Challenges of heterotopic ossification-Molecular background and current treatment strategies.

Heterotopic ossification (HO) is an abnormal formation of mature lamellar bone within extraskeletal soft tissues, such as muscles, tendons, and ligaments. This process is thought to be induced by inflammation associated with tissue injuries. HO is classified using two subtypes: resulting from injury or genetically inherited. HO formation is associated with polytrauma patients with traumatic brain injuries and spinal cord injuries. Moreover, HO is also considered to be a post-operative risk factor in some orthopaedic procedures. In this review, we summarize our current understanding of the pathology of different types of HO and discuss its current and future therapies. Thus far, research has revealed cellular and molecular pathways leading to HO formation and proposed several possible mechanisms leading to HO and conserved signalling pathways common in the different HO subtypes. Non-steroidal anti-inflammatory drug treatment and localized low-dose irradiation are currently the only available prophylactic treatments for HO. However, they are not always effective and do not target the osteogenic processes directly. New therapeutic strategies targeting the pathological processes of HO, such as bone morphogenetic protein (BMP) inhibitors like noggin, BMP type 1 receptor inhibitor, and nuclear retinoid acid receptor-gamma (RARγ) agonists, are currently being investigated. In-depth understanding of the HO pathological process could help to develop effective therapeutic strategies.