Heterotopic ossification (HO) is a pathological condition of abnormal bone formation in soft tissue. HO causes pain and restricts range of motion, greatly impairing functional mobility of the patient. Blast trauma-induced HO, in particular, has the highest incidence, and is the single most significant obstacle for combat-injured military personnel to return to active duty. Despite the clinical importance, the underlying mechanism of HO is poorly understood and effective treatment options are lacking.

This study aims to explore the cellular and molecular mechanism of blast trauma-induced HO. Using a live animal blast trauma model, we firstly showed that muscle trauma resulting from blast wave exposure is associated with HO formation. We then investigated the role of muscle injury in HO pathogenesis and showed that bone morphogenetic protein-7 (BMP-7) expression was highly upregulated following muscle injury. Inhibition of BMP-7 activity in vitro suppressed the osteogenesis-promoting effect of culture medium conditioned by injured muscle tissue on muscle-derived stromal cells (MDSCs), and in vivo reduced the volume of HO. These results indicate that BMP-7 is a key osteoinductive factor in injured muscle that facilitates HO formation.

Next, we sought to identify additional factors that could contribute to the pathogenesis of blast trauma-induced HO. We investigated the impact of glucocorticoids on HO formation since excessive endogenous glucocorticoid production due to stress is found in severe trauma patients. Our results showed that dexamethasone treatment together with cardiotoxin (CTX)-induced muscle injury led to a significant amount of dystrophic calcification (DC) in muscle. This effect was likely related to decreased circulating transforming growth factor-beta 1 (TGF-β1) level, as supplementation of recombinant TGF-β1 markedly rescued this phenomenon.

Lastly, we investigated the involvement of muscle tissue death in HO pathogenesis. We observed that implantation of devitalized muscle tissue together with muscle injury induced by CTX injection caused DC formation, accompanied by a systemic effect of lowered circulating TGF-β1 level.

Taken together, these results strongly suggest that a two-hit mechanism of blast traumainduced HO, namely muscle injury-induced BMP-7 upregulation combined with pathological condition-induced downregulation of circulating TGF-;​21 level, is an important causative mechanism of DC/HO formation.