Myositis Ossificans vs Heterotropic Ossification

Attachment 1420
During discussion, we were given the x-ray of a 35-year old quadriplegic male patient due to a C7 complete spinal cord injury that happened over a year ago. My groupmates thought that the patient acquired myositis ossificans and I thought that it was heterotrophic ossification but I was not sure about it. So in the end, we presented the case and was told that it was heterotropic ossification. But it was not explained clearly why it was heterotropic ossification. Can someone explain why it is heterotropic ossification and not myositis ossificans? Also, I really can't find a distinct difference between the two. Does myositis ossificans really fall under the category of heterotropic ossification?