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Introduction
A patient who presents with a painless bony mass either complain about the mass itself or the effects of the mass on surrounding structures such as nerves and joints. The absence of pain is usually a good prognostic feature, since aggressive and malignant bone tumors are generally painful. The typical patient is an adolescent or young adult with a solitary exostosis, or osteochondroma. Osteochondromas are benign surface lesions of bone that are usually found attached to the metaphyses of bones with the greatest growth potential: distal femur, proximal tibia, proximal humerus.
History
- Age, history of recent skeletal growth. The exostosis responds to pediatric growth cycles similarly to the physis, and growth of the lesion may parallel normal skeletal growth. A pediatric patient, therefore, can be simply monitored to ensure that the lesion stops growing when skeletal growth ends. Recent growth of a bone lesion in an adult is more worrisome. Although a solitary exostosis rarely undergoes malignant transformation, recent growth of a previously dormant bony mass warrants further investigations. Additionally, the growth of a painless mass in an adult could suggest a low-grade malignant bone tumor. Most malignancies are associated with pain; however, slow-growing tumors -- such as parosteal osteosarcoma or a low-grade surface chondrosarcoma -- may present as a painless hard mass. Parosteal osteosarcoma, by virtue of its usual location around the knee or shoulder, may initially present with symptoms of limited motion in the joint affected by the mass.
- Mechanical symptoms. Around the knee, growth of an osteochondroma may result in discomfort as the quadriceps muscle slides back and forth over the lesion. Distal to the knee, the pes anserine tendons on the medial side of the tibia may "snap" as they glide over the exostosis. Although most osteochondromas originate in the long bones, they may also develop from growth centers in the flat bones. Lesions arising on the deep surface of the scapula, for example, may cause irritation as they rub against the ribs during shoulder motion.
- Neurovascular impairment. Painless bony masses rarely cause a fixed neurological deficit, although the location of some lesions may result in transient impairment. For example, a large proximal humeral osteochondroma may cause transient radial nerve compression, and a posterior distal femoral parosteal osteosarcoma may cause sciatic nerve symptoms while the patient is seated or driving an automobile. Similarly, large lesions at the inguinal or popliteal regions may result in venous outflow obstruction, especially while the patient is seated for long periods of time. Rare cases of pseudoaneurysm have been caused by osteochondromas, usually in the popliteal artery.
- Possible non-tumor causes of a bone mass. Ask the patient whether or she has a history of trauma or infection. Exuberant callus formation after a traumatic fracture or even a stress fracture may have caused the presenting complaint. Soft tissue trauma resulting in myositis ossification situated close to the surface of the bone may be mistaken for a bony mass. A history of neurological injury associated with extremity trauma is particularly suggestive of myositis since both head and spinal cord injury increase the tendency to form heterotopic bone. Patients with chronic osteomyelitis may also have marked periosteal new bone formation. A history of sepsis should also be elicited.
- Multiple exostosis or familial osteochondromatosis. This autosomal dominant, variable penetrance condition may be associated with short stature, characteristic skeletal deformity (Madelung deformity of the forearm for example), and multiple osteochondromas, and it puts the patient at higher risk of developing secondary malignancy than patients with solitary exostoses. Counsel the patient to report clinical changes in the lesions as he or she ages. Although a family history of the condition is usually present, some patients may have a spontaneous mutation. For example, Ollier's disease (multiple enchondromatosis of bone) is not familial. However, the presence of bony deformity with this condition may result in patients presenting with complaints of a painless mass. This condition should be differentiated from hereditary osteochondromatosis because the risk of malignant transformation is higher in Ollier's disease.
Physical Examination
- Site of origination. Determine whether the lesion is, indeed, originating in the bone, not overlying soft tissues (as in myositis ossificans). This may be difficult if a soft tissue lesion is arising close to the bone, necessitating cross-sectional imaging to confirm the site of origination.
- Extent of the local symptoms. Assess the level of disability associated with the lesion during joint motion and with direct compression over the lesion. Provocative testing (for example compression of the lesion to evaluate a neural deficit) will assist in this effort. The severity of the symptoms may determine whether surgery is indicated.
- Additional exostoses. Examine the patient for evidence of other exostoses. Document short stature or skeletal deformity associated with multiple exostoses.
- Prior trauma, infection. Local evidence of prior trauma or infection should be sought. Evidence of inflammation in the soft tissues surrounding the lesion may also suggest malignant transformation, although this is usually associated with pain or tenderness in the lesion.
Investigations
- Plain radiographs. The most valuable initial investigation for a patient presenting with a painless bony mass is the plain radiograph, as osteochondromas, parosteal osteosarcoma, and fracture callus all have characteristic features on X-ray.
- Bone scans. They may be useful in determining the extent of involvement in multiple osteochondromatosis, and serial bone scans may detect a future change in activity at a newly symptomatic lesion. Do not rely on a bone scan in determining whether an osteochondroma has undergone malignant transformation; bone remodeling associated with a mature osteochondroma may result in persistent uptake for years after skeletal growth has ceased.
- CT scans. A CT scan is useful in differentiating osteochondroma, parosteal osteosarcoma, and myositis ossificans. It can also determine the anatomical relationships of a large lesion to surrounding neurovascular structures prior to removing a large osteochondroma.
- MRI. It is critical to order an MRI when investigating parosteal osteosarcoma to determine the peripheral extent of the lesion and to assess whether it penetrates into the medullary canal. MRI is also useful for assessing the thickness of the cartilaginous cap on the surface of adult osteochondromas. In most adults, this surface cartilage is converted to bone through a process of enchondral ossification. If the cartilaginous cap is greater than 1-2 cm in thickness in adults, there is reason for concern that malignant transformation may be present.
- Blood work. Although not generally helpful in the diagnosis of painless bone masses, a complete blood count, ESR and C-reactive protein may be helpful if infection is suspected. Alkaline phosphatase may be elevated in myositis ossificans.
- Biopsy. The need to biopsy a painless bony mass depends on the imaging investigations described above. If there is reason to suspect malignancy, such as a thick cartilaginous cap, biopsy the mass to assist in planning therapy.
The Other Presenting Complaints
