Osteoid osteoma of the femur
Osteoid osteoma is a benign osteoblastic tumor that is usually less than 2 cm in size. It consists of a central vascularized nidus that represents the neoplastic tissue. The nidus is surrounded by normal reactive bone. It is most frequently found in long bones, such as the femur and tibia, but can occur at any site. Cortical thickening surrounding a small central core of lower density, the nidus, are the classical radiological finding.
X-rays may be normal; however, the cortical thickening is generally well visualized with this imaging modality. The central “nidus” is sometimes visible on x-rays as a well-circumscribed lucent region, occasionally with a central sclerotic dot. CT imaging considered to be the imaging modality of choice. On this imaging modality appearing as a focally lucent area within surrounding sclerotic reactive bone. In juxta-articular localization, the reactive sclerosis may be absent.
Kayser et al. have classified osteoid osteoma into four categories according to the relation of the lesion with the bone cortex: subperiosteal (located on the external aspect of the cortex, surrounded by periosteal reaction), intracortical (located within the cortex), endosteal (on the internal of the cortex) and medullary (located within the medullary bone).