Osteoid osteoma

This is a benign circumscribed lesion that may arise in the cortex of long bones or occasionally in the cancellous bone of the spine. It affects young patients aged 10–35 and is three-times commoner in males.

Pathology. The characteristic feature is the formation of a small nidus of osteoid tissue, usually less than 0.5 cm diameter, surrounded by a reactive zone of dense sclerotic new bone formation (Fig. 8.1).

Clinical features. They usually present with increasingly severe, but well-localised, deep aching pain and sometimes local bone tenderness. The pain is worse at night and is eased by aspirin or NSAIDs, a diagnostic feature.

Imaging. Plain radiographs typically show local sclerotic thickening of the shaft that may obscure the small central nidus within the area of rarefaction (Fig. 8.5). The nidus is best seen on a fine cut CT scan (Fig. 8.6) and also exhibits intense uptake on an isotope bone scan.

Fig. 8.5 A and B AP and lateral radiographs of tibia. There is an area of dense cortical thickening and sclerosis of the posterior tibial diaphysis. A small central lucency is only just visible. These are typical appearances of osteoid osteoma.

Fig. 8.6 CT scan demonstrates a central small lucency (arrow) containing a more dense central nidus within the thickened cortex. The appearance is diagnostic of an osteoid osteoma.

Treatment. In younger patients some osteomas may resolve spontaneously after several months, but most require surgical treatment. Removal of the central nidus results in resolution of the reactive bone formation and dramatic relief of symptoms. This can be achieved by open surgical excision or curettage, but increasingly less-invasive methods are being used. Where the necessary equipment is available, a CT-guided needle can be inserted into the nidus and the lesion ablated with radiofrequency coagulation.

Chondroma

This is a tumour composed of translucent hyaline cartilage, usually presenting in the 15–50 age group.

Pathology. There are two forms of chondroma: in the commonest type the tumour grows within a bone and expands it (enchondroma) (Fig. 8.2); in the other more rare form it grows outward from a bone (periosteal chondroma or ecchondroma). Most periosteal chondromas arise in the hands or feet, or from flat bones such as the scapula or ilium. They often reach a large size and are more prone to develop malignant change. Enchondromas are fairly common in the long bones, but over 50% occur in the small bones of the hands and feet. The affected bone is expanded by the tumour and its cortex is much thinned; so pathological fracture is common and usually the presenting feature. Many remain asymptomatic and are only discovered as chance findings on radiographs taken for other purposes.

Imaging. Central enchondromas expand the bone with thinning, but not erosion, of the cortex and exhibit a variable degree of mineralisation or speckled calcification (Fig. 8.7).

Fig. 8.7 Radiograph of metacarpo-phalangeal joint of the little finger with a typical enchondroma in the proximal phalanx. The lucent lesion has produced slight medullary expansion with thinning of the overlying cortex. There is some calcification within the lesion indicating a lesion of cartilage origin.

Multiple enchondromata of the major long bones occur mainly in the distinct, but rare, clinical condition known as dyschondroplasia (multiple chondromatosis or Ollier’s disease) (p. 65). In this disorder, which begins in childhood, enchondromata arise in the region of the growing epiphysial cartilages (growth plates) of several bones: they interfere with normal growth at the epiphysial plate and consequently may lead to shortening or deformity (see Fig. 6.4A).

Occasionally an enchondroma undergoes malignant change, becoming a chondrosarcoma, usually in one of the major long bones rather than in the small bones of the hands or feet. Clinically this should be suspected when there is a sudden increase in size of the swelling or if the lesion becomes painful.

Treatment. A chondroma is often best left alone. If it causes a fracture or is unsightly it should be removed by curettage and the defect filled with bone graft.

Osteochondroma (osteocartilaginous exostosis)

This is the commonest benign tumour of bone, usually presenting in the 10–20 age group.

Pathology. The tumour originates in childhood from the growing epiphysial cartilage plate, but as the bone grows in length the outgrowth gets ‘left behind’ and tends to point away from the adjacent joint. It frequently grows outwards from the bone like a mushroom with a bony stalk in continuity with the cortex of the underlying bone (Fig. 8.3). Less commonly the lesion may be sessile with a more broad-based origin. The bony stalk has a larger cap of cartilage which continues to grow until the cessation of skeletal growth.

The ordinary osteochondroma is single; but in the condition known as diaphysial aclasis (multiple exostoses) (p. 63) the tumours affect several or many bones. The risk of malignant change to a chondrosarcoma is higher in these multiple lesions than in the solitary lesion and should be suspected if the tumour continues to enlarge or becomes painful after puberty.

Clinical features. The tumour may be noticed as a circumscribed hard swelling near a joint, but is usually painless. It may become painful due to pressure effects on adjacent nerve or vascular structures, or from the formation of an overlying pseudobursa, though increase in size and pain should always be regarded with suspicion.

Imaging. Plain radiographs show the mushroom-like stalk of the bony tumour (Fig. 8.8), but not the larger cartilaginous cap until this calcifies once skeletal maturity is reached. Patients with known lesions should be warned to seek referral for further imaging if their swelling enlarges or becomes painful.

Fig. 8.8 Osteochondroma of the distal femur. The radiograph shows a bony exostosis arising from the posterior femur and directed away from the adjacent joint. The cartilage cap is not visible.

Treatment. The tumour should be excised if it causes pain or if it enlarges after puberty and must be sent for routine histological examination to exclude any malignant change.

Giant-cell tumour (osteoclastoma)

This is an important tumour because, though generally classed as benign, it tends to recur after local removal or curettage. It occurs most commonly in young adults in the 20–40 age group. In about 10% of cases it behaves as a frankly malignant tumour, metastasising through the blood stream to the lungs.

Pathology. The commonest sites are the lower end of the femur, the upper end of the tibia, the lower end of the radius, and the upper end of the humerus – that is, at those ends of the long bones at which most growth occurs. It may also occur in the spine and sacrum. Characteristically it occurs in the end of the bone, occupying the epiphysial region and often extending almost to the joint surface (Fig. 8.4). It destroys the bone substance, but new bone forms beneath the raised periosteum, so that the bone end becomes expanded and pathological fracture is common.

Histologically the tumour consists of abundant mononuclear oval or spindle-shaped stromal cells profusely interspersed with giant cells that may contain as many as fifty nuclei (Fig. 8.9), hence the name ‘giant cell tumour’. The giant cells possibly represent fused conglomerations of the oval or spindle-shaped stromal cells, which may frequently show mitotic figures, though this is not necessarily indicative of malignant change.

Fig. 8.9 Histology of giant cell tumour showing large osteoclastic multinucleate giant cells (arrows) with interspersed mononuclear tumour cells. The nuclei of the two cell types are very similar. (Haematoxylin and eosin ×400.)

Clinical features. The symptoms are pain at the site of the tumour and a gradually increasing local swelling. Sometimes the patient is made suddenly aware that something is wrong by the occurrence of a pathological fracture. Examination reveals a bony swelling which may be tender on firm palpation.

Imaging. Radiographs show lytic destruction of the bone substance, with expansion of the cortex, but no sclerotic rim or periosteal reaction (Fig. 8.10). A few bony trabeculae may remain within the tumour giving a faintly loculated appearance. The tumour tends to grow eccentrically, and often extends as far as the articular surface of the bone. Magnetic resonance imaging will help to determine the amount of soft tissue extension of the tumour (Fig. 8.11).

Fig. 8.10 A and B Radiographs of giant cell tumour in the proximal tibia. There is a lucent lesion in the metaphysis, with extension into the epiphyseal region extending to the articular surface of the knee. In a patient with fused epiphyses the most likely diagnosis is a giant cell tumour.

Fig. 8.11 MRI of the same patient confirms that the tumour reaches superiorly to the articular cortex. The lesion is however confined to the bone with no evidence of extension through the bony cortex.

Treatment. This depends upon the site of the tumour. Curettage of the contents with a high-speed burr is the standard method of treatment for most giant cell tumours, though this is associated with a high rate (20–25%) of recurrence. This rate can be reduced to less than 10% by the use of adjuvant treatment applied to the lining of the cavity after curettage. Methods used include the chemical phenol, freezing with liquid nitrogen, or the insertion of polymethylmethacrylate bone cement. In addition to its exothermic reaction on any residual cells, the cement has the added advantage of providing support to the subchondral bone and cartilage of the articular surface of the joint. In some bones, or after multiple recurrences, it may be possible to excise the tumour and replace it with allograft bone or a metal prosthesis, without undue functional compromise. Sites where this is possible include the distal radius, distal femur, and proximal tibia.

Radiotherapy is sometimes used and is capable of bringing about permanent cure, but there is a risk that it may itself induce malignant change, perhaps many years later. It should therefore be confined to tumours at sites that are inaccessible to operation, particularly in the spinal column and sacrum.

Chondromyxoid fibroma and chondroblastoma

Both of these benign bone tumours are extremely rare and readers should consult larger textbooks for details of their clinical presentation, imaging and treatment.