Myositis ossificans

Myositis ossificans comprises two syndromes characterized by heterotopic ossification (calcification) of muscle. In 2020, the World Health Organization classified myositis ossificans together with fibro-osseous pseudotumor of digits as a single specific entity in the category of fibroblastic and myofibroblastic tumors.

Classification

In the first, and by far most common type, nonhereditary myositis ossificans (commonly referred to simply as "myositis ossificans", as in the remainder of this article), calcifications occur at the site of injured muscle, most commonly in the arms or in the quadriceps of the thighs.

The term myositis ossificans traumatica is sometimes used when the condition is due to trauma. Also myositis ossificans circumscripta is another synonym of myositis ossificans traumatica refers to the new extraosseous bone that appears after trauma.

The second condition, myositis ossificans progressiva (also referred to as fibrodysplasia ossificans progressiva) is an inherited affliction, autosomal dominant pattern, in which the ossification can occur without injury, and typically grows in a predictable pattern. Although this disorder can be passed to offspring by those afflicted with FOP, it is also classified as nonhereditary, as it is most often attributed to a spontaneous genetic mutation upon conception.

Most (i.e. 80%) ossifications arise in the thigh or arm, and are caused by a premature return to activity after an injury. Other sites include intercostal spaces, erector spinae, pectoralis muscles, glutei, and the chest.