Neurogenic Heterotopic Ossification

Heterotopic ossification is defined as the formation within the soft tissues of abnormal, ectopic lamellar bone containing bone marrow.

Extensive ossification in the soft tissues around both shoulders in a patient with head injury an year ago
however the underlying bones are un affected.  

It has 3 etiologies:
Trauma (fractures, dislocations, post-surgery, burns)
Genetic (fibrodysplasia ossificans progressiva)
Neurogenic (mainly spinal cord injury and traumatic brain injury)

In a patient with such soft tissue ossification, ask for history of either a direct trauma to that site or trauma/surgery to brain/ spinal cord, one of which is the etiology in most of the cases.