Osteochondroma (exostosis) is the most common benign bone tumor and is a cartilage-capped bony projection growing outward from the bone surface, showing cortical and medullary continuity. It accounts for approximately 30-40% of all bone tumors. It originates from the growth plate and typically locates in the metaphysis of long bones, especially around the knee (distal femur, proximal tibia). Radiologically, it is characterized by cortical and medullary continuity with the parent bone (pathognomonic finding) and overlying cartilage cap. It can be solitary or multiple (hereditary multiple exostoses/osteochondromatosis).
Age Range
10-30
Peak Age
15
Gender
Male predominant
Prevalence
Very Common
Osteochondroma forms through aberrant direction of cartilage cells from the growth plate (physis) toward the bone surface (physeal herniation); this aberrant cartilage tissue follows the normal endochondral ossification process, forming a bony projection with cortical and medullary continuity. The cortical and medullary continuity proves that the lesion is not a distinct tumor from the parent bone but a physeal growth defect and forms the basis of radiological diagnosis — this continuity is directly observed on CT and conventional radiography. The cartilage cap is a growth plate-like structure that ossifies through endochondral ossification; when skeletal maturity is complete, the cartilage cap thins and becomes <1-2 cm — cap thickness >2 cm or thickening raises suspicion for malignant transformation (secondary chondrosarcoma). On MRI T2-weighted sequences, the cartilage cap shows high signal because hyaline cartilage contains 70-80% water; this T2 signal characteristic is the most accurate method for measuring cap thickness.
The cortex and medullary cavity of the lesion continue without interruption with the cortex and medullary cavity of the parent bone — this pathognomonic finding is diagnostic for osteochondroma and is not seen in any other bone lesion. This continuity is most reliably confirmed on CT multiplanar reformats.
The cortex and medullary cavity of the lesion continue without interruption with the cortex and medullary cavity of the parent bone — this is a pathognomonic finding and diagnostic of osteochondroma. This continuity is confirmed three-dimensionally on CT multiplanar reformats. This finding, also visible on conventional radiography, is more reliably evaluated with CT in complex anatomic regions (pelvis, scapula, vertebra).
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The lesion shows cortical and medullary continuity with the parent bone, supporting the diagnosis of osteochondroma.
The cartilage cap shows markedly high signal on MRI T2-weighted sequences — due to the 70-80% water content of hyaline cartilage. Measurement of cartilage cap thickness is critically important: <1 cm benign, 1-2 cm borderline, >2 cm suspicious for malignant transformation (secondary chondrosarcoma). MRI T2 sequence is the most accurate modality for measuring cartilage cap thickness. Lobular irregular thickening should be carefully evaluated for focal malignant focus.
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The cartilage cap shows high signal on T2-weighted sequences with a measured thickness of [X] mm; below/above the malignant transformation threshold.
Fatty marrow in the lesion's medullary cavity shows the same high T1 signal as the parent bone's fatty marrow on T1-weighted sequences — this medullary continuity is best evaluated on T1. The cartilage cap shows low-to-intermediate signal on T1 (long T1 relaxation due to high water content). Pedicle structure can be evaluated on T1.
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Fatty marrow with the same high signal as the parent bone is seen in the medullary cavity on T1-weighted sequences, confirming medullary continuity.
Osteochondroma appears in two morphological forms: sessile (broad-based, spreading on bone surface) and pedunculated (stalked, attached to bone by a narrow pedicle). Cortical and medullary continuity is preserved in both forms. Pedunculated type generally points away from the joint. Sessile type may cover a wider surface area and may interact more with adjacent structures (neurovascular bundle, tendons). CT shows the morphology and relationship with adjacent structures in detail.
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Sessile/pedunculated bony projection with preserved cortical and medullary continuity with the parent bone.
On STIR sequence, the cartilage cap shows high signal clearly separated from surrounding tissues thanks to fat suppression. Cap boundaries and thickness can also be evaluated on STIR but T2 is the gold standard. Complications — bursa formation (friction bursitis), perichondrial edema, or inflammation in surrounding tissue — are detected with markedly high signal on STIR. Signs of neurovascular compression can be evaluated.
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Cartilage cap shows high signal on STIR; surrounding tissues should be evaluated for complication findings (bursa, edema).
Normal osteochondroma shows minimal or no enhancement of the cartilage cap (avascular cartilage). The presence of enhancement should be carefully evaluated — prominent enhancement raises suspicion for malignant transformation (secondary chondrosarcoma). Enhancement may be seen in surrounding bursa or inflammation related to complications but should be distinguished from cartilage cap enhancement.
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Enhancement is/is not seen in the cartilage cap on post-contrast sequences; should be evaluated for malignant transformation.
Criteria
Single lesion. Most common form (85%). Malignant transformation risk 1-2%. Usually asymptomatic, incidental finding.
Distinct Features
Typical radiological findings: cortical/medullary continuity, thin cartilage cap (<1 cm), metaphyseal location. Complications: mechanical symptoms (nerve/vessel compression), friction bursitis, fracture.
Criteria
Multiple osteochondromas. Autosomal dominant inheritance (EXT1/EXT2 gene mutation). Malignant transformation risk 5-25% (significantly higher than solitary). Short stature and extremity deformities may accompany.
Distinct Features
Osteochondromas in multiple bones, may have bilateral and symmetric distribution. Bone deformities (ulna shortening, Madelung-like deformity). Cartilage cap thickness should be monitored for each lesion individually — thickening in even a single lesion raises malignancy suspicion. Whole-body MRI screening protocol is recommended.
Criteria
Malignant transformation in osteochondroma background. Risk 1-2% in solitary, 5-25% in HME. Cartilage cap >2 cm, calcification irregularity, growth (after skeletal maturity), pain.
Distinct Features
Cartilage cap thickening >2 cm (measured on T2). Mineralization irregularity within cap. Development of enhancement. Size increase after skeletal maturity. Development of lobular irregular cap contour. Surrounding soft tissue mass.
Distinguishing Feature
Secondary chondrosarcoma shows cartilage cap >2 cm, enhancement, growth after skeletal maturity, and calcification irregularity. Normal osteochondroma has cartilage cap <1-2 cm, minimal enhancement, stable size.
Distinguishing Feature
Parosteal osteosarcoma appears as a densely mineralized mass on the bone surface but lacks medullary continuity. Osteochondroma shows cortical and medullary continuity with the parent bone — this pathognomonic difference is decisive in differential diagnosis.
Distinguishing Feature
Enchondroma is intramedullary (within bone), osteochondroma grows outward from the bone surface (exophytic). Enchondroma lacks exophytic bony projection and cortical continuity.
Distinguishing Feature
Periosteal desmoid (cortical desmoid) appears as an irregular cortical defect on the bone surface at the posterior distal femur. Unlike osteochondroma, there is no medullary continuity and no cartilage cap.
Urgency
lowManagement
Asemptomatik lezyonlar izleme ile takip edilir. Semptomatik lezyonlarda (ağrı, sinir/damar basısı, deformite) cerrahi eksizyon uygulanır. Kıkırdak kapağı tamamen çıkarılmalıdır, aksi halde nüks riski artar. Malign transformasyon şüphesinde geniş rezeksiyon yapılır.Biopsy
Not NeededFollow-up
Soliter asemptomatik lezyon izlem gerektirmez. Semptomatik, büyüme gösteren veya HME hastalarında kıkırdak kapağı kalınlığı yıllık MR ile izlenir. İskelet matüritesi sonrası herhangi bir büyüme veya semptom gelişmesi halinde acil MR değerlendirmesi önerilir.Osteochondroma is generally benign and treatment is not needed in most cases. Confirmation of cortical and medullary continuity with CT is usually sufficient for diagnosis. Critical follow-up parameters: cartilage cap thickness (<2 cm), growth status after skeletal maturity, and symptom development. In HME patients, malignant transformation risk is 5-25% and regular screening is required.
Osteochondroma is the most common benign bone tumor and most are asymptomatic. Malignant transformation (chondrosarcoma) risk is 1% for solitary and 5-10% for hereditary multiple exostoses (HME). Cartilage cap >2cm, new growth in adults, or pain are signs of malignant transformation.