Osteoid Osteoma is a benign, bone forming neoplasm. It has a small nidus of neoplastic tissue surrounded by heavy zone of reactive mature bone. This benign tumor is usually located intracortically within the diaphysis of long bones. Osteoid osteoma affects males more than females, and 70% of patients are younger than 20 years old.
The proximal femur around the area of the lesser trochanter is a common location for osteoid osteoma to form. The femur and tibia are the most common locations for osteoid osteoma to occur, as well as the posterior elements of the spine. For example, the pedicles or the lamina. In addition, osteoid osteoma is the most common benign tumor of the carpal bones.
Osteoid osteoma is a painful condition that is worse at night and has no history of trauma. The painful symptoms are mediated by Prostaglandin E2. There will also be increased Cyclooxygenase (COX) activity, which is why the pain caused by the lesion is relieved by aspirin and anti-inflammatory drugs.
With osteoid osteoma, you will have a central lucent nidus area surrounded by a sclerotic area. The nidus, which is the lytic lesion, is oval or round and it is well demarcated. The diameter of the nidus is usually less than 1.5 cm, and has a self-limiting growth.
CT scan and MRI will show the lesion as well circumscribed and a cortically base lesion with significant surrounding edema. Increased uptake will be found (aka hot bone scan). Osteoid osteoma can look like a stress fracture. If the lesions has large bone reaction, stress fracture can be ruled out. In addition, a stress fracture produces more linear radiolucency.
There are a few differential diagnoses. Brodie’s abscess is one of these. Osteoid osteoma is located within the cortex, while Brodie’s abscess is located within the medullary canal or in the cancellous bone. However, the abscess may be surrounded with fibrous tissue and sclerotic bone. In this case, it may be difficult to differentiate the Brodie’s abscess from the osteoid osteoma. Osteosarcoma may also need to be differentiated. In this case, new bone formation will be seen. Finally, osteoblastoma would be more than 2 cm. in size and provoke less reactive host sclerosis than osteoid osteoma would.
Osteoid osteoma is the most common cause of painful scoliosis in young patients. The curvature of the scoliosis is concave towards the site of the lesion. Osteoid osteoma of the thoracic spine will cause the apex of the scoliosis to correspond to the level of the lesion. However, in the lumbar spine, specifically in the lower lumbar region, the apex of the scoliosis may be above the lesion. Surgical excision of the lesion can help the scoliosis, except if the curvature is large or, in younger patients, if the excision is delayed.
The pathology will show very cellular and vascular stroma with plump, but not atypical osteoblast cells, making a matrix of immature woven bone. The heavy, mature reactive trabeculae encircles the nidus. There will be no inflammatory cells or dead bone to suggest Brodie’s abscess or osteomyelitis. There will be demarcation between the nidus and the bone, and the woven bone will have rimming osteoblasts.
Osteoid osteoma usually becomes asymptomatic and spontaneously heals. The most important step in the treatment of osteoid osteoma is observation and oral anti-inflammatory medications. If the conservative treatment fails, a CT guided radiofrequency ablation can be performed. The radiofrequency ablation is done in a majority of cases that are painful, except in the spine, because of the proximity to the dura and the nerve roots. Radiofrequency ablation should also not be used in the hands. Radiofrequency is not used in the digits because of the thermal necrosis of the overlying skin and due to the proximity of the neurovascular structures. Since radiofrequency ablation cannot be used in the spine and hands, surgical resection with curettage is used in these locations.
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