Osteochondroma

Osteochondromas, also known as osteocartilaginous exostoses, are among the most prevalent benign bone tumors. These lesions manifest as projections of bone and cartilage that most commonly emerge from the metaphyseal region of long bones but occur in flat bones as well. Their most distinctive feature is the seamless transition between the bone and marrow of the parent bone and the bony stalk of the osteochondroma, also known as corticomedullary continuity. The lesion is topped by a cap of cartilage. Most osteochondromas occur as solitary lesions, but multiple lesions throughout the skeleton occur in patients with Multiple Hereditary Exostoses, an inherited genetic syndrome with an autosomal dominant inherited pattern. While osteochondromas are neoplastic in nature, they typically follow a benign course and often remain asymptomatic, requiring no treatment. Surgical intervention becomes necessary only when symptoms develop, usually from mechanical irritation of adjacent structures, soft tissue impingement, or nerve compression. Malignant transformation of a solitary osteochondroma is uncommon, occurring in roughly 1% of cases over a lifetime. When malignant change occurs, the cartilaginous cap transforms into secondary chondrosarcoma, while the underlying bone remains benign.

Epidemiology

Solitary osteochondromas are commonly encountered bone tumors that typically present during childhood and adolescence. They occur sporadically, without genetic predisposition, and demonstrate a male predilection with a male-to-female ratio of approximately 1.5 to 1. Their true prevalence remains uncertain since many lesions are asymptomatic and discovered incidentally. These growths characteristically develop in the metaphyseal regions of long bones, with the greatest predilection for the bones around the knee joint. The distal femur and proximal tibia together account for approximately 30% of cases, while the distal tibia and humerus represent other frequent sites. Less commonly, about 5% of osteochondromas arise in flat bones such as the scapula and ilium. Some osteochondromas develop secondary to local trauma or treatment, including surgical procedures, growth plate fractures in children, or radiation therapy. Notably, these secondary lesions carry no increased risk of malignant transformation compared to primary osteochondromas.

Multiple Hereditary Exostoses (MHE) represents an autosomal dominant disorder characterized by the development of multiple osteochondromas throughout the skeleton. MHE affects approximately 2 individuals per 100,000 population and is found in males and females with equal frequency.

Clinical Features

Osteochondromas are frequently asymptomatic, especially when they appear as solitary lesions. When symptoms do occur, they often present as mechanical pain from irritation of surrounding soft tissues. Common examples include pain at the distal femur due to repetitive trauma to the extensor mechanism or iliotibial band. Also common is snapping of the pes tendons with medial tibial lesions. Nerve or vascular compression may also develop causing distinctive neurologic or vascular symptoms. In rare cases of malignant transformation, patients experience a unique, deep, toothache-like pain that intensifies progressively and is unrelated to activity. This pattern differs significantly from the typical symptoms of benign lesions and warrants careful evaluation through patient history and physical examination.

The clinical presentation of Multiple Hereditary Exostoses (MHE) varies with the severity of the disease. Beyond pain from the osteochondromas themselves, affected individuals often develop significant skeletal deformities, including short stature, valgus deformities of the knee and ankle, bowing of the forearm with ulnar deviation at the wrist, scoliosis, and hip dysplasia. Presentations range widely: some individuals exhibit only a few lesions, while others experience extensive skeletal involvement. Visible lesions can raise cosmetic concerns, and many patients undergo multiple surgical procedures in childhood to address both functional and aesthetic issues.

Radiologic Features

Osteochondromas demonstrate characteristic anatomical findings, typically arising near joints where they project perpendicularly from the metaphysis, growing outward from the adjacent joint. These lesions appear in two distinct forms on imaging: pedunculated variants have a narrower stalk and cap projection (Figure 1), while sessile variants present as broad-based, flattened outgrowths from the metaphyseal cortex (Figure 2). The defining imaging feature of osteochondromas is corticomedullary continuity—the seamless flow of the cortical bone and marrow space from the parent bone into the lesion. While this diagnostic feature may be less visible on conventional radiographs due to beam angle relative to the lesion, it is consistently seen on cross-sectional imaging.

Figure 1: AP radiograph of the right proximal tibia, demonstrating pedunculated osteochondroma.

Figure 2: Lateral radiograph of the left distal femur, demonstrating a sessile osteochondroma.

CT scans or MRI are useful for a detailed evaluation of osteochondromas. On axial sequences, these imaging modalities reliably demonstrate corticomedullary continuity, which helps differentiate osteochondromas from other bone lesions. Fluid-sensitive MRI sequences are particularly effective for visualizing the cartilage cap, whose thickness is not effectively measured on standard radiographs. The thickness of the cartilage cap, an important determinant of malignant transformation, is best assessed with MRI (Figure 3). A cartilage cap thicker than 2 cm is very concerning for potential malignant transformation. Cauliflower osteochondromas appear quite striking and indeed may resemble a head of cauliflower. They tend to occur on the large, flat bones and the proximal femur. The cartilage caps associated with these lesions are commonly thicker than those seen in sessile or pedunculated osteochondromas in adults but should still be less than 2 cm thick in the absence of malignant transformation (Figure 4). Contrast-enhanced MRI series are also helpful as they can differentiate an overlying bursa from a thick cartilage cap.

Figure 3: Axial STIR MRI of left distal femur in child with MHE. Multiple osteochondromas of the distal femur are displayed. The osteochondroma emanating from the anterolateral distal femur has a cartilage cap that is 3mm thick.

Figure 4: Sagittal STIR MRI (left) and sagittal CT scan (right) of a cauliflower osteochondroma arising from the anterior border of the scapula. Note the thin cartilage cap, despite the large shape of the bony mass. Corticomedullary continuity can be appreciated.

The most distinctive radiographic feature of MHE is the presence of multiple bony outgrowths (exostoses) projecting from the metaphyseal regions of long bones, often resulting in metaphyseal widening (Figure 5). MHE frequently affects both sides of the body, so these abnormalities are typically observed bilaterally on x-rays.

Figure 5: AP x-ray of bilateral knees demonstrating Multiple Hereditary Exostoses.

Pathology

Solitary osteochondromas develop through a distinct process: physeal cartilage herniates through the periosteum surrounding the growth plate and subsequently separates. This displaced cartilage fragment continues to grow and undergoes ossification, creating the characteristic outward projection of bone with its cartilaginous cap.

The pathogenesis differs in Multiple Hereditary Exostoses (MHE), where mutations affecting the EXT1, EXT2, and EXT3 genes drive the formation of multiple lesions. The condition shows near-complete penetrance, resulting in consistent phenotypic expression in affected individuals.

Grossly, osteochondromas present as bony projections covered by a distinctive blue-grey cartilage cap. Some lesions may present as pure bony exostoses without cartilage, though these often develop an overlying reactive bursa. Microscopic examination (Figure 6) reveals an outer layer of normal cortical bone surrounding a core of cancellous bone. The cartilaginous cap contains chondrocytes arranged in a columnar pattern that resembles, but does not perfectly replicate, normal growth plate organization. Benign lesions should show no cellular atypia. Concerning features that suggest malignant transformation to secondary chondrosarcoma include cartilage cap thickness exceeding 2 cm or cellular changes such as increased cellularity, pleomorphism or atypia.

Figure 6: Histopathologic examination of osteochondroma, low power H&E stain showing cartilage cap adjacent to underlying bone. (Image courtesy of OrthopaedicsOne- The Orthopaedic Knowledge Network. Created Jan 13, 2009. Retrieved Feb 1, 2022)

Differential Diagnosis

Several surface bone lesions may radiographically resemble osteochondromas, including periosteal chondroma, parosteal osteosarcoma, and periosteal osteosarcoma. The critical differentiating feature is corticomedullary continuity: in osteochondromas, the cortex and marrow of the parent bone continue seamlessly into the lesion, whereas in these other entities, the lesions form on top of an intact cortical surface without this continuity. This radiographic finding is essential for accurate diagnosis.

Histologically, when examining the cartilaginous component, several other conditions may also enter the differential, including periosteal chondroma, chondrosarcoma, and enchondroma. Reaching an accurate diagnosis requires correlating the microscopic findings with clinical presentation, anatomic location, and detailed imaging features.

Disease Course: Treatment And Prognosis

The growth of osteochondromas during childhood is typical and generally not cause for concern. However, continued cartilaginous growth after skeletal maturity, or the appearance of new growth in adulthood, typically signals malignant transformation.

Solitary osteochondromas are often found incidentally and, if asymptomatic, generally require only observation. Given the approximate 1% lifetime risk of malignant transformation, annual surveillance with radiographs or MRI is reasonable. In contrast, patients with Multiple Hereditary Exostoses (MHE) have a notably higher lifetime risk of 5-10% for malignant transformation.

For symptomatic lesions causing mechanical issues, marginal excision is typically effective. This surgical technique involves removing the lesion through its base, excising both bony and cartilaginous components, to ensure complete symptom relief. When excising sessile osteochondromas from weight-bearing bones, surgeons should consider the risk of pathologic fractures due to the resulting cortical defect. In such cases, prophylactic fixation and/or temporary weight-bearing restrictions may be necessary at the surgeon’s discretion (Figures 7 and 8).

Figure 7: AP radiograph of a sessile osteochondroma off the medial distal left femur.

Figure 8: Postoperative AP (left) and lateral (right) x-rays of left distal femur after tumor excision and prophylactic plating. Over the following year, the medial cortex reconstituted and remodeled at the corticotomy site.

In cases where secondary chondrosarcoma is suspected, management includes biopsy followed by wide excision if malignancy is confirmed. Local recurrence of osteochondromas is rare and usually results from incomplete removal of the cartilage cap in sessile lesions, especially in younger children.

Key Test Topics

Corticomedullary continuity distinguishes osteochondroma from other surface-based bone tumors
Lesions that grow in adulthood or new lesions that develop in adulthood as well as those with a cartilage cap >2cm suggest malignant transformation.
Asymptomatic osteochondromas are treated with observation, while those that are symptomatic are treated with marginal excision of the bone and cartilage cap.