Myositis ossificans comprises two syndromes characterized by heterotopic ossification (calcification) of muscle.

• 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 second condition, myositis ossificans progressiva (also referred to as fibrodysplasia ossificans progressiva) is an inherited affliction, autosomal dominant pattern, in which the calcification occurs without injury, and in a predictable pattern.

Myositis ossificans usually presents with pain, tenderness, focal swelling, and joint muscle reduction, in the aftermath of a painful muscle contusion which resolved more slowly than expected, if at all.

The condition rarely is asymptomatic, and may sometimes be diagnosed from radiographs obtained for unrelated problems.

Most (ie, 80%) ossifications arise in the thigh or arm, and are predisposed to by a too-early return to activity after an injury. Other sites include intercostal spaces, erector spinae, pectoralis muscles, glutei, and the chest. Hazy densities are sometimes noted ca. one month after injury, while the denser opacities eventually seen may not be apparent until two months have passed

Treatment is initially conservative, as some patients' calcifications will spontaneously be reabsorbed, and others will have minimal symptoms. In occasional cases, surgical debridement of the abnormal tissue is required, although success of such therapy is limited.