bubble_chart Overview

Solitary osteochondroma is a hamartoma of bone, originating from abnormal proliferation of the growth cartilage beneath the periosteum. It is a common benign bone tumor, more prevalent in males with a male-to-female ratio of 1.5–2:1. During skeletal growth, the slow enlargement of a painless bony protrusion is virtually the only clinical symptom, and a correct diagnosis can often be made based solely on radiographic imaging.

bubble_chart Pathological Changes

1. Grossly, the top of the osteochondroma is covered by a cartilaginous cap, which changes with age. In solitary osteochondromas of young children, the osteochondroma is primarily composed of cartilage, and its initial ossification merges with the margins of newly formed trabeculae in the metaphysis. During childhood, the cartilaginous cap still covers the entire top of the osteochondroma, appearing as a white to bright blue hyaline cartilage resembling normal infantile cartilage, with a thickness ranging from a few millimeters to 1 cm or more. With advancing age, the thickness of the cartilaginous cap decreases and may disappear in many areas. In adulthood, the original cartilaginous cap often leaves behind a small, irregular remnant, no more than a few millimeters thick. The thickness and morphology of the cartilaginous cap are crucial in determining whether malignant transformation has occurred in a solitary osteochondroma.

On the cut surface of a solitary osteochondroma, beneath the cartilaginous cap lies cancellous bone with irregular meshwork and yellow marrow, and occasionally red marrow. Cartilaginous islands may extend a short distance from the surface of the osteochondroma or be embedded within it. Over time, both exhibit significant calcification and/or ossification, appearing as yellow-white spots that are hard and gritty.

A bony membrane covers the entire osteochondroma, extending from the metaphysis to the cartilaginous cap and adhering to it. Synovial bursae may be observed at the top of the osteochondroma, sometimes with a pseudosynovial membrane.

2. Microscopically, during the growth phase of a solitary osteochondroma, the cartilaginous cap resembles normal growth cartilage but is less regular, displaying layers of proliferation, columnar arrangement, hypertrophy, and calcification from the surface inward. Below this is trabecular bone formed by endochondral ossification, with irregular distribution, orientation, and shape, occasionally containing cartilaginous islands. In adults, the discontinuous and thin cartilaginous cap resembles mature hyaline cartilage, similar to articular cartilage.

The only significant aspect of histological examination in solitary osteochondroma is confirming the absence of fleshy tumor-like tissue within the cartilage.

bubble_chart Clinical Manifestations

Because solitary osteochondroma does not produce symptoms until it reaches a considerable size, it is rarely seen before the age of 8–10 years. Moreover, since its growth parallels that of the skeleton, it attains its maximum volume around puberty, so most cases of solitary osteochondroma present symptoms between the ages of 10 and 18, after which it is uncommon.

Age is an important factor in diagnosis and prognosis. Before puberty, the growth of solitary osteochondroma is normal, and it is almost impossible for it to develop into peripheral chondrosarcoma. However, if solitary osteochondroma resumes growth during adulthood, it almost certainly indicates transformation into chondrosarcoma.

Nearly 90% of solitary osteochondromas are located in the metaphyses of the long bones of the limbs. They most commonly occur around the knee (especially the distal femur) and shoulder (proximal humerus), followed by the ankle, proximal femur, and wrist. Solitary osteochondroma is less frequently found in the bones of the trunk, primarily appearing in the scapula, pelvis (mostly the ilium), and spine (posterior arch). Rare cases occur in the short tubular bones of the hands and feet. Some deeply located solitary osteochondromas, such as those in the vertebrae or ilium, may remain asymptomatic and undetected.

During skeletal growth, a slowly enlarging, painless bony protrusion is almost the only symptom. Solitary osteochondroma originates from and remains attached to the bone surface without adhering to soft tissues. Some cases are entirely asymptomatic and may be incidentally discovered during imaging for other reasons. Solitary osteochondroma can cause friction with overlying muscles, tendon sheaths, or tendons, leading to the formation of a bursa on its surface.

bubble_chart Auxiliary Examination

X-ray findings: The imaging is highly typical, and a correct diagnosis can be made based solely on radiographic images.

On X-ray films, the diagnostic feature of a solitary osteochondroma is the outward flaring of the host bone's cortex, resembling the fingers of a glove, forming the cortical bone of the osteochondroma. The cancellous bone directly continues with the cancellous bone of the metaphysis. Therefore, an osteochondroma is an outgrowth of bone composed of the following components: ① a thin outer layer of cortical bone; ② internal cancellous bone; and ③ an uneven, expanded apex. The boundary between a solitary osteochondroma and soft tissue is very distinct.

The internal structure of a solitary osteochondroma consists of mature cancellous bone, but its pressure lines are neither regular nor orderly, sometimes appearing as broad and irregularly meshed cancellous bone. The shape and size of osteochondromas vary widely. When an osteochondroma is pedunculated, its direction often tilts toward the diaphysis, away from the epiphysis.

Bone scans are often positive in active osteochondromas during childhood but weakly positive or negative in inactive osteochondromas in adults.

bubble_chart Treatment Measures

Most solitary osteochondroma cases do not require surgical excision, and patients can engage in normal work and life. However, when a solitary osteochondroma is located in the trunk bones, it may be excised even if asymptomatic to prevent the not uncommon transformation into fleshy osteosarcoma.

Apart from the above scenario, surgery is only indicated for cosmetic reasons or when symptoms are present. Like all hamartomas, solitary osteochondroma surgery is best avoided during childhood because the indications for surgery are difficult to determine before the osteochondroma has fully developed, and excision during childhood may lead to local recurrence. However, in rare cases, such as when the osteochondroma causes significant functional impairment and deformity or grows near neurovascular bundles, surgery may be performed during childhood.

Surgery for solitary osteochondroma must involve the complete removal of the osteochondroma along with its covering membrane (marginal excision) to avoid leaving behind part of the cartilage cap, which could lead to regrowth. In adults, there is no need to excise the stalk and base of the solitary osteochondroma from its root, as the bony part of the solitary osteochondroma has no proliferative potential, and even intracapsular excision of the osteochondroma apex will not result in recurrence. In children, surgical excision must be extracapsular, and the base of the osteochondroma and surrounding bone membrane must also be removed, as they may contain viable cartilage remnants that could lead to postoperative recurrence.

When clinical and radiographic findings raise suspicion of early fleshy tumor transformation in a solitary osteochondroma, surgical excision must strictly follow extracapsular, marginal, or wide excision principles. Care must be taken to avoid stripping the covering membrane of the osteochondroma's cartilage surface and to prevent fragmentation of the osteochondroma during surgery.

bubble_chart Differentiation

The diagnosis can be made based solely on clinical and imaging findings, with the only challenge being the differentiation between benign osteochondroma and osteochondroma that has undergone fleshy tumor transformation. A comprehensive analysis of all clinical, radiographic, bone scan, gross pathological, and histological features is required.