Myositis ossificans is an unusual condition that causes bone to form deep within muscles of the body. Often this condition is found in young athletes who sustain a traumatic injury, or sometimes as a result of repetitive injury to the muscle. Most commonly found in the thigh, and sometimes in the forearm, myositis ossificans often occurs in athletes such as football or soccer players.

Exactly why myositis ossificans occurs is not entirely clear. The current theory is that cells important in the healing response of injury called fibroblasts 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 known as a self-limited disease process, meaning that given time it will resolve on its own.

Signs

Aching pain within the muscle that persists longer than expected for a normal muscle contusion
Limited mobility of joints surrounding the injured muscle
Swelling of the muscle group, and sometimes extending throughout the extremity

Many tests can be performed to evaluate patients who have a bone mass found within a muscle. Most often, the initial test performed is an x-ray. The common concern when abnormal bone is seen on a x-ray is that there could be a tumor within the soft-tissues. Fortunately, myositis ossificans has some typical clues that usually make it easily differentiated from a tumor.

If there is any question about the diagnosis, repeat x-rays will be obtained several weeks later to ensure the bone mass is a typical myositis ossificans. Other imaging tests including ultrasound, CT scan, MRIs, and bone scans can also be performed to differentiate myositis ossificans from other conditions. In addition, some doctors will perform laboratory tests. These tests include the alkaline phosphatase which can be detected in the bloodstream. This test may be normal in the early stages of myositis ossificans, followed by elevated levels peaking within 2-3 months of the injury, and resolving within 6 months of the injury.

Biopsies are not typically necessary, but if performed the test will confirm the finding of a thin rim of bone surrounding a central cavity of fibroblast cells. Biopsies can be performed as a surgical procedure or can be done through a needle inserted into the mass. As mentioned before, this test is typically performed in situations where there may be a concern about the diagnosis, and if the mass could be a tumor and not myositis.

Treatment

The early stages of treatment are focused on limiting any further bleeding or inflammation within the muscle. Therefore, early steps include:

Rest
Ice application
Immobilization
Anti-Inflammatory Medication

Rarely is surgical excision of the myositis ossificans warranted. If the myositis ossificans is removed too soon, there are concerns about it returning. Therefore, most surgeons wait between 6 and 12 months before considering removal. That said, there is little evidence that a specific time of waiting is necessary. Furthermore, there is a chance of return even when removed very late. Myositis ossificans is only removed surgically if there are persistent symptoms despite appropriate nonsurgical treatment such as interference with joint motion or pressure from the mass on a nerve.