Plain Radiography
Plain radiography is the mainstay of imaging for osteochondroma. Good-quality radiographs should be obtained in two perpendicular planes to characterize the lesion fully. Classic radiographic features include orientation of the lesion away from the physis and medullary continuity. [41] (See the images below.)
Solitary osteochondroma. Anteroposterior radiograph of a pedunculated osteochondroma of the distal femur.
Solitary osteochondroma. Lateral radiograph of a pedunculated osteochondroma of the distal femur. Orientation is away from the growth plate, and medullary continuity is clear.
Solitary osteochondroma. Anteroposterior radiograph of sessile osteochondroma of the humerus.
CT and PET
In certain bones, such as the pelvis and the scapula, computed tomography (CT) is a useful adjunct for localizing the lesion. CT localization can be helpful in planning resection. [42] (See the images below.)
Solitary osteochondroma. CT scan of the pelvis depicting a massive solitary osteochondroma.
Solitary osteochondroma. CT scan of the same sessile osteochondroma of the humerus as in Image 6.
Purandare et al studied the role of whole-body fluorodeoxyglucose (FDG) positron emission tomography (PET)-CT in evaluating sarcomatous transformation of osteochondromas in 12 patients with a diagnosis of osteocartilaginous lesions, [43] with the following results:
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Seven patients with histopathologic evidence of sarcomatous transformation to grade II chondrosarcoma had moderate-to-high FDG uptake
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One patient with a dedifferentiated chondrosarcoma had a focus of very intense uptake
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Four patients with histopathologic or clinical diagnosis of a benign osteocartilaginous lesion had low FDG uptake
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FDG uptake was also noted in an asymptomatic osteochondroma, which on histopathology revealed a grade II chondrosarcoma
Magnetic Resonance Imaging
Magnetic resonance imaging (MRI) is needed only in cases where malignancy is a concern or where relevant soft-tissue anatomy must be delineated. MRI is the modality of choice for assessing the thickness of the cartilage cap (see the image below). Although not an absolute indication, cartilage-cap thickness is related to malignancy. Thick cartilage caps (eg, >2 cm in adults and >3 cm in children [17] ) are suggestive of malignant degeneration, especially when they are associated with pain.
Solitary osteochondroma. MRI of sessile osteochondroma of the femur demonstrating the thickness of the cartilage cap.
Bone Scanning
As a rule, bone scans are not useful in the workup of osteochondromas or in preoperative planning for resection. [44]
Histologic Findings
Grossly, the stalk is contiguous with the intramedullary marrow (see the images below). By definition, the medullary canal of the affected bone and the canal of the tumor are connected. The stalk is made up of mature bone. The cartilage cap, which tops the lesion and can be quite thick in children, is replaced by enchondral bone formation in maturing patients.
Solitary osteochondroma. Gross osteochondroma specimen at the time of resection. Bone stalk and overlying membrane on cartilage cap.
Solitary osteochondroma. Cut surface of surgical osteochondroma specimen. Cartilage cap and underlying bone with medullary continuity.
On microscopic examination, the cartilage cap can exhibit varying amounts of cellularity. The cap has an overlying fibrous layer that contains mesenchymal cells, which are thought to be responsible for the lesion's growth. [45] The cells in the cartilage are orientated vertically, as in a growth plate (see the image below).
Solitary osteochondroma. High-power view of benign cartilage cells arranged in vertical growth plate pattern.
In skeletally immature patients, the cells undergo enchondral bone formation (see the image below). Although no specific cartilage-cap thickness is an absolute indicator of risk for malignancy, a thickness of less than 4 cm generally is thought to be in the range of normal. Furthermore, the cap should not thicken in persons older than 30 years.
Solitary osteochondroma. Histology of cut osteochondroma specimen. Cartilage cap and orientation of enchondral bone formation.
Staging
Osteochondromas are benign lesions and can be staged according to the Enneking or Musculoskeletal Tumor Society (MSTS) classification for benign lesions, as follows:
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Stage 1 (latent) - Inactive or static lesions
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Stage 2 (active) - Actively growing lesions
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Stage 3 (aggressive) - Actively growing lesions that are locally destructive/aggressive
Most osteochondromas are stage I or II. However, significant deformity secondary to mass effect can occur in areas such as the radioulnar joint and the tibiofibular joint (see the image below); although the classification is not perfect, such lesions could reasonably be considered stage III. These cases likely represent a pressure-erosive process rather than a truly invasive one (as the staging for benign lesions is defined), a subtle but significant biologic distinction.
Solitary osteochondroma. Radiograph demonstrating the deformation of the distal tibiofibular joint in a patient with a solitary osteochondroma.
In the rare case of malignant degeneration of the cartilage cap, the lesion is usually a low-grade chondrosarcoma that would be staged as a low-grade extracompartmental lesion (MSTS stage 1B).
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Solitary osteochondroma. Anteroposterior radiograph of a pedunculated osteochondroma of the distal femur.
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Solitary osteochondroma. Lateral radiograph of a pedunculated osteochondroma of the distal femur. Orientation is away from the growth plate, and medullary continuity is clear.
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Solitary osteochondroma. Lateral radiograph of a sessile osteochondroma of the distal femur.
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Solitary osteochondroma. Anatomic and age distribution of solitary osteochondromas.
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Solitary osteochondroma. CT scan of the pelvis depicting a massive solitary osteochondroma.
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Solitary osteochondroma. Anteroposterior radiograph of sessile osteochondroma of the humerus.
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Solitary osteochondroma. CT scan of the same sessile osteochondroma of the humerus as in Image 6.
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Solitary osteochondroma. MRI of sessile osteochondroma of the femur demonstrating the thickness of the cartilage cap.
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Solitary osteochondroma. Gross osteochondroma specimen at the time of resection. Bone stalk and overlying membrane on cartilage cap.
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Solitary osteochondroma. Cut surface of surgical osteochondroma specimen. Cartilage cap and underlying bone with medullary continuity.
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Solitary osteochondroma. Histology of cut osteochondroma specimen. Cartilage cap and orientation of enchondral bone formation.
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Solitary osteochondroma. High-power view of benign cartilage cells arranged in vertical growth plate pattern.
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Solitary osteochondroma. Radiograph demonstrating the deformation of the distal tibiofibular joint in a patient with a solitary osteochondroma.
