cartilagecap

Cartilagecap refers to the thin layer of hyaline cartilage that covers the exophytic bone projection known as an osteochondroma. The cartilage cap sits atop the bony stalk or sessile lesion and represents the active growth layer from which the osteochondroma extends during skeletal development.

The cap is composed of mature cartilage with chondrocytes in lacunae and varies in thickness with age and growth. In skeletally immature individuals, the cap can be relatively thick and may decrease in size after maturation. The size and integrity of the cap help distinguish benign osteochondromas from other lesions and can influence imaging interpretation.

On imaging, osteochondromas appear as bone protuberances continuous with the cortex and medullary cavity of the host bone, capped by cartilage. Radiographs can estimate cap thickness, but MRI is preferred for precise measurement and assessment of the cap’s integrity, marrow continuity, and surrounding soft tissues. A cartilage cap thicker than about 2 cm in adults is considered suspicious for malignant transformation, while in children larger caps can occur normally. Other red flags include cap irregularity, rapid growth, new pain, or soft-tissue mass.

Clinical significance centers on the small risk of malignant transformation to chondrosarcoma, higher in cases of hereditary multiple exostoses and in adults. Most osteochondromas are asymptomatic and require no treatment unless symptomatic or suspicious for malignancy. Treatment, when indicated, is surgical excision of the lesion and the cartilage cap. Recurrence is possible if the cap is not completely removed.