Kemedy K. McQuillen
• Many benign bone tumors are painless and are incidental findings on radiographs.
• Osteoid osteoma is a relatively common benign tumor. It frequently causes pain that is worse at night and is exquisitely responsive to nonsteroidal anti-inflammatory drugs (NSAIDs).
• Nonossifying fibromas (NOF) are common fibrous lesions. They are often incidental findings but can also cause chronic pain.
• Osteochondromas tend to present as a bony, nonpainful mass. Radiographically they appear as sessile or pedunculated lesions of the long bones.
• Patients with enchondromas may present with a mass or pathologic fracture, but most are asymptomatic. The hands are most commonly involved.
• Solitary bone cysts in the lower extremity are prone to fracture and require excision.
• Aneurysmal bone cysts (ABCs) commonly involve the long bones. They are associated with rapidly progressive pain and swelling and can cause significant morbidity.
Osteoid osteoma is a very common benign tumor of bone and accounts for 2% to 3% of all bone tumors and 10% to 20% of benign bone tumors.1 It is two to three times more frequent in males and is most common between 5 and 20 years of age. Osteoid osteoma most commonly involves the long bones of the legs but may occur in any bone including the hands and the spine where it is the most common cause of painful scoliosis in skeletally immature individuals.2
Pain is the most common presentation of osteoid osteoma. It is mediated by the proliferation of nerve endings in the tumor and a high level of prostaglandins in the nidus, which accounts for the exquisite responsiveness of the pain to NSAIDs.3 The pain is usually mild and intermittent but then becomes continuous and severe and tends to be worse at night. Patients may have point tenderness, a swollen limb, and/or a tender palpable mass; or they may present with a painless limp. Osteoid osteomas in the joints may mimic arthropathy whereas those in the spine may present with scoliosis, torticollis, hyperlordosis, or kyphoscoliosis. Osteoid osteomas that are close to the growth plates may lead to growth disturbance and limb-length discrepancies or angular deviations. If the diagnosis of osteoid osteoma is delayed, patients may present with chronic pain or limping as well as atrophy of the affected limb.
Clinical history and radiographs are usually sufficient to diagnose osteoid osteoma (histologic confirmation is not necessary). The typical radiographic appearance of a cortical osteoid osteoma is of a small (<1 cm) radiolucent round or oval area of osteolysis (nidus), surrounded by a regular ring of bony sclerosis (Fig. 111-1). The entire entity rarely exceeds 1.5 cm. In some cases, the center of the nidus may have an irregular nucleus of bone density giving a cockade appearance. The bone circumference may be increased. Cortical diaphyseal lesions may produce an oblong thickening to one side of the shaft. In these lesions, the nidus lies at the center of the thickening and may be contained within the primary cortex or oriented toward the endosteal or periosteal side of the bone. The surrounding reactive bone may be sufficiently dense as to obscure the nidus. With a medullary osteoid osteoma, osteosclerosis may be minimal or absent because of the lack of adjacent periosteum as the absence of periosteum limits the bone’s ability to mount a proliferative response. If osteosclerosis is present with a medullary osteoid osteoma, it may be located away from the lesion. Joint space widening may occur in the presence of synovitis. Osteoid osteomas of the spine typically involve the posterior elements and may be difficult to visualize on plain radiographs; films may be normal or demonstrate only osteosclerosis.
FIGURE 111-1. Osteoid osteoma of the proximal tibia.
If conventional radiography is insufficient, computed tomography (CT) and isotope bone scans may aid in the diagnosis of osteoid osteoma. CT using bone windows will show the nidus of the osteoid osteoma as a well-defined, rounded area of decreased attenuation with a smooth periphery surrounded by a variable amount of reactive sclerosis. The hypervascular nidus of an osteoid osteoma may also be demonstrated on a contrast-enhanced CT scan. Because of the small size of the tumor, however, the CT scan should be performed in thin sections (1–2 mm). On bone scans, the double density sign is described from the small, rounded area of intense uptake (the nidus) centered in a less intensely positive and more diffuse halo (peripheral hypervascularization and sclerosis). Magnetic resonance imaging (MRI) shows intense bone marrow edema adjacent to the nidus although the nidus itself may not be visible.2
Osteoid osteomas tend to be nonprogressive and may resolve over the course of years. Medical management with prolonged use of NSAIDs is an option but, severe pain, intolerance of NSAIDs, and disuse muscle atrophy mandates removal of the nidus as a curative intervention. Techniques include radiofrequency ablation (considered by many to be the treatment of choice4); wide surgical excision; intralesional excision; arthroscopic, CT and MRI-guided core drill excision; injection of ethanol; interstitial laser photocoagulation; and a variety of minimally invasive surgeries. If the nidus is not removed completely, the patient may have persistent pain.
NOF, also known as fibrous cortical defects, metaphyseal fibrous defects, and cortical desmoids, are extremely common fibrous lesions seen in up to 40% of preadolescents and adolescents.5 They are rarely found in adults and, when followed radiographically over time, usually fill in with normal bone during adolescence.6 They tend to be asymptomatic and are usually discovered incidentally but occasionally present with pain from a pathologic fracture. Infrequently, the fibroma may cause chronic pain. NOFs may occur in multiple bones.
Radiographs reveal an eccentric lucency in the metaphyseal cortex that has sharp margins and surrounding sclerosis. Lesions may be multilocular and expansive with extension into the medullary bone.5 The long axis of the fibroma parallels the long axis of the bone. With an overlying stress fracture and associated periosteal reaction, NOFs may be mistaken for osteogenic sarcomas. NOFs may also have an atypical appearance if they are discovered as they are filling in with normal bone. CT scanning may be helpful if plain radiographs are not diagnostic. NOFs with a characteristic radiographic appearance do not require biopsy.5
Routine follow-up for asymptomatic NOFs is controversial. Fibromas found in young children may increase in size relative to the growth of the adjacent bone and place the bone at risk for fracture. For this reason, yearly or semi-annual radiographs may be considered in younger patients until the lesion is stable in size relative to the involved bone. NOFs discovered in adolescence do not require follow-up.
The majority of NOFs do not require treatment. Lesions greater than 50% of the width of the bone or smaller lesions in areas of high stress may be considered for curettage and bone grafting because of the risk of associated pathologic fracture.6 Injection, extensive burring, chemical, thermal, or other adjuvants are not required. Recurrence is rare.
Osteochondromas, also called cartilaginous exostoses, are the most common benign bone tumor in children and adolescents.5 Most osteochondromas occur in the metaphysis of long bones, particularly the proximal tibia, proximal humerus, and distal femur. When they occur in flat bones, the ilium and scapula are most commonly affected. They are cartilaginous in growing children, enlarge during skeletal growth, and ossify at skeletal maturity. It likely arises as a result of physeal cartilage displacement onto the bone surface.2 Osteochondromas are usually discovered between 5 and 15 years of age and present as a bony, nonpainful mass. If pain is present, it is usually owing to friction or pressure on the adjacent structures. Occasionally, exostoses have been associated with neurovascular impingement, fracture, pseudoaneurysms, vascular obstruction, and nerve damage.
Although most exostoses occur singly, rarely some patients have autosomal dominant multiple hereditary exostoses (MHE) resulting in multiple osteochondromas.6 Although the degree of involvement tends to run in families, phenotypic expression is variable. Severely affected individuals may have short stature, limb-length discrepancies, premature partial physeal arrests, extremity deformities, and deterioration of the hip joint.
Radiographs demonstrate sessile or pedunculated projections arising from the surface of the bone usually directed away from the joint (Fig. 111-2). The lesion’s cartilage cap, which can be more than 2-cm thick in childhood, is not seen on x-ray, making the osteochondroma appear smaller on radiographs than it is on physical examination. The cortex and marrow space of the involved bone are continuous with the lesion. CT or MRI may be needed to demonstrate it clearly. The radiographic appearance is usually pathognomonic, and biopsy is rarely needed to establish the diagnosis.6 Resection of osteochondromas may be considered for cosmetic reasons; pain relief; or if they cause growth retardation, vascular obstruction, or pseudoaneurysms.
FIGURE 111-2. Osteochondroma of the proximal humerus.
Malignant transformation of a single lesion is exceptionally rare. In families with MHE, however, there is a 1% to 8% lifetime risk of developing chondrosarcomas in the exostoses. Malignant transformation occurs in adulthood.7Growth of the cartilage cap or a cap that is thicker than 2 cm after skeletal maturity is worrisome for malignant transformation.2
Enchondroma is a benign lesion of hyaline cartilage that occurs centrally in the bone and most commonly affects the hands.5 The majority of lesions are asymptomatic but some patients may present with a mass or pathologic fracture. Radiographs show a radiolucent, sharply marginated lesion in the medullary canal with associated thinning of bone and cortical bulging (Fig. 111-3). Punctate or stippled calcification may be seen; however, this finding is more common in adults. Orthopedic consultation is indicated since curettage and bone grafting may be considered for large or symptomatic lesions.
FIGURE 111-3. Enchondroma of the pelvis.
Most enchondromas are solitary with a low risk for malignant transformation. The risk of malignant transformation is much greater in the multifocal enchondromas of Ollier disease (enchondromas, bony dysplasia, short stature, limb-length abnormalities, and joint deformity) (Fig. 111-4) and Maffucci syndrome (enchondromas and soft-tissue angiomas). Pain is the most sensitive indicator of sarcomatous transformation and, radiographically, there is a destructive lytic lesion with a soft-tissue mass.2
FIGURE 111-4. Pelvis radiograph in a patient with Ollier disease. Note the stippled calcification.
UNICAMERAL BONE CYSTS
Unicameral bone cysts, also known as simple or multiloculated bone cysts, are expansile, usually serous fluid-containing defects of tubular and flat bones. The majority of unicameral bone cysts occur in the proximal humerus (60%) and proximal femur (30%).2,8 They tend to be discovered in the first 2 decades of life; less than 15% occur after 20 years of age.2 Males are affected twice as frequently as females. In most patients, only one bone is affected.
Unicameral bone cysts start near the epiphyseal plate and extend toward the diaphysis during growth. They tend to be active in children younger than 10 years and inactive in children beyond 10 years of age. Active cysts have a higher recurrence rate than do inactive cysts.
Unicameral bone cysts can be asymptomatic and found incidentally or may be discovered as a result of a pathologic fracture. Patients may also present with pain, limp, or failure to use the extremity normally.
Radiographically, long bone unicameral cysts are seen as concentrically located lytic lesions in the medullary cavity of the metaphysis with expansion in all directions (Fig. 111-5). This creates an expanded, thinned but unpenetrated cortex. Cysts in the flat bones are centered between the inner and outer tables of the ilium and midportion of the superior pubic ramus. The long axis of the lesion tends to exceed its width. Unicameral bone cysts do not induce a periosteal reaction and, if one is seen, it suggests an associated fracture. The “fallen leaf” or “fallen fragment” sign is pathognomonic of a multiloculated bone cyst and represents a broken piece of cortex that has fallen into the fluid-containing cavity of the cyst; similarly, there may be a “rising bubbles” sign.9 If the radiographic diagnosis is in question, CT scanning can be helpful.
FIGURE 111-5. Unicameral bone cyst of the distal tibia.
These lesions are prone to fracture and require excision, especially in the lower extremity. Orthopedic consultation is indicated. Injection of steroid into the lesion is also therapeutic, but two to three injections may be required for complete healing. More recently, a single percutaneous injection of demineralized bone matrix has been found to result in cyst resolution in 80% to 90% of patients.6 Regardless of treatment regimen, children older than 10 years heal at a higher rate (90%) than those younger than 10 years (60%).8 Radiographs may never normalize, and the goal of treatment is a functionally stable bone, not a normal-appearing x-ray.
ANEURYSMAL BONE CYSTS
ABC is a rare, rapidly growing, destructive bone tumor that most commonly involves the long bones (50%) and the spine (20%); pelvic bones are the most commonly affected flat bones.2 The majority of ABCs present in the first two decades of life. They are slightly more common in females.10 Although they are considered benign, ABCs have the potential to cause significant morbidity. Most patients present with pain and swelling although ABCs near the joint may limit range of motion and spinal involvement can result in neurologic deficits. Up to 20% of patients can have pathologic fractures. Less than 10% of patients will have involvement of multiple bones. Up to 30% of ABCs may arise with a coexisting bone lesion; the most common underlying lesions include fibrous dysplasia, chondroblastoma, giant cell tumor, and osteosarcoma.11
On plain radiographs, ABCs demonstrate an eccentric, fusiform, aneurysmal dilation of the bone with a thin rim of cortical bone seen peripherally (Fig. 111-6). They are often located near the end of the metaphysis and may penetrate into the epiphysis through the growth cartilage. In most instances, there is aggressive lytic bony destruction, and laminated periosteal reaction is common in the long bones. Faint trabeculations are usually present giving the tumor a “soap bubble” appearance. ABCs may also demonstrate a soft-tissue mass extending out of the bone into the soft tissues.
FIGURE 111-6. Aneurysmal bone cyst of the proximal fibula. Note the faint trabeculations that give it a “soap bubble” appearance.
ABCs are filled with blood and, on cross-sectional CT imaging, fluid–fluid levels may be seen as red cells and serous components separate when the patient lies still for the examination. Fluid–fluid levels are highly suggestive of ABCs but are not pathognomonic as they may be associated with telangiectatic osteosarcoma, giant cell tumor, and simple cyst with fracture. The pathologic diagnosis is based on a biopsy done at the time of definitive surgery.12 The mainstay of treatment is curettage and bone grafting and/or cementation.11 Surgical treatment of ABCs may be complicated by copious bleeding; in patients with lesions that are central or difficult to access, preoperative embolization should be considered.6 Low-dose radiation therapy is used for incompletely resectable, aggressive, or recurrent ABCs with local control in 90% of cases.11 Ten to fifty percent of ABCs will recur, more commonly in younger patients.6 Most recurrences are seen within 2 years of treatment. Recurrent lesions are treated with surgery or radiation.
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