Osteoid Osteoma

Osteoid osteomas are common, benign, bone-forming tumors that are characterized by small size (< 1.5 cm), limited growth potential, and disproportionate pain. Histologically, they are composed of osteoid and woven bone and are often surrounded by a rim of plump osteoblasts, findings that can also be seen in osteoblastomas.

Patients are young, more often white and male, and present with pain that is initially intermittent and mild that eventually becomes more constant and severe. The pain is usually more intense at night and usually (75% of the time) responds to aspirin. The pain may precede radiographic findings, and may be referred to a nearby joint.

When in the spine, patients commonly have scoliosis, which is less likely to occur with an osteoblastoma involving the spine. In fact, osteoblastoma is the most common cause of painful scoliosis in adolescents. Unlike osteoblastomas of the spine, osteoid osteomas are rarely associated with neurological symptoms.

Before continuing, a word is warranted on usage of the term "nidus." Properly, the nidus refers to the lucent lesion of the osteoid osteoma itself, and not the central mineralization that can be seen in some osteoid osteomas.

Three types of osteoid osteomas have been described by Edeiken et al:

• Cortical/classic: The most common type, seen in 75% of cases. A lucent lesion, the nidus, is located in the center of reactive sclerosis and periosteal reaction. The periosteal reaction is usually solid, but may be lamellated. Tends to affect the tibia and femur.
• Cancellous/medullary (shown above): About 25% of cases. Tends to affect the femoral neck, small bones of the hands and feet, and the spine. Sclerosis, if present, is mild or moderate. The lucent nidus is not necessarily in the center of the sclerosis, which may be located some distance away. In the spine, the differential diagnosis would include osteoblastoma.
• Subperiosteal: The rarest type. Presents as soft tissue mass adjacent to bone, which may demonstrate pressure erosion or resorption, but little or no significant reactive sclerosis. Tends to affect the medial aspect of the femoral neck, the hands and feet, and the talar neck. Patients with a painful joint may initially be evaluated with ultrasound, and the finding of cortical irregularity and focal synovitis should raise the possibility of an intraarticular osteoid osteoma.
  
  Subperiosteal osteoid osteomas may actually represent the early stage of a cortical osteoid osteoma, with the subperiosteal getting incorporated into the cortex by periosteal new bone formation.

Depending on their relationship to the joint, osteoid osteomas can also be classified as intraarticular. Intraarticular osteoid osteomas are generally of the cancellous or subperiosteal type, and most commonly occur at the hip. Imaging findings may be subtle due to the absence of periosteal reaction. Indeed, when the nidus is within 1 cm of the articular surface, little or no periosteal reaction may be present. Imaging studies may reveal nonspecific joint effusion, synovitis, and regional osteoporosis. Radiographs may be normal at first.

Bone scintigraphy reveals increased activity on all three phases. The double density sign describes focal increased uptake (nidus) surrounded by a larger area of increased uptake. The double-density sign can help differentiate an osteoid osteoma from osteomyelitis. In osteomyelitis, there may be an area of decreased uptake corresponding to the lucent sinus or abscess.

On CT, the nidus of a cortical or cancellous lesion is a smooth low-attenuation lesion with or without surrounding sclerosis and periosteal reaction. The nidus may contain central mineralization, which may be punctate, amorphous, ringlike, or dense.

MRI is not as good as CT for detection of osteoid osteomas in about 2/3 of cases. MRI reveals the nidus to be intermediate signal intensity on T1-weighted images, and intermediate-to-high on T2-weighted images. Central mineralization, if present, can cause low signal intensity. The surrounding marrow may show increased T2 signal. Lesions show early enhancement with early washout compared to the adjacent marrow on dynamic MRI.

The images above depict a cancellous osteoid osteoma presenting as a low-attenuation lesion with central mineralization, located within the femoral neck. Note the mild surrounding sclerosis. The mineralized center is low signal intensity on the T1- and T2-weighted images and is located within an intermediate signal intensity nidus, which is surrounded by bone marrow edema.

Treatment consists of radiofrequency ablation, which has a success rate similar to that of surgery. The target is the lucent nidus. After treatment, the surrounding sclerosis may regress.

References

• Ebrahim FS, Jacobson JA, Lin J, Housner JA, Hayes CW, Resnick D. Intraarticular osteoid osteoma: sonographic findings in three patients with radiographic, CT, and MR imaging correlation. AJR Am J Roentgenol. 2001 Dec;177(6):1391-5.
• Edeiken J, DePalma AF, Hodes PJ. Osteoid osteoma. (Roentgenographic emphasis). Clin Orthop Relat Res. 1966 Nov-Dec;49:201-6.
• Helms CA, Hattner RS, Vogler JB 3rd. Osteoid osteoma: radionuclide diagnosis. Radiology. 1984 Jun;151(3):779-84.
• Kransdord MJ and Murphey MD. Chapter 14: Osseous tumors. in Imaging of Bone Tumors and Tumor-Like Lesions. Davies AM, Sundaram M, and James SLJ (eds). Springer-Verlag Berlin Heidelberg (2009); pp 261-269.
• Pettine KA, Klassen RA. Osteoid-osteoma and osteoblastoma of the spine. J Bone Joint Surg Am. 1986 Mar;68(3):354-61.