A 16-year-old boy presents with a painless bony lump over the distal femur that has been gradually enlarging over the past 2 years. On examination, a firm, immobile, bony hard mass is palpable over the posterolateral aspect of the distal femur. Plain radiographs show a sessile bony projection with a cartilage cap arising from the metaphyseal region. The lesion is oriented away from the joint. What is the most likely diagnosis?

## Diagnosis: Osteochondroma ### Clinical Presentation **Key Point:** Osteochondroma is the most common benign bone tumor, typically presenting in adolescents and young adults with a painless, slowly enlarging bony mass. ### Characteristic Features | Feature | Osteochondroma | Other Benign Tumors | |---------|-----------------|---------------------| | **Age of onset** | 10–20 years (growth plate active) | Varies | | **Location** | Metaphyseal region (near growth plate) | Diaphyseal or epiphyseal | | **Growth pattern** | Grows with skeleton; stops at skeletal maturity | May continue growing | | **Pain** | Usually painless unless impinging on soft tissue | Can be painful | | **Orientation** | Away from joint (exophytic) | Variable | | **Cartilage cap** | Present (hyaline cartilage) | Absent in most | | **Malignant potential** | <1% (chondrosarcoma) | Very rare | ### Pathology **High-Yield:** Osteochondroma is a cartilage-capped bony projection arising from the metaphysis, composed of hyaline cartilage superficially and mature bone internally. The cartilage cap is essential for diagnosis and growth. ### Imaging Findings - **Plain radiographs:** Sessile or pedunculated bony mass with well-defined margins - **Orientation:** Directed away from the joint (exophytic) - **Location:** Metaphyseal region, commonly around the knee (distal femur, proximal tibia) - **CT/MRI:** Cartilage cap thickness <3 cm is reassuring; cap >3 cm or rapid growth raises concern for malignant transformation ### Clinical Pearl **Key Point:** Painless bony lump in a teenager near the knee with exophytic orientation = osteochondroma until proven otherwise. Pain suggests either mechanical irritation of adjacent structures or (rarely) malignant transformation. ### Management 1. **Observation** — if asymptomatic and no functional impairment 2. **Surgical excision** — if symptomatic (pain, mechanical irritation, nerve/vessel compression) or concern for malignant transformation 3. **Follow-up imaging** — annually until skeletal maturity; then only if symptoms develop ### Malignant Transformation **Warning:** Malignant transformation to chondrosarcoma occurs in <1% of solitary osteochondromas but in up to 5% of hereditary multiple osteochondromas (HME/EXT). Red flags include: - Rapid growth after skeletal maturity - Cartilage cap >3 cm - New onset pain - Imaging evidence of aggressive features [cite:Rockwood & Green's Fractures in Adults Ch 32]