, the most common being the calcaneus. With growth, the growth plate moves away from a bone cyst. Lesions are centered within the medullary space, well-defined and have a mildly sclerotic cyst wall. Overlying cortex may be thinned and mildly expanded. Septations may be seen. Bone cysts are the most common cause for pathologic fracture in children. The fallen fragment sign is diagnostic of a simple bone cyst (Fig. 5). On MRI, the cyst wall enhances mildly with uniform thickness. Adjacent bone marrow edema is absent in the absence of fracture or stress related to the cyst.

Aneurysmal bone cysts (ABCs) are composed of communicating loculations contain blood and lined by fibrous walls containing spicules of bone, fibrous tissue, red blood cells and hemosiderin [9]. An underlying lesion may be found in up to one third of cases.

ABC most commonly occurs in children and young adults. It is rare under 5 years of age. The lesion is most common in long bone metaphyses, but can occur in flat bones. Most ABCs are intramedullary but eccentric, with cortical and subperiosteal (surface) lesions being much less common.

ABCs appear as well-defined, multiloculated, expansile, lucent lesions, classically described as a “soap bubble” [9]. Overlying cortex is remodeled and thinned. With CT or MR, fluid-fluid levels due to blood products within locules of the lesion are characteristic. Septations enhance; however, any solid component may signal the presence of an underlying lesion.