This two-part review will focus on the cartilage lesions, both benign and malignant, that we have encountered in our clinical practice in recent years. In part one, I will discuss the typical appearances and locations of benign cartilage lesions as well as the patient populations that present with these tumors. I have included radiographic and cross sectional images showing examples of various lesion types.

BENIGN CARTILAGE LESIONS

Enchondroma

The most common solitary cartilage-containing lesion encountered in our practice is the enchondroma. It is also considered the most common cartilage-containing lesion of the phalanx. Enchondromas account for 1 % of all bone tumors and are found more frequently in the hands than the feet. They occur with equal frequency in males and females, and have a peak incidence of diagnosis in the third decade. Typically, enchondromas are incidental findings. However, as they are lytic expansile lesions, patients sometimes present with pathological fractures (Fig 1).

Histologically these neoplasms are composed of hyaline cartilage, commonly with myxoid areas that contain amorphous calcification with a variable matrix mineralization pattern. The cartilage matrix does not show the overt induction of neovascularity that is characteristic of malignant lesions. These patterns of calcification have been described as stippled, speckled, and ring or arc-like on radiographs (Fig 2 and Fig 3).

The plain film radiographic appearance of an enchondroma usually shows a well-defined geographic, lytic lesion, with or without sclerotic boarders, located in the metaphysis. These lesions can be central, where they may widen the bone contour, or eccentric, where they can bulge and thin the cortex. This growth can weaken the bone and lead to pathological fractures. Computed tomography (CT) and magnetic resonance imaging (MRI) studies are rarely obtained unless a pathological fracture is suspected and not demonstrated on the radiographs, or if the diagnosis of an enchondroma is in doubt. As hyaline cartilage has a high water content, these lesions have similar intensities to fluid and show increased signal intensity on long TR sequences. The mineralized portions of the cartilage matrix will remain low on all pulse sequences (Fig 4).

Occasionally, a patient is encountered with multiple enchondromas of varied severity. This entity, known as Ollier disease or Dyschondroplasia, has a slight male predilection, is not hereditary, and is usually diagnosed in childhood. In this instance, the enchondromas tend to be unilateral and may stabilize at puberty. There is a known increase in malignant transformation to chondrosarcoma in patients with Ollier disease, with the lesions closest to the axial skeleton at greatest risk for the malignant change. Radiographically, there are multiple lytic lesions similar in appearance to single enchondromas. An additional, sometimes confusing, appearance in skeletally immature patients can look like vertical lytic columns, arising from the epiphyseal plate, that extend into the metadiaphysis of the bone.

A second syndrome involving multiple enchondromas with soft tissue cavernous hemangiomata is Maffucci syndrome. This entity also shows a slight male predilection and is not hereditary. The distal extremities seem to have the greatest involvement with marked skeletal deformity reported. Patients with Maffucci syndrome are at greater risk for developing chondrosarcoma than those with Ollier disease, and also have an additional risk for developing vascular sarcomas, ovarian malignancies, and gliomas. The radiographic hallmark for this entity is the numerous soft tissue masses that contain calcified phleboliths.

Osteochondroma

Osteochondroma is a lesion many consider to be an abnormality of growth rather than a true neoplasm. Some debate persists, however, and this lesion continues to be classified as a neoplasm because it acts like a tumor. These lesions are seen in young (under twenty years of age) males and females with equal frequency. Nearly half of the cases are diagnosed in bones around the knee where the chief complaint is that of a palpated mass. An osteochondroma is an expansile process that arises in the metaphysis and grows away from the epiphysis. A critical feature for correct diagnosis of the mass is that the cortex and medullary portion is continuous with the underlying bone. A hyaline cartilage cap, that involutes after growth, covers this lesion.

Although the growth of an osteochondroma can cause symptoms from pressure effects to cosmetic concerns, it is the cartilage cap that deserves the most attention as it is here where malignant degeneration to a chondrosarcoma or osteosarcoma can occur. Unfortunately, this cartilage cap is not easily seen on plain films unless there is internal calcification, an uncommon finding (Fig 5). The cap thickness is related to malignant degeneration. A thickness of less than 1 cm is said to be present in benign lesions while a thickness of greater than 1 cm is worrisome for malignant change; some use an absolute limit of 1.5 cm. Assessment of cap thickness is not difficult as the average reported thickness for malignant lesions is 6 cm. If the patient is skeletally immature, the cap can be thicker than 1.5 cm and still indicate a growing lesion rather than a malignant change.

Additional features suggesting malignant degeneration include pain, changes in the radiographic or cross-sectional imaging appearance, and invasion into nearby structures. CT can have some of the same problems that radiographs have in measuring the thickness of non-calcified caps because cartilage can appear similar to surrounding muscle. At our institution, we find MRI to be the most useful for evaluating cap thickness; the cartilage increases in signal on the longer TR pulse sequences while the underlying cortex and nearby muscles are lower in signal intensity (Fig 6a & Fig 6b).

Multiple hereditary exostoses is an autosomal dominantly inherited entity, affecting males more commonly than females (3:1). It is composed of multiple osteochondromas of varying size and may involve any bone in the skeleton pre-formed in cartilage. These lesions tend to be bilateral and symmetrical, however one side may predominate. There is a higher rate of malignant transformation in patients with this entity compared to those who have a single osteochondroma present. The lesions closest to the axial skeleton appear to be at increased risk for malignant degeneration.

Chondroblastoma

Chondroblastoma, or Codman tumor, is a benign cartilage-containing tumor that is most commonly encountered around the knee. The most common location within the bone is centered at the growth plate, with both the epiphysis and metaphysis involved. These lesions can be found isolated in the epiphysis or apophysis with almost as much frequency as the growth plate location. They are rarely found in the metaphysis only. Chondroblastoma is twice as common in males compared to females and is usually diagnosed between 10 and 25 years of age. The chief complaint is pain. Histologically these tumors contain chondroblasts and multinucleated giant cells. These lesions can be misdiagnosed as osteosarcoma or chondrosarcoma.

Radiographically, these appear as geographic lytic lesions with or without sclerotic margins (Fig 7a). Approximately 67 % of these tumors are eccentric in location. There is rarely bone expansion, however an internal calcified chondroid matrix and periosteal reaction is commonly seen (30-50 %). Cross sectional imaging with CT (Fig 7b) or MRI can show fluid-fluid levels. The signal intensity is similar to other cartilage lesions, although chondroblastoma can have prominent surrounding edema. Although benign, 25% of chondroblastomas will recur after curretage.

Periosteal chondroma

Periosteal or juxtacortical chondroma is a benign lesion composed of hyaline cartilage that forms deep to the periosteum adjacent to the cortex of bone (Fig 8). These lesions are twice as common in men than women, and the peak age at diagnosis is in the second decade. Patients usually present with a palpable mass, sometimes with pain. The most common bones involved tend to be the long tubular bones, typically the humerus and femur, and the lesions are located in the metaphysis. These neoplasms can erode the bone cortex and internal calcification is seen in half of the cases radiographically. The sclerotic margins associated with the erosions favor a benign diagnosis; aggressive features such as cortical destruction or soft tissue invasion would favor a malignant diagnosis such as chondrosarcoma.

Chondromyxoid Fibroma

Chondromyxoid Fibroma (CMF) is considered the rarest of the benign cartilage-containing neoplasms of bone. Histologically, these tumors contain various amounts of chondroid, myxoid, and fibrous elements. The histologic appearance can be misleading and often this lesion can be mistaken for a chondrosarcoma. This tumor affects men slightly more frequently than women, with a peak incidence in the second and third decades of life. Patients typically are symptomatic, complaining of pain, swelling, or decreased range of motion, which tends to be chronically progressive.

CMF is most often found in the metaphysis of long tubular bones in the lower extremities, especially the femur and tibia. Typically these lesions appear as geographic lytic lesions with a peculiar common finding of thickened trabeculations (Fig 9a). Cortical expansion with endosteal sclerosis or thinning is common, while pathological fractures, internal calcifications, and periosteal elevations are rare (Fig 9b).

Selected Readings

1. McFarland GB, Morden ML. Benign Cartilaginous Lesions. Orthop Clin North Am 1977; 8:737.
2. Schajowicz F, Gallardo H. Epiphyseal Chondroblastoma of Bone. A Clinico-pathological Study of Sixty-nine cases. J Bone Joint Surg [Br] 1970; 52:205-26.
3. Kilpatrick SE, Pike EJ, Ward WG, Pope TL. Dedifferentiated Chondrosarcoma in patients with Multiple Osteochondromatosis: report of a case and review of the literature. Skeletal Radiol. 1997; 26:370-4.
4. Murphey MD, Flemming DJ, Boyea SR, Bojescul JA, Sweet DE, Temple HT. Enchondroma versus Chondrosarcoma in the Appendicular Skeleton: Differentiating Features. Radiographics. 1998; 18:1213-37.
5. Resnick D, Kyriakos M, Greenway G. Tumors and Tumor-Like Lesions of Bone. Diagnosis of Bone and Joint Disorders. W.B. Saunders, Phila 1988, 3602.
6. Greenfield GB. The Solitary Lesion. Radiology of Bone Diseases, 3^rd ed. Phila: J.B. Lippincott; 1980:515.
7. Murphey MD, Choi JJ, Kransdorf MJ, Flemming DJ, Gannon FH. Imaging of Osteochondroma: Variants and Complications with Radiologic-Pathologic Correlation. Radiographics 2000; 20:1407-1434.