Osteochondroma

Epidemiology

· Common, approximately 35% of benign bone tumors and 10-15% of all bone tumors

· Originating in early infancy, most cases are diagnosed in the first 2 decades of life

· More common in males, 2 to 1

· They are most commonly sporadic 

· 15% of osteochondroma patients also have MHE , while some may have Trevor disease or Langer-Giedion syndrome.

Localization

· Most commonly occur in the long bones; femur > humerus > tibia

· The most common area is around the knee (proximal tibia and distal femur) 

· Originating from the metaphysis, it tends to grow toward the diaphysis with skeletal growth

· In flat bones, the ilium and scapula are frequent locations 

· Rare in the small bones of the hand and foot, ribs, and spine 

·Solitary osteochondromas

o Can arise because of trauma (e.g., Salter-Harris fracture), iatrogenic (e.g., surgery), and radiotherapy (the most typical radiation-induced benign bone tumor) [figure 6]

o The inactivation of the EXT gene is restricted to the cartilage cap and occurs somatically

· Both solitary and MHE are associated with loss-of-function mutations in the EXT1 (8q24) and EXT2 (11p11) genes, leading to decreased production of heparan sulfate by chondrocytes found at the physis

· Individuals with the EXT1 mutation exhibit a more severe presentation than those with the EXT2 mutation, including a higher incidence of chondrosarcoma, a greater number of exostoses, increased limb malalignment, reduced range of motion in the forearms and knees, and more involvement of the pelvis and flat bones

· MHE 

o Heredity is present in 2/3 of the cases

o Shows AD inheritance; patients have a heterozygous germline EXT1/EXT2 mutation, while their tumors usually show a homozygous EXT mutation

o The penetrance is estimated to be 96% in females and 100% in males

Presentation

· Most of the lesions are asymptomatic 

· The most common symptom is a painless mass that increases in size during skeletal growth

· Occasionally, it may cause pain because of mechanical symptoms, including bursitis, tendinitis, or neurovascular compression 

Imaging

Radiograph

o Lesions on bone surfaces can be either pedunculated (narrow stalk) or sessile (broad base) , and they are continuous with the medullary cavity of the native bone, which is pathognomonic.

o Pedunculated lesions grow toward the diaphysis 

o Sessile lesions have a higher risk of malignant transformation [

o Usually, the cartilage cap is radiolucent

CT

CT imaging confirms the findings visible on radiographs but provides superior visualization of medullary continuity and the cartilage cap. In some cases, enhancement may be observed along the septa between cartilage lobules.

Ultrasound

Ultrasonography can delineate the cartilage cap with high accuracy, appearing as a hypoechoic layer bordered by bone on the deep aspect and by overlying muscle or fat superficially.

MRI

MRI is the most sensitive modality for evaluating cartilage cap thickness (which is crucial for identifying malignant transformation), assessing bone or soft-tissue edema, and depicting adjacent neurovascular structures.

On MRI, the cartilaginous cap demonstrates low to intermediate signal intensity on T1-weighted images and high signal intensity on T2-weighted sequences, consistent with cartilage elsewhere. A cap thickness exceeding 1.5 cm after skeletal maturity suggests possible malignant change, whereas in younger individuals, the cap can normally measure up to 3 cm.

Following gadolinium contrast administration, benign osteochondromas typically show enhancement confined to the peripheral fibrovascular tissue covering the cartilage cap, while the cap itself remains non-enhancing. In contrast, chondrosarcomas often display internal septal enhancement between cartilage lobules.

Pathology

· Usually, a biopsy is not necessary for diagnosis [figure 13]

· The cartilage cap consists of mature hyaline cartilage with well-defined fibrous perichondrium around the cartilage cap

· In children, the transition between the bone and cartilage cap resembles a growth plate, showing endochondral ossification into mature bone; cartilage cap thickness is not a reliable indicator of malignant transformation in children (up to 1-3 cm thickness) [figure 14]

· Cartilage cap thickness diminishes and may even be absent in older adults (usually 2-3 mm thickness)

· In adults, a cartilage cap thickness of > 2 cm increases the risk of chondrosarcoma

Staging

· Growing stops after skeletal maturity

· Malignant transformation to chondrosarcoma

o The sudden onset of pain in adults with MHE should raise concerns about possible malignant transformation

o Histological features alone are often inadequate for a definitive diagnosis of secondary chondrosarcoma arising from osteochondroma, which requires correlation with clinical and imaging findings

o Mostly low-grade chondrosarcoma (90%) [figure 15]

o Proximal lesions have a higher risk of undergoing malignant transformation compared to distal lesions; sessile lesions have a higher risk of malignant transformation; a cartilage cap thickness of > 2 cm increases the risk of chondrosarcoma

o Occurs mostly in >50 years old

o Malignant transformation of solitary osteochondroma to chondrosarcoma is rare, occurring in < 1% of cases [figure 16]

o This transformation does not happen before puberty, and the risk increases to 5-10% in MHE

o The risk of malignant transformation depends on the site, with the pelvis being the most common location [figure 17]

Differential diagnosis

· Bizarre parosteal osteochondromatous proliferation (Nora lesion): Typically located in the hands; arises from the bone cortex and lacks medullary continuity

· Florid reactive periostitis: Typically located in the hands, especially in proximal phalanxes; the cortex of the bone is typically intact

· Supracondylar humerus spur: projects towards the elbow joint; the cortex of the bone is typically intact

Treatment

· Asymptomatic lesions [figure 18]

o There is no indication for surgery; observation alone

o The lesions in the trunk and limb girdles can be surgically removed to prevent the risk of secondary chondrosarcoma transformation [figure 19]

· Symptomatic lesions

o Marginal resection at the base of the stalk, including cartilage cap [figure 20]

· Surgical removal of lesions near the epiphysis may cause injury to the growth plate; if feasible, surgery should be postponed until skeletal maturity

· Secondary chondrosarcomas are treated as typical chondrosarcomas with wide surgical resection

· Recurrence may be seen in 2-5% cases after resection

References 

1. Fletcher CDM, Bridge JA, Hogendoorn PCW, Mertens F (eds). WHO Classification of Soft Tissue and Bone Tumours, 5th Edition. Lyon: IARC Press; 2020.

2. Bovée JVMG. Multiple osteochondromas. Orphanet J Rare Dis. 2008;3:3. doi:10.1186/1750-1172-3-3.

3. Ahmed AR, Tan TS, Unni KK, Collins MS, Wenger DE, Sim FH. Secondary chondrosarcoma in osteochondroma: Report of 107 patients. Clin Orthop Relat Res. 2003;(411):193–206.

4. Murphey MD, Choi JJ, Kransdorf MJ, Flemming DJ, Gannon FH. Imaging of osteochondroma: Variants and complications with radiologic-pathologic correlation. Radiographics. 2000;20(5):1407–1434.

5. Stieber JR, Dormans JP. Manifestations of hereditary multiple exostoses. J Am Acad Orthop Surg. 2005;13(2):110–120.

6. Bovée JVMG, Hogendoorn PCW. Cartilage-forming tumours of bone and soft tissue and their differential diagnosis. Curr Diagn Pathol. 2002;8(6):332–339.

7. Czajka CM, DiCaprio MR. What is the proportion of malignant transformation in hereditary multiple exostoses? Clin Orthop Relat Res. 2015;473(7):2355–2361.

8. Legeai-Mallet L, Munnich A, Maroteaux P, Le Merrer M. Genetic counselling in hereditary multiple exostoses. J Med Genet. 1997;34(4):265–268.