Abstract
Osteoid osteoma is a common benign bone-forming lesion that is composed of a nidus of vascular osteoid tissue and woven bone lined by osteoblasts. It is frequently associated with considerable surrounding inflammation. The diagnosis is usually straightforward when imaging reveals a radiolucent nidus surrounded by variable degrees of reactive sclerosis. However, the diagnosis can be elusive when osteoid osteomas occur in atypical locations, as they may have a nonspecific and misleading appearance on different imaging modalities, particularly on MRI. The purpose of this pictorial essay is to review the typical and atypical features of osteoid osteomas on different imaging modalities, and the appearance of osteoid osteomas in different locations. We also review growth disturbances caused by osteoid osteomas and potential mimickers, with imaging characteristics that can aid in diagnosis.
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Introduction
In 1935, Jaffe [1] coined the term osteoid osteoma. Osteoid osteoma is the most common benign bone-forming lesion, comprising approximately 12% of benign bone tumors [2, 3]. It is the third-most-common benign bone neoplasm after osteochondroma and nonossifying fibroma. Osteoid osteomas are much more frequent in boys, and usually present between ages 7 and 25 [3, 4]. The classic presentation is of pain that is worse at night and ameliorated by nonsteroidal anti-inflammatory drugs (NSAIDs). Although pain is often referred to a nearby joint, it may be so distant from the lesion that radiographic examinations are misdirected. Localized tenderness to touch and pressure and local swelling might be present, particularly when the bone involved is in close proximity to the skin [5]. Much of this swelling may be secondary to the highly vascular nature of the tumor, which can be confirmed by angiography, scintigraphy, Doppler sonography and MRI [6–10]. This may also be mediated by the production of prostaglandins, which can affect soft tissue and vascular permeability. Prostaglandin levels in the nidus are 100–1,000 times that of normal bone and are thought to be important in the perception of pain [11]. The pain relief associated with NSAIDs has also been attributed to the inhibition of this prostaglandin production [12]. Intra-articular prostaglandins are thought to be responsible for the related lymphofollicular synovitis [13]. Prostaglandins are also believed to lead to vasodilatation and edema in the surrounding medullary tissues, reflected as hyperintense areas on T2-weighted sequences on MRI [14–16]. Depending on the location of the lesion, other less common presentations include swelling, limp, growth disturbance, painful scoliosis, joint stiffness or contracture [3]. Painless presentations are also recognized, particularly those involving the phalanges of the hands and feet [17, 18].
Osteoid osteomas are composed of a nidus of vascular osteoid tissue and woven bone lined by osteoblasts. Even though there may be considerable inflammation associated with osteoid osteomas, the tumor itself (nidus) rarely exceeds 1.5 cm. The precise location of the nidus must be determined, since its removal is necessary for cure [2]. The pathogenesis of this lesion is controversial. While some authors believe it to be a benign neoplasm, others believe it may represent an unusual healing process or arise on an inflammatory basis [19, 20]. Whether osteoid osteomas are a true neoplasm is also questioned because its growth potential is limited. There is no known malignant potential.
With special immunohistochemical techniques, abundant nerve fibers can be demonstrated within the nidus and particularly within the adjacent zone of sclerosis [21, 22]. These nerve fibers belong to the osseous nerve supply, which plays an important role in skeletal development and homeostasis. Large amounts of nerve fibers have been encountered in healing fractures as well. It is believed that nerve fibers proliferate to provide information about the fracture site and that they might also be actively involved in bone formation and remodeling by the exertion of a paracrine effect on osteoblasts through the production and release of cytokines, such as calcitonin gene-related peptide [23]. It is possible that the same mechanism applies with regard to the innervation of osteoid osteomas, as the greatest concentration of nerves is found in the reactive zone, which might explain the intense sclerosis at this site [3, 21, 22].
Classification
Osteoid osteomas can be classified based on their location within bone (Fig. 1) [24]. The most common presentation, or “classic” type, is cortical with the radiolucent nidus located in the center of fusiform cortical thickening (Fig. 2). This is usually seen in the diaphysis or metaphysis of long tubular bones, especially the tibia and femur. The value of CT in the diagnosis of osteoid osteoma has been well documented in the literature [25–28]. Although recent literature recommends the use of non-ionizing imaging modalities in children, the role of MRI in diagnosing osteoid osteoma remains controversial, despite recent reports advocating the utility of it for the diagnosis of osteoid osteomas (Fig. 3) [29–31].
Nonetheless, the differentiation of osteoid osteomas from other masses may still be problematic, especially when they occur in less common and atypical locations, such as the subperiosteal space, medullary cavity, spine or non-long bone lesions, or in lesions that have atypical features [25–28].
On CT, the nidus may be lucent or may have central calcifications. The recently described “CT vessel” or “vascular groove” sign is highly specific for osteoid osteomas, and can be seen as curvilinear low-density grooves entering or in proximity to the nidus (Fig. 4) [32, 33]. On MRI, the nidus demonstrates low to intermediate signal intensity on T1-weighted images with enhancement, following intravenous contrast injection. In the authors’ experience [34], a rim of low SI on all pulse sequences is often seen surrounding the nidus, which corresponds to the peripheral rim of sclerosis seen on pathology. Adjacent reactive bone marrow and soft-tissue edema appear hyperintense on T2-weighted images and may also demonstrate enhancement after the administration of intravenous contrast agent.
Medullary osteoid osteomas are typically juxta-articular in location, and are often seen in the femoral neck, hands, feet and posterior elements of the spine (Fig. 5). Medullary lesions show mild to moderate reactive sclerosis, which may be remote from the nidus [2].
Subperiosteal lesions are located on the external aspect of the cortex, and are frequently seen along the medial aspect of the femoral neck, hands, feet and neck of the talus (Fig. 6). These lesions cause pressure, atrophy or erosion of the underlying cortical bone and may have less compact periosteal reaction than typical cortical osteoid osteomas. Endosteal osteoid osteomas are located on the internal aspect of the cortex and may cause an atypical cortical circumferential thickening. It has been suggested that cortical, medullary and endosteal lesions migrate from a subperiosteal location [24].
Intra-articular
While not part of the classification described by Kayser et al. [24], intra-articular osteoid osteomas have an atypical presentation and imaging appearance. The clinical symptoms include joint tenderness, stiffness, swelling and effusion, which can mimic infection or arthritis. The pain may not be worse at night. The most frequently affected joint is the hip, with the ankle, elbow, wrist and knee less common [31, 35]. On imaging, there is minimal or absent reactive cortical thickening and sclerosis due to lack of cambium in the inner layer of periosteum (Fig. 7) [4]. There is frequently a reactive joint effusion with a lymphoproliferative synovitis and widening of the joint space [36]. Soft-tissue abnormalities can be observed in up to 50% of patients. Atrophy of adjacent muscles, evidenced by fatty replacement, has also been reported. When involving the proximal femur, extensive bone marrow edema may be appreciated extending from the femoral neck into the proximal diaphysis [31]. If the nidus is not identified, these findings can lead to misdiagnosis of infection or neoplasm (Fig. 8) [31].
Spine
Spinal osteoid osteomas are more frequently located in the posterior elements (Fig. 9). The lumbar spine is the most common location, followed by the cervical and thoracic spine, and the sacrum. Patients may present with radicular pain, painful scoliosis or gait disturbances. Plain films can show reactive sclerosis, but due to the anatomy and overlapping structures, CT or MRI is usually needed for localization of the lesion [4].
Hands and feet
Carpal and tarsal osteoid osteomas are usually intramedullary and are considered intra-articular in these locations (Fig. 10). There is less reactive sclerosis than a typical cortical lesion and inflammatory changes may involve the adjacent bones, soft tissues and joints, causing synovitis and joint effusion (Fig. 11). Lesions of the tubular bones of the hands and feet are most commonly cortical, and can present with soft-tissue swelling, clinically mimicking infection or arthritis (Fig. 12) [4].
Growth disturbances
Osteoid osteomas can cause failure of normal tubulation of long bones, bony overgrowth and disuse osteopenia, resulting in growth disturbances (Fig. 13). When located near a physis, an osteoid osteoma can lead to a leg length discrepancy, elongating the affected limb [22]. Other complications of growth caused by osteoid osteomas include angular deformity, joint contracture and scoliosis with atrophy of the paraspinal muscles (Fig. 14).
Differential considerations
While imaging characteristics of osteoid osteomas can often be straightforward, other lesions can mimic their radiologic appearance. A Brodie abscess of chronic osteomyelitis with a radiolucent center and surrounding reactive sclerosis can appear much like an osteoid osteoma when small in size (Fig. 15). In an intracortical abscess, the sequestrum is often irregular in shape and the inner margin of the area of lucency is not smooth, in contrast to an osteoid osteoma, which often has smooth inner margins (Fig. 16) [4]. Tumors can also pose a diagnostic challenge. Chondroblastomas occur in the epiphyses of children and can appear as an osteolytic lesion associated with extensive bone marrow edema as well as periosteal reaction, similar to an osteoid osteoma. An epiphyseal and intramedullary location can help differentiate chondroblastomas from osteoid osteomas, which the latter are more commonly diaphyseal and intracortical (Fig. 17) [4, 37]. In children, cortical tibial lesions require special consideration because a variety of pathologies within the cortex of the tibia can produce cortical thickening and proliferation, such as osteofibrous dysplasia (Fig. 18), adamantinoma and stress fractures [38]. Stress fractures, commonly seen within the lower extremities, can mimic osteoid osteomas on conventional radiography, as both can produce focal cortical thickening of varying degrees. Cross-sectional imaging is helpful to delineate a discrete fracture line in an area of cortical thickening, as opposed to the central nidus of an osteoid osteoma (Fig. 19).
Treatment
Although there is some evidence to suggest that osteoid osteomas may regress spontaneously, this may take several years of observation and pain management with nonsteroidal anti-inflammatory medication, and thus is not a suitable option for children [39, 40]. Complete surgical resection has proved to be curative when the nidus is removed; however, when this approach is used in the weight-bearing bones, a prolonged period of restricted activities may be required. In addition, the surgically created bone defect may lead to pathologic fracture [41]. CT-guided percutaneous resection, a less invasive method, is performed with CT guidance to minimize the amount of excised bone and ensure that the entire nidus is resected, as incomplete resection can lead to recurrence. At our institution, surgically resected osteoid osteomas are all performed with CT guidance, with 40% of children requiring an overnight hospital stay for pain management. When the lower extremities are involved, activities are restricted to toe touch weight-bearing for approximately 4 weeks, with slow progression to full weight-bearing over the next 4 weeks.
Radiofrequency (RF) ablation has been successfully used to treat osteoid osteomas for more than 20 years [42] and is currently the choice of treatment for most lesions [41]. RF ablation is performed with the use of CT guidance, and the entire procedure generally takes 90 min to perform. The RF electrode is inserted with the tip directed toward the nidus and thermal heating is applied for 4–6 min. Larger lesions may require multiple ablation cycles with repositioning of the electrode tip. Success rates have been reported as high as 90% (Fig. 20) [41]. RF is preferred over a surgical approach in treating epiphyseal lesions that may require arthrotomy with resultant impairment of bone growth and/or joint mobility. Intra-articular lesions, located on or immediately beneath joint surfaces, adjacent to the articular cartilage have also successfully been treated with RF ablation and have been well tolerated (Fig. 21) [39]. Percutaneous RF ablation may not be advisable for spinal osteoid osteomas or lesions within the hand, as the cortex may not be a reliable insulator to protect the adjacent neural tissue. In our experience, patients treated with RF ablation are discharged to home the same day and very few require pain medications. All daily activities are resumed immediately and there are no restricted activities.
Conclusion
In most cases, the classic appearance of an osteoid osteoma does not pose a diagnostic challenge. Throbbing pain at night that responds to salicylates and the radiolucent nidus surrounded by variable degrees of reactive sclerosis are distinctive features of osteoid osteomas that usually lead to a straightforward diagnosis. However, on occasion, the clinical picture and the imaging findings are atypical and may lead to difficulties in diagnosis. Lesions in atypical locations can have a confusing clinical presentation and appearance on MR imaging. Incorrect diagnosis may delay treatment, which can result in long-term morbidity, especially with intra-articular lesions. Knowledge of these atypical findings should lead to a careful search for a nidus, prompting a subsequent CT. Mimics of osteoid osteoma should be considered in the differential diagnosis, as imaging appearances can have overlapping findings. Radiologists should be familiar with these less common forms of osteoid osteoma to avoid extensive workups, unnecessary invasive procedures and delayed treatment.
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Acknowledgements
This work was awarded an honorable mention as an educational poster at SPR 2012, San Francisco, CA.
We thank Dr. Anne Marie Cahill from the Department of Radiology and Dr. John Dormans from the Department of Orthopedics at the Children’s Hospital of Philadelphia for their help and contributions to this manuscript.
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Laurence, N., Epelman, M., Markowitz, R.I. et al. Osteoid osteomas: a pain in the night diagnosis. Pediatr Radiol 42, 1490–1501 (2012). https://doi.org/10.1007/s00247-012-2495-y
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DOI: https://doi.org/10.1007/s00247-012-2495-y