Myositis ossificans (my-uh-SY-tuss uh-SIH-fuh-kanz) is when a bone forms inside your muscle or other soft tissue. Usually, myositis ossificans develops after a traumatic injury. Most often, it affects large muscles, such as in your arms or legs.
When bone forms where it shouldn’t, you may develop a painful, tender lump. Myositis ossificans that develops after an injury is the most common type of heterotopic ossification.
MO is typically caused by direct trauma to the muscle, like a knee to the thigh in rugby or landing hard on the ground in a bike accident. If you develop a large contusion (bruise) and a pool of blood forms a lump over the injury (hematoma), bone may grow at the injury site. This is responsible for 20 percent
Repeated minor trauma, sometimes seen on the inside thighs of horseback riders — can also result in MO. Fractures and surgery, especially hip replacement, are other possible causes.
Although larger muscles of the upper arm and thigh are most commonly affected, MO can also occur in fat or connective tissue like tendons, ligaments, and fascia.
Sports injuries or accidents usually initiate MO. Adolescents and young adults in their 20s are most likely to develop MO. It’s rare for children 10 and under to get the condition. People who have paraplegia are also prone to MO, but usually with no evidence of trauma.
The most common sign of myositis ossificans is a large lump beneath your skin. In around 4 in 5 people, the bump forms in your arm or leg muscle. People with paraplegia are more likely to have lumps grow around their hips or knees.
The lump may be:
As the lump gets bigger, it may reduce your range of motion. You’re more likely to have reduced range of motion if the growth is near a joint.
Before diagnosing myositis ossificans, a doctor will ask about what happened, how long ago it happened, the symptoms, and what steps a person has taken to manage the pain or injury.
If it has been at least 2–3 weeks since the pain or other symptoms started, the doctor may order some imaging tests to look for evidence of bone growth in the soft tissue.
The types of tests that the doctor may use include:
Sometimes, early diagnostic testing can confuse myositis ossificans with certain types of soft tissue cancers. As a result, the doctor may order additional testing to confirm the diagnosis.
This condition will be rated on limitation of motion of affected body part.
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