What is Myositis Ossificans including its epidemiology and clinical presentation?
What is Myositis Ossificans?
Myositis if we break the word down, myo means muscle, and ositis is bone; so it is the formation of bone within the muscle. However, the extraskeletal bone formation may be found in not just a muscle but also the fascia, tendon, ligaments, in and under the skin, vascular wall, or virtually any site where there is connective tissue. The extra bone growth can occur in any soft tissue. The bone formation may be intramembranous, or it may be present in the endochondral pathways. That means they could be within the belly or within the structure of the soft tissue, but they might be present outside of those areas as well
Nongenetic heterotrophic ossification occurs most often in young adults with a history of trauma or surgery. So what does this mean for us? Half of the patients are in the second to third decades of their life, so about 20- to 30-year-olds. Men are more affected than women and non-genetic form is more prevalent than genetic. So, it is more common to have heterotrophic ossification from complications of trauma, surgery, and other systemic issues. Having said that, that already gives us a patient profile at this point.
Heterotrophic ossification occurs at an increased frequency for some conditions. For example. it occurs in approximately forty percent of patients with post-hip arthroplasty and in about 30% of cases of bone fractures or dislocation, with elbow trauma or dislocation being a common site. It is not just an orthopedic situation, it could occur in up to 50% of spinal cord injuries, and up to 20% of third-degree burns. For severe traumatic amputations, this incidence rises to above 90%.
Patients with ankylosing spondylitis, Paget's disease, and hypertrophic osteoarthritis are at risk of developing myositis ossificans post-arthroplasty.
The early stage is from zero to four weeks. We see the laying down of fibroblasts and myofibroblasts. We are seeing a little bit of cellular activity at this point. Between four to eight weeks, we start seeing osteoblasts and gradually this starts turning into mature bone tissue. After 8 weeks (the mature period) it turns into mature lamellar bone.
In the early/inflammatory phase, your patients will complain of localized pain, tenderness, and swelling. A rapid increase in size, and it might mimic a soft tissue sarcoma. In later stages and with gradual maturation of the bone tissue, the swelling becomes more localized, firm, and when adjacent to a joint may restrict motion. Lesions resembling myositis ossificans have been reported within nerves or the abdominal mesentery and fascia, and their presentation is site specific. The overall presentation, in the sense of what clinical symptoms they show, depends on the site that they affect.
This Ask the Expert is an edited excerpt from the course, Myositis Ossificans: Classification, Pathology, Case Study, and Rehabilitation presented by Rina Pandya, PT, DPT