It’s not entirely clear how myositis ossificans forms. It’s believed that fibroblasts, which are cells that are involved in the healing response of injury might incorrectly differentiate into bone forming cells. The word myositis ossificans means that bone forms within the muscle, and this occurs at the site of the injury. Myositis ossificans is a self-limited disease process that will generally resolve on its own.
Signs
Aching pain within the muscle that persists longer than expected with a normal muscle contusion Limited mobility of joints surrounding the injured muscle Swelling or firmness of the muscle group, sometimes extending throughout the extremity
Many tests can be performed if you have a bone mass within one of your muscles. Most often, the initial test is an X-ray. When an X-ray shows bone within muscle, the most serious concern is that it could be a tumor. Fortunately, myositis ossificans has some features that usually differentiate it from a tumor.
If there is any uncertainty about your diagnosis, repeat X-rays may be obtained several weeks later to determine whether the bone mass is typical of myositis ossificans. Other imaging tests, including ultrasound, computerized tomography (CT) scan, magnetic resonance imaging (MRI), or bone scan can also differentiate myositis ossificans from other conditions.
In addition, your healthcare provider might order laboratory tests. These tests might include an alkaline phosphatase level, which can be detected in the bloodstream. This test may be normal in the early stages of myositis ossificans, and later elevated levels may peak within two or three months of the injury, and resolve within six months of the injury.
A biopsy is not typically necessary, but if it’s done, it will confirm the finding of a thin rim of bone surrounding a central cavity of fibroblast cells. A biopsy can be obtained with a surgical procedure, or a biopsy sample can be obtained using a needle that’s inserted into the mass. This test is typically done in situations the diagnosis is unclear, or if the mass might be a tumor—and not myositis.
Treatment
The early stages of treatment are focused on limiting any further bleeding or inflammation within the muscle.
Early steps include:
Rest Ice application Immobilization Anti-Inflammatory Medication Gentle stretching
Rarely is surgical excision of the myositis ossificans needed. If excision is warranted, most surgeons wait between six and 12 months before considering removal. There are some concerns that when myositis ossificans is removed too soon, it can return. That said, there is little evidence that a specific waiting period is necessary, and there is still a chance of return even when the bone is removed long after it develops.
Myositis ossificans is only removed surgically if it causes persistent symptoms despite appropriate nonsurgical treatment. Surgery may be considered if things like impaired joint motion or pressure from the mass on a nerve develop.
