Osteoid osteoma

What is an osteoid osteoma?

An osteoid osteoma is a relatively common non-cancerous bone tumour. These tumours are usually small (less than 2 cm) and are most frequently found in the bones of the legs, arms, spine, hands, and feet. They generally occur in children and young adults but can occasionally occur in older adults. The most common symptom is pain, which may be severe and is often worse at night.


Bones are a special type of hard tissue that provide our bodies with support and protection. In total there are 206 bones in each of our bodies. Some bones (like the bones in our fingers) are very small while other bones (like the bones in our upper legs) are very big.

Normal bones develop when specialized cells called osteoblasts produce a substance called osteoid. Osteoid is initially soft but becomes hard over time as minerals (such as calcium) are added to it. Osteoid that is still soft is called ‘immature’ bone while mineralized, hard osteoid is called ‘mature’ bone. The immature bone starts out with a woven, disorganized appearance under the microscope before undergoing mineralization and remodelling, which makes it hard and strong.

How do pathologists make this diagnosis?

This diagnosis can be made after a small tissue sample is removed in a procedure called a biopsy or when the entire tumour is removed in a procedure called a resection. The tissue is then sent to a pathologist for examination under the microscope. However, in some cases, the diagnosis can be made without a tissue examination because the symptoms and the appearance on X-rays are very characteristic.

Under the microscope, an osteoid osteoma is made up of disorganized, immature new bone called osteoid surrounded by osteoblasts. Pathologists use the phrase “osteoblastic rimming” to describe osteoid surrounded by osteoblasts. This feature is important because some types of bone cancer can look similar to osteoid osteoma under the microscope, but bone cancer does not show osteoblastic rimming.

osteoid osteoma

This diagnosis can only be made if the tumour is less than 2 cm in size. For this reason, your pathologist may also look at your X-ray or other imaging results before making the diagnosis of osteoid osteoma. A tumour that looks similar when examined under the microscope but measures more than 2 cm is called an osteoblastoma.


Treatment options include radio-frequency ablation or surgical removal of the tumour. A tiny piece of the tumour might also be removed by a special biopsy needle during radio-frequency ablation. That tissue will then be sent to a pathologist for examination under the microscope.

by Ashley Flaman MD and Bibianna Purgina MD FRCPC (July 20, 2021)
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