Heterotopic ossification is a medical condition whereby bone tissue forms outside the normal skeleton - e.g. in muscles and other organs.

It can be caused by trauma - for example, it's estimated that more than half of patients who undergo hip replacement surgery will go on to develop it (though it's only rarely problematic to the patient). Other types of trauma and injury can lead to the condition :

Heterotopic ossification often develops in patients with traumatic brain or spinal cord injuries, other severe neurologic disorders or severe burns, most commonly around the hips. The mechanism is unknown.

Source :Wikipedia

The ossification is common in the case of spinal injuries, and usually manifests itself below the site of the injury. (ref.)