An osteochondroma is a non-cancerous bone tumor, representing an overgrowth of bone and cartilage that typically forms near a growth plate. While these growths develop during childhood and adolescence, their presence often extends into adulthood. Since skeletal maturity marks the closure of growth plates, the continued stability of these benign tumors is the primary focus for adults who have them. Understanding what an osteochondroma is and how it may change is important for management and monitoring.
Understanding Osteochondroma
An osteochondroma is the most common form of benign bone tumor, characterized by a bony projection capped by hyaline cartilage. This outgrowth forms on the external surface of the bone, usually near the ends of long bones where growth plates are active. The structure is unique because its inner bone marrow and outer cortical layer are continuous with the underlying parent bone.
These tumors originate when a fragment of growth plate cartilage is displaced and grows outward. Since the lesion’s growth is driven by this displaced cartilage, an osteochondroma generally stops growing once the patient reaches full skeletal maturity and the growth plates close. While most cases involve a single lesion (solitary), about 15% of individuals have multiple osteochondromas, a genetic condition known as Hereditary Multiple Exostoses (HME).
How Osteochondroma Presents in Adults
After skeletal growth has ceased, an osteochondroma’s impact on an adult is primarily mechanical, related to its size and location. Many adults with solitary lesions remain asymptomatic, often discovered incidentally on imaging performed for other reasons.
Symptoms typically arise when the bony mass physically interferes with surrounding soft tissues. This interference can cause pain by pressing on nearby nerves, leading to sensations like tingling or numbness. Tendons may also snap over the bony prominence during movement, creating irritation and soreness.
A common complication is bursitis, which occurs when a fluid-filled sac (bursa) forms over the tumor and becomes inflamed. The severity of mechanical symptoms depends on the size and position of the osteochondroma, with common sites including the long bones around the knee, hip, and shoulder.
Assessing the Risk of Malignant Transformation
The primary concern for an adult with an osteochondroma is the rare possibility of malignant transformation into a secondary peripheral chondrosarcoma. This change is infrequent, estimated to occur in about 1% of solitary osteochondromas. The risk is higher for individuals with Hereditary Multiple Exostoses, with reported transformation rates ranging from 2% to 5%.
The development of new or worsening pain unrelated to mechanical activity is an important warning sign in a skeletally mature adult. Since benign osteochondromas stop growing after growth plates close, any documented increase in the tumor’s size warrants immediate investigation.
Imaging plays a significant role in assessing this risk, particularly by evaluating the thickness of the cartilaginous cap. A cap thickness greater than 2 centimeters in an adult is highly suggestive of malignant transformation. Lesions located in the axial skeleton (pelvis, scapula, and spine) generally carry a slightly higher risk and require close monitoring.
Diagnosis and Management Options
The diagnostic process begins with conventional X-rays, which are often sufficient to characterize the bony outgrowth and its continuity with the parent bone. If symptoms are present or malignant change is suspected, cross-sectional imaging is necessary.
Magnetic Resonance Imaging (MRI) is the preferred method for assessing soft tissue structures and accurately measuring the thickness of the cartilage cap. This detailed visualization helps differentiate between a stable, benign lesion and one that may be undergoing transformation.
Management is determined by symptoms and the risk profile. Asymptomatic, stable lesions are typically managed with observation and regular clinical follow-up. Surgical excision is reserved for cases involving persistent pain, nerve or vascular compression, or when imaging findings suggest malignant transformation. Complete surgical removal of the tumor, including the entire cartilaginous cap, is the definitive and curative treatment for symptom relief.