Laboratory Studies
Patients with osteoid osteoma usually have normal blood and chemistry findings.
Osteoid osteoma has been linked to prostaglandins, which may explain the inflammatory features of the lesion. It has been suggested that prostaglandins may play a fundamental role in the development of osteoid osteoma. Several authors have noted extremely high levels of prostaglandin metabolites, especially prostaglandins E2, I2, and F1α, which have been seen at levels 100-1000 times normal, especially in the nidus.
Two pathways have been postulated for pain generation in osteoid osteoma secondary to the effects of prostaglandin. One pathway involves vasodilatory and permeability effects related to an increase in both the size of blood vessels and the flow within vessels in the bony lesion that increases pain and pressure. In the second pathway, the effects of the bradykinin system potentiate pain akin to that due to injured soft tissue because of increased capillary permeability.
Plain Radiography
Osteoid osteoma elicits a profound osteoblastic response in surrounding medullary and cortical bone and shows the characteristic picture of sclerosis around a lucent nidus. It appears as cortical thickening and diffuse medullary sclerosis on radiographs (see the image below). Radiography usually reveals a radiolucent area about 1 cm in diameter, called the nidus, with a center that is sometimes calcified, resulting in a radiopaque point called the bell. The nidus is surrounded by a rim or halo of radiodense cortical hypertrophy or hyperostosis.
Anteroposterior (AP) and lateral radiographs of osteoid osteoma. Courtesy of Cincinnati Children's Hospital Medical Center, Department of Pediatric Orthopaedics.
Cortical lesions may appear with sclerosis that may obscure the lucent nidus.
Intracapsular lesions usually manifest as proliferative synovitis with joint effusion and soft-tissue swelling and no sclerosis. Atar et al noted that lesions in the femoral neck caused widening and foreshortening of the femoral neck, with a reduction in the height of the capital femoral epiphysis and with osteoporosis of the proximal femur. [19]
Subperiosteal lesions may not be apparent on plain radiographs.
Epiphyseal and metaphyseal lesions may show only minimal sclerotic changes around the nidus.
Variation in radiographic appearance is possible. Lesions can appear as only a radiolucent zone, as only a radiopaque area, or as a central sclerosis with surrounding radiopacity.
Regarding the diagnostic value of radiography, some authors have reported that the combination of clinical and radiographic features confirms the diagnosis. However, Georgoulis et al noted that plain radiographs alone have low diagnostic value in detecting the lesion. [26]
Osteoid osteoma may be present months to years before radiographic confirmation. Radiographs may be normal during the first months after the onset of complaint. Therefore, repeat radiographs should be obtained from time to time to document osseous manifestations.
Radionuclide Scanning
Radionuclide scans are reliable tools when radiographic findings are not diagnostic. Wells et al urged that bone scans be performed when radiographic findings are normal or inconclusive, especially in pediatric patients with back pain. Radionuclide concentrates principally in the nidus of osteoid osteoma and minimally in perifocal bone. [9]
Osteoid osteoma is associated with a nonspecific but intense well-defined uptake of activity on bone scans. This focus of intense uptake is correlated with the nidus. Meire et al reported the presence of a nonspecific hot spot long before radiographs depict an alternation of bone texture that represents an osteoblastic lesion. [27]
Technetium bone scanning aids in establishing the diagnosis of osteoid osteoma. Rinsky et al reported that technetium-99m scintigraphy is sensitive for osteoid osteoma in its early stages. [28] In fact, the diagnostic delay is reduced from 35 months to 12 months with the use of bone scans. Swee et al reported that 25% of patients with osteoid osteoma presented with negative radiographic findings but positive bone scans. [29] They recommended that bone scanning be done in suspected cases when radiographic findings are normal.
Radionuclide scanning also has the advantage of aiding the surgeon and the pathologist in confirming resection of the tumor and locating the lesion for histologic examination. Specimen autoradiography and scintigraphy are accurate in localizing the nidus, thus facilitating histologic examination. [30]
Bone scintigraphy also is helpful in ruling out multicentric processes.
To date, no negative bone-scan findings have been reported in patients with osteoid osteoma. Bone scanning is currently the most accurate means of localizing the tumor. Wells et al noted that the sensitivity of skeletal scintigraphy for osteoid osteoma is 100%. [9]
Osteoid osteoma can sometimes be confused with spondylolysis, especially in the lower lumbar spine, as both result in a hot spot on bone scans. Wells et al observed two basic differences between the diseases, as follows. [9]
Images obtained immediately after injection showed minimal or no abnormal activity in spondylolysis. [9] By comparison, images of osteoid osteoma demonstrated easily detectable uptake of the tracer in all cases. However, abnormal activity was noted with spondylolysis when a delay occurred between injection and imaging. On delayed images, both spondylolysis and osteoid osteoma showed hot spots, but spondylolysis resulted in unilateral or bilateral abnormalities, whereas osteoid osteoma led to intense tracer accumulation.
Arteriography
Arteriography can be used when other examinations fail to give sufficient information. Meire et al described three phases of osteoid osteoma on the basis of arteriographic findings [27] :
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Early arterial phase
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Late arterial phase
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Venous phase that generates the bluish picture due to the storage of dye
Computed Tomography
Computed tomography (CT) is helpful in precisely delineating the nidus. It is recommended when the nidus is not visible on conventional radiography, when residual or recurrent tumor is present, or when the tumor is located in a critical area (eg, spine or femoral neck). CT increases specificity for calcified lesions and allows visualization of the nidus. It is useful for precisely defining the location of the tumor and the extent of osseous involvement, especially in areas deep in complex joints (eg, the hip). (See the images below.)
Axial CT scan shows osteoid osteoma. Courtesy of Cincinnati Children's Hospital Medical Center, Department of Pediatric Orthopaedics.
Reconstructed axial CT scan shows osteoid osteoma. Courtesy of Cincinnati Children's Hospital Medical Center, Department of Pediatric Orthopaedics.
Coronal CT scan shows osteoid osteoma. Courtesy of Cincinnati Children's Hospital Medical Center, Department of Pediatric Orthopaedics.
Sagittal CT scan shows osteoid osteoma. Courtesy of Cincinnati Children's Hospital Medical Center, Department of Pediatric Orthopaedics.
CT is more accurate than magnetic resonance imaging (MRI). Sans et al reported that CT helped in confirming the diagnosis of osteoid osteoma in 74% of cases. [31] Szendroi et al reported accuracies of about 66% in the diagnosis of intra-articular lesions and 90% in extra-articular lesions. [32] At present, CT is the primary investigational tool for the definitive diagnosis of osteoid osteoma.
On CT scans, osteoid osteoma appears as a circumscribed anular lesion with a double-attenuating sign. When CT is performed with intravenous contrast material, scans of osteoid osteoma typically show a rapid, early arterial phase of enhancement and then a slow exit of the contrast material from the nidus, consistent with delayed flow in the venous phase.
Magnetic Resonance Imaging
A noncalcified nidus has homogeneous enhancement on contrast-enhanced, fat-suppressed T1-weighted MRI. By comparison, a calcified nidus has ring enhancement, the intensity of which is proportional to the extent of the remaining part of the vascularized nidus.
On T1-weighted MRI, the lesion is an area of decreased signal intensity, sometimes with a bell of highly decreased signal intensity at its center. The degree of bone marrow and soft-tissue enhancement is directly correlated with the size and reactive inflammatory changes of the lesion. [33] (See the images below.)
MRI shows osteoid osteoma. Courtesy of Cincinnati Children's Hospital Medical Center, Department of Pediatric Orthopaedics.
MRI shows osteoid osteoma. Courtesy of Cincinnati Children's Hospital Medical Center, Department of Pediatric Orthopaedics.
MRI has not been greatly useful in the diagnosis of osteoid osteoma. It has generally been reserved for equivocal cases because it can suggest the diagnosis of osteoid osteoma. It is sensitive in detecting bone marrow, peritumoral edema, and soft-tissue abnormalities. Localization of the nidus is often difficult because of an abundance of reactive bone and edema surrounding the lesion that is occasionally misleading.
Assoun et al noted that MRI interpretation resulted in notable errors in diagnosis, most often confusion with malignancies. [34] Guzey et al confirmed this finding. They reported that spinal osteoid osteoma with changes on paravertebral soft tissue can mimic malignant soft-tissue tumors on MRI. [35]
The potential rate of missed diagnosis with MRI is 35%. The tumor is identified on only 65% of axial images. Hence, reliance on MRI alone may lead to clinically significant misdiagnoses. This is particularly true for medullary lesions. [36]
Histologic Findings
The diagnosis of osteoid osteoma can be confirmed only with pathologic examination.
Osteoid osteoma typically consists of a discrete central nidus, usually smaller than 1 cm with diffuse peripheral sclerosis. The nidus is usually a distinct, well-circumscribed cavity, surrounded by dense reactive bone of varying thickness. It is typically cherry-red in color and can be shelled out of the surrounding reactive bone. The nidus varies in consistency from vascular, soft, friable, gritty, and granular to densely sclerotic.
During surgery, an increased number of fine punctate vessels and adherent periosteum overlying the lesion may be observed. Osteoid osteoma is usually cortical and may extend into the periosteal or endosteal surface of the bone. It is rare in the spongiosa.
On microscopic evaluation, the nidus is typically composed of a mass of irregular osteoid tissues that lie in a highly vascular stroma of connective tissue containing osteoblastic cells. It consists of irregular lacelike osteoid and calcified matrix lined by plump osteoblasts and osteoclasts with a well-vascularized but bland stroma.
The microscopic appearance of osteoid osteoma may vary with the degree of lesional maturity. During the initial stage of the disease, proliferation occurs in osteoblasts and vascularized spindle cell stroma with minimal new bone formation. In the intermediate stage, patches of calcified osteoid between the neoplastic stromal cells appear. The mature stage manifests with densely packed atypical bony trabeculae with decreased vascularity and stroma.
The histologic stage is not correlated with the patient's clinical picture.
Pathologic confirmation of the diagnosis of osteoid osteoma by means of percutaneous needle biopsy yields reliable results in only 50% of cases. Use of autoimaging and scintigraphy has increased in the pathologic examination of osteoid osteoma specimens. Autoimaging helps direct attention to the hottest fragment, which corresponds to the nidus. Thus, it decreases false-negative reports, which most commonly occur because of sampling error.
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Anteroposterior (AP) and lateral radiographs of osteoid osteoma. Courtesy of Cincinnati Children's Hospital Medical Center, Department of Pediatric Orthopaedics.
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Axial CT scan shows osteoid osteoma. Courtesy of Cincinnati Children's Hospital Medical Center, Department of Pediatric Orthopaedics.
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Reconstructed axial CT scan shows osteoid osteoma. Courtesy of Cincinnati Children's Hospital Medical Center, Department of Pediatric Orthopaedics.
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Coronal CT scan shows osteoid osteoma. Courtesy of Cincinnati Children's Hospital Medical Center, Department of Pediatric Orthopaedics.
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Sagittal CT scan shows osteoid osteoma. Courtesy of Cincinnati Children's Hospital Medical Center, Department of Pediatric Orthopaedics.
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MRI shows osteoid osteoma. Courtesy of Cincinnati Children's Hospital Medical Center, Department of Pediatric Orthopaedics.
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MRI shows osteoid osteoma. Courtesy of Cincinnati Children's Hospital Medical Center, Department of Pediatric Orthopaedics.
