Heterotopic Ossification: Cellular Basis, Symptoms, and Treatment

Heterotopic ossification (HO) is the process by which calcified bone develops in soft tissues. Because of the abnormal calcification, complications such as bone deformation, loss of range of motion, and joint immobility adversely affect patients. There are many genetic types of heterotopic ossification, namely fibrodysplasia ossificans progressiva, progressive osseous heteroplasia, and Albright hereditary osteodystrophy. However, this condition can also arise from surgery, burns, or traumatic injuries, so it is seen as an important area for research in the future. There are various treatments available such as non-steroidal anti-inflammatory drugs and radiation therapy, as well as combinations of the two. The molecular basis of HO is currently being explored in hopes of developing drugs that prevent the development of heterotopic bone. Additionally, new methods of treatment that offer fewer side effects may be promising for patients in the future. HETEROTOPIC OSSIFICATION 4 Heterotopic Ossification: Cellular Basis, Symptoms, and Treatment Background Heterotopic ossification (HO) is the process by which bone calcifies in soft tissues where it does not belong. In contrast to orthotopic ossification, which is the formation of tissue in the correct anatomical positions, HO causes bone to grow between muscle planes, eventually leading to some form of physical debilitation for patients (Mania et al., 2009). Because heterotopic bone can form on muscles, ligaments, and tendons, there are many different complications that can arise from its presence, depending on the location of the body. This process of bone formation can arise in different ways, including genetics, traumatic injury, and surgery. Heterotopic ossification was first reported in literature in 1692 by Patin as myositis ossificans progressiva syndrome, but there were similar reports given later on from different cases. There were records of HO occurring from neurological injuries and traumas, and in paraplegic patients. Although various types of heterotopic ossification have been recognized and treatments have been utilized for years, there is still not a complete understanding of this condition. A fully successful prevention of heterotopic bone is still of interest to researchers, as this problem affects the lifestyle of those who have it (Baird & Kang, 2009). As a result of the unclear differentiation between the naturally occurring ossification process and the heterotopic calcification, researchers and physicians find it hard to investigate what actually causes the buildup of bone to occur. Since there are multiple factors that are contributing to this process, especially on a molecular basis, the HETEROTOPIC OSSIFICATION 5 challenge of preventing this condition will continue to exist for years to come. However, society now has access to the latest advances in technology that were not available when HO was first reported. Advancement has come through imaging techniques and medical technology, such as being able to prepare tissue samples for analysis. Because heterotopic bone may interact with proximal tissues, it is important to identify differences in composition. The configuration of the periosteum, a membrane that lines the outermost surface of bone, is not affected by the development of new bone, although the existing skeletal structure can be covered with more calcified bone (Baird & Kang, 2009). Types of HO Since heterotopic ossification is the process by which bone tissue develops outside of the skeleton, it is common for HO to occur in the vicinity of joints. Overall, it decreases range of motion and can cause complete joint ankylosis in which surgical intervention is required. Usually, the development of HO occurs through one of three routes: genetic predisposition, post-traumatic injury, or post-surgery. Most cases of HO come from the two latter categories, so studies focused on preventing HO is especially important for the medical field. HO may be classified in a rare hereditary form or a most common acquired form, due to injuries or joint replacements. To this day, there are three distinct genetic hereditary forms known to humans: fibrodysplasia ossificans progressiva, progressive osseous heteroplasia, and Albright hereditary osteodystrophy. When it is acquired, HO occurs frequently after neurologic injuries including spinal trauma or a head injury. In addition, it can occur in areas of the body where a joint is replaced, such as the hip,