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DIAGNOSIS: Osteoid osteoma
DISCUSSION:
Osteoid osteomas are a benign skeletal lesion composed of osteoid and woven bone. Most average 1cm in size, but by definition they should be less then 1.5cm. If the lesion measures greater then 2 cm, an alternate diagnosis, specifically osteoblastoma, should be considered.
Osteoid osteoma represents 12% of all benign skeletal neoplasms. Most lesions occur in patients less then 25 years of age and are twice as common in males as females. The exact etiology is not known but is thought to be an inflammatory response.
Patients classically present with localized pain with tenderness to palpation. This pain is worse at night and is relieved by small doses of aspirin. Ninety percent of osteoid osteomas occur in the femur and tibia. Twenty percent of all lesions occur around the knee. The metadiaphysis is the most common location in the long bone.
Osteoid osteomas are classified into three subtypes: cortical, cancellous and subperiosteal.
Cortical osteoid osteoma are the most common and present as a nidus within the cortex with associated solid or laminated periosteal reaction and prominent sclerotic cortical thickening.
Cancellous osteoid osteomas are intramedullary in location and are most commonly located in the femoral neck. Cancellous tumors present with little to no associated sclerosis.
Subperiosteal osteoid osteomas are rare and present as a round soft-tissue mass adjacent to bone with associated bony sclerosis and are most commonly located at the medial aspect of the femoral neck.
Although some consider CT the study of choice for nidus localization, CT may fail to identify medullary (cancellous) lesions, due to lack of associated bony sclerosis. The typical cortical osteoid osteomas present on CT as a small well-defined oval nidus surrounded by a variable amount of sclerosis.
MR imaging can reliably demonstrate the nidus of an osteoid osteoma. Furthermore, a large majority of osteoid osteomas encountered by MR are of the cancellous variety that are, as discussed above, difficult to identify on CT.
On MR, osteoid osteoma most commonly present with a ring pattern: hypointense peripherally and variable signal intensity centrally. Perilesional reactive bone marrow edema is seen in 63% with adjacent soft-tissue edema seen in 47%. Synovitis and joint effusion can be seen with periarticular lesions.
Treatment is complete surgical excision of the nidus or percutaneous CT guided radiofrequency ablation.
Differential diagnosis for cortical lesions includes: stress fracture, osteoblastoma, Brodie abscess, skeletal form of reflex sympathetic dystrophy and more aggressive lesions such as osteosarcoma and Ewing sarcoma. Periarticular lesions must be differentiated from infectious and inflammatory arthritis.
References:
Pomeranz Stephen J. Gamuts and Pearls in MRI & Orthopedics.
MRI-EFI Publications; 1997.
Resnick Donald, Kang Heung. Internal Derangements of Joints: Emphasis on MR Imaging.
W.B. Saunders Co. 1997. pg 745.
SpougeAR. Thain LM. Osteoid osteoma: MR imaging revisited.
Clinical Imaging. 2000 Jan-Feb; 24 (1). pg 19-27.
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