More than 70 % in long tubular bones. Most frequent sites: upper end of femur neck and trochanter and lower end of femur, tibia, and humerus. Metacarpal, metatarsal, and phalanges sites are relatively frequent. Osteoid osteomas arise more frequently in metaphysis and diaphysis. Epiphyseal locations are infrequent. More common in cortex or subperiosteal location. Less common intramedullary.
Fifteen percent in spine. In the vertebral column, it tends to involve the posterior elements – 90 %.
Clinical Symptoms and Signs
Pain is the classic and most frequent symptom and is practically diagnostic.
More intense at night.
Sometimes referred to a nearby joint – mimicking inflammatory arthritis.
Relieved dramatically by aspirin and nonsteroidal anti-inflammatory medication.
Painful local swelling in superficial bones.
Atrophy of muscles in affected limb, localized swelling, and tenderness.
In joints: swelling, joint effusions, joint pain, and limited motion in tumors localized in periarticular bones, mimicking inflammatory arthritis.
When in a lower extremity, the patient presents with a limp.
Lesions located near the growth plate may cause overgrowth of bone and bone length discrepancy.
In small bones of the hands and feet, important swelling may mimic infection.
When in spine: painful scoliosis with the concavity at the side of the lesion, due to paravertebral muscle spasm. Symptoms may mimic lumbar disc disease.
Osteoid osteoma is usually a solitary lesion.
Nidus – less than 2 cm – is characteristic, round and osteolytic in most cases. Twenty-five percent present with mineralization of the central area or a ringlike calcified central area – “target pattern.” The nidus is surrounded by sclerosis.
Three types of location:
Cortical: the most frequent and classic location. Characteristic and striking perifocal reactive sclerosis.
Subperiosteal: the nidus abuts to the soft tissue, raising the periosteum, which produces a usually thick layer of reactive bone formation.
Medullary: mild perifocal bone sclerosis.
Extensive sclerosis may mask the nidus in nearly 25 % of cases.
The most useful method to show the nidus, especially when nidus is masked by sclerosis.
Round small, well-demarcated lesion. Often presents with central mineralization.
Of limited utility
Low signal in T1
High signal intensity in T2 with peripheral and soft tissue edema –sometimes in great amounts
Angio-MRI: useful in demonstrating the hypervascularized nidus
Extremely hot spot
Image Differential Diagnosis
Intraosseous Abscess (Brodie Abscess)
Lack central calcification
Massive bone peripheral sclerosis
Mature periosteal bone formation
In angio-MRI: less vascularized lesion
Circumscribed small osteolytic intracortical lesion
Surrounded by a large zone of sclerotic bone
Transverse fracture: on the tension side.
Oblique fracture: on the compression side.
Later on, an intramedullary horizontal sclerotic zone repairs the lucent line.
Diffuse periosteal bone reaction overlying the sclerotic area.
In resection specimens, the nidus is comprised in a block of sclerotic bone.
In most cases the lesion is round or oval, less than 1 cm in greatest diameter and no more than 2 cm.
Fragmented specimens are frequently submitted.
The nidus has a reddish and hyperemic aspect.
The center of the nidus is often chalky and gritty white or grayish.
In cases in which the nidus is difficult to identify, the pathologist must cut the specimen in slabs no more than 5 mm thick and radiograph it.
Another procedure is to label the nidus with preoperative tetracycline, because the nidus will stand out under ultraviolet light.
Histologically, the nidus is composed by haphazard anastomosing immature osteoid and bone trabeculae, lined by uniform osteoblasts, with round regular nuclei and abundant cytoplasm and a few benign multinucleated giant cells of osteoclastic type embedded in a loose fibrovascular stroma.
Isolated osteoblasts may show an epithelioid aspect, but nuclear pleomorphism or atypia is absent.
In the central part of the nidus, immature mineralized tissue may predominate, and it is responsible for the radiographic images of more dense central area.
Cartilage is not present.
The surrounding sclerotic area is composed of cancellous bone tissue with thickened trabeculae separated by a richly vascularized fibrous connective tissue, varying to a densely sclerotic compact bone.
Occasionally, long-standing lesions may present central areas of the nidus with a more mature bone tissue, with typical “mosaic” pattern similar of Paget’s disease of bone.
Tumors localized in periarticular sites frequently are associated with reactive angiohyperplastic synovitis, mimicking rheumatoid arthritis.
Findings associated with the mechanism of pain are:
Extremely vascularized lesion.
Fibrous zones surrounding the nidus with increase of blood vessels.
Special silver impregnation techniques demonstrate axons of nerve fibers associated with blood vessels within the nidus. This feature is uncommon in other bone tumors and practically seen exclusively in osteoid osteomas.
Expression of S100 and neurofilament by immunohistochemistry confirms the former findings.
Prostaglandin E2, prostacyclins, and COX-2 overexpression in tumoral osteoblasts.
Pathologic Differential Diagnosis
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Both tumors are morphologically similar.
More than 2 cm in diameter
Lacks sclerotic peripheral reactive area
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