A boy aged 4 years presented with a bump on his left lower leg near the knee. The asymptomatic bump had been there for the past two years, and the family was concerned about its appearance. There was no pain, redness, or limitation of activity.

1. Patient history

The boy was born full-term with no known perinatal complications. His developmental history was within normal limits, and he had achieved all his milestones at the appropriate ages. Medical and family history was unremarkable. No history of trauma, surgical procedures, or rapid growth was reported.

2. Diagnosis

Physical examination, including full musculoskeletal exam, was unremarkable except for a 5-cm, hard, and nontender prominence extending downward from the medial aspect of the knee (Figure 1). The overlying skin was normal, and no bruit was heard over the lesion. There was no distortion of the long bones or limitation of movement of the knee joint.

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Possible etiologies considered included an old displaced fracture; such benign lesions as osteochondroma, osteoid osteoma, or fibroma in neurofibromatosis; and such malignant lesions as osteosarcoma or osteochondroma. However, the lack of history of trauma-associated local or systemic abnormalities, pain, or rapid growth made most of these diagnoses unlikely.

The boy was sent for radiographic imaging of the affected knee. Lateral x-ray revealed a bony protuberance on the medial aspect of the left leg below the knee. The growth arose from the medial aspect of the proximal left tibial metaphysis and extended downward (Figure 2). The cortical bone and medulla were continuous with that of the tibia. An overlying cartilage cap measured <0.5 cm in thickness.

Based on the clinical and radiographic appearance, the boy was diagnosed with an osteochondroma (exostosis) of the medial aspect of his left tibia. The family was reassured that his knee prominence was a benign lesion and did not require any intervention at this point. The extremely low risk of malignant transformation was explained.

3. Treatment and outcome

The family was bothered by the appearance and risk (although rare) of malignant transformation and requested that the lesion be removed. The boy was referred to pediatric orthopedic surgery for evaluation and possible excision of the lesion.

4. Discussion

Solitary osteochondromas are the most commonly seen bony tumors in children, representing approximately 20% to 50% of benign bone tumors and 10% to 15% of all bone tumors. They are incidentally found in 1%-2% of people undergoing extensive radiologic evaluation and may be unrecognized in a number of patients, as they are usually asymptomatic. Osteochondromas have a male predilection and are usually found in boys younger than age 20 years.

Osteochondromas are thought to arise as a result of displacement of a fragment of the epiphyseal growth plate cartilage during early growth of the bone. These lesions are most likely to occur in the long bones, particularly the distal femur, proximal humerus, and/or proximal tibia. The lower extremities are more likely to be involved than the upper extremities, and the knee is the most frequently affected structure. The epiphyseal fragment continues to grow and undergoes enchondral ossification, resulting in a subperiosteal osseous excrescence with a cartilage cap that projects from the surface of the bone. These lesions cease to enlarge after adolescence and skeletal maturity.

Radiology of exostoses is very typical because of their unique composition of cortical and medullary bone with an overlying hyaline cartilage cap. Plain films typically reveal a solitary, pedunculated or sessile, exophytic outgrowth from the bone surface. The growth is usually directing away from the adjacent joint thanks to the forces of the overlying tendons and ligaments. Continuity of the lesion with the underlying native bone cortex and the medullary canal is pathognomonic. This osseous connection may be broader than the height of the lesion, or narrow with a bulbous tip (pedunculated osteochondroma). The radiographic size of the lesion is usually smaller than the clinical appearance, as the overlying hyaline cap is usually not visualized on plain films unless calcified. Occasionally, plain radiographic films may not offer enough evidence to diagnose exostoses, especially in such flat bones as the scapula. In these cases, a CT scan may be necessary to make the definitive diagnosis.

Solitary osteochondromas are usually asymptomatic and present as a bony and nonpainful prominence. But, they may cause symptoms if they undergo such complications as fracture, vascular compromise, neurologic sequelae, or overlying bursa formation. The overlying vessels may get displaced or compressed by the lesion growing rapidly during puberty. This may result in erosion of the vessel wall and pseudo-aneurysm formation, which is most commonly seen in the popliteal vessels and can result in a pulsatile mass or, rarely, claudication. Compression of the overlying nerves in peripheral lesions often manifests as atrophy of the muscles supplied by the affected nerves. Foot drop is the most commonly seen manifestation. Lesions of the skull or vertebrae can cause cranial nerve palsies, radiculopathy, or cauda equina syndrome.

Malignant transformation is rare in solitary osteochondromas and occurs in approximately 1% of cases. Continued growth or a persistent cartilage cap larger than 1.5 cm after skeletal maturity is suggestive of malignant transformation. The malignant transformation is almost always attributable to a chondrosarcoma arising in the cartilage cap of the lesion. In rare cases, however, an osteosarcoma may occur at the base of the lesion. Central lesions (pelvis, shoulders) are more prone to malignant transformation. Radiologic features suggestive of malignancy include an irregular or indistinct surface, focal regions of radiolucency in the interior of the lesion, and erosion or destruction of the underlying bone.

Multiple osteochondromas are rare and inherited as an autosomal dominant condition called hereditary multiple exostoses (HME). Unlike solitary lesions, HME is associated with significant skeletal deformities, limb-length inequalities, and short stature. In most cases, HME is diagnosed before age 5 years. The risk of malignant transformation is also higher (3%-5%), and the tumor may be of a more dedifferentiated histologic type.

Management of osteochondromas is rarely needed, and asymptomatic small lesions can be followed clinically. Larger, symptomatic lesions may be resected at the attachment point to the underlying normal bone. It is important to resect the overlying perichondrium completely to prevent recurrence. Although the recurrence rate is low (2%), there is a significant risk of complications with surgical excision, including neuropraxia, arterial laceration, compartment syndrome, and fracture. Management of HME is more complex and includes treatment of the exostoses as well as the associated deformities. These patients also require continued surveillance of progressive deformities as well as monitoring for malignant transformation. Management of chondrosarcoma is a wide surgical resection and limb salvage. Radiation therapy and chemotherapy are generally not indicated except in cases of dedifferentiated tumors. As these tumors are usually low-grade, the prognosis is typically good.

Dr. Sanjeev Tuli is associate professor and chief, division of ambulatory pediatrics; Dr. Kelly and Dr. Parker are assistant professors, division of ambulatory pediatrics; and Dr. Sonal Tuli is associate professor, division of ophthalmology, all at the University of Florida College of Medicine in Gainesville.


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All electronic documents accessed February 15, 2011.